U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the California State Water Resources Control Board

A product of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program

Variations on a Method for Evaluating Decadal-Scale Changes in the 

Groundwater Quality of Two GAMA Coastal Study Units 2004–14, 

California GAMA Priority Basin Project

Scientific Investigations Report 2018–5088
Front Cover Map: Groundwater basins categorized by sampling priority. Location of groundwater basin 

boundaries from California Department of Water Resources (CDWR, 2003).

Cover photographs:

Front cover: Vineyard in Napa Valley, California (Photograph taken by George L. Bennett, 

U.S. Geological Survey.)

Back cover: View from Point Lobos State Natural Reserve, California (Photograph taken by Robert Kent, U.S. 

Geological Survey.)

Sampling priority

Study area

Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins
Variations on a Method for Evaluating 

Decadal-Scale Changes in the 

Groundwater Quality of Two GAMA Coastal 

Study Units 2004–14, California GAMA 

Priority Basin Project

By Robert Kent

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2018–5088

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Kent, R., 2018, Variations on a method for evaluating decadal-scale changes in the groundwater quality of two GAMA 

coastal study units 2004–14, California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations 

Report 2018–5088, 75 p., https://doi.org/10.3133/sir20185088.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Descriptions of the Evaluated Coastal Study Units.........................................................................2

North San Francisco Bay Study Unit........................................................................................2

Monterey Bay and Salinas Valley Basins Study Unit ............................................................2

Methods...........................................................................................................................................................6

Well Selection .......................................................................................................................................6

Sample Collection and Analysis.........................................................................................................6

Data Processing....................................................................................................................................7

Censored Results.........................................................................................................................8

Difference Thresholds ................................................................................................................8

Difference Thresholds Set by the GAMA-PBP Replicate Acceptability Criteria .....9

Difference Threshold Set by Confidence Intervals.......................................................9

Reporting Levels ...............................................................................................................10

Hypothesis Tests on the Differences......................................................................................10

Additional Factors Considered ................................................................................................10

Quality Control.....................................................................................................................................11

Field Blanks.................................................................................................................................11

Replicates....................................................................................................................................11

Laboratory Matrix Spikes .........................................................................................................12

Results ...........................................................................................................................................................12

Water-Quality Indicators ...................................................................................................................13

Nutrients...............................................................................................................................................18

Major and Minor Ions and Silica......................................................................................................20

Trace Elements....................................................................................................................................31

Isotopic Constituents .........................................................................................................................43

Summary and Conclusions.........................................................................................................................47

Constituents Exhibiting Conclusive Evidence of Step Trends .....................................................47

Inconclusive Step-Trend Findings....................................................................................................49

Comparison of the Three Evaluation Methods ..............................................................................50

Implications for Water Managers....................................................................................................51

References Cited..........................................................................................................................................51

Tables ...........................................................................................................................................................54

Acknowledgments

We thank the following cooperators for their support: The California State Water Resources 

Control Board, the California Department of Public Health, and the California Department of 

Water Resources. We especially thank the cooperating well owners and water purveyors for 

their generosity in allowing the U.S. Geological Survey to collect groundwater samples from 

their wells. 
iv

Figures

1. Map showing the study units sampled for the Groundwater Ambient Monitoring 

and Assessment Priority Basin Project, 2004–14, highlighting the two coastal 

study units that are the subject of this report..........................................................................3

2. Map showing the North San Francisco Bay study unit of the Groundwater 

Ambient Monitoring and Assessment Priority Basin Project, the wells sampled 

in 2004, and the wells resampled in 2014..................................................................................4

3. Map showing the Monterey Bay and Salinas Valley study unit of the Groundwater 

Ambient Monitoring and Assessment Priority Basin Project, the wells sampled in 

2005, and the wells resampled in 2014 ......................................................................................5

4. Graph showing paired results of water temperatures measured in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................13

5. Graph showing paired results of dissolved oxygen measured in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................14

6. Graph showing paired results of field-measured specific conductance of 

groundwater during the collection of initial samples (2004–05) and decadal samples 

(2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley 

Basins Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................14

7. Graphs showing paired results of laboratory-measured specific conductance of 

groundwater during the collection of paired initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas 

Valley Basins Groundwater Ambient Monitoring and Assessment Priority Basin 

Project study units......................................................................................................................15

8. Graph showing paired results of field-measured pH of groundwater during the 

collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................16

9. Graphs showing paired results of laboratory-measured pH of groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................17

10. Graphs showing paired results of carbonate concentrations in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................18

11. Graphs showing paired results of total nitrogen concentrations in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................19

12. Graphs showing paired results of nitrate concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................20
v

13. Graphs showing paired results of orthophosphate concentrations in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................21

14. Graphs showing paired results of calcium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................22

15. Graphs showing paired results of magnesium concentrations in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................24

16. Graphs showing paired results of potassium concentrations in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................25

17. Graphs showing paired results of sodium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................26

18. Graphs showing paired results of chloride concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................27

19. Graphs showing paired results of iodide concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................28

20. Graphs showing paired results of sulfate concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................29

21. Graphs showing paired results of fluoride concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................30

22. Graphs showing paired results of silica concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................32

23. Graphs showing paired results of aluminum concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................33

24. Graphs showing paired results of arsenic concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................34

Figures—Continued
vi

25. Graphs showing paired results of barium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................35

26. Graphs showing paired results of iron concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................36

27. Graphs showing paired results of lead concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................37

28. Graphs showing paired results of lithium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................38

29. Graphs showing paired results of manganese concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................39

30. Graphs showing paired results of nickel concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................40

31. Graphs showing paired results of selenium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................41

32. Graphs showing paired results of strontium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................42

33. Graphs showing paired results of vanadium concentrations in groundwater during 

the collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................43

34. Graphs showing paired results of the isotopic ratio of oxygen-18 in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................44

35. Graphs showing paired results of the isotopic ratio of carbon-13 in groundwater 

during the collection of initial samples (2004–05) and decadal samples (2014) in 

the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

study units....................................................................................................................................45

36. Graphs showing paired results of tritium activities in groundwater during the 

collection of initial samples (2004–05) and decadal samples (2014) in the North 

San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project study units .........................46

Figures—Continued
vii

Tables

1. Sample dates, elevations, and construction information for the 50 trend wells 

that were sampled two times, approximately 9–10 years apart, in the North San 

Francisco Bay and Monterey Bay and Salinas Valley Basins study units........................54

2A. Constituents detected in at least 10 percent of the samples collected during 

initial sampling or resampling of trend wells, the number of paired results, number 

increased, number decreased, and p values from Wilcoxon signed-rank test with 

Pratt correction using different threshold criteria for the combined study units............56

2B. Constituents detected in at least 10 percent of the samples collected during 

initial sampling or resampling of trend wells, the number of paired results, number 

increased, number decreased, and p values from Wilcoxon signed-rank test with 

Pratt correction using different threshold criteria for the North San Francisco Bay 

study unit......................................................................................................................................62

2C. Constituents detected in at least 10 percent of the samples collected during 

initial sampling or resampling of trend wells, the number of paired results, number 

increased, number decreased, and p values from Wilcoxon signed-rank test 

with Pratt correction using different threshold criteria for the Monterey Bay and 

Salinas Valley Basins study unit...............................................................................................68

2D. Constituents detected in at least 10 percent of the samples collected during 

initial sampling or resampling of trend wells, the number of paired results, number 

increased, number decreased, and p values from Wilcoxon signed-rank test with 

Pratt correction using different threshold criteria: Summary of statistical evaluation 

results for constituents exhibiting step trends in this report ..............................................74

3. Constituents for which one of three circumstances indicate step-trend results are 

inconclusive.................................................................................................................................75

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8.
viii

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

National Geodetic Vertical Datum of 1929 (NGVD 29).

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Activities for radioactive constituents in water are given in picocuries per liter (pCi/L).

Results for measurements of stable isotopes of an element (with symbol E) in water, solids, 

and dissolved constituents commonly are expressed as the relative difference in the ratio of 

the number of the less abundant isotope (i

E) to the number of the more abundant isotope of a 

sample with respect to a measurement standard.

Delta notation, δiE, the ratio of a heavier isotope of an element (iE) to the more common lighter 

isotope of that element, relative to a standard reference material, expressed as per mil.

Abbreviations and Acronyms

CDWR California Department of Water Resources

CI confidence interval

EPA U.S. Environmental Protection Agency

GAMA Groundwater Ambient Monitoring and Assessment Program

GAMA PBP Groundwater Ambient Monitoring and Assessment Program Priority Basin

 Project

GRAC replicate acceptability criteria 

HAL-US lifetime health advisory level (USEPA)

MCL-US maximum contaminant level (USEPA

MCL-CA California maximum contaminant level

NFM National Field Manual (USGS)

NAWQA National Water-Quality Assessment 

NSF North San Francisco Bay

NWQL National Water Quality Laboratory (USGS)

PBP Priority Basin Project

QA quality assurance

QC quality control
ix

QSB Quality Systems Branch (USGS)

SALMON Monterey and Salinas Valley Basins 

SMCL-CA California secondary maximum contaminant level

SRL study reporting level 

TDS total dissolved solids

USGS U.S. Geological Survey

VOCs volatile organic compounds

Abbreviations and Acronyms—Continued

Variations on a Method for Evaluating Decadal-Scale 

Changes in the Groundwater Quality of Two GAMA 

Coastal Study Units 2004–14, California GAMA Priority 

Basin Project

By Robert Kent

Abstract

Decadal changes in groundwater quality in two study 

units on the north-central California coast were evaluated 

by the Priority Basin Project (PBP) of the Groundwater 

Ambient Monitoring and Assessment (GAMA) Program. 

Groundwater samples collected from wells during 2004–05 

were compared on a pair-wise basis to samples collected 

from the same wells during 2014. The data set consisted of 

paired-samples from 50 public supply wells in 2 GAMAPBP study units (25 wells each in the North San Francisco 

Bay and the Monterey Bay and Salinas Valley Basins), with 

analytical results for 160 water-quality constituents. Statistical 

analysis was done on grouped results for the 59 constituents 

that were detected in at least 10 percent of the samples during 

either sampling period to evaluate decadal-scale change by a 

step-trend analysis.

The data for both of the sampling periods were processed 

three different ways, resulting in three variations of the 

paired results to be submitted for statistical analyses. The first 

evaluation method variation processed data only to facilitate 

comparison of data when one or both of the results was a 

non-detection. The second and third variations applied the 

additional requirement that differences between initial and 

decadal-sample results exceed a defined threshold to prevent 

small differences from supporting the conclusion of a step 

trend. One method for setting the difference threshold between 

initial-sampling and resampling results is based on criteria 

used by the GAMA-PBP to determine whether or not replicate 

results are acceptable. The other difference-threshold-setting 

method uses a calculated confidence interval around each 

result based on demonstrated analytical variability for the 

constituents during each sampling period. Finally, constituents 

for which decadal-scale changes were statistically significant 

were identified using the Wilcoxon-Pratt signed-rank test on 

each of the three evaluation method variations.

Step trends were identified by at least 1 of the 3 method 

variations for 33 constituents. After considering other factors, 

however, such as water-quality context and the results of 

quality-control samples, it was concluded that decadal changes 

were meaningful for 14 constituents in at least 1 of the 2 study 

units. Constituents for which step trends indicated meaningful 

increases were dissolved oxygen, total nitrogen, nitrate, 

orthophosphate, calcium, chloride, sulfate, iron, and lithium. 

Constituents for which step trends indicated meaningful 

decreases were temperature, arsenic, lead, the isotopic ratio of 

carbon-13, and tritium.

Introduction

The California State Water Resources Control 

Board implemented the Groundwater Ambient 

Monitoring and Assessment (GAMA) program to assess 

California groundwater quality (California State Water 

Resources Control Board, 2003). Part of the GAMA 

program is the Priority Basin Project (GAMA-PBP), 

undertaken in cooperation with the U.S. Geological Survey 

(USGS; http://ca.water.usgs.gov/gama/; Belitz and others, 

2003; Kulongoski and Belitz, 2004). The GAMA-PBP 

is assessing water quality in three ways: (1) status of 

groundwater quality, (2) understanding of factors that affect 

groundwater quality, and (3) trends in groundwater quality. 

The statewide assessments for status and understanding of 

groundwater quality were done by sequentially sampling 

more than 2,300 wells in 35 defined “study units” ranging in 

area from less than 80 square kilometers (km2

) for the Santa 

Barbara study unit to more than 40,000 km2 for the Sierra 

Nevada study unit. Sampling for the statewide assessment of 

temporal trends in California groundwater quality is ongoing 

(Kent and Landon, 2013; Kent and others, 2014; Kent, 2015; 

Kent and Landon, 2016; Mathany, 2017).
2 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Purpose and Scope

The purpose of this report is to present variations on a 

method to identify step trends for water-quality constituents 

measured in groundwater samples collected during two 

distinct periods about 10 years apart. Differences were 

evaluated using a step-trend analysis comparing results of 

paired samples collected from 50 wells in two coastal GAMAPBP study units during 2004–05 with those collected in 2014. 

The method to identify step trends presented here consists 

of data processing and statistical testing to determine the 

significance of the measured changes, and it is illustrated using 

paired sample data from the two sampling periods. The present 

study builds on methods used to evaluate step trends in “Major 

Aquifer Studies” by the National Water-Quality Assessment 

(NAWQA) Project (Rosen and Lapham, 2008; Rupert, 2008; 

Lindsey and Rupert, 2012). The report is not intended to 

explore the causative factors leading to the observed changes, 

and the evaluations are not intended to predict whether the 

observed changes might continue in the future.

Descriptions of the Evaluated Coastal Study 

Units

Groundwater-quality trend assessments are facilitated 

by grouping study units into regions with relatively similar 

geologic, climatic, and hydrologic characteristics. The two 

coastal study units—the North San Francisco Bay study 

unit and the Monterey Bay and Salinas Valley Basins study 

unit—are separated by about 160 miles of the north-central 

California coast (fig. 1), are nearly the same size, and share 

similar climates, although the geologic and hydrologic 

characteristics differ. Water-bearing deposits in both study 

units include Quaternary alluvium. Pliocene volcanic 

deposits are important in the North San Francisco Bay study 

unit, however, whereas sandstones are important in the 

Monterey Bay and Salinas Valley Basins study unit. Note 

that these differences in geology and hydrology between the 

two study units limit the usefulness of the grouped study 

unit evaluations. 

North San Francisco Bay Study Unit

The GAMA-PBP assessed area of the North San 

Francisco Bay study unit (figs. 1, 2) is nearly 2,600 square 

kilometers (km2), mostly in Napa, Sonoma, and Marin 

Counties, and is in the Northern Coast Ranges hydrogeologic 

province (Belitz and others, 2003). The climate is 

characterized by warm, dry summers and cool, moist 

winters, with an average precipitation of 76 centimeters 

falling primarily during the winter and early spring 

(Kulongoski and others, 2006). The study unit consists of 

seven California Department of Water Resources (CDWR)-

defined groundwater basins (California Department of Water 

Resources, 2003). The hydrogeologic settings of the North San 

Francisco Bay study unit, its groundwater basins, its individual 

study areas, and groundwater-quality results for the GAMAPBP wells in the study unit are described by Kulongoski 

and others (2006). The status of groundwater quality in the 

primary aquifers in the North San Francisco Bay study unit 

and an understanding assessment of the natural and human 

factors affecting groundwater quality in the study unit are 

provided by Kulongoski and others (2010). Results of 3-year 

trend sampling in the North San Francisco Bay study unit are 

provided by Kent and others (2014).

Monterey Bay and Salinas Valley Basins Study 

Unit

The GAMA-PBP assessed area of the Monterey Bay and 

Salinas Valley Basins study unit (figs. 1, 3) is nearly 2,600 km2

in Monterey, Santa Cruz, and San Luis Obispo Counties and 

is in the Southern Coast Ranges hydrogeologic province 

(Belitz and others, 2003). The climate is characterized by 

warm, dry summers and cool, moist winters, with an average 

precipitation of 50 centimeters falling primarily during the 

winter and early spring (Kulongoski and Belitz, 2007). The 

study unit consists of eight CDWR-defined groundwater 

basins (California Department of Water Resources, 2003). The 

hydrogeologic settings of the Monterey Bay and Salinas Valley 

Basins study unit, its groundwater basins and subbasins, and 

its individual study areas are described by Kulongoski and 

Belitz (2007). The status of groundwater quality in the primary 

aquifers in the Monterey Bay and Salinas Valley Basins 

study unit and an understanding assessment of the natural 

and human factors affecting groundwater quality in the study 

unit are provided by Kulongoski and Belitz (2011). Results of 

3-year trend sampling in the Monterey Bay and Salinas Valley 

Basins study unit are provided by Kent and others (2014).
Introduction 3

OREGON

NEVADA

MEXICO

ARIZONA

40 ̊

42 ̊ 124 ̊ 122 ̊ 120 ̊ 118 ̊ 116 ̊ 114 ̊

38 ̊

36 ̊

34 ̊

PA C I F I C O C E A N

Bakersfield

Los Angeles

San Diego

Redding

SacramentoSacramento

San

Francisco

Valley

Klamath

Mountains

Transverse Ranges and

selected Peninsular Ranges

Southern Coast

Ranges

Sierra

Nevada

Northern

Coast

Ranges

Desert

Central

Basin and Range

San Diego

Drainages

0 200 MILES

0 200 KILOMETERS

100

100

Monterey Bay/Salinas

study unit (fig. 3)

Cascades and 

Modoc Plateau

North San

Francisco

Bay study

unit

(fig. 2)

Base modified from U.S. Geological Survey and other Federal and State 

digital data, various scales; Albers Equal-Area Conic projection, standard 

parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

sac16-0618_fig 01

Figure 1. Study units sampled for the Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project, 2004–14, 

highlighting the two coastal study units that are the subject of this report.
4 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 02

Lake

Berryessa

Lake

Sonoma

Napa River

1

80 680

Napa Sonoma

Benicia

Petaluma

Calistoga

Sebastopol

Santa

Rosa

80

780

1

116

128 101 175

16

101

116

37

29

29

121

121

128

12

29

128

101

SONOMA CO SOLANO CO

MARIN CO CONTRA COSTA CO

NAPA CO

SOLANO

CO

MENDOCINO LAKE CO YOLO CO

CO

SONOMA CO

LAKE CO

NAPA CO

SONOMA

CO

SONOMA CO

MARIN CO

San Pablo

Bay

Pacific

Ocean

123° 122°30'

38°

30'

38°

North San Francisco Bay study unit

0 5 10 MILES

0 5 10 KILOMETERS

Base modified from U.S. Geological Survey and other Federal and State 

digital data, various scales; Albers Equal-Area Conic projection, standard 

parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

EXPLANATION

Valleys and Plains

Volcanic

Wilson Grove

Sonoma Creek

Russian River

Putah Creek

Cache Creek

Site sample frequency

Decadal trend well

Triennial trend well

Decadal and triennial

 trend well

Status well only

Figure 2. North San Francisco Bay study unit of the Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project, 

the wells sampled in 2004, and the wells resampled in 2014.
Introduction 5

sac16-0630_fig 03

Fresno Slough

California Aqueduct

Lake

Nacimiento

Lake 

San 

Antonio

SAN LUIS OBISPO CO KERN

CO

KINGS CO

MONTEREY CO

SAN BENITO CO

FRESNO CO

MERCED CO

MERCED CO

MADERA CO

MADERA CO

FRESNO CO

101 1

1

101

46

46

198

25

25

152

152

156

165

33

33

33

99

17

9

5

San Benito River

S alinas River

Gilroy

Salinas

Monterey

Los Banos

Santa

Cruz

Soledad

King City

Pacific

Ocean

Monterey

Bay

San Luis

Reservoir

Anderson

 Lake

Carmel River

122° 121°30' 121° 120°30'

37°

36°

30'

36°

35°

30'

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey and other Federal and State 

digital data, various scales; Albers Equal-Area Conic projection, standard 

parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

EXPLANATION

Monterey Bay and Salinas Valley Basins study unit

Santa Cruz

Monterey Bay

Site sample frequency

Salinas Valley

Paso Robles

Decadal trend well

Triennial trend well

Decadal and triennial

 trend well

Status well only

Figure 3. Monterey Bay and Salinas Valley study unit of the Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin 

Project, the wells sampled in 2005, and the wells resampled in 2014.
6 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Methods

This study evaluated changes in the groundwater 

quality of two coastal GAMA study units using five broad 

steps: (1) select wells for initial sampling and select a 

subset of these same wells approximately 10 years later for 

resampling; (2) collect, process, and have samples analyzed 

(including quality-control, or QC, samples) for the waterquality constituents of interest; (3) compile the groundwater 

and QC sample results and perform data verification on the 

two data sets to characterize any potential bias that could 

affect the groundwater sample results from one or both of 

the sampling periods; (4) for all constituents detected in 

more than 10 percent of the samples for either sampling 

period, process the quality-assured, paired results iteratively 

to handle censored data (non-detections) and to ensure that 

differences between the initial and resampling results meet 

a sufficient threshold to support a conclusion of a change in 

groundwater quality; and (5) submit the processed grouped 

data pairs to a nonparametric hypothesis test on differences 

between the initial and resampling results. The method builds 

on methods used by the USGS NAWQA program to identify 

step trends in groundwater quality (Rosen and Lapham, 2008; 

Rupert, 2008; Lindsey and Rupert, 2012; Lindsey and others, 

2016). Differences between the NAWQA method and the 

method used in the present study are described in the relevant 

subsections of this “Methods” section.

Well Selection

The well networks were designed to provide a spatially 

unbiased assessment of the quality of untreated groundwater 

used for public water supplies. The relatively deep production 

wells were selected by using a spatially distributed, 

randomized grid-based method (Belitz and others, 2010) to 

provide statistical representation of the water quality in the 

primary aquifers in the areas assessed. Wells selected for 

sampling in this way are referred to as “grid wells.” They are 

also referred to as “status wells,” because the results from their 

sampling were used to characterize the status of groundwater 

quality in the study units (Kulongoski and others, 2010; 

Kulongoski and Belitz, 2011). Additional wells were sampled 

to improve understanding of specific groundwater-quality 

issues. Such wells are referred to as “understanding wells.” 

For the 2 coastal study units included in this report, 175 status 

wells were initially sampled: 84 wells in the North San 

Francisco Bay study unit (Kulongoski and others, 2006) and 

91 in the Monterey Bay and Salinas Valley Basins study unit 

(Kulongoski and Belitz, 2007). 

Approximately 10 years after the initial sampling, 

50 previously sampled wells (approximately 25 percent of the 

number of status wells) were selected for resampling to assess 

step trends (“trend wells”). Twenty-five wells were selected 

from each of the two study units. As with the initial selection 

of status wells, priority for the selection of trend wells was 

randomly assigned to the status wells; however, other factors 

were considered during the process of selecting trend wells. 

An effort was made to maintain spatial distribution and avoid 

areal clustering of trend wells. Also, preference was given to 

wells that had already been resampled once to assess trends 

approximately 3 years after the initial sampling (Kent and 

others, 2014; Kent and Landon, 2016). In addition, preference 

was given to wells for which the initial samples were analyzed 

for the most complete suites of constituents, including organic 

constituents, inorganic constituents, and selected isotopes. 

Many initial samples collected from the GAMA-PBP status 

wells in 2004–05 were not analyzed for inorganic constituents, 

such as major ions, trace elements, nutrients, or the selected 

isotopes (Kulongoski and others, 2006; Kulongoski and Belitz, 

2007). Status wells that were sampled for these constituents 

were given preference when selecting trend wells so that a 

greater number of constituents could be evaluated for step 

trends. Well identifiers, sample dates, elevations, and depth 

information for the 50 trend wells are provided in table 1. 

Table 1 also provides links to the analytical results for the 

trend wells.

Sample Collection and Analysis

Samples were collected following modified USGS 

National Field Manual (NFM; U.S. Geological Survey, 

variously dated) and modified USGS NAWQA Project 

(Koterba and others, 1995) sampling protocols. These 

sampling protocols were followed so that samples would 

represent groundwater in the aquifer and the potential for 

contamination would be minimal. Following these protocols 

also allows for comparison to data collected by the GAMAPBP throughout California and to other USGS projects in 

California and the Nation. For detailed descriptions of specific 

sampling procedures and analytical methods used during 

initial sampling in the two study units, see Kulongoski and 

others (2006) and Kulongoski and Belitz (2007). For detailed 

descriptions of the trend-sample collection (resampling) and 

analyses, see Kent and others (2014).

Table 1 of the data release (Kent, 2018 https://doi.

org/10.5066/F7GH9GF5) lists the 160 water-quality indicators 

and chemical constituents, by class, that were analyzed for 

most initial and decadal samples. Included were 4 fieldmeasured parameters, 6 laboratory-measured water-quality 

indicators, 5 nutrient species of nitrogen or phosphorus, 

perchlorate, 9 major ions, silica, 22 trace elements, 44 volatile 

organic compounds, 63 pesticide compounds, and 5 isotopic 

constituents.
Methods 7

Five laboratories were used for chemical analyses 

of samples collected during the initial-sampling period 

(Kulongoski and others, 2006, Kulongoski and Belitz, 2007) 

and the decadal-sampling period (Kent and others, 2014). 

Most of the analyses were done at the USGS National Water 

Quality Laboratory (NWQL). The NWQL maintains a qualityassurance (QA) program (Pirkey and Glodt, 1998; Maloney, 

2005). The USGS Quality Systems Branch (QSB) maintains 

independent oversight of QA at the NWQL. The QSB 

documents bias and variability for the NWQL through the 

use of blind QA samples and provides periodic data-quality 

assessment summaries based on the sample results. Steptrend evaluations potentially affected by bias or variability 

documented in these summaries are discussed in the “Results” 

and the “Summary and Conclusions” sections.

Initial and decadal-sample data for 50 wells were 

used for the present study. During decadal sampling, all 

160 constituents were measured for samples from all 50 wells, 

with only a few exceptions. In contrast, initial samples lacked 

analyses for some constituents, particularly for field-measured 

parameters and inorganic constituents. For this reason, the 

number of paired-sample measurements for field-measured 

parameters, water-quality indicators, nutrients, perchlorate, 

major ions, trace elements, and carbon isotopes varied 

from 21 to 50 (Kent, 2018, table 1, https://doi.org/10.5066/

F7GH9GF5).

Some of the trend wells lacking initial-sample data were 

resampled approximately 3 years later (Kent and Landon, 

2016). Data from 12 wells, resampled in 2007–08, were 

substituted as initial samples to supplement some of the 

lacking data for nutrients (11 trend wells), perchlorate (4), 

major ions and trace elements (6), and carbon isotopes (9). 

The initial-sample counts provided for these constituents in 

table 1 of the data release (Kent, 2018, https://doi.org/10.5066/

F7GH9GF5) include these supplemental data.

During the initial-sampling period, samples were 

analyzed for 88 volatile organic compounds (VOCs; 

Kulongoski and others, 2006; Kulongoski and Belitz, 2007). 

During the trend-sampling period, samples were analyzed 

for 87 VOCs (Kent and others, 2014). Forty-four VOCs were 

common to the analyses used during each period and could 

be evaluated for step trends (Kent, 2018, table 1, https://doi.

org/10.5066/F7GH9GF5). Samples from all 50 wells in each 

study period were analyzed for most (42) of these constituents. 

For 2 constituents—methyl acetate and tert-butyl alcohol—

only 36 initial samples were analyzed.

During the initial-sampling period, samples were 

analyzed for 63 pesticide compounds (Kulongoski and others, 

2006; Kulongoski and Belitz, 2007). During the decadalsampling period, samples were analyzed for 80 pesticide 

compounds (Kent and others, 2014). All of the samples were 

analyzed for pesticide compounds during both sampling 

periods, with one exception. The sample bottle for the initial 

sample from one well (NSFVOL-06) was broken during 

transit to the NWQL. The 63 compounds common to the 

analyses used during each period could be evaluated for 

step trends (Kent, 2018, table 1, https://doi.org/10.5066/

F7GH9GF5). 

Five isotopes were evaluated for step trends. Nearly 

all of the samples were analyzed for the stable isotopes of 

hydrogen and oxygen in water, as wells as for tritium, during 

both sampling periods. Only 21 of the initial samples from 

the 50 trend wells were analyzed for the 2 carbon-isotope 

parameters, however. Carbon-isotope samples collected from 

an additional 9 trend wells in 2007–08 to evaluate 3-yr trends 

were used as the initial samples, to make a total of 30 carbonisotope sample pairs (Kent, 2018, table 1, 

https://doi.org/10.5066/F7GH9GF5).

Data Processing

The results from initial and decadal sampling for all 

constituents detected in at least 10 percent of the samples from 

either sampling period were processed before performing 

statistical hypothesis tests on the differences between the 

grouped result pairs in an effort to prevent false detections of 

step trends. Constituents detected in fewer than 10 percent 

of the samples were unlikely to have sufficient data to detect 

step trends, so these constituents were not evaluated for step 

trends. The NAWQA program imposes a similar restriction on 

the number of statistical evaluations and considers constituents 

for which there are fewer than 10 untied, paired results 

“insufficient data”; constituents with insufficient data are not 

submitted to the statistical test (Lindsey and others, 2016). 

Data processing for the present study was done to achieve two 

objectives. The first objective was to manage censored results 

so that the magnitude of differences between sample pairs 

that included a censored result (reported only as less than the 

reporting level) was not overestimated. The second objective 

was to establish minimum thresholds for the differences 

between the paired results so that small differences due to 

analytical variability would not support the conclusion of a 

step trend. An example of the procedure used for processing 

the data for each constituent is provided in table 2 of the data 

release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5), an 

Excel workbook using the constituent lead as an example. 

The Excel workbook processes the input variables for each 

trend well. The input variables consist of the initial and 

10-year sampling results for the constituent, the result-specific 

reporting levels, the effective study reporting levels (when 

applicable), and the standard deviations and relative standard 

deviations determined by replicate samples collected during 

each sampling period. In the table, the formulas used to 

process the input variables are given in row 2 for the first trend 

well (MSSC-06). The data for this well are processed in row 3.
8 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Censored Results

Many sample results in this study, especially those for 

VOCs and pesticide compounds, were expressed as nondetections and were censored. That is, they were expressed 

as less than their reporting level. Conservative approaches 

were used to process censored data to avoid overestimating 

observed changes. In cases where the initial and decadal 

results were both censored, the results were considered 

analytically identical (tied), regardless of whether or not the 

two results were censored at the same reporting level. In 

cases where only the result for one of the two samples from 

a well was censored, the reporting level in effect at the time 

of analysis was substituted for the censored result so that the 

measured difference between the censored and uncensored 

results was minimized. For example, if the result for one 

of the two samples was 5 milligrams per liter (mg/L) and 

the result for the other was less than 1 mg/L, the difference 

between these sample results would be calculated as 4 mg/L, 

even though the difference could be as great as 5 mg/L if the 

constituent was absent in the sample with the censored result. 

In cases where an uncensored (detected) result was less than 

the reporting level of the censored result, however, they were 

considered tied, just as if both results were censored, because 

it would be impossible to discern a difference between 

them. The steps for processing censored data in this way are 

illustrated in columns K through M of table 2 of the data 

release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5). 

The data in columns K and L are submitted to the hypothesis 

tests to determine step trends based on differences with no 

threshold requirement.

An alternative approach used by Bexfield (2008), 

Barlow and others (2012), Lindsey and Rupert (2012), 

Lindsey and others (2016), and other researchers substitutes 

a uniform concentration value less than the lowest detected 

concentration for all results not detected by the laboratory. 

This is the approach used by the NAWQA program (Lindsey 

and others, 2016). Although this approach overcomes 

difficulties associated with variable laboratory reporting 

limits, it can overestimate the true difference between paired 

results. The true concentration of a constituent result censored 

by a relatively high reporting limit could actually be closer 

to that relatively high reporting limit than to the lower, 

substituted value. 

The potential difference overestimation could lead 

to perceived differences in paired results even though, for 

practical purposes, they are not different. When a censored 

result is paired with a detected result, the censored result 

is perceived to be the lesser of the two results because of 

the substitution. This is true even if the detected result is a 

concentration less than the reporting limit of the censored 

result. For example, if a constituent concentration of 

0.003 micrograms per liter (μg/L) is paired with a censored 

result of less than 0.007 μg/L, the censored result is perceived 

to be less than the detected result, even though the two results 

are consistent, because it would be substituted with a value 

lower than any detected result.

Difference Thresholds

Difference thresholds were set so that small differences 

between results of the initial and decadal samples as a result 

of analytical variability would not support a conclusion 

that a step trend had occurred. Two methods were used 

to set difference thresholds. Both methods were based 

on results from replicate quality-control samples and 

distinguished results with low concentrations from results 

with high concentrations relative to the reporting limits for 

each constituent. For purposes of these evaluations, a low 

concentration was less than five times the reporting limit, and 

a high concentration was greater than or equal to five times the 

reporting limit. This result concentration boundary between 

low and high concentrations of five times the reporting level 

has been used by GAMA-PBP since 2009 for evaluating 

replicate results (Densmore and others, 2009). It is based 

on the observation described by Mueller and Titus (2005) 

that “Variability over a large range of concentrations can 

be approximated by dividing this range into segments over 

which either the standard deviation or the relative standard 

deviation are reasonably constant” (Anderson, 1987). The 

steps for processing the sample-pair data to categorize the 

concentrations as low or high are illustrated in columns N 

through P of table 2 of the data release (Kent, 2018, 

https://doi.org/10.5066/F7GH9GF5). 

Other researchers apply no difference threshold. The 

justification for that approach could be that the random error 

associated with the results is expected to be expressed equally 

as slight positive or slight negative bias, and step-trend 

signals overcome such “noise” because the nonparametric 

statistical test assigns higher ranks to greater differences. Such 

an approach fails to take into consideration factors causing 

small analytical biases, such as changes in analytical methods 

over time and short-term laboratory issues. Short-term bias 

can affect the outcome of the statistical evaluation because 

a predominant number of the small (analytically identical) 

differences would be in just one direction. 

Results for constituents for which step trends were 

indicated, but short-term analytical bias was documented 

by the QSB, were subjected to additional evaluation before 

concluding that step trends had occurred. Data collected 

during the period(s) affected by short-term analytical bias were 

adjusted by the magnitude of bias estimated by the QSB. The 

hypothesis tests on the differences between the grouped results 

were repeated with the adjusted data to see if the bias could 

have affected the outcome of the tests. If a step trend was no 

longer indicated after performing the test on the adjusted data, 

the step-trend finding was considered inconclusive. 
Methods 9

Difference Thresholds Set by the GAMA-PBP Replicate 

Acceptability Criteria

The first method for setting the difference threshold 

was based on methods used by the GAMA-PBP to determine 

whether the replicate results are acceptable (Kent and 

others, 2014, p. 148). This is not to say that the GAMAPBP deems any results “unacceptable.” In cases where 

replicate results indicated high variability for a constituent 

during a study, mention of that high variability is included 

in reports prepared for that study. The GAMA-PBP replicate 

acceptability criteria (GRAC) was used to define high 

variability. The GRAC threshold-setting method asserts that 

if two results are similar enough to constitute a replicate that 

represents variability that is acceptable for project objectives, 

the small difference between them should not support the 

conclusion that there is a trend. The GAMA-PBP replicate 

results were considered acceptable (sufficiently similar) for 

low constituent concentrations if the standard deviation for 

the replicate pair was less than one half of the greater of the 

two reporting limits at the time of the analyses. For high 

concentrations (greater than five times the reporting limit), 

replicate results were acceptable if the relative standard 

deviation (also known as the coefficient of variation) for the 

replicate pair was less than 10 percent. When the difference 

between paired sampling results did not exceed the applicable 

threshold for the water-quality constituent in question, the 

two sample results were considered analytically identical, 

and each result was substituted with the sample-pair median 

so that a zero difference was indicated for that sample for the 

purpose of group tests on differences. The steps for processing 

constituent sample pairs for difference thresholds based on 

the GAMA-PBP replicate acceptability criteria (GRAC) 

are illustrated in columns Q through T of table 2 of the data 

release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5). 

Results in columns R and S are submitted to the hypothesis 

tests to determine step trends.

Difference thresholds, set by GRAC, were calculated as 

described for all of the evaluated water-quality constituents, 

except for pH, carbon-14 (in percent modern carbon), and the 

isotopic ratios of hydrogen and oxygen in water. The concepts 

of detection level and reporting limit are not relevant for the 

measurements of pH, carbon-14, or the isotopic ratios of 

hydrogen and oxygen in natural water. Because of this, some 

modifications were made to calculate difference thresholds 

for these constituents. The manufacturer of the field meters 

used by the GAMA-PBP for the measurement of pH, specifies 

precision is within 0.2 standard pH units (http://www.ysi.

com/productsdetail.php?556MPS-21). Because this level of 

precision implies that it would be possible for one of the two 

paired results to be as much as 0.2 units greater than the true 

value and the other to be as much as 0.2 units less than the true 

value, the difference threshold applied for field and laboratorymeasured pH was 0.4 standard pH units. The three laboratories 

that analyzed samples for carbon isotopes provided estimates 

of counting error in their measurements of carbon-14. The 

difference threshold for carbon-14 pairs was the greatest 

estimated error for the two results of percent modern carbon. 

No estimated error nor uncertainty was provided by the Reston 

Stable Isotope Laboratory for the results for isotopic ratios 

of hydrogen and oxygen in water. Therefore, the difference 

threshold between initial-sampling and resampling results for 

these isotopic ratios was set at a relative standard deviation 

greater than or equal to 10 percent.

Difference Threshold Set by Confidence Intervals

The second method for setting the difference threshold 

between initial-sampling and resampling results, the 

confidence interval (CI) method, uses the variability between 

replicate samples collected during initial and decadal-sampling 

periods for each constituent to estimate the (period specific) 

uncertainty for each sample. A 95-percent confidence interval 

was used for each result, based on comparisons of actual 

replicates collected by the GAMA-PBP. The CI difference 

threshold was exceeded if confidence intervals around the 

initial and decadal-sample results did not overlap. Similar 

to the GRAC difference-threshold-setting method, this CI 

method used a two-tiered approach to establish difference 

thresholds for each result. The appropriate confidence interval 

was selected on the basis of result concentration relative to the 

reporting level. Again, for purposes of this evaluation, a low 

concentration was less than five times the reporting limit. For 

low concentrations, the CI was calculated using the following 

formula (Mueller and Titus, 2005):

CI   Result Z S   1a  Dreplicates (1)

where

Result is the reported analytical result for the 

constituent evaluated,

Z(1-a) is the two-tailed z-score (1.96 for the 

95 percent CI), and

SDreplicates is the mean standard deviation of replicate 

result pairs.

For high constituent concentrations, the CI was calculated 

using the following formula (Mueller and Titus, 2005):

CI   Result Z R   1a   esult RSD 100 replicates / (2)

where

Result is the reported analytical result for the 

constituent being evaluated,

Z(1-a) is the two-tailed z-score (1.96 for the 

95 percent CI), and 

RSDreplicates is the mean relative standard deviation of 

replicate result pairs.
10 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

The steps for processing constituent sample pairs for 

difference thresholds based on confidence intervals are 

illustrated in columns U through AJ in table 2 of the data 

release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5). 

First, two separate confidence intervals were calculated 

for each result: one for the case that the concentration was 

categorized (column P) as low (columns U through X), 

another for the case that the concentration was categorized 

as high (columns Y through AB). In cases when confidence 

intervals for initial results overlapped confidence intervals 

for 10-year trend results, both results were replaced with 

their median value so that the results were considered tied 

(columns AC through AF), The difference threshold is not met 

for this method if the confidence intervals overlap. Finally, the 

applicable results, determined by whether the concentrations 

were categorized as low or high (column P), are applied 

(columns AG and AH) for submission to the hypothesis tests 

to determine step trends. 

Similar to what was done with the GRAC differencethreshold-setting method, modifications were made to 

calculate confidence intervals for pH, carbon-14, and the 

isotopic ratios of hydrogen and oxygen in water. Because the 

concepts of detection level and reporting limit are not relevant 

for these constituents, the confidence intervals were calculated 

using only the high-concentration formula based on relative 

standard deviations.

Reporting Levels

Reporting levels were used for both differencethreshold-setting methods described earlier. In the 10 years 

between sampling periods in this study, reporting levels at 

the laboratories changed for many of the constituents. If the 

reporting levels changed, the greater of the two reporting 

levels was used for the purpose of determining the threshold. 

The GAMA-PBP has periodically evaluated the 

detections of constituents in project blank samples to define 

threshold concentrations above which one can be reasonably 

certain that detections in groundwater samples were not 

the result of contamination (Olsen and others, 2010; Fram 

and others, 2012; Davis and others, 2014). Based on such 

threshold concentrations, the program established study 

reporting levels (SRLs), which are used to censor results at 

concentrations sometimes greater than the laboratory reporting 

levels. Results at concentrations less than an established SRL 

are published in GAMA-PBP reports as less than or equal 

to the concentration reported by the laboratory. In this way, 

results censored by SRLs were treated differently than results 

censored by laboratory reporting levels, because the result was 

considered a range from zero to the detected concentration, 

rather than from zero to the reporting level. The application 

of SRLs is illustrated in columns E and I in table 2 of the data 

release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5).

Hypothesis Tests on the Differences

After processing the data in this way, a Wilcoxon signedrank test with a modification proposed by Pratt (1959) was 

used to compare the final paired results for each evaluated 

constituent to determine whether or not there was a step trend. 

The Wilcoxon signed-rank test is a nonparametric alternative 

to a paired t-test that is not dependent upon data distribution 

(Helsel and Hirsch, 2002). It is used to test whether the 

median difference between paired observations equals zero 

(null hypothesis). The absolute values of the differences 

are ranked, so that the relative magnitudes and the relative 

number of changes in each direction (increases or decreases) 

are both taken into account. When there is no difference 

between paired results, the traditional Wilcoxon signed-rank 

test discards that pair before the evaluation. Data used for 

the present study included many pairs with zero differences 

because of the prevalence of censored results (non-detections) 

and the imposed requirement that differences exceed the 

previously defined thresholds. The Wilcoxon signed-rank test 

with the modification proposed by Pratt (1959) was preferred 

for this study because it ranks all of the differences, including 

the zero differences, although it only uses the non-zero 

differences to calculate a p value. A trend was considered 

detected at a significance level greater than or equal to 

95 percent (attained significance level, p=0.05). The NAWQA 

program evaluates step trends with this statistical test for the 

24 parameters assessed in common with this study, but applies 

a less conservative significance level of p=0.1 (Lindsey and 

others, 2016).

Additional Factors Considered

In recognition of the need to be conservative in 

determining trends when the evidence is limited to groups of 

paired data, additional factors were considered. Step-trend 

findings for a constituent were deemed inconclusive under any 

one of the following three circumstances: (1) analytical bias 

was documented by the USGS Quality Systems Branch (QSB) 

during one or both sampling periods, and, when the data were 

adjusted for the bias, the step trend was no longer statistically 

significant; (2) the step-trend finding was not consistent with 

the evaluated results of a different, generally correlated, 

water-quality constituent; and (3) the concentrations of the 

constituent for which the step trend was indicated were 

negligible relative to their potential effect on groundwater 

quality.
Methods 11

Quality Control

The results from quality-control samples collected during 

each sampling period were evaluated for constituents detected 

in at least 10 percent of the samples during either sampling 

period to determine the extent to which analytical bias or 

variability could have affected step-trend findings. Three 

types of quality-control samples—field blanks, replicates, 

and spikes—were collected around the times of initial and 

decadal sampling. Replicate results were further used to 

calculate confidence intervals around the results for one of 

the two methods described previously to establish difference 

thresholds. The GAMA-PBP quality-control results are 

presented in tables 10, 11, 12, and 13 of the accompanying 

data release (Kent, 2018, https://doi.org/10.5066/F7GH9GF5) 

and discussed in the “Results” section of each of the 

potentially affected constituents. The periodic data-quality 

assessment summaries provided by the QSB provided an 

additional quality control used for this study. Step-trend 

evaluations of constituents potentially affected by bias or 

variability documented in these summaries were repeated 

after adjusting data by the bias estimated by the QSB. 

Results for these repeated evaluations are included in the 

“Results” section, and their implications are discussed in the 

“Summary and Conclusions” section. Quality-control results 

for constituents detected in less than 10 percent of the samples 

during both sampling periods are not presented in this report.

Field Blanks

Field blanks were collected to assess inadvertent 

contamination (positive bias) of samples during collection, 

processing, transport, and analysis. Certified clean blank 

water was pumped through the sampling equipment (fittings 

and tubing) used to collect groundwater samples; then, it 

was processed and transported using the same protocols used 

for the groundwater samples. Field blanks were collected 

during both sampling periods for all constituents evaluated for 

step trends, except for the field-measured parameters (water 

temperature, dissolved oxygen, specific conductance, and 

pH) and the isotopes. Only the field blanks associated with 

sampling of the North San Francisco Bay and Monterey Bay 

and Salinas Valley Basins study units were used to evaluate 

positive bias during the initial-sampling period. For the 

initial-sampling period, the number of field-blank samples 

collected for each of the evaluated constituents ranged from 

4 (alkalinity) to 22 (organic constituents). During the decadalsampling period, field-blank samples collected for the North 

San Francisco Bay and Monterey Bay and Salinas Valley 

Basins study units, as well as for an additional nine study 

units, were evaluated to provide a similar number of fieldblank samples to that of the initial-sampling period. This was 

necessary because only about 25 percent of the wells sampled 

during the initial-sampling period were resampled for trends, 

with a correspondingly lower number of quality-control 

samples collected for each study unit. Decadal-sampling was 

done in the North San Francisco Bay and Monterey Bay and 

Salinas Valley Basins study units from August to November 

2014. The first and last field-blank samples evaluated for the 

decadal-sampling period were collected in January 2014 and 

May 2016, respectively. Sample results and summary statistics 

for the field-blank samples evaluated are provided in the data 

release (Kent, 2018, tables 10 and 13, https://doi.org/10.5066/

F7GH9GF5). The summary statistics for field-blank samples 

include, by constituent and sampling period, the number 

of field blanks collected, the number of field blanks with 

a detection of the constituent, the maximum concentration 

detected in a field blank, and the minimum concentration 

detected in a groundwater sample used to evaluate step 

trends. In cases where there were no field-blank detections 

for a constituent, or where the maximum concentration of the 

constituent detected in a field blank was small relative to the 

minimum concentration detected in a groundwater sample, it 

was assumed that inadvertent contamination was not a source 

of positive bias, which could have contributed to the false 

finding of a step trend for that constituent.

Replicates

Sequential replicate samples were collected to assess 

variability that can result from the processing and analyses of 

the constituents. For the present study, replicate results were 

also used to calculate the standard deviations and relative 

standard deviations for each constituent so that confidence 

intervals for the CI difference thresholds could be established. 

Replicates were collected during both sampling periods 

for all constituents evaluated for step trends, except for the 

field-measured parameters (water temperature, dissolved 

oxygen, specific conductance, and pH). Replicates of specific 

conductance and pH measured at the NWQL were evaluated. 

Replicates evaluated for the initial-sampling period included 

those collected during sampling of the North San Francisco 

Bay and Monterey Bay and Salinas Valley Basins study units, 

along with those collected during the sampling of an additional 

study unit (the Upper Los Angeles Basin study unit), which 

was sampled between sampling the North San Francisco Bay 

and Monterey Bay and Salinas Valley Basins study units. 

Replicates evaluated for the decadal-sampling period 

included replicates collected for the North San Francisco Bay 

and Monterey Bay and Salinas Valley Basins study units, 

as well as for an additional nine study units (Kent, 2018, 

table 11, https://doi.org/10.5066/F7GH9GF5). Three of the 

additional decadal-trend study units contributing replicates for 

evaluation during this study were sampled before sampling 

for the North San Francisco Bay and Monterey Bay and 

Salinas Valley Basins study units. The three additional study 

units were the Northern San Joaquin Basin, the Southern 

Sacramento Valley, and the San Diego Drainages study units. 
12 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Six of the additional decadal-trend study units contributing 

replicates for evaluation during this study were sampled after 

sampling for the North San Francisco Bay and Monterey 

Bay and Salinas Valley Basins study units. The six additional 

decadal-trend study units were the Cascade Range and Modoc 

Plateau, the Klamath Mountains, the Southeast San Joaquin 

Valley, the Kern County Subbasin, the Central Eastside San 

Joaquin Basin, and the Santa Barbara study units. Replicates 

were evaluated from the additional study units for each study 

period to improve the standard deviation and relative standard 

deviation estimates used for the confidence-interval difference 

thresholds. The first and last replicate samples evaluated for 

the decadal-sampling period were collected in January 2014 

and April 2016, respectively. Only those replicate pairs for 

which the constituent was detected in both samples were 

used to calculate standard deviations and relative standard 

deviations, because these could be underestimated if replicate 

pairs for which the constituent was undetected in one or both 

samples was used. Individual standard deviations and relative 

standard deviations were calculated for each of the two 

sampling periods. The summary statistics for replicate samples 

include, by constituent and sampling period, the total numbers 

of replicates and the numbers of replicates with the constituent 

detected in both samples for each sampling period along 

with the standard deviations and relative standard deviations 

calculated from these replicates (Kent, 2018, table 13, 

https://doi.org/10.5066/F7GH9GF5).

Laboratory Matrix Spikes

Laboratory matrix spikes are replicate samples to which 

the laboratory adds known concentrations of a constituents of 

analytical interest to assess bias (positive or negative) caused 

by characteristics of the groundwater matrix. Results from 

laboratory matrix spikes are evaluated against corresponding 

environmental sample results to calculate a percentage 

recovery. During this study, laboratory matrix spikes were 

only used for volatile, organic, and pesticide compounds. 

Evaluations and statistical summaries are only presented 

for the five volatile organic compounds and three pesticide 

compounds detected in at least 10 percent of the initial or 

decadal samples (Kent, 2018, tables 12 and 13, 

https://doi.org/10.5066/F7GH9GF5).

Only the laboratory matrix spikes associated with 

sampling of the North San Francisco Bay and Monterey Bay 

and Salinas Valley Basins study units were used to evaluate 

positive and negative bias during the initial-sampling period. 

Laboratory matrix spikes evaluated for the decadal-sampling 

period included those collected for the North San Francisco 

Bay and Monterey Bay and Salinas Valley Basins study 

units, as well as for an additional nine study units (Kent, 

2018, table 12, https://doi.org/10.5066/F7GH9GF5). Two 

of the additional decadal-trend study units contributing 

laboratory matrix spikes for evaluation during this study 

were sampled before sampling for the North San Francisco 

Bay and Monterey Bay and Salinas Valley Basins study 

units. These were the Southern Sacramento Valley and the 

San Diego Drainages study units. The other seven additional 

decadal-trend study units contributing laboratory matrix 

spikes for evaluation during this study were sampled after 

sampling for the North San Francisco Bay and Monterey Bay 

and Salinas Valley Basins study units. These were the San 

Fernando-San Gabriel, the Western San Joaquin Valley, the 

Southeast San Joaquin Valley, the Kern County Subbasin, 

the Central Eastside San Joaquin Basin, the Santa Barbara, 

and the Southern Sierra study units. The summary statistics 

for laboratory matrix-spike samples include, by constituent 

and sampling period, the numbers of laboratory matrix spikes 

evaluated to represent each sampling period along with the 

mean, median, minimum, and maximum percentage recoveries 

for the constituents of interest (Kent, 2018, table 13, 

https://doi.org/10.5066/F7GH9GF5). 

Results

Of the 160 water-quality constituents that were analyzed 

both in initial and in decadal samples, 59 were detected 

in at least 10 percent of the samples during either period 

(Kent, 2018, table 1, https://doi.org/10.5066/F7GH9GF5). 

Analytical results for initial and decadal samples, as well as 

for other samples collected at the trend wells, are linked to 

the USGS station identification numbers in table 1. Some of 

the additional results for these trend wells are from samples 

that were collected to evaluate 3-year trends (Kent and others, 

2014; Kent and Landon, 2016) and groundwater quality in 

shallow aquifers (Goldrath and others, 2016). Some of the data 

collected to evaluate triennial trends were used to supplement 

the initial-sample data (table 1).

The 59 constituents detected in at least 10 percent of 

initial or decadal samples were evaluated for step trends 

by processing the data and applying the statistical tests as 

described in the “Methods” section. Table 2A gives the results 

of 3 step-trend test methods applied to the results of the 2 

study units combined for each of the 59 constituents: (1) no 

difference threshold applied, (2) the difference threshold 

established by the GAMA replicate acceptability criteria 

(GRAC) applied, and (3) the difference threshold established 

by confidence intervals (CI) applied. Tables 2B and C give 

the same types of results from the step-trend tests for the 

individual study units, North San Francisco Bay (NSF) and the 

Monterey and Salinas Valley Basins (SALMON). Table 2D

summarizes the findings provided in tables 2A, B, and C.
Results 13

Step trends were identified by at least 1 of the evaluations 

in at least 1 of the 2 study units or in the combined study 

units for 33 of the 59 constituents (table 2). Step trends 

were indicated for more constituents when no difference 

threshold was applied (27 constituents) than when the GRAC 

(15 constituents) or the CI (23 constituents) difference 

threshold was applied. The CI difference threshold could 

not be applied to 11 of the constituents, however, because it 

was not possible to calculate confidence intervals for them 

(table 2). Replicate samples for the five field-measured 

parameters of air and water temperature, dissolved oxygen, 

specific conductance, and pH were not collected. Six of 

the evaluated constituents—thallium, carbon disulfide, 

methyl-tert-butyl ether (MTBE), tetrachloroethene (PCE), 

trichloroethene (TCE), and simazine—were not detected 

in both samples of the replicated pairs during the decadalsampling period. Statistical analyses of the results for all 

59 constituents detected in at least 10 percent of initial or 

decadal samples are presented in tables 2A, B, and C. Only the 

33 constituents for which step trends were indicated by at least 

1 of the evaluations are discussed.

One-to-one plots showing initial and trend results are 

presented for each of the 33 constituents in the “Results” 

sections that follow. Data point symbols that lie close to the 

one-to-one line indicate no step trend. Data points below the 

one-to-one line indicate a decrease in the decadal sample 

compared to the initial sample, and those above the line 

indicate an increase. Relevant findings from quality-control 

results, including from the USGS Quality Systems Branch 

(QSB) data-quality assessment summaries, are presented 

and discussed in the “Results” section for each potentially 

affected constituent.

Results for constituents that did not exhibit step trends 

by any of the evaluations are not discussed. The lack of 

step trends for the five volatile organic compounds (VOCs) 

and three pesticide compounds with sufficient detection 

frequency to be evaluated is notable (table 2). A national 

evaluation of decadal trends has evaluated all of these 

compounds, except carbon disulfide, for decadal step trends 

in 67 groundwater networks nationwide (Lindsey and others, 

2016). Step trends were found in fewer than 10 networks 

for each of the VOCs, chloroform, PCE, TCE, and MTBE. 

More step trends were found for the pesticide compounds, 

however. Atrazine increased in six networks and decreased 

in seven of the networks. Simazine increased in seven 

networks and decreased in two of the networks. The herbicide 

degradate, 2-Chloro-4-isopropylamino-6-amino-s-triazine 

(deethylatrazine) increased in 17 networks and decreased in 

6 of the networks.

Water-Quality Indicators

The water-quality indicators that were evaluated for step 

trends were water temperature, dissolved oxygen, specific 

conductance, total dissolved solids, pH, alkalinity, bicarbonate, 

and carbonate. Step trends were evaluated for specific 

conductance and pH measurements collected both in the field 

and at the NWQL. Step trends were found for at least one of 

the study units or the combined study units by at least one 

of the evaluation methods for water temperature, dissolved 

oxygen, specific conductance, pH, and carbonate (table 2).

A step trend was indicated for decreased temperature in 

groundwater from the combined study units and the SALMON 

study unit when no difference threshold was applied, but not 

when the GRAC difference threshold was applied (tables 2A, 

C, D; fig. 4). Differences in water temperature did not exceed 

the GRAC difference threshold for any of the trend wells. The 

few wells where groundwater temperatures increased in the 

SALMON study unit were located in the central part of the 

Monterey Bay study area. No spatial pattern was observed for 

the few NSF wells where groundwater temperatures increased.

Decadal samples, in degrees Celsius

Initial samples, in degrees Celsius

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied — —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

EXPLANATION

Decrease

10

15

20

25

30

35

40

10 15 20 25 30 35 40

Figure 4. Paired results of water temperatures measured in 

groundwater during the collection of initial samples (2004–05) and 

decadal samples (2014) in the North San Francisco Bay and the 

Monterey Bay and Salinas Valley Basins Groundwater Ambient 

Monitoring and Assessment Priority Basin Project (GAMA-PBP) 

study units.
14 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

A step trend was only found for increased concentrations 

of dissolved oxygen for wells in the NSF study unit when 

no difference threshold applied (table 2B, D; fig. 5). With 

the GRAC difference threshold applied, no step trends were 

found for dissolved oxygen. Replicate measurements of water 

temperature and dissolved oxygen were not done, so no CI 

difference threshold could be applied to these parameters. 

Field and laboratory specific conductance values for 

samples from the combined study units and for the NSF 

study unit indicated step trends of increased values, except 

for field-measured specific conductance in NSF samples 

with the GRAC difference threshold applied (tables 2A, B, 

D; figs. 6, 7A, B). Replicate measurements of field-measured 

specific conductance were not done, so no CI difference 

threshold could be applied to this parameter. The QSB 

data-quality assessment summaries for the time of the decadalsampling period indicated a positive bias of 5 to 8 percent 

for laboratory-measured specific conductance. Thus, specific 

conductance measured by the NWQL might have been 

greater than the true concentrations by up to 8 percent during 

the decadal sampling. This potential positive bias during 

the decadal-sampling period could have contributed to the 

findings of increased values. To test the potential effect of 

this documented bias on the step trends, the evaluations were 

repeated on the paired-sample data after subtracting 8 percent 

from each decadal-trend period result. After adjusting the 

data in this way, none of the evaluations showed a step trend 

for laboratory-measured specific conductance (table 2D; 

figs. 7C, D).

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.1

1.0

10.0

0.1 1.0 10.0

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

—

— —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

EXPLANATION

Figure 5. Paired results of dissolved oxygen measured in 

groundwater during the collection of initial samples (2004–05) and 

decadal samples (2014) in the North San Francisco Bay and the 

Monterey Bay and Salinas Valley Basins Groundwater Ambient 

Monitoring and Assessment Priority Basin Project (GAMA-PBP) 

study units.

Decadal samples, in microsiemens per centimeter

Initial samples, in microsiemens per centimeter

100

1,000

10,000

100 1,000 10,000

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

EXPLANATION

Increase

Increase

Figure 6. Paired results of field-measured specific conductance 

of groundwater during the collection of initial samples (2004–05) 

and decadal samples (2014) in the North San Francisco Bay 

and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project 

(GAMA-PBP) study units.
Results 15

sac16-0630_fig 07abcd

A B

C D Decadal samples, in microsiemens per centimeter

Initial samples, in microsiemens per centimeter

100

1,000

10,000

100 1,000 10,000

Decadal samples, in microsiemens per centimeter

Initial samples, in microsiemens per centimeter

100

1,000

10,000

100 1,000 10,000

Decadal samples, in microsiemens per centimeter

Initial samples, in microsiemens per centimeter

100

1,000

10,000

100 1,000 10,000

Decadal samples, in microsiemens per centimeter

Initial samples, in microsiemens per centimeter

100

1,000

10,000

100 1,000 10,000

Data adjusted for QSB results Data adjusted for QSB results

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase Increase Increase

—

—

—

—

— —

Figure 7. Paired results of laboratory-measured specific conductance of groundwater during the collection of paired initial samples 

(2004–05) and decadal samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater 

Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria 

(GRAC) difference threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference 

threshold applied after subtracting 8 percent (maximum demonstrated bias approximation) from the decadal-sample concentrations;

and D, with the CI difference threshold applied after subtracting 8 percent (maximum demonstrated bias approximation) from the 

decadal-sample concentrations. Bias approximations are based on periodic data-quality assessment summaries provided by the 

Inorganic Blind Sample Project of the USGS Quality Systems Branch (QSB).
16 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

applied and when the GRAC difference threshold was applied 

(tables 2A, B, C, D; fig. 10A). No step trend for carbonate 

was indicated when the CI difference threshold was applied, 

and this appeared to be because differences in paired results 

from well samples where carbonate was less than 0.5 mg/L 

were all less than the CI difference threshold (tables 2A, B, C, 

D; fig. 10B). 

Decadal samples, in standard pH units

Initial samples, in standard pH units

6.0 6.5 7.0 7.5 8.0 8.5

6.0

6.5

7.0

7.5

8.0

8.5

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

—

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Decrease

—

Decrease

Figure 8. Paired results of field-measured pH of groundwater 

during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey 

Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units.

Interestingly, no step trends were found for total 

dissolved solids (TDS) by any of the evaluations in either of 

the study units nor the combined study units. There is typically 

a correlation between specific conductance and TDS in water 

samples, and specific conductance is often used as a surrogate 

for TDS. It would be expected, therefore, that step trends 

for specific conductance would also be indicated for TDS. 

Nevertheless, in the present study, TDS concentrations in 

groundwater from about as many wells increased as decreased. 

The TDS concentrations in samples from several trend 

wells in the SALMON study unit were equal to or greater 

than the California secondary maximum contaminant level 

(SMCL-CA) of 500 mg/L. In contrast, TDS concentrations 

in groundwater from most NSF study-unit trend wells were 

less than the SMCL-CA, and a study in the area of the NSF 

study unit concluded that TDS concentrations are not changing 

in a meaningful way there (RMC Water and Environment, 

2013). A national evaluation of decadal trends evaluated 

TDS (but not specific conductance) for decadal step trends in 

67 groundwater networks nationwide (Lindsey and Rupert, 

2012; Lindsey and others, 2016). Concentrations of TDS were 

found to have increased in 25 of these networks and to have 

decreased in 2 networks.

Field and laboratory measurements of pH in groundwater 

samples did not indicate a consistent change over time. Field 

measurements of pH are more reliable than are laboratory 

measurements because the pH of a water sample can change 

substantially within hours or even minutes after sample 

collection (U.S. Geological Survey, variously dated). No 

step trend by any evaluation method was found for field or 

laboratory-measured pH for samples from the NSF study 

unit (tables 2B, D; figs. 8, 9A, B). A step trend of decreasing 

field-measured pH was indicated for samples from the 

SALMON study unit and from the combined study units when 

no difference threshold was applied (tables 2C, D; fig. 8). An 

opposite step trend of increased pH was found for samples 

from the SALMON study unit for laboratory-measured pH 

when no difference threshold was applied and when the CI 

difference threshold was applied (tables 2C, D; figs. 9A, B). 

No step trend was found for pH measured in the field or at the 

laboratory for the SALMON study unit and for the combined 

study units when no difference threshold was applied (tables 

2A, B, C, D; figs. 8, 9A). Replicate measurements of fieldmeasured pH were not made, so no CI difference threshold 

could be calculated. 

Carbonate concentrations decreased for the SALMON 

and combined study units when no difference threshold was 
Results 17

sac16-0630_fig 09ab Decadal samples, in standard pH units

Initial samples, in standard pH units

Decadal samples, in standard pH units

Initial samples, in standard pH units

6.0 6.5 7.0 7.5 8.0 8.5 9.0

6.0

6.5

7.0

7.5

8.0

8.5

9.0

6.0 6.5 7.0 7.5 8.0 8.5 9.0

6.0

6.5

7.0

7.5

8.0

8.5

9.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase —

—

—

— Increase

—

—

—

—

A B

Figure 9. Paired results of laboratory-measured pH of groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
18 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Nutrients

Five nutrient species of nitrogen and phosphorus were 

evaluated for step trends. Analytical results for the nutrient 

species of nitrogen and phosphorus are reported in mg/L 

as N and P, respectively. Step trends were found in the 

SALMON study unit or the combined study units by at least 

one of the evaluation methods for total nitrogen, nitrate, and 

orthophosphate (table 2).

An increase in total nitrogen concentrations was indicated 

only for the SALMON study unit when the CI difference 

threshold was applied (tables 2C, D; fig. 11B). It is apparent 

in figure 11 (A, B) that the decadal-sampling result for one 

well in the SALMON study unit (MSSV-03) was an outlier, 

perhaps not representative of the study unit as a whole. 

Because, the concentrations of several trace elements and 

major ions in groundwater from well MSSV-03, along with 

nitrate, all increased significantly, it could be suspected that 

samples were mixed up at the laboratory. An additional sample 

that was collected from this well for a separate GAMA study 

in 2013, however, confirmed the changes in the quality of 

water from this well (Kent, 2018, tables 4 and 5, 

https://doi.org/10.5066/F7GH9GF5). The concentration 

of total nitrogen in the samples from this well increased 

from 0.1 to 26 mg/L. The Central Coast Regional Water 

Quality Control Board (State of California) attributes the 

rapid deterioration of water quality in this well to a land-use 

shift in its area of recharge from non-irrigated vineyard and 

scrub to intensively irrigated vegetable crops. Details of this 

land-use shift and remediation steps that are being taken by 

stakeholders in the area are available at a County of Monterey 

web link (http://www.co.monterey.ca.us/government/

departments-i-z/resource-management-agency-rma-/planning/

current-major-projects/san-lucas-water-district).

sac16-0630_fig 10ab Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.1

1.0

10.0

0.1 1.0 10.0

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.1

1.0

10.0

0.1 1.0 10.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied —

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

Decrease

—

—

Decrease

Decrease

—

—

Decrease

—

A B

Figure 10. Paired results of carbonate concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 19

An increase in nitrate concentrations was also found 

only for the SALMON study unit. In contrast to the increased 

concentration for total nitrogen by only one evaluation 

method, however, the step trend of increased nitrate in the 

SALMON study unit was indicated by all three evaluation 

methods, and these findings did not change when the 

evaluations excluded data from MSSV-03 (tables 2C, D; 

figs. 12A, B). This finding was consistent with the findings of 

a recent study (Harter and Lund, 2012) that found significant 

nitrate increases in the area of the SALMON study unit. That 

study concluded that nitrate-source reduction actions might 

not reverse this trend for several decades because of the long 

groundwater travel times in this area. The QSB data-quality 

assessment summaries published around the decadal-sampling 

period indicated a negative bias of 6 to 8 percent for nitrate. 

Decadal-sampling period nitrate measured by the NWQL 

might have been less than the true concentration by up to 

8 percent. The step trends of increasing nitrate concentrations 

were found despite this potential decadal-sampling period 

negative bias, which could have obscured the trend.

With the exception of the 2014 sample from MSSV-03, 

nitrate concentrations in groundwater from wells sampled 

were all less than the EPA maximum contaminant level 

(MCL-US) of 10 mg/L as nitrogen. Nitrate concentrations in 

groundwater from all NSF study-unit trend wells were less 

than the MCL-US. No step trend was found for nitrate in the 

NSF study unit, and there is a published study that concluded 

that nitrate concentrations in this area are not changing in a 

meaningful way (RMC Water and Environment, 2013). A 

national evaluation of decadal trends has evaluated nitrate for 

decadal step trends in 67 groundwater networks nationwide 

(Lindsey and Rupert, 2012; Lindsey and others, 2016). Nitrate 

was found to have increased in 14 of these networks and to 

have decreased in 6 networks.

sac16-0630_fig 11ab Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.0

0.1

1.0

10.0

100.0

0.0 0.1 1.0 10.0 100.0

0.0

0.1

1.0

10.0

100.0

0.0 0.1 1.0 10.0 100.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

—

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

— — Increase

—

—

—

—

—

A B

Figure 11. Paired results of total nitrogen concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
20 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Orthophosphate concentrations in groundwater increased 

according to all three evaluations for the SALMON study 

unit and for the combined study units (tables 2A, C, D; 

figs. 13A, B). Orthophosphate concentrations were less than 

0.6 mg/L as phosphorus during both sampling periods, with 

one exception. Orthophosphate concentrations in water 

from well NSFVP-48 were 2.09 mg/L as phosphorus in the 

initial sample and 2.11 mg/L as phosphorus in the decadal 

sample. This difference did not exceed either difference 

threshold (figs. 13A, B). A national evaluation of decadal 

trends has evaluated orthophosphate for decadal step trends 

in 67 groundwater networks nationwide (Lindsey and others, 

2016). Orthophosphate was found to have increased in 13 of 

these networks and to have decreased in 9 networks.

Major and Minor Ions and Silica

Nine major and minor ions, and silica, were evaluated 

for step trends. Step trends were found in at least one of the 

study units or the combined study units by at least one of 

the evaluation methods for all of these constituents except 

bromide (tables 2A, B, C, D).

sac16-0630_fig 12ab Decadal samples, in milligrams per liter as nitrogen

Initial samples, in milligrams per liter as nitrogen

Decadal samples, in milligrams per liter as nitrogen

Initial samples, in milligrams per liter as nitrogen

0.0

0.1

1.0

10.0

100.0

0.0 0.1 1.0 10.0 100.0

0.0

0.1

1.0

10.0

100.0

0.0 0.1 1.0 10.0 100.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

Increase

—

— —

— Increase

Increase

Increase

A B

Figure 12. Paired results of nitrate concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 21

Calcium concentrations increased in groundwater 

samples from the NSF study unit and the combined study units 

with no difference threshold applied and with the CI difference 

threshold applied (tables 2A, B, D; figs. 14A, B). For the 

SALMON study unit, increased concentrations were indicated 

by all three step-trend evaluation approaches (tables 2C, 

D). The QSB data-quality assessment summaries published 

around the initial-sampling period indicated a positive bias 

of about 5 percent for calcium. Calcium measured by the 

NWQL during the initial period might have been greater than 

the true concentration. Summaries published around the time 

of the decadal-sampling period did not indicate any bias in 

calcium analyses at the NWQL. This potential positive bias 

during the initial period could have decreased the magnitude 

of the measured increases in calcium concentrations. To test 

the potential effect of this documented bias on the findings of 

step trends, the three evaluations were repeated on the paired 

sample data after subtracting 5 percent from each initial-period 

result. After adjusting the data in this way, calcium showed 

a step trend of increased concentrations for the combined 

study units by all three evaluations (table 2D; figs. 14 C, D). 

After the data adjustment, the NSF study unit still showed no 

step trend for calcium when the GRAC difference threshold 

was applied (fig. 14C). While no spatial pattern was noted 

for calcium increases in the SALMON study unit, calcium 

increases in the NSF study unit groundwater primarily 

occurred in the eastern side of the study unit.

Magnesium concentrations increased in groundwater 

from the SALMON study unit when no difference threshold 

was applied and when the CI difference threshold was applied 

(tables 2A, C, D; figs. 15A, B). For the NSF study unit and the 

combined study units, the step trend of increased magnesium 

concentrations was indicated by all three evaluation methods 

(tables 2A, B, D). All paired differences in magnesium 

exceeded the CI difference threshold, so this threshold had 

no effect on the statistical test. The QSB reported biases for 

analyses performed by the NWQL that could have influenced 

the finding of increased magnesium concentrations. The 

sac16-0630_fig 13ab Decadal samples, in milligrams per liter as phosphorus

Initial samples, in milligrams per liter as phosphorus

Decadal samples, in milligrams per liter as phosphorus

Initial samples, in milligrams per liter as phosphorus

0.00

0.01

0.10

1.00

10.00

0.00 0.01 0.10 1.00 10.00

0.00

0.01

0.10

1.00

10.00

0.00 0.01 0.10 1.00 10.00

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

Increase

Increase

Increase

Increase

Increase

Increase

Increase

Increase

A B

Figure 13. Paired results of orthophosphate concentrations in groundwater during the collection of initial samples (2004–05) and 

decadal samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient 

Monitoring and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference 

threshold applied, and B, with the confidence interval difference threshold applied.
22 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 14abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

Data adjusted by QSB results

1

10

100

1,000

1 10 100 1,000

Data adjusted by QSB results

A B

C D

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

— Increase

Increase

— Increase

Increase

Increase

Increase Increase

Increase Increase

Increase

Increase

Increase Increase

Increase

Increase

— Increase

Figure 14. Paired results of calcium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied after 

subtracting 5 percent (demonstrated bias approximation) from the initial sample concentrations; and D, with the CI difference threshold 

applied after subtracting 5 percent (demonstrated bias approximation) from the initial sample concentrations. Bias approximations are 

based on periodic data-quality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey 

Quality Systems Branch.
Results 23

QSB data-quality assessment summaries published around 

the initial-sampling period indicated a negative bias of about 

5.5 percent for magnesium, whereas the summaries published 

around the time of the decadal-sampling period indicated a 

positive bias of about 9 percent. Magnesium measured by the 

NWQL during the initial period might have been less than the 

true concentration, and decadal-sampling period magnesium 

measurements might have been greater than the true 

concentrations. To test the potential effect of this documented 

bias on the findings of step trends, the three evaluations were 

repeated on the paired sample data after adding 5.5 percent 

to each initial-period result and subtracting 9 percent from 

each decadal-sampling period result. After adjusting the data 

in this way, none of the evaluations indicated a step trend for 

magnesium (table 2D; figs. 15C, D).

Potassium concentrations increased only for the 

combined study units, and only when the GRAC difference 

threshold was applied (tables 2A, D; figs. 16A, B). Paired 

differences in potassium concentrations only exceeded the 

GRAC difference threshold in four wells (three of them in 

the SALMON study unit), but all four were increases. The 

QSB reported a positive bias for analyses done by the NWQL 

during both sampling periods. All potassium measured by the 

NWQL for this study might have been greater than the true 

concentration. The QSB data-quality assessment summaries 

published around the initial-sampling period indicated a 

positive bias of about 6 percent, whereas the summaries 

published around the time of the decadal-sampling period 

indicated a positive bias of about 8 percent. To test the 

potential effect of this documented bias on the findings of 

step trends, the three evaluations were repeated on the paired 

sample data after subtracting 6 percent from each initial-period 

result and subtracting 8 percent from each decadal-sampling 

sampling period result. After adjusting the data in this way, 

potassium concentrations decreased (opposite direction from 

the combined study unit result when the GRAC difference 

threshold was applied before adjustment) in samples from the 

NSF study unit when no difference threshold was applied and 

when the CI difference threshold was applied (tables 2B, D; 

fig. 16D).

Sodium concentrations increased only in groundwater 

from the combined study units when no difference threshold 

was applied and when the CI difference threshold was applied 

(tables 2A, D; figs. 17A, B). No step trend was found for 

sodium with the GRAC difference threshold applied. The 

QSB data-quality assessment summaries published around 

the decadal-sampling period indicate a positive bias of about 

8 percent for sodium. Decadal-sampling period sodium 

measured by the NWQL could have been greater than the true 

concentration. Summaries published around the time of the 

initial-sampling period did not indicate any bias in sodium 

analyses at the NWQL. This potential positive bias during 

the decadal-sampling period might have contributed to the 

increased concentrations indicated by two of the evaluations 

for the combined study units. To test the potential effect of 

this documented bias on the findings of step trends, the three 

evaluations were repeated on the paired sample data after 

subtracting 8 percent from each decadal-sampling period 

result. After adjusting the data in this way, the step trend of 

increased sodium concentrations for the combined study units 

was no longer indicated, and a step trend of decreased sodium 

concentrations (opposite direction from the combined study 

unit result before adjustment) was indicated for the NSF study 

unit when no difference threshold was applied and when the 

CI difference threshold was applied (table 2D; figs. 17C, D).

Chloride concentrations increased in groundwater from 

the NSF study unit when no difference threshold was applied 

and when the CI difference threshold was applied (tables 2A, 

B, D; figs. 18A, B). All paired differences in chloride exceeded 

the CI difference threshold, so this threshold had no effect on 

the statistical test. No step trends were found when the GRAC 

difference threshold was applied. One well from each study 

unit had chloride concentrations greater than the secondary 

maximum contaminant level of 250 mg/L established by the 

California Department of Public Health (SMCL-CA). Chloride 

concentrations increased in both of these wells (figs. 18A, B). 

A national evaluation of decadal trends has evaluated chloride 

for decadal step trends in 67 groundwater networks nationwide 

(Lindsey and Rupert, 2012; Lindsey and others, 2016). 

Chloride was found to have increased in 31 of these networks 

and to have decreased in 2 networks.

Iodide concentrations decreased in groundwater from 

the NSF study unit only when the CI difference threshold was 

applied (tables 2B, D; fig. 19B). The CI difference threshold 

was an effective threshold for iodide, but only when sample 

concentrations were less than 0.01 mg/L in the paired samples 

(fig. 19B). The threshold was exceeded for all sample pairs 

from wells where iodide concentrations were greater than 

0.01 mg/L. Neither field nor laboratory quality-control results 

revealed bias nor unusual variability in iodide results during 

the relevant sampling periods.

Sulfate concentrations increased by all three evaluations 

for the combined study units (tables 2A, D; figs. 20A, B) 

and with the GRAC difference threshold for the NSF study 

unit (tables 2B, D; fig. 20A). All paired differences in sulfate 

exceeded the CI difference threshold, so this threshold had 

no effect on the statistical test. While no spatial pattern was 

noted for sulfate increases in the SALMON study unit, 

sulfate increases in the NSF study unit groundwater primarily 

occurred in the eastern side of the study unit. A national 

evaluation of decadal trends has evaluated sulfate for decadal 

step trends in 67 groundwater networks nationwide (Lindsey 

and others, 2016). Sulfate was found to have increased in 

16 of these networks and to have decreased in 6 networks. 

Neither field nor laboratory quality-control data indicated bias 

or significant variability in sulfate results during the sampling 

periods.
24 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 15abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

1

10

100

1 10 100

1

10

100

1 10 100

1

10

100

1 10 100

1

10

100

1 10 100

Data adjusted by QSB results Data adjusted by QSB results

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Increase Increase

Increase Increase — Increase

—

—

—

—

—

—

—

—

—

—

—

—

A B

C D

Figure 15. Paired results of magnesium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied after 

adding 5 percent (demonstrated bias approximation) to initial sample concentrations and subtracting 9 percent (demonstrated bias 

approximation) from the decadal sample concentrations; and D, with the CI difference threshold applied after adding 5 percent 

(demonstrated bias approximation) to initial sample concentrations and subtracting 9 percent (demonstrated bias approximation) from 

the decadal sample concentrations. Bias approximations are based on periodic data-quality assessment summaries provided by the 

Inorganic Blind Sample Project of the U.S. Geological Survey Quality Systems Branch.
Results 25

sac16-0630_fig 16abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0

1

10

100

0 1 10 100

0

1

10

100

0 1 10 100

0 1 10 100

Data adjusted by QSB results

0

1

10

100

0 1 10 100

Data adjusted by QSB results

0

1

10

100

A B

C D

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

—

Decrease

Decrease

Decrease

—

—

—

—

—

—

—

—

—

—

—

— —

— —

—

—

—

—

Figure 16. Paired results of potassium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied after 

subtracting 6 percent (demonstrated bias approximation) from initial sample concentrations and subtracting 8 percent from the decadal 

sample concentrations; and D, with the CI difference threshold applied after subtracting 6 percent (demonstrated bias approximation) 

from initial sample concentrations and subtracting 8 percent from the decadal sample concentrations. Bias approximations are based 

on periodic data-quality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey Quality 

Systems Branch.
26 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 17abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000 1 10 100 1,000

Data adjusted by QSB results Data adjusted by QSB results

1

10

100

1,000

A B

C D

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

—

Decrease

Decrease

Decrease

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

Figure 17. Paired results of sodium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied 

after subtracting 8 percent (demonstrated bias approximation) from the decadal sample concentrations; and D, with the CI difference 

threshold applied after subtracting 8 percent (demonstrated bias approximation) from the decadal sample concentrations. Bias 

approximations are based on periodic data-quality assessment summaries provided by the Inorganic Blind Sample Project of the 

U.S. Geological Survey Quality Systems Branch.
Results 27

sac16-0630_fig 18ab Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

— Increase

—

—

—

—

—

—

—

—

Figure 18. Paired results of chloride concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
28 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 19abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.00

0.01

0.10

1.00

0.00 0.01 0.10 1.00

0.00

0.01

0.10

1.00

0.00 0.01 0.10 1.00

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

—

—

—

—

—

—

— —

—

—

—

A B

Figure 19. Paired results of iodide concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 29

Fluoride concentrations decreased in groundwater from 

the NSF study unit and the combined study units when no 

difference threshold was applied and when the CI difference 

threshold was applied (tables 2A, B, D; fig. 21B). No step 

trend was found for fluoride when the GRAC difference 

threshold was applied. The QSB data-quality assessment 

summaries published around the decadal-sampling period 

indicated a positive bias in fluoride analyses at the NWQL of 

about 6 percent during the initial-sampling period and from 

7 to 13 percent during the decadal-sampling period. Fluoride 

measured by the NWQL could have been greater than the 

true concentrations during both sampling periods, but the 

documented bias was larger during the decadal-sampling 

period. To test the potential effect of this documented bias on 

the findings of step trends, the three evaluations were repeated 

on the paired sample data after subtracting 6 percent from each 

initial-sampling period result and subtracting, first, 7 percent, 

then 13 percent, from each trend period result. Results of 

the three evaluations changed little after adjusting the data 

in this way. In each case, decreased fluoride concentrations 

were indicated when no difference threshold was applied 

and when the CI difference threshold was applied for the 

NSF study unit and the combined study units. Additionally, 

decreased concentrations of fluoride were indicated for the 

SALMON study unit with both levels of data adjustment 

with no difference threshold applied and, at the 13 percent 

adjustment, with the CI difference threshold applied (table 2D; 

figs. 21D, F). Data adjustments did not change the finding of 

no step trend for fluoride when the GRAC difference threshold 

was applied (table 2D; figs. 21C, E). A national evaluation of 

decadal trends has evaluated fluoride for decadal step trends 

in 67 groundwater networks nationwide (Lindsey and others, 

2016). Fluoride increased in 4 of these networks and decreased 

in 10 networks.

sac16-0630_fig 20ab Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0

10

100

1,000

10,000

0 10 100 1,000 10,000

0

10

100

1,000

10,000

0 10 100 1,000 10,000

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

— —

Increase —

Increase

Increase

Increase

Increase

—

—

—

—

Figure 20. Paired results of sulfate concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
30 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 21abcd Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Trend sample results QSB

adjustment 7 percent

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

Trend sample results QSB

adjustment 7 percent

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

Decrease

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

— — Decrease

Decrease Decrease

Decrease Decrease

Decrease Decrease Decrease Decrease

—

—

—

—

—

— — —

A B

C D

Figure 21. Paired results of fluoride concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied 

after subtracting 6 percent from initial sample concentrations and 7 percent (demonstrated bias approximation) from the decadal 

sample concentrations; D, with the CI difference threshold applied after subtracting 6 percent from initial sample concentrations and 

7 percent (demonstrated bias approximation) from the decadal sample concentrations; E, with the GRAC difference threshold applied 

after subtracting 6 percent from initial sample concentrations and 13 percent (demonstrated bias approximation) from the decadal 

sample concentrations; and F, with the CI difference threshold applied after subtracting 6 percent from initial sample concentrations 

and 13 percent (demonstrated bias approximation) from the decadal sample concentrations. Bias approximations are based on periodic 

data-quality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey Quality Systems 

Branch.
Results 31

Silica concentrations in groundwater decreased in each 

study unit and in the combined study units when no difference 

threshold was applied and when the CI difference threshold 

was applied (tables 2A, B, C, D; fig. 22B). No step trend was 

found for silica when the GRAC difference threshold was 

applied; only two of the paired sample differences exceeded 

the GRAC difference threshold. The QSB data-quality 

assessment summaries published around the initial-sampling 

period indicated a positive bias of about 7 percent for silica. 

Initial-sampling period silica measured by the NWQL could 

have been greater than the true concentrations. Summaries 

published around the time of the decadal-sampling period 

did not indicate any bias in silica analyses at the NWQL. The 

potential positive bias during the initial-sampling period could 

have contributed to the findings of decreased concentrations 

indicated by two of the evaluations. To test the potential effect 

of this documented bias on the findings of step trends, the 

three evaluations were repeated on the paired sample data after 

subtracting 7 percent from each initial-sampling period result. 

After adjusting the data in this way, none of the evaluations 

indicated a step trend for silica (table 2D; figs. 22C, D).

Trace Elements

Twenty trace elements were evaluated for step trends. 

Step trends were found in at least 1 of the study units or the 

combined study units by at least 1 of the evaluation methods 

for 11 of these trace elements (tables 2A, B, C, D).

Aluminum concentrations increased in groundwater 

from the NSF study unit and the combined study units by all 

three evaluation methods (tables 2A, B, C, D; figs. 23A, B). 

No step trends were indicated for aluminum in the SALMON 

study unit by any of the evaluation methods. The GAMAPBB field-blank results indicated a potential positive bias 

for initial-sampling period aluminum concentrations (Kent, 

2018, tables 10 and 13, https://doi.org/10.5066/F7GH9GF5), 

and a study reporting level (SRL) of 1.6 μg/L was applied 

to data collected during this period (Davis and others, 

2014). The increased aluminum concentrations were found 

despite this potential initial-sampling period positive bias, 

which could have obscured the trend. During the decadalsampling period, however, the GAMA-PBP replicate results 

sac16-0630_fig 21ef Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

Decadal samples, in milligrams per liter

Initial samples, in milligrams per liter

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5

Trend sample results QSB

adjustment 13 percent

Trend sample results QSB

adjustment 13 percent

E F

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

Decrease

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

— — — Decrease

Decrease

Decrease

Decrease

Figure 21. —Continued
32 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 22abcd Decadal samples, in milligrams per liter as silica

Initial samples, in milligrams per liter as silica

Decadal samples, in milligrams per liter as silica

Initial samples, in milligrams per liter as silica

Decadal samples, in milligrams per liter as silica

Initial samples, in milligrams per liter as silica

Decadal samples, in milligrams per liter as silica

Initial samples, in milligrams per liter as silica

0

50

100

150

0 50 100 150

0

50

100

150

0 50 100 150

0

50

100

150

0 50 100 150 0 50 100 150

Data adjusted by QSB results Data adjusted by QSB results

0

50

100

150

A B

C D

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

Decrease

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

— Decrease

Decrease

—

Decrease

— Decrease

—

—

—

—

—

—

—

—

—

—

—

—

Figure 22. Paired results of silica concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with differences within the GAMA replicate acceptability criteria (GRAC) 

difference threshold applied; B, with differences within the confidence interval (CI) difference threshold applied; C, with the GRAC 

difference threshold applied after subtracting 7 percent (demonstrated bias approximation) from the initial sample concentrations; 

and D, with the CI difference threshold applied after subtracting 7 percent (demonstrated bias approximation) from the initial sample 

concentrations. Bias approximations are based on periodic data-quality assessment summaries provided by the Inorganic Blind Sample 

Project of the U.S. Geological Survey Quality Systems Branch.
Results 33

(Kent, 2018, tables 11 and 13, https://doi.org/10.5066/

F7GH9GF5) indicated that analytical results for aluminum 

were unusually variable. Project replicate results for aluminum 

had a mean relative standard deviation of 6.6 percent during 

decadal-sampling compared to 1.5 percent during the initial 

sampling period. 

Arsenic concentrations decreased in groundwater from 

the NSF study unit and the combined study units when no 

difference threshold was applied and when the CI difference 

threshold was applied (tables 2A, B, D; fig. 24B). No step 

trend was found for arsenic when the GRAC difference 

threshold was applied. Arsenic has a California maximum 

contaminant level (MCL-CA) of 10 μg/L; concentrations 

in samples from most of the wells evaluated for this study 

were less than this benchmark. Arsenic concentrations in 

initial samples from wells NSFVP-38 and MSPR-03 were 

greater than the MCL-CA, however. Arsenic concentrations 

in NSFVP-38 were 17.2 μg/L in 2004 and 15.9 μg/L in 2014. 

The difference between these concentrations did not exceed 

the GRAC difference threshold (fig. 24A). In contrast, the 

arsenic concentration measured in samples from MSPR03 decreased from17.7 μg/L to 7.5 μg/L. While no spatial 

pattern was noted for arsenic changes in the SALMON study 

unit, arsenic decreases in the NSF study unit groundwater 

primarily occurred in the southern-central and eastern areas 

of the study unit.A national evaluation of decadal trends has 

evaluated arsenic for decadal step trends in 67 groundwater 

networks nationwide (Lindsey and others, 2016). Arsenic was 

found to have increased in three of these networks and to have 

decreased in eight networks. 

Barium concentrations increased in groundwater from 

the NSF study unit and the combined study units when no 

difference threshold was applied and when the CI difference 

threshold was applied (tables 2A, B; fig. 25B). No step trend 

was indicated for barium when the GRAC difference threshold 

was applied. Barium has a MCL-CA of 1,000 μg/L, and 

barium concentrations were all less than this benchmark. 

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

0

2

4

6

8

0 2 4 6 8

0

2

4

6

8

0 2 4 6 8

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase

Increase

Increase

Increase

Increase

—

—

—

—

Increase

Increase

Increase

Figure 23. Paired results of aluminum concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
34 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

The QSB data-quality assessment summaries published 

around the initial-sampling period indicated a negative bias 

of about 7 percent for barium. Initial-sampling period barium 

measured by the NWQL could have been less than the true 

concentrations. Summaries published around the time of the 

decadal-sampling period did not indicate any bias for barium 

analyses by the NWQL. The potential negative bias during the 

initial-sampling period could have contributed to step trend of 

increased concentrations indicated by two of the evaluations. 

To test the potential effect of this documented bias on the 

findings of step trends, the three evaluations were repeated on 

the paired sample data after adding 7 percent to each initialsampling period result. After adjusting the data in this way, 

none of the evaluation methods indicated a step trend for 

barium (table 2D; figs. 25C, D). 

Iron concentrations in groundwater increased according 

to all three evaluations for the SALMON study unit and the 

combined study units (tables 2A, C, D; figs. 26A, B). The 

GAMA-PBP replicate variability for iron was greater than 

that of most other constituents. As a result of this, the standard 

deviations calculated from replicate variability during each 

sampling period were greater than those for other constituents, 

and therefore, the CI difference threshold was comparable to 

the GRAC difference threshold for iron. Nevertheless, paired 

sample differences only failed to exceed the CI difference 

threshold when groundwater iron concentrations were less 

than about 20 μg/L (fig. 26B). All paired sample differences 

exceeded the CI difference threshold when groundwater iron 

concentrations were greater than 20 μg/L. The GAMA-PBB 

field-blank results indicated a potential positive bias in initialsampling period iron concentrations (Kent, 2018, tables 10 

and 13, https://doi.org/10.5066/F7GH9GF5). The step trend 

of increased iron concentrations was indicated despite the 

potential initial-sampling period positive bias, which might 

have obscured the trend. While no spatial pattern was noted 

for iron increases in the NSF study unit, the largest iron 

increases in the SALMON study unit were observed in wells 

in the northern part of the study unit. A national evaluation 

of decadal trends has evaluated iron for decadal step trends 

in 67 groundwater networks nationwide (Lindsey and others, 

sac16-0630_fig 24ab Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

0

1

10

100

0 1 10 100

0

1

10

100

0 1 10 100

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

—

—

—

—

—

Decrease

—

Decrease Decrease

Decrease Decrease

Figure 24. Paired results of arsenic concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 35

sac16-0630_fig 25abcd Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

A B Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

1

10

100

1,000

1 10 100 1,000

1

10

100

1,000

1 10 100 1,000

C D

Data adjusted by QSB results Data adjusted by QSB results

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

— — Increase

—

—

—

—

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

—

—

—

—

—

—

—

—

Figure 25. Paired results of barium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate criteria GRAC) difference threshold applied; 

B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied after adding 7 percent 

(demonstrated bias approximation) to the initial sample concentrations; and D, with the CI difference threshold applied after adding 

7 percent (demonstrated bias approximation) to the initial sample concentrations. Bias approximations are based on periodic dataquality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey Quality Systems Branch.
36 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

2016). Iron was found to have increased in 7 of these networks 

and to have decreased in 10 networks. 

Lead concentrations in groundwater decreased only for 

the combined study units when the GRAC difference threshold 

was applied (tables 2A, D; fig. 27A). Based on GAMA-PBP 

field-blank results, study reporting levels (SRL) were applied 

to lead results from both sampling periods (Olsen and others, 

2010; Davis and others, 2014). The SRL for the decadalsampling period (0.82 μg/L) was slightly greater than the 

SRL for the initial-sampling period (0.65 μg/L). In addition, 

GAMA-PBB field-blank results indicated a potential positive 

bias in decadal-sampling period lead concentrations (Kent, 

2018, tables 10 and 13, https://doi.org/10.5066/F7GH9GF5). 

The step trend of decreased lead concentrations for the 

combined study units was indicated despite the potential 

decadal-sampling period positive bias, which might have 

obscured the trend. Lead has a EPA action level of 15 μg/L, 

and lead concentrations in trend wells sampled for this study 

were all less than half this benchmark. While no spatial pattern 

was noted for lead changes in the SALMON study unit, 

lead decreases in the NSF study unit groundwater primarily 

occurred in the southern part of the study unit.

Lithium concentrations in groundwater increased for 

the NSF study unit and the combined study units when no 

difference threshold was applied and when the CI difference 

threshold was applied (tables 2A, B, D; fig. 28B). All paired 

differences in lithium exceeded the CI difference threshold, so 

this threshold had no effect on the statistical test. No step trend 

was found for lithium when the GRAC difference threshold 

was applied. There was no evidence of analytical bias for 

lithium from project quality-control samples or the QSB dataquality assessment summaries.

Manganese concentrations in groundwater increased 

for the combined study units when no difference threshold 

was applied and when the CI difference threshold was 

applied; concentrations increased for the SALMON study 

unit according to all three evaluation methods (tables 2A, 

C, D; figs. 29A, B). Manganese concentrations were greater 

than the SMCL-CA of 50 μg/L in decadal samples from 

14 wells. Of these 14 wells, 10 were in the NSF study unit. In 

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

1 10 100 1,000 10,000

1

10

100

1,000

10,000

1 10 100 1,000 10,000

1

10

100

1,000

10,000

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

Increase Increase

Increase Increase

—

—

Increase Increase

Increase Increase

Figure 26. Paired results of iron concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, B, with the confidence interval difference threshold applied.
Results 37

addition, manganese concentrations in initial samples from all 

but 1 of the 14 wells (MSPR-03) were also greater than the 

SMCL-CA. That is, manganese in groundwater from only one 

well changed from a concentration less than the SMCL-CA to 

a concentration greater than the benchmark between sampling 

periods.

Based on GAMA-PBP field-blank results, study reporting 

levels (SRL) were applied to manganese results for both 

sampling periods (Olsen and others, 2010; Davis and others, 

2014). The SRL for the decadal-sampling period (0.66 μg/L) 

was greater than the SRL for the initial-sampling period 

(0.2 μg/L). In contrast, the QSB data-quality assessment 

summaries published around the initial-sampling period 

indicated a negative bias of about 8 percent for manganese. 

Initial-sampling period manganese measured by the NWQL 

could have been less than the true concentrations. Summaries 

published around the time of the decadal-sampling period did 

not indicate any bias in manganese analyses by the NWQL. 

The negative bias during the initial-sampling period could 

have contributed to the findings of increased concentrations 

of manganese. To test the potential effect of this documented 

bias on the findings of step trends, the three evaluations were 

repeated on the paired-sample data after adding 8 percent to 

each initial-sampling period result. After adjusting the data in 

this way, none of the evaluation methods indicated a step trend 

for manganese (table 2D; figs. 29C, D). A national evaluation 

of decadal trends has evaluated manganese for decadal step 

trends in 67 groundwater networks nationwide (Lindsey and 

others, 2016). Manganese was found to have increased in 6 of 

these networks and to have decreased in 21 networks.

Nickel concentrations increased in only the NSF study 

unit when no difference threshold was applied and when the 

CI difference threshold was applied (tables 2B, C, D; fig. 30B). 

Nickel has a MCL-CA of 100 μg/L, and nickel concentrations 

in trend wells sampled for this study were all substantially less 

than this benchmark. Groundwater in only one well—MSSV03 in the SALMON study unit—had a nickel concentration 

greater than 10 μg/L. The nickel concentration in groundwater 

from that well increased from 3.5 μg/L in the initial sample to 

32 μg/L in the decadal sample. The concentrations of several 

other trace elements and major ions, along with nitrate, all 

increased substantially in groundwater from MSSV-03. The 

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

0

2

4

6

8

0 2 4 6 8

0

2

4

6

8

0 2 4 6 8

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease

—

—

—

—

—

—

—

—

—

—

—

Figure 27. Paired results of lead concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, B, with the confidence interval difference threshold applied.
38 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

QSB data-quality assessment summaries published around 

the decadal-sampling period indicated a positive bias of 

about 17 percent for nickel. Decadal-sample period nickel 

measured by the NWQL could have been greater than the 

true concentrations. Summaries published around the time of 

the initial-sampling period did not indicate any bias in nickel 

analyses at the NWQL. The potential positive bias during the 

decadal trend period could have contributed to the increased 

concentrations found by two of the evaluations for the NSF 

study unit. To test the potential effect of this documented 

bias on the findings of step trends, the three evaluations were 

repeated on the paired-sample data after subtracting 17 percent 

from each decadal-sample period result. After adjusting the 

data in this way, none of the evaluation methods indicated a 

step trend for nickel (table 2D; figs. 30C, D).

Selenium concentrations decreased in groundwater 

from for the NSF study unit by all three evaluation methods 

(tables 2B, D; figs. 31A, B). No step trends were exhibited 

for selenium in the SALMON study unit nor in the combined 

study units by any of the evaluation methods. There was 

no evidence of analytical bias for selenium from project 

quality-control samples or the QSB data-quality assessment 

summaries.

Strontium concentrations increased for only the combined 

study units when the GRAC difference threshold was applied 

(tables 2A, D; figs. 32A). The QSB data-quality assessment 

summaries from the initial-sampling period indicated a 

positive bias of about 7 percent for strontium. Initial period 

strontium measured by the NWQL could have been greater 

than the true concentration. Summaries published around 

the time of the decadal-sampling period did not indicate any 

bias in strontium analyses at the NWQL. The step trend of 

increased strontium concentrations for the combined study 

units was indicated despite the potential initial-sampling 

period positive bias, which might have obscured the trend. A 

national evaluation of decadal trends has evaluated strontium 

for decadal step trends in 67 groundwater networks nationwide 

(Lindsey and others, 2016). Strontium was found to have 

increased in three of these networks and to have decreased in 

two networks.

sac16-0630_fig 28abcd Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

1

10

100

1 10 100

1

10

100

1 10 100

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Increase Increase

— Increase

—

— — —

—

Figure 28. Paired results of lithium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 39

sac16-0630_fig 29abcd Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

1 10 100 1,000 10,000

1

10

100

1,000

10,000

1 10 100 1,000 10,000

1

10

100

1,000

10,000

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

C D

1 10 100 1,000 10,000

1

10

100

1,000

10,000

1 10 100 1,000 10,000

1

10

100

1,000

10,000

Data adjusted by QSB results Data adjusted by QSB results

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

Increase Increase

Increase Increase

—

—

Increase Increase

— Increase

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

—

—

—

—

—

—

—

—

Figure 29. Paired results of manganese concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied 

after adding 8 percent (demonstrated bias approximation) to the initial sample concentrations; and D, with the CI difference threshold 

applied after adding 8 percent (demonstrated bias approximation) to the initial sample concentrations. Bias approximations are based 

on periodic data-quality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey Quality 

Systems Branch.
40 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 30abcd Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

C D

Data adjusted by QSB results Data adjusted by QSB results

0.1 1.0 10.0 100.0

0.1

1.0

10.0

100.0

0.1 1.0 10.0 100.0

0.1

1.0

10.0

100.0

0.01 1.00 10.00 100.00

0.01

0.10

1.00

100.00

10.00

0.10 0.01 1.00 10.00 100.00

0.01

0.10

1.00

100.00

0.10

10.00

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

Increase

Increase

—

—

Increase

—

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

Figure 30. Paired results of nickel concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring 

and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria (GRAC) difference 

threshold applied; B, with the confidence interval (CI) difference threshold applied; C, with the GRAC difference threshold applied after 

subtracting 17 percent (demonstrated bias approximation) from the trend sample concentrations; and D, with the CI difference threshold 

applied after subtracting 17 percent (demonstrated bias approximation) from the trend sample concentrations. Bias approximations are 

based on periodic data-quality assessment summaries provided by the Inorganic Blind Sample Project of the U.S. Geological Survey 

Quality Systems Branch.
Results 41

sac16-0630_fig 31ab Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

0.01 1.00 10.00 100.00

0.01

0.10

1.00

100.00

10.00

0.10 0.01 1.00 10.00 100.00

0.01

0.10

1.00

100.00

0.10

10.00

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

—

—

—

—

Decrease

Decrease

Decrease

Decrease

Figure 31. Paired results of selenium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, B, with the confidence interval difference threshold applied.
42 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Vanadium concentrations decreased for only the NSF 

study unit when the GRAC difference threshold was applied 

(tables 2A, D; fig. 33A). The GAMA-PBP field-blank results 

indicated a potential positive bias in decadal-sampling 

period vanadium concentrations (Kent, 2018, tables 10 and 

13, https://doi.org/10.5066/F7GH9GF5). The step trend of 

decreasing vanadium concentrations was indicated despite this 

potential positive bias in decadal samples, which might have 

obscured the trend.

sac16-0630_fig 32ab Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

10

100

1,000

10,000

10 100 1,000 10,000

10

100

1,000

10,000

10 100 1,000 10,000

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

—

—

Increase —

— — —

—

—

—

Figure 32. Paired results of strontium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Results 43

Isotopic Constituents

Five isotopic constituents were evaluated for step trends. 

Step trends were found for at least one of the study units or the 

combined study units by at least one of the evaluation methods 

for the ratio of oxygen-18 in water (δ18O-H2O), the ratio of the 

stable isotope carbon-13 (δ13C), and tritium (tables 2A, B, C, 

D).

The isotopic ratio of oxygen-18 in water (δ18O-H2O) 

increased in groundwater from each study unit and for the 

combined study units when no difference threshold was 

applied and when the CI difference threshold was applied 

(tables 2A, B, C, D; fig. 34B). An increase in an isotopic ratio 

is defined here to mean that the isotopic composition has 

become less negative or heavier (enriched in regard to the 

heavier isotope). All paired differences in δ18O-H2

O exceeded 

the CI difference threshold, so this threshold had no effect on 

the statistical test. In contrast, no paired sample differences 

in δ18O-H2

O exceeded the GRAC difference threshold; thus, 

no step trend was indicated when this difference threshold 

was applied.

The ratio of the stable isotope carbon-13 (δ13C) in 

groundwater decreased in the SALMON study unit when no 

difference threshold was applied and when the CI difference 

threshold was applied (tables 2A, C, D; fig. 35B). A decrease 

in an isotopic ratio is defined here to mean that the isotopic 

composition has become more negative or lighter (depleted in 

regard to the heavier isotope). As was the case with the stable 

isotope in water δ18O-H2O, all paired differences for δ13C 

exceeded the CI difference threshold, but no paired differences 

for δ13C exceeded the GRAC difference threshold.

sac16-0630_fig 33ab Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

Decadal samples, in micrograms per liter

Initial samples, in micrograms per liter

A B

0.1 1.0 10.0 100.0

0.1

1.0

10.0

100.0

0.1 1.0 10.0 100.0

0.1

1.0

10.0

100.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

— —

— Decrease

—

—

—

—

—

—

Figure 33. Paired results of vanadium concentrations in groundwater during the collection of initial samples (2004–05) and decadal 

samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
44 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Tritium in groundwater decreased in each study unit and 

the combined study units according to all three evaluations 

(tables 2A, B, C, D; figs. 36A, B). Tritium decreases were 

far more common than increases, which would be expected. 

Tritium is an unstable (radioactive) isotope of hydrogen with 

a half-life of approximately 12.3 years (Plummer and others, 

1993). Figure 36 (A, B) includes a dotted line representing the 

sac16-0630_fig 34ab Decadal samples, in per mil

Initial samples, in per mil

Decadal samples, in per mil

Initial samples, in per mil

A B

−9.0

−8.5

−8.0

−7.5

−7.0

−6.5

−6.0

−5.5

−5.0

−4.5

−4.0

−9.0 −8.5 −8.0 −7.5 −7.0 −6.5 −6.0 −5.5 −5.0 −4.5 −4.0

−9.0

−8.5

−8.0

−7.5

−7.0

−6.5

−6.0

−5.5

−5.0

−4.5

−4.0

−9.0 −8.5 −8.0 −7.5 −7.0 −6.5 −6.0 −5.5 −5.0 −4.5 −4.0

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

— — —

Increase Increase

Increase Increase

Increase

Increase

Increase Increase Increase

Figure 34. Paired results of the isotopic ratio of oxygen-18 (δ18O) in groundwater during the collection of initial samples (2004–05) 

and decadal samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient 

Monitoring and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference 

threshold applied, and B, with the confidence interval difference threshold applied.

decrease in tritium activity predicted by radioactive decay. 

It is interesting that when initial activities in groundwater 

were less than about 6 picocuries per liter (pCi/L), tritium 

decreased more than expected by radioactive decay alone, but 

when initial activities were greater than about 6 pCi/L, tritium 

decreased less than expected.
Results 45

sac16-0630_fig 35ab Decadal samples, in per mil

Initial samples, in per mil

Decadal samples, in per mil

Initial samples, in per mil

A B

−25 −20 −15 −10

−25

−20

−15

−10

−25 −20 −15 −10

−25

−20

−15

−10

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

—

—

— — Decrease

— — —

—

—

Decrease

Decrease

Figure 35. Paired results of the isotopic ratio of carbon-13 (δ13C) in groundwater during the collection of initial samples (2004–05) 

and decadal samples (2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient 

Monitoring and Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference 

threshold applied, and B, with the confidence interval difference threshold applied.
46 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

sac16-0630_fig 36ab

Half-life decay

Half-life decay

Decadal samples, in picocuries per liter

Initial samples, in picocuries per liter

Decadal samples, in picocuries per liter

Initial samples, in picocuries per liter

0

2

4

6

8

10

0 2 4 6 8 10

0

2

4

6

8

10

0 2 4 6 8 10

A B

EXPLANATION

Salinas-Monterey difference greater than threshold

North San Francisco Bay difference greater than threshold

Salinas-Monterey difference less than threshold

North San Francisco Bay difference less than threshold

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

Decrease

Step-trend findings (—, no trend)

Combined

study units

No threshold

Threshold applied

North San Francisco

Bay study unit

Monterey Bay and Salinas 

Valley Basins study unit

Decrease Decrease

Decrease

Decrease

Decrease

Decrease

Decrease Decrease Decrease

Figure 36. Paired results of tritium activities in groundwater during the collection of initial samples (2004–05) and decadal samples 

(2014) in the North San Francisco Bay and the Monterey Bay and Salinas Valley Basins Groundwater Ambient Monitoring and 

Assessment Priority Basin Project (GAMA-PBP) study units: A, with the GAMA replicate acceptability criteria difference threshold 

applied, and B, with the confidence interval difference threshold applied.
Summary and Conclusions 47

Summary and Conclusions

Three variations of a method to identify step trends 

in groundwater quality are presented for a case study on 

the north-central California coast as part of a statewide 

investigation of groundwater quality by the U.S. Geological 

Survey (USGS) for the Groundwater Ambient Monitoring 

and Assessment (GAMA) Priority Basin Project (PBP). The 

GAMA-PBP is being carried out in cooperation with the 

California State Water Resources Control Board to assess 

and monitor the quality of groundwater resources applied 

for drinking-water supply and to improve public knowledge 

of groundwater quality in California. Paired samples were 

collected from 50 public supply wells in 2 GAMA-PBP study 

units during 2 sampling periods separated by approximately 

10 years. During both sampling periods, 160 water-quality 

constituents were measured. The initial-sampling period was 

from 2004 to 2005. For a few wells, data collected in 2007–08 

supplemented the initial-sample data for inorganic constituents 

that were not analyzed in the samples collected in 2004–05. 

The decadal-sampling period to assess step trends was in 

2014; decadal samples were generally analyzed for all of the 

common constituents. 

To identify step trends, a nonparametric hypothesis test 

(Wilcoxon signed-rank test with the Pratt correction) on the 

differences in constituent results between the two sampling 

periods was used. Before doing the statistical evaluations (the 

hypothesis tests), the data were processed three different ways, 

resulting in three variations of the evaluation method. Data 

was processed to prevent small differences from supporting 

the conclusion of a step trend and to overcome challenges 

of comparing result pairs in which one or both of the results 

was a non-detection. The first evaluation method variation 

was limited to processing the data to overcome challenges 

comparing result pairs in which one or both of the results 

was a non-detection (a result expressed simply as less than 

the reporting level). The second and third variations added a 

requirement that the paired-result differences exceed a defined 

threshold. If the threshold was not exceeded, the results were 

considered analytically identical (tied) for the statistical 

evaluation. The objective of both difference-threshold-setting 

methods was to prevent small differences from supporting 

the conclusion of a step trend. The GAMA-PBP replicate 

acceptability criteria (GRAC) method for setting the difference 

threshold between initial-sampling and resampling results 

is based on criteria used by the GAMA-PBP to determine 

whether or not replicate results are acceptable (Kent and 

others, 2014, p. 148); that is, if two results are similar enough 

to constitute an acceptable replicate, the small difference 

between them should not support the conclusion that there is 

a trend. In contrast, the confidence interval (CI) differencethreshold-setting method uses the actual results of replicate 

samples collected during each sampling period to calculate 

a 95-percent confidence interval around each result. This 

threshold is exceeded if the confidence intervals for the initial 

and decadal-sample results do not overlap.

Step trends were identified by at least 1 of the 

3 evaluations for 33 of the 59 constituents detected in at least 

10 percent of samples analyzed (a requirement imposed for 

evaluation; tables 2A, B, C, D). The statistical evaluations 

alone do not provide sufficient evidence to conclude that all 

33 constituents with statistically significant results for a step 

trend are actually changing in groundwater on the northcentral California coast, however. Therefore, results of the 

statistical evaluations were further examined critically and 

in context. After such examination, some step-trend findings 

were deemed inconclusive. 

Constituents Exhibiting Conclusive Evidence of 

Step Trends

There is evidence to support the findings of meaningful 

changes for 14 of the 33 constituents for which step trends 

were indicated by 1 or more of the evaluation methods. 

Constituent concentrations changed in one or both of the 

study units or in the combined study units. The constituents 

that increased were dissolved oxygen, total nitrogen, nitrate, 

orthophosphate, calcium, chloride, sulfate, iron, and lithium. 

The constituents that decreased were temperature, arsenic, 

lead, the isotopic ratio of carbon-13 (δ13C), and tritium. 

Dissolved oxygen concentrations increased in 

groundwater of the North San Francisco Bay (NSF) study 

unit; concentrations increased in paired samples from 13 wells 

and decreased in paired samples from 4 wells. Changes 

in dissolved oxygen concentrations are most meaningful 

when they indicate the groundwater converted from oxic 

to anoxic conditions or vice versa. McMahon and Chapelle 

(2008) defined anoxic groundwater as having a dissolved 

oxygen concentration less than 0.5 mg/L. Using this criterion, 

groundwater from four of the NSF wells changed from anoxic 

to oxic between sampling periods, and groundwater from one 

NSF well changed from oxic to anoxic.

Total nitrogen, mostly in the form of nitrate, increased in 

groundwater of the SALMON study unit. This is consistent 

with a recent study (Harter and Lund, 2012) that found nitrate 

increased significantly in the area of the SALMON study unit. 

That study concluded that nitrate source reduction actions 

might not reverse this trend for several decades because of 

the long groundwater travel times in this area. No step trend 

was indicated for nitrate in the NSF study unit. A national 

evaluation of decadal step trends found that nitrate had 

increased in 14 groundwater networks and had decreased in 

6 networks after approximately 10-year intervals (Lindsey and 

Rupert, 2012; Lindsey and others, 2016).
48 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Orthophosphate increased in groundwater from 

the SALMON study unit and from the combined study 

units. A study that evaluated 3-year step trends found that 

orthophosphate increased statewide in California (Kent and 

Landon, 2016). Interestingly, that study found no step trend 

for orthophosphate in the hydrogeologic area (coastal) that 

included wells from the NSF and the SALMON GAMA-PBP 

study units. That study included only two wells from the 

SALMON study unit, however, and orthophosphate in both 

of those wells increased by more than the GRAC difference 

threshold after 3 years. A national evaluation of decadal step 

trends found that orthophosphate increased in 13 groundwater 

networks and decreased in 9 networks after approximately 

10-year intervals (Lindsey and others, 2016).

Calcium concentrations increased in groundwater from 

both study units and from the combined study units. Calcium 

and magnesium are the primary contributors to water hardness 

(Hem, 1985), and calcium was the greatest contributor 

to hardness in the groundwater of 30 out of the 34 wells 

evaluated during this study. Moreover, the groundwater in 

26 out of these 34 wells would be classified as “hard” or “very 

hard” on the basis of the combined concentrations of calcium 

and magnesium (Durfor and Becker, 1964). Although there 

is no human health threshold for calcium concentrations in 

water, increases in calcium concentration pose a potentially 

important groundwater-quality concern in the study area. 

While no spatial pattern was noted for calcium increases in 

the SALMON study unit, calcium increases in the NSF study 

unit groundwater primarily occurred in the eastern side of the 

study unit.

Chloride concentrations increased in groundwater from 

the NSF study unit. At the time of this study, most chloride 

concentrations in the study areas were not at levels of concern. 

Chloride concentrations in groundwater from two trend 

wells were greater than the California secondary maximum 

contaminant level (SMCL-CA) of 250 mg/L. A national 

evaluation of decadal step trends found that chloride increased 

in 31 groundwater networks and decreased in 2 networks after 

approximately 10-year intervals (Lindsey and others, 2016).

Sulfate concentrations increased in groundwater from the 

NSF study unit and from the combined study units. At the time 

of this study, most sulfate concentrations in the study areas 

were not at levels of concern. In one trend well, the sulfate 

concentration was greater than the SMCL-CA of 250 mg/L. 

That well (MSSV-03) was in the SALMON study unit, for 

which a step trend for sulfate concentrations was not indicated. 

A national evaluation of decadal step trends found that 

sulfate concentrations increased in 16 groundwater networks 

and decreased in 6 networks after approximately 10-year 

intervals (Lindsey and others, 2016). While no spatial pattern 

was noted for sulfate increases in the SALMON study unit, 

sulfate increases in the NSF study unit groundwater primarily 

occurred in the eastern side of the study unit.

Iron concentrations increased in groundwater from the 

SALMON study unit and from the combined study units. Iron 

concentrations were greater than the SMCL-CA of 300 μg/L 

in samples from seven wells, and iron in one of these wells 

(MSSC-08) increased from 218 to 308 μg/L to exceed this 

non-regulatory benchmark during the study period. Iron 

concentrations generally increased in groundwater from 

wells for which the initial concentration was greater than 

20 μg/L. In wells from which the initial concentration was 

less than 20 μg/L, there were about as many increases as there 

were decreases in iron concentrations, and the changes did 

not generally exceed either difference threshold. A national 

evaluation of decadal step trends found that iron increased in 

7 groundwater networks and decreased in 10 networks after 

approximately 10-year intervals (Lindsey and others, 2016). 

While no spatial pattern was noted for iron increases in the 

NSF study unit, the largest iron increases in the SALMON 

study unit were observed in wells in the northern part of the 

study unit.

Lithium concentrations increased in groundwater from 

the NSF study unit and from the combined study units. There 

is no human health threshold for lithium concentrations in 

water. Lithium can be toxic to plants, however, and citrus trees 

can be damaged by irrigation water containing 60–100 μg/L 

of lithium (Bradford, 1963). Lithium concentrations in 

groundwater from four trend wells were in this range during 

the initial sampling, and concentrations in groundwater 

from five trend wells were in this range during the 

decadal sampling.

The temperature of groundwater from most wells 

sampled for this study decreased by a mean of about 

0.7 degrees Celsius (oC). The measured decreases in 

groundwater temperature cannot be explained by differences 

in sampling conditions between initial and decadal sampling. 

Groundwater was pumped to land surface before field 

parameters were measured, so air temperature could affect 

the water-temperature measurement. Air temperatures were 

greater at the time of the decadal sampling than at the time of 

the initial sampling for 9 of the 13 wells in which groundwater 

temperatures decreased. The few wells where groundwater 

temperatures increased in the SALMON study unit were 

located in the central part of the Monterey Bay study area. 

No spatial pattern was observed for the few NSF wells where 

groundwater temperatures increased.

Arsenic concentrations decreased in groundwater from 

the NSF study unit and from the combined study units. 

Arsenic in California groundwater is generally naturally 

present, but can also derive from agricultural activities, 

including pesticide application, and industrial activities 

(Balazs and others, 2012). In 2001, the EPA adopted a new 

maximum contaminant level for arsenic of 10 μg/L, which 

replaced the old standard of 50 μg/L (U.S. Environmental 

Protection Agency, 2001), and compliance with the new 

standard has been burdensome for some (particularly small) 

water agencies (Jones and Joy, 2006). A national evaluation 

of decadal step trends found that arsenic increased in three 

groundwater networks and decreased in eight networks after 

approximately 10-year intervals (Lindsey and others, 2016). 
Summary and Conclusions 49

While no spatial pattern was noted for arsenic changes in the 

SALMON study unit, arsenic decreases in the NSF study unit 

groundwater primarily occurred in the southern-central and 

eastern areas of the study unit.

Lead concentrations decreased in groundwater from 

the combined study units. A study that evaluated 3-year step 

trends found that lead decreased statewide in California (Kent 

and Landon, 2016). As was the case for orthophosphate, that 

study found no step trend for lead in the hydrogeologic area 

(coastal) that included wells from the North San Francisco 

Bay and the Monterey Bay and Salinas Valley Basins GAMAPBP study units. Only three wells in the present study area 

were included in that 3-year evaluation, however, so the data 

were insufficient to find step trends. Lead concentrations in 

two of those three wells were less than the applied reporting 

level (an SRL) in both samples (non-detections in all cases). 

The lead concentration in the third well decreased from 2.68 

to 0.45 μg/L in 3 years. While no spatial pattern was noted 

for lead changes in the SALMON study unit, lead decreases 

in the NSF study unit groundwater primarily occurred in the 

southern part of the study unit.

The isotopic ratio of carbon-13 (δ13C) decreased in 

groundwater from the SALMON study unit. Changes in 

δ13C do not indicate a change in groundwater quality, but 

could indicate that groundwater recharge or mineralization 

conditions changed in the aquifer (Awad, 2014). The decrease 

in δ13C in groundwater can also reflect atmospheric decreases 

in δ13C caused by the Seuss Effect (Keeling, 1979). The Suess 

Effect is the decrease in atmospheric δ13C and δ14C values due 

to fossil fuel emissions, which are depleted in 13C and do not 

contain 14C.

Tritium decreased in groundwater from both study units 

and from the combined study units. Tritium is an unstable 

(radioactive) isotope of hydrogen and has a half-life of 

approximately 12.3 years (Plummer and others, 1993). As 

a result, tritium activity in an isolated parcel of water is 

expected to decrease. Because a finding of decreasing tritium 

is expected, the ability to systematically detect the decrease 

demonstrates that the trend-evaluation method is capable of 

detecting change.

Inconclusive Step-Trend Findings

There is insufficient evidence to conclude that there 

were meaningful changes in groundwater quality for 19 of 

the 33 constituents exhibiting a step trend by at least 1 of 

the evaluation methods. Step-trend findings for a constituent 

were deemed inconclusive under any one of the following 

three circumstances: (1) analytical bias was documented by 

the USGS Quality Systems Branch (QSB) during one or both 

sampling periods, and, when the data were adjusted for the 

bias, the step trend was no longer statistically significant; 

(2) the step-trend finding was not consistent with the evaluated 

results of a different, generally correlated, water-quality 

constituent; and (3) the concentrations of the constituent for 

which the step trend was indicated were negligible relative to 

their potential effect on groundwater quality. In consideration 

of these circumstances, findings were deemed inconclusive 

for 19 of the 33 constituents for which step-trend results were 

initially significant.

Analytical bias that could have affected step-trend 

findings was documented by the QSB for seven of the waterquality constituents for which step trends were indicated 

by at least one of the evaluation methods (table 3). These 

constituents included the major ions magnesium (figs. 15C, 

D), potassium (figs. 16C, D), sodium (figs. 17C, D), silica 

(figs. 22C, D), and the trace elements barium (figs. 25C, D), 

manganese (figs. 29C, D), and nickel (figs. 30C, D). In each 

of these cases, after adjusting the data for the bias, step trends 

were no longer indicated.

Step-trend findings for five constituents were deemed 

inconclusive because of a lack of support from the evaluations 

of other constituents that are generally correlated with these 

constituents (table 3). An increase indicated for field (fig. 6) 

and laboratory-measured (fig. 7) specific conductance was not 

supported by the evaluations of total dissolved solids, which 

failed to indicate a statistically significant increase. A decrease 

in field-measured pH for the SALMON study unit and the 

combined study units (fig. 8) was partially contradicted 

by the indication of increased laboratory-measured pH in 

groundwater from the SALMON study unit (fig. 9). Finally, 

the increased isotopic ratio of oxygen-18 in water (δ18O-H2

O; 

fig. 34) was not accompanied by a corresponding increase in 

the isotopic ratio of deuterium in water (δ2H-H2O). Almost 

invariably, δ18O-H2O co-varies with δ2H-H2O (Ingraham, 

1998). No step trend was found by any of the evaluation 

methods for δ2H-H2O, however.

The concentrations of seven constituents for which step 

trends were indicated were small relative to their potential 

effect on groundwater quality (table 3). As a result, the step 

trends were not considered important. Neither carbonate 

(fig. 10) nor iodide (fig. 19) has a health-based or non-healthbased drinking water threshold. Carbonate is most important 

in natural waters for its contribution, along with bicarbonate, 

to alkalinity (Hem, 1985). The contribution of carbonate to 

alkalinity in samples from trend wells during this study was 

negligible, however. Iodide concentrations measured during 

this study were about average for drinking water in the United 

States and were much less than concentrations shown to cause 

adverse health effects in humans (World Health Organization, 

1996). Fluoride has a maximum contaminant level in 

California (MCL-CA) of 2 mg/L, and fluoride concentrations 

in groundwater from trend wells sampled for this study were 

all less than 0.5 mg/L (fig. 21). Aluminum has a MCL-CA 

of 1,000 μg/L, and aluminum concentrations in groundwater 

from trend wells sampled for this study were all less than 

10 μg/L (fig. 23). Selenium has an EPA maximum contaminant 

level (MCL-US) of 50 μg/L, and the greatest concentration 

measured for selenium during this study was 15.6 μg/L 
50 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

(fig. 31). Strontium has an EPA lifetime health advisory level 

(HAL-US) of 4,000 μg/L, and groundwater from only one 

trend well had a strontium concentration as much as half of 

the HAL-US (fig. 32). Vanadium has a California Department 

of Public Health notification level (NL-CA) of 50 μg/L, and 

vanadium concentrations in NSF study unit wells, where the 

step trend of decreased concentrations was indicated, were all 

less than 20 μg/L (fig. 33).

Comparison of the Three Evaluation Methods

The three evaluation methods differed in their findings of 

step trends for the water-quality constituents analyzed during 

this study. Thirty-three constituents exhibited step trends in 

groundwater from one or both of the study units, or from 

the combined study units, by at least one of the evaluation 

methods (tables 2A, B, C, D). As expected, the evaluation 

method with no threshold difference requirement yielded 

the greatest number of significant step trends, at 50. The 

evaluation method that applied the CI difference threshold 

resulted in slightly fewer step-trend findings, at 41. One reason 

for this, however, is that the four field-measured parameters 

for which eight step trends were indicated when no difference 

threshold was applied could not be evaluated by the CI 

difference threshold method. When the CI difference threshold 

could be applied, the finding of step trends with this difference 

threshold was the same as the findings with no difference 

threshold applied, with only three exceptions. The step 

trends indicated decreased concentrations of carbonate in the 

SALMON study unit and in the combined study units when 

no difference threshold was applied were not found when the 

CI difference threshold was applied. In contrast, the step trend 

of decreasing concentrations for iodide in the NSF study unit 

when the CI difference threshold was applied was not found 

when no difference threshold was applied. The evaluation 

method that applied the GRAC difference threshold resulted 

in 26 step trends, and it appeared to be effective at screening 

differences in constituent concentrations that did not represent 

meaningful changes.

This study showed that applying a difference threshold 

helped to identify step trends in groundwater quality by 

preventing small differences (analytical variability) from 

indicating a significant step trend. Judging from the one-toone plots provided in figures 4–36, the GRAC difference 

threshold was generally effective at distinguishing meaningful 

differences in the paired samples from data scattered 

throughout the range of concentrations measured for nearly 

all of the constituents in this study. The effectiveness of 

the GRAC difference threshold was shown particularly 

clearly in the evaluations of the two isotopic constituents 

that exhibited step trends when no difference threshold was 

applied and when the CI difference threshold was applied. 

By these evaluations, the isotopic ratio of oxygen in water 

(δ18O-H2

O) increased in groundwater from both study units 

and in groundwater from the combined study units, and the 

isotopic ratio of carbon-13 in dissolved carbonates (δ13C) 

decreased in groundwater from the SALMON study unit. The 

isotopes of water δ18O-H2O and δ2H-H2O usually co-vary, 

however, and stable isotopic compositions are not expected 

to change appreciably in groundwater unless the source of the 

groundwater recharge changes (Ingraham, 1998), so these step 

trends are considered to be unlikely. No paired differences for 

δ18O-H2O or δ13C exceeded the GRAC difference threshold, 

so no step trends for δ18O-H2O or δ13C were indicated by this 

evaluation method.

In contrast to the GRAC difference threshold, the CI 

difference threshold, as applied in this study, was usually not 

an effective threshold, as was evident by the nearly identical 

step-trend findings for the evaluations with no difference 

threshold and those using the CI difference threshold. The 

CI difference threshold appeared to be effective for some 

constituents, but only for sample pairs for which constituent 

concentrations were low relative to reporting levels. For 

example, all of the differences between arsenic concentrations 

that failed to exceed the CI difference threshold were for 

paired samples from wells where arsenic concentrations 

were less than 1 μg/L. The maximum reporting level for 

arsenic used in this study was 0.2 μg/L. For iron, which had 

a maximum reporting level used in this study of 6.4 μg/L, 

the effective limit under which the CI difference threshold 

appeared effective was about 20 μg/L. For manganese, with 

a maximum reporting level used in this study of 0.66, the 

effective limit was about 2 μg/L. 

There are various possible reasons why the CI difference 

threshold was less effective than the GRAC difference 

threshold. The CI difference threshold relies on the assumption 

that analytical variability observed during each of the two 

separate periods indicates the analytical variability between 

the periods. This is not a valid assumption based on what was 

observed during this study. Variability was often substantially 

different from one sampling period to the next, for trace 

elements, in particular (Kent, 2018, tables 11 and 13, 

https://doi.org/10.5066/F7GH9GF5). In addition, analytical 

bias was documented for several constituents during one 

sampling period, but not for the other. Another issue with 

confidence intervals is that if they surround a value close 

to zero, they can extend to negative values. This illogical 

outcome makes the confidence intervals effectively smaller, so 

that it is easier for difference thresholds based on confidence 

intervals to be exceeded. A threshold is ineffective if it is 

easily exceeded. It could be possible to overcome some of 

the problems with difference thresholds calculated using 

confidence intervals by assigning a significance level less 

than 0.05 or by calculating the confidence intervals in a 

different way.
References Cited 51

Implications for Water Managers

The step-trend changes in constituent concentrations 

identified in this study, while low to moderate in magnitude, 

will be of interest to water managers and stakeholders in the 

areas of the North San Francisco Bay and the Monterey Bay 

and Salinas Valley Basins GAMA-PBP study units. The most 

relevant findings for water managers in the area of the North 

San Francisco Bay study unit are the increasing step trend in 

sulfate concentrations and the decreasing step trend in arsenic 

concentrations. The most relevant findings for water managers 

in the area of the Monterey Bay and Salinas Valley Basins 

study unit are the increasing step trends in groundwater nitrate 

and iron concentrations. Future monitoring efforts may seek 

explanations for the observed changes. Such explanations are 

beyond the scope of the present study. 

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54 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Tables

Table 1. Sample dates, elevations, and construction information for the 50 trend wells that were sampled two times, approximately 

9–10 years apart, in the North San Francisco Bay and Monterey Bay and Salinas Valley Basins study units.

[LSD, land surface datum; mm/dd/yyyy, month/day/year; NA, not applicable; NGVD 29, National Geodetic Vertical Datum of 1929; NWIS, National Water 

Information System; USGS, U.S. Geological Survey. GAMA well identification number acronyms: Monterey Bay and Salinas Valley Basins study 

unit: MSMB, Monterey Bay study area; MSPR, Paso Robles study area; MSSC, Santa Cruz study area; MSSV, Salinas Valley study area. North San Francisco 

Bay study unit: NSFVOL, Volcanic Highlands study area; NSFVP, Valley and Plains study area; NSFWG, Wilson Grove Formation Highlands study area; 

NSFWGFP, Wilson Grove Formation Highlands study area flow-path well]

GAMA well 

identification 

number

USGS station 

identification 

number 

(hyperlinked to data 

in the USGS NWIS)

Initial 

sample date

(mm/dd/yyyy)

Triennial samples 

(supplement some 

initial data) date

(mm/dd/yyyy)

Decadal 

sample date

(mm/dd/yyyy)

Elevation of 

LSD 

(meters 

above 

NGVD 29)1

Well 

depth 

(meters 

below 

LSD)

Top of 

highest 

perforation 

(meters 

below LSD)

Bottom 

of lowest 

perforation 

(meters 

below LSD)

North San Francisco Bay study unit

NSFVOL-05 382832122354301 9/27/2004 NA 10/21/2014 305 155 101 155

NSFVOL-06 381120122262901 9/28/2004 08/23/20072 11/18/2014 30 85 61 85

NSFVOL-14 381639122150801 10/7/2004 08/27/2007, 

10/19/2011

11/19/2014 42 127 17 127

NSFVOL-17 383501122302201 10/20/2004 NA 9/16/2014 171 81 NA NA

NSFVOL-18 383038122271301 10/20/2004 11/28/20072,3,4, 

10/26/2011

11/5/2014 61 204 41 201

NSFVOL-20 381906122274901 11/4/2004 NA 11/20/2014 91 76 52 76

NSFVP-06 381934122403601 9/1/2004 NA 10/22/2014 43 213 52 207

NSFVP-10 383034122590701 9/13/2004 NA 9/17/2014 11 30 23 29

NSFVP-19 383630122512601 9/16/2004 NA 9/16/2014 25 30 6 18

NSFVP-22 381422122352001 9/20/2004 NA 11/19/2014 15 153 18 153

NSFVP-26 383916122473501 9/27/2004 NA 9/15/2014 58 183 38 183

NSFVP-29 384238122541201 9/28/2004 08/27/2007, 

10/24/2011

9/15/2014 63 37 19 37

NSFVP-32 382553122232501 10/7/2004 11/20/20072,4 11/4/2014 38 122 NA NA

NSFVP-34 382307122311301 10/18/2004 08/22/2007, 

10/18/2011

10/20/2014 98 79 12 79

NSFVP-36 381153122185701 10/19/2004 08/20/20072,4, 

10/17/2011

11/18/2014 2 93 44 91

NSFVP-37 381808122293801 10/19/2004 08/22/2007 10/20/2014 36 110 18 107

NSFVP-38 381544122263801 10/20/2004 08/22/2007, 

10/18/2011

10/21/2014 9 235 64 235

NSFVP-39 383148122292901 10/21/2004 11/16/20072,4, 

10/26/2011

10/23/2014 75 140 17 140

NSFVP-41 381932122172601 10/21/2004 08/20/20072,4, 

10/19/2011

11/6/2014 14 72 18 72

NSFVP-46 382720122245701 11/3/2004 NA 11/4/2014 46 55 12 55

NSFVP-48 381440122191101 11/4/2004 11/15/20072,4 11/5/2014 20 61 18 61

NSFWG-03 382318122511401 9/21/2004 08/29/20072,3,4, 

10/25/2011

9/18/2014 83 168 82 168

NSFWG-04 382153122480301 9/22/2004 NA 11/3/2014 59 138 132 138

NSFWG-06 381524122573701 9/23/2004 NA 11/3/2014 136 49 NA NA

NSFWGFP-01 382345122490701 10/5/2004 08/29/2007, 

10/25/2011

9/17/2014 24 161 42 161
Tables 55

GAMA well 

identification 

number

USGS station 

identification 

number 

(hyperlinked to data 

in the USGS NWIS)

Initial 

sample date

(mm/dd/yyyy)

Triennial samples 

(supplement some 

initial data) date

(mm/dd/yyyy)

Decadal 

sample date

(mm/dd/yyyy)

Elevation of 

LSD 

(meters 

above 

NGVD 29)1

Well 

depth 

(meters 

below 

LSD)

Top of 

highest 

perforation 

(meters 

below LSD)

Bottom 

of lowest 

perforation 

(meters 

below LSD)

Monterey Bay and Salinas Valley Basins study unit

MSMB-02 363700121500001 8/10/2005 NA 9/8/2014 12 69 57 66

MSMB-04 365218121490301 8/17/2005 08/20/2008 8/4/2014 2 244 61 244

MSMB-09 365500121470001 8/15/2005 NA 8/5/2014 30 142 60 136

MSMB-11 364616121451301 8/18/2005 NA 8/20/2014 3 183 69 183

MSMB-13 363900121450001 8/17/2005 NA 8/19/2014 58 170 96 163

MSMB-17 364500121440001 8/9/2005 NA 8/20/2014 6 192 113 186

MSMB-20 365425121452201 8/16/2005 NA 8/5/2014 8 54 31 45

MSMB-26 363955121401001 8/11/2005 NA 8/14/2014 13 198 137 190

MSMB-28 363618121381901 8/3/2005 08/21/20082,3,4,5 8/13/2014 18 149 126 142

MSMB-37 364100121360001 9/1/2005 NA 8/21/2014 34 247 94 247

MSMB-38 363900121360001 8/11/2005 NA 8/13/2014 21 192 110 186

MSPR-01 353041120394501 7/19/2005 NA 8/11/2014 247 152 46 152

MSPR-03 353800120380001 7/28/2005 11/14/20083,4,5 8/11/2014 229 207 79 201

MSPR-09 354805120453601 7/18/2005 11/14/20083,4,5 9/10/2014 176 153 22 151

MSSC-04 370304122014201 8/25/2005 NA 8/6/2014 140 110 47 108

MSSC-05 370349121594801 8/30/2005 NA 8/7/2014 207 518 213 509

MSSC-06 365700121580001 8/24/2005 08/18/2008 8/4/20147 12 70 34 61

MSSC-08 370150121565301 9/15/2005 08/18/20088 8/6/2014 67 73 62 73

MSSC-10 365933121552601 8/29/2005 NA 9/9/2014 55 165 116 158

MSSV-02 360000120540001 8/4/2005 NA 8/12/2014 143 40 24 40

MSSV-03 360600121000001 8/2/20056 NA 9/10/2014 110 43 27 43

MSSV-06 361100121070001 8/2/2005 11/13/20083,4,5 8/18/2014 91 67 49 67

MSSV-07 361207121075501 8/2/2005 NA 8/18/2014 91 65 40 62

MSSV-11 361900121160001 7/25/2005 NA 9/8/2014 98 269 95 263

MSSV-16 362300121210001 9/14/2005 NA 9/11/2014 84 244 61 241

1

LSD is a datum plane that approximates land surface at each well. The altitude of the LSD is described in feet above the NGVD 29.

2

This sample, collected to evaluate triennial trends, was used as the initial carbon isotope sample for this well.

3

This sample, collected to evaluate triennial trends, was used as the initial major ion and trace element sample for this well.

4

This sample, collected to evaluate triennial trends, was used as the initial nutrient sample for this well.

5

This sample, collected to evaluate triennial trends, was used as the initial perchlorate sample for this well.

6

Inorganic constituents were sampled September 12, 2005, at this well.

7

Sample for analysis of volatile organic compounds collected August 6, 2014, at this well.

8

Sample only analyzed for carbon isotopes and stable isotopes of water.

Table 1. Sample dates, elevations, and construction information for the 50 trend wells that were sampled two times, approximately 

9–10 years apart, in the North San Francisco Bay and Monterey Bay and Salinas Valley Basins study units.—Continued

[LSD, land surface datum; mm/dd/yyyy, month/day/year; NA, not applicable; NGVD 29, National Geodetic Vertical Datum of 1929; NWIS, National Water 

Information System; USGS, U.S. Geological Survey. GAMA well identification number acronyms: Monterey Bay and Salinas Valley Basins study 

unit: MSMB, Monterey Bay study area; MSPR, Paso Robles study area; MSSC, Santa Cruz study area; MSSV, Salinas Valley study area. North San Francisco 

Bay study unit: NSFVOL, Volcanic Highlands study area; NSFVP, Valley and Plains study area; NSFWG, Wilson Grove Formation Highlands study area; 

NSFWGFP, Wilson Grove Formation Highlands study area flow-path well]
56 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 50 8 10 32 0.010▼

Dissolved oxygen 00300 44 4 23 17 0.674

Field specific conductance 00095 49 1 27 21 0.040▲

Lab specific conductance1 90095 34 0 25 9 0.010▲

Total dissolved solids 70300 34 0 16 18 0.427

Field pH 00400 35 1 12 22 0.006▼

Lab pH 00403 34 0 22 12 0.293

Alkalinity 29801 32 0 17 15 0.308

Bicarbonate 63786 34 0 16 16 0.614

Carbonate 63788 32 8 5 19 0.002▼

Total nitrogen 62854 34 3 16 15 0.478

Nitrate1 00631 34 10 14 10 0.260

Nitrite 00613 34 28 2 4 0.461

Ammonia 00608 34 20 8 6 0.545

Orthophosphate 00671 34 0 28 6 0.002▲

Perchlorate 61209 43 39 3 1 0.312

Calcium1 00915 34 0 24 10 0.007▲

Magnesium1 00925 34 0 25 9 0.001▲

Potassium1 00935 34 0 17 17 0.885

Sodium1 00930 34 0 25 9 0.007▲

Bromide 71870 34 2 12 20 0.266

Chloride 00940 34 0 25 9 0.083

Iodide 71865 34 2 12 20 0.073

Sulfate 00945 34 0 23 11 0.011▲

Fluoride1 00950 32 1 9 22 0.002▼

Silica1 00955 34 0 6 28 <0.001▼

Aluminum1,2 01106 34 22 11 1 0.003▲

Antimony 01095 34 28 2 4 0.430

Arsenic 01000 34 3 8 23 0.004▼

Barium1 01005 34 0 20 14 0.046▲

Boron1 01020 34 0 18 16 0.638

Cadmium1 01025 34 22 4 8 0.296

Chromium 01030 34 22 7 5 0.520

Copper 01040 34 22 7 5 0.658

Iron2 01046 34 9 19 6 0.006▲

Lead 01049 34 21 4 9 0.116
Tables 57

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 34 0 24 10 0.024▲

Manganese1 01056 34 7 18 9 0.030▲

Molybdenum 01060 34 0 25 29 0.939

Nickel1 01065 34 1 22 11 0.669

Selenium1 01145 34 14 9 11 0.488

Strontium1 01080 34 0 22 12 0.092

Thallium 01057 34 31 1 2 0.588

Uranium1 22703 34 6 13 15 0.824

Vanadium 01085 34 1 14 19 0.758

Zinc1 01090 34 25 5 4 0.683

Carbon disulfide 77041 50 41 5 4 0.664

Chloroform (trichloromethane) 32106 50 38 8 4 0.260

Methyl tert-butyl ether (MTBE) 78032 50 45 1 4 0.180

Tetrachloroethene (PCE) 34475 50 45 1 4 0.168

Trichloroethene (TCE) 39180 50 45 2 3 0.668

2-Chloro-4-isopropylamino-6-aminos-triazine04040 49 46 3 0 0.083

Atrazine 39632 49 46 1 2 0.548

Simazine 04035 49 40 2 7 0.093

δ2

H of water 82082 48 0 27 21 0.587

δ18O of water 82085 48 0 42 6 <0.001▲

δ13C of dissolved carbonates 82081 30 0 10 20 0.111

Carbon-14 (modern carbon) 49933 30 0 14 16 0.504

Tritium 07000 49 15 4 30 <0.001▼

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]
58 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 50 50 0 0 NA

Dissolved oxygen 00300 44 27 8 9 0.719

Field specific conductance 00095 49 40 8 1 0.021▲

Lab specific conductance1 90095 34 25 8 1 0.023▲

Total dissolved solids 70300 34 30 3 1 0.325

Field pH 00400 35 33 0 2 0.157

Lab pH 00403 34 32 1 1 0.983

Alkalinity 29801 32 29 3 0 0.083

Bicarbonate 63786 34 29 3 0 0.083

Carbonate 63788 32 22 1 9 0.011▼

Total nitrogen 62854 34 17 10 7 0.389

Nitrate1 00631 34 18 11 5 0.123

Nitrite 00613 34 30 2 2 0.976

Ammonia 00608 34 31 1 2 0.540

Orthophosphate 00671 34 15 16 3 0.007▲

Perchlorate 61209 43 41 1 1 0.987

Calcium1 00915 34 28 5 1 0.095

Magnesium1 00925 34 25 8 1 0.018▲

Potassium1 00935 34 30 4 0 0.046▲

Sodium1 00930 34 30 3 1 0.325

Bromide 71870 34 20 4 10 0.158

Chloride 00940 34 22 7 5 0.494

Iodide 71865 34 23 4 7 0.326

Sulfate 00945 34 18 13 3 0.013▲

Fluoride1 00950 32 31 0 1 0.317

Silica1 00955 34 32 0 2 0.157

Aluminum1,2 01106 34 28 6 0 0.014▲

Antimony 01095 34 33 1 0 0.317

Arsenic 01000 34 27 2 5 0.268

Barium1 01005 34 27 5 2 0.210

Boron1 01020 34 26 4 4 0.926

Cadmium1 01025 34 31 1 2 0.588

Chromium 01030 34 32 2 0 0.157

Copper 01040 34 30 1 3 0.340

Iron2 01046 34 21 11 2 0.011▲

Lead 01049 34 26 1 7 0.037▼

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]
Tables 59

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 34 28 5 1 0.095

Manganese1 01056 34 22 8 4 0.198

Molybdenum 01060 34 26 4 4 0.853

Nickel1 01065 34 20 6 8 0.483

Selenium1 01145 34 25 3 6 0.330

Strontium1 01080 34 30 4 0 0.046▲

Thallium 01057 34 34 0 0 NA

Uranium1 22703 34 23 6 5 0.687

Vanadium 01085 34 16 7 11 0.351

Zinc1 01090 34 25 5 4 0.808

Carbon disulfide 77041 50 43 4 3 0.636

Chloroform 

(trichloromethane)

32106 50 44 4 2 0.434

Methyl tert-butyl ether (MTBE) 78032 50 48 1 1 0.989

Tetrachloroethene (PCE) 34475 50 47 0 3 0.083

Trichloroethene (TCE) 39180 50 46 2 2 1.000

2-Chloro-4-isopropylamino-6-aminos-triazine

04040 49 49 0 0 NA

Atrazine 39632 49 48 0 1 0.317

Simazine 04035 49 48 0 1 0.317

δ2

H of water 82082 48 48 0 0 NA

δ18O of water 82085 48 48 0 0 NA

δ13C of dissolved carbonates 82081 30 30 0 0 NA

Carbon-14 

(modern carbon)

49933 30 2 12 16 0.459

Tritium 07000 49 24 3 22 <0.001▼

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]
60 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 50 No results for field parameters

Dissolved oxygen 00300 44 No results for field parameters

Field specific conductance 00095 49 No results for field parameters

Lab specific conductance1 90095 34 0 25 9 0.010▲

Total dissolved solids 70300 34 0 16 18 0.427

Field pH 00400 35 No results for field parameters

Lab pH 00403 34 0 22 12 0.293

Alkalinity 29801 32 0 17 15 0.308

Bicarbonate 63786 34 0 16 16 0.614

Carbonate 63788 32 25 1 5 0.095

Total nitrogen 62854 34 12 14 8 0.244

Nitrate1 00631 34 14 13 7 0.166

Nitrite 00613 34 28 2 4 0.461

Ammonia 00608 34 22 7 5 0.546

Orthophosphate 00671 34 8 22 4 0.003▲

Perchlorate 61209 43 42 0 1 0.317

Calcium1 00915 34 0 24 10 0.007▲

Magnesium1 00925 34 0 25 9 0.001▲

Potassium1 00935 34 1 16 17 0.878

Sodium1 00930 34 0 25 9 0.007▲

Bromide 71870 34 10 9 15 0.290

Chloride 00940 34 0 25 9 0.083

Iodide 71865 34 18 4 12 0.051

Sulfate 00945 34 0 23 11 0.011▲

Fluoride1 00950 32 10 4 18 0.002▼

Silica1 00955 34 0 6 28 <0.001▼

Aluminum1,2 01106 34 23 11 0 <0.001▲

Antimony 01095 34 30 1 3 0.340

Arsenic 01000 34 8 5 21 0.002▼

Barium1 01005 34 0 20 14 0.046▲

Boron1 01020 34 3 16 15 0.590

Cadmium1 01025 34 23 4 7 0.403

Chromium 01030 34 28 4 2 0.415

Copper 01040 34 24 5 5 0.975

Iron2 01046 34 17 14 3 0.006▲

Lead 01049 34 22 3 9 0.067

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]
Tables 61

Constituent

USGS 

parameter 

code

n 

Difference threshold based on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 34 0 24 10 0.024▲

Manganese1 01056 34 11 16 7 0.029▲

Molybdenum 01060 34 7 12 15 0.897

Nickel1 01065 34 14 11 9 0.986

Selenium1 01145 34 16 8 10 0.486

Strontium1 01080 34 0 22 12 0.092

Thallium 01057 34 No qualifying replicates to establish CI

Uranium1 22703 34 16 11 7 0.273

Vanadium 01085 34 4 11 19 0.555

Zinc1 01090 34 25 5 4 0.808

Carbon disulfide 77041 50 No qualifying replicates to establish CI

Chloroform 

(trichloromethane)

32106 50 42 5 3 0.485

Methyl tert-butyl ether (MTBE) 78032 50 No qualifying replicates to establish CI

Tetrachloroethene (PCE) 34475 50 No qualifying replicates to establish CI

Trichloroethene (TCE) 39180 50 No qualifying replicates to establish CI

2-Chloro-4-isopropylamino-6-aminos-triazine04040 49 48 1 0 0.317

Atrazine 39632 49 48 0 1 0.317

Simazine 04035 49 No qualifying replicates to establish CI

δ2

H of water 82082 48 0 27 21 0.587

δ18O of water 82085 48 0 42 6 <0.001▲

δ13C of dissolved carbonates 82081 30 0 10 20 0.111

Carbon-14 

(modern carbon)

49933 30 0 14 16 0.504

Tritium 07000 49 19 2 28 <0.001▼

1

Data-quality assessment summaries published by the QSB found measurement bias or variability at the NWQL for this constituent during one or both sampling periods.

2

Project field-blank results indicate a potential positive bias in initial period concentrations for this constituent.

Table 2A. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the combined study units.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NWQL, USGS National Water Quality Laboratory; O, oxygen; p value, calculated 

probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; ▲, increasing trend or 

▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the ratio of a heavier 

isotope to more common lighter isotope of that element, relative to a standard reference material]
62 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 3 6 16 0.092

Dissolved oxygen 00300 19 2 13 4 0.025▲

Field specific conductance 00095 25 0 15 10 0.028▲

Lab specific conductance1 90095 18 0 16 2 0.002▲

Total dissolved solids 70300 18 0 8 10 0.306

Field pH 00400 19 1 12 6 0.376

Lab pH 00403 18 0 8 10 0.327

Alkalinity 29801 16 0 9 7 0.278

Bicarbonate 63786 16 0 9 7 0.278

Carbonate 63788 16 7 5 4 0.745

Total nitrogen 62854 18 2 6 10 0.295

Nitrate1 00631 18 6 5 7 0.350

Nitrite 00613 18 15 0 3 0.084

Ammonia 00608 18 9 5 4 0.482

Orthophosphate 00671 18 0 14 4 0.064

Perchlorate 61209 26 26 0 0 NA

Calcium1 00915 18 0 12 6 0.005▲

Magnesium1 00925 18 0 13 5 0.011▲

Potassium1 00935 18 0 9 9 0.557

Sodium1 00930 18 0 13 5 0.053

Bromide 71870 18 2 6 10 0.316

Chloride 00940 18 0 16 2 0.011▲

Iodide 71865 18 2 6 10 0.089

Sulfate 00945 18 0 11 7 0.064

Fluoride1 00950 16 1 3 12 0.008▼

Silica1 00955 18 0 1 17 <0.001▼

Aluminum1,2 01106 18 8 9 1 0.007▲

Antimony 01095 18 18 0 0 NA

Arsenic 01000 18 1 3 14 0.012▼

Barium1 01005 18 0 11 7 0.048▲

Boron1 01020 18 0 10 8 0.777

Cadmium1 01025 18 16 0 2 0.158

Chromium 01030 18 15 2 1 0.564

Copper 01040 18 11 4 3 0.822

Iron2 01046 18 4 10 4 0.069

Lead 01049 18 9 3 6 0.261
Tables 63

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 18 0 14 4 0.012▲

Manganese1 01056 18 2 9 7 0.239

Molybdenum 01060 18 0 6 12 0.184

Nickel1 01065 18 1 15 2 0.010▲

Selenium1 01145 18 12 0 6 0.015▼

Strontium1 01080 18 0 13 5 0.112

Thallium 01057 18 16 1 1 0.968

Uranium1 22703 18 4 5 9 0.369

Vanadium 01085 18 1 4 13 0.081

Zinc1 01090 18 5 5 8 0.402

Carbon disulfide 77041 25 20 2 3 0.742

Chloroform (trichloromethane) 32106 25 19 4 2 0.437

Methyl tert-butyl ether (MTBE) 78032 25 22 1 2 0.564

Tetrachloroethene (PCE) 34475 25 24 0 1 0.317

Trichloroethene (TCE) 39180 25 24 0 1 0.317

2-Chloro-4-isopropylamino-6-aminos-triazine04040 24 23 1 0 0.317

Atrazine 39632 24 23 0 1 0.317

Simazine 04035 24 21 0 3 0.083

δ2

H of water 82082 24 0 14 10 0.310

δ18O of water 82085 24 0 22 2 <0.001▲

δ13C of dissolved carbonates 82081 18 0 8 10 0.586

Carbon-14 (modern carbon) 49933 18 0 9 9 0.845

Tritium 07000 25 7 3 15 0.003▼

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]
64 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 25 0 0 NA

Dissolved oxygen 00300 19 13 5 1 0.081

Field specific conductance 00095 25 22 3 0 0.083

Lab specific conductance1 90095 18 13 5 0 0.026▲

Total dissolved solids 70300 18 17 1 0 0.317

Field pH 00400 19 19 0 0 NA

Lab pH 00403 18 16 1 1 0.968

Alkalinity 29801 16 15 1 0 0.317

Bicarbonate 63786 16 15 1 0 0.317

Carbonate 63788 16 14 1 1 0.964

Total nitrogen 62854 18 11 3 4 0.317

Nitrate1 00631 18 11 3 4 0.500

Nitrite 00613 18 17 0 1 0.317

Ammonia 00608 18 17 1 0 0.317

Orthophosphate 00671 18 9 7 2 0.134

Perchlorate 61209 26 26 0 0 NA

Calcium1 00915 18 17 1 0 0.158

Magnesium1 00925 18 14 4 0 0.046▲

Potassium1 00935 18 17 1 0 0.317

Sodium1 00930 18 17 1 0 0.317

Bromide 71870 18 13 1 4 0.200

Chloride 00940 18 13 4 1 0.164

Iodide 71865 18 13 1 4 0.148

Sulfate 00945 18 9 8 1 0.013▲

Fluoride1 00950 16 16 0 0 NA

Silica1 00955 18 17 0 1 0.317

Aluminum1,2 01106 18 13 5 0 0.026▲

Antimony 01095 18 18 0 0 NA

Arsenic 01000 18 14 1 3 0.364

Barium1 01005 18 15 2 1 0.519

Boron1 01020 18 15 1 2 0.611

Cadmium1 01025 18 18 0 0 NA

Chromium 01030 18 18 0 0 NA

Copper 01040 18 16 0 2 0.158

Iron2 01046 18 12 5 1 0.110

Lead 01049 18 12 1 5 0.136

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]
Tables 65

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 18 16 2 0 0.158

Manganese1 01056 18 11 3 4 0.901

Molybdenum 01060 18 14 1 3 0.364

Nickel1 01065 18 13 4 1 0.221

Selenium1 01145 18 14 0 4 0.046▼

Strontium1 01080 18 17 1 0 0.317

Thallium 01057 18 18 0 0 NA

Uranium1 22703 18 12 3 3 0.896

Vanadium 01085 18 7 2 9 0.036▼

Zinc1 01090 18 12 3 3 0.896

Carbon disulfide 77041 25 21 2 2 0.932

Chloroform (trichloromethane) 32106 25 22 2 1 0.597

Methyl tert-butyl ether (MTBE) 78032 25 23 1 1 0.977

Tetrachloroethene (PCE) 34475 25 24 0 1 0.317

Trichloroethene (TCE) 39180 25 24 0 1 0.317

2-Chloro-4-isopropylamino-6-aminos-triazine04040 24 24 0 0 NA

Atrazine 39632 24 24 0 0 NA

Simazine 04035 24 23 0 1 0.317

δ2

H of water 82082 24 24 0 0 NA

δ18O of water 82085 24 24 0 0 NA

δ13C of dissolved carbonates 82081 18 18 0 0 NA

Carbon-14 (modern carbon) 49933 18 2 8 9 0.896

Tritium 07000 25 12 2 11 0.011▼

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]
66 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based 

on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 No results for field parameters

Dissolved oxygen 00300 19 No results for field parameters

Field specific conductance 00095 25 No results for field parameters

Lab specific conductance1 90095 18 0 16 2 0.002▲

Total dissolved solids 70300 18 0 8 10 0.306

Field pH 00400 19 No results for field parameters

Lab pH 00403 18 0 8 10 0.327

Alkalinity 29801 16 0 9 7 0.278

Bicarbonate 63786 16 0 9 16 0.278

Carbonate 63788 16 13 1 2 0.564

Total nitrogen 62854 18 7 6 5 0.598

Nitrate1 00631 18 9 4 5 0.426

Nitrite 00613 18 15 0 3 0.084

Ammonia 00608 18 10 5 3 0.335

Orthophosphate 00671 18 5 10 3 0.103

Perchlorate 61209 26 26 0 0 NA

Calcium1 00915 18 0 12 6 0.005▲

Magnesium1 00925 18 0 13 5 0.011▲

Potassium1 00935 18 1 8 9 0.542

Sodium1 00930 18 0 13 5 0.053

Bromide 71870 18 8 3 7 0.262

Chloride 00940 18 0 16 2 0.011▲

Iodide 71865 18 9 1 8 0.022▼

Sulfate 00945 18 0 11 7 0.064

Fluoride1 00950 16 3 2 11 0.008▼

Silica1 00955 18 0 1 17 <0.001▼

Aluminum1,2 01106 18 9 9 0 0.003▲

Antimony 01095 18 18 0 0 NA

Arsenic 01000 18 3 2 13 0.009▼

Barium1 01005 18 0 11 7 0.048▲

Boron1 01020 18 3 8 7 0.678

Cadmium1 01025 18 17 0 1 0.317

Chromium 01030 18 18 0 0 NA

Copper 01040 18 12 3 3 0.937

Iron2 01046 18 8 7 3 0.136

Lead 01049 18 9 3 6 0.261

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]
Tables 67

Constituent

USGS 

parameter 

code

n 

Difference threshold based 

on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 18 0 14 4 0.012▲

Manganese1 01056 18 3 9 6 0.213

Molybdenum 01060 18 7 3 8 0.131

Nickel1 01065 18 8 9 1 0.023▲

Selenium1 01145 18 12 0 6 0.015▼

Strontium1 01080 18 0 13 5 0.112

Thallium 01057 18 No qualifying replicates to establish CI

Uranium1 22703 18 13 3 2 0.578

Vanadium 01085 18 2 3 13 0.055

Zinc1 01090 18 12 3 3 0.896

Carbon disulfide 77041 25 No qualifying replicates to establish CI

Chloroform (trichloromethane) 32106 25 22 2 1 0.597

Methyl tert-butyl ether (MTBE) 78032 25 No qualifying replicates to establish CI

Tetrachloroethene (PCE) 34475 25 No qualifying replicates to establish CI

Trichloroethene (TCE) 39180 25 No qualifying replicates to establish CI

2-Chloro-4-isopropylamino-6-aminos-triazine04040 24 24 0 0 NA

Atrazine 39632 24 24 0 0 NA

Simazine 04035 24 No qualifying replicates to establish CI

δ2

H of water 82082 24 0 14 10 0.310

δ18O of water 82085 24 0 22 2 <0.001▲

δ13C of dissolved carbonates 82081 18 0 8 10 0.586

Carbon-14 (modern carbon) 49933 18 0 9 9 0.845

Tritium 07000 25 10 1 14 0.002▼

1

Data-quality assessment summaries published by the QSB found measurement bias or variability at the NWQL for this constituent during one or both sampling periods.

2

Project field-blank results indicate a potential positive bias in initial period concentrations for this constituent.

Table 2B. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the North San Francisco Bay study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygem; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material.]
68 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 5 4 16 0.040▼

Dissolved oxygen 00300 25 2 10 13 0.236

Field specific conductance 00095 24 1 12 11 0.407

Lab specific conductance1 90095 16 0 9 7 0.570

Total dissolved solids 70300 16 0 8 8 0.796

Field pH 00400 16 0 0 16 <0.001▼

Lab pH 00403 16 0 14 2 0.007▲

Alkalinity 29801 16 0 8 8 0.717

Bicarbonate 63786 16 0 7 9 0.796

Carbonate 63788 16 1 0 15 <0.001▼

Total nitrogen 62854 16 1 10 5 0.052

Nitrate1 00631 16 4 9 3 0.025▲

Nitrite 00613 16 13 2 1 0.514

Ammonia 00608 16 11 3 2 0.799

Orthophosphate 00671 16 0 14 2 0.007▲

Perchlorate 61209 17 13 3 1 0.303

Calcium1 00915 16 0 12 4 0.006▲

Magnesium1 00925 16 0 12 4 0.021▲

Potassium1 00935 16 0 8 8 0.642

Sodium1 00930 16 0 12 4 0.070

Bromide 71870 16 9 6 10 0.221

Chloride 00940 16 0 9 7 0.756

Iodide 71865 16 0 6 10 0.501

Sulfate 00945 16 0 12 4 0.088

Fluoride1 00950 16 0 6 10 0.088

Silica1 00955 16 0 5 11 0.015▼

Aluminum1,2 01106 16 14 2 0 0.158

Antimony 01095 16 10 2 4 0.453

Arsenic 01000 16 2 5 9 0.313

Barium1 01005 16 0 9 7 0.570

Boron1 01020 16 0 8 8 0.717

Cadmium1 01025 16 6 4 6 0.612

Chromium 01030 16 7 5 4 0.625

Copper 01040 16 11 3 2 0.703

Iron2 01046 16 5 9 2 0.029▲

Lead 01049 16 12 1 3 0.271
Tables 69

Constituent

USGS 

parameter 

code

n 

No difference threshold

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 16 0 10 6 0.501

Manganese1 01056 16 5 9 2 0.019▲

Molybdenum 01060 16 0 9 7 0.278

Nickel1 01065 16 0 7 9 0.179

Selenium1 01145 16 2 9 5 0.453

Strontium1 01080 16 0 9 7 0.301

Thallium 01057 16 15 0 1 0.317

Uranium1 22703 16 2 8 6 0.422

Vanadium 01085 16 0 10 6 0.301

Zinc1 01090 16 9 4 3 0.666

Carbon disulfide 77041 25 21 3 1 0.308

Chloroform (trichloromethane) 32106 25 19 4 2 0.416

Methyl tert-butyl ether (MTBE) 78032 25 23 0 2 0.157

Tetrachloroethene (PCE) 34475 25 21 1 3 0.288

Trichloroethene (TCE) 39180 25 21 2 2 0.966

2-Chloro-4-isopropylamino-6-aminos-triazine04040 25 23 2 0 0.157

Atrazine 39632 25 23 1 1 0.977

Simazine 04035 25 19 2 4 0.416

δ2

H of water 82082 24 0 13 11 0.909

δ18O of water 82085 24 0 20 2 <0.001▲

δ13C of dissolved carbonates 82081 12 0 2 10 0.041▼

Carbon-14 (modern carbon) 49933 12 0 5 7 0.209

Tritium 07000 24 8 1 15 <0.001▼

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]
70 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 25 0 0 NA

Dissolved oxygen 00300 25 14 3 8 0.108

Field specific conductance 00095 24 18 5 1 0.108

Lab specific conductance1 90095 16 12 3 1 0.302

Total dissolved solids 70300 16 13 2 1 0.564

Field pH 00400 16 14 0 2 0.158

Lab pH 00403 16 16 0 0 NA

Alkalinity 29801 16 14 2 0 0.157

Bicarbonate 63786 16 14 2 0 0.158

Carbonate 63788 16 8 0 8 0.005▼

Total nitrogen 62854 16 15 0 1 0.074

Nitrate1 00631 16 7 8 1 0.015▲

Nitrite 00613 16 13 2 1 0.514

Ammonia 00608 16 14 0 2 0.158

Orthophosphate 00671 16 6 9 1 0.020▲

Perchlorate 61209 17 15 1 1 0.966

Calcium1 00915 16 11 4 1 0.026▲

Magnesium1 00925 16 11 4 1 0.162

Potassium1 00935 16 13 3 0 0.084

Sodium1 00930 16 13 2 1 0.564

Bromide 71870 16 7 3 6 0.156

Chloride 00940 16 9 3 4 0.886

Iodide 71865 16 10 3 3 0.928

Sulfate 00945 16 9 5 2 0.239

Fluoride1 00950 16 15 0 1 0.317

Silica1 00955 16 15 0 1 0.317

Aluminum1,2 01106 16 15 1 0 0.317

Antimony 01095 16 15 1 0 0.317

Arsenic 01000 16 13 1 2 0.564

Barium1 01005 16 12 3 1 0.271

Boron1 01020 16 11 3 2 0.656

Cadmium1 01025 16 13 1 2 0.617

Chromium 01030 16 14 2 0 0.158

Copper 01040 16 14 1 1 0.964

Iron2 01046 16 9 6 1 0.041▲

Lead 01049 16 14 0 2 0.158

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]
Tables 71

Constituent

USGS 

parameter 

code

n 

Difference threshold based on 

replicate acceptability criteria

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 16 12 3 1 0.302

Manganese1 01056 16 11 5 0 0.026▲

Molybdenum 01060 16 12 3 1 0.271

Nickel1 01065 16 7 2 7 0.143

Selenium1 01145 16 11 3 2 0.656

Strontium1 01080 16 13 3 0 0.084

Thallium 01057 16 16 0 0 NA

Uranium1 22703 16 11 3 2 0.611

Vanadium 01085 16 9 5 2 0.405

Zinc1 01090 16 13 2 1 0.564

Carbon disulfide 77041 25 22 2 1 0.532

Chloroform (trichloromethane) 32106 25 22 2 1 0.564

Methyl tert-butyl ether (MTBE) 78032 25 25 0 0 NA

Tetrachloroethene (PCE) 34475 25 23 0 2 0.157

Trichloroethene (TCE) 39180 25 22 2 1 0.564

2-Chloro-4-isopropylamino-6-aminos-triazine04040 25 25 0 0 NA

Atrazine 39632 25 24 0 1 0.317

Simazine 04035 25 25 0 0 NA

δ2

H of water 82082 24 24 0 0 NA

δ18O of water 82085 24 24 0 0 NA

δ13C of dissolved carbonates 82081 12 12 0 0 NA

Carbon-14 (modern carbon) 49933 12 1 4 7 0.195

Tritium 07000 24 12 1 11 0.003▼

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]
72 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Constituent

USGS 

parameter 

code

n 

Difference threshold based 

on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Water temperature 00010 25 No results for field parameters

Dissolved oxygen 00300 25 No results for field parameters

Field specific conductance 00095 24 No results for field parameters

Lab specific conductance1 90095 16 0 9 7 0.570

Total dissolved solids 70300 16 0 8 8 0.796

Field pH 00400 16 No results for field parameters

Lab pH 00403 16 0 14 2 0.007▲

Alkalinity 29801 16 0 8 8 0.717

Bicarbonate 63786 16 0 7 0 0.796

Carbonate 63788 16 13 0 3 0.084

Total nitrogen 62854 16 4 9 3 0.037▲

Nitrate1 00631 16 5 9 2 0.017▲

Nitrite 00613 16 13 2 1 0.514

Ammonia 00608 16 12 2 2 0.891

Orthophosphate 00671 16 3 12 1 0.008▲

Perchlorate 61209 17 16 0 1 0.317

Calcium1 00915 16 0 12 4 0.006▲

Magnesium1 00925 16 0 12 4 0.021▲

Potassium1 00935 16 1 16 17 0.642

Sodium1 00930 16 0 12 4 0.070

Bromide 71870 16 2 6 8 0.293

Chloride 00940 16 0 9 7 0.756

Iodide 71865 16 9 3 4 0.752

Sulfate 00945 16 0 12 4 0.088

Fluoride1 00950 16 7 2 7 0.092

Silica1 00955 16 0 5 11 0.015▼

Aluminum1,2 01106 16 14 2 0 0.158

Antimony 01095 16 12 1 3 0.371

Arsenic 01000 16 5 3 8 0.179

Barium1 01005 16 0 9 7 0.570

Boron1 01020 16 0 8 8 0.717

Cadmium1 01025 16 6 4 6 0.612

Chromium 01030 16 10 4 2 0.418

Copper 01040 16 12 2 2 1.000

Iron2 01046 16 9 7 0 0.009▲

Lead 01049 16 13 0 3 0.084

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]
Tables 73

Constituent

USGS 

parameter 

code

n 

Difference threshold based 

on confidence intervals

Number of wells 

with no change

Number of wells 

with increase

Number of wells 

with decrease

p value and trend direction 

(if applicable)

Lithium 01130 16 0 10 6 0.501

Manganese1 01056 16 8 7 1 0.026▲

Molybdenum 01060 16 0 9 7 0.278

Nickel1 01065 16 6 2 8 0.104

Selenium1 01145 16 4 8 4 0.433

Strontium1 01080 16 0 9 7 0.301

Thallium 01057 16 No qualifying replicates to establish CI

Uranium1 22703 16 3 8 5 0.377

Vanadium 01085 16 2 8 6 0.422

Zinc1 01090 16 13 2 1 0.564

Carbon disulfide 77041 25 No qualifying replicates to establish CI

Chloroform (trichloromethane) 32106 25 20 3 2 0.628

Methyl tert-butyl ether (MTBE) 78032 25 No qualifying replicates to establish CI

Tetrachloroethene (PCE) 34475 25 No qualifying replicates to establish CI

Trichloroethene (TCE) 39180 25 No qualifying replicates to establish CI

2-Chloro-4-isopropylamino-6-aminos-triazine04040 25 24 1 0 0.317

Atrazine 39632 25 24 0 1 0.317

Simazine 04035 25 No qualifying replicates to establish CI

δ2

H of water 82082 24 0 13 11 0.909

δ18O of water 82085 24 0 20 4 <0.001▲

δ13C of dissolved carbonates 82081 12 0 2 10 0.041▼

Carbon-14 (modern carbon) 49933 12 0 5 7 0.209

Tritium 07000 24 9 1 14 <0.001▼

1

Data-quality assessment summaries published by the QSB found measurement bias or variability at the NWQL for this constituent during one or both sampling periods.

2

Project field-blank results indicate a potential positive bias in initial period concentrations for this constituent.

Table 2C. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria for the Monterey Bay and Salinas Valley Basins study unit.—Continued

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; CI, confidence intervals; GAMA-PBP, USGS Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project; 

H, hydrogen; n, number of wells with paired results for constituent; NA, not applicable; NWQL, USGS National Water Quality Laboratory; O, oxygen; 

p value, calculated probability of the obtained Wilcoxon-Pratt test result if the null hypothesis is true; QSB, USGS Quality Systems Branch; <, less than; 

▲, increasing trend or ▼, decreasing trend with p value less than 0.05, considered statistically significant indication of step trend; δ, standard delta notation-the 

ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]
74 Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14

Table 2D. Constituents detected in at least 10 percent of the samples collected during initial sampling or resampling of trend wells, 

the number of paired results, number increased, number decreased, and p values from Wilcoxon signed-rank test with Pratt correction 

using different threshold criteria: Summary of statistical evaluation results for constituents exhibiting step trends in this report.

[The five-digit United States Geological Survey (USGS) parameter code is used to uniquely identify a specific constituent or property. 

Abbreviations: C, carbon; NSF, North San Francisco Bay; O, oxygen; ▲, increase; ▼, decrease; —, no step trend. Double entry divided by a forward slash (/) 

indicates a change in the step-trend finding before (in front of slash) and after (under slash) additional quality-control interpretation; δ, standard delta notationthe ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material]

Constituent

USGS 

parameter 

code

No difference 

threshold

Replicate criteria 

threshold

Confidence intervals 

threshold

Combined 

study 

units

NSF 

study 

unit

SALMON 

study 

unit

Combined 

study 

units

NSF 

study 

unit

SALMON 

study 

unit

Combined 

study 

units

NSF 

study 

unit

SALMON 

study 

unit

Water temperature 00010 ▼ — ▼ — — — No results for field parameters

Dissolved oxygen 00300 — ▲ — — — — No results for field parameters

Field specific conductance 00095 ▲ ▲ — ▲ — — No results for field parameters

Lab specific conductance2 90095 ▲ / — ▲ / — — ▲ / — ▲ / — — ▲ / — ▲ / — —

Field pH 00400 ▼ — ▼ — — — No results for field parameters

Lab pH 00403 — — ▲ — — — — — ▲

Carbonate 63788 ▼ — ▼ ▼ — ▼ — — —

Total nitrogen 62854 — — — — — — — — ▲

Nitrate1 00631 — — ▲ — — ▲ — — ▲

Orthophosphate 00671 ▲ — ▲ ▲ — ▲ ▲ — ▲

Calcium1 00915 ▲ ▲ ▲ — / ▲ — ▲ ▲ ▲ ▲

Magnesium1 00925 ▲ / — ▲ / — ▲ / — ▲ / — ▲ / — — ▲ / — ▲ / — ▲ / —

Potassium1 00935 — — / ▼ — ▲ / — — — — — / ▼ —

Sodium1 00930 ▲ / — — / ▼ — — — — ▲ / — — / ▼ —

Chloride 00940 — ▲ — — — — — ▲ —

Iodide 71865 — — — — — — — ▼ —

Sulfate 00945 ▲ — — ▲ ▲ — ▲ — —

Fluoride1 00950 ▼ ▼ — / ▼ — — — ▼ ▼ — / ▼

Silica2 00955 ▼ / — ▼ / — ▼ / — — — — ▼ / — ▼ / — ▼ / —

Aluminum1,2 01106 ▲ ▲ — ▲ ▲ — ▲ ▲ —

Arsenic 01000 ▼ ▼ — — — — ▼ ▼ —

Barium1 01005 ▲ / — ▲ / — — — — — ▲ / — ▲ / — —

Iron2 01046 ▲ — ▲ ▲ — ▲ ▲ — ▲

Lead 01049 — — — ▼ — — — — —

Lithium 01130 ▲ ▲ — — — — ▲ ▲ —

Manganese1 01056 ▲ / — — ▲ / — — — ▲ / — ▲ / — — ▲ / —

Nickel1 01065 — ▲ / — — — — — — ▲ / — —

Selenium1 01145 — ▼ — — ▼ — — ▼ —

Strontium1 01080 — — — ▲ — — — — —

Vanadium 01085 — — — — ▼ — — — —

δ18O of water 82085 ▲ ▲ ▲ — — — ▲ ▲ ▲

δ13C of dissolved 

carbonates

82081 — — ▼ — — — — — ▼

Tritium 07000 ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼

1

Data-quality assessment summaries published by the QSB found measurement bias or variability at the NWQL for this constituent during one or both 

sampling periods.

2

Project field-blank results indicate a potential positive bias in initial period concentrations for this constituent.
Table 3. Constituents for which one of three circumstances indicate step-trend results are inconclusive.[CaCO3, calcium carbonate; HAL-US, Enviromental Protection Agency lifetime health-advisory level; MCL-CA, California maximum contaminant level; mg/L, milligram per l

notification level; NSF, North San Francisco Bay; SALMON, Monterey Bay and Salinas Valley Basins; USGS, U.S. Geological Survey; μg/L, microgram per liter; =, equal to; >

Constituent Step-trend findings before data adjustment Bias period Bias direction 

Circumstance 1: Analytical bias documented by the USGS Quality Systems Branch (QSB), which may have contributed to step-trend f

After adjusting the data for the documented bias the step-trend findings for these constituents were no longer significant.

Magnesium Increase in both study units and combined Initial Negative 

Magnesium Increase in both study units and combined Decadal Positive 

Potassium Increase in combined study units Initial Positive 

Potassium Increase in combined study units Decadal Positive 

Sodium Increase in combined study units Decadal Positive 

Silica Decrease in both study units and combined Initial Positive 

Barium Increase in NSF study unit and combined Initial Negative 

Manganese Increase in SALMON study unit and combined Initial Negative 

Nickel Increase in NSF study unit Decadal Positive 

Constituent Step-trend finding Correlated constituent Step-trend finding 

Circumstance 2: The step-trend finding is not consistent with the evaluated results of a different, generally correlated water-quality con

Field-measured specific conductance Increase in NSF study unit and combined Total dissolved solids None

Laboratory-measured specific 

conductance

Increase in NSF study unit and combined Total dissolved solids None

Field-measured pH Decrease in SALMON study unit and combined Laboratory-measured pH Increase in SALMO

Laboratory-measured pH Increase in SALMON study unit Field-measured pH Decrease in SALMO

Isotopic ratio of oxygen-18 in water 

(δ18O)

Increase in both study units and combined Isotopic ratio of deuterium in water (δ2H) None

Constituent Step-trend finding Maximum concentration Benchmark fo

Circumstance 3: Constituent concentrations observed in this study are negligible relative to their potential effect on water quality

Carbonate Decrease in SALMON study unit and combined 3.3 mg/L Alkalinity range=38–391 m

Iodide Decrease in NSF study unit 0.26 mg/L >1 mg/L (World Health Or

Fluoride Decrease in NSF study unit and combined 0.5 mg/L MCL-CA=2 mg/L

Aluminum Increase in NSF study unit and combined 7 μg/L MCL-CA=1,000 μg/L

Selenium Decrease in NSF study unit 15.6 μg/L MCL-US=50 μg/L

Strontium Increase in combined study units 2,214 μg/L HAL-US=4,000 μg/L

Vanadium Decrease in NSF study unit 18 μg/L (in NSF study unit) NL-CA=50 μg/L

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Sacramento Publishing Service Center

For more information concerning the research in this report, contact the

 Director, California Water Science Center

 U.S. Geological Survey

 6000 J Street, Placer Hall

 Sacramento, CA 95819

 https://ca.water.usgs.gov

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185088
Kent—Variations on a Method for Evaluating Decadal-Scale Changes in the Groundwater Quality of Two GAMA Coastal Study Units 2004–14, CA GAMA Priority Basin Project—SIR 2018–5088