U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5034

A product of the California Groundwater Ambient Monitoring and Assessment Program 

Prepared in cooperation with the California State Water Resources Control Board

Updated Study Reporting Levels (SRLs) for Trace-Element 

Data Collected for the California Groundwater Ambient 

Monitoring and Assessment (GAMA) Program Priority Basin 

Project, October 2009–October 2018
Cover: A GAMA field crew member collecting a field blank, photograph taken by George L. Bennett V, July 2019.
Updated Study Reporting Levels (SRLs) 

for Trace-Element Data Collected for 

the California Groundwater Ambient 

Monitoring and Assessment (GAMA) 

Program Priority Basin Project, 

October 2009–October 2018

By George L. Bennett V

A product of the California Groundwater Ambient Monitoring and Assessment Program

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2020–5034

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Bennett, G.L. V, 2020, Updated study reporting levels (SRLs) for trace-element data collected for the 

California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project, 

October 2009–October 2018: U.S. Geological Survey Scientific Investigations Report 2020–5034, 24 p., 

https://doi.org/ 10.3133/ sir20205034.

Associated data for this publication: Bennett, G.L., V, 2020, Blank results for trace-element data collected for 

the California Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project, October 2009–

October 2018: U.S. Geological Survey data release, https://doi.org/ 10.5066/ P9TRFPUO.

ISSN 2328-0328 (online)
iii

Contents

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

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

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

Methods...........................................................................................................................................................5

Methods for Collecting Samples for Trace Element and Hexavalent Chromium Analysis .......5

Laboratory Quality Control and Certification ....................................................................................5

Data Analysis Methods for Evaluating Bias .....................................................................................5

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

Trace Elements Detected in Fewer Than 5 Percent of Field Blanks ..........................................13

Aluminum (Al) .............................................................................................................................13

Antimony (Sb) .............................................................................................................................13

Arsenic (As) ................................................................................................................................13

Barium (Ba) .................................................................................................................................13

Beryllium (Be) .............................................................................................................................15

Boron (B) .....................................................................................................................................15

Cadmium (Cd) .............................................................................................................................15

Chromium (Cr) .............................................................................................................................15

Iron (Fe)........................................................................................................................................15

Lithium (Li) ...................................................................................................................................15

Manganese (Mn) .......................................................................................................................15

Molybdenum (Mo) .....................................................................................................................16

Nickel (Ni)....................................................................................................................................17

Selenium (Se) .............................................................................................................................17

Silver (Ag) ....................................................................................................................................17

Strontium (Sr) .............................................................................................................................17

Thallium (Tl).................................................................................................................................17

Uranium (U) .................................................................................................................................17

Vanadium (V)...............................................................................................................................17

Trace Elements Detected in Greater Than 5 Percent of Field Blanks ........................................18

Hexavalent Chromium (Cr[VI]) .................................................................................................18

Cobalt (Co) ...................................................................................................................................19

Copper (Cu) .................................................................................................................................19

Lead (Pb)......................................................................................................................................20

Zinc (Zn) .......................................................................................................................................21

Summary........................................................................................................................................................22

Acknowledgments.......................................................................................................................................24

References Cited..........................................................................................................................................24
iv

Figures

1. Map showing hydrologic provinces, locations of wells sampled for the 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

Aquifer Assessments 2004–18, and sample locations of trace-element field 

blanks between October 2009 and September 2013 ...............................................................3

2. Map showing hydrologic provinces, locations of wells sampled for the 

Groundwater Ambient Monitoring and Assessment Priority Basin Project 

Aquifer Assessments 2004–18, and sample locations of trace-element field 

blanks between October 2013 and October 2018 ....................................................................4

3. Graphs showing trace-element concentrations in field, source-solution, and 

equipment blanks and study reporting levels, for constituents detected in 

fewer than 5 percent of the field blanks collected from October 2013 through 

October 2018, Groundwater Ambient Monitoring and Assessment (GAMA) 

Priority Basin Project ...................................................................................................................6

4. Graph showing chromium (VI) concentrations in field, source-solution, and 

equipment blanks collected September 2015 through October 2018 conducted 

and analyzed at Weck Laboratories, Inc., total chromium concentrations in 

blank water tests conducted at the National Water Quality Laboratory plotted 

in time series and study reporting level, Groundwater Ambient Monitoring and 

Assessment Priority Basin Project ..........................................................................................10

5. Graph showing cobalt concentrations in field, source-solution, and equipment 

blanks collected October 2009 through October 2018 plotted in time series, 

study reporting levels, and cobalt concentrations in groundwater samples 

collected between October 2013 and October 2018, Groundwater Ambient 

Monitoring and Assessment Priority Basin Project ..............................................................12

6. Graph showing copper concentrations in field, source-solution, and equipment 

blanks collected October 2009 through October 2018 plotted in time series and 

study reporting levels, Groundwater Ambient Monitoring and Assessment 

Priority Basin Project .................................................................................................................20

7. Graph showing lead concentrations in field, source-solution, and equipment 

blanks collected October 2009 through October 2018 plotted in time series and 

study reporting levels, Groundwater Ambient Monitoring and Assessment 

Priority Basin Project .................................................................................................................21

8. Graph showing zinc concentrations in field, source-solution, and equipment 

blanks collected October 2009 through October 2018 plotted in time series and 

study reporting levels, Groundwater Ambient Monitoring and Assessment 

Priority Basin Project .................................................................................................................22
v

Tables

1. Detection frequencies for trace elements in field blanks collected for the 

U.S. Geological Survey Groundwater Ambient Monitoring and Assessment 

Priority Basin Project between October 2009 and March 2013, and between 

October 2013 and October 2018 ................................................................................................11

2. Parameter codes, analytical methods, benchmark type and value, range of 

detection limits, and study reporting levels (SRLs) for trace element data 

collected for the U.S Geological Survey Groundwater Ambient Monitoring and 

Assessment Priority Basin Project, May 2004–September 2009, October 2009–

September 2013, and updated SRL starting October 2013 ...................................................14

3. Concentrations of cobalt and manganese in high-capacity filters tested by the 

U.S. Geological Survey National Water Quality Laboratory (NWQL) for use in 

the collection of trace-element blanks—December 2012–March 2019 ............................16

4. Concentrations of chromium in blank water tested by the U.S. Geological 

Survey National Water Quality Laboratory (NWQL) for use in the collection of 

trace-element blanks—September 2015–May 2018 .............................................................18

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Volume

liter (L) 33.81402 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

Mass

microgram (μg) 3.527e−8 ounce, avoirdupois (oz)

Supplemental Information

Concentrations of chemical constituents in water are given in micrograms per liter (μg/L).
vi

Abbreviations

AL-US U.S. Environmental Protenction Agency Action Level

BD-90/90 90th percentile concentrations calculated using the binomial probability distribution 

for greater than 90 percent confidence

GAMA Groundwater Ambient Monitoring and Assessment Program

IBW inorganic blank water

LEP laboratory evaluation process

NAWQA National Water-Quality Assessment Program (USGS)

NWQL National Water Quality Lab

OWQ Office of Water Quality

PBP Priority Basin Project

QA quality-assurance

QC quality-control

QSB Quality Systems Branch

SRL study reporting level

SWRCB California State Water Resources Control Board

USGS U.S. Geological Survey
Updated Study Reporting Levels (SRLs) for Trace-Element 

Data Collected for the California Groundwater Ambient 

Monitoring and Assessment (GAMA) Program Priority 

Basin Project, October 2009–October 2018

By George L. Bennett V

Abstract

Groundwater samples have been collected in California 

as part of statewide investigations of groundwater quality 

conducted by the U.S. Geological Survey for the Groundwater 

Ambient Monitoring and Assessment (GAMA) Priority 

Basin Project (PBP) since 2004. The GAMA-PBP is being 

conducted in cooperation with the California State Water 

Resources Control Board to assess and monitor the quality 

of groundwater resources used for public and domestic 

drinking-water supply and to improve public knowledge 

of groundwater quality in California. Quality-control 

samples (including but not limited to field, equipment, 

and source-solution blanks) were collected to evaluate and 

quantify the quality of the groundwater sample results.

The GAMA-PBP previously determined study reporting 

levels (SRLs) for trace-element results based primarily on 

field blanks collected in California from May 2004 through 

March 2013. SRLs are raised reporting levels used to reduce 

the likelihood of reporting false detections attributable to 

contamination bias. The purpose of this report is to identify 

any changes in the pattern or magnitude of concentrations or 

detections in field blanks since the last evaluation that would 

require changing or ending the use of SRLs implemented 

in October 2009. Constituents analyzed were aluminum, 

antimony, arsenic, barium, beryllium, boron, cadmium, 

chromium, hexavalent chromium, cobalt, copper, iron, lead, 

lithium, manganese, molybdenum, nickel, selenium, silver, 

strontium, thallium, uranium, vanadium, and zinc.

For this review, data from 167 field blanks collected 

from October 2009 through October 2018 by the GAMA-PBP 

for trace elements were compiled. Based on a consistent 

pattern of decreasing cobalt and manganese concentrations in 

field blanks from 2009 to 2013, the GAMA-PBP decided to 

reevaluate all trace-element SRLs, effectively setting an end 

date for previously defined SRLs. Beginning October 2013, 

SRLs would be determined from field-blank data collected 

through October 2018. The detection frequency and upper 

limit of potential contamination bias (BD-90/90) were 

determined from field blanks for each trace element. The 

BD-90/90, that is, the upper 90-percent confidence limit 

of the 90th percentile concentration of potential extrinsic 

contamination, was calculated by assuming the binomial 

probability distribution. These results were compared to each 

constituent’s detection limit to determine whether an SRL 

was necessary to minimize the potential for detections in the 

groundwater samples, attributed principally to contamination 

bias. Results of the evaluation were used to set SRLs for 

trace-element data collected by the GAMA-PBP between 

October 2013 and October 2018. Trace elements prescribed 

an SRL based on this review were hexavalent chromium, 

cobalt, copper, lead, and zinc. This review also resulted in 

the removal of SRLs from iron, manganese, molybdenum, 

and nickel. Although an SRL for hexavalent chromium could 

not be evaluated in the earlier reviews because the data were 

not collected regularly until 2015, one was established herein 

as 0.34 micrograms per liter (μg/L). The SRL for cobalt, 

as previously implemented, had been to reject all results; it 

was changed to 0.16 μg/L following a reduction in cobalt 

field-blank detection frequency resulting from mitigation 

steps, starting in 2014, aimed at reducing contamination bias 

introduced by high-capacity capsule filters used during sample 

collection. The SRL for copper did not change, and the SRL 

for lead changed very little based on this review. Lastly, the 

SRL for zinc was lowered from 6.2 μg/L to 3.9 μg/L.

Introduction

To assess the quality of ambient groundwater in aquifers 

used for public and domestic drinking-water supply and 

to establish a baseline groundwater-quality monitoring 

program on a statewide scale, the California State Water 

Resources Control Board (SWRCB), in cooperation with 

the U.S. Geological Survey (USGS), implemented the 

Groundwater Ambient Monitoring and Assessment (GAMA) 

Priority Basin Project (PBP).
2 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

The GAMA-PBP follows a rigorous quality-assurance 

(QA) plan based on modified versions of protocols 

defined by the USGS National Water-Quality Assessment 

(NAWQA) Program (Koterba and others, 1995) and the 

National Field Manual for the Collection of Water-Quality 

Data (U.S. Geological Survey, variously dated). Specific 

modifications are detailed in Bennett and Fram (2014) and 

Shelton and Fram (2017). Quality-control (QC) samples were 

collected at about 10 percent of the wells to evaluate potential 

contamination, as well as bias and variability of the data 

that may have resulted from sample collection, processing, 

storage, transportation, and laboratory analysis. The QC 

samples included field, source-solution, and equipment blanks, 

replicates, and matrix spikes. This report focuses on results 

from field, source-solution, and equipment blanks. A field 

blank is a QC sample collected in the field in the same manner 

as the environmental sample that exposes it to all aspects of 

sample collection while using bottled water certified to be free 

of the target analytes. A source-solution blank is collected in 

the field using a freshly opened bottle of blank water (source 

solution) that is poured directly into the sample bottles and 

sent to the lab to confirm the source solution is free of the 

target analytes. An equipment blank is a sample collected in 

a clean environment using just the portable pump used in the 

field to collect field blanks. This allows for the assessment 

of contamination of samples by the portable pump and the 

effectiveness of the pump cleaning procedures.

Groundwater samples collected for the GAMA-PBP 

were analyzed for a suite of constituents, including trace 

elements. Trace elements occur naturally and help define 

water characteristics and quality. Trace-element occurrence 

and concentration can also be affected by anthropogenic 

influences. The primary laboratory used to analyze most 

of the samples is the USGS National Water Quality 

Laboratory (NWQL) in Denver, Colorado. Since 2015, Weck 

Laboratories, Inc. (hereinafter referred to as Weck), in City 

of Industry, California, was contracted to analyze samples 

for hexavalent chromium (Cr[VI]) for the GAMA-PBP. 

Between October 2009 and September 2013, 90 trace-element 

field blanks were collected by the GAMA-PBP (fig. 1). 

Trace-element field blanks were also collected from 77 sites 

between October 2013 and October 2018 (fig. 2). Of those 

77 sites, 51 had Cr(VI) field blanks collected at them.

This QC evaluation examines blanks collected for trace 

element and Cr(VI) analysis and is focused on controlling 

the risk of reporting false positives. The GAMA-PBP uses 

laboratories that have very-low method detection limits (parts 

per billion), and previous QC evaluations of field blanks by the 

GAMA-PBP identified low-level contamination of select trace 

elements and established study unit reporting levels (SRLs) for 

those constituents (Olsen and others, 2010; Davis and others, 

2014). This report, like Olsen and others (2010) and Davis 

and others (2014), establishes SRLs in order to reduce false 

positives. Established SRLs are far less than any regulatory or 

non-regulatory benchmark for drinking water and have little 

effect on the interpretation of trace-element results where the 

focus is generally on detections with concentrations greater 

than one-half of the benchmark concentrations used to provide 

context to the results.

Purpose and Scope

This report describes the evaluation of data derived from 

167 trace-element field blanks and 19 source-solution blanks 

collected along with groundwater samples from October 2009 

through October 2018 by the GAMA-PBP. Data from 51 field 

and 7 source-solution blanks for Cr(VI) were also included 

in the evaluation. Additionally, data from 8 blanks collected 

to test the portable pumps used to collect blanks in the field 

(equipment blanks) were also compiled and analyzed. Only 5 

of the equipment blanks included analyses of Cr(VI). Using 

a simplified version of the methodology developed and 

detailed in Olsen and others (2010) and revisited in Davis and 

others (2014), this evaluation determined when trace-element 

contamination bias in field blanks changed and updates the 

SRLs most recently defined by Davis and others (2014) based 

on the available blank results. The updated SRLs were set to 

minimize potential contamination bias in groundwater-quality 

results while maintaining consistent and accurate reporting 

levels for comparison and interpretation of data over extended 

periods of time. Trace-element field blanks collected by the 

GAMA-PBP during the previous evaluation of Davis and 

others (2014) were included as a basis of comparison for more 

recent field blanks presented in this report.
Purpose and Scope 3

PACIFIC OCEAN

ARIZONA

MEXICO

CALIFORNIA

Northern

Coast

Ranges

Cascade and

Modoc Plateau

Sierra Nevada

Basin and Range

Desert

San Diego

Drainages

Southern

Coast

Ranges

Transverse Ranges and

selected Peninsular Ranges

Central Valley

124° 122° 120° 118° 116° 114°

42°

40°

38°

36°

34°

32°

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

NEVADA

OREGON IDAHO

Klamath

Mountains

UTAH

0

0 50 100 150 200 KILOMETERS

50 100 150 200 MILES

EXPLANATION

Sample location with a field blank 

collected between October 2009 

and September 2013

GAMA-PBP well sample location

Provinces from Belitz and others, 2003

Figure 1. Hydrologic provinces, locations of wells sampled for the Groundwater Ambient Monitoring and Assessment 

Priority Basin Project (GAMA-PBP) Aquifer Assessments 2004–18, and sample locations of trace-element field blanks 

between October 2009 and September 2013.
4 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

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

Provinces from Belitz and others, 2003

124° 122° 120° 118° 116° 114°

42°

40°

38°

36°

34°

32°

Klamath

Mountains

NEVADA

OREGON IDAHO

ARIZONA

CALIFORNIA

MEXICO

Central Valley

Northern

Coast

Ranges

Southern

Coast

Ranges

Transverse Ranges and

selected Peninsular Ranges

San Diego

Drainages

Desert

Cascade and

Modoc Plateau

Sierra Nevada

Basin and Range

PACIFIC OCEAN

UTAH

0

0 50 100 150 200 KILOMETERS

50 100 150 200 MILES

EXPLANATION

Sample location with a field blank 

collected between October 2013 

and October 2018

GAMA-PBP well sample location

Figure 2. Hydrologic provinces, locations of wells sampled for the Groundwater Ambient Monitoring and Assessment 

Priority Basin Project (GAMA-PBP) Aquifer Assessments 2004–18, and sample locations of trace-element field blanks 

between October 2013 and October 2018.
Methods 5

Methods

This investigation used specific methods to collect 

groundwater and associated QC samples, analyze these 

samples and report the results, compile the field QC data, 

and evaluate the combined data to characterize any potential 

bias that could affect the groundwater sample results. These 

methods are described below.

Methods for Collecting Samples for Trace 

Element and Hexavalent Chromium Analysis

Environmental sample collection for trace-element 

analysis (NWQL) and Cr(VI) analysis (Weck) followed 

modified protocols developed by the NAWQA Program 

(Koterba and others, 1995) and the USGS National Field 

Manual (U.S. Geological Survey, variously dated; Bennett 

and Fram, 2014; Shelton and Fram, 2017). The protocols 

were developed to ensure that the samples are representative 

of the aquifer being sampled and that they are handled 

consistently from site to site thereby minimizing the potential 

for contamination. A thorough description of the entire process 

is provided in Davis and others (2014).

Field blanks were collected at about 10 percent of 

GAMA-PBP sites in order to determine if contamination of the 

samples is occurring that could be attributed to the equipment 

or procedures used in the field or at the laboratory. Field 

blanks for trace elements and Cr(VI) were collected using a 

portable diaphragm pump. Inorganic blank water (IBW) was 

pumped through the fittings of the pump, spigot, and sample 

line. Certified IBW was obtained from the NWQL. Teflon 

tubing was used at the pump inlet and outlet to connect to the 

sample-collection equipment. Field blanks were collected at 

well or spring sites by pumping at least 4 to 8 liters (L) of IBW 

through the clean sample-collection equipment to simulate 

the well-purging step before collecting the sample. Following 

this purge, IBW was pumped through the equipment to be 

collected as blank samples, following the same protocols as 

were used for the groundwater samples, including filtration 

and preservation. For this assessment and that of Davis 

and others (2014), it was assumed that the portable pump 

equipment used to pump IBW through the sample equipment 

was not a significant source of contamination when cleaned 

regularly. Eight blanks were collected to test this assumption; 

for these, the IBW contacted only the pump equipment 

(equipment blanks).

Source-solution blanks were collected whenever a new 

lot of IBW was shipped from the NWQL to verify that the 

IBW used for the field blanks was free of analytes of interest. 

Source-solution blanks were collected by pouring IBW 

directly into sample containers that were then preserved, 

stored, and shipped in the same manner as the groundwater 

samples. Source-solution blanks were used to supplement 

analyses that had been performed at the NWQL in certifying 

the quality of the IBW.

Laboratory Quality Control and Certification

The NWQL (D.L. Stevenson, U.S. Geological Survey, 

written commun., 2013) and Weck follow thorough QA plans, 

and both labs conduct laboratory QC, including laboratory 

method blanks, continuing calibration verification checks, 

reagent spikes, certified standard reference materials, and 

external blind proficiency samples. The NWQL maintains 

certification by the National Environmental Laboratory 

Accreditation Program (NELAP) and other certifications 

(U.S. Geological Survey, 2019a), and Weck maintains 

certification with the California Department of Public Health, 

Environmental Laboratory Accreditation Program (ELAP), 

and other certifications (Weck Laboratories, Inc., 2019).

The Quality Systems Branch (QSB) within the USGS 

Water Mission Area (formerly the Branch of Quality 

Systems [BQS] within the Office of Water Quality) maintains 

independent oversight of QA at the NWQL and coordinates 

blind testing of blanks and reference samples through the 

Inorganic Blind Sample Project (U.S. Geological Survey, 

2019b). The GAMA-PBP prepares an annual Laboratory 

Evaluation Process (LEP) for USGS and non-USGS 

laboratories including Weck. The LEPs present a compilation 

and review of the in-house QC results provided by the 

laboratory. LEPs are designed to ensure projects are 

continuously working with laboratories to ensure data-quality 

objectives defined by each project are being met.

Data Analysis Methods for Evaluating Bias

Four tools were used to evaluate the trace-element 

field-blank data: (1) time-series plots, (2) detection 

frequencies, (3) 90th percentile concentrations calculated 

by using the binomial probability distribution for greater 

than 90 percent confidence, referred to as the “BD-90/90 

concentration” by Olsen and others (2010) and Davis 

and others (2014), and (4) the Wilcoxon rank-sum test, a 

non-parametric statistical method used to compare two 

independent groups of data to determine if there are significant 

differences between the groups (Helsel and Hirsch, 2002). 

These tools were used to identify any changes in the frequency 

and concentrations of detections in field blanks so that SRLs 

could be revised, if necessary.

Time-series plots facilitate visual identification of trends 

or temporal components in the data, which might correspond 

to changes in the conditions under which the field blanks or 

groundwater samples were collected. Time-series plots of 

blank results were prepared for all constituents (figs. 3–8). 
6 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018 Aluminum concentration, in micrograms per liter Antimony concentration, in micrograms per liter Arsenic concentration, in micrograms per liter Barium concentration, in micrograms per liter Beryllium concentration, in micrograms per liter

4

3.5

3

2.5

2

1.5

1

0.5

0

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

1.2

1

0.8

0.6

0.4

0.2

0

07/16/09

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

Collection date

07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

No SRL needed

Previous review period Current review period

Previous review period Current review period

Not shown is one detection of aluminium in a field 

blank with a concentration of 30 μg/L on 8/22/2012.

No study reporting levels (SRL) needed

A

No SRL needed

Previous review period Current review period

B

C

Previous review period Current review period

No SRL needed

No SRL needed

Previous review period Current review period

D

E

EXPLANATION

Aluminum non-detections (n = 165)

Aluminum field-blank detections (n = 2)

EXPLANATION

Antimony non-detections (n = 163)

Antimony field-blank detections (n = 4)

EXPLANATION

Arsenic non-detections (n = 165)

Arsenic source-solution detections (n = 1)

Arsenic field-blank detections (n = 2)

EXPLANATION

Barium non-detections (n = 163)

Barium source-solution detections (n = 2)

Barium equipment-blank detections (n = 2)

Barium field-blank detections (n = 4)

EXPLANATION

Beryllium non-detections (n = 166)

Beryllium field-blank detections (n = 1)

Figure 3. Trace-element concentrations in field, source-solution, and equipment blanks (when detected) and study reporting 

levels (SRLs) for constituents detected in fewer than 5 percent of the field blanks collected from October 2013 through October 2018, 

Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project: A, aluminum; B, antimony; C, arsenic; D, barium; 

E, beryllium; F, boron; G, cadmium; H, chromium; I, iron; J, lithium; K, manganese; L, molybdenum; M, nickel; N, selenium; O, silver; 

P, strontium; Q, thallium; R, uranium; and S, vanadium. [μg/L, microgram per liter]
Methods 7

No SRL needed

No SRL needed

No SRL needed

No SRL needed

35

30

25

20

15

10

5

0

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

0.6

0.5

0.4

0.3

0.2

0.1

0

14

12

10

8

6

4

2

0

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

Boron concentration,

in micrograms per liter

Cadmium concentration,

in micrograms per liter

Chromium concentration,

in micrograms per liter

Iron concentration,

in micrograms per liter

Lithium concentration,

in micrograms per liter

Collection date

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

F

G

Previous review period Current review period

Previous review period Current review period

H

I

J

Previous review period Current review period

SRL=6 μg/L 

Previous review period Current review period

Previous review period Current review period

EXPLANATION

Boron non-detections (n = 162)

Boron field-blank detections (n = 5)

EXPLANATION

Cadmium non-detections (n = 167)

Cadmium equipment-blank detections (n = 1)

EXPLANATION

Lithium non-detections (n = 167)

EXPLANATION

Chromium non-detections (n = 166)

Chromium equipment-blank detections (n = 1)

Chromium source-solution detections (n = 1)

Chromium field-blank detections (n = 1)

No SRL needed

EXPLANATION

Iron non-detections (n = 161)

Study reporting levels (SRL)

Iron field-blank detections (n = 6)

Figure 3. —Continued
8 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

1.2

1

0.8

0.6

0.4

0.2

0

Manganese concentration,

in micrograms per liter

Molybdenum concentration,

in micrograms per liter

Nickel concentration,

in micrograms per liter

0.06

0.05

0.04

0.03

0.02

0.01

0

1

0.1

0.01

0.001

Selenium concentration,

in micrograms per liter

Silver concentration,

in micrograms per liter

Collection date

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

K

Previous review period Current review period

L

Previous review period Current review period

M

Previous review period Current review period

N

Previous review period Current review period

O

Previous review period Current review period

SRL = 0.66 μg/L

SRL = 0.023 μg/L

SRL = 0.21 μg/L

No SRL needed

No SRL needed

No SRL needed

No SRL needed

No SRL needed

EXPLANATION

Selenium non-detections (n = 166)

Selenium field-blank detections (n = 1)

EXPLANATION

Silver non-detections (n = 163)

Silver field-blank detections (n = 4)

EXPLANATION

Manganese non-detections (n = 100)

Manganese equipment-blank detections (n = 2)

Manganese field-blank detections (n = 67)

Study reporting levels (SRL)

EXPLANATION

Molybdenum non-detections (n = 160)

Molybdenum field-blank detections (n = 7)

SRL

EXPLANATION

Nickel non-detections (n = 160)

Nickel source-solution detections (n = 1)

Nickel equipment-blank detections (n = 1)

Nickel field-blank detections (n = 7)

SRL

Figure 3. —Continued
Methods 9

1.4

1.2

1

0.8

0.6

0.4

0.2

0

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Strontium concentration,

in micrograms per liter

Thallium concentration,

in micrograms per liter

Uranium concentration,

in micrograms per liter

Vanadium concentration,

in micrograms per liter

Collection date

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

P

Q

R

S

Previous review period Current review period

Previous review period Current review period

Previous review period Current review period

Previous review period Current review period

No study reporting levels (SRL) needed

No SRL needed

No SRL needed

No SRL needed

EXPLANATION

Vanadium non-detections (n = 164)

Vanadium source-solution detections (n = 1)

Vanadium equipment-blank detections (n = 1)

Vanadium field-blank detections (n = 3)

EXPLANATION

Thallium non-detections (n = 166)

Thallium field-blank detections (n = 1)

EXPLANATION

Uranium non-detections (n = 165)

Uranium field-blank detections (n = 2)

EXPLANATION

Strontium non-detections (n = 163)

Strontium source-solution detections (n = 1)

Strontium equipment-blank detections (n = 1)

Strontium field-blank detections (n = 4)

Figure 3. —Continued
10 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Non-detections were plotted at their respective detection limit 

concentrations by using a different symbol than was used for 

the detections. Detections in source-solution and equipment 

blanks were plotted to aid in identifying instances of potential 

contamination in the source solution (IBW) used for the 

field blanks. Blanks collected between October 2009 and 

October 2018 were included in the time series plots, which 

cover the entirety of the Davis and others (2014) review period 

and the current SRL review periods. The collection of Cr(VI) 

samples didn’t begin until September 2015, therefore the 

time-series available to be plotted is shorter than other trace 

elements (fig. 4).

Detection frequencies in field blanks, calculated for each 

trace element by dividing the number of detections by the total 

number of field blanks, are an easy way to identify potential 

contamination bias. Trace-element detection frequencies in 

field blanks from Davis and others, 2014, and from the current 

review are shown in table 1. For the time period used to 

evaluate recent contamination bias (hereinafter referred to as 

the “current review period”) and update SRLs—October 2013 

through October 2018—77 field blanks were analyzed for each 

of the trace elements, except for Cr(VI), which had 51. For 77 

ranked values, the 4th highest value statistically defines the 

BD-90/90 concentration. Thus, the detection frequency in a 

population of 77 field blanks must be less than 5.1 percent (no 

more than 3 detections) for the BD-90/90 concentration to be 

a non-detection. Therefore, a detection frequency of 5 percent 

was used as a threshold for identifying trace elements 

(including Cr[VI]) requiring additional scrutiny and possible 

establishment or modification of an SRL.

BD-90/90 concentrations were calculated by using the 

binomial probability distribution from the method reported 

by Hahn and Meeker (1991). This approach was used to 

determine the upper 90-percent confidence limit for the 

90th percentile of potential extrinsic contamination for 

each trace element. Calculations were made by using the 

BINOMDIST function in Excel for Office 365 (Microsoft 

Corporation, Redmond, Washington), which takes the form

CL BINOMDIST number s trials

probability s cumulative

= ( _ , ,

_ , )

 

(1)

where

CL is the confidence limit for the specified rank 

and for the percentile of interest;

number_s is the number of successes in trials, in this 

case, the specified rank minus 1;

trials is the number of trials, in this case, the 

number of field blanks;

probability_s the probability of success on each trial, in this 

case, 0.90 for the 90th percentile; and

cumulative is a logical value that determines the form 

of the function, in this case TRUE, such 

that BINOMDIST returns the cumulative 

distribution function, which assumes 

that there are at most (cumulatively) 

number_s successes.

SRL = 0.34 μg/L

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

EXPLANATION

Chromium (VI) detections in source-solution blanks (n = 4)

Study reporting levels (SRL)

Chromium (total) concentrations in blank water (n = 32)

Chromium (VI) non-detections (n = 18) Chromium (VI) field-blank detections (n = 33)

Chromium (VI) equipment-blank detections (n = 4)

Chromium (VI) concentration, in micrograms per liter

12/05/14 06/05/15 12/05/15 06/04/16 12/04/16 06/04/17 12/04/17 06/04/18 12/04/18

Collection date

Figure 4. Chromium (VI) concentrations in field, source-solution, and equipment blanks collected September 2015 through 

October 2018 and analyzed at Weck Laboratories, Inc., total chromium concentrations in blank water tests conducted at the National 

Water Quality Laboratory plotted in time series, and study reporting level (SRL), Groundwater Ambient Monitoring and Assessment 

(GAMA) Priority Basin Project.
Methods 11

Table 1. Detection frequencies for trace elements in field blanks 

collected for the U.S. Geological Survey (USGS) Groundwater 

Ambient Monitoring and Assessment (GAMA) Priority Basin 

Project between October 2009 and March 2013, and between 

October 2013 and October 2018.

[<, less than; >, greater than; na, not applicable]

Constituent

Detection frequency in field blanks

(percent)

October 2009–

March 2013

(n = 69)1

October 2013–

October 2018

(n = 77)

Detected in <5 percent of field blanks (2013–18)

Aluminium 1.4 1.3

Antimony 1.4 3.9

Arsenic 1.4 0

Barium 2.9 2.6

Beryllium 0 1.3

Boron na2 0

Cadmium 0 0

Chromium 1.4 0

Iron 7.2 0

Lithium 0 0

Manganese 73 2.6

Molybdenum 8.7 0

Nickel 5.8 1.3

Selenium 0 1.3

Silver 1.4 2.6

Strontium 0 2.6

Thallium 0 0

Uranium 0 2.6

Vanadium 0 3.9

Detected in >5 percent of field blanks (2013–18)

Hexavalent chromium na 365

Cobalt 99 42

Copper 36 44

Lead 57 30

Zinc 36 32

1As reported in Davis and others (2014) for samples collected using the 

short sampling lines.

2Detections of boron in field blanks during this review period were 

attributed to contaminated blank water.

3Detection frequency calculated from 51 field blanks with hexavalent 

chromium analysis from Weck Laboratories, Inc., during the current 

review period.

The BD-90/90 concentrations represent the upper 

threshold of potential contamination bias of trace-element 

results. For trace elements that had BD-90/90 concentrations 

above their respective detection limit, the chance that 

a groundwater concentration less than the respective 

BD-90/90 concentration was significantly affected by 

extrinsic contamination was greater than 10 percent. For 

these trace elements, the likely source(s) of contamination 

was determined, and an SRL was set equal to the BD-90/90 

concentration. A statistically robust BD-90/90 is calculated 

with enough data points to reduce sensitivity to outliers. 

Having from 52 to 64 samples results in the 3rd highest value 

in the dataset being set as the BD-90/90, while for 65–77 it 

is the 4th highest value, for 78–99 it is the 5th highest value, 

and for 100–102 it is the 6th highest value. The GAMA-PBP 

considers 52 or more samples to be an ideal number of blank 

results for determination of an SRL.

In Olsen and others (2010) and Davis and others (2014), 

the evaluation of field blanks was supplemented by evaluation 

of internal and third-party laboratory QC data collected by the 

NWQL and the USGS Quality Systems Branch (QSB). They 

calculated the range of concentrations, mean concentration, 

and standard deviation for laboratory-blank analyses and 

compared them with the detection limit for each trace element. 

This was done to evaluate random error that could affect the 

analytical method results and to evaluate systematic errors that 

can create positive or negative deviations in results from the 

target value (Davis and others, 2014). Analysis of laboratory 

QC data collected by NWQL and QSB were not included 

in the reevaluation of SRLs in this report. In the previous 

reports, the potential biases (positive or negative) observed 

in the laboratory QC data affected the ultimate decision to 

implement an SRL or at what level the SRL was set for only 

three of the trace elements: iron, molybdenum, and nickel. For 

the other trace elements, the laboratory QC results indicated 

which required some caution when interpreting results near 

the detection limit. For instance, Davis and others (2014) list 

aluminum, arsenic, barium, chromium, silver, strontium, and 

vanadium as having laboratory QC data suggesting either 

variability based on laboratory blank results or bias based on 

QSB blind blanks that might cause concerns. Ultimately, for 

these constituents an SRL was not determined to be necessary 

based on field-blank results (Davis and others, 2014).

Certificates of analysis for the IBW used for the field, 

source-solution, and equipment blanks and certificates of 

analysis for the capsule filters used for the field blanks and the 

groundwater samples were reviewed to determine whether any 

bias could be attributed to the IBW or the capsule filters.
12 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Groundwater detections at concentrations less than an 

SRL are considered to have an unacceptably high likelihood 

of positive bias, and therefore, should be reported as less than 

the SRL. This is contrary to the way detections less than the 

SRL were reported in Olsen and others (2010) and Davis and 

others (2014). In those reports, detections were reported as 

less than or equal to the reported value. The idea was that 

users of the data would know the magnitude of each detected 

concentration and that the true concentration could be less than 

or equal to the detected concentration. This report rejects that 

guidance and follows traditional laboratory reporting practices 

with respect to established detection limits. Detections less 

then the established SRL will be reported as less than the SRL.

Results

The dataset used for the evaluation of SRLs and the 

examination of potential contamination bias included all 

field, source-solution, and equipment blanks collected for 

the GAMA-PBP from October 2009 through October 2018. 

This dataset was compiled and is presented in Bennett 

(2020). Associated environmental groundwater samples and 

other ancillary data, including sample notes, blank-water lot 

numbers, and filter lot numbers for the blanks collected by 

the GAMA-PBP were also compiled to help interpret the QC 

data. The dataset includes 167 field and 19 source-solution 

blanks and their associated groundwater sample results plus an 

additional 8 equipment blanks.

After reviewing the time-series plots, a notable shift 

in the contamination bias (decreasing blank concentrations 

and detection frequency) for cobalt (fig. 5) and manganese 

(fig. 3K) was observed. Concentrations of each generally 

decreased from about 2011 to 2013, and detections of 

manganese in blanks became much more infrequent starting 

late in 2013. Part of the reduction of manganese detections 

in blanks is likely related to the fact that the detection limit 

for manganese was raised from 0.1 to 0.4 μg/L beginning 

October 2013. Detection limit updates are generally 

implemented by the NWQL and occur at the beginning of a 

new fiscal year (October). The shift in cobalt and manganese 

patterns and the timing of new reporting levels being 

implemented by the NWQL were used to mark an obvious 

point from which to begin a reevaluation of SRLs by the 

GAMA-PBP. The previously defined SRLs may be overly 

conservative if trends in contamination bias are declining over 

time and periodically should be reevaluated to ensure they are 

still relevant.

Cobalt concentration, in micrograms per liter

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Previous review period Current review period

07/16/09 07/16/11 07/15/13 07/15/15 07/14/17 07/14/19

Not pictured are 16 cobalt

detections in groundwater 

greater than 1 μg/L.

SRL = 0.16 μg/L

SRL = “reject all samples”

Study reporting levels (SRL)

EXPLANATION

Cobalt detections in groundwater (n = 341) Cobalt field-blank detections (n = 116)

Cobalt non-detections (n = 51) Cobalt equipment-blank detections (n = 5)

Collection date

Figure 5. Cobalt concentrations in field, source-solution, and equipment blanks collected October 2009 through October 2018 

plotted in time series, study reporting level (SRL), and cobalt concentrations in groundwater samples collected between 

October 2013 and October 2018, Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project. [μg/L, microgram 

per liter]
Results 13

Results of the reevaluation of contamination bias between 

October 2013 and October 2018 for each trace element were 

categorized into two groups: (1) trace elements detected in less 

than 5 percent of field blanks and (2) trace elements detected 

in greater than 5 percent of the field blanks (table 1). Detection 

frequencies for each trace element in field blanks collected 

by the GAMA-PBP between October 2009 and March 2013, 

the period reviewed by Davis and others (2014), are tabulated 

in table 1 for comparison with those from the current review 

period. Time-series plots of the field, source-solution, 

and equipment blank results for each trace element (and 

groundwater detections for cobalt), along with the SRLs from 

October 2009 to October 2018, are displayed in figures 3A–S

and 4–8. SRLs for trace elements based on results from the 

first two assessments (Olsen and others, 2010 and Davis and 

others, 2014) and the SRLs based on results from the current 

review period are compiled in table 2.

Trace Elements Detected in Fewer Than 

5 Percent of Field Blanks

Of the 24 trace elements analyzed (including Cr[VI]) 

for samples collected from October 2013 to October 2018 

for the GAMA-PBP, 8 were not detected in any of the field 

blanks: arsenic, boron, cadmium, chromium, iron, lithium, 

molybdenum, and thallium (table 1). Eleven trace elements—

aluminum, antimony, barium, beryllium, manganese, nickel, 

selenium, silver, strontium, uranium, and vanadium—had 

detection frequencies of less than 5 percent (table 1). For a 

trace element detected in less than 5 percent of the 77 field 

blanks collected between October 2013 and October 2018, 

the BD-90/90 value was a non-detection (lower than the 

trace-element detection limit); therefore, no SRLs were 

defined for the 19 trace elements detected in less than 

5 percent of the field blanks.

Aluminum (Al)

From October 2013 through October 2018, 1 of 77 field 

blanks (1.3 percent) collected for aluminum had a detection 

(fig. 3A; table 1). The detection limit for aluminum changed 

from 2.2 μg/L to 3 μg/L beginning October 2014. The one 

detection of aluminum in the field blanks was 3.6 μg/L. There 

were no detections of aluminum in the 19 source-solution or 

8 equipment blanks collected October 2009–October 2018 

(fig. 3A). Based on the field blanks collected during the current 

review period, the SRL for aluminum was not updated from 

the previous SRL of “none” (table 2).

Antimony (Sb)

From October 2013 through October 2018, 3 of the 

77 field blanks (3.9 percent) collected for antimony had a 

detection (fig. 3B; table 1). The three detections ranged in 

concentration from 0.07 μg/L to 0.16 μg/L. There were no 

detections of antimony in 19 source-solution or 8 equipment 

blanks collected October 2009–October 2018. There were 

changes in the detection limit for antimony from 0.027 μg/L 

to 0.03 μg/L beginning August 2016 and from 0.03 μg/L 

to 0.06 μg/L beginning March 2018. Based on the field 

blanks collected during the current review period, the SRL 

for antimony was not updated from the previous SRL of 

“none” (table 2).

Arsenic (As)

From October 2013 through October 2018, none of the 

77 field blanks collected for arsenic had a detection (fig. 3C; 

table 1). Arsenic was detected in one source-solution blank 

collected in 2014 at a concentration of 0.17 μg/L and none 

of the equipment blanks (fig. 3C). There was a change in 

the detection limit for arsenic from 0.1 μg/L to 0.05 μg/L 

beginning September 2016. The detection limit changed 

back to 0.1 μg/L beginning March 2018. From October 2009 

through March 2013, arsenic was only detected twice in field 

blanks with both detections having concentrations less than 

the current 0.1 μg/L detection limit (Davis and others, 2014). 

Arsenic was detected in one source-solution blank collected 

in 2014 at a concentration of 0.14 μg/L and none of the 

equipment blanks (fig. 3C). Based on the field blanks collected 

during the current review period, the SRL for arsenic was not 

updated from the previous SRL of “none” (table 2).

Barium (Ba)

From October 2013 through October 2018, barium 

was detected in 2 of 77 field blanks (2.6 percent) with 

concentrations of 0.77 μg/L and 1.01 μg/L (fig. 3D; table 1). 

From October 2009 through March 2013, 2 of the 69 field 

blanks (2.9 percent) collected for barium had detections 

(Davis and others, 2014). The detection limit for barium 

changed from 0.25 μg/L to 0.1 μg/L in August 2016; however, 

both blank detections occurred before the detection limit was 

lowered. Barium was detected in two source-solution blanks, 

once in 2011 and again in 2017, at low concentrations close 

to the detection limit (fig. 3D). Barium was detected in two 

equipment blanks during the previous review periods at low 

concentrations. Based on the field blanks collected during the 

current review period, the SRL for barium was not updated 

from the previous SRL of “none” (table 2).
14 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Table 2. Parameter codes, analytical methods, benchmark type and value, range of detection limits, and study reporting levels (SRLs) 

for trace element data collected for the U.S. Geological Survey (USGS) Groundwater Ambient Monitoring and Assessment (GAMA) 

Priority Basin Project, May 2004–September 2009, October 2009–September 2013, and updated SRLs starting October 2013.

[The five-digit parameter code is used to uniquely identify a specific constituent or property. Analytical method abbreviations: cICP-MS, collision/reaction 

cell inductively coupled plasma-mass spectrometry; ICP-MS, inductively coupled plasma-mass spectrometry; ICP-AES, inductively coupled plasma-atomic 

emission spectrometry; IC-UV-Vis, ion chromatography with post-column derivatization and UV-visable spectroscopic detection. Benchmark types: 

Regulatory health-based benchmarks: AL-US, EPA action level; HAL-US, EPA lifetime health advisory level; MCL-CA, SWRCB-DDW maximum 

contaminant level; MCL-US, EPA maximum contaminant level. Non-regulatory health-based benchmarks: HBSL, USGS Health Based Screening Level; 

RL-CA, SWRCB-DDW response level. Non-regulatory aesthetic-based benchmarks: SMCL-CA, SWRCB-DDW secondary maximum contaminant level. 

Abbreviations: μg/L, microgram per liter; EPA, Environmental Protection Agency; na, not available; SWRCB-DDW, California State Water Resources Control 

Board Division of Drinking Water.]

Constituent Parameter 

code

Analytical 

method

Benchmark 

type

Benchmark 

value

(μg/L)

Range of 

detection 

limits

(μg/L)

SRLs

May 2004–

September 2009

(μg/L)1

October 2009–

September 2013

(μg/L)2

Starting 

October 2013

(μg/L)3

Aluminium 01106 cICP-MS MCL-CA 1,000 3.4–6.0 1.6 None None

Antomony 01095 cICP-MS MCL-US 6 0.04–0.12 None None None

Arsenic 01000 cICP-MS MCL-US 10 0.04–0.20 None None None

Barium 01005 cICP-MS MCL-CA 1,000 0.14–0.50 0.36 None None

Beryllium 01010 cICP-MS MCL-US 4 0.01–0.04 None None None

Boron 01020 ICP-MS HAL-US 6,000 2.8–10 None None None

Cadmium 01025 cICP-MS MCL-US 5 0.02–0.06 None None None

Chromium 01030 cICP-MS MCL-CA 50 0.12–1.0 0.42 None None

Hexavalent 

chromium

01032 IC-UV-Vis HBSL 20 40.0044–0.015 Not calculated Not calculated 0.34

Cobalt 01035 cICP-MS na na 0.01–0.10 None Report all results 

as “reviewed and 

rejected.”

0.16

Copper 01040 cICP-MS AL-US 1,300 0.4–1.6 1.7 2.1 2.1

Iron 01046 ICP-AES SMCL-CA 300 4–20 6 6 None

Lead 01049 cICP-MS AL-US 15 0.03–0.08 0.65 0.82 0.83

Lithium 01130 ICP-MS na na 0.3–1.0 None None None

Manganese 01056 cICP-MS SMCL-CA 50 0.2–0.8 0.2 0.66 None

Molybdenum 01060 cICP-MS HAL-US 40 0.02–0.10 None 0.023 None

Nickel 01065 cICP-MS MCL-CA 100 0.12–0.40 0.36 0.21 None

Selenium 01145 cICP-MS MCL-US 50 0.04–0.10 None None None

Silver 01075 cICP-MS MCL-US 100 0.008–2 None None None

Strontium 01080 ICP-MS HAL-US 4,000 0.4–1.6 0.99 None None

Thallium 01057 ICP-MS MCL-US 2 0.02–0.08 None None None

Uranium 22703 cICP-MS MCL-US 30 0.006–0.06 None None None

Vanadium 01085 cICP-MS RL-CA 500 0.16–0.20 0.1 None None

Zinc 01090 cICP-MS SMCL-CA 5,000 2–4 4.8 6.2 3.9

1SRL defined by Olsen and others, 2010.

2SRL defined by Davis and others, 2014.

3SRL calculated using the 90th percentile concentration of potential extrinsic contamination calculated by using the binomial probability distribution for 

greater than 90 percent confidence (BD-90/90).

4Detection limit ranges reported for hexavalent chromium determination using IC-UV-Vis (EPA method 218.7).
Results 15

Beryllium (Be)

From October 2013 through October 2018, 1 of the 

77 field blanks (1.3 percent) collected for beryllium had a 

detection at a concentration of 0.03 μg/L (fig. 3E; table 1). 

Beryllium was not detected in any of the 19 source-solution 

or 8 equipment blanks collected October 2009–October 2018 

(fig. 3E). The detection limit for beryllium changed from 

0.02 μg/L to 0.1 μg/L in August 2016. Based on the field 

blanks collected during the current review period, the SRL 

for beryllium was not updated from the previous SRL of 

“none” (table 2).

Boron (B)

From October 2013 through October 2018, none of the 

77 field blanks collected had a detection of boron (fig. 3F; 

table 1). Boron had been detected in five blanks during the 

previous review (fig. 3F). The lot number of IBW used to 

collect the field blanks having detections of boron in 2012 

greater than 25 μg/L (fig. 3F) was reported to have boron 

at concentrations up to 30 μg/L (Davis and others, 2014). 

Boron was not detected in any of the 19 source-solution or 

8 equipment blanks collected October 2009–October 2018 

(fig. 3F). The detection limit for boron during the current 

review period remained constant at 5 μg/L. Based on the 

field blanks collected during the current review period, the 

SRL for boron was not updated from the previous SRL of 

“none” (table 2).

Cadmium (Cd)

From October 2013 through October 2018, none of 

the 77 field blanks collected had a detection of cadmium 

(fig. 3G; table 1). Cadmium was not detected in any of the 

19 source-solution blanks and only 1 of 8 equipment blanks 

(0.029 μg/L) collected October 2009–October 2018 (fig. 3G). 

The detection limit for cadmium during the current review 

period remained constant at 0.03 μg/L. Based on the field 

blanks collected during the current review period, the SRL 

for cadmium was not updated from the previous SRL of 

“none” (table 2).

Chromium (Cr)

From October 2013 through October 2018, none of the 

77 field blanks collected had a detection of chromium (fig. 3H; 

table 1). Chromium was detected in 1 of 19 source-solution 

blanks (0.4 μg/L) and 1 of 8 equipment blanks (0.15 μg/L) 

collected October 2009–October 2018 (fig. 3H). The detection 

limit for chromium during the current review period changed 

from 0.3 μg/L to 0.5 μg/L in August 2016. Based on the field 

blanks collected during the current review period, the SRL 

for chromium was not updated from the previous SRL of 

“none” (table 2).

Iron (Fe)

From October 2013 through October 2018, none 

of the 77 field blanks collected had a detection of iron 

(fig. 3I; table 1). Iron was not detected in any of the 

19 source-solution blanks or 8 equipment blanks collected 

from October 2009 to October 2018 (fig. 3I). Iron was 

detected in 7.2 percent of blanks analyzed during the previous 

review by Davis and others (2014), and they established an 

SRL for iron of 6 μg/L (table 2, fig. 3I). The detection limit 

for iron increased twice during the current review period, 

changing from 4 μg/L to 5 μg/L in August 2016 and from 

5 μg/L to 10 μg/L in June 2017. Based on the current detection 

limit of 10 μg/L and field-blank data during the current review 

period, the SRL for iron has been updated from the previous 

SRL of 6 μg/L to “none” (table 2).

Lithium (Li)

From October 2013 through October 2018, none of 

the 77 field blanks collected had a detection of lithium 

(fig. 3J; table 1). Lithium was not detected in any of the 

19 source-solution or 8 equipment blanks collected from 

October 2009 to October 2018 (fig. 3J). The detection limit for 

lithium was lowered from 0.22 μg/L to 0.15 μg/L in August 

of 2015. Based on the field blanks collected during the current 

review period, the SRL for lithium was not updated from the 

previous SRL of “none” (table 2).

Manganese (Mn)

Between October 2009 and September 2013, manganese 

was detected in 65 of 90 field blanks (73 percent) with 

concentrations ranging from 0.14 to 0.91 μg/L (fig 3K;

table 1). The median manganese concentration in field 

blanks during this time was 0.3 μg/L. Based on the blank 

results, an SRL for manganese of 0.66 μg/L was established 

by Davis and others (2014; table 2). The occurrence of 

manganese in field blanks during this time coincides with the 

observation of contamination bias of cobalt and manganese 

samples attributed to the high-capacity capsule filters used 

during sample collection from October 1, 2008, to about 

September 30, 2014 (U.S. Geological Survey, 2016). 

Nationally, manganese was being detected in approximately 

33 percent of field blanks (U.S. Geological Survey, 2016). 
16 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Recommendations from the USGS Office of Water Quality 

(OWQ) to address low-level detections of manganese in field 

blanks collected using high-capacity capsule filters have 

been to increase the deionized water rinse volume, track filter 

lot numbers, review certificates of analysis for detections 

of analytes of interest, and to evaluate environmental 

concentrations with respect to blank data. Filter lot numbers 

and the results of quality-control tests conducted by the 

NWQL on the high-capacity capsule filters used for the 

collection of trace elements between late 2012 and early 

2019 are shown in table 3. It has been noted that despite the 

prescribed increase in field rinsing, the contamination has not 

always been mitigated (U.S. Geological Survey, 2016).

From October 2013 through October 2018, 2 of the 

77 field blanks collected had a detection of manganese 

(fig. 3K; table 1). Manganese was not detected in any of 

the 19 source-solution blanks collected October 2009–

October 2018 but was detected in 2 equipment blanks 

collected during the previous review period (fig. 3K). In 

October 2013, the detection limit for manganese was raised 

from 0.15 μg/L to 0.4 μg/L, and the number of manganese 

detections in field blanks dropped significantly. Based on the 

field blanks collected during the current review period, the 

SRL for manganese was updated from the previous SRL of 

0.66 μg/L to “none” (table 2).

Molybdenum (Mo)

From October 2013 through October 2018, none of the 

77 field blanks collected had a detection of molybdenum 

(fig. 3L; table 1). Molybdenum was not detected in any 

of the 19 source-solution or 8 equipment blanks collected 

from October 2009 to October 2018. The detection limit 

for molybdenum was raised beginning October 2013 from 

0.014 μg/L to 0.05 μg/L, which may account for the decrease 

in blank detection starting in October 2013. Davis and 

others (2014) had established an SRL for molybdenum of 

0.023 μg/L; however, based on the field blanks collected 

during the current review period, the SRL for molybdenum 

was updated to “none” (table 2).

Table 3. Concentrations of cobalt and manganese in high-capacity filters tested by the U.S. Geological Survey National Water Quality 

Laboratory (NWQL) for use in the collection of trace-element blanks—December 2012–March 2019. 

[mm/dd/yyyy, month/day/year; μg/L, microgram per liter; na, not available; nd, not detected]

Manufacturer 

lot number

Certificate 

date

(mm/dd/yyyy)

Cobalt concentration1

(μg/L)

Manganese concentration1

(μg/L)

Laboratory 

reporting 

level

Reported 

concentration2 Minimum Maximum

Laboratory 

reporting 

level

Reported 

concentration2 Minimum Maximum

FW7427 12/03/2012 0.023 0.0016 na na 0.15 0.0071 na na

FZ8372 03/08/2013 0.023 0.011 0.0041 0.015 0.15 0.029 0.019 0.036

FZ0324 02/25/2014 0.05 0.0034 nd 0.0018 0.4 0.021 0.012 0.016

FZ1395 04/29/2014 0.05 0.0034 0.0023 0.0044 0.4 0.021 0.018 0.033

FZ1557 05/28/2014 0.05 0.0024 0.0001 0.0082 0.4 0.062 0.023 0.18

FZ2194 11/11/2014 0.05 0.0046 0.0019 0.0077 0.4 0.024 0.011 0.048

FZ4118 04/20/2015 0.05 0.0055 0.0008 0.012 0.4 0.021 0.01 0.025

FZ4362 10/26/2015 0.05 0.0022 0.0003 0.0066 0.4 nd nd 0.0054

FZ8418 04/22/2016 0.05 0.0022 0.0003 0.0066 0.4 nd nd 0.0054

FC0692 01/05/2017 0.03 nd nd nd 0.4 0.011 nd 0.02

FC2587 05/25/2017 0.03 0.002 0.0014 0.0027 0.4 nd nd 0.0058

FC4706 11/14/2017 0.03 0.007 na na 0.4 0.014 na na

FC7580 04/19/2018 0.03 0.0048 0.0023 0.0077 0.04 0.022 0.0066 0.088

FE1643 03/01/2019 0.03 0.0048 0.0023 0.0077 0.4 0.022 0.0066 0.088

1Reported concentrations are uncensored instrument readings provided by NWQL. Values are not significantly different from “zero” and should be interpreted 

to be “non-detections.”

2Ten capsule filters are tested from each new lot, as well as a capsule filter from a previously approved lot for a control, which is not included in the reported 

data. The “reported concentration” is the average result of the 10 new lot filters.
Results 17

Nickel (Ni)

From October 2013 through October 2018, 1 of the 

77 field blanks (1.3 percent) collected had a detection 

of nickel (fig. 3M; table 1). Nickel was detected in 1 of 

19 source-solution and 1 of 8 equipment blanks collected 

from October 2009 to October 2018 (fig. 3M). Beginning 

October 2013, the detection limit for nickel was raised from 

0.09 μg/L to 0.2 μg/L. Davis and others (2014) had established 

an SRL for nickel of 0.21 μg/L; however, based on the field 

blanks collected during the current review period, the SRL for 

nickel was updated to “none” (table 2).

Selenium (Se)

From October 2013 through October 2018, 1 of the 

77 field blanks (1.3 percent) collected had a detection of 

selenium (fig. 3N; table 1). This was the only detection of 

selenium in field blanks going back to October 2009 (fig. 3N). 

Selenium was not detected in any of the 19 source-solution 

or 8 equipment blanks collected October 2009–October 2018 

(fig. 3N). Beginning in October 2013, the NWQL detection 

limit for selenium was raised from 0.03 μg/L to 0.05 μg/L. 

Based on the field blanks collected during the current review 

period, the SRL for selenium was not updated from the 

previous SRL of “none” (table 2).

Silver (Ag)

From October 2013 through October 2018, 2 of the 

77 field blanks (2.6 percent) collected had a detection of 

silver (fig. 3O; table 1). Silver was also detected in two field 

blanks between October 2009 and September 2013 (fig. 3O). 

Silver was not detected in any of the 19 source-solution or 

8 equipment blanks collected October 2009–October 2018 

(fig. 3O). Beginning in October 2013, the detection limit for 

silver was raised from 0.005 μg/L to 0.02 μg/L, and it was 

raised again in August 2016 from 0.02 μg/L to 1 μg/L. Based 

on the field blanks collected during the current review period, 

the SRL for silver was not updated from the previous SRL of 

“none” (table 2).

Strontium (Sr)

From October 2013 through October 2018, 2 of the 

77 field blanks (2.6 percent) collected had a detection of 

strontium (fig. 3P; table 1). Strontium was also detected in 

two field blanks between October 2009 and September 2013 

(fig. 3P). Strontium was detected in 1 of the 19 source-solution 

blanks at a concentration of 1.3 μg/L, a higher concentration 

than any of the field blanks but far less than concentrations 

typically observed in groundwater. The median 

concentration of strontium in groundwater samples collected 

contemporaneously with the field blanks in the current 

review period was 282 μg/L. Beginning in October 2013, 

the detection limit for strontium was raised from 0.2 μg/L to 

0.8 μg/L and was later lowered in August 2016 from 0.8 μg/L 

to 0.5 μg/L. Based on the field blanks collected during the 

current review period, the SRL for strontium was not updated 

from the previous SRL of “none” (table 2).

Thallium (Tl)

From October 2013 through October 2018, none of 

the 77 field blanks collected had a detection of thallium 

(fig. 3Q; table 1). Thallium was not detected in any of 

the 19 source-solution or 8 equipment blanks collected 

October 2009–October 2018 (fig. 3Q). Beginning in 

October 2013, the detection limit for thallium was raised 

from 0.01 μg/L to 0.03 μg/L and was later lowered in 

August 2016 from 0.03 μg/L to 0.02 μg/L. In October 2018, 

the detection limit was raised to 0.04 μg/L. Based on the 

field blanks collected during the current review period, the 

SRL for thallium was not updated from the previous SRL of 

“none” (table 2).

Uranium (U)

From October 2013 through October 2018, 2 of the 

77 field blanks (2.6 percent) collected had a detection of 

uranium (fig. 3R; table 1). Concentrations of uranium in the 

two blanks were 0.062 and 0.045 μg/L. Uranium was not 

detected in any of the 19 source-solution or 8 equipment 

blanks collected October 2009–October 2018 (fig. 3R). The 

detection limit for uranium changed twice during the current 

review period. Beginning in October 2013, the detection limit 

for uranium was 0.014 μg/L. In August 2016, it was lowered 

to 0.01 μg/L, and it was raised in March 2018 to 0.03 μg/L. 

Based on the field blanks collected during the current review 

period, the SRL for uranium was not updated from the 

previous SRL of “none” (table 2).

Vanadium (V)

From October 2013 through October 2018, 3 of the 

77 field blanks (3.9 percent) collected had a detection of 

vanadium (fig. 3S; table 1). Concentrations of vanadium 

in the three blanks were 0.19, 0.26, and 0.81 μg/L, and all 

occurred relatively early in 2014. Vanadium was detected 

in 1 of the 19 source-solution blanks and 1 of 8 equipment 

blanks collected October 2009–October 2018 (fig. 3S). The 

detection limit for vanadium changed once during the current 

review period. Beginning in October 2013, the detection limit 

for vanadium was 0.08 μg/L. In October 2014, it was raised 

to 0.1 μg/L. Based on the field blanks collected during the 

current review period, the SRL for vanadium was not updated 

from the previous SRL of “none” (table 2).
18 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Trace Elements Detected in Greater Than 

5 Percent of Field Blanks

One species of the trace element chromium (Cr[VI]), 

and cobalt, copper, lead, and zinc had detection frequencies 

in field blanks greater than 5 percent between October 2013 

and October 2018 (table 1). Detection frequencies of cobalt, 

lead, and zinc decreased when compared to the previous 

review period (October 2009–March 2013), while copper 

detections increased (table 1). Detected concentrations of 

cobalt in field blanks decreased significantly (Wilcoxon 

rank-sum p < 0.0001, α = 0.05) when comparing the 

previous and current review periods. Copper, lead, and zinc 

had insignificant differences between the median detected 

concentrations when comparing the review periods, with 

rank-sum p = 0.74, p = 0.11, and p = 0.70 (α = 0.05), 

respectively (figs. 6–8). Collection of Cr(VI) samples 

submitted to Weck for analysis began during the second half 

of 2015 and were therefore not part of the previous review by 

Davis and others (2014).

Except for Cr(VI), SRLs for the trace elements with 

detection levels greater than 5 percent were determined based 

on the BD-90/90 of each trace element’s concentrations 

in field blanks collected during the current review period. 

For Cr(VI), additional data from the equipment and 

source-solution blanks were included when calculating the 

BD-90/90 concentration that defined the SRL. Groundwater 

results with concentrations less than the SRL have an 

unacceptably high likelihood of significant contamination bias 

and will be reported as less than the SRL.

Hexavalent Chromium (Cr[VI])

From August 2015 through October 2018, 33 of the 

51 field blanks (65 percent) collected had a detection of 

Cr(VI) (fig. 4; table 1). Concentrations of Cr(VI) in the field 

blank detections ranged from 0.02 to 0.60 μg/L (fig. 4). 

Cr(VI) was detected in 4 of the 7 source-solution blanks with 

concentrations ranging from 0.03 to 0.34 μg/L. Cr(VI) was 

also detected in 4 of 5 equipment blanks with concentrations 

ranging from 0.02 to 0.20 μg/L. The detection limit for Cr(VI) 

used by Weck for Cr(VI) analyses is 0.02 μg/L. The median 

concentration of detections observed in field, source-solution, 

and equipment blanks was 0.04 μg/L.

The high detection frequency of Cr(VI) in field, 

source-solution, and equipment blanks indicates a persistent 

source of low-level Cr(VI) contamination. An examination 

of the IBW used found that total chromium (a measure which 

includes all chromium species) was detected in QC tests 

conducted by the USGS National Water Quality Laboratory 

(NWQL) in all eight lots of IBW used over the current review 

period (Lot numbers 81506, 81507, 81601, 81608, 81705, 

81708, 81802, and 81805; table 4).

Table 4. Concentrations of chromium in blank water tested by the U.S. Geological 

Survey National Water Quality Laboratory (NWQL) for use in the collection of 

trace-element blanks—September 2015–May 2018.

[mm/dd/yyyy, month/day/year; μg/L, microgram per liter; nd, not detected]

Blank 

water lot 

number

Date tested

(mm/dd/yyyy)

Laboratory 

detection 

level

Chromium concentration1

(μg/L)

Bottle

#1

Bottle

#2

Bottle

#3

Bottle

#4

Average

81506 09/30/2015 0.3 0.013 0.0089 0.19 0.0088 0.054

81507 03/31/2015 0.3 0.0013 0.0019 0.0011 0.0003 0.0012

81608 08/31/2016 0.3 nd 0.0009 0.0073 0.018 0.0039

81601 10/31/2016 0.5 0.05 0.07 0.03 0.05 0.05

81705 04/30/2017 0.5 0.33 0.31 0.20 0.24 0.27

81708 07/31/2018 0.5 0.12 0.079 0.087 0.06 0.087

81802 01/31/2018 0.5 0.068 0.03 nd nd 0.02

81805 05/31/2018 0.5 0.16 0.05 nd nd nd

1Reported concentrations are uncensored instrument readings provided by NWQL. Values are 

not significantly different from “zero” and should be interpreted to be “non-detections.”
Results 19

The NWQL ran tests on four separate bottles from each 

IBW lot to determine average trace-element concentrations 

in each lot (table 4). Concentrations of total chromium 

reported by the NWQL from these tests ranged from <0.003 to 

0.326 μg/L, which is very close to the range of concentrations 

measured in the field, source-solution, and equipment blanks 

collected (fig. 4). The NWQLs testing of IBW does not 

include Cr(VI). Although the IBW tests only reported total 

chromium, a statewide assessment of Cr(VI) occurrence in 

California found that 90 percent of the dissolved chromium 

in groundwater samples was Cr(VI), suggesting that Cr(VI) 

is the dominant chromium species in most environmental 

water samples (Izbicki and others, 2015). While the IBW 

isn’t an environmental water sample it is assumed that 

the predominant chromium species in the IBW is Cr(VI). 

The IBW certified by NWQL for use when conducting 

trace-element blanks was not flagged for low-level chromium 

concentrations because the detection limit for total chromium 

analyses conducted by the NWQL is 0.5 μg/L, which is greater 

than the highest concentration of total chromium measured 

in the eight IBW lots (0.326 μg/L) tested by NWQL during 

this review.

The SRL for Cr(VI) was determined by calculating 

the BD-90/90 concentration from the 63 Cr(VI) results 

from the combined dataset of field, source-solution, and 

equipment blanks. Using this combined dataset, the SRL 

for Cr(VI) reported from Weck was determined to be 

0.34 μg/L (table 2). For 63 ranked values, the 3rd highest 

value statistically defines the BD-90/90 concentration. The 

3rd highest concentration of Cr(VI) from the combined 

field, source-solution, and equipment-blank dataset was 

from a source-solution blank collected in April 2018 (fig. 4). 

Because the IBW itself contains low level concentrations 

of Cr(VI), it was not possible to determine if low-level field 

contamination of Cr(VI) is occurring. Field and equipment 

blanks did not have noticeably higher concentrations of Cr(VI) 

than the source-solution blanks (fig. 4). Despite low-level 

detections of Cr(VI) in blanks not being attributed to field 

contamination, an SRL of 0.34 μg/L was still implemented to 

be conservative when reporting low-level detections of Cr(VI) 

in environmental samples. The range of Cr(VI) concentrations 

in the environmental samples associated with the field blanks 

was 0.03 to 24 μg/L. About 24 percent of those samples 

had Cr(VI) concentrations less than 0.34 μg/L (Jurgens and 

others, 2018).

Cobalt (Co)

Cobalt was detected in 99 percent of field blanks 

between October 2009 and March 2013 (Davis and others, 

2014). As was discussed in the manganese section, the high 

detection frequency of cobalt in field blanks is attributed to 

the high-capacity capsule filters used to collect samples for 

trace-element analyses starting October 2009 and continuing 

through September 2014 (U.S. Geological Survey, 2016). The 

calculated BD-90/90 concentration using field blanks collected 

October 2009 through March 2013 was 0.038 μg/L; however, 

that SRL was not assigned. Instead, the samples were 

considered “reviewed and rejected” (Davis and others, 2014). 

This was done in part because of the pervasiveness of cobalt 

detections in field blanks during this time period and because 

cobalt does not have a regulatory benchmark for drinking 

water, meaning that rejecting the data did not affect the ability 

of the GAMA-PBP to assess groundwater quality within the 

framework of drinking-water benchmarks.

From October 2013 through October 2018, 32 of the 

77 field blanks (42 percent) collected had a detection of cobalt 

(fig. 5; table 1). Concentrations in the field blanks ranged 

from 0.03 to 0.21 μg/L. There were no detections of cobalt 

in the 19 source-solution blanks collected October 2009–

October 2018. Cobalt was detected in three of five equipment 

blanks with concentrations ranging from 0.04 to 0.08 μg/L 

within the current review period and two of three equipment 

blanks during the previous review period (fig. 5). The 

frequency and concentration of cobalt in field blanks dropped 

considerably in 2014, suggesting the increased awareness 

of contamination bias associated with the capsule filters and 

suggesting mitigation strategies by the USGS OWQ have 

been effective to some degree (fig. 5; table 3). This resulted 

in a reconsideration of the previous determination by Davis 

and others (2014) that all cobalt detections in groundwater 

samples be reported as “reviewed and rejected.” Cobalt was 

detected in about 40 percent (339 of 849) of the groundwater 

samples collected by GAMA during the current review 

period with concentrations ranging from 0.03 to 22.5 μg/L 

and a median concentration of 0.09 μg/L (Jurgens and others, 

2018). The BD-90/90 concentration for cobalt based on the 

field-blank data for the current review period is 0.16 μg/L. 

Applying an SRL of 0.16 μg/L for cobalt to the dataset reduces 

the detection frequency to about 10 percent (84 of 849), 

effectively screening low-level detections of cobalt for which 

contamination bias cannot be ruled out.

Copper (Cu)

During the previous review period, 36 percent of 

field blanks collected for copper had detections, and the 

BD-90/90 concentration calculated based on those field 

blanks was 2.1 μg/L (Davis and others, 2014; tables 1, 2). 

From October 2013 through October 2018, 34 of 77 field 

blanks (44 percent) had detections of copper with 

concentrations ranging from 0.21 to 6.31 μg/L (fig. 6). One 

of 19 source-solution blanks and 6 of 8 equipment blanks had 

detections of copper between October 2009 and October 2018 

(fig. 6). The copper concentration in the source-solution blank 

was 1.50 μg/L, and concentrations in the equipment blanks 

ranged from 0.51 to 6.7 μg/L (fig. 6).
20 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

EXPLANATION

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

Copper non-detections (n = 108) Copper field-blank detections (n = 59)

Copper equipment-blank detections (n = 6) Copper source-solution detections (n = 1)

Copper concentration, in micrograms per liter

Collection date

Study reporting levels (SRL)

7

6

5

4

3

2

1

0

Previous review period Current review period

SRL = 2.1 μg/L SRL = 2.1 μg/L

Figure 6. Copper concentrations in field, source-solution, and equipment blanks collected October 2009 through October 2018 

plotted in time series and study reporting levels (SRLs), Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin 

Project. [μg/L, microgram per liter]

The contamination bias for copper was determined to 

be more likely caused by the equipment and processes used 

to collect samples in the field than by laboratory processes 

(Davis and others, 2014). Copper is one of the primary 

metals in the brass alloy plumbing hardware used to connect 

sampling equipment to the well. Some stainless steels also can 

contain low concentrations of copper. This bias has showed no 

improvement since the last review. The SRL for copper based 

on the BD-90/90 concentration using field-blank detections 

during the current review period (2.1 μg/L) is the same as 

the SRL determined after the previous review by Davis and 

others (2014; table 2). The SRL of 2.1 μg/L is far less than the 

U.S. Environmental Protection Agency Action Level (AL-US) 

for copper of 1,300 μg/L.

Lead (Pb)

During the previous review period (October 2009 through 

March 2013), 57 percent of field blanks collected for lead 

had detections, and the BD-90/90 concentration calculated 

from those field blanks was 0.82 μg/L (tables 1, 2). From 

October 2013 through October 2018, 23 of 77 field blanks 

(30 percent) had detections of lead with concentrations 

ranging from 0.021 to 1.29 μg/L (fig. 7). Lead was not 

detected in any of the 19 source-solution blanks collected 

but was detected in 4 of 8 equipment blanks collected over 

the entire review period with concentrations ranging from 

0.04 μg/L to 0.35 μg/L (fig. 7). The detection limit for lead 

dropped from 0.04 to 0.02 μg/L in August of 2016; however, 

this change did not appear to result in an increase in detections 

in field blanks. Concentrations of lead in field blanks during 

the current review period were highest between 2015 and 

2016 with more recent field blanks having relatively low 

concentrations or non-detections (fig. 7).

As noted in Davis and others (2014), the sampling 

equipment is likely the main source of lead contamination 

observed in the field blanks. The contamination bias for lead 

observed by Davis and others (2014) showed little sign of 

improvement between October 2013 and December 2016, with 

39 percent of field blanks (19 of 49) having detections of lead 

(fig. 7). However, since the beginning of 2017, only 14 percent 

(4 of 28) of the field blanks had a lead detection. Despite this 

apparent improvement, an SRL for lead was calculated based 

on the BD-90/90 concentration using field-blank data from the 

current review period. The resulting SRL, 0.83 μg/L, is nearly 

the same as the SRL calculated during the previous review 

period (0.82 μg/L; Davis and others, 2014). As before, the 

newly established SRL of 0.83 μg/L is far less than the AL-US 

for lead of 15 μg/L.
Results 21

EXPLANATION

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

Lead concentration, in micrograms per liter

Collection date

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

Previous review period Current review period

SRL = 0.82 μg/L SRL = 0.83 μg/L

Lead non-detections (n = 100)

Study reporting levels (SRL)

Lead field-blank detections (n = 67)

Lead equipment-blank detections (n = 4)

Figure 7. Lead concentrations in field, source-solution, and equipment blanks collected October 2009 through October 2018 plotted 

in time series and study reporting levels (SRLs), Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project. 

[μg/L, microgram per liter]

Zinc (Zn)

During the previous review period (October 2009 through 

March 2013) 36 percent of field blanks collected for zinc 

had detections and the BD-90/90 concentration calculated 

from those field blanks was 6.2 μg/L (tables 1, 2). From 

October 2013 through October 2018, 25 of 77 field blanks 

(32 percent) had detections of zinc with concentrations 

ranging from 2.02 to 7.08 μg/L (fig. 8). Zinc was detected in 

1 of 19 source-solution blanks and 4 of 8 equipment blanks 

collected between October 2009 and October 2018 (fig. 8). 

Like lead, concentrations of zinc in field blanks during 

the current review period were highest between 2015 and 

2016 with more recent field blanks having relatively low 

concentrations of zinc or non-detections (fig. 8).

The contamination bias for zinc observed by Davis 

and others (2014) showed little sign of improvement. It is 

likely that zinc detections in blanks (like lead and copper) 

are associated with the sampling equipment, specifically 

the brass (an alloy of copper and zinc) plumbing hardware 

used to connect sampling equipment to the wells. Between 

October 2013 and December 2016, 43 percent of blanks (21 

of 49) had detections of zinc (fig. 8). However, since the 

beginning of 2017, only 14 percent of field blanks (4 of 28) 

had a zinc detection. Despite this apparent improvement, 

an SRL for zinc was calculated based on the BD-90/90 

concentration using field blank data from the current review 

period. The calculated SRL was 3.9 μg/L, which is less than 

the SRL of 6.2 μg/L determined during the previous review 

period. The updated SRL of 3.9 μg/L is far less than the 

SWRCB Division of Drinking Water Secondary Maximum 

Contaminant Level (SMCL-CA) for zinc of 5,000 μg/L.
22 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Zinc non-detections (n = 117) Zinc field-blank detections (n = 50)

Zinc equipment-blank detections (n = 4) Zinc source-solution detections (n = 1)

10

9

8

7

6

5

4

3

2

1

0

Collection date

07/16/09 07/16/10 07/16/11 07/15/12 07/15/13 07/15/14 07/15/15 07/14/16 07/14/17 07/14/18 07/14/19

Previous review period Current review period

SRL = 6.2 μg/L

SRL = 3.9 μg/L

EXPLANATION

Zinc concentration, in micrograms per liter

Study reporting levels (SRL)

Figure 8. Zinc concentrations in field, source-solution, and equipment blanks collected October 2009 through October 2018 

plotted in time series and study reporting levels (SRLs), Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin 

Project. [μg/L, microgram per liter]

Summary

The California State Water Resources Control Board 

in cooperation with the U.S. Geological Survey have been 

analyzing groundwater quality statewide since 2004 as part 

of the Groundwater Ambient Monitoring and Assessment 

(GAMA) Program Priority Basin Project (PBP). The 

program is a comprehensive assessment of statewide 

groundwater quality designed to identify and characterize 

risks to groundwater resources and to increase the availability 

of information about groundwater quality to the public. 

Trace-element field blanks were collected as part of the 

quality-control (QC) process for this program and were 

reviewed to determine potential bias to environmental 

sample results. Bias in the environmental data could be 

attributed to contamination in the field from contact between 

groundwater and sampling equipment or other contaminant 

sources, to contamination during processing or shipping, or 

to method bias during the analysis of samples. Bias can affect 

the interpretation of results, particularly if any constituents 

are present in the samples solely because of extrinsic 

contamination that would otherwise be absent from the 

groundwater sampled.

Olsen and others (2010) had previously determined 

study reporting levels (SRLs) for trace-element results based 

primarily on field-blank samples collected May 2004 through 

January 2008. Using the same approach as Olsen and others 

(2010) the SRLs were re-evaluated by Davis and others 

(2014). Using a modified approach in this report, the SRLs 

were again reviewed by analyzing field, source-solution, 

and equipment blanks collected October 2009 through 

October 2018. As in the previous two reports, results from 

the field blanks were used to calculate detection frequencies 

and to estimate the upper 90-percent confidence limit of 

contamination bias (BD-90/90). Unlike the previous reports, 

the BD-90/90 was used as the exclusive determinant of 

the SRL for the current review period (October 2013–

October 2018). The BD-90/90 is the upper 90-percent 

confidence limit of the 90th percentile concentration of 

potential extrinsic contamination, with greater than 90 percent 

confidence derived by assuming the binomial probability 

distribution, from the method reported by Hahn and 

Meeker (1991).

Ten trace elements analyzed—antimony (Sb), arsenic 

(As), beryllium (Be), boron (B), cadmium (Cd), lithium (Li), 

selenium (Se), silver (Ag), thallium (Tl), and uranium (U)—

had field-blank results that did not necessitate establishing 

SRLs during this review or the two previous reviews. Four 

trace elements analyzed—iron (Fe), manganese (Mn), 

molybdenum (Mo), and nickel (Ni)—had field-blank results 

that necessitated establishing an SRL during the previous 

review but were determined to no longer need an SRL. Copper 

(Cu) had field-blank results that necessitated keeping the 

previous SRL (2.1 μg/L). Two trace elements analyzed, cobalt 

(Co) and zinc (Zn), had field-blank results that necessitated 

decreasing the previous SRLs, from “reject all results” to 

0.16 μg/L for Co and from 6.2 μg/L to 3.9 μg/L for Zn. Lead 

(Pb) had field-blank results that necessitated retention of 

an SRL at a nearly identical concentration (0.83 μg/L) to 

the one established during the previous review (0.82 μg/L). 

Hexavalent chromium (Cr[VI]) had field-blank results 

that necessitated establishing an SRL of 0.34 μg/L for the 

first time.
Summary 23

The SRLs for Cr(VI) (0.34 μg/L), Co (0.16 μg/L), 

Cu (2.1 μg/L), Pb (0.83 μg/L), and Zn (3.9 μg/L) were 

changed to these concentrations beginning October 2013, 

based on the BD-90/90 concentration in field blanks. The 

BD-90/90 calculation for Cr(VI) included source-solution 

and equipment blank results. October 2013 was selected as 

the start of the current review period because it was around 

this time that a noteworthy shift occurred with respect to 

detection frequencies and concentrations of manganese and 

cobalt. Additionally, between October 2013 and October 2018 

(the current review period), there were enough blanks to 

calculate statistically robust BD-90/90 values. At all times, 

SRLs for the constituents were at least an order of magnitude 

less than the benchmarks established for drinking water 

for health and aesthetic purposes (cobalt does not currently 

have a benchmark); therefore, the practice of reporting 

concentrations below the SRLs as less than the SRL would 

not prevent the identification of values greater than the 

drinking-water benchmarks.

The primary sources of contamination inferred from 

this review are biases either from the inorganic blank water 

(IBW) used to collect the samples or from the equipment and 

processes used in the field to collect the samples. Reductions 

in field-blank detection frequencies for Fe, Mn, Mo, and Ni 

are likely related to increasing laboratory detection limits over 

time. Contamination in field blanks with Cr(VI), analyzed 

by Weck Laboratories, Inc., was inferred from this review 

to be linked to the IBW used when collecting blanks for 

trace elements.

Contamination in field blanks with Co and Mn reported 

in a 2014 review of GAMA-PBP blank-sample results was 

attributed to the high-capacity capsule filters that are still in 

use by the GAMA-PBP. As of October 2013, issues with the 

capsule filter appear to have improved with the application of 

mitigation strategies provided by the U.S. Geological Survey. 

The abrupt decreases in the frequency and magnitude of cobalt 

and manganese detections in blanks starting in October 2013 

helped define the start date for SRLs resulting from this latest 

re-evaluation. Even though Co detections in field blanks have 

seen a significant reduction, Co was still detected in greater 

than 5 percent of the field blanks, and an SRL was established. 

The Mn detection frequency in blanks dropped from over 

70 percent during the previous review to less than 3 percent 

during the current review and Mn no longer needs an SRL. 

Lastly, the sampling equipment is likely the main source of 

contamination for Co, Pb, and Zn. All three are components 

of brass, which is consistently a part of the sampling process. 

For each of these trace elements, there was either little change 

or a minor reduction in the contamination bias with respect to 

SRLs determined during this review as compared to the two 

previous reviews.

The SRLs determined in this report are intended to 

be used for GAMA groundwater-quality data for samples 

collected from October 2013 onward, for as long as QC data 

are consistent with what was observed in this study. The 

QC data should be continuously reviewed in the context 

of the findings in this report to reassess the SRLs used by 

the GAMA-PBP.
24 Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018

Acknowledgments

Funding was provided by the California State Water 

Resources Control Board (SWRCB) and U.S. Geological 

Survey (USGS) Cooperative Matching funds. This report is a 

product of the California Groundwater Ambient Monitoring 

and Assessment Program Priority Basin Project. A very special 

thanks is given to all the cooperating well owners and water 

purveyors for their generosity in allowing the USGS to collect 

groundwater samples from their wells.

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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, California 95819

https://ca.water.usgs.gov

Publishing support provided by the U.S. Geological Survey 

Science Publishing Network, Sacramento Publishing Service Center
Bennett—Updated SRLs for Trace-Element Data Collected for the GAMA Program Priority Basin Project, October 2009–October 2018—SIR 2020–5034

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205034