Quality-Control Results for Ground-Water and 

Surface-Water Data, Sacramento River Basin, 

California, National Water-Quality Assessment, 

1996–1998

By Cathy Munday and Joseph L. Domagalski

Sacramento, California

2002

Water-Resources Investigations Report 02-4201

U.S. GEOLOGICAL SURVEY

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 6230-32
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall – Suite 2012

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
 iii

FOREWORD

The U.S. Geological Survey (USGS) is 

committed to serve the Nation with accurate and timely 

scientific information that helps enhance and protect 

the overall quality of life, and facilitates effective 

management of water, biological, energy, and mineral 

resources. Information on the quality of the Nation's 

water resources is of critical interest to the USGS 

because it is so integrally linked to the long-term 

availability of water that is clean and safe for drinking 

and recreation and that is suitable for industry, 

irrigation, and habitat for fish and wildlife. Escalating 

population growth and increasing demands for the 

multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

critical to the long-term sustainability of our 

communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. Shaped by and coordinated with ongoing efforts 

of other Federal, State, and local agencies, the 

NAWQA Program is designed to answer: What is the 

condition of our Nation's streams and ground water? 

How are the conditions changing over time? How do 

natural features and human activities affect the quality 

of streams and ground water, and where are those 

effects most pronounced? By combining information 

on water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims to 

provide science-based insights for current and 

emerging water issues. NAWQA results can 

contribute to informed decisions that result in practical 

and effective water-resource management and 

strategies that protect and restore water quality.

Since 1991, the NAWQA Program has 

implemented interdisciplinary assessments in more 

than 50 of the Nation's most important river basins and 

aquifers, referred to as Study Units. Collectively, these 

Study Units account for more than 60 percent of the 

overall water use and population served by public 

water supply, and are representative of the Nation's 

major hydrologic landscapes, priority ecological 

resources, and agricultural, urban, and natural sources 

of contamination. 

Each assessment is guided by a nationally 

consistent study design and methods of sampling and 

analysis. The assessments thereby build local 

knowledge about water-quality issues and trends in a 

particular stream or aquifer while providing an 

understanding of how and why water quality varies 

regionally and nationally. The consistent, multi-scale 

approach helps to determine if certain types of waterquality issues are isolated or pervasive, and allows 

direct comparisons of how human activities and natural 

processes affect water quality and ecological health in 

the Nation's diverse geographic and environmental 

settings. Comprehensive assessments on pesticides, 

nutrients, volatile organic compounds, trace metals, 

and aquatic ecology are developed at the national scale 

through comparative analysis of the Study-Unit 

findings. 

The USGS places high value on the 

communication and dissemination of credible, timely, 

and relevant science so that the most recent and 

available knowledge about water resources can be 

applied in management and policy decisions. We hope 

this NAWQA publication will provide you the needed 

insights and information to meet your needs, and 

thereby foster increased awareness and involvement in 

the protection and restoration of our Nation's waters. 

The NAWQA Program recognizes that a 

national assessment by a single program cannot 

address all water-resource issues of interest. External 

coordination at all levels is critical for a fully integrated 

understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation's water resources. The Program, therefore, 

depends extensively on the advice, cooperation, and 

information from other Federal, State, interstate, 

Tribal, and local agencies, non-government 

organizations, industry, academia, and other 

stakeholder groups. The assistance and suggestions of 

all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
iv Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

CONTENTS

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

Introduction .......................................................................................................................................................... 2

Quality-Control Sample Types............................................................................................................................. 6

Blank Samples............................................................................................................................................. 6

Field Blanks ....................................................................................................................................... 7

Equipment Blanks.............................................................................................................................. 7

Trip Blanks......................................................................................................................................... 7

Source Solution Blanks...................................................................................................................... 7

Ambient Blanks.................................................................................................................................. 7

Spiked Samples ........................................................................................................................................... 7

Replicate Samples ....................................................................................................................................... 8

Analysis....................................................................................................................................................... 8

Quality-Assurance and Quality-Control Design .................................................................................................. 11

Ground Water.............................................................................................................................................. 11

Blank Samples.................................................................................................................................... 11

Major Ions................................................................................................................................. 11

Dissolved Organic Carbon........................................................................................................ 13

Nutrients.................................................................................................................................... 14

Trace Elements.......................................................................................................................... 14

Arsenic............................................................................................................................. 16

Aluminum ........................................................................................................................ 16

Copper.............................................................................................................................. 16

Chromium ........................................................................................................................ 16

Cadmium.......................................................................................................................... 17

Barium ............................................................................................................................. 17

Pesticides in Filtered Water Analyzed by Gas Chromatography/Mass Spectrometry ............. 17

Pesticides in Filtered Water Analyzed by High Performance Liquid Chromatography........... 17

Volatile Organic Compounds ................................................................................................... 18

Replicate Samples.............................................................................................................................. 19

Surrogate Recovery............................................................................................................................ 22

Field Spiked Samples......................................................................................................................... 23

Pesticides in Filtered Water Analyzed by Gas Chromatography/Mass Spectrometry ............. 23

Pesticides in Filtered Water Analyzed by High Performance Liquid Chromatography........... 25

Volatile Organic Compounds ................................................................................................... 27

Surface Water.............................................................................................................................................. 27

Blank Samples.................................................................................................................................... 29

Major Ions................................................................................................................................. 29

Dissolved Organic Carbon........................................................................................................ 29

Suspended Organic Carbon....................................................................................................... 31

Nutrients.................................................................................................................................... 31

Trace Elements.......................................................................................................................... 32

Aluminum ........................................................................................................................ 32

Chromium, Copper, Manganese, Nickel, Zinc, and Iron................................................. 33

Total Mercury ........................................................................................................................... 33

Methylmercury.......................................................................................................................... 33

Pesticides in Filtered Water Analyzed by Gas Chromatography/Mass Spectrometry ............. 33
Contents v

Pesticides in Filtered Water Analyzed by High Performance Liquid Chromatography........... 33

Volatile Organic Compounds ................................................................................................... 33

Replicate Samples.............................................................................................................................. 33

Surrogate Recovery............................................................................................................................ 36

Field Spiked Samples......................................................................................................................... 37

Pesticides in Filtered Water Analyzed by Gas Chromatography/Mass Spectrometry ............. 37

Pesticides in Filtered Water Analyzed by High Performance Liquid Chromatography........... 39

Volatile Organic Compounds ................................................................................................... 41

Summary and Conclusion .................................................................................................................................... 44

References Cited .................................................................................................................................................. 45
vi Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

FIGURES

Figure 1. Map showing National Water-Quality Assessment ground-water sampling sites in the 

Sacramento River Basin, California .................................................................................................. 3

Figure 2. Map showing National Water-Quality Assessment surface-water sampling sites in the 

Sacramento River Basin, California .................................................................................................. 4

Figure 3. Graph showing boxplots of boron concentration in ground-water environmental samples and 

field blanks in the Sacramento River Basin, California, 1996–1998................................................. 13

Figure 4. Graph showing boxplots of dissolved organic carbon concentration in ground-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–1998............................ 13

Figure 5. Graph showing boxplots of ammonia concentration in ground-water environmental samples 

and field blanks in the Sacramento River Basin, California, 1996–1998.......................................... 15

Figure 6. Graph showing boxplots of aluminum concentration in ground-water environmental samples 

and field blanks in the Sacramento River Basin, California, 1996–1998.......................................... 17

Figure 7. Graph showing boxplots of copper concentration in ground-water environmental samples 

and field blanks in the Sacramento River Basin, California, 1996–1998.......................................... 17

Figure 8. Graph showing boxplots of trichloromethane (chloroform) concentration in ground-water 

environmental samples and field blanks in the Sacramento River Basin, California, 1996–1998.... 19

Figure 9. Graph showing boxplots of dissolved organic carbon concentration in surface-water 

environmental samples, field blanks, and source solution blanks in the Sacramento River Basin, 

California, 1996–1998 ....................................................................................................................... 31

Figure 10. Graph showing boxplots of suspended organic carbon concentration in surface-water 

environmental samples and field blanks in the Sacramento River Basin, California, 1996–1998.... 31

Figure 11. Graph showing boxplots of aluminum concentration in surface-water environmental samples 

and field blanks in the Sacramento River Basin, California, 1996–1998.......................................... 32
Tables vii

TABLES

Table 1. Critical constituents in the analysis and interpretation of ground- and surface-water data, 

Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998................... 5

Table 2. Analytical methods by constituent category used during the Sacramento River Basin, 

California, National Water-Quality Assessment, 1996–1998............................................................ 9

Table 3. Detections of critical constituents in ground-water field blank samples, Sacramento River 

Basin, California, National Water-Quality Assessment, 1996–1998 ................................................ 12

Table 4. Comparison of ammonia detections in ground-water samples and field blanks by study type, 

Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998................... 15

Table 5. Method reporting limit comparison between inductively coupled plasma/mass spectrometry 

analysis of ground-water environmental samples and low-level analysis of blank samples for 

selected constituents, Sacramento River Basin, California, National Water-Quality Assessment, 

1996–1998 ......................................................................................................................................... 16

Table 6. Variability of ground-water replicate samples collected during the Sacramento River Basin 

National Water-Quality Assessment, 1996–1998.............................................................................. 20

Table 7. Recovery of surrogate compounds in Sacramento River Basin, California, ground-water samples, 

1996–1998 ......................................................................................................................................... 23

Table 8. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, 

ground-water samples analyzed by gas chromatography/mass spectrometry, 1996–1998 ............... 24

Table 9. Spike recovery data for selected pesticides from Sacramento River Basin, California, 

ground-water samples, 1996–1998, and recovery and precision data published in the methods 

of analysis report................................................................................................................................ 25

Table 10. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, 

ground-water samples analyzed by high performance liquid chromatography, 1996–1998............. 26

Table 11. Spike recovery data for selected pesticides from Sacramento River Basin, California, 

ground-water samples, 1996–1998, and recovery and precision data published in the methods 

of analysis report................................................................................................................................ 27

Table 12. Relative percentage difference and recovery of field matrix spikes for volatile organic 

compounds in ground-water samples collected during the Sacramento River Basin, California, 

National Water-Quality Assessment, 1996–1998.............................................................................. 28

Table 13. Detections of critical constituents in surface-water field blank samples collected during the 

Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998................... 30

Table 14. Variability of surface-water replicate samples collected during the Sacramento River Basin, 

California, National Water-Quality Assessment, 1996–1998............................................................ 34

Table 15. Recovery of surrogate compounds in Sacramento River Basin, California, surface-water 

samples, 1996–1998........................................................................................................................... 36

Table 16. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, 

surface-water samples, 1996–1998, analyzed by gas chromatography/mass spectrometry .............. 38

Table 17. Spike recovery data for selected critical constituents from Sacramento River Basin, California, 

surface-water samples, 1996–1997, with recovery and precision data published in the methods 

of analysis report................................................................................................................................ 39

Table 18. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, 

surface-water samples, 1996–1998, analyzed by high performance liquid chromatography............ 40

Table 19. Spike recovery data for selected pesticides from Sacramento River Basin, California, 

surface-water samples, 1996–1998, and field matrix spike recovery and precision data 

published in the methods of analysis report....................................................................................... 41

Table 20. Recovery of field matrix spikes for volatile organic compounds from Sacramento River Basin, 

California, surface-water samples, 1996–1998.................................................................................. 42
viii Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS

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

°F = (1.8)°C + 32

VERTICAL DATUM

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 

1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both the 

United States and Canada, formerly called Sea Level Datum of 1929.

Abbreviations and Acronyms

μg/L microgram per liter

μS/cm microsiemen per centimeter

mg/L milligram per liter

DCP/AES direct current plasma-atomic emission spectrometry

DOC dissolved organic carbon

GC/MS gas chromatography/mass spectrometry

HPLC high performance liquid chromatography

ICP/AES inductively coupled plasma/atomic emission spectrometry

ICP/MS inductively coupled plasma/mass spectrometry

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment (Program)

NWQL National Water Quality Laboratory

RPD relative percentage difference

SOC suspended organic carbon

USGS U.S. Geological Survey

VOC volatile organic compound

Multiply By To obtain

centimeter (cm) 0.3937 inch

centimeter per year (cm/y) 0.3937 inch per year

cubic meter per day (m3/d) 0.000811 cubic foot per day

meter (m) 3.281 foot

meter per day (m/d) 3.281 foot per day

square kilometer (km2) 0.3861 square mile
Abstract 1

Quality-Control Results for Ground-Water and SurfaceWater Data, Sacramento River Basin, California, 

National Water-Quality Assessment, 1996–1998

By Cathy Munday and Joseph L. Domagalski

ABSTRACT

Evaluating the extent that bias and 

variability affect the interpretation of ground- and 

surface-water data is necessary to meet the 

objectives of the National Water-Quality 

Assessment (NAWQA) Program. Quality-control 

samples used to evaluate the bias and variability 

include annual equipment blanks, field blanks, 

field matrix spikes, surrogates, and replicates. This 

report contains quality-control results for the 

constituents critical to the ground- and surfacewater components of the Sacramento River Basin 

study unit of the NAWQA Program. A critical 

constituent is one that was detected frequently 

(more than 50 percent of the time in blank 

samples), was detected at amounts exceeding 

water-quality standards or goals, or was important 

for the interpretation of water-quality data. 

Quality-control samples were collected along with 

ground- and surface-water samples during the high 

intensity phase (cycle 1) of the Sacramento River 

Basin NAWQA beginning early in 1996 and 

ending in 1998.

Ground-water field blanks indicated 

contamination of varying levels of significance 

when compared with concentrations detected in 

environmental ground-water samples for 

ammonia, dissolved organic carbon, aluminum, 

and copper. Concentrations of aluminum in 

surface-water field blanks were significant when 

compared with environmental samples. Field 

blank samples collected for pesticide and volatile 

organic compound analyses revealed no 

contamination in either ground- or surface-water 

samples that would effect the interpretation of 

environmental data, with the possible exception of 

the volatile organic compound trichloromethane 

(chloroform) in ground water.

Replicate samples for ground water and 

surface water indicate that variability resulting 

from sample collection, processing, and analysis 

was generally low. Some of the larger maximum 

relative percentage differences calculated for 

replicate samples occurred between samples 

having lowest absolute concentration differences 

and(or) values near the reporting limit.

Surrogate recoveries for pesticides analyzed 

by gas chromatography/mass spectrometry 

(GC/MS), pesticides analyzed by high 

performance liquid chromatography (HPLC), and 

volatile organic compounds in ground- and 

surface-water samples were within the acceptable 

limits of 70 to 130 percent and median recovery 

values between 82 and 113 percent. The recovery 

percentages for surrogate compounds analyzed by 

HPLC had the highest standard deviation, 

20 percent for ground-water samples and 

16 percent for surface-water samples, and the 

lowest median values, 82 percent for ground-water 

samples and 91 percent for surface-water samples. 

Results were consistent with the recovery results 

described for the analytical methods.

Field matrix spike recoveries for pesticide 

compounds analyzed using GC/MS in ground- and 

surface-water samples were comparable with 

published recovery data. Recoveries of 

carbofuran, a critical constituent in ground- and 

surface-water studies, and desethyl atrazine, a 

critical constituent in the ground-water study, 
2 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

could not be calculated because of problems with 

the analytical method. Recoveries of pesticides 

analyzed using HPLC in ground- and surfacewater samples were generally low and comparable 

with published recovery data. Other 

methodological problems for HPLC analytes 

included nondetection of the spike compounds and 

estimated values of spike concentrations.

Recovery of field matrix spikes for volatile 

organic compounds generally were within the 

acceptable range, 70 and 130 percent for both 

ground- and surface-water samples, and median 

recoveries from 62 to 127 percent. High or low 

recoveries could be related to errors in the field, 

such as double spiking or using spike solution past 

its expiration date, rather than problems during 

analysis. The methodological changes in the field 

spike protocol during the course of the Sacramento 

River Basin study, which included decreasing the 

amount of spike solution added to volatile organic 

compound samples and changing the method of 

spike delivery, had no apparent effect on recovery 

results.

INTRODUCTION

The U.S. Geological Survey (USGS) began the 

National Water-Quality Assessment (NAWQA) 

Program in 1991. The goals of the NAWQA Program 

are to describe water-quality conditions and trends in a 

representative part of the nation's surface- and groundwater resources, and to identify the natural and human 

factors affecting the quality of these resources (Leahy 

and others, 1990).

In 1994, the Sacramento River Basin in northern 

California was among the NAWQA study units 

selected for implementation of the first cycle of the 

program. Three ground-water studies were undertaken 

between 1996 and 1998: a study of randomly selected 

wells in the southeast Sacramento Valley aquifer 

referred to as the Sacramento River Basin subunit 

survey (hereinafter referred to as subunit survey), an 

agricultural land-use study in rice growing areas, and 

an urban land-use study (fig. 1). The surface-water 

study began in February 1996 and ended in April 1998 

(fig. 2). Design details for ground- and surface-water 

studies for the NAWQA Program, including the 

Sacramento River Basin, are presented in Gilliom and 

others (1995).

As part of the NAWQA Program, surface- and 

ground-water samples were collected and analyzed for 

selected chemical constituents according to published 

protocols [Koterba and others (1995) for ground water; 

Shelton (1994) and Mueller and others (1997) for 

surface water]. Dissolved organic carbon (DOC) 

samples of ground water and surface water were 

collected using NAWQA protocol and processed using 

methods described by Alpers and others (2000). A 

study of mercury in surface water was incorporated 

into the Sacramento River Basin study unit plan. 

Collection and analysis of mercury samples in surface 

water followed protocols of the USGS Mercury 

Research Laboratory in Middleton, Wis. (Olson and 

DeWild, 1999). The water-quality and quality-control 

data collected during the Sacramento River Basin study 

area are given in Domagalski and others (2000). 

Additional infomation regarding the surface water 

quality study in the Sacramento River Basin during 

1996–1998 can be found in Domagalski and Dileanis 

(2000). Also available are interpretive analyses of 

pesticides in surface water (Domagalski, 2001) and 

analyses of ground-water data collected during the 

subuint survey (Dawson, 2001a) and the agricultural 

land-use study (Dawson, 2001b).

 Data obtained from field quality-control 

samples are used to estimate the bias and variability 

that result from sample collection, processing, and 

analysis. Bias refers to a systematic, consistent positive 

or negative deviation from the known or true value. 

Variability is random error in independent 

measurements, the result of repeated application of the 

process under specified conditions. Estimates of bias 

and variability were based on quality-control sample 

analysis for constituents considered critical to this 

study (table 1).
Introduction 3

38

30'

39

30'

39

122 121 30'

T10N

R2W R1E R5E

T5N T15N T20N

EXPLANATION

Agricultural Land-Use Study

(Rice areas)

Sacramento River Basin

Subunit Survey

Urban Land-Use Study

Ground-water wells

Quality-control samples

Public Land Survey System

Township/Range

Sacramento

Valley

Ground Water

Study Area

Sutter

Buttes

Agricultural Land-Use Study

(Rice areas)

Sacramento River Basin

Subunit Survey

Urban Land-Use Study

Feather River

Yuba River

Sacramento River

American Riv

Arcad

er

e Creek

Cache Cr eek

Putah Creek

Bear River

Figure 1. National Water-Quality Assessment ground-water sampling sites in the Sacramento River Basin, California.
4 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

39

40

123

41

122

121

Sacramento River above Bend 8

Bridge near Red Bluff

Sacramento River at Colusa

Yuba River at Marysville

Feather River near Nicolaus

Cache Creek near Rumsey

Colusa Basin Drain at Road 99E near West Sacramento

Sacramento Slough near Knights Landing

Sacramento River at Verona

Arcade Creek near Del Paso Heights

Yolo Bypass at Interstate 80 near West Sacramento

American River at Sacramento

Sacramento River at Freeport 0 20 40 Miles

0 20 40 Kilometers

7

2

1

4

3

Sampling Sites

EXPLANATION

5

6

9

10

11

12

Study area

Valley floor

Yolo Bypass

Colusa

Freeport

Knights Landing

Marysville

Nicolaus

Red Bluff

Rumsey

Verona

Del Paso Heights

Sacramento

Cache Creek

Putah Creek

Colusa Basin Drain

Feather River

Sacramento River

Yuba River

Bear River

American R iver

Pit River

1

2

3

4

5

6

7

8

9

10

11

12

Figure 2. National Water-Quality Assessment surface-water sampling sites in the Sacramento River Basin, California.
Introduction 5

Table 1. Critical constituents in the analysis and interpretation of ground- and surface-water data, Sacramento 

River Basin, California, National Water-Quality Assessment, 1996–1998

[A constituent was determined to be critical if it was detected in more than 50 percent of the blank samples, was 

detected in environmental samples at amounts exceeding water-quality standards or goals, or was otherwise 

important to the interpretation of water-quality data. GC/MS, gas chromatography/mass spectrometry; HPLC, high 

performance liquid chromatography] 

GROUND WATER

Major ions 

Calcium, magnesium, sodium, potassium, chloride, sulfate, boron, silica

Dissolved organic carbon

Nutrients

Nitrite plus nitrate, ammonia

Trace Elements

Arsenic, aluminum, copper, chromium, cadmium, barium

Pesticides analyzed by GC/MS

Atrazine, carbofuran, desethyl atrazine, molinate, simazine, thiobencarb

Pesticides analyzed by HPLC

Bentazon, carbofuran 

Volatile organic compounds

Trichloromethane (chloroform), methyl tert-butyl ether, trichloroethene

SURFACE WATER

Major ions

Calcium, magnesium, sodium, potassium, chloride, sulfate, silica

Dissolved organic carbon/suspended organic carbon 

Nutrients

Nitrite plus nitrate, phosphorus (total phosphorus, dissolved phosphorus, and orthophosphorus)

Trace elements

Aluminum, chromium, copper, manganese, nickel, zinc, iron

Mercury

Total mercury, methylmercury

Pesticides analyzed by GC/MS

Chlorpyrifos, carbofuran, diazinon, metolachlor, molinate, simazine, thiobencarb 

Pesticides analyzed by HPLC

Carbofuran, diuron 

Volatile organic compounds

Methyl tert-butyl ether 
6 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

A critical constituent is one that was detected 

frequently (greater than 50 percent of blank samples), 

was detected in environmental samples at amounts 

exceeding water-quality standards or goals, or was 

important to the interpretation of water-quality data. 

For example, DOC is a critical constituent because it 

can react with chlorine to form disinfection byproducts, 

most commonly trihalomethanes, which are dominated 

by trichloromethane (chloroform) (Thurman, 1985). 

Trihalomethanes are of concern to human heath and are 

regulated under the U.S. Environmental Protection 

Agency’s drinking water standards and health 

advisories (U.S. Environmental Protection Agency, 

2000). Nitrate also is considered a critical constituent 

because of its potential affect on human health when 

standards are exceeded. Phosphorus is an important 

component in aquatic health, whereas mercury has a 

potential affect on human health and is of particular 

interest in the Sacramento Valley because of its wide 

occurrence and distribution both from natural sources 

and as a remnant of gold and mercury mining. Mercury 

and the pesticides diazinon and chlorpyrifos are critical 

constituents because they are being considered for 

future regulation. The pesticides molinate, thiobencarb, 

and carbofuran are critical constituents because of 

ongoing regulatory controls.

This report describes and interprets field level 

quality-control data collected in the Sacramento River 

Basin during water years 1996–1998. Analysis and 

interpretation of laboratory quality-control results and 

performance evaluation for the USGS National Water 

Quality Laboratory (NWQL) is available online 

(U.S. Geological Survey, accessed October 8, 2001). 

The quality assurance manual for the USGS Wisconsin 

District Office’s Mercury Laboratory (U.S. Geological 

Survey, accessed October 15, 2001) provides 

information on laboratory quality-control results.

The authors thank the following USGS 

employees for their assistance in preparing and 

reviewing this report: Sharon Fitzgerald, 

Norman Spahr, Rick Iwatsubo, Gail Keeter, 

Glenn Schwegmann, Donna Knifong, Yvonne Roque, 

and Susan Davis.

QUALITY-CONTROL SAMPLE TYPES

Three types of field quality-control samples are 

routinely collected during NAWQA studies: blanks 

(field, source solution, ambient, and trip), field matrix 

spikes, and field replicates. Blanks and field matrix 

spikes estimate bias, and field replicates estimate 

variability. Equipment blanks, required annually, are 

collected in a laboratory setting rather than in the field 

to evaluate contamination introduced by samplecollection and sample-processing equipment. The 

number, type, and sites for quality-control sampling 

were chosen in accordance with published protocols 

(Koterba and others, 1995; Mueller and others, 1997). 

Information about the quality-control samples 

collected during the Sacramento River Basin study 

(appendix A-1 through A-7) include sample types, 

quantities, dates, and locations.

Surrogate compounds, which are added to all 

environmental, spike, and blank samples analyzed for 

pesticides and volatile organic compounds (VOC), help 

detect sample handling problems throughout the 

analytical processes. Surrogate compounds are similar 

in chemical properties to some of the target analytes, 

but are not expected in the environmental samples. 

Surrogates also can be used to evaluate matrix effects 

on analyte recovery when compared with recovery in 

reagent spike samples (Fitzgerald, 1997).

Blank Samples

A blank sample consists of water that has 

undetectable concentrations of measured constituents. 

Inorganic-grade deionized water used for the major 

ion, nutrient, and trace element blank samples was 

from the USGS Water Quality and Research 

Laboratory in Ocala, Fla. Pesticide-grade water for 

pesticide and DOC blank samples, and volatile-grade 

water for VOC blank samples, was from the NWQL in 

Denver, Colo., (Mueller and others, 1997). Blank water 

for mercury analysis was from the USGS Mercury 

Research Laboratory in Middleton, Wis. Blank 

samples were evaluated to determine any bias due to 

contamination introduced during sample collection, 

processing, shipping, or analysis. Once collected, the 

blank samples were processed and analyzed as a 

typical environmental water-quality sample.
Quality-Control Sample Types 7

Field Blanks

Field blanks, which were prepared at 

environmental sampling sites, help ensure that 

equipment had been adequately cleaned prior to sample 

collection. Field blanks also verify that onsite sample 

collection and processing, and sample handling and 

transport, had not introduced contamination (Mueller 

and others, 1997). For the surface-water study, blank 

water was passed through sampling and processing 

equipment at an environmental sampling site after the 

equipment had been used and field cleaned. The blank 

samples were collected in a manner similar to 

environmental water-quality sample collection 

procedures. For the ground-water study, the pump was 

placed in a clean 1,500-mL glass graduated cylinder. 

Blank water was then poured into the cylinder and 

pumped through the field-cleaned equipment. Field 

blank samples were collected onsite following groundwater sample collection and field cleaning.

Equipment Blanks

Equipment blanks evaluate contamination 

introduced during sample collection or by the 

processing equipment (Mueller and others, 1997) and 

confirm the effectiveness of cleaning procedures used 

prior to sampling. Blank water was poured through 

clean equipment routinely used for environmental 

sample collection and processing. The equipment 

blanks were collected and processed in the USGS 

California District laboratory, at the field office, or in 

the mobile laboratory and, therefore, were not subject 

to the ambient conditions associated with the 

environmental sampling sites.

Trip Blanks

Trip blanks identify contamination that might 

occur during sample transport, interim storage, and 

analysis rather than during sample collection and 

processing in the field (Mueller and others, 1997). The 

blanks were submitted for VOC analysis. The trip 

blanks were prepared by the NWQL using VOC grade 

water, shipped to Sacramento, and transported 

unopened to the field with other VOC bottles. They 

were then shipped back to NWQL with the 

environmental VOC samples for analysis.

Source Solution Blanks

Source solution blanks verify that blank water is 

contaminant-free prior to use as a field blank. The 

blanks were collected in a clean environment by 

placing the stock solution directly into the sample 

container.

Ambient Blanks

Ambient blanks identify any contaminants in the 

sampling and processing areas that might affect the 

environmental samples. One ambient blank sample was 

submitted for analysis of VOCs. The blank was 

collected by placing the stock solution directly into the 

VOC vials and exposing the open vials to the 

laboratory environment when the equipment blank was 

collected and processed.

Spiked Samples

Spiked samples measure bias caused by analyte 

degradation or sample matrix interference, or test the 

effects of sample matrix on the analyses of specific 

constituent groups. A spike is an environmental sample 

fortified with a known concentration of selected 

analytes. Pesticide and VOC samples were spiked and 

submitted for analysis. Ground-water pesticide 

samples, ground-water VOC samples, and surfacewater VOC samples were collected as sequential 

replicate sample sets. One of the samples was fortified 

with analysis-specific spike solution from the NWQL, 

and the other was designated as the environmental 

sample. Routinely, a third ground-water VOC sample 

was collected, spiked, and submitted as a spike 

replicate. Surface-water samples submitted for 

pesticide analysis were prepared by dividing a single 

volume of water into two subsamples. One of those 

subsamples was fortified with a spike solution 

appropriate to either the gas chromatography/mass 

spectrometry (GC/MS) or the high performance liquid 

chromatography (HPLC) analytical method and 

submitted to the NWQL along with the unfortified 

environmental subsample. (Mueller and others, 1997; 

Domagalski and others, 2000).
8 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Replicate Samples

Replicates measure the variability in water 

samples during sample collection, processing, and 

analysis. The samples are collected and processed so 

that the samples are virtually identical in composition. 

Split replicates were collected for surface-water 

pesticide samples and prepared by dividing a single 

volume of water into two subsamples. Sequential 

replicates, multiple samples collected at the same 

location, were collected for ground-water pesticide and 

VOC samples and for surface-water VOC samples.

Analysis

The NWQL analyzed all ground- and surfacewater samples for the Sacramento River Basin study, 

with the exception of total mercury and methylmercury 

analyses (table 2). The comprehensive quality-control 

program in place at the NWQL is outlined in Pirkey 

and Glodt (1998). Analytical data from the NWQL are 

presented in the following ways: 

1. A measured value.

2. A value preceded by a “<” (less than) 

annotation; this means the analyte was not 

detected at the laboratory at the method 

reporting limit.

3. A “U-delete” comment means that the value 

was determined to be invalid at the laboratory 

level and deleted from the database.

4. An “E” preceding a value for a pesticide or 

VOC compound is used to signify that a 

measured concentration is estimated by the 

NWQL (Connor and others, 1998; Childress 

and others, 1999).

When analyzing for VOCs, laboratory qualitycontrol procedures may include the dilution of samples 

prior to analysis. To minimize instrument 

contamination, samples are diluted when a selected 

compound is present at a concentration greater than the 

highest calibration standard. Samples that foam when 

shaken also will be diluted to prevent instrument 

malfunction; all of the VOC samples collected at the 

Arcade Creek site were diluted at the laboratory 

because of foaming. Samples containing hydrogen 

sulfide also should be diluted to prevent damage to 

analytical instruments. Analytical results from diluted 

samples are reported with raised reporting limits 

(Connor and others, 1998).

The USGS Mercury Research Laboratory 

conducted total mercury and methylmercury analyses. 

However, at the time of testing during this study, the 

analytical methods for total and methylmercury had not 

yet received approval by the U.S. Environmental 

Protection Agency and, therefore, data must be 

considered provisional. Information about the 

laboratory's quality-assurance plan is available online 

(U.S. Geological Survey, accessed October 15, 2001).
Quality-Control Sample Types 9

Table 2. Analytical methods by constituent category used during the Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 [N, nitrogen; P, phosphorus; CVAFS, cold vapor atomic fluorescence spectrometry] 

Constituent category Method of Analysis Reference

Inorganic compounds

Boron Inductively coupled plasma-atomic emission spectrometry Struzeski and others, 1996Bromide Colorimetry, automated-segmented flow, fluroscein Fishman and Friedman, 1989

Chloride, sulfate Ion chromatography Fishman and Friedman, 1989Fluoride Colorimetry, automated-segmented flow, ion-selective electrode Fishman and Friedman, 1989Potassium Atomic absorption, flame Fishman and Friedman, 1989Iron, magnesium, silica, sodium, calcium Inductively coupled plasma Fishman, 1993

Trace metals 

Arsenic and selenium Graphite furnace-atomic absorption spectrometry Jones and Garbarino, 1999

Cobalt, cadmium, nickel, beryllium, antimony, silver, 

barium, uranium (natural), zinc, molybdenum, lead, aluminum, chromium, copper, manganese

Inductively coupled plasma/mass spectrometry Faires, 1993

Low level blank sample analysis

Inorganic compounds: Major ions and trace metalsArsenic, selenium (added to analysis) See Trace metals above - Arsenic and selenium were added as “lab codes” and 

analyzed under the “trace metals” methodology

Jones and Garbarino, 1999

Zinc, chromium, beryllium, barium, manganese, 

molybdenum, cadmium, cobalt, silver, antimony, copper, uranium (natural), lead 

Inductively coupled plasma/mass spectrometry Faires, 1993

Magnesium, silica, iron, sodium, boron, calcium Inductively coupled plasma Fishman, 1993Aluminum, nickel Inductively coupled plasma/mass spectrometry Faires, 1993

Nutrient compounds

Phosphorus (total) Low level total phosphorus, filtered U.S. Environmental Protection Agency, 1993

Phosphorus, phosphate, dissolved phosphorus, 

orthophosphorus

Colorimetry, automated-segmented flow, phosphomolybdate, P, 

orthophosphate as P

Fishman, 1993

Nitrogen, ammonia + organic nitrogen Colorimetry, automated-segmented flow, Microkjeldahl digestion, N, 

ammonia + organic nitrogen as N

Fishman, 1993

Nitrogen, nitrite Colorimetry, automated-segmented flow, N, nitrite as N, Fishman, 1993Nitrogen, ammonia Colorimetry, automated-segmented flow, salicylate-hypochlorite, N, 

ammonia

Fishman, 1993
10 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 Table 2. Analytical methods by constituent category used during the Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 —Continued Nitrogen, nitrite + nitrate Colorimetry, automated-segmented flow, cadmium reduction– diazotization, N, nitrite + nitrate, N Fishman, 1993 Dissolved organic carbon Organic carbon Unacidified uv-promoted persulfate oxidation and infrared spectrometry Brenton and Arnett, 1993 Pesticides Gas chromatography/mass spectrometry with selected ion monitoring Zaugg and others, 1995 Pesticide (acetechlor only) Automated solid-phase extraction and gas chromatography with mass- selective detection Lindley and others, 1996 Pesticides High-performance liquid chromatography Werner and others, 1996 Volatile organic compounds Gas chromatography/mass spectrometry Connor and others, 1998 Total mercury and methylmercury Total mercury U.S. Environmental Protection Agency Method 1631:Mercury in water by oxidation, purge and trap, and CVAFS with modifications Olson and DeWild, 1999 Methylmercury U.S. Environmental Protection Agency Method 1630: Methylmercury in water by distillation, aqueous ethylation, purge and trap, and CVAFS with minor modifications Olson and DeWild, 1999 Constituent category Method of Analysis Reference
Quality-Assurance and Quality-Control Design 11

QUALITY-ASSURANCE AND QUALITYCONTROL DESIGN

Ground Water

The ground water study had three components: a 

subunit survey, an agricultural land-use study, and an 

urban land-use study. For the subunit survey, 

31 shallow, domestic wells were sampled from May 

through August 1996; most were privately owned. 

Wells were selected randomly in both agricultural and 

urban settings. Equipment blank samples were 

collected and processed at the field office prior to the 

initial environmental sampling. In addition, qualitycontrol samples were collected at 3 of the 

31 environmental sites.

For the agricultural land-use study, 30 shallow 

observation wells were drilled at randomly selected 

sites in the rice growing regions of the Sacramento 

Valley, of which 28 were sampled from July through 

October 1997. Prior to sampling the first environmental 

site, equipment blank samples were collected and 

processed at the field office. Quality-control samples 

also were collected at 4 of the 28 environmental 

sampling sites.

To assess the effects of recent urbanization on 

shallow ground water, 19 shallow observation wells 

were drilled in the Sacramento metropolitan area at 

sites randomly chosen in areas developed between 5 to 

25 years ago. The wells were sampled from June 

through August 1998. Equipment blank samples were 

collected and processed at the field office prior to 

sampling the urban wells. Quality-control samples 

were collected from 4 of the 19 urban wells.

The collection and processing of all groundwater quality-control samples followed published 

protocols (Koterba and others, 1995). Corrective action 

was taken when quality-control samples indicated the 

introduction of systematic contamination during 

sample collection and(or) processing. Environmental 

and quality-control data for the Sacramento River 

Basin study are presented in Domagalski and others 

(2000).

Blank Samples

Results of field blank sample analyses for critical 

constituents in the ground-water study are given in 

table 3.

Major Ions

Two equipment blanks and 11 field blanks 

(table 3) were submitted for analysis of major ions and 

trace metals between May 23, 1996, and August 19, 

1998. Sodium, potassium, chloride, and sulfate were 

undetectable in all equipment and field blank samples, 

but were detected in every environmental sample. Of 

the two equipment blanks, calcium was detected in 

both, magnesium was detected in neither, and silica 

was detected in one. Calcium, magnesium, and silica 

were detected in fewer than 50 percent of the field 

blank samples (table 3), but were detected in all 

78 environmental samples (Domagalski and others, 

2000). At least 2 orders of magnitude separate the 

highest concentration of calcium, magnesium, or silica 

in the blank samples from the lowest value reported in 

ground-water samples; therefore, there is no indication 

of bias in the analytical method that would affect data 

for these major ions.

Boron was detected in 5 of the 11 (45 percent) 

field blanks, but in neither of the 2 equipment blanks 

(table 3). The method reporting limit changed three 

times during the study. Until June 1, 1996, the direct 

current plasma/atomic emission spectrometry 

(DCP/AES) method was used, which has a reporting 

limit of less than 10 μg/L. After that date, the 

inductively coupled plasma/atomic emission 

spectrometry (ICP/AES) technique was used, which 

has a reporting limit of less than 4 μg/L 

(U.S. Geological Survey, 1996). Finally, the systematic 

evaluation of reporting levels for NWQL methods 

resulted in a change of the minimum reporting level for 

boron analysis by ICP/AES from 4 to 16 μg/L, 

effective December 22, 1997 (U.S. Geological Survey, 

1997).

Boron was detected in all 78 of the 

environmental samples (Domagalski and others, 2000); 

concentrations ranged from 13 to 1,790 μg/L. Boxplot 

analysis (fig. 3) and the Mann–Whitney statistical test 

of the median values were used to compare blank and 

environmental sample results. Boxplots indicate no 

significant overlap between blank and environmental 

data. The Mann–Whitney statistical test also shows that 

the medians of the environmental and blank data are 

dissimilar (p=0.0001). Thus, the environmental data 

can be used without qualification.
12 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 3. Detections of critical constituents in ground-water field blank samples, Sacramento River Basin, California, National Water-Quality 

Assessment, 1996–1998

[All data are given in concentration units. Changes in the method reporting limit (MRL) occurred during the course of the data collecton for some of 

the analyses as indicated by multiple values. Number of significant figures do not reflect analytical MRLs. D, detections expressed as percentages; 

maximum, maximum observed value or concentration; median, median observed value or concentration; N, nitrogen. mg/L, milligram per liter; 

μg/L, microgram per liter; <, less than; E-value, laboratory estimated result; NA, not applicable] 

Constituent or compound Number of field 

blank samples D

Concentration

MRL(s) Maximum Median

Major ions

Calcium (mg/L) 11 36 0.011 0.062 <0.020

Magnesium (mg/L) 11 18 0.010, 0.004 0.016 <0.005

Sodium (mg/L) 11 0 0.20, 0.10

Potassium (mg/L) 11 0 0.1

Chloride (mg/L) 11 0 0.1

Sulfate (mg/L) 11 0 0.1

Silica (mg/L) 11 45 0.01, 0.1 0.045 0.04

Boron (μg/L) 11 45 4, 10, 16 11 7.80

Dissolved Organic Carbon (mg/L as carbon) 11 91 0.1 1.6 0.30

Nutrients

Ammonia (mg/L as N) 11 64 0.015, 0.020 0.098 0.025

Nitrite (NO2) + nitrate (NO3) (mg/L as N) 11 36 0.05 0.085 <0.050

Trace Elements

Arsenic (μg/L) 8 0 1

Aluminum (μg/L) 8 100 0.3 4 3.55

Copper (μg/L) 8 62 0.2 0.65 0.28

Chromium (μg/L) 8 25 0.2 0.89 <0.20

Cadmium (μg/L) 8 12 0.3 0.42 <0.30

Barium (μg/L) 8 75 0.2 0.60 0.28

Pesticides

Analyzed by gas chromatography/mass spectrometry 

Atrazine (μg/L) 11 NA 0.001 1 E-value only

Carbofuran (μg/L) 11 0 0.003

Desethyl atrazine (μg/L) 11 0 0.002

Molinate (μg/L) 11 0 0.004

Simazine (μg/L) 11 0 0.005

Thiobencarb (μg/L) 11 0 0.002

Analyzed by high performance liquid chromatography 

Bentazon (μg/L) 11 NA 0.014 1 E-value only

Carbofuran (μg/L) 11 0 0.0280; 0.120

Volatile Organic Compounds

Trichloromethane (Chloroform) (μg/L) 7 NA 0.052 2 nondetects; 5 E-values

Methyl tert-butyl ether (μg/L) 7 0 0.1, 0.112, 0.166

Trichloroethene (μg/L) 7 0 0.050, 0.038
Quality-Assurance and Quality-Control Design 13

The boron in blank samples probably was 

solubilized from the borosilicate glass ampoules that 

contained the nitric acid used to preserve water 

samples. To alleviate this problem, beginning in 

September 1998, nitric acid has been dispensed in 

polypropylene vials (U.S. Geological Survey, 1998).

Dissolved Organic Carbon

Of the 28 blank samples submitted for DOC 

analysis, 3 were equipment blanks and 11 were field 

blanks. A source solution blank was submitted with 

each blank, for a total of 28 samples. DOC was not 

detected at the reporting limit of 0.1 mg/L (as carbon) 

in 10 of the 11 source solution blanks. One source 

solution blank had a reportable detection of 0.2 mg/L, 

just above the reporting limit. These results effectively 

eliminate organic-free water as the source of 

contamination and show the laboratory analytical 

method to be free of positive bias. However, DOC was 

detected in 10 of the 11 (91 percent) field blanks 

(table 3) at concentrations from 0.2 to 1.6 mg/L, and in 

74 of the 78 environmental samples (Domagalski and 

others, 2000), with more than 50 percent of detections 

in the environmental samples at 0.6 mg/L or less. 

Boxplot analysis of the ground-water environmental 

samples and the field blanks (fig. 4) reveals overlap 

between the data sets (p=0.0411, Mann–Whitney 

statistical test).

Although the data make the majority of 

environmental DOC values suspect, qualifying the 

environmental data may not be necessary when taking 

field procedures into consideration. DOC 

contamination in the field blank samples is likely due 

to cleaning methods that used other organic 

constituents to prevent carryover contamination 

between sampling sites. The soap and methanol used to 

clean sampling equipment could have contributed 

DOC in detectable amounts. Sampling equipment, 

including about 61 m of tubing, was cleaned according 

to NAWQA protocol at each site following groundwater sampling. After field cleaning, if a field blank 

was to be collected, 4 liters of organic-free water was 

pumped through the lines to flush the tubing of any 

residual cleaning media. It is possible that more 

organic-free water is needed to effectively purge 

Figure 3. Boxplots of boron concentration in ground-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–

1998.

200

0

400

600

800

1,000

1,200

1,400

1,600

1,800

Boron, in micrograms per liter

Environmental

Samples

Field

Blanks

(78)

(11)

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples

Figure 4. Boxplots of dissolved organic carbon concentration in groundwater environmental samples and field blanks in the Sacramento River 

Basin, California, 1996–1998.

0.0

0.75

1.5

2.25

3.0

3.75

4.5

5.25

6.0

6.75

7.5

8.25

Dissolved organic carbon as carbon, in milligrams per liter

Environmental

Samples

Field

Blanks

(11)

(78) 75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples
14 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

cleaning media residue. While other sources of 

contamination, including processing equipment, cannot 

be completely eliminated, review of these data suggests 

that blank sample collection procedures rather than 

processing is the source of contamination. Insufficient 

purging is not considered an issue with environmental 

samples because a minimum of three well casing 

volumes of water are pumped through the sampling 

lines and tubing prior to sample collection. Although 

the environmental data for DOC were not qualified or 

deleted from the database, and the source of 

contamination in blank samples has probably been 

identifed, environmental data should be used with 

caution in interpretive analyses; for example, Dawson 

(2001a, p. 9) did not use DOC data for analytical 

purposes and stated, “DOC in field blanks indicate that 

ground-water sample concentrations measured below 

1.8 mg/L in this study may be partly or entirely due to 

sample contamination introduced during sample 

collection or analysis” (Dawson, 2001b, p. 17).

Nutrients

Fourteen blank samples—3 equipment blanks 

and 11 field blank samples—were analyzed for 

nutrients, including ammonia and nitrite plus nitrate. 

The frequency and magnitude of ammonia detections 

in blanks indicate contamination that would affect the 

interpretation of ground-water data. Ammonia was 

detected in 2 of the 3 equipment blanks and in 7 of the 

11 (64 percent) field blanks. The median concentration 

of ammonia in field blank samples is greater than the 

concentration reported for 46 of the 78 (about 

59 percent) ground-water samples (Domagalski and 

others, 2000). The boxplot comparison of the analytical 

results for ammonia in the ground-water environmental 

samples and field blanks is shown on figure 5. The 

Mann–Whitney statistical test results (p=0.9630) 

indicate that there is no significant difference between 

the environmental and blank data sets. There are 

temporal and land-use pattern variations (table 4) in the 

number of detections for ammonia in both the groundwater samples and field blanks, with the highest 

number of detections reported during the urban study 

of 1998. The source of ammonia contamination has not 

been determined. Environmental data for ammonia 

were not deleted from the database; however, the 

quality-control data indicate that ammonia in the 

environmental samples may be due wholly, or in part, 

to contamination as indicated by the ‘V’ code applied 

to those values in the database. Because of 

contamination in the blank samples, ground-water 

ammonia data were not used in either report published 

by Dawson (2001a,b).

Analysis of blank samples for nitrite plus nitrate 

(NO2 + NO3) indicates an absence of systematic 

contamination. Although nitrite plus nitrate was 

detected in 4 of the 11 (36 percent) field blanks at 

levels ranging from 0.05 to 0.085 mg/L (table 3), it was 

in 66 of the 78 (85 percent) environmental samples at 

concentrations ranging from 0.06 to 12 mg/L. The 

majority of those environmental detections (43 of 78) 

were at levels exceeding 0.86 mg/L (Domagalski and 

others, 2000). Because nitrite plus nitrate was detected 

in fewer than 50 percent of the blank samples, neither a 

boxplot nor statistical analyses were applied to the 

data. Environmental data may be used without 

qualification.

Trace Elements

Blank samples were submitted for arsenic 

analysis by graphite furnace-atomic absorption (Jones 

and Garbarino, 1999) at the same method reporting 

limit as the environmental samples (1.0 μg/L). 

Aluminum, cadmium, copper, chromium, and barium 

samples were analyzed by an inductively coupled 

plasma/mass spectrometry (ICP/MS) method used 

specifically to detect low levels of contamination in 

blank samples (Faires, 1993); low-level analysis was 

not available for arsenic. During the subunit survey of 

1996, an equipment blank was submitted, but no 

environmental trace element samples were collected. 

During the rice land-use study of 1997 and the urban 

land-use study of 1998, equipment blanks and field 

blank samples were submitted. Ground-water samples 

were submitted for trace metal analysis during the 1997 

and 1998 studies. The method reporting limits for lowlevel blank sample analysis were 3 to 5 times lower 

than those for environmental samples analyzed by 

ICP/MS (Faires, 1993). The method reporting limit 

comparison between ICP/MS environmental sample 

analysis and ICP/MS low-level blank sample analysis 

is given in table 5.
Quality-Assurance and Quality-Control Design 15

0.000

0.075

0.150

0.225

0.300

0.375

0.450

0.525

0.600

0.675

0.750

0.825

Ammonia as nitrogen, in milligrams per liter

Environmental

Samples

Field

Blanks

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples (78)

(11)

Figure 5. Boxplots of ammonia concentration in ground-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–1998.

Table 4. Comparison of ammonia detections in ground-water samples and field blanks by study type, Sacramento River Basin, California, National WaterQuality Assessment, 1996–1998

[mg/L, milligram per liter] 

Study (method reporting limit) 

 Ground-water samples Field blanks

Number of 

samples

Number of 

detections

Detections 

in percent

Number of 

samples

Number of 

detections

Detections in 

percent

1996 Subunit Survey (0.015 mg/L) 31 24 74 5 3 60

 

1997 Agricultural Land-Use Study [rice areas (0.015 mg/L)] 28 10 36 5 1 20

1998 Urban Land-Use Study (0.02 mg/L) 19 17 89 5 5 100
16 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 5. Method reporting limit comparison between inductively 

coupled plasma/mass spectrometry analysis of ground-water 

environmental samples and low-level analysis of blank samples for 

selected constituents, Sacramento River Basin, California, National WaterQuality Assessment, 1996–1998

[All reporting limits are in microgram per liter (μg/L). Blank samples were 

analyzed for arsenic at 1.0 μg/L. NA, low-level analysis not available for 

arsenic blanks] 

Arsenic

Arsenic (1 μg/L detection limit) was not detected 

in the blank samples (table 3), but was detected in 75 of 

78 ground water samples at concentrations ranging 

from 1 to 62 μg/L (Domagalski and others, 2000). 

Environmental data may be used without qualification.

Aluminum

Ground-water samples may have been affected 

by aluminum contamination, which was detected in the 

field blank samples. All eight of the field blank 

samples (table 3) had aluminum at concentrations 

ranging from 2.5 to 4 μg/L. Ground-water samples 

were not analyzed for aluminum in 1996 during the 

subunit survey, but were tested for aluminum and other 

trace elements during the rice land-use and urban landuse studies undertaken in 1997 and 1998, respectively. 

All ground-water samples contained aluminum ranging 

from 2.2 to 10.37 μg/L, with 17 of 47 (36 percent) of 

the detections below 4 μg/L and within the range of the 

blank sample results (Domagalski and others, 2000).

A comparison of aluminum data for field blanks 

and environmental samples is shown in figure 6. 

Although the boxplot shows some overlap between the 

environmental and blank data sets, the Mann–Whitney 

statistical test (p=0.0269) shows the medians of the 

data sets to be dissimilar. The aluminum detected in 

field blank and ground-water samples may have been 

solubilized from glass ampoules containing the nitric 

acid preservative used in sample processing 

(U.S. Geological Survey, 1998); nitric acid 

preservative has been dispensed from polypropylene 

vials since September 1998 to eliminate the 

solubilization problem. Environmental data for 

aluminum were not deleted from the database; 

however, the quality-control data indicate that some 

concentrations of aluminum may be due wholly, or in 

part, to contamination as indicated by the ‘V’ code 

applied to those values in the database. Because of 

contamination in the blank samples, ground-water 

aluminum data were not used by Dawson (2001a) for 

interpretive analysis.

Copper

Low-level analysis detected copper (0.2 μg/L 

detection level) in 5 of the 8 (62 percent) field blank 

samples at concentrations ranging from 0.28 to 

0.65 μg/L (table 3). Environmental samples contained 

copper concentrations (1.0 μg/L detection level) 

ranging from 1.0 to 8.6 μg/L for 16 wells, and 

nondetections for the remaining 31 (Domagalski and 

others, 2000). Although the source of copper 

contamination in the blank samples is unknown, 

detections of copper in blank samples do not appear to 

affect the validity of the environmental data. Results of 

the Mann–Whitney statistical test show that the 

medians of the environmental and blank data are 

dissimilar (p=0.0090). A boxplot comparison of field 

blanks and environmental samples data for copper is 

shown in figure 7. Environmental data for copper were 

not qualified or deleted from the database, and 

environmental data can be considered without 

qualification.

Chromium

Blank sample analysis does not provide evidence 

of systematic chromium contamination that affected 

ground-water samples. Chromium was detected in 2 of 

the 8 (25 percent) field blank samples (table 3) at 0.72 

and 0.89 μg/L, but was in 46 of the 47 environmental 

samples at levels ranging from 1.8 to 16 μg/L 

(Domagalski and others, 2000). Environmental data for 

chromium can be considered without qualification.

Environmental 

samples

Blank 

samples

Arsenic 1.0 NA

Aluminum 1.0 0.3

Copper 1.0 0.2 

Chromium 1.0 0.2 

Cadmium 1.0 0.3 

Barium 1.0 0.2 
Quality-Assurance and Quality-Control Design 17

Cadmium

Blank samples do not provide evidence of 

cadmium contamination affecting ground-water 

samples. Cadmium was detected in only 1 of the 8 

(12 percent) blank samples at 0.42 μg/L and in 3 of the 

47 environmental samples at concentrations ranging 

from 6.1 to 7.4 μg/L (Domagalski and others, 2000). 

Environmental data for cadmium can be considered 

without qualification.

Barium

Barium was reported in 1 of the 3 equipment 

blanks and in 6 of the 8 (75 percent) field blanks 

(table 3) at concentrations ranging from 0.22 to 

0.6 μg/L. Barium was detected in all ground-water 

samples at concentrations ranging from 10 to 

5,050 μg/L (Domagalski and others, 2000). Because 

there is more than an order of magnitude between the 

highest barium concentration in a field blank and the 

lowest barium concentration in an environmental 

sample, ground-water barium data can be considered 

unaffected by contamination and useful without 

qualification.

Pesticides in Filtered Water Analyzed by Gas 

Chromatography/Mass Spectrometry

Three equipment blanks and 11 field blanks were 

submitted to the NWQL for analysis. Blank sample 

analysis provided no evidence of contamination for any 

of the 47 analytes, including the critical constituents 

atrazine, carbofuran, desethyl atrazine, molinate, 

simazine, and thiobencarb. Data can be used without 

qualification.

Pesticides in Filtered Water Analyzed by High Performance 

Liquid Chromatography

Three equipment blanks and 11 field blanks were 

analyzed for 39 pesticide compounds by the NWQL. 

Blank sample analysis provided no evidence of 

Figure 6. Boxplots of aluminum concentration in ground-water 

environmental samples and field blanks in the Sacramento River Basin, 

California, 1996–1998.

0.00

1.25

2.50

3.75

5.00

6.25

7.50

8.75

10.00

11.25

12.50

Aluminum, in micrograms per liter

Environmental

Samples

Field

Blanks

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples

(47)

(8)

Figure 7. Boxplots of copper concentration in ground-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–1998.

0

1

2

3

4

5

6

7

8

9

Copper, in micrograms per liter

Detection limit for environmental

samples only

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples

Environmental

Samples

Field

Blanks

(8)

(47)
18 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

contamination for any of the compounds, including the 

critical constituents bentazon and carbofuran. Data can 

be used without qualification.

Volatile Organic Compounds

Ground-water samples were analyzed for 

85 VOCs during the subunit survey of 1996 and the 

urban study of 1998. The 12 blank samples included 

2 equipment blanks, 2 trip blanks, 1 ambient blank, and 

7 field blanks. Critical constituents include 

trichloromethane (chloroform), methyl tert-butyl ether 

(MTBE), and trichloroethene.

Trichloromethane was reported as a low-level 

estimated value (E0.030 to E0.050 μg/L) in the 

equipment blank, ambient blank, and the three field 

blank samples submitted in 1996 during the subunit 

survey. Only the trip blank from 1996 had a 

nondetection result for trichloromethane. The 

equipment blank collected prior to the urban land-use 

study of 1998 had the highest trichloromethane 

concentration (0.334 μg/L) (Domagalski and others, 

2000) and the only nonestimated detection reported 

among any of the blank samples. Two of the four 

subsequent field blanks submitted during the 1998 

study had estimated low-level concentrations of 0.064 

and 0.006 μg/L. Trichloromethane was not detected in 

the other two field blanks or in the trip blank sample 

during the 1998 study.

The most likely source of trichloromethane in 

blank samples is the blank water itself or residual 

contamination from chlorinated tap water used during 

the initial rinse of the sampling lines. Blank water is 

suspected because, although it was not labeled at the 

time, nitrogen-purged VOC blank water has a 

recommended shelf life of only 2 weeks from the 

nitrogen purge date. The water used in the equipment 

blank submitted on June 5, 1998, which had the highest 

reported trichloromethane concentration in a blank 

sample at 0.334 μg/L, was laboratory certified as 

nitrogen purged on March 27, 1998.

Contamination from tap water also is suspected 

because, following the review of the June 5, 1998, 

equipment blank results and subsequent elimination of 

tap water from the cleaning protocol, trichloromethane 

was not detected in field blank samples submitted on 

July 22 or August 19, 1998. An estimated 

trichloromethane value was reported for the blank 

sample submitted on August 6, 1998, but at an 

estimated concentration of 0.006 μg/L it was the lowest 

value reported for any blank or environmental sample 

collected during the subunit survey of 1996 or urban 

land-use study of 1998.

Boxplot comparison (fig. 8) of the field blanks 

and environmental samples show an overlap in 

concentration between the two data sets, and the 

Mann–Whitney statistical test shows similarity 

between their median values (p=0.4852). The effect of 

this contamination on trichloromethane concentrations 

in environmental samples is unclear, however, because 

the compound was estimated or detected in only 3 of 

the 31 subunit survey wells at concentrations ranging 

from 0.03 (estimated) to 1.1μg/L (Domagalski and 

others, 2000). In addition, of the 19 urban land-use 

wells sampled, five trichloromethane detections ranged 

from 0.119 to 5.05 μg/L and estimated values for 

11 wells ranged from 0.010 to 0.14 μg/L. Although 

environmental data was not qualified, the data should 

be used with caution.

There were no detections of MTBE or 

trichloroethene in any of the blank samples. Reporting 

limits changed for both of these compounds and for 

trichloromethane during the course of the study, as 

indicated by different “less-than” values in the groundwater blank sample data (Domagalski and others, 

2000). Environmental data for both trichlorethene and 

MTBE can be used without qualification.
Quality-Assurance and Quality-Control Design 19

Replicate Samples

The variability of ground-water replicate 

samples in the Sacramento River Basin NAWQA, 

represented as the relative percentage difference (RPD) 

and as the absolute difference in concentration units, is 

given in table 6. The RPD is calculated as the absolute 

difference between values of the replicate pair divided 

by their average value and multiplied by 100. For 

calculation purposes, if a less-than value result was 

paired with a detection, 50 percent of the method 

reporting limit value was used in place of the less-than 

value. The number of detections varied among the 

critical constituents; for example, arsenic was detected 

in all 8 replicate sets (16 samples), whereas cadmium 

replicate results included only 3 detections 

(16 samples).

The median RPD for critical constituents in 

ground water ranged from 0 percent for calcium, 

magnesium, sodium, sulfate, silica, ammonia, arsenic, 

and cadmium to 17 percent for aluminum. The 

maximum RPD ranged from 3.4 percent for silica 

(detections in all samples) to 170 percent for cadmium. 

Other constituents that have maximum RPDs greater 

than 100 include boron (detections in all samples), 

nitrite plus nitrate (1 set of nondetects and 1 set with 

only one detection out of 11 sets), copper (4 sets of 

nondetects and 1 split set out of 8 sample sets), and 

chromium (detections in all samples) (table 6).

The maximum RPD of 170 percent for cadmium 

(table 6) was based on only three detections out of 

16 samples: One replicate pair had cadmium 

concentrations of 7.1 and 1.3 μg/L, and another pair 

had 6.1 μg/L of cadmium detected in the 

environmental sample, but no detectable concentration 

in the replicate (Domagalski and others, 2000). The 

reason for the variability of the ground-water cadmium 

data is unknown because so few detections of the 

compound were in both the replicate sample sets and in 

the environmental ground-water samples. Cadmium 

samples were not submitted during the subunit survey 

of 1996. Cadmium was detected in only 3 of the 

28 samples submitted during the rice land-use study of 

1997, but was not in any of the 19 samples submitted 

during the urban land-use study of 1998 (Domagalski 

and others, 2000).

Trichloromethane, in micrograms per liter

Environmental

Samples

Field

Blanks

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

(n) Number of samples

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08 (50)

(7)

Figure 8. Boxplots of trichloromethane (chloroform) concentration in ground-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–1998. Outliers are not 

shown.
20 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 Table 6. Variability of ground-water replicate samples collected during the Sacramento River Basin National Water-Quality Assessment, 1996–1998 [If one of the 2 results for a replicate pair was reported as a “<“ (less than the reporting limit) value, a value equal to 1/2 the reporting limit was used to complete the calculations used for this table. Number of significant figures do not reflect analytical method reporting levels. NWQL, National Water Quality Laboratory. mg/L, milligram per liter; μg/L, microgram per liter] Analyte (number of detections) Number of sets Relative difference, in percent Difference, in concentration units Minimum Maximum Median Minimum Maximum Median Major ions Calcium (mg/L) (detections in all samples) 11 0 6.5 0 0 10 0 Magnesium (mg/L) (detections in all samples) 11 0 6.9 0 0 2 0 Sodium (mg/L) (detections in all samples) 11 0 6.9 0 0 3 0 Potassium (mg/L) (detections in all samples) 11 0 18 3.1 0 0.2 0.02 Chloride (mg/L) (detections in all samples) 11 0 30 1.7 0 31 1 Sulfate (mg/L) (detections in all samples) 11 0 18 0 0 300 0 Silica (mg/L) (detections in all samples) 11 0 3.4 0 0 2 0 Boron (μg/L) (detections in all samples) 11 0 109 4.7 0 65 3 Dissolved organic carbon (mg/L as carbon) (detections in all samples) 11 0 67 10.7 0 0.2 0.07 Nutrients Ammonia (mg/L as nitrogen) (4 sets of nondetects out of 11 sample sets) 11 0 30 0 0 0.07 0.01 Nitrite (NO2) + nitrate (NO3) (detections in mg/L as nitrogen) [1 set of nondetects; 1 split set (1 of the 2 samples has a nondetect result)] 11 0 117 0.5 0 1 0.02 Trace elements Arsenic (μg/L) (detections in all samples) 8 0 5.7 0 0 2 0 Aluminum (μg/L) (detections in all samples) 8 2.5 88 17 0.1 4.9 0.95 Copper (detections in μg/L) [4 sets of nondetects; 1 split set (1 of the 2 samples has a nondetect result)] 8 0 150 3.6 0 3 0.05 Chromium (μg/L) (detections in all samples) 8 0 114 7.5 0 8.7 0.5 Cadmium (detections in μg/L) [6 sets of nondetects; 1 split set (1 of the 2 samples has a nondetect result)] 8 0 170 0 0 5.8 0 Barium (detections in all samples in μg/L) 8 0 8 2.7 0 13 1.5 Pesticides analyzed by gas chromatography/mass spectrometry No replicates collected
Quality-Assurance and Quality-Control Design 21

Table 6. Variability of ground-water replicate samples collected during the Sacramento River Basin National Water-Quality Assessment, 1996–1998—Continued

Pesticides analyzed by high performance liquid chromatography No replicates collected

Volatile Organic Compounds No replicates collectedAlkalinity as CaCO3 (calcium carbonate): Comparison of field alkalinity values (mg/L)

Sacramento River Basin Subunit Survey 1996 29 0 10 1 0 9 2Rice Land-Use Study 1997 20 0 19 1 0 87 3.5Urban Land-Use Study 1998 19 0 15 1 0 17 3

Specific conductance: NWQL values versus field values (microsiemens per centimeter at 25 degrees Celsius)

Sacramento River Basin Subunit Survey 1996 31 0 17 1 0 57 6Rice Land-Use Study 1997 28 0 10 3 0 1200 34Urban Land-Use Study 1998 19 0 10 2 1 320 10

Analyte (number of detections) Number of sets Relative difference, in percent Difference, in concentration units Minimum Maximum Median Minimum Maximum Median
22 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

The maximum RPD of 109 percent for boron 

(table 6) is the result of one anomalous sample set in 

which boron was reported at 27 μg/L in the 

environmental sample and 92 μg/L in the replicate 

(Domagalski and others, 2000). While the reason for 

the anomalous result is unknown, the majority of the 

boron replicate samples indicate that environmental 

data for boron are not affected by variability in the 

analytical method. The median RPD calculated for 

boron in the 11 replicate sample sets was 4.7 percent.

Similarly, the maximum RPD of 117 percent for 

nitrite plus nitrate (table 6) is not indicative of 

variability in the analytical method that would affect 

the interpretation of environmental data. The replicate 

set resulting in this RPD had a nondetection result for 

nitrite plus nitrate in the environmental sample and a 

detection of 0.095 mg/L in the replicate sample 

(Domagalski and others, 2000). To apply the relative 

percentage difference calculation, 50 percent of the 

method reporting limit (0.025 mg/L) was used as the 

environmental sample value. The result was the largest 

RPD value for nitrite plus nitrate between samples 

whose difference in concentration units was only 

0.07 mg/L and where one of the two values assigned 

was based on a nondetection result. The median RPD 

calculated for nitrate plus nitrite in the 11 replicate 

sample sets was 0.5 percent.

Like nitrite plus nitrate, the maximum RPD of 

150 percent for copper (table 6) is the result of a 

detectable concentration in the environmental sample 

paired with a nondetection result for the replicate 

sample. With a median RPD value of 3.6 percent, there 

is not enough evidence of variability to indicate that the 

environmental data interpretation would be affected.

The maximum RPD of 114 percent for 

chromium (table 6) was between an environmental and 

replicate sample set with detection results of 12 and 

3.3 μg/L, respectively (Domagalski and others, 2000). 

The cause of the variation between these two values is 

unknown. It is uncertain whether this level of 

variability in the analytical method would affect 

interpretation of the chromium environmental data.

The median difference in concentrations of 

critical constituents ranged from 0 for calcium, 

magnesium, sodium, sulfate, silica, arsenic and 

cadmium to 3 μg/L for boron. Arsenic had the lowest 

maximum difference (2 μg/L) in concentration. The 

highest maximum difference in concentration, sulfate 

at 300 mg/L, was between an environmental and 

replicate sample that also had the highest sulfate 

concentrations detected during the ground-water study 

(1,500 and 1,800 mg/L, respectively). The 

corresponding RPD of the samples was 18 percent. The 

other 10 replicate pairs show no difference in sulfate 

concentration for 6 of the sets, differences of between 

0.1 and 1 mg/L for 3 sets, and a difference of 10 mg/L 

for one replicate set (220 and 230 mg/L) (Domagalski 

and others, 2000).

Although not listed on the critical constituents 

table, replicate data also are included on table 6 for 

alkalinity and specific conductance. These data may 

facilitate the interpretation of other analytical data 

collected during this study.

Surrogate Recovery

Three surrogate compounds were added to each 

of the 103 ground-water samples analyzed for 

pesticides by GC/MS. Median recoveries were 100, 

113, and 96 percent for diazinon-d10, terbuthylazine, 

and alpha HCH-d6 (hexachlorocyclohexane), 

respectively (table 7). The surrogate 4-bromo-3,5-

dimethylphenyl-n-methylcarbamate (BDMC) was 

added to 103 ground-water samples submitted for 

pesticide analysis by the HPLC method. For the 

103 samples analyzed by HPLC, 83 had reportable 

results with a median recovery of 82 percent. The 

laboratory deleted 3 of the surrogate recoveries from 

the database and reported 17 others as estimates. In 

addition, the surrogate toluic acid was deleted from the 

HPLC procedure because of variability in performance 

(Werner and others, 1996). Three surrogates added to 

VOC samples analyzed by GC/MS—1,2,-

dichloroethane-d4, p-bromofluorobenzene, and 

toluene-d8—had median recoveries of 104, 84, and 

98 percent, respectively.
Quality-Assurance and Quality-Control Design 23

Although all three of the analyses returned 

median recovery results between 80 and 120 percent, 

within the recovery limits considered acceptable for 

this report (70 to 130 percent), the variable recoveries 

of surrogates analyzed using HPLC illustrate the 

tendency toward negatively biased and inconsistent 

results. However, the affect on the interpretation of 

data for environmental ground-water samples analyzed 

by HPLC probably is negligible. Of the 78 groundwater samples submitted for analysis of 39 compounds, 

only 4 compounds were detected. The majority of 

ground-water samples analyzed for 48 pesticides by 

GC/MS also resulted in nondetections (Domagalski 

and others, 2000). Although environmental pesticide 

data are not being qualified on the basis of the 

surrogate recovery results, the data should be used with 

caution; the lack of detection of some compounds may 

be due to poor analytical performance.

Field Spiked Samples

Pesticide and VOC samples were spiked and 

submitted for analysis; percentage of spike recovered 

was calculated according to instructions provided by 

the NWQL (Mueller and others, 1997). The 

corresponding environmental sample submitted with 

each spiked sample was used to determine detectable 

background concentrations of the spiked analytes. If an 

analyte was detected in the environmental sample, the 

concentration was subtracted from the concentration 

result of the spiked sample to provide the adjusted 

values for calculating the spike recovery percentages 

(tables 8 through 12).

Pesticides in Filtered Water Analyzed by Gas 

Chromatography/Mass Spectrometry

Ten environmental samples spiked with 

47 compounds were analyzed for selected pesticides by 

GC/MS (table 8). Mean recoveries for all compounds 

with reported detection values (not estimates) were 

within the acceptable limits of 70 to 130 percent, 

except for permethrin and p,p′-DDE, which had the 

lowest mean recoveries at 57 and 64 percent, 

respectively. Similar results for permethrin and 

p,p′-DDE were obtained during the method of analysis 

study done at the laboratory (Zaugg and others, 1995).

Table 7. Recovery of surrogate compounds in Sacramento River Basin, California, ground-water samples, 1996–1998

[The minimum, maximum, and median values are in percentage recovered. HPLC, high performance liquid chromatography] 

1 103 samples were analyzed by HPLC; however, 17 estimated (E) values and 3 laboratory deleted resultes were omitted from these calculations.

Samples Minimum Maximum Median Standard 

deviation

GROUND-WATER PESTICIDES

Pesticides analyzed by gas chromatography/mass spectrometry

Diazinon-d10 103 66 132 100 14

Terbuthylazine 103 69 141 113 15

alpha HCH-d6 (hexachlorocyclohexane) 103 59 117 96 12

Pesticides analyzed by HPLC

4-Bromo-3,5-dimethylphenyl-n-methylcarbamate (BDMC) 183 33 148 82 20

GROUND-WATER VOLATILE ORGANIC COMPOUNDS

1,2,-Dichloroethane-d4 74 80 153 104 12

p-Bromofluorobenzene 74 72 104 84 8

Toluene-d8 74 93 107 98 2
24 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 8. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, ground-water samples analyzed by gas 

chromatography/mass spectrometry, 1996–1998

[Recovery data are given in percentages. Spike recovery was calculated according to protocols of the National Water Quality Laboratory (Mueller 

and others, 1997). Spike recovery calculations were not included for estimated values. Critical constituents are in bold. No. samples, number of 

samples where recovery values could be calculated] 

Compound No. 

samples

Spike recovery in percent

Minimum Maximum Median Mean

2,6-Diethylaniline 9 80 105 86 90

Acetochlor 9 73 132 113 106

Alachlor 10 73 133 109 107

Atrazine 10 66 120 96 97

Methyl azinphos 0 Estimated values 

Benfluralin 9 44 104 73 70

Butylate 9 85 121 94 98

Carbaryl 0 Estimated values 

Carbofuran 0 Estimated values 

Chlorpyrifos 10 66 103 89 87

Cyanazine 10 46 146 111 107

DCPA 10 87 122 99 101

Desethyl atrazine 0 Estimated values

Diazinon 10 62 113 103 97

Dieldrin 10 72 142 94 101

Disulfoton 10 56 100 85 85

EPTC 9 83 110 92 94

Ethalfluralin 9 56 122 91 85

Ethoprop 10 82 130 94 98

Fonofos 10 69 136 93 98

Lindane 10 72 135 92 95

Linuron 9 94 166 128 130

Malathion 10 43 140 99 96

Metolachlor 10 73 143 113 112

Metribuzin 10 53 122 85 86

Molinate 9 85 106 92 94

Napropamide 10 61 121 103 95

Parathion 10 56 135 96 98

Methyl parathion 9 43 120 87 86

Pebulate 9 83 105 92 93

Pendimethalin 10 45 141 67 74

Phorate 10 54 105 88 85

Prometon 10 73 122 95 99

Propachlor 9 74 112 103 98

Propanil 10 83 136 104 107

Propargite 10 62 128 101 96

Pronamide 10 70 117 98 98

Simazine 10 60 121 95 95

Tebuthiuron 8 64 159 108 111

Terbacil 0 Estimated values 

Terbufos 10 53 113 75 78

Thiobencarb 10 79 128 106 105

Triallate 10 81 109 99 97

Trifluralin 9 50 116 77 75

Alpha-BHC 10 72 115 94 94

Permethrin 9 33 97 46 57

p,p′-DDE 10 47 84 63 64
Quality-Assurance and Quality-Control Design 25

Of the compounds considered critical for this 

study (table 1), recovery of carbofuran and desethyl 

atrazine could not be evaluated because all results were 

estimated (table 8). These compounds performed 

poorly during the methods of analysis study (Zaugg 

and others, 1995), and results were reported as 

estimates for the reagent, environmental ground-water, 

and environmental surface-water samples tested. 

Table 9 gives the recovery data for the critical 

constituents atrazine, molinate, simazine, and 

thiobencarb. During this study, all four of these 

compounds performed better (mean recoveries from 94 

to 105 percent for ground-water samples) than during 

the methods of analysis study (Zaugg and others, 

1995), which reported mean spike recoveries from 73 

to 82 percent for ground-water samples and from 76 to 

89 percent for reagent (blank) water samples. Although 

environmental data are not being qualified owing to 

spike recovery results, the data should be used with 

caution; the lack of detection of some compounds may 

be due to poor analytical performance. [See 

interpretive analyses of spike recovery data for the 

subunit survey (Dawson, 2001a) and the agricultural 

land-use study (Dawson, 2001b).]

Pesticides in Filtered Water Analyzed by High Performance 

Liquid Chromatography

Seven ground-water samples were spiked with 

39 compounds and analyzed for pesticides by HPLC 

(table 10). Mean recoveries for compounds that had 

reported detections (not estimated) ranged from 

3 percent for chloramben (1 recovery value) to 

87 percent recovery for fluometuron (7 recovery 

values) and propham (4 recovery values). The 

pesticides DNOC and dichlobenil had only estimated 

values and could not be included in the spike recovery 

calculations. Chlorothalonil also was omitted from the 

summary because of estimated or undetectable 

concentrations in the spiked samples.

The HPLC methods of analysis study (Werner 

and others, 1996) indicates a tendency toward 

negatively biased results. The mean recovery for 

30 field matrix spike compounds in 81 samples “spiked 

at 1.0 μg/L” ranged from 9 percent for chlorothalonil to 

101 percent for propham. Twenty-five of the 

30 compounds in the field matrix spike solution had 

mean recoveries of 65 percent or less, and 10 of the 

30 compounds had mean recoveries less than 

50 percent. Bentazon and carbofuran, two critical 

constituents, had comparable low recovery results in 

both the HPLC methods study and in the Sacramento 

River Basin study (table 11).

Table 9. Spike recovery data for selected pesticides from Sacramento River Basin, California, ground-water samples, 1996–1998, and 

recovery and precision data published in the methods of analysis report

[Recovery and precision data are in Zaugg and others (1995). Spike recovery data are in percentages. Pesticides added to water samples at 0.1 

microgram per liter] 

 Compound

Sacramento River Basin Methods of analysis report

Ground water 

(10 samples)

Ground water 

(7 samples)

Reagent (blank) water 

(6 samples)

Minimum Maximum Median Mean Mean

Relative 

standard 

deviation 

(percent)

Mean

Relative 

standard 

deviation 

(percent)

Atrazine 66 120 96 97 79 3 89 6

Molinate 85 106 92 94 82 4 82 3

Simazine 60 121 95 95 73 4 76 3

Thiobencarb 79 128 106 105 74 4 85 3
26 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 10. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, ground-water samples analyzed by high 

performance liquid chromatography, 1996–1998

[Recovery data are in percentages. Spike recovery was calculated according to protocols of the National Water Quality Laboratory (Mueller and 

others, 1997). Spike recovery calculations were not included for estimated values. Critical constituents are in bold. Some spike solutions did not 

contain all compounds. No. samples, number of samples where recovery values could be calculated] 

Compound No. 

samples

Spike recovery in percent

Minimum Maximun Median Mean

2,4,5-T 7 14 98 76 69

2,4-D 6 60 71 68 67

2,4-DB 7 44 78 64 64

Silvex 7 63 87 72 72

3-Hydroxycarbofuran 5 52 77 62 64

DNOC 0 Estimated values 

Acifluorfen 6 41 75 70 66

Aldicarb 2 51 53 52 52

Aldicarb sulfone 3 8 41 22 24

Aldicarb sulfoxide 4 27 82 51 53

Bentazon 7 55 71 63 63

Bromacil 7 62 93 72 75

Bromoxynil 7 63 77 71 70

Carbaryl 6 63 82 68 70

Carbofuran 6 66 99 76 78

Chloramben 1 3 3 3 3

Chlorothalonil 0 Estimated values 

Clopyralid 5 8 57 41 38

Dacthal 6 49 78 72 69

Dicamba 7 5 89 67 52

Dichlobenil 0 Estimated values

Dichlorprop 7 57 74 68 66

Dinoseb 7 49 76 67 65

Diuron 6 49 94 73 74

Fenuron 7 56 136 70 80

Fluometuron 7 70 120 77 87

Linuron 7 69 127 72 83

MCPA 7 51 80 62 61

MCPB 6 48 67 64 60

Methiocarb 6 62 85 68 71

Methomyl 6 61 93 77 76

Neburon 7 54 132 71 78

Norfluorazon 6 69 91 73 76

Oryzalin 5 45 84 72 69

Oxamyl 6 37 72 54 55

Picloram 6 60 75 65 67

Propham 4 55 133 80 87

Propoxur 6 53 84 67 68

Triclopyr 6 46 79 69 65
Quality-Assurance and Quality-Control Design 27

Although ground-water data were not qualified 

because of spike recovery results, only 14 of the 

39 compounds analyzed by the HPLC method returned 

acceptable recoveries of 70 percent or greater. While 

there were few detections of pesticides in ground-water 

samples analyzed by HPLC, the analytical data should 

be used with caution; the lack of detection of some 

compounds in the environmental samples may be due 

to poor analytical performance. [See interpretive 

analyses of spike recovery data for the subunit survey 

(Dawson, 2001a) and the agricultural land-use study 

(Dawson, 2001b).]

Volatile Organic Compounds

A total of 12 ground-water samples, each spiked 

with a solution containing 13 VOCs, were submitted 

for analysis during the subunit survey of 1996 and the 

urban study of 1998; VOC samples were not collected 

during the rice land-use study of 1997. Each spiked 

sample was submitted with a spike replicate. The 

relative percentage difference between the recovery 

results for each of the replicate spike pairs and the 

minimum, maximum, median, and mean percentage of 

spike recovered by compound for each sample are 

given in table 12. Two sets of data also are given, with 

and without the inclusion of an anomalous set of data 

collected on June 22, 1998, during the urban land-use 

study. The most likely explanation for those anomalous 

recoveries is that one of the samples was spiked twice, 

and the other sample was improperly spiked.

The median relative percentage difference 

between spike replicate pairs ranged from 3 percent for 

1,2-dichloroethane to 22 percent (19 percent without 

anomalous recoveries) for chloroethene. The median 

recovery percentages spanned from 72 for 

chloroethene to 113 percent for 1,2-dichloroethane 

(table 12).

Of the three constituents listed as critical for the 

ground-water studies—trichloromethane, MTBE, and 

trichloroethene—only MTBE and trichloroethene were 

in the spike solutions. Data in the methods of analysis 

study (Connor and others, 1998) show the percentage 

of recovery as the mean recovery of seven replicates, 

each spiked at “1 and 10 μg/L”. Recovery results for 

the ground-water and blank water samples analyzed for 

MTBE ranged from 98 to 122 percent; tricholoethene 

ranged from 98 to 102 percent (Connor and others, 

1998). The mean recovery results for 10 ground-water 

samples spiked with MTBE and trichloroethene during 

the Sacramento River Basin NAWQA were 93 and 

97 percent, respectively (table 12 data without 

anomalous sample set), well within acceptable 

recovery limits (70 to 130 percent).

The samples submitted for the Sacramento River 

Basin study were spiked with 100 μL of spike solution 

during the subunit survey of 1996; because of an 

adjustment in protocol, the samples collected during 

the urban study of 1998 were spiked with 20 μL of 

spike solution using a 25-μL syringe (Connor and 

others, 1998). The changes had no apparent effect on 

recovery results. [See interpretive analyses of spike 

recovery data for the subunit survey (Dawson, 2001a) 

and the agricultural land-use study (Dawson, 2001b).]

Surface Water

Surface-water samples were collected from 

12 sites throughout the Sacramento River Basin from 

February 1996 to April 1998 (fig. 2). Three of these 

sites—Colusa Basin Drain at Road 99E near Knights 

Landing, Arcade Creek near Del Paso Heights, and 

Sacramento River at Freeport—had increased sampling 

frequency for pesticides and(or) VOCs during selected 

seasonal periods (Domagalski and others, 2000).

Table 11. Spike recovery data for selected pesticides from Sacramento River Basin, California, ground-water samples, 1996–1998, and 

recovery and precision data published in the methods of analysis report

[Recovery and precision data are in Werner and others (1996). Spike recovery data are in percentages] 

 Compound Sacramento River Basin (10 samples) Methods of analyses report (81 samples)

Minimum Maximum Median Mean Mean recovery

Bentazon 55 71 63 63 60 (relative standard deviation ± 23)

Carbofuran 66 99 76 78 62 (relative standard deviation ± 32)
28 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 Table 12. Relative percentage difference and recovery of field matrix spikes for volatile organic compounds in ground-water samples collected during the Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 [Difference and recovery data are given in percentages. Spike recovery was calculated according to protocols of the National Water Quality Laboratory (Mueller and others, 1997). In 1996 spike volume was 100 microliters according to protocol; by 1998, spike volume had been adjusted to 20 microliters (Connor and others, 1998). Percentage recovery calculations were based on values adjusted relative to concentrations detected in corresponding environmental samples, if applicable. Spike recovery results were not calculated when concentrations were reported by the laboratory as estimated values. VOC, volatile organic compound].Critical constituents are in bold] Compound VOCs Number of spike replicate pairs Relative percentage difference in recovery results for spike replicate pairs Number of samples Spike recovery Minimum Maximum Median Mean Minimum Maximum Median Mean Results calculated with the anomalous sample set: 1,1,1-Trichloroethane 5 4 20 5 10 10 83 124 108 106 1,1-Dichloroethene 5 1 20 8 10 10 64 117 89 92 1,2-Dichloroethane 4 0 5 3 3 8 88 128 113 109 1,4-Dichlorobenzene 5 5 25 6 11 10 59 98 76 81 Bromodichloromethane 5 4 19 6 9 10 92 117 103 103 Tribromomethane 6 0 79 8 19 12 80 185 94 100 Chlorodibromomethane 6 0 74 6 18 12 80 175 94 100 Ethylbenzene 6 0 99 8 25 12 45 134 95 93 Tetrachloroethene 6 0 90 10 23 12 45 123 81 88 Tetrachloromethane 5 0 22 4 8 10 75 128 98 102 Trichloroethene 6 0 81 7 20 12 70 165 100 100 Chloroethene 6 6 77 22 28 12 47 154 72 82 Methyl tert-butyl ether 6 0 80 7 18 12 67 156 95 96 Results calculated without the anomalous sample set: 1,1,1-Trichloroethane 5 4 20 5 10 10 83 124 108 106 1,1-Dichloroethene 5 1 20 8 10 10 64 117 89 92 1,2-Dichloroethane 4 0 5 3 3 8 88 128 113 109 1,4-Dichlorobenzene 5 5 25 6 11 10 59 98 76 81 Bromodichloromethane 5 4 19 6 9 10 92 117 103 103 Tribromomethane 5 0 13 6 7 10 88 105 94 94 Chlorodibromomethane 5 0 20 4 7 10 84 103 94 94 Ethylbenzene 5 0 29 6 10 10 72 108 95 94 Tetrachloroethene 5 0 24 9 10 10 59 123 81 89 Tetrachloromethane 5 0 22 4 8 10 75 128 98 102 Trichloroethene 5 0 26 4 8 10 81 108 100 97 Chloroethene 5 6 26 19 18 10 47 106 72 76 Methyl tert-butyl ether 5 0 13 5 6 10 68 108 95 93
Quality-Assurance and Quality-Control Design 29

Equipment blanks were collected under 

controlled conditions and submitted annually. Field 

blanks, spikes, and replicate samples were scheduled in 

conjunction with the monthly environmental sampling 

schedules. Quality-control data were reviewed to 

determine whether systematic contamination was being 

introduced that might require corrective action.

Blank Samples

Results of field blank sample analyses for critical 

constituents in the surface water study are given in 

table 13. These include analytical results for major 

ions, DOC, nutrients, trace elements, pesticides in 

filtered water, and VOCs.

Major Ions

Two equipment blanks and six field blank 

samples (table 13) were submitted for major ion 

analysis. Magnesium, sodium, and potassium were not 

detected in the equipment or field blank samples. 

Calcium was not detected in either of the equipment 

blanks, but was in one of the six field blanks at about 

0.05 mg/L and in all surface-water samples at 

concentrations from 4 to 47 mg/L (Domagalski and 

others, 2000). Chloride and sulfate were not detected in 

either of the equipment blanks, but were in the same 

field blank. The detections of sulfate and chloride, 

along with a relatively high specific conductance of 

73 μS/cm for the blank sample from Sacramento River 

at Verona on February 24, 1998 (Domagalski and 

others, 2000), are probably due to soap residue in the 

sampling equipment. Because the corresponding 

environmental sample data for these constituents are 

similar to data at that site throughout the study, 

contamination appears to be limited to the field blank 

sample.

Silica was detected in one of the two equipment 

blanks and in three of the six field blank samples at 

concentrations ranging from 0.01 to 0.022 mg/L. Of the 

320 environmental samples analyzed for major ions, 

319 had silica detections ranging from 3 to 45 mg/L, 

and one sample had a concentration of 1.1 mg/L 

(Domagalski and others, 2000). More than 2 orders of 

magnitude separate over 99 percent of the 

environmental data from the blank data. Silica found in 

blank samples was most likely solubilized from the 

glass ampoules containing the nitric acid used as a 

sample preservative (U.S. Geological Survey, 1998); 

polypropylene vials are now being used for nitric acid.

Detections of major ions in blank samples 

submitted during the surface-water study were few and 

at relatively low levels, reflecting random rather than 

systematic contamination. Therefore, environmental 

data can be used without qualification.

Dissolved Organic Carbon

Twenty-four DOC quality-control samples—

13 field blanks (table 13) and 11 source solution 

blanks—were submitted for analysis. DOC was 

detected in all field blank samples at concentrations 

ranging from the reporting limit of 0.1 to 2.5 mg/L. 

DOC was in six of the source solution blanks, five at 

the detection limit of 0.1 mg/L, and one at 0.2 mg/L. 

The boxplot (fig. 9) shows no overlap between the field 

blank and environmental sample results, and the 

median values of the two data sets are not similar 

(p=0.0001, Mann–Whitney statistical test). With the 

environmental samples having concentrations ranging 

from 0.3 to 18 mg/L, however, some of the 

environmental data values near the reporting limit 

could have a positive bias. The source of the DOC 

contamination in blank samples collected in 

conjunction with surface-water sampling is unknown. 

Certificates of analysis that accompanied the five lots 

of blank water used during the course of the study did 

not provide conclusive evidence that blank water was 

the source of contamination. Although evidence of 

positive bias is insufficient to substantiate a 

qualification of the environmental data, environmental 

data near the reporting limit should be used with 

caution.
30 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 13. Detections of critical constituents in surface-water field blank samples collected during the Sacramento River Basin, California, National 

Water-Quality Assessment, 1996–1998

[All data are given in concentration units. Number of significant figures do not reflect analytical method reporting levels. Detections are in percentages. 

Changes in the method reporting limit (MRL) occurred during the course of the data collection for some of the analyses as indicated by multiple values. 

Maximum, maximum observed value or concentration; median, median observed value or concentration. mg/L, milligram per liter; μg/L, microgram 

per liter; <, less than; E-value, laboratory estimated result. NA, not applicable] 

Constituent or compound

Number 

of field 

blank 

samples

Detections MRL(s)

Concentration

Maximum Median

Major ions

Calcium (detections in mg/L) 6 17 0.02 0.049 <0.020

Magnesium (detections in mg/L) 6 0 0.01 NA NA

Sodium (detections in mg/L) 6 0 0.20; 0.10 NA NA

Potassium (detections in mg/L) 6 0 0.1 NA NA

Chloride (detections in mg/L) 6 17 0.1 1.8 <0.10

Sulfate(detections in mg/L) 6 17 0.1 0.13 <0.10

Silica (detections in mg/L) 6 50 0.01; 0.10 0.022 0.01

Dissolved organic carbon (detections in mg/L as carbon) 13 100 0.1 2.5 0.2

Suspended organic carbon (detections in mg/L as carbon) 7 57 0.1; 0.2 0.2 0.1

Nutrients

Nitrite (NO2) + Nitrate (NO3) (detections in mg/L as nitrogen) 7 29 0.05 0.125 <0.050

Whole-water phosphorus (detections in mg/L as phosphorus) 7 14 0.01 0.02 <0.010

Dissolved phosphorus (detections in mg/L as phosphorus) 7 0 0.01 NA NA

Orthophosphorus (detections in mg/L as phosphorus) 7 43 0.01 0.022 <0.010

Trace elements

Aluminum (detections in μg/L) 13 92 1.0 7 4

Chromium (detections in μg/L) 13 0 1.0 NA NA

Copper (detections in μg/L) 13 8 1.0 1 <1.0

Manganese (detections in μg/L) 13 0 1.0 NA NA

Nickel (detections in μg/L) 13 15 1.0 2 <1.0

Zinc (detections in μg/L) 13 23 1.0 3 <1.0

Iron (detections in μg/L) 6 33 3; 10 4.3 2.75

Mercury

Total mercury (ng/L) 4 100 0.03 0.31 0.06

Methylmercury (ng/L) 1 0 0.02 NA NA

Pesticides analyzed by gas chromatography/mass spectrometry 

Carbofuran (detections in μg/L) 6 0 0.003 NA NA

Chlorpyrifos (detections in μg/L) 6 0 0.004 NA NA

Diazinon (detections in μg/L) 6 0 0.002 NA NA

Metolachlor (detections in μg/L) 6 0 0.002 NA NA

Molinate (detections in μg/L) 6 NA 0.004 1 E-value only

Simazine (detections in μg/L) 6 0 0.005 NA NA

Thiobencarb (detections in μg/L) 6 0 0.002 NA NA

Pesticides analyzed by high performace liquid chromatography

Carbofuran (detections in μg/L) 6 0 0.280; 0.120 NA NA

Diuron (detections in μg/L) 6 0 0.02 NA NA

Volatile organic compounds

Methyl tert-butyl ether (MTBE) (detections in μg/L) 7 NA 0.200; 0.100; 0.112 2 E-values only NA
Quality-Assurance and Quality-Control Design 31

Suspended Organic Carbon

Of the seven field blank samples submitted for 

suspended organic carbon analysis, low-level 

detections (0.1 and 0.2 mg/L) were reported for four of 

the samples. Although boxplot comparison (fig. 10) of 

environmental samples and field blanks reveals no 

overlap, and the median values of the two data sets are 

significantly different (p=0.0001, Mann–Whitney 

statistical test), a positive bias probably exists at 

concentrations near the reporting limit. This may affect 

72 of the 298 environmental samples that had 

suspended organic carbon (SOC) concentrations of 

0.2 mg/L or less.

Nutrients

Nine blank samples—seven field blanks and two 

equipment blanks—were collected and analyzed for 

eight nutrient compounds. Field blanks include two 

detections (29 percent) of nitrite plus nitrate (0.053 and 

0.125 mg/L), a single detection (14 percent) of wholewater phosphorus (0.02 mg/L), and three detections 

(43 percent) of orthophosphorus (0.01–0.022 mg/L) 

(table 13). Dissolved phosphorus was not detected in 

field blanks, but was detected at 0.01 mg/L in one 

equipment blank. Orthophosphorus also was detected 

at 0.01 mg/L in one equipment blank. The maximum 

concentrations (table ) for nitrite plus nitrate 

(0.125 mg/L as nitrogen) and orthophosphorus 

(0.022 mg/L as phosphorus), both of which are from 

the same field blank, are greater than twice the 

reporting limit. Although the source of the 

contamination is unknown, the three maximum 

concentrations appear to be anomalies and do not affect 

the environmental data.

Figure 9. Boxplots of dissolved organic carbon concentration in 

surface-water environmental samples, field blanks, and source solution 

blanks in the Sacramento River Basin, California, 1996–1998.

0

2

4

6

8

10

12

14

16

18

Dissolved organic carbon as carbon, in milligrams per liter

Environmental

Samples

Source Solution

Blanks

Field

Blanks

(308)

(13)

(11)

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Extreme outlier

Outlier

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values are more than 3 times the

interquartile range from the top or bottom

of the rectangle

(n) Number of samples

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

Suspended organic carbon as carbon, in milligrams per liter

Environmental

Samples

Field

Blanks

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

Outlier values deleted for clarity

(n) Number of samples

(298)

(7)

Figure 10. Boxplots of suspended organic carbon concentration in surfacewater environmental samples and field blanks in the Sacramento River 

Basin, California, 1996–1998.
32 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Trace Elements

The trace elements analyzed by the NWQL 

considered critical to the surface-water component of 

the Sacramento River Basin NAWQA study include 

aluminum, chromium, copper, manganese, nickel, zinc, 

and iron (table 13). Sixteen blank samples were 

collected and analyzed for trace elements, excluding 

iron—13 field blanks and 3 equipment blanks. Six field 

blank samples and two equipment blanks were 

submitted for iron analysis using the analytical method 

[inductively coupled plasma (Fishman, 1993)] used for 

major ions.

Aluminum

Aluminum was detected in 12 of the 13 field 

blanks (92 percent) (table 13) and in all three 

equipment blanks at levels ranging from 3 to 7 μg/L. 

Aluminum in the 294 environmental samples ranged 

from 3 to 325 μg/L, and about 51percent had detections 

of 7 μg/L or less. Boxplot analysis (fig. 11) of these 

data shows minor overlap between the two data sets. 

Although the Mann–Whitney statistical test shows the 

medians of the environmental and blank data sets to be 

dissimilar (p=0.0001), environmental data have a 

positive bias, especially for low–end detection values. 

Some contamination in the blank samples could be 

attributed to the solubilization of aluminum from glass 

ampoules containing the nitric acid used as a sample 

preservative (U.S. Geological Survey, 1998); 

polypropylene vials are currently being used for nitric 

acid preservative. Because the boxplot provides 

justification for qualifying the aluminum values in the 

database, environmental samples that have low 

aluminum concentrations of 12 μg/L or less will be 

qualified with a V code in the database to indicate that 

the value may be due wholly, or in part, to 

contamination.

0.0

1.5

3.0

4.5

6.0

7.5

9.0

10.5

12.0

13.5

Aluminum, in milligrams per liter

Environmental

Samples

Field

Blanks

(294)

(13)

75th percentile

25th percentile

Median

Vertical Line

Vertical Line

Interquartile

Range

Vertical lines extend a distance equal to 1.5

times the interquartile range away from the

top or bottom of the rectangle or to the

limit of the data, whichever is least

(n) Number of samples

Figure 11. Boxplots of aluminum concentration in surface-water environmental 

samples and field blanks in the Sacramento River Basin, California, 1996–1998. 

Outliers are not shown.
Quality-Assurance and Quality-Control Design 33

Chromium, Copper, Manganese, Nickel, Zinc, and Iron

Blank samples were submitted to the NWQL and 

analyzed by ICP/MS for the trace elements chromium, 

copper, manganese, nickel, and zinc (Faires, 1993). 

Neither chromium nor manganese were detected in the 

blank samples (table 13). Of the 13 field samples, 

copper was detected in 1 sample (8 percent) at 1 μg/L, 

nickel was detected in 2 field blanks (15 percent) at 

values of 2 and 1μg/L, and zinc was detected in 

1 equipment blank and in 3 field blanks (23 percent) at 

concentrations ranging from 1 to 3 μg/L. Iron was not 

detected in either equipment blank, but was detected in 

two of the six field blank samples (33 percent) at 4.0 

and 4.3 μg/L. The infrequent detections of these 

analytes at low levels indicate no systematic 

contamination and, therefore, environmental data may 

be used without qualification.

Total Mercury

Six blank samples were analyzed for total 

mercury—four field blanks and two trip blanks. 

Mercury was detected in all six samples at 

concentrations ranging from the reporting limit of 0.03 

to 0.31 ng/L; all of the 296 environmental samples 

contained concentrations of mercury ranging from 0.75 

to 2,248 ng/L (Domagalski and others, 2000). The 

Mann–Whitney statistical test results (p=0.0001) 

provide no evidence of contamination that would effect 

the interpretation of environmental data.

Methylmercury

Only one blank sample was submitted for 

methylmercury analysis (table 13). Methylmercury was 

not detected in that sample; however, one sample 

provides insufficient data to make a determination 

about bias or any inference about the environmental 

data.

Pesticides in Filtered Water Analyzed by Gas 

Chromatography/Mass Spectrometry

Six blank samples were collected and analyzed 

for 47 pesticide compounds by GC/MS (table 13). 

Except for two estimated values—one for EPTC at 

0.0011 μg/L (method reporting limit 0.0020 μg/L) and 

one for the critical constituent molinate at 0.0014 μg/L 

(method reporting limit 0.0040 μg/L)— no pesticides 

were detected in the blank samples. The blank data 

provide no evidence of contamination that would affect 

the interpretation of environmental data.

Pesticides in Filtered Water Analyzed by High Performance 

Liquid Chromatography

Six blank samples were collected and analyzed 

for 39 compounds by HPLC; no pesticides were 

detected in any of the samples (table 13). The blank 

data provide no evidence of contamination that would 

affect the interpretation of environmental data.

Volatile Organic Compounds

Thirteen blank samples were analyzed for 

VOCs—7 field blanks, 3 trip blanks, 2 source solution 

blanks, and 1 equipment blank. The blank samples 

provide no evidence of systematic contamination that 

would affect environmental data analysis. There were 

detections of acetone, dichloromethane, and 

methylbenzene—all noncritical constituents—in one 

trip blank sample set (sample date January 13, 1997). 

All of the vials in that set contained air bubbles when 

they arrived from the laboratory, suggesting the 

introduction of contamination at the point of origin or 

during transport. The environmental sample from the 

same date showed no anomalous detections.

Methyl tert-butyl ether (MTBE), a critical 

constituent, was not detected in 11 of the 13 blanks. 

Estimated values were reported for MTBE in two field 

blanks, but those estimated concentrations were less 

than 50 percent of the method reporting limit. The 

environmental data can be used without qualification.

Replicate Samples

Variability based on analysis of replicate 

samples is presented for critical constituents in surfacewater samples (table 14). The variability between 

replicates is presented as the RPD and as the absolute 

difference in concentration units. For calculation 

purposes, if a less-than result was paired with a 

detection, 50 percent of the reporting limit was used in 

place of the less-than value.
34 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 Table 14. Variability of surface-water replicate samples collected during the Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998 [If one of the two results for a replicate pair was reported as a “<” (less than the reporting limit) value, a value equal to 50 percent the reporting limit was used to complete the calculations used in this table. If reported as such, “E” (estimated) values were used in calculations for pesticides and the volatile organic compound methyl tert-butyl ether (MTBE). NWQL, National Water Quality Laboratory. Number of significant figures do not reflect analytical method reporting levels. μg/L, microgram per liter; mg/L, milligram per liter; ng/L, nanogram per liter] Analyte (number of detections) Total number of sets Relative difference, in percentage Difference, in concentration units Minimum Maximum Median Minimum Maximum Median Major ions Calcium (detections in all samples in mg/L) 6 0 1.9 0 0 0.1 0 Magnesium (detections in all samples in mg/L) 6 0 1.1 0 0 0.1 0 Sodium (detections in all samples in mg/L) 6 0 6.1 0 0 1 0 Potassium (detections in all samples in mg/L) 6 0 7.8 1.8 0 0.1 0 Chloride (detections in all samples in mg/L) 6 0 7.4 0 0 0.1 0 Sulfate (detections in all samples in mg/L) 6 0 4.7 1.3 0 0.2 0.1 Silica (detections in all samples in mg/L) 6 0 3.8 0 0 1 0 Dissolved organic carbon (detections in all samples in mg/L as carbon) 7 0 8.7 3.4 0 0.3 0.1 Suspended organic carbon [detections in mg/L as carbon; 1 split set (1 of the 2 samples has a nondetect result)] 7 0 67 0 0 0.3 0 Nutrients Nitrite (NO2) + nitrate (NO3) [detections in mg/L as nitrogen (1 set of nondetects)] 10 0 10 3.6 0 0.1 0.003 Whole-water phosphorus [detections in mg/L as phosphorus (2 sets of nondetects)] 10 0 40 8.2 0 0.031 0.008 Dissolved phosphorus [detections in mg/L as phosphorus (2 sets of nondetects; 2 split sets (1 of the 2 samples has a nondetect result)] 10 0 120 4 0 0.015 0.001 Orthophosphorus [detections in mg/L as phosphorus (2 sets of nondetects)] 10 0 32 0 0 0.006 0 Trace elements Aluminum (detections in all samples in μg/L) 4 0 32 5 0 11 0.35 Copper [detections in μg/L (1 set of nondetects)] 4 0 40 14 0 1 0.2 Manganese (detections in all samples in μg/L) 4 0 9.5 0.9 0 1 0.05 Nickel [detections in μg/L (2 sets of nondetects)] 4 0 0 0 0 0 0 Zinc [detections in μg/L (1 set of nondetects; 1 split set (1 of the two samples has a nondetect result)] 4 0 67 42 0 0.6 0.5 Iron (detections in all samples in μg/L) 6 17 93 37 2 26 8.6 Total mercury (detections in all samples in ng/L) 72 0 171 6 0.01 13.231 0.305
Quality-Assurance and Quality-Control Design 35

Table 14. Variability of surface-water replicate samples collected during the Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Methylmercury (detections in all samples in ng/L) 24 1 148 17 0.002 0.553 0.0165

Pesticides analyzed by gas chromatography/mass spectrometry

Carbofuran [detections in μg/L (6 nondetect sets; 6 sets with E values)] 12 0 15 0 0 0.0056 0

Chlorpyrifos [detections in μg/L (6 nondetect sets; 2 split sets (1 of the 2 samples has a nondetect 

result)]

12 0 100 1.2 0 0.01 0.0002

Diazinon [detections in μg/L (1 nondetect set; 1 set with a nondetect and an E value)] 12 0 67 4.7 0 0.017 0.0015Metolachlor [detections in μg/L (1 nondetect set; 2 sets with E values)] 12 0 18 3.9 0 0.025 0.001Molinate [detections in μg/L (5 nondetect sets)] 12 0 3 0 0 0.016 0

Simazine [detections in μg/L (2 nondetect sets; 1 set with a nondetect and a detection; 1 set with a 

nondetect and an E value)]

12 0 186 3.5 0 0.0644 0.0006

Thiobencarb [detections in μg/L (6 nondetect sets)] 12 0 15 1.1 0 0.03 0.0002

Pesticides analyzed by high performace liquid chromatography 

Carbofuran [detections in μg/L (9 nondetect sets)] 10 0 22 0 0 0.05 0

Diuron [detections in μg/L (2 nondetect sets; 1 set with E values)] 10 0 25 4.5 0 0.9 0.01

Volatile organic compounds

Methyl tert-butyl ether (MTBE) [detections in μg/L (1 nondetect set; 3 sets with E values)] 6 0 2.4 0 0 0.006 0

Alkalinity as CaCO3 [calcium carbonate (comparison of field alkalinity values)] 215 0 33 2 0 33 1

Specific conductance: NWQL values versus field values (in microsiemen per centimeter at 25 degrees 

Celsius)

318 0 16 3 0 37 4

Analyte (number of detections)

Total

number

of sets

Relative difference, in percentage Difference, in concentration units

Minimum Maximum Median Minimum Maximum Median
36 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

The median RPD is 0 percent for calcium, 

magnesium, sodium, chloride, silica, suspended 

organic carbon, orthophosphorus, nickel, the pesticides 

carbofuran (GC/MS and HPLC analyses) and molinate 

(GC/MS analysis), and the VOC MTBE. The highest 

median RPD is 42 percent for zinc followed by 

37 percent for iron. Five analytes have maximum RPD 

values at or exceeding 100 percent, including dissolved 

phosphorus (120 percent), total mercury, (171 percent), 

methylmercury (148 percent), and the pesticides 

chlorpyrifos and simazine (100 and 186 percent, 

respectively). Although dissolved phosphorus has a 

maximum RPD value of 120 percent, the maximum 

difference in concentration was small (0.015 mg/L) and 

near the reporting limit (0.010 mg/L). Replicate data 

for alkalinity and specific conductance are also in 

table 14. Although not listed as critical constituents in 

table 1, these data may facilitate the interpretation of 

other analytical data collected during this study.

Surrogate Recovery

The three surrogate compounds—diazinon-d10, 

terbuthylazine, and alpha HCH-d6

(hexachlorocyclohexane)—added to the 104 samples 

analyzed for pesticides by GC/MS had median 

recoveries of 102, 109, and 99 percent, respectively 

(table 15); median recovery figures include anomalous 

results for one sample that probably had surrogate 

added improperly. The surrogate compound 4-bromo3,5-dimethylphenyl-n-methylcarbamate (BDMC), 

which was added to 100 samples analyzed by HPLC, 

had a median recovery of 91 percent based on recovery 

results for 86 of the 100 samples. The laboratory 

deleted surrogate recovery results for 11 of the samples 

with the notation “unable to determine due to 

interference” (Wayne Nitta, U.S. Geological Survey, 

National Water Quality Laboratory, written commun., 

1999). Three samples with estimated values were not 

included in the surrogate recovery summary. A second 

surrogate, toluic acid, was deleted from the HPLC 

procedure because of variable performance (Werner 

and others, 1996). The three surrogate compounds—

1,2,-dichloroethane-d4, p-bromofluorobenzene, and 

toluene-d8—added to 64 surface-water samples 

analyzed for VOCs, had median recoveries of 105, 99, 

and 99 percent, respectively.

Table 15. Recovery of surrogate compounds in Sacramento River Basin, California, surface-water samples, 1996–1998

[The minimum, maximum, and median values are in percentage recovered. Minimum values (in bold) were verified as correct by National Water Quality 

Laboratory and are all from one sample, indicating that surrogate was improperly added to that sample. Except for the median value (in bold italics), the 

median values were unaffected. Standard deviation values (in bold) were calculated using both sets of minimum values. HPLC, high performance liquid 

chromatography] 

1100 samples were analyzed by HPLC; however, 3 estimated (E) values and 11 laboratory deleted results were omitted from these calculations.

Surface water pesticides Samples Minimum Maximum Median Standard 

deviation

SURFACE WATER PESTICIDES

Pesticides analyzed by gas chromatography/mass spectrometry

Diazinon-d10 104/103 8.8/102 126 102 13/9

Terbuthylazine 104/103 11.4/88 148 109/110 15/12

alpha HCH-d6 (hexachlorocyclohexane) 104/103 10.5/78 133 99 14/11

Pesticides analyzed by HPLC 

4-Bromo-3,5-dimethylphenyl-n-methylcarbamate (BDMC) 186 47 138 91 16

SURFACE WATER VOLATILE ORGANIC COMPOUNDS

1,2-Dichloroethane-d4 64 89 133 105 9

p-Bromofluorobenzene 64 65 111 99 9

Toluene-d8 64 87 105 99 3
Quality-Assurance and Quality-Control Design 37

Median recovery results for all surrogates are 

between 91 and 110 percent, which are within 

acceptable limits (70 to 130 percent). The performance 

of the HPLC method, however, provides further 

evidence that the procedure tends to be problematic. 

Although environmental pesticide data is not being 

qualified on the basis of the surrogate recovery results, 

lack of detection of some compounds analyzed by 

HPLC may be due to poor analytical performance, and 

environmental data should be used with caution.

Field Spiked Samples

Pesticide and VOC samples were spiked, 

submitted for analysis, and the percentage of spike 

recovered was calculated according to protocols of the 

National Water Quality Laboratory (Mueller and 

others, 1997). The corresponding environmental 

sample submitted with each spiked sample was used to 

estimate detectable background concentrations of the 

spiked analytes. If an analyte was detected in the 

environmental sample, that concentration was 

subtracted from the concentration of the spiked sample 

and the adjusted value was used to calculate spike 

recovery percentages (tables 16 through 20).

Pesticides in Filtered Water Analyzed by Gas 

Chromatography/Mass Spectrometry

Surface-water samples were spiked with 

47 pesticide compounds and submitted for analysis by 

GC/MS during the Sacramento River Basin study 

(table 16). Median recoveries for all compounds that 

had detection values (not estimates) ranged from 

48 percent for p,p′-DDE to 118 percent for tebuthiuron, 

except for permethrin, which had the lowest median 

recovery (29 percent) of all the compounds. Permethrin 

also had the lowest median spike recovery result for 

ground-water samples submitted during the 

Sacramento River Basin NAWQA (table 8). Similar 

results for permethrin were obtained during a methods 

of analysis study (Zaugg and others, 1995) done at the 

laboratory in which six reagent water samples spiked at 

0.1 μg/L yielded low mean recovery results (37 

percent); seven surface-water samples spiked at 0.1 

μg/L also had low mean recovery results (39 percent) 

for permethrin. The only compound having a lower 

mean recovery in the surface water sample set during 

method analysis was linuron (37 percent) (Zaugg and 

others, 1995; Lindley and others, 1996).

Of the pesticide compounds considered critical 

to the surface water component of the Sacramento 

River Basin NAWQA, carbofuran recovery could not 

be evaluated because all results were estimated 

(Domagalski, 2000, p. 15). Carbofuran also performed 

poorly during the method analysis study (Zaugg and 

others, 1995). Recovery data for the other critical 

constituents are given in table 17. Comparison of mean 

recovery data shows that spike recoveries for surfacewater samples submitted by the Sacramento River 

Basin study were higher and nearer to 100 percent than 

recoveries for those compounds obtained by Zaugg and 

others (1995), which were spiked at similar 

concentrations.
38 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 16. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, surface-water samples, 1996–1998, 

analyzed by gas chromatography/mass spectrometry

[Recovery data are given in percentages. Spike recovery was calculated according to protocols of the National Water Quality Laboratory 

(Mueller and others, 1997). Spike recovery calculations were not included for estimated values. Critical constituents are in bold. No. 

samples, number of samples where recovery values could be calculated] 

Compound No.

samples

Spike recovery in percent

Minimum Maximum Median Mean

2,6-Diethylaniline 6 77 91 86 85

Acetochlor 6 92 106 97 97

Alachlor 6 88 118 100 101

Atrazine 6 79 111 94 95

Methyl azinphos 0 Estimated values

Benfluralin 6 79 97 89 88

Butylate 6 90 118 98 100

Carbaryl 0 Estimated values

Carbofuran 0 Estimated values

Chlorpyrifos 6 63 104 85 87

Cyanazine 6 88 131 110 110

DCPA 6 76 122 98 99

Desethyl atrazine 0 Estimated values

Diazinon 6 40 101 90 81

Dieldrin 6 86 107 94 95

Disulfoton 6 42 84 59 60

EPTC 6 85 106 97 96

Ethalfluralin 6 96 129 103 109

Ethoprop 6 81 106 98 96

Fonofos 6 80 99 86 89

Lindane 6 79 105 92 91

Linuron 6 40 104 79 76

Malathion 6 71 112 99 93

Metolachlor 6 90 124 105 106

Metribuzin 6 82 127 100 102

Molinate 6 90 109 101 100

Napropamide 6 78 112 96 93

Parathion 6 96 139 116 116

Methyl parathion 6 91 178 104 122

Pebulate 6 89 107 99 99

Pendimethalin 6 66 103 94 91

Phorate 6 40 81 65 64

Prometon 6 22 119 94 86

Propachlor 6 93 129 109 109

Propanil 6 90 113 106 104

Propargite 6 60 92 82 81

Pronamide 6 93 113 101 102

Simazine 6 62 114 91 90

Tebuthiuron 6 36 158 118 108

Terbacil 0 Estimated values

Terbufos 6 45 110 83 82

Thiobencarb 6 84 101 95 94

Triallate 6 80 102 92 91

Trifluralin 6 82 123 97 99

Alpha-BHC 6 84 102 96 95

Permethrin 6 16 59 29 31

p,p′-DDE 6 37 82 48 52
Quality-Assurance and Quality-Control Design 39

Of the 47 compounds analyzed by GC/MS, spike 

recoveries could not be calculated for five compounds 

because detection values were estimated (table 16). 

Spike recovery results for four other compounds were 

below the acceptable limit (70 percent). Although 

environmental data was not qualified because of spike 

recovery results, lack of detection of some compounds 

may be due to poor analytical performance, and data 

should be used with caution.

Pesticides in Filtered Water Analyzed by High Performance 

Liquid Chromatography

Five surface-water samples were spiked and 

submitted for pesticide analysis using the HPLC 

method (table 18). Median recovery values for 

compounds that had detections (not estimates) ranged 

from a minimum of 3 percent for dicamba, which had 

only one reportable recovery, to a maximum recovery 

of 89 percent for silvex, which had five recoveries. 

Recovery values were not calculated for aldicarb, 

aldicarb sulfone, or aldicarb sulfoxide because spike 

compounds were not detected in some of the samples 

or were reported only as estimated values. Recovery 

values also could not be calculated for DNOC, 

chlorothalonil, and dichlobenil because all detection 

results for these compounds were estimated; or for 

chloramben because some of the values were deleted 

from the database by the laboratory (U-delete), and the 

compound was not detected in the remaining samples. 

Altogether there were 22 incidents where spiked 

compounds were not detected in surface water samples. 

Of the 32 compounds where the spike recovery 

percentages were calculated, recovery values for all 

five samples were determined for only 16 (Domagalski 

and others, 2000).

The HPLC method tends to yield results that 

have a negative bias. The mean recovery range for 

30 field matrix spike compounds in 81 samples spiked 

at “1.0 μg/L” during the laboratory evaluation of the 

HPLC method (Werner and others, 1996) was from 

9 percent for chlorothalonil to 101 percent for 

propham. Twenty-five of the 30 compounds in the field 

matrix spike solution had mean recoveries of 

65 percent or less, and 10 of those 30 compounds had 

mean recoveries under 50 percent. Recoveries of 

carbofuran and diuron, the constituents targeted as 

critical in surface water, are given in table 19, along 

with comparable mean accuracy results from the 

methods study (Werner and others, 1996).

Table 17. Spike recovery data for selected critical constituents from Sacramento River Basin, California, surface-water samples, 1996–1997, 

with recovery and precision data published in the methods of analysis report

[Recovery and precision data are in Zaugg and others (1995). Spike recovery data are in percentages] 

1 Corrected for background concentrations of compound in surface water.

Compound 

Sacramento River Basin Methods of analysis report mean 

recoveries

Minimum Maximum Median Mean Surface water Reagent water

Chlorpyrifos 63 104 85 87 80 83

Diazinon 40 101 90 81 168 77

Metolachlor 90 124 105 106 87 92

Molinate 90 109 101 100 81 82

Simazine 62 114 91 90 158 76

Thiobencarb 84 101 95 94 76 85
40 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 18. Recovery of field matrix spikes for pesticides from Sacramento River Basin, California, surface-water samples, 1996–1998, analyzed by 

high performance liquid chromatography

[Recovery data are given in percentages. Spike recovery was calculated according to protocols of the National Water Quality Laboratory (Mueller and 

others, 1997). Spike recovery calculations were not included for estimated values. Results deleted by the laboratory are designated “U-delete.” Several 

spiked compounds returned nondetect results. Critical constituents are in bold. No. samples, number of samples where recovery values could be 

calculated] 

Compound No.

samples

Spike recovery in percent

Minimum Maximum Median Mean

2,4,5-T 5 43 92 73 70

2,4-D 3 41 84 54 60

2,4-DB 4 41 83 62 62

Silvex 5 51 100 89 81

3-Hydroxycarbofuran 5 28 100 71 62

DNOC 0 Estimated values

Acifluorfen 3 23 94 75 64

Aldicarb 0 Spike nondetects and estimated values

Aldicarb sulfone 0 Spike nondetects and estimated values

Aldicarb sulfoxide 0 Spike nondetects and estimated values

Bentazon 2 47 79 63 63

Bromacil 4 58 83 74 72

Bromoxynil 5 39 86 84 69

Carbaryl 4 66 88 73 75

Carbofuran 5 47 92 69 68

Chloramben 0 U-delete and spike nondetect values—cannot be calculated

Chlorothalonil 0 Estimated values

Clopyralid 1 25 25 25 25

Dacthal 3 43 79 54 59

Dicamba 1 3 3 3 3

Dichlobenil 0 Estimated values

Dichlorprop 5 25 87 73 62

Dinoseb 5 51 103 84 77

Diuron 4 38 91 84 74

Fenuron 5 47 97 83 74

Fluometuron 5 64 93 80 78

Linuron 5 65 86 81 76

MCPA 4 27 78 76 64

MCPB 3 46 77 65 62

Methiocarb 5 69 92 82 80

Methomyl 5 51 133 88 85

Neburon 5 67 82 78 75

Norfluorazon 5 63 97 87 82

Oryzalin 5 0 68 64 51

Oxamyl 4 26 80 63 58

Picloram 1 60 60 60 60

Propham 1 67 67 67 67

Propoxur 3 38 86 41 55

Triclopyr 5 35 87 80 72
Quality-Assurance and Quality-Control Design 41

Only 14 of the 39 analytes had mean spike 

recovery results within the acceptable limits of 70 to 

130 percent (table 18). Although the data are not 

qualified based on spike recovery results, the lack of 

detection of some compounds may be due to poor 

analytical performance, and the data should be used 

with caution.

Volatile Organic Compounds

All surface-water VOC samples were collected 

at Arcade Creek near Del Paso Heights (table 20). 

Three of the samples collected in 1997 were spiked 

with 13 VOC analytes and submitted to the NWQL for 

analysis. In 1998, one replicate VOC sample set was 

spiked with a solution containing 84 of the 85 VOC 

analytes and submitted for analysis.

The VOC samples collected during 1997 were 

spiked with 13 analytes in 100 μL of spike solution 

delivered using a 100 μL syringe. By 1998, protocol 

had changed and samples were spiked with 85 analytes 

delivered in 20 μL of spike solution using a gas-tight 

25-μL syringe. The VOC samples collected on 

March 9, 1998, however, were spiked with a solution 

that had an expiration date of May 22, 1997. Therefore, 

the relative percentage differences are not reported.

A maximum of five VOC spike recovery values 

could be calculated. Median recoveries ranged from a 

minimum of about 62 percent for methyl methacrylate, 

which had two reportable results, to a maximum of 

about 127 percent for methylbenzene, also having two 

reportable results. For those compounds where five 

recovery values were calculated, the median recovery 

results ranged from 70 percent for ethylbenzene to 

100 percent for 1,2-dichloroethane and 

tetrachloroethene (table 20). Performance of VOCs 

spiked into seven surface water replicate samples 

tested during the methods of analysis evaluation 

(Connor and others, 1998) shows recovery percentages 

ranging from 89.2 percent for 1,1,2-trichloro-1,2,2-

trifluoroethane (spiked at “1 μg/L”) to 119.4 percent 

for trans-1,4-dichloro-2-butene (spiked at “10 μg/L”).

MTBE was the only VOC listed as critical for 

the surface-water component of the Sacramento River 

Basin NAWQA (table 1). There were three spiked 

samples submitted during 1997 and two submitted in 

1998. These five samples returned a median recovery 

of 85 percent and a mean recovery of 82 percent 

(table 20). During laboratory evaluation of the GC/MS 

method of VOC analysis (Connor and others, 1998), 

the mean recovery ranged from 100 to 121 percent for 

seven surface water and seven volatile blank water 

replicate samples spiked using “1 and 10 μg/L” MTBE 

solutions. Although the spike recovery results for the 

Sacramento River Basin surface-water samples appear 

to have a slightly negative bias, they are within 

acceptable limits. The relatively low spike recovery 

results are more likely attributable to problems with the 

spike procedures in the field rather than problems at the 

laboratory. Environmental data should be usable 

without qualification.

Table 19. Spike recovery data for selected pesticides from Sacramento River Basin, California, surface-water samples, 1996–1998, and field 

matrix spike recovery and precision data published in the methods of analysis report

[Field matrix spike recovery and precision data are in Werner and others (1996). Spike recovery data are in percentages] 

1Mean recoveries for the carbofuran and diuron spikes collected during the Sacramento surface-water study were compiled from data contained 

in Domagalski and others (2000).

Compound

Sacramento River Basin Methods of analysis report: 

field matrix spike

Minimum Maximum Median Mean1 Mean recovery—81 samples

Carbofuran 47 92 69 68 62 (standard deviation +/- 32)

Diuron 38 91 84 74 43 (standard deviation +/- 18)
42 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Table 20. Recovery of field matrix spikes for volatile organic compounds from Sacramento River Basin, California, surface-water samples,

1996–1998

[Recovery data are given in percentage. In 1997 spike volume was 100 microliters according to protocol; by 1998, spike volume had been adjusted 

to 20 microliters (Connor and others, 1998). Spike recovery was calculated according to protocols of the National Water Quality Laboratory 

(Mueller and others, 1997). Spike recovery calculations were not included for estimated values. Critical constituents are in bold. No. samples, 

number of samples where recovery values could be calculated] 

Volatile organic compound No.

samples

Spike recovery in percent 

Minimum Maximum Median Mean

1,1,1,2-Tetrachloroethane 2 77 87 82 82

1,1,1-Trichloroethane 5 52 104 95 88

1,1,2,2-Tetrachloroethane 2 98 113 106 106

1,1,2-Trichloroethane 2 94 110 102 102

1,1,2-Trichloro-1,2,2-trifluoroethane 2 70 77 73 73

1,1-Dichloroethane 2 95 100 98 98

1,1-Dichloroethene 5 45 90 82 78

1,1-Dichloropropene 2 80 83 81 81

1,2,3,4-Tetramethylbenzene 0 Compound not in spike

1,2,3,5-Tetramethylbenzene 2 82 101 92 92

1,2,3-Trichlorobenzene 2 76 94 85 85

1,2,3-Trichloropropane 2 84 101 93 93

1,2,3-Trimethylbenzene 2 85 106 95 95

1,2,4-Trichlorobenzene 2 62 76 69 69

1,2,4-Trimethylbenzene 2 83 99 91 91

1,2-Dibromo-3-chloropropane 2 84 103 94 94

1,2-Dibromoethane 2 80 85 83 83

1,2-Dichlorobenzene 2 85 100 93 93

1,2-Dichloroethane 5 60 117 100 96

1,2-Dichloropropane 2 82 96 89 89

1,3,5-Trimethylbenzene 1 92 92 92 92

1,3-Dichlorobenzene 2 78 95 86 86

1,3-Dichloropropane 2 88 99 93 93

1,4-Dichlorobenzene 5 58 92 83 79

2,2-Dichloropropane 2 84 89 86 86

2-Butanone 2 83 102 92 92

2-Chlorotoluene 2 69 80 75 75

2-Hexanone 2 80 95 87 87

3-Chloro-1-propene 2 77 79 78 78

4-Chlorotoluene 2 70 82 76 76

p-Isopropyltoluene 2 73 89 81 81

4-Methyl-2-pentanone 1 98 98 98 98

Acetone 2 118 120 119 119

2-Propenenitrile 2 119 129 124 124

Benzene 2 86 92 89 89

Bromobenzene 2 69 78 73 73

Bromochloromethane 2 88 93 90 90

Bromodichloromethane 5 59 105 97 91

Bromoethene 2 69 84 77 77

Tribromomethane 5 68 100 89 87

Bromomethane 0 Estimated values

n-Butylbenzene 2 68 84 76 76

Carbon disulfide 2 58 71 65 65
Quality-Assurance and Quality-Control Design 43

Table 20. Recovery of field matrix spikes for volatile organic compounds from Sacramento River Basin, California, surface-water samples,

1996–1998—Continued

Chlorobenzene 2 66 77 71 71

Chloroethane 2 69 91 80 80

Trichloromethane 2 101 111 106 106

Chloromethane 0 Estimated values

Chlorodibromomethane 5 65 101 95 89

Dibromomethane 2 91 100 96 96

Dichlorodifluoromethane 0 Estimated values

Dichloromethane 2 104 109 107 107

Diethyl ether 2 79 84 82 82

Diisopropyl ether 2 73 83 78 78

Ethyl methacrylate 2 68 83 75 75

Ethyl tert-butyl ether 2 64 73 68 68

Ethylbenzene 5 53 96 70 74

Hexachlorobutadiene 2 76 91 83 83

Hexachloroethane 2 91 103 97 97

Isopropylbenzene 2 67 76 72 72

Methyl acrylate 2 81 84 83 83

Methyl acrylonitrile 2 82 89 86 86

Iodomethane 0 Estimated values

Methyl methacrylate 2 57 67.3 62 62

Naphthalene 2 65 80 72 72

Styrene 2 78 90 84 84

Tetrachloroethene 5 52 114 100 90

Tetrachloromethane 5 49 99 90 85

Tetrahydrofuran 1 84 84 84 84

Methylbenzene 2 118 135 127 127

Trichloroethene 5 52 102 89 84

Trichlorofluoromethane 2 78 96 87 87

Chloroethene 5 36 83 71 62

cis-1,2-Dichloroethene 2 81 86 84 84

cis-1,3-Dichloropropene 2 68 75 71 71

1,4-Dimethylbenzene 2 73 85 79 79

n-Propylbenzene 2 75 85 80 80

2-Ethyltoluene 2 77 92 85 85

1,2-Dimethylbenzene 2 66 80 73 73

sec-Butylbenzene 1 92 92 92 92

Methyl tert-butyl ether 5 64 93 85 82

tert-Butylbenzene 1 104 104 104 104

tert-Amyl methyl ether 2 66 75 71 71

trans-1,2-Dichloroethene 2 86 89 88 88

trans-1,3-Dichloropropene 2 66 81 73 73

trans-1,4-Dichloro-2-butene 1 105 105 105 105

Volatile organic compound No.

samples

Spike recovery in percent 

Minimum Maximum Median Mean
44 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

SUMMARY AND CONCLUSION

Quality-control data discussed in this report are 

intended for use in conjunction with the environmental 

data collected during the ground- and surface-water 

components of the Sacramento River Basin cycle 1 

study. Quality-control data can be used to quantify bias 

and variability resulting from sample collection, 

processing, or analytical practices, and to add 

appropriate qualifications to interpretations of the 

environmental data. Data from blank samples collected 

during the ground-water study revealed levels of 

ammonia and dissolved organic carbon (DOC) 

contamination that would affect the interpretation of 

ground-water data. Residual cleaning media was the 

likely source of DOC contamination in the blanks, but 

purging the sampling lines with three casing volumes 

of ground water likely would have removed residue. 

Therefore, the environmental samples would not have 

been exposed to the contamination detected in the 

DOC blanks. The source of ammonia in blank samples 

was undetermined. However, because of the frequency 

and levels of detection in the blank samples, at least 

some of the measured detections of ammonia in 

ground-water samples, especially those near the 

detection limit, may be due to contamination; 

interpretations using these data should include 

qualifying statements. Aluminum and copper 

contamination in blank samples indicate a bias in 

ground-water samples. Surface-water data have a 

positive bias based on aluminum contamination in the 

blank samples. The source of copper contamination is 

unknown. Additional testing is needed before any 

corrective measures can be taken to mitigate 

contamination. Adjustments to the cleaning procedures 

should result in fewer detections of DOC and 

suspended organic carbon in blank samples, and the 

use of polypropylene vials to dispense nitric acid 

preservative should reduce the number of detections of 

aluminum. Blank samples collected for pesticide and 

volatile organic compound analyses revealed no 

systematic contamination that would affect ground- or 

surface-water environmental data.

Replicate samples for ground and surface water 

indicate that variability resulting from sample 

collection, processing, and analysis is generally low. In 

addition, some of the larger relative percentage 

differences for replicate samples were between 

samples having lowest absolute concentration 

differences and(or) between samples having 

concentrations near the reporting limit.

Overall, surrogate recoveries for pesticides and 

volatile organic compounds in ground- and surfacewater samples were within acceptable limits. As 

expected from laboratory method performance and 

from results published in the methods of analysis 

report, the greatest variability, least consistency among 

recoveries, and a tendency toward a negative bias were 

reported for the surrogate compound analyzed using 

high performance liquid chromatography (HPLC).

Field matrix spikes submitted for pesticide 

analysis by gas chromatography/mass spectrometry 

(GC/MS) during the ground- and surface-water studies 

had recovery values for critical constituents that were 

comparable with published recoveries. Carbofuran, a 

critical constituent for the ground- and surface-water 

studies, and desethyl atrazine, a critical constituent for 

the ground-water component, had only estimated 

concentrations and could not be evaluated. These 

compounds also performed poorly during the 

laboratory methods of analysis study and were 

published as estimates. Field matrix spikes also were 

submitted for pesticide analysis using HPLC during 

ground- and surface-water sampling. Again, results for 

both ground- and surface-water samples were 

comparable with published results in the methods of 

analysis studies, including results for critical 

constituents. Recovery values indicate a predominately 

negative bias for all constituents analyzed with this 

method. In addition to the negative bias, there were 

instances where the spiked compound was not detected 

(22 nondetects in surface-water samples), compounds 

for which values could only be estimated, or cases of 

laboratory-deleted data. Neither the ground- nor 

surface-water pesticide data analyzed by the GC/MS or 

HPLC methods will be qualified in the database as a 

result of field spike recovery calculations. However, 

lack of detection of some compounds may be due to 

poor analytical performance, and the environmental 

data should be used with caution because results may 

be underreported.

Field matrix spikes for VOCs generally showed 

acceptable recovery results for both ground- and 

surface-water samples. Problems affecting recoveries 

appear to be related to errors in the field, such as 

double spiking or using a spike solution past the 

expiration date, rather than problems with the 

analytical method. Because the adjustments made to 
References Cited 45

the method of spike delivery and the amount of spike 

solution added to a sample during the Sacramento 

River Basin NAWQA did not affect measured 

recoveries, data for both protocols were combined in 

this report.

REFERENCES CITED

Alpers, C.N., Taylor, H.E., and Domagalski, J.L., eds., 2000, 

Metals transport in the Sacramento River, California, 

1996–1997. Volume 1: Methods and data: 

U.S. Geological Survey Water-Resources 

Investigations Report 99-4286, 430 p.

Brenton, R.W., and Arnett, T.L., 1993, Methods of analysis 

by the U.S. Geological Survey National Water Quality 

Laboratory—Determination of dissolved organic 

carbon by UV-promoted persulfate oxidation and 

infrared spectrometry: U.S. Geological Survey OpenFile Report 92-480, 12 p.

Childress, C.J.O., Foreman, W.T., Connor, B.F., and 

Maloney, T.J., 1999, New reporting procedures based 

on long-term method detection levels and some 

considerations for interpretations of water-quality data 

provided by the U.S. Geological Survey National Water 

Quality Laboratory: U.S. Geological Survey Open-File 

Report 99-193, 19 p.

Connor, B.F., Rose, D.L., Noriega, M.C., Murtagh, L.K., 

and Abney, S.R., 1998, Methods of analysis by the 

U.S. Geological Survey National Water Quality 

Laboratory—Determination of 86 volatile organic 

compounds in water by gas chromatography/mass 

spectrometry, including detections less than reporting 

limits: U.S. Geological Survey Open-File 

Report 97-829, 78 p.

Dawson, B.J.M., 2001a, Ground-water quality in the 

southeastern Sacramento Valley aquifer, California, 

1996: U.S. Geological Survey Water-Resources 

Investigations Report 01-4125, 24 p.

———2001b, Shallow ground water quality beneath rice 

areas in the Sacramento Valley, California, 1997: 

U.S. Geological Survey Water-Resources 

Investigations Report 01-4000, 33 p.

Domagalski, J.L., 2000, Pesticides in surface water 

measured at select sited in the Sacramento River Basin, 

California, 1996–1998: U.S. Geological Survey WaterResources Investigations Report 00-4203, 24 p.

Domagalski, J.L., and Dileanis, P.D., 2000, Water-quality 

assessment of the Sacramento River Basin, 

California—Water quality of fixed sites, 1996–1998: 

U.S. Geological Survey Water-Resources 

Investigations Report 00-4247, 60 p.

Domagalski, J.L., Dileanis, P.D., Knifong, D.L., 

Munday, C.M., May, J.T., Dawson, B.J., Shelton, J.L., 

and Alpers, C.N., 2000, Water-quality assessment of 

the Sacramento River Basin, California: Water-quality, 

sediment and tissue chemistry, and biological data, 

1995–1998: U.S. Geological Survey Open-File 

Report 00-391, available at 

http://ca.water.usgs.gov/sac_nawqa/waterindex.html.

Faires, L.M., 1993, Methods of analysis by the 

U.S. Geological Survey National Water Quality 

Laboratory—Determination of metals in water by 

inductively coupled plasma-mass spectrometry: 

U.S. Geological Survey Open-File Report 92-634, 28 p.

Fishman, M.J., 1993, Methods of analysis by the 

U.S. Geological Survey National Water Quality 

Laboratory—Determination of inorganic and organic 

constituents in water and fluvial sediments: 

U.S. Geological Survey Open-File Report 93-125, 

217 p.

Fishman, M.J., and Friedman, L.C., 1989, eds., Methods for 

determination of inorganic substances in water and 

fluvial sediments: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. A1, 

545 p.

Fitzgerald, S.A., 1997, Results of quality-control sampling 

of water, bed sediment, and tissue in the western Lake 

Michigan drainages study unit of the National WaterQuality Assessment Program: U.S. Geological Survey 

Water-Resources Investigations Report 97-4148, 24 p.

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design 

of the National Water-Quality Assessment Program: 

Occurrence and distribution of water-quality 

conditions: U.S. Geological Survey Circular 1112, 33 p.

Jones, S.R., and Garbarino, J.K., 1999, Methods of analysis 

of the U.S. Geological Survey National Water Quality 

Laboratory—Determination of arsenic and selenium in 

water and sediment by graphite furnace atomic 

absorption spectrometry: U.S. Geological Survey 

Open-File Report 98-639, 39 p.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water data-collection protocols and procedures 

for the National Water-Quality Assessment Program: 

Collection and documentation of water-quality samples 

and related data: U.S. Geological Survey Open-File 

Report 95-399, 113 p.

Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990, 

Implementation plan for the National Water-Quality 

Assessment Program: U.S. Geological Survey OpenFile Report 90-174, 10 p.

Lindley, C.E., Stewart, J.T., and Sandstrom, M.W., 1996, 

Determination of low concentrations of acetochlor in 

water by automated solid-phase extraction and gas 

chromatography with mass-selected detection: Journal 

of AOAC International, v. 79, no. 4, p. 962–966.
46 Quality-Control Results for Ground-Water and Surface-Water Data, Sacramento River Basin, California, National Water-Quality Assessment, 1996–1998

Mueller, D.K., Martin, J.D., and Lopes, T.J., 1997, Qualitycontrol design for surface-water sampling in the 

National Water-Quality Assessment Program: 

U.S. Geological Survey Open-File Report 97-223, 17 p.

Olson, M.L., and DeWild, J.F., 1999, Techniques for the 

collection and species-specific analysis of low levels of 

mercury in water, sediment, and biota, in

Contamination of hydrologic systems and related 

ecosystems, v. 2 of Morganwalp, D.W., and 

Buxton, H.T., eds., U.S. Geological Survey Toxic 

Substances Hydrology Program—Proceedings of the 

technical meeting, Charleston, South Carolina, 

March 8–12, 1999: U.S. Geological Survey WaterResources Investigations Report 99-4018B, p. 191–

199.

Pirkey, K.D., and Glodt, S.R., 1998, Quality control at the 

U.S. Geological Survey National Water Quality 

Laboratory: U.S. Geological Survey Fact Sheet 

FS-026-98, 4 p. [also available on at 

http://wwwnwql.cr.usgs.gov/Public/pubs/QC_Fact/text.

html].

Shelton, L.R., 1994, Field guide for collecting and 

processing stream-water samples for the National 

Water-Quality Assessment Program: U.S. Geological 

Survey Open-File Report 94-455, 42 p.

Struzeski, T.M., DeGiacomo, W.J., and Zayhowski, E.J., 

1996, Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—

Determination of dissolved aluminum and boron in 

water by inductively coupled plasma-atomic emission 

spectrometry: U.S. Geological Survey Open-File 

Report 96-149, 17 p.

Thurman, E.M., 1985, Organic Geochemistry of natural 

waters: Dordrecht, Holland, Kluwer Academic, 

Development in Biogeochemistry 2, 497 p.

U.S. Environmental Protection Agency, 2000, Drinking 

water standards and health advisories: 

U.S. Environmental Protection Agency, Office of 

Water, EPA-822-B-00-001, accessed August 21, 2001, 

at 

http://www.epa.gov/waterscience/drinking/standards/i

ndex.html.

U.S. Environmental Protection Agency, 1993, Methods for 

the determination of inorganic substances in 

environmental samples: Cincinnati, Ohio, 

Environmental Monitoring and Support Laboratory, 

EPA/600/R-93/100, 79 p.

U.S. Geological Survey, Laboratory performance evaluation 

studies: National Water Quality Laboratory, accessed 

October 8, 2001, at 

http://wwwnwql.cr.usgs.gov/USGS/Performance/labpe.

html.

———Quality assurance: Wisconsin District Mercury 

Laboratory, accessed October 15, 2001, at 

http://infotrek.er.usgs.gov/doc/mercury/qa.html.

———1996, Inductively coupled plasma-atomic emission 

spectrometry (ICP-AES) to replace direct current 

plasma-atomic emission spectrometry (DCP-AES) for 

the determination of dissolved aluminum and boron in 

water: National Water Quality Laboratory Technical 

Memorandum 96-10, accessed September 12, 2001, at 

http://wwwnwql.cr.usgs.gov/Public/nwql_memo.html.

———1997, Changes in minimum reporting levels from 

inductively coupled plasma—Atomic emission 

spectrometry: National Water Quality Laboratory 

Technical Memorandum 98.05, accessed September 12, 

2001, at 

http://wwwnwql.cr.usgs.gov/Public/nwql_memo.html.

———1998, Change in nitric acid preservation for trace 

element samples: Office of Water Quality Technical 

Memorandum 98.06, accessed September 12, 2001, at 

http://water.usgs.gov/admin/memo/QW/auto.html.

Werner, S.L, Burkhardt, M.R., and DeRusseau, S.N., 1996, 

Methods of analysis by the U.S. Geological Survey 

National Water Quality Laboratory—Determination of 

pesticides in water by carbopak-B solid-phase 

extraction and high-performance liquid 

chromatography: U.S. Geological Survey Open-File 

Report 96-216, 42 p.

Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and 

Fehlberg, K.M., 1995, Methods of analysis by the 

U.S. Geological Survey National Water Quality 

Laboratory—Determination of pesticides in water by 

C-18 solid-phase extraction and capillary-column gas 

chromatography/mass spectrometry with selected-ion 

monitoring: U.S. Geological Survey Open-File Report 

95-181, 49 p.