U.S. Geological Survey

U.S. Department of the Interior

Review of Trace Element Blank and Replicate Data Collected

in Ground and Surface Water for the National Water-Quality

Assessment Program, 1991–2002

Scientific Investigations Report 2006–5093

National Water-Quality Assesssment Program

Metals Metalloids Nonmetals Transition Metals Noble Gases

Li Be B

Al

H

Na Mg

K

Rb Y

 C N O F Ne

 Si P S Cl Ar

 He

Zr Nb Tc Ru Rh Pd In Sn Te I Xe

Cs Lu Hf Ta W Re Os Ir Pt Au Hg Bi Po At

Fr Ra Lr

La

Ac

Ce

Th

Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

Pa Np Pu Am Cm Bk Cf Es Fm Md No

Ca Sc Ti V Ga Ge Br Kr

U

Cr

Sr Mo

Ba

Mn Fe Co Ni Cu

Ag Cd

Tl Pb Rn

Zn As

Sb

Se
Cover: Periodic table showing the trace elements, in bold, that are covered in this report. Periodic table adapted from the 

American Chemical Society’s online periodic table, accessed on December 8, 2005, at 

http://www.chemistry.org/portal/a/c/s/1/acsdisplay.html?DOC=sitetools%5Cperiodic_table.html#
Review of Trace Element Blank and 

Replicate Data Collected in Ground and 

Surface Water for the National Water-Quality

Assessment Program, 1991–2002

by Lori E. Apodaca1, David K. Mueller2, and Michael T. Koterba3

Scientific Investigations Report 2006-5093

1

Formerly with the U.S. Geological Survey; currently with the U.S. Environmental Protection Agency, Washington, D.C.

2

U.S. Geological Survey, Denver, Colorado.

3

U.S. Geological Survey, Beltsville, Maryland.

Sacramento, California

2006

National Water-Quality Assessment Program
U.S. Department of the Interior

Dirk Kempthorne, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

For product and ordering information:

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natural hazards, and the environment:

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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested reference:

Apodaca, L.E., Mueller, D.K., and Koterba, M.T., 2006, Review of trace element blank and replicate data collected in 

ground and surface water for the National Water-Quality Assessment Program, 1991–2002: U.S. Geological Survey 

Scientific Investigations Report 2006-5093, 32 p.
v

FOREWORD

The U.S. Geological Survey (USGS) is committed to providing the Nation with accurate and timely scientific information that helps enhance and protect the overall quality of life and that facilitates effective 

management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on 

the quality of the Nation’s water resources is critical to assuring the long-term availability of water that is 

safe for drinking and recreation and suitable for industry, irrigation, and habitat for fish and wildlife. Population growth and increasing demands for multiple water uses make water availability, now measured in 

terms of quantity and quality, even more essential to the long-term sustainability of our communities and 

ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support 

national, regional, and local information needs and decisions related to water-quality management and 

policy (http://water.usgs.gov/nawqa). 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 and priorities. 

From 1991-2001, the NAWQA Program completed interdisciplinary assessments in 51 of the Nation’s 

major river basins and aquifer systems, referred to as Study Units (http://water.usgs.gov/nawqa/

studyu.html). Baseline conditions were established for comparison to future assessments, and long-term 

monitoring was initiated in many of the basins. During the next decade, 42 of the 51 Study Units will be 

reassessed so that 10 years of comparable monitoring data will be available to determine trends at many 

of the Nation’s streams and aquifers. The next 10 years of study also will fill in critical gaps in characterizing water-quality conditions, enhance understanding of factors that affect water quality, and establish 

links between sources of contaminants, the transport of those contaminants through the hydrologic 

system, and the potential effects of contaminants on humans and aquatic ecosystems.

The USGS aims to disseminate credible, timely, and relevant science information to inform practical and 

effective water-resource management and strategies that protect and restore water quality. We hope this 

NAWQA publication will provide you with insights and information to meet your needs, and will foster 

increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS 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 NAWQA Program, therefore, depends on advice and information from other agencies—Federal, State, interstate, Tribal, and local—as well as nongovernmental organizations, industry, 

academia, and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

 Robert M. Hirsch

Associate Director for Water
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Contents

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

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

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

Acknowledgments ................................................................................................2

Trace Element Data Collected ..........................................................................................4

Procedures to Aggregate Blank and Replicate Data ...............................................................4

Methods Used for Data Analysis.......................................................................................5

Methods to Determine Bias .......................................................................................5

Methods to Determine Variability.................................................................................6

Trace Element Contamination Bias ....................................................................................6

Ground Water ......................................................................................................9

Surface Water ......................................................................................................9

Trace Element and Radon Sample Variability ..................................................................... 15

Ground Water ................................................................................................... 15

Surface Water ................................................................................................... 22

Implications of Quality Control Results to Environmental Data ................................................ 26

Potential Effects of Contamination Bias ..................................................................... 26

Potential Effects of Sample Variability ........................................................................ 28

Confidence Limit for an Individual Water-Quality Measurement or Multiple Measurements ... 28

Exceedance of a Water-Quality Standard............................................................... 28

Two Water-Quality Measurements are Different ...................................................... 28

Summary ............................................................................................................... 30

Glossary of Data-Quality Terms .................................................................................... 31

References ............................................................................................................ 31

Note: The data sets used for analysis in this report are posted on the Web site for this report 

(http://pubs.water.usgs.gov/sir/2006-5093/). 

vii
viii Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002

Figures

1. Locations of the 52 National Water-Quality Assessment Program study units that contributed 

the data analyzed in this report .................................................................................3

2. Upper 95-percent confidence limits for contamination of aluminum, barium, boron, chromium, 

cobalt, copper, iron, manganese, nickel, strontium, and zinc that are based on data from 

ground-water field blanks .................................................................................... 10

3. Upper 95-percent confidence limits for contamination of aluminum, barium, boron, copper, 

iron, manganese, strontium, and zinc that are based on data from surface-water field blanks ...... 13

4. Ground-water data used to select low and high ranges of replicate concentrations for 

select trace elements and radon.............................................................................. 17

5. Surface-water data used to select low and high ranges of replicate concentrations for 

select trace elements .......................................................................................... 23

Tables

1. Upper 95-percent confidence limit by percentiles for contamination in trace elements from 

ground-water field-blank data ....................................................................................... 7

2. Upper 95-percent confidence limit by percentiles for contamination in trace elements from 

surface-water field-blank data .................................................................................8

3. Number of trace element ground-water and surface-water replicate sample sets and number of 

consistent detections within the replicate sets .................................................................. 15

4. Estimated sample variability at measured concentrations in ground-water trace element data ......... 16

5. Estimated sample variability at measured concentrations in surface-water trace element data ......... 25

6. Summary of trace-element environmental ground-water and surface-water data collected 

from 1991 to 2002...................................................................................................... 27

7. Estimated sample variability and confidence intervals around U.S. Environmental Protection Agency 

drinking-water standards for select trace elements in ground water and surface water. .................. 29
ix

Abbreviations

AL, action level

AMCL, alternative maximum contaminant level

CRL, common reporting level

GFAA, graphite-furnace atomic absorption

HA, health advisory

HGAA, hydride generation atomic absorption

IBSP, Inorganic Blind Sample Project

ICP-AES, inductively coupled plasma-atomic emission spectroscopy

ICP-MS, inductively coupled plasma-mass spectrometry

loess, local regression nonparametric smoothing technique

LRL, laboratory reporting level

MCL, Maximum Contaminant Level

MRL, method reporting level

NAWQA, National Water-Quality Assessment (U.S. Geological Survey) program

NWQL, National Water Quality Laboratory (U.S. Geological Survey)

pCi/L, picocuries per liter

QC, quality control

RSD, relative standard deviation

SD, standard deviation

SDWR, Secondary Drinking Water Regulations

UCL, upper confidence limit

USEPA, U.S. Environmental Protection Agency

USGS, U.S. Geological Survey

μg/L, microgram per liter

Notes: Concentrations of chemical constituents in water are given in microgram per liter (μg/L).

ix Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
This page intentionally left blank.
Abstract

In the process of interpreting and analyzing trace element 

water-quality data for ground and surface water, it is important 

to determine the bias and variability that may be associated 

with these data. Trace element quality control samples (blanks 

and replicates) collected in the field for the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) 

Program from 1991 to 2002 were reviewed to determine the 

potential bias and variability that may be associated with the 

environmental samples. Bias in the data may be related to 

contamination from the field or laboratory during the collection, processing, shipping, or analysis of the samples. Sample 

variability can affect the interpretation of differences between 

individual measurements or mean concentrations. Trace element quality control data are available for 23 trace elements: 

aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), 

beryllium (Be), boron (B), cadmium (Cd), chromium (Cr), 

cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), 

manganese (Mn), molybdenum (Mo), nickel (Ni), selenium 

(Se), silver (Ag), strontium (Sr), thallium (Tl), uranium (U), 

vanadium (V), and zinc (Zn). In addition, replicate data for 

radon (Rn) in ground water were reviewed. Statistical analyses 

were used to estimate the likelihood of contamination bias 

and sample variability that could occur in the environmental 

samples. The 95-percent upper confidence limit was calculated 

at select percentiles to assess the potential for trace element 

contamination. The 95-percent confidence intervals were 

calculated for sample variability.

The trace elements Sb, Be, and Tl in ground water and 

Sb, Be, Co, Mo, and U in surface water are unaffected by 

contamination. Limited quality control data (blanks) for Li and 

V in ground water and surface water do not allow for a good 

assessment on the potential contamination associated with 

these trace elements. Potential contamination was identified 

for Al, As, Ba, B, Cd, Cr, Cu, Fe, Pb, Mn, Ni, Se, Ag, Sr, and 

Zn in ground water and surface water. Evidence of potential 

contamination was shown for Co, Mo, and U in ground water; 

potential contamination was shown for T1 in surface water. In 

comparing the potential contamination for these trace elements 

with the U.S. Environmental Protection Agency’s (USEPA) 

drinking-water standards, the contamination for most of these 

trace elements is less than 10 percent of the drinking-water 

standard; therefore, contamination would have little or no 

effect when comparing trace element concentrations with 

the USEPA drinking-water standards. The exceptions are Al, 

Cd, and possibly Pb in ground water, and As and possibly 

Pb in surface water. Potential contamination identified for 

these trace elements is greater than 10 percent of the USEPA 

drinking-water standard, but affects only 5 percent or less of 

the As, Cd, and Pb samples. For most trace elements, the level 

of potential contamination is not large enough to significantly affect the measured concentration of the environmental 

sample. The exceptions may be Fe in ground water and Al in 

surface water, which have concentrations for at least 10 percent of the environmental samples that exceeded the USEPA 

drinking-water standards.

Review of Trace Element Blank and Replicate Data 

Collected in Ground and Surface Water for the National 

Water-Quality Assessment Program, 1991–2002

By Lori E. Apodaca, David K. Mueller, and Michael T. Koterba
Sample variability for some of the trace elements could 

not be determined because there were either no detected 

concentrations, or there were less than 10 replicate sets with 

detected concentrations. These trace elements are Be, Ag, 

and Tl for ground water and Sb, Be, Cr, Co, Pb, Ag, and Tl 

for surface water. For most trace elements, sample variability 

was less than 10 percent, which would have little or no affect 

on the reported concentrations. The exceptions are Al, Cd, 

Cu, Pb, Rn (at concentrations less than about 700 picocuries 

per liter), Se, and Zn in ground water and Cu, Se, and Zn in 

surface water, all of which have sample variability ranging 

from 10 to 20 percent. Sample variability should be considered 

when evaluating the potential error associated with a sample 

measurement.

Collection of additional quality control samples for some 

of these trace elements to determine bias and variability is 

probably warranted particularly for those trace elements that 

the NAWQA Program did not begin sampling until 1998. 

Results obtained from the analysis of the quality control data 

can be applied to the interpretation of the environmental data 

collected from 1991 to 2002 and for water-quality data that are 

currently being collected as part of the NAWQA Program.

Introduction

The U.S. Geological Survey’s (USGS) National WaterQuality Assessment (NAWQA) Program was implemented 

in 1991 to improve the scientific and public understanding of 

water quality in the Nation’s major river basins and groundwater systems. The goals of the NAWQA Program are to 

describe current water-quality conditions and trends in the 

Nation’s rivers, streams, and ground water to understand the 

natural characteristics and human influences that affect water 

quality (Hirsch and others, 1988). In Cycle I (1991–2001), 

the first decade of extensive monitoring within 52 study units 

(fig. 1), work concentrated primarily on gathering comparable 

information on water quality in surface water and ground 

water.

To interpret trace element environmental water-quality 

data, information is needed to determine the bias and variability in the water-quality data that can result from sample 

collection, processing, shipping, and analysis. Sample bias and 

variability can be evaluated by collecting quality control (QC) 

samples such as blank and replicate samples that are collected 

with the environmental samples. Bias is a systematic error and 

can be either positive or negative. An example of positive bias 

is contamination of the samples. Variability is a random error 

that affects the ability to reproduce an analysis.

Purpose and Scope

This report describes the results of the analysis of the 

trace element blank and replicate data collected with the 

environmental samples from 1991 to 2002 of the NAWQA 

Program. The QC data analysis is used to (1) describe the frequency and magnitude of trace element contamination using 

field blank data; (2) evaluate variability of the water-quality 

data using field replicate data; and (3) identify potential effects 

of bias and variability in interpreting the trace element data.

Trace elements reviewed in this report include aluminum 

(Al), antimony (Sb), arsenic (As), barium (Ba), beryllium 

(Be), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), 

copper (Cu), iron (Fe), lead (Pb), lithium (Li), manganese 

(Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silver 

(Ag), strontium (Sr), thallium (Tl), uranium (U), vanadium 

(V), and, zinc (Zn). In addition, radon (Rn) replicate sample 

analyses in ground water were reviewed.

The results of the QC data analysis are compared with 

the U.S. Environmental Protection Agency’s (USEPA) drinking-water standards to assess the potential effects of bias and 

variability on the environmental data (U.S. Environmental 

Protection Agency, 2004). This report does not address QC or 

trace element data that are contaminated or trace element data 

errors related to the coding of trace element data.

Acknowledgments

The authors would like to thank Leslie DeSimone and 

George Groschen for their technical reviews of this manuscript. Also, the work done by the Cycle I NAWQA study 

unit hydrologists and hydrologic technicians who collected 

the trace element water-quality data is greatly appreciated. 

Without their efforts, the analysis of the trace element QC data 

would not have been possible.

2 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
CCPT

WILL REDN

WMIC CONN HDSN

USNK

LSUS

POTO SPLT

WHIT SANJ

ALBE

OZRK

RIOG

TRIN

GAFL ACFB

NVBR

PUGT

SACR EIWA LINJ LERI

UCOL KANA

SANT

MISE

SOFL

SCTX

UTEN

ALMN

UMIS

CAZB

LIRB

NROK

GRSL

DELR

UIRB

MIAM

SANA

LTEN

MOBL

ACAD

YELL

NECB

YAKI

DLMV

CNBR

HPGW

1992–1995

1996–1998

1999–2001

1999–2001

Samples collected during 

 water years

EXPLANATION

COOK

OAHU

MAP

ACRONYM STUDY UNIT

ACAD

ACFB

ALBE

ALMN

CAZB

CCPT

CNBR

CONN

COOK

DELR

DLMV

EIWA

GAFL

GRSL

HDSN

HPGW

KANA

LERI

Acadian–Pontchartrain Drainages

Apalachicola–Chattahoochee–Flint River Basins

Albemarle–Pamlico Drainages

Allegheny and Monongahela River Basins

Central Arizona Basins

Central Columbia Plateau

Central Nebraska Basins

Connecticut, Housatonic, and Thames River Basins

Cook Inlet Basin

Delaware River Basin

Delmarva Peninsula

Eastern Iowa Basins

Georgia–Florida Coastal Plain Drainages

Great Salt Lake Basins

Hudson River Basin

High Plains Regional Ground Water Study

Kanawha–New River Basin

Lake Erie–Lake Saint Clair Drainages

MAP

ACRONYM STUDY UNIT

MAP

ACRONYM STUDY UNIT

LINJ

LIRB

LSUS

LTEN

MIAM

MISE

MOBL

NECB

NROK

NVBR

OAHU

OZRK

POTO

PUGT

REDN

RIOG

SACR

SANA

Long Island–New Jersey Coastal Drainages

Lower Illinois River Basin

Lower Susquehanna River Basin

Lower Tennessee River Basin

Great and Little Miami River Basins

Mississippi Embayment

Mobile River Basin

New England Coastal Basins

Northern Rockies Intermontane Basins

Nevada Basin and Range

Oahu

Ozark Plateaus

Potomac River Basin

Puget Sound Drainages

Red River of the North Basin

Rio Grande Valley

Sacramento River Basin

Santa Ana Basin

SANJ

SANT

SCTX

SOFL

SPLT

TRIN

UCOL

UIRB

UMIS

USNK

UTEN

WHIT

WILL

WMIC

YAKI

YELL

San Joaquin–Tulare Basins

Santee River Basin and Coastal Drainages

South-Central Texas

South Florida Drainages

South Platte River Basin

Trinity River Basin

Upper Colorado River Basin

Upper Illinois River Basin

Upper Mississippi River Basin

Upper Snake River Basin

Upper Tennessee River Basin

White River Basin

Willamette Basin

Western Lake Michigan Drainages

Yakima River Basin

Yellowstone River Basin

Figure 1. Locations of the 52 National Water-Quality Assessment Program study units that contributed the data analyzed in this report.

Introduction 3
Trace Element Data Collected

For the 52 study units that were sampled in Cycle I 

(1992–2001), implementation of the study-unit investigations 

were phased so that the water-quality data were collected in 

about one-third of the study units at a time. During Cycle I 

of the NAWQA Program, sampling occurred over a period 

of 3 water years (water year is the period from October 1 

through September 30 and identified by the year in which 

it ends) in each study unit. After sampling was completed 

during the three year period, sampling was implemented that 

involved water-quality sampling at a few select surface-water 

and ground-water sites. For the first group of 20 study units, 

sampling occurred mainly during water years 1993–95, but 

some limited sampling did occur in water year 1992, followed 

by sampling at selected sites during water years 1996–2001. 

The second group of 16 study units began sampling during 

water years 1996–1998, followed by sampling at selected 

sites during water years 1999–2001. The third group, consisting of 16 study units, completed sampling during water years 

1999–2001. Four study units from the third group completed 

water-quality sampling in water year 2002. Data from these 

study units have been included in the analysis of the trace 

element QC data.

During Cycle I, trace element sampling initially was 

optional and focused mainly on ground water (1992/3–95). 

Surface-water sampling generally consisted of synoptic 

sampling to address local trace element issues. Ground-water 

samples were collected for Rn analysis by most study units 

during Cycle I. From 1996 to 1998, trace element sampling 

became a more integrated part of the NAWQA Program and 

was fully implemented into the NAWQA Program from 1999 

to 2001.

The differences in the timing as to when trace elements 

were collected in Cycle I, in the media targeted for sampling, 

and in the laboratory methods used during Cycle I, have lead 

to differences in the number of QC samples available for each 

trace element. From 1992 to 1993, about 15 percent of the 

water-quality samples that were collected consisted of QC 

samples (P. Patrick Leahy, U.S. Geological Survey, written 

commun., 1992 and 1993). QC sampling was fully integrated 

into the sampling design beginning in 1994 (Koterba and others, 1995; Mueller and others, 1997). QC sampling required 

that blanks and replicates be taken at regular intervals 

throughout a sampling effort.

Trace element environmental and QC samples were 

analyzed at the USGS’s National Water Quality Laboratory 

(NWQL), Denver, Colorado. The trace elements discussed in 

this report typically were analyzed by two different analytical techniques: inductively coupled plasma-atomic emission 

spectroscopy (ICP-AES) and inductively coupled plasma-mass 

spectrometry (ICP-MS) (Garbarino, 1999). Additional 

analytical techniques were used for As, Cr, and Se. As and 

Se were analyzed using graphite-furnace atomic absorption 

(GFAA), hydride generation atomic absorption (HGAA), and 

ICP-MS. Cr was analyzed using GFAA, and ICP-AES or ICPMS analytical techniques. Review of the analytical data for 

As, Cr, and Se from the NWQL Inorganic Blind Sample Project (IBSP) indicates that these methods are comparable, with 

the relative error between the analytical value and the most 

probable value averaging less than 0.2 percent for the period 

when samples were collected during Cycle I for both methods 

(Ludtke and Woodworth, 1997).

Analysis of trace elements, prior to 1993, involved the 

use of analytical methods that were available at the NWQL. 

In 1993, a low-level (1 μg/L or less) trace element analytical 

schedule was developed for the NAWQA Program along with 

a low-level (0.5 μg/L or less) blank schedule (Garbarino and 

Taylor, 1996). B, Sr, and Tl were added to the NAWQA lowlevel trace element schedule in 1996; in addition, reporting 

levels decreased, thus, eliminating the need for the low-level 

blank schedule. In 1998, Li and V were added to the trace element schedule (Garbarino, 1999). Beginning in October 1999, 

the NWQL also provided semiqualified (E-remarked) values 

below the laboratory reporting level (LRL) for select trace 

elements (Childress and others, 1999). Rn was analyzed using 

scintillation vial counting throughout Cycle I. Fe and Mn were 

analyzed using the NAWQA Program’s major-ions schedule; 

however, Mn concentrations from the trace element schedule 

were used in this report because the trace element analytical 

schedule had a lower method reporting level (MRL).

Procedures to Aggregate Blank and 

Replicate Data

The main objective in the aggregation of blank and replicate data for trace elements was to obtain a data set that could 

be used to assess bias and variability in the trace element 

environmental data. Because of differences in sample collection methods, when blanks and replicates were collected, and 

the emphasis on trace elements in ground water rather than 

on surface water, QC data for these two media are analyzed 

separately. For each medium, however, identical procedures 

are used to aggregate the QC blank data for each trace element 

in relation to a single reporting level, which is referred to in 

this report as a “common reporting level” (CRL). The use of 

a single common reporting level, however, can result in the 

loss of data, particularly for those trace elements with several 

MRLs that are below the selected reporting level. Therefore, 

for some of the trace elements, the blank data have also been 

censored and analyzed at the second most CRL.

4 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
The first CRL was selected as the most frequently used 

MRL in the environmental and blank samples from among 

those used for a particular trace element in Cycle I, except 

for Sr, where the MRL of the blank samples was used. This 

approach was used to obtain the maximum number of uncensored values at or above the CRL. In selecting the CRL for a 

particular trace element, the QC data for each trace element 

were processed independent of one other. Quantified (not 

E-remarked), semi-quantified (E-remarked), or less than (<) 

values below the CRL became a “<CRL” value for statistical 

purposes. Samples that are V-marked (samples identified as 

being contaminated) were not used in determining the CRL. 

If possible, a second CRL was selected that was often the 

next lowest common MRL. The second CRL is used to better 

define the potential contamination, but consists of a smaller 

sample size.

Water that was used for collecting the blank samples 

essentially was free of the analytes of interest; analyte concentrations in the blank water samples are documented by the 

NWQL prior to distribution to the NAWQA study units. To 

assess potential causes of bias in the environmental samples, 

all blank sample types collected during Cycle I were reviewed. 

These samples included source solution, equipment, and field 

blanks; however, the field blanks were the most useful in interpreting the bias associated with the environmental samples. 

Generally, no more than about a dozen suitable source-solution blanks or equipment blanks are available for a particular 

trace element in ground water or surface water.

The trace element replicate data set that is used in this 

report includes samples that are identified as being either 

environmental or replicate samples that have the same site and 

date. The type of replicate samples collected for ground water 

were most often identified as a “sequential” replicate (96 

percent of the replicates collected). Sequential replicates are 

samples collected immediately following the environmental 

sample. For surface water, however, several types of replicate 

samples were collected that included concurrent, sequential, 

and split replicate samples. All replicate sample types in the 

surface-water samples were combined to increase the number 

of replicate samples available for a particular trace element, 

as most trace elements had limited data with detected values. 

Other reasons for combining the replicate data are that the 

variability in the sample data are most likely a result of laboratory processing and analysis. Mueller (1998) reviewed the 

standard deviation for nitrogen and phophorus concentrations 

of split and other types of replicate samples and determined 

that variability could not be attributed to the type of replicate 

sample collected. In this report, replicate trace element data 

are not censored to a CRL.

All QC data used in this report were retrieved from 

the USGS’s NAWQA Data Warehouse (http://water.usgs.

gov/nawqa/data). Retrievals for blank data were conducted 

in March 2002 and a later retrieval for replicate data was 

conducted in October 2003. The only difference between these 

two data retrievals is that there are replicate samples from 

study units that completed their Cycle I sampling in water year 

2002. The data sets used for analysis in this report are posted 

on the Web site for this report (http://pubs.water.usgs.gov/

sir2006-5093/). The glossary located in the back of this report 

defines QC terms that might be unfamiliar to some readers.

Methods Used for Data Analysis

Bias and variability associated with trace element waterquality data can be determined by statistical analysis of field 

blank and replicate samples. The trace element field blank data 

were censored to a CRL before analysis because of the different MRLs used and inconsistencies early in Cycle I as to the 

schedules used for analysis. Trace element replicate data were 

not given a CRL.

Methods to Determine Bias

A graphical approach (consisting of time series plots of 

contaminant concentrations) was initially used to determine 

whether blank contamination occurred above or below the 

CRL during a specified period in Cycle I. In addition, to assess 

the potential for trace element blank contamination at or above 

the selected CRL, a one-sided nonparametric 95-percent upper 

confidence limit (UCL) was calculated at select percentiles 

(50th, 75th, 85th, and 90th through 99th) for the trace element concentrations using the method of Hahn and Meeker 

(1991). Hahn and Meeker (1991) used a distribution-free UCL 

for a percentile, which is appropriate for skewed data. The 

method uses order statistics, which is based on ranking the 

data concentration from large to small, then uses a binomial 

probability to determine the UCL. The SAS statistical program 

(SAS Institute, Inc, 1990) was used to calculate the UCL at the 

selected percentiles. The UCL is used to determine the maximum contamination expected for a specified percentage of 

water samples. For example, the 95-percent UCL confidence 

limit for the 90th percentile indicates that, with 95-percent 

confidence, the specified amount of contamination would be 

exceeded in no more than 10 percent of the samples. The 95-

percent confidence at the 90th percentile can also be described 

as the maximum contamination expected in 90 percent of the 

samples with only a 5-percent chance that the contamination 

has been underestimated. The amount of contamination identified in the blank samples relates to the environmental samples, 

which are collected, processed, shipped, and analyzed in the 

same manner as the blank samples.

Methods Used for Data Analysais 5
Methods to Determine Variability

Information on the ground-water and surface-water 

replicate data was summarized by the number of replicate 

samples collected and the consistency of detections. Replicate 

variability can be determined only when both samples in a 

replicate pair have detections. Thus, replicate data sets with 

measured concentrations less than the MRL were excluded. In 

addition, replicate data sets with inconsistent detections (that 

is, where the trace element was detected in one sample but not 

in both) also were excluded. The reason for excluding these 

samples is that concentrations less than the MRL typically 

were rounded to a whole number, such that the calculation of 

the relative standard deviation (RSD) would be quite variable 

between replicate sets, even if the difference in concentration 

between the environmental and the replicate sample was relatively small. Also, only data that were reported to at least one 

decimal place were used in evaluating the variability between 

replicate sets for a particular trace element. Rn data, which 

are reported primarily as whole numbers, however, were not 

excluded in the replicate data analysis.

Sample variability of replicate samples is best characterized by different statistical measures over different concentration ranges. Variability over a large range of concentrations 

can be approximated by dividing the concentration range into 

segments where either the standard deviation (SD) or RSD is 

relatively constant (Anderson, 1987). Concentration ranges for 

the replicate sets were determined by plotting the mean sample 

concentration and the SD and RSD. The RSD or coefficient of 

variance, in percent, is defined as the SD divided by the mean 

concentration (# ) times 100:

RSD = [(SD/C ) × 100].	 (1)

An approximate boundary between concentration ranges 

can be determined by the change in slope of a loess (local 

regression nonparametric smoothing technique) curve 

(S-Plus, 2002) through the center of the data. For low concentrations—those close to the MRL—the mean SD is approximately constant and therefore, is an appropriate measure of 

the sample variability. The mean SD generally increases with 

concentration; therefore, variability is most appropriately represented by the mean RSD for the upper range of concentrations. Because of the small number of replicate sets for some 

of the trace elements, a change in the slope of the loess curve 

was not observed; therefore, the mean RSD was used to evaluate the variability over the entire concentration range. Using 

the mean RSD over the entire range most likely overestimates 

the variability at high concentrations and underestimates the 

variability at low concentrations.

After the sample variability has been determined for the 

low and high concentration ranges, then confidence intervals 

can be determined for any one measurement such as an 

environmental sample. The confidence interval for an 

individual measurement is defined as:

[ ] , (1- /2) C ,C C Z L U = ± ×

α

σ 	 (2)

where

CL ,CU = the lower and upper limits of concentration for 

the 100(1 – α) percent confidence interval,

C = individual measured concentration,

α = probability that the confidence interval does not 

include the true concentration,

Z = the ordinate of the normal curve (Z-value) that 

contains 100(1 – α) percent of the distribution, 

and

σ = sampling variability for the measured concentration: low range σ = SD and high range 

σ = C (RSD/100).

For multiple samples, the error associated with the mean concentration as a result of sampling variability is reduced, as 

indicated in the following equation:

C ,C C Z L U





 



 = ± (1- / α 2)

σ

n ,

	 (3)

where

 n = the number of samples,

 # = the mean concentration for these samples; and the 

other variables are as previously defined above.

Trace Element Contamination Bias

The potential contamination for the 23 different trace 

elements was determined separately for ground-water (table 

1) and surface-water (table 2) data. For most of the trace elements sampled in Cycle I, there is about one third to one-half 

as many surface-water blanks as compared with ground-water 

blanks.

In evaluating the QC data for each trace element, time 

series plots showed no evidence of contamination for specific 

time periods for any of the 23 trace elements. The distribution of the trace element concentrations greater than the 

CRL in blanks is more likely to be a result of changes in the 

sampling frequency or reporting level (number of significant 

decimal places) rather than by an overall increase or decrease 

in concentrations during Cycle I. This is similar to the results 

of Mueller and Titus (2005) who reviewed nutrient QC data 

collected by the NAWQA Program from water years 1992 

to 2001. For example, systematic contamination among the 

study units is apparent for Al, B, Fe, and Zn (tables 1 and 2). 

For other trace elements, high concentrations in blanks were 

observed in only a few study units; no temporal pattern in 

contamination was observed, and so, these cases are likely not 

the result of systematic contamination.

6 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Table 1. Upper 95-percent confidence limit by percentiles for contamination in trace elements from ground-water field-blank data. 

[Bold faced values indicate contamination greater than the common reporting level. Al, aluminum; Ag, silver; As, arsenic; B, boron; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cr, chromium; Cu, copper; Fe, iron; Pb, lead; Li, lithium; Mn, manganese; Mo, molybdenum; Ni, nickel; Sb, antimony; Se, selenium; Sr, strontium; Tl, thallium; U, uranium; USEPA, U.S. Environmental Protection Agency; V, vanadium; Zn, zinc; na, not applicable; μg/L, microgram per liter; <, less than; —, too few data to compute]

1U.S. Environmental Protection Agency, 2004. 2Secondary Drinking Water Regulation (SDWR): is a non-enforceable Federal guideline to address cosmetic or aesthetic effects of drinking water. 3Maximum Contaminant Level (MCL): the highest level of contaminant that is allowed in drinking water. 4Health Advisory (HA): is an acceptable drinking-water level for a chemical substance that is based on health effects information. HAs are not legally enforceable, but are a standard that serves as a guidance 

to assist Federal, state, and local officials. 

5Action Level (AL): concentration of a contaminant that triggers treatment or other requirements that a water system must follow. For lead and copper, the action level is the level where 10 percent of the 

homes tested exceed the standard.

Al Sb As Ba Be B Cd Cr Co Cu Fe Pb Li Mn Mo Ni Se Ag Sr Tl U V Zn

USEPA Drinking-Water Standard1 250–200 36 310 32,000 34 4600 35 3100 — 51,300 2300 15 — 4300 440 41,00 350 4100 44,000 32 330 — 42,000

Common Reporting Level (μg/L) 1.0 1.0 1.0 1.0 1.0 4.0 1.0 1.0 1.0 1.0 3.0 1.0 0.3 1.0 1.0 1.0 1.0 1.0 0.1 0.1 1.0 1.0 1.0

Number of blanks 330 347 329 344 346 164 353 343 343 345 423 343 37 546 341 339 302 343 186 151 368 37 341

Percentile 95-percent Upper Confidence Limit (μg/L)

50th (median) 3.0 <1.0 <1.0 <1.0 <1.0 5.9 <1.0 <1.0 <1.0 <1.0 <3.0 <1.0 <0.3 <1.0 <1.0 <1.0 <1.0 <1.0 <0.1 <0.1 <1.0 <1.0 1.0

75th 4.0 <1.0 <1.0 <1.0 <1.0 9.2 <1.0 <1.0 <1.0 <1.0 5.0 <1.0 <0.3 <1.0 <1.0 <1.0 <1.0 <1.0 0.16 <0.1 <1.0 <1.0 3.085th 4.9 <1.0 <1.0 <1.0 <1.0 12 <1.0 <1.0 <1.0 1.0 8.0 <1.0 0.36 <1.0 <1.0 <1.0 <1.0 <1.0 0.3 <0.1 <1.0 <1.0 6.090th 6.0 <1.0 <1.0 <1.0 <1.0 13 <1.0 <1.0 <1.0 2.0 11 <1.0 0.78 1.0 <1.0 <1.0 <1.0 <1.0 0.78 <0.1 <1.0 <1.0 8.0

91st 6.5 <1.0 <1.0 <1.0 <1.0 13 <1.0 <1.0 <1.0 2.1 12 <1.0 0.78 1.0 <1.0 <1.0 <1.0 <1.0 0.78 <0.1 <1.0 <1.0 9.0

92nd 7.0 <1.0 <1.0 <1.0 <1.0 13 <1.0 <1.0 <1.0 3.0 15 <1.0 0.78 1.0 <1.0 <1.0 <1.0 <1.0 1.6 <0.1 <1.0 <1.0 11

93rd 8.0 <1.0 <1.0 1 <1.0 14 <1.0 <1.0 <1.0 3.0 20 <1.0 — 1.0 <1.0 <1.0 <1.0 <1.0 2.0 <0.1 <1.0 — 14

94th 8.3 <1.0 <1.0 1 <1.0 15 <1.0 1 <1.0 3.8 21 <1.0 — 1.0 <1.0 <1.0 <1.0 <1.0 2.3 <0.1 <1.0 — 1495th 9.0 <1.0 <1.0 1.1 <1.0 15 <1.0 1 <1.0 4.0 29 <1.0 — 1.2 <1.0 <1.0 <1.0 <1.0 3.0 <0.1 <1.0 — 1896th 10 <1.0 <1.0 1.5 <1.0 19 <1.0 1.1 <1.0 4.4 36 <1.0 — 1.6 <1.0 <1.0 <1.0 <1.0 3.4 <0.1 <1.0 — 2197th 12 <1.0 <1.0 3.9 <1.0 19 <1.0 2 <1.0 5.3 42 <1.0 — 2.0 <1.0 1.0 <1.0 <1.0 6.1 <0.1 <1.0 — 3698th 14 <1.0 <1.0 5 <1.0 22 1 2 1 12 46 <1.0 — 2.0 <1.0 2.0 1.2 <1.0 19 <0.1 <1.0 — 3899th 91 <1.0 1.0 7.9 <1.0 — 1 4.5 1.5 101 100 7 — 4.0 <1.0 2.5 2.4 1.0 — — 1.6 — 68

Common Reporting Level (μg/L) 0.3 0.2 0.18 0.2 0.2 2 0.3 0.2 0.2 0.2 na 0.3 na 0.1 0.2 0.5 0.7 0.2 na na 0.2 0.21 0.5

Number of blanks 147 152 9 147 152 142 153 144 152 154 na 152 na 190 152 149 40 148 na na 153 4 146

Percentile 95-percent Upper Confidence Limit (μg/L)

50th (median) 2.9 <0.2 na <0.2 <0.2 5.1 <0.3 <0.2 <0.2 0.24 na <0.3 na <0.1 <0.2 <0.5 <0.7 <0.2 na na <0.2 na 0.92

75th 3.6 <0.2 na 0.27 <0.2 7.9 <0.3 0.27 <0.2 0.56 na <0.3 na 0.15 <0.2 <0.5 <0.7 <0.2 na na <0.2 na 2.185th 4.1 <0.2 na 0.47 <0.2 10 <0.3 0.33 <0.2 0.79 na <0.3 na 0.25 <0.2 <0.5 <0.7 <0.2 na na <0.2 na 3.590th 4.8 <0.2 na 0.6 <0.2 11 <0.3 0.5 0.2 1.4 na <0.3 na 0.35 <0.2 <0.5 <0.7 <0.2 na na <0.2 na 6.8

91st 4.8 <0.2 na 0.77 <0.2 12 <0.3 0.54 0.22 1.8 na <0.3 na 0.44 <0.2 <0.5 <0.7 <0.2 na na <0.2 na 7

92nd 5 <0.2 na 1 <0.2 12 <0.3 0.68 0.36 1.9 na <0.3 na 0.47 <0.2 0.51 <0.7 <0.2 na na <0.2 na 7.2

93rd 5.2 <0.2 na 1.3 <0.2 12 <0.3 0.72 0.37 2.2 na <0.3 na 0.54 <0.2 0.51 — <0.2 na na <0.2 na 12

94th 5.6 <0.2 na 1.9 <0.2 12 <0.3 0.76 0.42 2.7 na 0.42 na 0.61 <0.2 0.62 — <0.2 na na <0.2 na 18

95th 6.4 <0.2 na 3.6 <0.2 13 <0.3 0.83 0.82 3.9 na 0.57 na 0.79 <0.2 0.75 — <0.2 na na <0.2 na 2596th 6.5 <0.2 na 3.9 <0.2 13 <0.3 0.89 0.89 4.3 na 0.7 na 0.86 <0.2 0.85 — <0.2 na na <0.2 na 38

97th 8.5 <0.2 na 5.2 <0.2 14 0.42 1.1 1.4 13 na 7.0 na 1.1 0.25 1.0 — <0.2 na na <0.2 na 6898th — <0.2 na — <0.2 — 0.42 — 1.4 13 na 7.0 na 1.2 0.25 1.0 — <0.2 na na <0.2 na —

99th — — na — — — — — — — na — na — — — — — na na — na —

Trace Element Contamination Bias7
Al Sb As Ba Be B Cd Cr Co Cu Fe Pb Li Mn Mo Ni Se Ag Sr Tl U V Zn

USEPA Drinking-Water Standard1 250–200 36 310 32,000 34 4600 35 3100

— 51,300 2300 515

— 4300 440 4100 350 4100 44,000 32 330

— 42,000

Common Reporting Level (μg/L) 1.0 1.0 1.0 1.0 1.0 4.0 1.0 1.0 1.0 1.0 3.0 1.0 0.3 1.0 1.0 1.0 1.0 1.0 0.1 0.1 1.0 1.0 1.0

Number of blanks 119 130 86 124 138 55 139 125 126 127 831 129 9 875 130 124 119 142 57 54 127 10 124

Percentile 95-percent Upper Confidence Limit (μg/L)

50th (median) 2.0 <1.0 <1.0 <1.0 <1.0 <4.0 <1.0 <1.0 <1.0 <1.0 <3.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 <0.1 <0.1 <1.0

— <1.0

75th 4.0 <1.0 <1.0 <1.0 <1.0 5.5 <1.0 <1.0 <1.0 <1.0 3.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 <0.1 <0.1 <1.0

— 3.0

85th 5.0 <1.0 <1.0 <1.0 <1.0 9.4 <1.0 <1.0 <1.0 <1.0 5.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 0.39 <0.1 <1.0

— 4.0

90th 7.0 <1.0 <1.0 <1.0 <1.0 11 <1.0 <1.0 <1.0 1.0 6.1 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 0.65 <0.1 <1.0

— 4.3

91st 7.0 <1.0 <1.0 <1.0 <1.0 14 <1.0 <1.0 <1.0 1.0 7.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 0.65 <0.1 <1.0

— 6.0

92nd 10 <1.0 <1.0 <1.0 <1.0 14 <1.0 <1.0 <1.0 1.0 8.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 3.9 0.17 <1.0

— 8.0

93rd 15 <1.0 <1.0 1.0 <1.0 14 <1.0 <1.0 <1.0 1.0 8.0 <1.0

— <1.0 <1.0 <1.0 <1.0 <1.0 3.9 0.17 <1.0

— 11

94th 34 <1.0 <1.0 1.1 <1.0

— <1.0 <1.0 <1.0 1.0 10 <1.0

— 1.0 <1.0 1.0 <1.0 1.0 3.9 0.17 <1.0

— 11

95th 45 <1.0 1.25 1.1 <1.0

— <1.0 <1.0 <1.0 2.0 10 <1.0

— 1.1 <1.0 1.0 <1.0 1.0

—

— <1.0

— 13

96th 45 <1.0 1.25 2.0 <1.0

— <1.0 <1.0 <1.0 2.3 20 1.8

— 1.6 <1.0 1.0 <1.0 2.0

—

— <1.0

— 13

97th 79 <1.0

— 3.0 <1.0

— <1.0 <1.0 <1.0 2.7 32 6.3

— 2.0 <1.0 2.0 2.0 3.0

—

— <1.0

— 13

98th

—

—

—

—

—

—

—

—

—

— 57

—

— 3.0

—

—

—

—

—

—

—

—

—

99th

—

—

—

—

—

—

—

—

—

— 75

—

— 4.0

—

—

—

—

—

—

—

—

—

Common Reporting Level (μg/L) 0.3 0.2 0.18 0.2 0.2 2 0.3 0.2 0.2 0.2 na 0.3 na 0.1 0.2 0.5 0.7 0.2 na na 0.2 0.21 0.5

Number of blanks 46 57 6 46 52 44 55 41 51 51 na 52 na 84 51 50 11 46 na na 55 5 48

Percentile 95-percent Upper Confidence Limit (μg/L)

50th (median) 0.36 <0.2 na <0.2 <0.2 <2.0 <0.3 <0.2 <0.2 <0.2 na <0.3 na <0.1 <0.2 <0.5 na <0.2 na na <0.2 na <0.5

75th 3.3 <0.2 na <0.2 <0.2 3.5 <0.3 <0.2 <0.2 0.34 na <0.3 na <0.1 <0.2 <0.5 na <0.2 na na <0.2 na 1.185th 7.0 <0.2 na 1.22 <0.2 9.4 <0.3 0.21 <0.2 0.49 na <0.3 na 0.12 <0.2 <0.5 na <0.2 na na <0.2 na 2.990th 15 <0.2 na 0.31 <0.2 11 0.42 0.34 <0.2 0.75 na <0.3 na 0.15 <0.2 <0.5 na <0.2 na na <0.2 na 11

91st 34 <0.2 na 0.9 <0.2 11 0.42 0.34 <0.2 0.75 na <0.3 na 0.15 <0.2 <0.5 na <0.2 na na <0.2 na 13

92nd 34 <0.2 na 0.9 <0.2 14 0.7 0.34 <0.2 2.3 na <0.3 na 0.2 <0.2 <0.5 na <0.2 na na <0.2 na 13

93rd 34 <0.2 na 0.9 <0.2 14 0.7

— <0.2 2.3 na <0.3 na 0.47 <0.2 <0.5 na <0.2 na na <0.2 na 13

94th

— <0.2 na

— <0.2 14 0.7

— <0.2 2.3 na <0.3 na 0.42 <0.2 <0.5 na

— na na <0.2 na

—

95th

—

— na

—

—

—

—

—

—

— na

— na 0.92

—

— na

— na na

— na

—

96th

—

— na

—

—

—

—

—

—

— na

— na 0.92

—

— na

— na na

— na

—

97th

—

— na

—

—

—

—

—

—

— na

— na

—

—

— na

— na na

— na

—

98th

—

— na

—

—

—

—

—

—

— na

— na

—

—

— na

— na na

— na

—

99th

—

— na

—

—

—

—

—

—

— na

— na

—

—

— na

— na na

— na

—

Table 2. Upper 95-percent confidence limit by percentiles for contamination in trace elements from surface-water field-blank data. [Bold faced values indicate contamination greater than the common reporting level. Al, aluminum; Ag, silver; As, arsenic; B, boron; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cr, chromium; Cu, copper; Fe, iron; Pb, lead; Li, lithium; Mn, manganese; Mo, molybdenum; Ni, nickel; Sb, antimony; Se, selenium; Sr, strontium; Tl, thallium; U, uranium; USEPA, U.S. EnvironmentalProtection Agency; V, vanadium; Zn, zinc; na, not applicable; μg/L, microgram per liter; <, less than —, too few data to compute]

1U.S. Environmental Protection Agency, 2004. 2Secondary Drinking Water Regulation (SDWR): is a non-enforceable Federal guideline to address cosmetic or aesthetic effects of drinking water. 3Maximum Contaminant Level (MCL): the highest level of contaminant that is allowed in drinking water. 4Health Advisory (HA): is an acceptable drinking-water level for a chemical substance that is based on health effects information. HAs are not legally enforceable, but is a standard that serves

as a guidance to assist Federal, state, and local officials. 5Action Level (AL): concentration of a contaminant that triggers treatment or other requirements that a water system must follow. For lead and copper, the action level is the level where 10percent of the homes tested exceed the standard.

8 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Ground Water

A UCL was calculated for trace element ground-water 

blank data collected during Cycle I. The 95-percent UCL was 

selected, and calculations were completed for the 50th, 75th, 

85th, and 90th through 99th percentiles, where possible. In 

addition, calculations of the 95-percent UCL were determined 

at two CRLs if there was more than one MRL reported for a 

particular trace element. The use of the lower CRL more accurately defines the potential contamination. Using the lower 

CRL allows for a determination of the contamination at a 

value ranging from 0 to 1 μg/L instead of <1 μg/L, which can 

be important for trace elements at low concentrations.

In ground water, the potential contamination is estimated 

to be no greater than the MRLs for Sb, Be, Tl, and V at the 

selected CRLs (table 1). At the highest CRL, an additional 

11 trace elements (As, Ba, Cd, Cr, Co, Pb, Mo, Ni, Se, Ag, 

and U) that were analyzed during Cycle I have an estimated 

contamination of less than the MRL for at least 90 percent of 

the samples (table 1). At the same CRL, Mn has potential contamination of less than the MRL for at least 85 percent of the 

samples; Cu and Li each have potential contamination of less 

than the MRL for at least 75 percent of the samples, and Fe 

and Sr have potential contamination of less than the MRL for 

at least 50 percent of the samples. The potential contamination 

for Pb at the UCL of the 99th percentile is likely related to one 

high value (7.0 μg/L). Similarly, the potential contamination 

for Cu at the UCL of the 85th percentile is high because of a 

measured value of 101 μg/L. These high values may be due to 

the sample being contaminated or being incorrectly coded as 

a blank sample. Al, B, and Zn, have potential contamination 

in at least 50 percent of all samples regardless of the MRL. 

Because of limited ground-water blank data for Li and V, additional data need to be collected to better assess the sample bias 

associated with these trace elements.

The lower CRL helps to provide a more accurate assessment of the potential contamination for 10 of the trace elements (Ba, Cd, Cr, Co, Cu, Pb, Mn, Mo, Ni, and Se) sampled 

in Cycle I (table 1). For example, Ba at the higher CRL has a 

potential contamination of <1 μg/L for at least 85 percent of 

the samples. The estimate of <1 μg/L as the potential contamination is probably closer to 0.47 μg/L when calculating the 

95-percent confidence limit at the lower CRL (table 1). The 

use of the lower CRL (lowering <1 μg/L to values ranging 

from <0.1 to <0.7 μg/L) helps to better quantify the potential 

contamination.

The 95-percent UCL has been plotted for Al, Ba, B, Cr, 

Co, Cu, Fe, Mn, Ni, Sr, and Zn field blank data to show the 

changes in the potential contamination for these trace elements at the selected CRLs (figs. 2A, 2B, and 2C and table 

1). For five of the trace elements (Ba, Cr, Co, Mn, and Ni), a 

lower CRL also has been plotted. The lower CRL has not been 

plotted for Cu because of the likelihood that the estimated 

contamination is biased as a result of one high value. The use 

of the lower CRL provides a more accurate assessment of the 

potential contamination, particularly for those values below 

the higher CRL. The potential contamination varies depending 

on the trace element analyzed as shown in figures 2A, 2B, 

and 2C.

Surface Water

As with the ground-water blank data, the 95-percent UCL 

was selected, and calculations were completed for the 50th, 

75th, 85th, and 90th through 99th percentiles (table 2). In 

addition, the 95-percent UCL was calculated for two CRLs, if 

more than one MRL was reported for a particular trace 

element.

For five of the trace elements (Sb, Be, Co, Mo, and U) in 

surface water, the potential contamination is estimated to be 

less than the MRL at the selected CRLs (table 2). As, Ba, Cd, 

Cr, Pb, Mn, Ni, Se, Ag, and Tl have potential contamination 

no greater than the MRL in 90 percent of the samples at the 

higher CRL. Similar to Pb in ground water, it appears that one 

sample that has a value of 6.3 μg/L in surface water is either 

contaminated or has been incorrectly coded as a blank sample. 

Also at the higher CRL, Cu has potential contamination of 

less than the MRL for at least 85 percent of the samples; Sr 

has potential contamination of less than the MRL for at least 

75 percent of the samples; and B, Fe, and Zn have potential 

contamination of less than the MRL for at least 50 percent of 

the samples. Al has the potential for contamination in at least 

50 percent of the samples, regardless of the MRL. Limited 

surface-water blank data for Li and V does not allow for an 

assessment on the potential contamination associated with the 

analysis of these trace elements in surface water.

The 95-percent UCL has been plotted for Al, Ba, B, Cu, 

Fe, Mn, Sr, and Zn for surface-water blank data at the selected 

CRLs (figs. 3A and 3B and table 2). In addition, the lower 

CRL has been plotted for Ba and Mn. As shown in figures 3A

and 3B, the potential contamination varies depending on the 

trace element analyzed.

Trace Element Contamination Bias 9
DISSOLVED ALUMINUM CONCENTRATION, 

IN MICROGRAMS PER LITER

DISSOLVED BARIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

100

80

60

40

20

0

DISSOLVED BORON CONCENTRATION, 

IN MICROGRAMS PER LITER

25

20

15

10

5

0

DISSOLVED CHROMIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

5

4

3

2

1

0

50 60 70 80 90 100

PERCENTILE

50 60 70 80 90 100

PERCENTILE

50 60 70 80 90 100

PERCENTILE

50 60 70 80 90 100

0

2

4

6

8

10

Higher common reporting level

Detection limit

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Higher common reporting level

Detection limit

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Figure 2A. Upper 95-percent confidence limits for contamination of aluminum, barium, boron, chromium, cobalt, copper, iron, 

manganese, nickel, strontium, and zinc that are based on data from ground-water field blanks.

10 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Higher common reporting level

Detection limit

0

3

6

9

12

15

DISSOLVED COPPER CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED IRON CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED COBALT CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

5

4

3

2

1

0

100

80

60

40

20

0

DISSOLVED MANGANESE CONCENTRATION, 

IN MICROGRAMS PER LITER

5

4

3

2

1

0

50 60 70 80 90 100

PERCENTILE

50 60 70 80 90 100

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Higher common reporting level

Detection limit

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Figure 2B. Continued.

Trace Element Contamination Bias 11
Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Higher common reporting level

Detection limit

0

4

8

12

16

20

DISSOLVED STRONTIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED ZINC CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED NICKEL CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

5

4

3

2

1

0

100

80

60

40

20

0

50 60 70 80 90 100

Higher common reporting level

Detection limit

Figure 2C. Continued.

12 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Higher common reporting level

Detection limit

DISSOLVED ALUMINUM CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE PERCENTILE

100

80

60

40

20

0

50 60 70 80 90 100

Higher common reporting level

Detection limit

PERCENTILE

50 60 70 80 90 100

DISSOLVED BORON CONCENTRATION, 

IN MICROGRAMS PER LITER

25

20

15

10

5

0

PERCENTILE

50 60 70 80 90 100

DISSOLVED COPPER CONCENTRATION, 

IN MICROGRAMS PER LITER

15

12

9

6

3

0

0

2

4

6

8

10

DISSOLVED BARIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

50 60 70 80 90 100

Higher common reporting level

Detection limit

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Figure 3A. Upper 95-percent confidence limits for contamination of aluminum, barium, boron, copper, iron, manganese, strontium, and 

zinc that are based on data from surface-water field blanks.

Trace Element Contamination Bias 13
0

1

2

3

4

5

DISSOLVED MANGANESE CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED STRONTIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

50 60 70 80 90 100

DISSOLVED IRON CONCENTRATION, 

IN MICROGRAMS PER LITER

PERCENTILE

100

80

60

40

20

0

20

16

12

8

4

0

DISSOLVED ZINC CONCENTRATION, 

IN MICROGRAMS PER LITER

100

80

60

40

20

0

50 60 70 80 90 100

PERCENTILE

50 60 70 80 90 100

Higher common reporting level

Detection limit

Higher common reporting level

Detection limit

Higher common reporting level

Detection limit

Higher common reporting level

Lower common reporting level

Detection limit higher and lower common reporting level

Figure 3B. Continued.

14 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Trace Element and Radon 

Sample Variability

Field replicate data can be used to assess the overall variability that can affect the environmental samples. Information 

on the variability is important in determining the reproducibility of individual measurements, the likelihood that a measured value exceeds a standard, and the probability that two 

measured values are different. The replicate data are reviewed 

separately for ground water and surface water because environmental sample data will likely be aggregated and analyzed 

by ground-water or surface-water data. The numbers of replicate sets for each trace element and the number of consistent 

detections in the replicate sets for ground water and surface 

water are listed in table 3.

Ground Water

Sample variability can be estimated by calculating the SD 

and RSD of the replicate data sets and plotting these values 

with the mean concentrations for the trace elements and for Rn 

determined for samples collected in Cycle I. The only trace 

element in ground water without any quantified concentrations 

in the replicate data sets is Ag; therefore, sampling variability 

cannot be determined for this trace element. The mean RSD 

was used to estimate the sample variability for Al, Sb, As, Ba, 

Be, Cd, Cu, Pb, Li, Mn, Mo, Se, Sr, Tl, and Zn (table 4). The 

reason for using this approach is that there is no consistency 

in the SD or RSD when plotted against the range of mean 

concentrations in the replicate data sets (see plots of Al and 

Cu; figs. 4A, 4B, 4C, 4D, and 4E). Using the mean RSD for 

estimating the sample variability for Be and Tl most likely is 

an overestimate because of the limited number of replicate 

data sets. Ideally, it is best to estimate the variability of the 

trace element detections for a range of concentrations over 

which the measure of variability is constant or relatively constant. Sample variability for B, Cr, Co, Fe, Ni, U, V, and Rn in 

ground water was determined for the low concentration range 

by using the SD and for the high concentration range by using 

the RSD (table 4 and figs. 4A, 4B, 4C, 4D, and 4E).

Table 3. Number of trace element ground-water and surface-water replicate sample sets 

and number of consistent detections within the replicate sets.

[—, no data]

Analytes

Number of replicate 

sample sets 

collected

Consistent 

detections 

(analyzed)

Number of 

replicate sample 

sets collected

Consistent 

detections 

(analyzed)

Ground water Surface water

Aluminum 278 147 154 127

Antimony 350 41 113 8

Arsenic 369 202 125 60

Barium 276 272 119 119

Beryllium 348 7 118 1

Boron 143 128 95 83

Cadmium 275 28 123 16

Chromium 272 149 120 33

Cobalt 276 85 119 9

Copper 276 149 126 91

Iron 564 302 543 453

Lead 348 55 129 11

Lithium 108 99 45 25

Manganese 700 453 295 258

Molybdenum 348 206 130 77

Nickel 276 157 121 70

Radon 343 336 — —

Selenium 350 112 156 58

Silver 345 0 123 1

Strontium 143 143 74 74

Thallium 102 8 13 1

Uranium 388 198 116 64

Vanadium 111 64 45 15

Zinc 276 180 126 85

Trace Element and Radon Sample Variability 15
Analytes Consistent detections

Consistent 

detections 

minus whole 

values

Low concentrations High concentrations Mean SD all 

ranges (μg/L 

or pCi/L)

Mean RSD 

all ranges 

(percent)

Mean concentration replicate sets

Range (μg/L 

or pCi/L)

Number of 

replicate 

sets

Mean SD (μg/

L or pCi/L)

Range (μg/L 

or pCi/L)

Number of 

replicate 

sets

Mean RSD 

(percent)

Minimum 

(μg/L or 

pCi/L)

Median 

(μg/L or 

pCi/L)

Maximum 

(μg/L or pCi/L)

Aluminum 147 123

—

—

—

—

—

— 0.98 11.8 1.09 3.93 467

Antimony 41 41

—

—

—

—

—

— 0.01 9.42 0.03 0.06 0.75

Arsenic 202 125

—

—

—

—

—

— 0.33 7.62 0.16 1.92 108

Barium 272 240

—

—

—

—

—

— 1.54 2.02 1.13 51.7 20,300

Beryllium 7 7 —

—

—

—

—

— 0.06 6.23 0.05 0.12 7.71

Boron 128 124 4.5–72.8 87 1.79 72.8–

1,780

37 3.5 6.21 5.86 4.58 38.4 1,780

Cadmium 28 28

—

—

—

—

—

— 0.15 13.1 0.02 0.08 4.18

Chromium 149 120 0.51–2.39 63 0.19 2.39–30.2 57 9.6 0.36 11.5 0.51 2.37 30.2

Cobalt 85 84 0.01–1.63 70 0.01 1.63–71.2 14 5.96 0.44 7.37 0.01 0.2 71.2

Copper 149 138

—

—

—

—

—

— 0.69 10.3 0.15 1.96 403

Iron 302 116 3.64–61.7 55 2.75 61.7–

6,830

61 2.62 15.8 8.97 3.64 88.9 6,830

Lead 55 48

—

—

—

—

—

— 0.23 12.5 0.04 0.39 9.12

Lithium 99 99

—

—

—

—

—

— 1.24 7.06 0.35 3.88 164

manganese 453 229

—

—

—

—

—

— 3.8 3.28 0.15 36.8 28,300

molybdenum 206 152

—

—

—

—

—

— 0.14 3.51 0.16 2.84 188

Nickel 157 131 0.03–1.20 36 0.08 1.20–52.3 95 9.01 0.26 11.2 0.03 1.96 52.3

Radon (pCi/L) 336 336 44.5–697 240 19.9 697–

211,000

96 4.13 49 5.85 44.5 435 211,000

Selenium 112 89

—

—

—

—

—

— 0.4 10.3 0.26 1.9 41.1

Silver 0 0 —

—

—

—

—

—

—

—

—

—

—

Strontium 143 134

—

—

—

—

—

— 41.7 2.35 15.4 243 6,910

Thallium 8 8 —

—

—

—

—

— 0.01 18.3 0.03 0.08 0.13

Uranium 198 155 0.01–13.3 128 0.07 13.3–97.1 27 0.98 0.11 2.46 0.01 2.6 97.1

Vanadium 64 64 0.12–1.77 23 0.16 1.77–171 41 10.9 1.17 15.2 0.12 2.35 171

Zinc 180 156

—

—

—

—

—

— 2.78 11.8 1.03 7.25 5,190

Table 4. Estimated sample variability at measured concentrations in ground-water trace element data. [pCi/L, picocuries per liter; RSD, relative standard deviation; SD, standard deviation; μg/L, microgram per liter; —, no data]

16 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

5

4

3

2

1

0

0 100 200 300 400 500

Boron

Smooth curve through data

Division between low and high concentrations

Boron

Smooth curve through data

Division between low and high concentrations

Aluminum

Smooth curve through data

Aluminum

Smooth curve through data

0

2

4

6

8

10

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

RELATIVE STANDARD DEVIATION, 

IN PERCENT

RELATIVE STANDARD DEVIATION, 

IN PERCENT

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 10 20 30 40 50

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

50

40

30

20

10

0

20

15

10

5

0

0 5 10 15 20 25

0 100 200 300 400 500

Figure 4A. Ground-water data used to select low and high ranges of replicate concentrations for select trace elements and radon.

Trace Element and Radon Sample Variability 17
Chromium

Smooth curve through data

Division between low and high

concentrations

Cobalt

Smooth curve through data

Division between low and high

concentrations

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

1.0

0.8

0.6

0.4

0.2

0.0 0.0

0.1

0.2

0.3

0.4

0.5

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

Chromium

Smooth curve through data

Division between low and high 

concentrations

Cobalt

Smooth curve through data

Division between low and high 

concentrations

0 1 2 3 4 5

RELATIVE STANDARD DEVIATION, 

IN PERCENT

RELATIVE STANDARD DEVIATION, 

IN PERCENT

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 5 10 15 20

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

30

24

18

12

6

0

20

15

10

5

0

0 2 4 6 8 10

0 2 4 6 8 10

Figure 4B. Continued.

18 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Iron

Smooth curve through data

Division between low and high

concentrations

Copper

Smooth curve through data

Iron

Smooth curve through data

Division between low and high 

concentrations

RELATIVE STANDARD DEVIATION, 

IN PERCENT

RELATIVE STANDARD DEVIATION, 

IN PERCENT

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 10 20 30 40 50

50

40

30

20

10

0

50

40

30

20

10

0

0 100 200 300 400 500

Copper

Smooth curve through data

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

5

4

3

2

1

0

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 5 10 15 20

0

6

12

18

24

30

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

0 100 200 300 400 500

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

Figure 4C. Continued.

Trace Element and Radon Sample Variability 19
0

50

100

150

Nickel

Smooth curve through data

Division between low 

and high concentrations

Nickel

Smooth curve through data

Division between low and high

concentrations

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

0 2 4 6 8 10

STANDARD DEVIATION,

IN MICROGRAMS PER LITER

1.0

0.8

0.6

0.4

0.2

0.0

Radon

Smooth curve through data

Division between low and high

concentrations

200

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

0 500 1,000 1,500 2,000

0 500 1,000 1,500 2,000

Radon

Smooth curve through data

Division between low and high 

concentrations

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

RELATIVE STANDARD DEVIATION,

IN PERCENT

20

15

10

5

0

RELATIVE STANDARD DEVIATION,

IN PERCENT

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

0 2 4 6 8 10

20

15

10

5

0

Figure 4D. Continued.

20 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Vanadium

Smooth curve through data

Division between 

low and high concentrations

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 6 12 18 24 30

RELATIVE STANDARD DEVIATION, 

IN PERCENT

0

1

2

3

4

5

Uranium

Smooth curve through data

Division between low and 

high concentrations

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 6 12 18 14 30

1.0

0.0

0.8

0.6

0.4

0.2

Vanadium

Smooth curve through data

Division between low and high

concentrations

0.0

0.5

1.0

1.5

2.0

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

0 6 12 18 24 30

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

Uranium

Smooth curve through data

Division between 

low and high concentrations

RELATIVE STANDARD DEVIATION, 

IN PERCENT

0 6 12 18 24 30

60

40

20

0

Figure 4E. Continued.

Trace Element and Radon Sample Variability 21
Confidence intervals for each trace element can be calculated using the estimated sample variability (table 4) and the 

appropriate statistic (Z) from a table of normal deviates. For 

a 95-percent confidence interval, the value of α = 0.05 and 

the value of Z(1-α/2) = 1.96. As shown previously, it is possible 

to calculate the confidence limits of an individual measurement using equation 2 and the mean of multiple measurements 

using equation 3. For example, if U has a measured concentration of 2.5 μg/L and an estimated sample variability for the 

concentration range of 0.07 μg/L (table 4), then the 95-percent 

confidence interval can be calculated using equation 2.

[C ,C ] 2.5 . 7 L U = ± 1 0 96	 	 × . . 0

	 (4)

The inherent error associated with this measurement is ±0.14 

μg/L or ±5.49 percent of the measured concentration.

For a higher U concentration, such as 30 μg/L, the 

95-percent confidence interval can be calculated using the 

same equation, but using the sample variability of 0.98 percent 

for the higher concentration.

[C ,C ] 3 0 . ). L U = ± 1 96	(30×

0.98

100 (5)

The inherent error associated with this measurement would 

be ±0.57 μg/L or ±1.93 percent of the measured concentration. The confidence interval is relatively smaller for the larger 

concentration range. 

In addition, a 95-percent confidence interval for a mean 

value of 2.5 μg/L obtained from 10 measurements would have 

a smaller confidence interval than for an individual measurement, and can be calculated using equation 3:

[ , ]	 2.5	 	1.96 0.07

10

C CL U = ± × .

	 (6)

The error for 10 measurements would be ±0.04 or ±1.74 

percent of the measured value. The sample variability for a 

mean of 10 measurements is less than it is for an individual 

measurement.

Surface Water

In surface water as with ground water, the sample variability can be determined by plotting the SD or RSD and 

the mean of the replicate data sets (figs. 5A, 5B, and 5C

and table 5); however, only the mean RSD could be used to 

determine the variability for 11 of the trace elements (As, Ba, 

Cd, Li, Mo, Ni, Se, Sr, U, V, and Zn) because there is not a 

range in the data where either the SD or RSD are reasonably 

constant for the concentrations measured in the replicates sets 

(table 5). Using the mean of the RSD over the entire range 

of concentrations most likely overestimates the variability at 

high concentrations and underestimates variability at the low 

concentrations. Determination of the sample variability for Sb, 

Be, Cr, Co, Pb, and Tl, all of which have less than 10 replicate sets, overestimates the sample variability. Therefore, an 

accurate assessment of the sample variability cannot be made 

for these trace elements. In addition, as with ground water, the 

sample variability cannot be determined for Ag. For five trace 

elements (Al, B, Cu, Fe, and Mn) in surface water, the variability was determined for the low concentration range by SD 

and the high concentration range by RSD (table 5 and figs. 5A, 

5B, and 5C).

22 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
8

10

0 100 200 300 400

0

0

10

20

30

40

0

10

20

30

50 40

2

4

6

0 100 200 300 400 500 500

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

8

10

0

2

4

6

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

RELATIVE STANDARD DEVIATION,

IN PERCENT

RELATIVE STANDARD DEVIATION,

IN PERCENT

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

Aluminum

Smooth curve through data

Division between low and high 

concentrations

Boron

Smooth curve through data

Division between low and high 

concentrations

Boron

Smooth curve through data

Division between low and high 

concentrations

Aluminum

Smooth curve through data

Division between low and high 

concentrations

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

0 100 200 300 400 500 0 100 200 300 400 500

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION, 

IN MICROGRAMS PER LITER

Figure 5A. Surface-water data used to select low and high ranges of replicate concentrations for select trace elements.

Trace Element and Radon Sample Variability 23
0 2 4 6 8 10 0 50 100 150 200

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

0

1

2

3

4

5

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

RELATIVE STANDARD DEVIATION, 

IN PERCENT

RELATIVE STANDARD DEVIATION, 

IN PERCENT

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

0 2 4 6 8 10 0 50 100 150 200

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

Copper

Smooth curve through data

Division between low and high 

concentrations

Copper

Smooth curve through data

Division between low and high 

concentrations

Iron

Smooth curve through data

Division between low and high 

concentrations

Iron

Smooth curve through data

Division between low and high 

concentrations

Figure 5B. Continued.

10

8

6

4

2

0

0 50 100 150 200 250

STANDARD DEVIATION, 

IN MICROGRAMS PER LITER

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

Manganese

Smooth curve through data

Division between low and high 

concentrations

5

10

15

20

0

0 50 100 150 200 250

RELATIVE STANDARD DEVIATION, 

IN PERCENT

MEAN REPLICATE CONCENTRATION,

IN MICROGRAMS PER LITER

Manganese

Smooth curve through data

Division between low and high 

concentrations

Figure 5C. Continued.

24 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Table 5. Estimated sample variability at measured concentrations in surface-water trace element data. 

[RSD, relative standard deviation; SD, standard deviation; μg/L, microgram per liter; —, no data]

Trace Element and Radon Sample Variability 25

Analytes Consistent detections

Consistent 

detections 

minus whole 

values

Low concentrations High concentrations Mean SD

all ranges 

(μg/L)

Mean RSD 

all ranges 

(percent)

Mean concentration replicate sets

Range (μg/L) Number of 

replicate 

sets 

Mean SD 

(μg/L)

Range (μg/L) Number of 

replicate 

sets

Mean RSD 

(percent)

Minimum 

(μg/L)

Median 

(μg/L)

Maximum 

(μg/L)

Aluminum 127 70 1.10–101 61 1.02 101–9,160 9 2.83 4.89 8.7 1.1 13.3 9,160

Antimony 8 8 — — — — — — 0.01 1.94 0.05 0.12 11.4

Arsenic 60 33 — — — — — — 0.22 4.67 0.54 4.54 41.4

Barium 119 56 — — — — — — 0.66 1.38 3.53 31.5 232

Beryllium 1 1 — — — — — — 0.07 6.15 — 1.15 —

Boron 83 69 7.67–128 58 1.91 128–424 11 1.4 2.06 6.65 7.67 41.9 424

Cadmium 16 11 — — — — — — 0.05 1.56 0.1 2.8 8.81

Chromium 33 7 — — — — — — 0.06 4.8 0.6 1.6 3.95

Cobalt 9 8 — — — — — — 0.06 3.17 0.05 1.02 15.7

Copper 91 48 0.75–4.39 37 0.23 4.39–336 11 13.1 0.42 11.6 0.75 2.01 336

Iron 453 381 1.42–33.3 235 1.07 33.3–5,730 146 5.53 4.81 7.23 1.42 23.2 5,730

Lead 11 9 — — — — — — 0.06 3.51 0.08 1.7 20.1

Lithium 25 22 — — — — — — 0.52 2.21 0.73 19 85

Manganese 258 226 1.44–61.9 173 0.58 61.9–3,030 53 2.76 1.34 4.97 1.44 17.6 3,030

Molybdenum 77 29 — — — — — — 0.05 1.76 0.84 2.66 11.8

Nickel 70 31 — — — — — — 0.23 8.17 0.26 1.97 11.8

Selenium 58 33 — — — — — — 0.34 10.2 0.24 1.96 20.5

Silver 1 0 — — — — — — — — — — —

Strontium 74 51 — — — — — — 4.18 1.2 24.3 110 2,200

Thallium 1 1 — — — — — — 0.03 31.1 — 0.09 —

Uranium 64 28 — — — — — — 0.17 1.51 0.18 7.41 50.2

Vanadium 15 14 — — — — — — 0.34 4.5 1.46 5.7 31.4

Zinc 85 37 — — — — — — 4.36 18.6 1.27 7.44 4,220
Implications of Quality Control Results 

to Environmental Data

Review of the QC data (blanks and replicates) is important when evaluating the effects of bias and variability 

associated with the environmental samples. Field blanks were 

collected in a manner similar to the collection of the environmental samples and, therefore, will reflect contamination bias 

associated with the environmental samples. Information on 

contamination can provide a more complete interpretation of 

the environmental data. Variability is important in determining 

the reproducibility of an individual measurement, the exceedance of a water-quality standard, or whether two measurements are different. Both of these variables are important 

when determining whether a water-quality standard has been 

exceeded.

Potential Effects of Contamination Bias

Calculation of the 95-percent UCL provides a measure 

of the uncertainty in the field blank contamination that can be 

used in evaluating the need to consider contamination in the 

analysis and interpretation of the trace element environmental 

data (tables 1 and 2). No evidence of contamination was identified for Sb, Be, Tl, and V in ground water and for Sb, Be, 

Co, Mo, and U in surface water; all concentrations were below 

the selected CRLs. For Li and V in ground water and surface 

water, additional data are needed to make an assessment of the 

potential contamination associated with these trace elements. 

For the other trace elements in ground water and surface water, 

evidence of contamination was identified, and this information can be used in evaluating the possible exceedances of the 

USEPA’s drinking-water standards. Overall, if the potential 

contamination is less than 10 percent of the measured concentration, the effects of contamination on the measured value can 

be determined to be not significant (Mueller and Titus, 2005).

Contamination will likely not affect the measured concentrations for trace elements with USEPA drinking-water 

standards greater than 1,000 μg/L, such as Ba (2,000 μg/L, 

Maximum Contaminant Level [MCL]), Cu (1,300 μg/L, 

action level [AL]), Sr (4,000 μg/L, health advisory [HA]), or 

Zn (2,000 μg/L, HA) (U.S. Environmental Protection Agency, 

2004). The potential contamination for these trace elements 

generally accounts for less than 1 percent of the drinkingwater standard and is never more than 10 percent. Contamination is unlikely to cause problems with identifying exceedances of the USEPA drinking-water standards for these trace 

elements.

The trace elements As, Cd, Cr, Se, Tl, and U all have 

USEPA MCL drinking-water standards that range from 2 to 

100 μg/L. In ground water, the maximum contamination 

identified for As, Cr, Se, Tl, and U is less than 10 percent of 

the USEPA’s drinking-water standard for each trace element. 

The trace element Cd, however, which has a USEPA MCL of 

5 μg/L (U.S. Environmental Protection Agency, 2004), has a 

potential contamination of 1 μg/L in 2 percent of the samples 

or 0.42 μg/L in 3 percent of the samples. The contamination 

can account for up to 20 percent of the reported Cd concentration. In this case, contamination may affect the interpretation 

of some of the Cd data with respect to the USEPA drinkingwater standard. For surface water, the maximum contamination identified for Cd, Cr, Se, and Tl is less than 10 percent of 

the USEPA drinking-water standard for each trace element. 

The maximum potential contamination observed for As in 

surface water is 1.25 μg/L for at least 5 percent of the environmental samples. Contamination in about 5 percent of the 

surface-water samples may account for about 13 percent of the 

concentration.

For the trace elements B, Pb, Mn, Mo, Ni, and Ag, the 

USEPA drinking-water standards are either HA levels (B, 

600 μg/L; Mn, 300 μg/L; Mo, 40 μg/L; Ni, 100 μg/L; and Ag, 

100 μg/L) or an AL (Pb, 15 μg/L) (U.S. Environmental Protection Agency, 2004). For B, Mn, Mo, and Ni, the potential 

contamination is less than 5 percent of their USEPA drinkingwater standard and will likely have little affect on the interpretation of the data with respect to this standard. The trace 

element Pb has a potential contamination in ground water and 

surface water that is greater than 10 percent of the USEPA 

drinking-water standard, but only in 3 percent or fewer of the 

blank samples.

The trace elements Al (50 to 200 μg/L) and Fe (300 μg/L) 

have USEPA Secondary Drinking Water Regulations (SDWR), 

which are non-enforceable Federal guidelines to address 

cosmetic and aesthetic effects of drinking water (U.S. Environmental Protection Agency, 2004). The potential contamination associated with Al at the 90th percentile in ground water 

and surface water is slightly greater than 10 percent of the 

lower end of the USEPA drinking-water standard. For Fe, the 

potential contamination in about 4 percent of the ground-water 

and surface-water samples is about 10 percent of the USEPA 

drinking-water standard.

The blank data for both ground water (table 1) and 

surface water (table 2) indicate that the maximum potential 

contamination for at least 95 percent of the samples is unlikely 

to be the cause of exceedances of the USEPA drinking-water 

standards for most of the trace element environmental data 

collected in Cycle I. Any issues related to contamination 

would likely affect only a small number of samples. In addition, the number of environmental samples that exceed the 

USEPA drinking-water standard (table 6) is typically less 

than 10 percent of the samples, and for about two-thirds of 

those trace elements, the potential contamination is less than 1 

percent. Concentrations of Fe in ground water are exceptions, 

exceeding the SDWR for more than 20 percent of samples 

(table 6). Also, concentrations of Al in surface water exceed 

the lower end of the USEPA drinking-water standard in greater 

than 10 percent of the samples, and the contamination identified could affect the measured concentrations that are close to 

the lower end of the USEPA drinking-water standard.

26 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Al Sb As Ba Be B Cd Cr Co Cu Fe Pb Li Mn Mo Ni Se Ag Sr Tl U V Zn

USEPA Drinking-Water

Standard1

250–200 36 310 32,000 34 4600 35 31,00 — 51,300 2300 515 — 4300 440 4100 350 4100 44,000 32 330 — 42,000

Common Reporting Level 

(mg/L)

1.0 1.0 1.0 1.0 1.0 4.0 1.0 1.0 1.0 1.0 3.0 1.0 0.3 1.0 1.0 1.0 1.0 1.0 0.1 0.1 1.0 1.0 1.0

Ground Water

Number of 

samples

2,927 3,016 3,308 3,129 3,032 1,120 3,132 3,144 3,033 3,182 4,066 3,128 895 5,036 3,070 3,083 3,063 2,995 1,310 542 3,548 835 3,092

Median 3.0 <1.0 <1.0 54.3 <1.0 37.5 <1.0 1.1 <1.0 1.05 14 <1.0 6.74 7 <1.0 1.1 <1.0 <1.0 270 <0.1 <1.0 1.4 4.89

Maximum 1,450 6.3 550 5,050 18 3,380 16 148 684 2,040 81,000 479 1,230 28,200 4,730 666 93.5 8 44,000 0.72 553 190 3,290

Number equal to 

or greater than 

the USEPA 

drinking-water 

standard

92 1 250 3 5 38 8 1 — 1 1,000 6 — 595 47 7 6 0 56 0 142 — 1

Percent exceeding the USEPA 

drinking-water 

standard

3.14 0.03 7.56 0.1 0.16 3.39 0.25 0.03 — 0.03 24.7 0.19 — 11.8 1.53 0.23 0.2 0 4.27 0 4 — 0.03

Surface Water

Number of 

samples

3,171 2,471 2,441 3,445 3,030 1,853 3,612 3,158 2,468 3,473 4,929 3,388 665 2,170 2,773 3,250 3,094 3,329 1,881 103 2,565 387 3,101

Median 13.0 <1.0 <1.0 35 <1.0 54.3 <1.0 <1.0 <1.0 1.4 23.3 <1.0 13.9 7.77 <1.0 <1.0 <1.0 <1.0 140 <0.1 <1.0 1.74 4

Maximum 20,600 16.9 284 410 11 4,200 18.2 89 53.2 871 54,100 39 230 1,460 157 216 150 8 3,490 2.22 74 54.8 4,450

Number equal to 

or greater than 

the USEPA 

drinking-water 

standard

492 15 126 0 1 61 88 0 — 0 227 9 — 23 8 3 13 0 0 1 27 — 52

Percent exceeding the USEPA 

drinking-water 

standard

15.5 0.61 5.16 0 0.03 3.29 2.44 0 — 0 4.61 0.2 — 1.06 0.29 0.09 0.42 0 0 0.97 1.05 — 1.68

Table 6. Summary of trace-element environmental ground-water and surface-water data collected from 1991 to 2002. [Ag, silver; As, arsenic; B, boron; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cr, chromium; Cu, copper; Fe, iron; Pb, lead; Li, lithium; mg/L, milligram per liter; Mn, manganese; Mo, molybdenum;Ni, nickel; Sb, antimony; Se, selenium; Sr, strontium; Tl, thallium; U, uranium; USEPA, U.S. Environmental Protection Agency; V, vanadium; Zn, zinc; μg/L, microgram per liter; <, less than; —, no data]

1U.S. Environmental Protection Agency, 2004. 2Secondary Drinking Water Regulation (SDWR): is a non-enforceable Federal guideline to address cosmetic or aesthetic effects of drinking water. 3Maximum Contaminant Level (MCL): the highest level of contaminant that is allowed in drinking water. 4Health Advisory (HA): is an acceptable drinking-water level for a chemical substance that is based on health effects information. HAs are not legally enforceable, but is a standard that serves as a guidance 

to assist Federal, state, and local officials. 

5Action Level (AL): concentration of a contaminant that triggers treatment or other requirements that a water system must follow. For lead and copper, the action level is the level where 10 percent of the 

homes tested exceed the standard.

Implications of Quality Control Results to Environmental Data 27
Potential Effects of Sample Variability

The sample variability determined for the trace elements 

analyzed in Cycle I can be used to (1) evaluate the confidence interval for an individual water-quality measurement or 

multiple measurements, (2) help determine whether a waterquality standard has been exceeded, and (3) help determine 

whether two water-quality measurements are different. These 

uses are explained in more detail below. Confidence intervals for trace elements and Rn (ground-water samples only) 

sampled in Cycle I that have USEPA drinking-water standards 

are calculated in table 7 for the particular standard. The variability associated with each of these trace elements and Rn, as 

determined for ground water and surface water, are indicated 

in tables 5 and 6, respectively.

Confidence Limit for an Individual Water-Quality 

Measurement or Multiple Measurements

The influence of potential contamination on an individual 

water-quality measurement can be illustrated for selected 

concentrations of As. The trace element As has an MCL of 

10 μg/L and a variability of 7.62 percent for ground water 

and 4.67 percent for surface water (table 7). The 95-percent 

confidence interval for As at a concentration of 10 μg/L ranges 

from 8.51 to 11.5 μg/L in ground water and from 9.08 to 10.9 

μg/L in surface water. The potential error in the measurement 

from sample variability is ±1.49 μg/L in ground water or ±0.92 

μg/L in surface water, which is a relative error of 14.9 percent 

for ground water and 9.2 percent for surface water. Although 

the relative error may be considered high, the absolute error 

is small (<1.5 μg/L). Table 7 lists the variability for the other 

trace elements in ground water and surface water and for Rn in 

ground water. The potential errors associated with the sample 

variability will depend on the trace element analyzed. 

The sample variability is less for a mean of ten measurements, and the confidence interval is smaller than it is for an 

individual water-quality measurement (table 7). For example, 

the 95-percent confidence interval for As in ground water at 

a concentration of 10 μg/L ranges from 9.54 to 10.5 μg/L, 

whereas the confidence interval ranges from 8.51 to 11.5 μg/L 

for an individual measurement. The relative error for 10 measurements is 4.7 percent compared with 14.9 percent for an 

individual measurement.

Exceedance of a Water-Quality Standard

Measured trace element concentrations are often compared with USEPA drinking-water standards to determine 

whether these standards have been exceeded. The variability 

calculated for a particular trace element can be used to determine how much greater an individual measurement must be to 

assure that the water-quality sample has exceeded a standard. 

An upper confidence limit can be used to determine if the 

standard has been exceeded at a specified level of certainty. 

For example, for Cd that has an MCL of 5 μg/L, it is necessary to consider the sample variability in determining whether 

the standard has been exceeded. The 95-percent UCL can be 

calculated using equation 2 such that α = 0.05 and 

Z(1–α/2) = 1.645 (Hahn and Meeker, 1991). For example, if the 

Cd concentration in ground water is 4.2 μg/L, then the UCL = 

4.2 + [1.645 × (4.2 × 0.13)] = 5.1 μg/L. If the Cd concentration in ground water is 4.1 μg/L, then the UCL = 4.1 + [1.645 

× (4.1 × 0.13)] = 4.98 μg/L. For Cd at a concentration of 4.1 

μg/L or less, there is a 95-percent confidence that the standard 

has not been exceeded.

If there is bias associated with the analytical methods, 

then the upper confidence limit is not very useful in determining whether a water-quality standard has been exceeded. 

Therefore, a correction for bias would need to be applied 

before determining whether a water-quality standard has been 

exceeded.

Two Water-Quality Measurements are Different

The confidence intervals calculated from the replicate 

sample variability can be used to determine if two measurements (or means) are different. For example, if there is an 

overlap between the confidence intervals of two samples, then 

the measurements cannot be considered significantly different. 

If the confidence intervals do not overlap, then the differences 

in the measurements are significantly different.

For example, if Cd in ground water had a concentration 

of 5.0 μg/L, then the 95-percent confidence interval can be 

calculated using equation 2 for Z(1–α/2) = 1.96, which gives a 

confidence interval of 3.72–6.28 μg/L. A concentration of Cd 

at 6 μg/L would have a confidence interval of 4.46–7.54 μg/L. 

The confidence intervals for Cd concentrations between 5.0 

and 6.0 μg/L overlap, and so, the two measurements cannot 

be considered significantly different. A concentration of 2.97 

μg/L, however, would have a confidence interval of 2.16–3.64 

μg/L and would be significantly different from a concentration 

of 5.0 μg/L. For other trace elements where the confidence 

intervals are much smaller, the change in concentration of a 

few tenths of a microgram per liter may indicate that the 

measurements were significantly different.

28 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Analytes

USEPA drinkingwater standard1 

(μg/L or pCi/L)

Estimated sample variability

(percent, μg/L or pCi/L)

95-percent confidence interval for concentrations at the USEPA 

drinking-water standards (μg/L)

Individual measurements Mean of 10 measurements

Ground water Surface Water Ground water Surface water Ground water Surface water

Aluminum2 350–200 11.8 1.02 μg/L 38.4–61.6 48.0–52.0 46.3–53.7 49.4–50.6

Antimony 4

6 9.42 — 4.89–7.11 — 5.65–6.35 —

Arsenic 4

10 7.62 4.67 8.51–11.5 9.08–10.9 9.53–10.5 9.71–10.3

Barium 4

2,000 2.02 1.38 1,920–2,080 1,950–2,050 1,980–2,030 1,980–2,020

Beryllium 4

4 — — — — — —

Boron2 5

600 3.50 1.40 559–641 584–616 587–613 595–605

Cadmium 4

5 13.1 1.56 3.72–6.28 4.85–5.15 4.59–5.41 4.95–5.05

Chromium2 4

100 9.60 — 81.2–119 — 99.1–106 —

Cobalt2 — — — — — — —

Copper2 6

1,300 10.3 13.1 1,040–1,560 966–1,630 1,220–1,380 1,190–1,410

Iron2 3300 2.62 5.53 285–315 267–333 295–305 290–310

Lead 6

15 12.5 — 11.3–18.7 — 13.8–16.2 —

Lithium — — — — — — —

Manganese2 5

300 3.28 0.58 μg/L 281–319 299–301 294–306 300

Molybdenum 5

40 3.51 1.76 37.3–42.8 38.6–41.4 39.1–40.9 39.6–40.4

Nickel2 100 0.08 μg/L 8.17 99.8–100 84.0–116 100 95.0–105

Radon 4

300 pCi/L 19.9 pCi/L — 261–449 — 288–412 —

Radon 7

4,000 pCi/L 4.13 — 3,680–4,320 — 3,900–4,100 —

Selenium 4

50 10.3 10.2 39.9–60.1 40.0–60.0 46.8–53.2 46.8–53.2

Silver 5

100 — — — — — —

Strontium 5

4,000 2.35 1.20 4,820–4,180 4,910–4,090 3,940–4,060 3,970–4,030

Thallium 4

2 — — — — — —

Uranium2 4

30 0.98 1.51 29.4–30.6 29.1–30.9 29.8–30.2 29.7–30.3

Vanadium2 — — — — — — —

Zinc 5

2,000 11.8 18.6 1,550–2,450 1,270–2,730 1,860–2,140 1,770–2,230

Table 7. Estimated sample variability and confidence intervals around U.S. Environmental Protection Agency drinking-water standards 

for select trace elements in ground water and surface water. 

[pCi/L, picocuries per liter; RSD, relative standard deviation; USEPA, U.S. Environmental Protection Agency; μg/L, microgram per liter; —, no data]

1

U.S. Environmental Protection Agency, 2004. 2

See table 4 or 5 for variability of both low and high concentrations. For comparison with USEPA drinking-water standards the mean relative standard

deviation (RSD) was used with the exceptions of nickel (Ni) and radon (Rn) in ground water and aluminum (Al) and manganese (Mn) in surface water where the

standard deviation (SD) was used. 3

Secondary Drinking Water Regulation (SDWR): is a non-enforceable Federal guideline to address cosmetic or aesthetic effects of drinking water. 4

Maximum Contaminant Level (MCL): the highest level of contaminant that is allowed in drinking water. 5

Health Advisory (HA): is an acceptable drinking-water level for a chemical substance that is based on health effects information. HAs are not legally

enforceable, but is a standard that serves as a guidance to assist Federal, state, and local officials. 6

Action Level (AL): concentration of a contaminant that triggers treatment or other requirements that a water system must follow. For lead and copper, the

action level is the level where 10 percent of the homes tested exceed the standard. 7

Alternative Maximum Contaminant Level (AMCL): Used for those states that create Multimedia Mitigation programs to address radon in indoor air. 

Implications of Quality Control Results to Environmental Data 29
Summary

Trace element field blank and replicate data collected for 

the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program during 1991–2002 were used to 

assess the bias and variability associated with trace element 

environmental concentrations in ground water and surface 

water. Field blanks are quality control (QC) samples that are 

used to measure the contamination that may be associated 

with the environmental water-quality samples. Field replicate 

QC samples can help evaluate the confidence interval for an 

individual water-quality measurement, determine whether 

a water-quality standard has been exceeded, and also determine whether two water-quality measurements are different. 

Trace element QC data are available for 23 trace elements: 

aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), 

beryllium (Be), boron (B), cadmium (Cd), chromium (Cr), 

cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), 

manganese (Mn), molybdenum (Mo), nickel (Ni), selenium 

(Se), silver (Ag), strontium (Sr), thallium (Tl), uranium (U), 

vanadium (V), and zinc (Zn). In addition, replicate data for Rn 

in ground water were reviewed. Statistical analyses were used 

to estimate the likelihood of contamination bias and sample 

variability that would occur in the environmental samples. 

The 95-percent upper confidence limit (UCL) was calculated 

at select percentiles to assess the potential for trace element 

contamination. The 95-percent confidence intervals were

calculated for sample variability.

Analysis of blank data indicates bias resulting from contamination is negligible for Sb, Be, and Tl in ground water and 

negligible for Sb, Be, Co, Mo, and U in surface water. Limited 

QC data for Li and V in ground water and surface water do not 

allow for a good assessment on the potential contamination 

associated with these trace elements.

Potential contamination was identified for Al, As, Ba, 

B, Cd, Cr, Cu, Fe, Pb, Mn, Ni, Se, Ag, Sr, and Zn in ground 

water and surface water. Evidence of potential contamination 

was shown for Co, Mo, and U in ground water, and potential contamination was shown for Tl in surface water. When 

comparing the potential contamination of these trace elements 

with the U.S. Environmental Protection Agency’s (USEPA) 

drinking-water standards, the contamination for most of these 

trace elements would have little or no effect on the trace element concentrations with respect to the USEPA drinking-water 

standards. Potential contamination associated with Al at the 

90th percentile in ground water and surface water is slightly 

greater than 10 percent at the lower end of the USEPA drinking-water standard. Potential contamination for As in ground 

water is less than 10 percent of the USEPA drinking-water 

standard, but in surface-water contamination, contamination 

may account for 13 percent of the concentration at the 95th 

percentile. The trace element Cd has a potential contamination 

within about 20 percent of the USEPA drinking-water 

standard at the 98th percentile in ground water and is less 

than the common reporting level (CRL) in surface water. The 

potential contamination identified for Pb may be biased high 

because of the presence of one sample in ground water and 

one sample in surface water that may either be contaminated 

or have been misidentified as QC blanks.

For most trace elements, there is no significant effect 

from contamination on the measured concentrations of the 

environmental samples in comparison to USEPA drinkingwater standards. Median concentrations of the environmental 

samples for many trace elements in ground water and surface 

water during Cycle I are at or near the CRL. The number of 

environmental samples that exceed the USEPA drinking-water 

standards is typically less than 10 percent of the samples, and 

for about two-thirds of those trace element samples, exceedances are less than 1 percent. The exceptions may be Fe in 

ground water and Al in surface water, which have concentrations for at least 10 percent of the environmental samples that 

exceed the USEPA drinking-water standards. For these trace 

elements, it is possible that samples with concentrations close 

to the USEPA drinking-water standard would be affected by 

contamination.

In ground water and surface water, sample variability for 

Ag could not be determined because there were no detections 

in the replicate sets. For Be and Tl in ground water, the sample 

variability is overestimated because these trace elements 

have less than 10 replicate sets with detected concentrations. 

In addition for surface water, the sample variability is overestimated for Sb, Be, Cr, Co, Pb, and Tl, all of which have 

less than 10 replicate sets. Therefore, the sample variability 

for these trace elements cannot be determined. Depending 

on the trace element, sample variability can be small (less 

than 10 percent), which would have little or no affect on the 

reported concentration. For ground water, the trace elements 

with sample variability less than 10 percent are Sb, As, Ba, B, 

Cr, Fe, Mn, Mo, Ni, Sr, U, and Rn (at concentrations greater 

than 700 picocuries per liter) and for surface water, the trace 

elements with sample variability less than 10 percent are Al, 

As, Ba, B, Cd, Fe, Mn, Mo, Ni, Sr, and U. The other trace 

elements (ground water: Al, Cd, Cu, Se, Rn [at concentrations less than 700 picocuries per liter] and Zn; and surface 

water: Cu, Se, and Zn) have higher sample variability, but the 

variability is less than 20 percent. Also, the uncertainty in the 

mean concentration of multiple samples as a result of sample 

variability would be less than it is for an individual measurement. Sample variability needs to be considered when evaluating the potential error associated with a measurement.

Analysis of the trace element QC data collected from 

1991 to 2002 can be used in the interpretation of the environmental water-quality data collected by the 52 NAWQA study 

units in Cycle I and in subsequent years. Additional QC data 

may be needed to obtain a better estimate on the bias and variability for some of the trace elements with limited data.

30 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Glossary of Data-Quality Terms

Blank sample A sample prepared from water that is free of 

the analyte(s) of interest for determining contamination.

Concurrent replicates A set of samples that are collected at 

the same time and location.

Equipment blank Blank water that is passed through the 

entire sampling equipment system collected in a controlled 

environment, such as a laboratory, and subjected to identical 

collection, processing, preservation, and laboratory handling 

as used for environmental samples.

Field blank Blank water that is passed through the entire sampling equipment system onsite and subjected to identical collection, processing, preservation, transportation, and storage 

procedures and laboratory handling as used for environmental 

samples.

Replicates A set of samples that are collected close in time 

and space and in a manner so that the samples are thought to 

be identical in composition.

Sequential replicates A set of samples that are collected consecutively, generally one after the other.

Source-solution blank A sample of blank water taken directly 

from the source container without exposure to any sampling 

equipment.

Split replicates A single sample that is divided into two or 

more equal subsamples.

Water year A continuous 12-month period selected to present data relative to hydrologic or meteorological phenomena 

during which a complete annual hydrologic cycle normally 

occurs. The water year used by the U.S. Geological Survey 

runs from October 1 through September 30 and is designated 

by the year in which it ends.

References

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interpretations of water-quality data provided by the U.S.

Geological Survey National Water Quality Laboratory: U.S.

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

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32 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002
Apodaca and others—Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002—SIR 2006–5093