Prepared in cooperation with the City of Independence, Missouri, Water Department

Concentration Comparison of Selected Constituents between 

Groundwater Samples Collected within the Missouri River 

Alluvial Aquifer using Purge and Pump and Grab-Sampling 

Methods, near the City of Independence, Missouri, 2013

Scientific Investigations Report 2015–5144

U.S. Department of the Interior

U.S. Geological Survey
Cover. Background: Groundwater sampling well within the Independence well field, near Independence, Missouri, August 2013. 

Upper right: Student technician observing water-quality properties at a groundwater sampling well within the Independence well 

field, near Independence, Missouri, August 2013. 

Lower right: Groundwater sampling well at the City of Independence, Missouri, Water Department, near Independence, Missouri, 

August 2013. 

Photographs by Shelley Niesen.
Concentration Comparison of Selected 

Constituents between Groundwater 

Samples Collected within the Missouri 

River Alluvial Aquifer using Purge and 

Pump and Grab-Sampling Methods, near 

the City of Independence, Missouri, 2013

By Heather M. Krempa

Prepared in cooperation with the City of Independence, Missouri, 

Water Department

Scientific Investigations Report 2015–5144

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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

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

Suggested citation:

Krempa, H.M., 2015, Concentration comparison of selected constituents between groundwater samples collected 

within the Missouri River alluvial aquifer using purge and pump and grab-sampling methods, near the City of 

Independence, Missouri, 2013: U.S. Geological Survey Scientific Investigations report 2015–5144, 19 p., 

http://dx.doi.org/10.3133/sir20155144.

ISSN 2328-0328 (online)
iii

Contents

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

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

Independence Well Field Background..............................................................................................2

Purpose and Scope ..............................................................................................................................4

Methods...........................................................................................................................................................4

Calculation of Equilibrium Period.......................................................................................................4

Continuous Groundwater Quality.......................................................................................................4

Grab Sampler Methods........................................................................................................................4

Purge and Pump Sampling Methods.................................................................................................5

Data Analysis.........................................................................................................................................5

Quality Assurance.................................................................................................................................6

Results of Concentration Comparison between Pump and Grab Samples..........................................7

Continuous Groundwater Quality and Equilibrium Period .............................................................7

Comparison of Nutrient Concentrations between Pump and Grab Samples .............................7

Comparison of Major Ion Concentrations between Pump and Grab Samples ........................12

Comparison of Trace Element Concentrations between Pump and Grab Samples ................12

Comparison of Fuel Compound Concentrations between Pump and Grab Samples ..............14

Summary and Conclusions.........................................................................................................................17

References Cited..........................................................................................................................................18

Figures

1. Map showing study area and location of supply and monitoring wells within 

the Independence well field .......................................................................................................3

2. Graphs showing concentrations of selected water-quality properties within 

three selected wells in the Independence well field before, during, and 

after pumping ..............................................................................................................................10

3. Graphs showing comparison of concentrations of selected compounds 

between samples collected using the purge and pump and samples 

collected using the grab-sampling method, with environmental and 

duplicate purge and pump samples ........................................................................................13
iv

Tables

1. Concentrations of quality assurance and environmental samples collected within

the Independence well field during 2013 with absolute and relative percent

differences near Independence, Missouri...............................................................................8

2. Summary statistics for selected constituents comparing purge and pump samples

with grab samples and environmental pump samples with historical duplicate

pump samples from the Independence well field during 2013 and current (2015)

drinking water standards near Independence, Missouri ......................................................9

3. Groundwater nutrient and major ion concentrations using pump and grab sampling

method from the Independence well field and calculated absolute and relative

percent differences near Independence, Missouri..............................................................11

4. Groundwater trace element concentrations using purge and pump and grab

sampling method from the Independence well field and calculated absolute and

relative percent differences near Independence, Missouri ...............................................15
v

Conversion Factors

[Inch/Pound to International System of Units]

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

inch (in.) 25,400 micrometer (μm)

foot (ft) 0.3048 meter (m)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

pint (pt) 0.4732 liter (L) 

quart (qt) 0.9464 liter (L) 

gallon (gal) 3.785 liter (L) 

cubic inch (in3) 0.01639 liter (L)

Mass

microgram 35.27 x 10-8 ounce, avoirdupois (oz)

milligram 35.27 x 10-5 ounce, avoirdupois (oz)

gram 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 Pound, avoirdupois (lb)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

Altitude, as used in this report, refers to distance above the vertical datum.

Supplemental Information

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

25 °C).

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

or micrograms per liter (μg/L).

Concentration Comparison of Selected Constituents 

between Groundwater Samples Collected within the 

Missouri River Alluvial Aquifer using Purge and Pump and 

Grab-Sampling Methods, near the City of Independence, 

Missouri, 2013

By Heather M. Krempa

Abstract

The U.S. Geological Survey, in cooperation with the City 

of Independence, Missouri, Water Department, has historically 

collected water-quality samples using the purge and pump 

method (hereafter referred to as pump method) to identify 

potential contamination in groundwater supply wells within 

the Independence well field. If grab sample results are comparable to the pump method, grab samplers may reduce time, 

labor, and overall cost. This study was designed to compare 

constituent concentrations between samples collected within 

the Independence well field using the pump method and the 

grab method.

Relative percent differences between environmental grab 

and duplicate grab samples were greater than 10 percent for 

80 percent of the constituents. Duplicate grab samples were 

collected by tethering two grab samplers together, because the 

amount of water collected by each grab sampler is close to the 

amount necessary for analysis. The screened interval lengths 

of monitoring wells within the Independence well field is not 

conducive to collecting multiple grab samples by tethering 

samplers. The inability to collect required duplicate quality 

assurance samples may limit the use of grab samplers. 

Concentrations between pump and grab samples were 

similar for analyzed nutrient species, the variability between 

methods was less than the variability between historical duplicate samples, and there were no significant differences determined. Major ion relative percent differences were less than 

10 percent and root mean square error differences between 

methods and between historical duplicate samples were less 

than 1 milligram per liter with the exception of sulfate. Statistically significant differences were determined between pump 

and grab samples for sodium and fluoride. There is a strong 

association between major ion pump and grab samples based 

on bivariate plots and simple linear regressions. Variability 

between pump and grab samples of analyzed nutrients and 

major ions may have minimal effect on the ability to monitor 

temporal changes and potential groundwater contamination 

threats.

Relative percent differences between methods were 

greater than 10 percent for most analyzed trace elements. 

Barium, cobalt, manganese, and boron had concentrations that 

were significantly different between sampling methods. Barium, molybdenum, boron, and uranium method concentrations 

indicate a close association between pump and grab samples 

based on bivariate plots and simple linear regressions. Grab 

sample concentrations were generally larger than pump concentrations for these elements and may be because of using a 

larger pore sized filter for grab samples. Analysis of zinc blank 

samples suggests zinc contamination in filtered grab samples. 

Variations of analyzed trace elements between pump and grab 

samples could reduce the ability to monitor temporal changes 

and potential groundwater contamination threats. The degree 

of precision necessary for monitoring potential groundwater 

threats and application objectives need to be considered when 

determining acceptable variation amounts. 

Introduction

Protecting groundwater used for drinking water reduces 

public health risks and minimizes treatment costs (U.S. Environmental Protection Agency, 1987). Public water supply is a 

primary groundwater use in the United States, and 46 percent 

of the Nation’s population uses groundwater for drinking 

water (U.S. Environmental Protection Agency, 2000). Amendments made in 1986 to the Safe Drinking Water Act established a nationwide program designed to protect groundwater 

used for public water supplies from a wide range of contaminants (U.S. Environmental Protection Agency, 1987). Contributing recharge areas and potential contaminants need to be 

identified to successfully protect these groundwater sources 
2 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

(U.S. Environmental Protection Agency, 1987). Groundwater 

contaminants, such as arsenic, iron, and fluoride, can leach 

into groundwater from naturally occurring deposits. Anthropogenic contamination can result from many sources, including fuel storage, waste disposal, agricultural, and industrial 

practices. Long-term ambient water-quality monitoring programs that assess groundwater quality can potentially identify 

contamination threats before reaching supply wells. 

The U.S. Geological Survey, in cooperation with the 

City of Independence, Missouri, Water Department (hereafter 

referred to as Independence), has historically collected waterquality samples using the purge and pump method (hereafter 

referred to as pump method) to identify potential contamination in groundwater supply wells within the Independence well 

field (Kelly, 2002a; Wilkison, 2011). Pump methods require 

purging the well before sampling by removing large volumes 

of water (generally at least three well casing volumes) until 

water-quality properties stabilize (see Purge and Pump Sampling Methods). Purging removes stagnant water and draws 

water into the well from the surrounding aquifer. The formation water that is drawn into the well is sampled. 

No-purge grab sampling methods (hereafter referred 

to as grab method) collect water directly from a section of 

the well screen interval without well purging or the use of a 

pump (Parker and Clark, 2004; Savoie and LeBlanc, 2012). 

High hydraulic conductivity is necessary to maintain the same 

chemical and physical properties between the surrounding 

aquifer and the water at the sampled well section (Savoie and 

LeBlanc, 2012; U.S. Environmental Protection Agency, 2013). 

If the necessary hydrogeologic conditions exist and grab 

sample results are comparable to the pump method, grab 

samplers may reduce time, labor, and overall cost. The pump 

method requires a pump, generator, and for the Independence 

well field, a spool of more than 100 feet of tubing, which can 

make sampling wells without vehicle access difficult. Grab 

sample equipment is lighter and more compact than pump 

equipment, allowing remote wells to be sampled more easily. 

Time required to deploy and collect grab samplers is minimal 

compared to the time necessary for pump method equipment 

setup and breakdown, purging, and equipment cleaning. This 

reduces the time required at each well, allowing more wells to 

be sampled within a given day, and reduces the total number 

of trips necessary to complete well sampling. Generators 

required to power pumps can introduce contamination from 

engine fumes and the large volumes of water removed during 

purging can result in pumping a well dry before samples are 

collected. Grab methods remove the risk of contamination 

from engine fumes because a generator is unnecessary and 

the potential for cross contamination is reduced because grab 

samplers are disposable. 

Independence Well Field Background

Independence provides approximately 250,000 residents 

and surrounding communities with water from supply wells 

completed within the Missouri River alluvial aquifer (Missouri Department of Natural Resources, 2014; fig. 1). The 

U.S. Geological Survey (USGS), in cooperation with Independence, quantified groundwater travel times, contributing 

recharge areas, potential contamination areas, and designed a 

groundwater monitoring program for source water within the 

pumping and contributing recharge areas (Kelly, 2002a; Kelly, 

2002b; Kelly, 2011). Several nonpoint and point contamination sources were identified within the contributing recharge 

area, including, but not limited to, landfill presence, sewage 

sludge application areas, commercial development, highway 

construction, a closed oil refinery, and agricultural activity (Kelly, 1996; Kelly, 2002a). These contamination threats 

increase the need to monitor the quality of groundwater 

supplied by Independence (Kelly, 1996; Kelly, 2002a; Kelly, 

2011; Wilkison, 2011). During 1997 and 1998 a network of 

monitoring wells (see table 1 in Kelly, 2002a for detailed 

monitoring well information) was established within the area 

of supply wells (Independence well field, fig. 1) and groundwater sampling began. The current groundwater monitoring 

program was designed to assess water quality and monitor 

temporal changes in groundwater that has a simulated travel 

time of less than 10 years from monitoring well to supply well 

(10-year zone of contribution; Wilkison, 2011). Water-quality 

samples were collected from specific wells at various time 

intervals using the pump method with a portable submersible 

pump according to the USGS National Field Manual for the 

Collection of Water-Quality Data (U.S. Geological Survey, 

variously dated) to target specific source areas, assess aquifer 

water quality, and identify potential contamination in sufficient 

time to take action before the contaminant reaches the supply 

well (Kelly, 2002a; Wilkison, 2011). 

Sand and gravel deposits within the Missouri River alluvial aquifer have large hydraulic conductivities that allow for 

rapid groundwater flow through the aquifer, and groundwater 

levels within the aquifer respond rapidly to changes in river 

stage and pumping (Kelly, 2011). Grab samplers rely on high 

hydraulic conductivities and natural groundwater flow through 

the well screen to maintain an equilibrium between well water 

at the screened interval and the surrounding aquifer (Savoie 

and LeBlanc, 2012; U.S. Environmental Protection Agency, 

2013). The rapid flow of groundwater within the Independence 

well field may create suitable conditions for the use of grab 

samplers, by maintaining a constant purged state, flushing 

wells with fresh alluvial groundwater rapidly after a disturbance or mixing (Kelly, 2002b; Kelly, 2011).
Introduction 3

Figure 1. Study area and location of supply and monitoring wells within the Independence well field.

MISSOURI

Study

area

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2A, 2B

5B

9A

11C

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18B

27A 19A

28A

94°25’ 94°24’ 94°23’

39°11’

39°10’

39°09’

Base from U.S. Geological Survey digital data, 2005, 1:100,000

Universal Transverse Mercator projection

Zone 15 North

Missouri River

0 0.25 0.5 1 KILOMETER

0 0.25 0.5 1 MILE

! Supply well

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Sampled monitoring well ! and identifier 

1B

EXPLANATION

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Study

area

MISSOURI RIVER

ALLUVIAL AQUIFER

Independence
4 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

Purpose and Scope

The purpose of this report is to compare differences in 

analytical results between groundwater samples collected from 

monitoring wells within the Independence well field using 

two sampling methods, the conventional and previously-used 

pump method and the grab method. The U.S. Geological Survey, in cooperation with Independence collected samples during September and October 2013 from 11 selected monitoring 

wells within the Independence well field and concentrations 

of selected constituents, including nutrients, major ions, trace 

elements, and fuel compounds were compared. 

Methods

This study was designed to compare constituent concentrations between samples collected within the Independence 

well field using the pump method and the grab method. 

Monitoring wells were sampled during 2013 and included 

those having groundwater travel times from monitoring well 

to supply well ranging from 0.5 years to 10 years (Kelly, 2011; 

Wilkison, 2011). Grab samples were collected by deploying 

HydraSleeveTM grab samplers in selected wells, allowing a 

calculated equilibrium period of 7 days, and retrieving samplers. Immediately following the collection of grab samples 

pump samples were collected. Because a 7-day equilibrium 

period was needed before the collection of grab samples, 

grab samples were collected before pump samples to reduce 

the duration between method sample collections. All sample 

bottles were rinsed and samples were preserved according 

to the USGS National Field Manual for the Collection of 

Water-Quality Data (U.S. Geological Survey, variously dated) 

for both pump and grab samples. Samples were analyzed by 

the USGS National Water Quality Laboratory for nutrients 

according to methods described in Fishman (1993) and Patton 

and Kryskalla (2011), major ions according to Fishman (1989, 

1993), trace elements according to Fishman (1989), Garbarino 

(1999), and Garbarino and others (2005), and fuel compounds 

according to Connor and others (1998).

Calculation of Equilibrium Period

Groundwater velocities determine the rate that fresh 

alluvial water will flush a monitoring well after a disturbance 

or mixing occurs. Deployment of grab samplers may cause 

a minimal amount of vertical mixing within the well. The 

duration required for well flushing after a disturbance was 

calculated using the lowest simulated groundwater velocities within the Independence well field (Kelly, 2011). The 

length of time required for groundwater to pass through the 

gravel pack and well casing of a monitoring well at the lowest 

simulated groundwater velocity (0.07 meters per day) within 

the 10-year contributing recharge area is 2.18 days. This is 

calculated using a 2-inch (0.0508-meter) diameter monitoring 

well with a 2-inch thick gravel pack surrounding the well casing (0.1524 meters [which accounts for the well and the two 

sides of the gravel pack] divided by 0.07 meters per day equals 

2.18 days). The greatest simulated duration for groundwater to 

pass through the gravel pack and well casing (2.18 days) was 

multiplied by three to account for differences in groundwater 

travel times outside of steady-state groundwater flow and grab 

samplers were left in each well at least 7 days before samples 

were collected. 

Continuous Groundwater Quality

To identify water-quality property changes within a 

well before, during, and after pumping, dissolved oxygen 

(DO), pH, specific conductance (SC), and temperature were 

measured continuously (every 30 minutes) using the YSI 

556 multiprobe system at the midpoints of the screened 

intervals in three selected wells (2B, 10A, and 28A; fig. 1; 

screened interval depths of 12.78 meters, 15.32 meters, 

and 27.43 meters, respectively). Water-quality probes were 

deployed before sampling and water-quality properties were 

recorded for approximately 2 weeks. Wells were then pumped 

with the water-quality probes in place. Probes were removed, 

inspected, and replaced after pumping. Recording continued 

for approximately another 2 weeks, after which probes were 

removed and data were reviewed. 

Grab Sampler Methods

Grab samples were collected using the 1-liter 

HydraSleeveTM SuperSleeve 2-inch sampler (GeoInsight, 

2010). The samplers were 91.4 centimeters long, consisted 

of a 4-mil-thick (0.004 inch) polyethylene collection chamber with a reed valve at the top that prevented filling before 

retrieval, was sealed at the bottom, and had an attached 

weight. An optional top weight was used to collapse the 

sampler, which allowed it to fill within a shorter water column 

height. Screened intervals of monitoring wells within the 

Independence well field were 1.5 meters long and were located 

at the bottom; therefore, a tether was used to lower and secure 

the empty sampler to rest just above the bottom of the well.

A small amount of vertical mixing of well water may 

occur during deployment; therefore, samplers were left in 

the well for at least 7 days to allow equilibrium and flushing 

of the well. After the equilibration period, each sampler was 

retrieved by quickly and smoothly pulling it through the water 

column by the tether. The reed valve opened as the sampler 

was drawn up, the collection chamber was filled, and the reed 

valve closed when the collection chamber was full to prevent 

water from other parts of the well from entering the sample. 

Water-quality samples were then collected from the collection 

chamber.

Samples included unfiltered samples (used to measure 

total concentrations) and filtered samples (used to measure dissolved concentrations). Filtered samples (nutrients, major ions, 
Methods 5

and trace elements) were collected using a disposable syringe 

and were filtered using a fitted Whatman glass 0.70-μm-poresize microfiber filter (hereafter referred to as 0.70-μm syringe 

filter). Filtered samples require the use of a 0.45-μm pore 

size capsule filter (see Purge and Pump Sampling Methods; U.S. Geological Survey, variously dated), which were 

not available for use with the disposable syringes. Use of 

0.70-μm filters can introduce increased microbial activity and 

particle sizes into samples but this is expected to have had 

minimal impact on concentration results (Teresa L. Kirschling, 

National Water Quality Laboratory, written commun., 2014). 

Fuel samples were not filtered and were collected by inserting 

a discharge tube through the collection chamber wall to allow 

sample water to flow through the tube into the sample bottle. 

Purge and Pump Sampling Methods

Pump samples were collected directly after grab sampling using standard pump sampling methods described in the 

USGS National Field Manual for the Collection of WaterQuality Data (U.S. Geological Survey, variously dated). Each 

well was pumped using a portable stainless steel pump with 

Teflon impeller. Field properties (DO, pH, SC, and temperature) were monitored using the YSI 556 handheld multiprobe 

system. Samples were collected after field property stabilization and included unfiltered samples (used to measure total 

concentrations) and filtered samples (used to measure dissolved concentrations). Filtered samples (nutrients, major ions, 

and trace elements) were filtered through a purged Pall GWV®

high-capacity capsule filter with acrylic copolymer (Versapor®) filter membrane material with a 0.45-micrometer (μm) 

pore size (hereafter referred to as 0.45-μm capsule filter). Fuel 

samples were collected directly from the pump tubing and 

were not filtered. Before sampling the next well, equipment 

was cleaned according to procedures described in the USGS 

National Field Manual for the Collection of Water-Quality 

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

Data Analysis

Constituent concentrations for pump and grab samples 

were compared using several diagnostic statistics. Some sample concentration variability is expected. As part of standard 

quality assurance procedures (U.S. Geological Survey, variously dated) duplicate pump samples were collected during 

historical (1998–2013) sampling of monitoring wells within 

the Independence well field. These duplicate pump samples 

(hereafter referred to as historical duplicate samples) were collected concurrently with environmental pump samples using 

standard pump methods (U.S. Geological Survey, variously 

dated) and were analyzed using the same methods. Historical 

duplicate pump samples include 23 nutrient samples during 

1998 through 2013, 10 major ion and trace element samples 

during 2005 through 2013, and 6 fuel compound samples during 2009 through 2013. The difference between environmental 

and historical duplicate samples represents the amount of variability between samples that can be expected under standard 

conditions from field and laboratory variability.

To quantify concentration differences between sample 

methods beyond what was expected, the variability between 

methods was compared to the variability between environmental and historical duplicate samples using root mean square 

error (hereafter referred to as RMSE) and relative percent 

difference (hereafter referred to as RPD). The RMSE measures 

the absolute (non-negative) value of the variation between 

environmental and historical duplicate sample concentrations 

(hereafter referred to as Historical Duplicate RMSE) and 

pump and grab sample concentrations (hereafter referred to as 

Method RMSE). The difference between the Method RMSE 

and the Historical Duplicate RMSE was calculated (hereafter 

referred to as RMSE Difference). A negative RMSE Difference is recorded when the Method RMSE is less than the Historical Duplicate RMSE and indicates less variation between 

methods than between environmental and duplicate samples. 

A positive RMSE Difference is recorded when the Method 

RMSE is greater than the Historical Duplicate RMSE and 

larger values indicate larger variation between methods compared to the variation between environmental and duplicate 

samples. A RMSE Difference that is close to zero indicates 

that the variance between methods is similar to the variance 

between environmental and duplicate samples. 

The equation to calculate RMSE follows:

222 2

1 23 n eee e RMSE

n

+ + +...+ = (1)

where 

e is the difference between sample 

concentrations; and

n is the total number of sample pairs.

The RPD measures the variation between sample concentrations as a percentage of the concentration pair’s average 

value and the mean RPD is the average of all RPDs for a given 

constituent between environmental and historical duplicate 

sample concentrations (hereafter referred to as Historical 

Duplicate Mean RPD) and pump and grab sample concentrations (hereafter referred to Method Mean RPD). A low Method 

Mean RPD indicates the differences between sample concentrations do not represent a large percentage of the mean concentrations. The differences between the Historical Duplicate 

Mean RPD and Method Mean RPD were calculated (hereafter 

referred to as RPD Difference). A negative RPD Difference is 

recorded when the Method Mean RPD is less than the Historical Duplicate Mean RPD and indicates less variation between 

methods than between environmental and duplicate samples. A 

positive RPD Difference is recorded when the Method Mean 

RPD is greater than the Historical Duplicate Mean RPD and 

larger values indicates larger variation between methods compared to the variation between environmental and duplicate 

samples. A RPD Difference that is close to zero indicates that 
6 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

the percent variance between methods is similar to the percent 

variance between environmental and historical duplicate 

samples. 

The equation to calculate RPD follows:

1 2

1 2

absolute value 100

2

x x RPD

x x

    − =  ×     +

       

(2)

where 

x1 is the pump or environmental sample 

concentration; and

x2 is the grab or historical duplicate sample 

concentration.

A Wilcoxon signed-rank test (hereafter referred to as 

Wilcoxon test; Wilcoxon, 1945) was used to determine if the 

median of the differences between the pump and grab samples 

was statistically different than zero. A significance level of 

0.05 (σ = 0.05) was used; therefore, if the p-value is less than 

or equal to 0.05, the differences between sampling methods 

were significantly different. 

Bivariate plots and simple linear regression techniques 

(Kenney and Keeping, 1962) were used for selected constituents to compare pump and grab sample concentrations. The 

equation for the linear line of best fit and coefficient of determination (R2

) were used to quantify the relation between sampling methods. A line of best fit slope value and a R2

 value that 

is close to 1.00 indicates a strong association between pump 

and grab sample concentrations. Concentrations of previously 

collected environmental and duplicate pump samples were 

included in plots to display the amount of expected variation 

and a line of equal concentration was included for reference. 

To avoid false-positive quantification of a constituent, 

low concentrations were left-censored and reported as less 

than the method detection limit by the laboratory (Childress 

and others, 1999). Constituent concentrations were less than 

the method detection limit (left-censored) in several samples. 

There are many ways to handle censored data including 

estimating, substituting, or excluding. The small sample size 

of this study makes estimating censored values unreliable and 

substituting or eliminating censored data can skew results 

(Helsel, 2006). To avoid skewing differences between concentrations that were detected below and close to the detection 

limit, datasets that contained a large amount of left-censored 

data (greater than 10 percent was arbitrarily selected) were 

evaluated qualitatively and the RMSE, RPD, and Wilcoxon 

test p-value were not calculated. There were two concentration pairs that compared a censored value to an uncensored 

value in datasets that contained less than 10 percent censored 

data, including concentrations of orthophosphate with a leftcensored environmental sample and an uncensored historical 

duplicate sample. Substituting the censored value with the 

detection limit (0.01; the largest potential value) or zero (the 

lowest potential value) resulted in no difference in the RMSE 

and a 1.5 percent difference in the mean RPD. Substituting these censored values did not skew results excessively; 

therefore a value of one-half of the detection limit was used 

in place of these censored values. In comparisons where both 

values were left-censored a difference of zero was used.

Several datasets contained values that were below the 

laboratory reporting level and above the method detection 

limit. These values are flagged with an “E” (estimated) and 

the reported values were used in calculations. Values reported 

with the E qualifier are considered firm detections, although 

the precision of the value is frequently less than for values 

without this qualifier (Childress and others, 1999).

Quality Assurance

Quality assurance and quality control samples were collected during the current (2013) sampling period in accordance 

with standard quality assurance and quality control procedures 

(U.S. Geological Survey, variously dated). A blank sample 

was collected for fuel compounds and a duplicate sample was 

collected for both sampling methods. No fuel compounds 

exceeded the detection limit in the blank sample (table 1). 

There were three constituents (orthophosphate, chromium, 

and selenium) with RPDs between the current (2013) environmental pump and the current (2013) duplicate pump samples 

greater than 10 percent; however, absolute differences for 

these constituents were low (table 1).

RPDs between environmental grab and duplicate grab 

samples were greater than 10 percent for 80 percent of the 

constituents (table 1). Also, RPDs between environmental grab 

and duplicate grab samples (table 1) were greater than Method 

Mean RPDs between pump and grab samples (table 2) for 

68 percent of the constituents that had calculated RPDs. This 

indicates that there was greater variability between environmental and duplicate grab samples (within method variability) than between the environmental pump and grab samples 

(between method variability) for most constituents.

Duplicate grab samples were collected by tethering two 

grab samplers together, because the amount of water collected by each grab sampler is close to the amount necessary 

for analysis. The grab samplers were left in the well for the 

calculated equilibrium period and, as the tether was pulled to 

retrieve the samplers, the duplicate sampler was lifted through 

the water column and filled directly after the environmental 

sampler was filled. The well screen length of monitoring wells 

within the Independence well field is 1.5 m, the sampler must 

be lifted 0.9 m to fill, and the top weight of the sampler is 

0.3 m in length. Therefore, the environmental sampler was 

filled within approximately 1.2 m or 0.3 m below the top of 

the well screen. The water remaining within the screened interval is not sufficient to fill the duplicate sampler; therefore it 

was filled with water that remained from the screened interval, 

water that was drawn into the well while the first sampler was 

lifted, and water from above the screened interval. Because of 

this and the duplicate data results, the screened interval lengths 
Results of Concentration Comparison between Pump and Grab Samples 7

of monitoring wells within the Independence well field are not 

conducive to collecting multiple grab samples by tethering 

samplers. Quality assurance duplicate samples are included in 

the groundwater monitoring plan for the Independence well 

field (Wilkison, 2011). The inability to collect required quality 

assurance samples using two tethered grab samplers may limit 

the use of grab samplers. However, limiting the number of 

analyzed constituents per sample event or reducing the volume 

required for sample analyses by the laboratory such that sufficient volume for both an environmental and quality assurance 

sample can be obtained from one grab sample or two smaller 

samplers may allow grab sampler use. Further study may be 

necessary to determine if the collection of quality assurance 

duplicate samples by this method is appropriate.

Several zinc blank samples were collected during 2015 

after the initial sample analysis because of large zinc concentration differences between pump and grab samples to 

determine if greater zinc concentrations in grab samples may 

have been caused by exposure to the grab sampler or from 

using a syringe and a 0.70-μm syringe filter instead of standard tubing and a 0.45-μm capsule filter. A grab sampler was 

soaked in deionized water in an enclosed Teflon container for 

7 days (the duration of the calculated equilibrium period). An 

unfiltered sample, a sample filtered with standard tubing and 

a 0.45-μm capsule filter, and a sample filtered with a syringe 

and a 0.70-μm syringe filter were collected from the sampler 

as well as an unfiltered sample that was unexposed to the grab 

sampler; all samples were analyzed for zinc. The unexposed 

blank sample had a zinc concentration less than the detection 

limit (less than 2.0 μg/L, which was different than the 2013 

detection limit presented in table 1). The unfiltered sample that 

was exposed to the sampler and the sample that was filtered 

with standard tubing and a 0.45-μm capsule filter had similar 

zinc concentrations below the laboratory reporting level and 

above the long-term method detection limit (2.16 μg/L and 

3.04 μg/L; Childress and others, 1999). The sample filtered 

with a syringe and a 0.70-μm syringe filter had the largest zinc 

concentration (13.15 μg). This suggests the syringe or filter 

may be a source of zinc contamination. Tracking temporal zinc 

changes within the Independence well field using zinc contaminated equipment could be problematic and further study 

may be necessary to remove the risk of zinc contamination. 

Results of Concentration Comparison 

between Pump and Grab Samples

Continuous (every 30 minutes) groundwater quality was 

monitored before, during, and after pumping to determine the 

response of groundwater properties during pumping and the 

duration for properties to return to prepump values. Pump and 

grab sample nutrient, major ion, trace element, and fuel compound concentrations were compared using RMSE, RPD, and 

the Wilcoxon test. Constituents with greater than 10 percent 

censored data were compared qualitatively and the RMSE, 

RPD, and Wilcoxon test p-value were not calculated. 

Continuous Groundwater Quality and 

Equilibrium Period

Water-quality properties remained fairly consistent during 

pumping in selected wells (Wells 2B, 10A, and 28A), with 

the exception of Well 2B specific conductance (SC; fig. 2), 

indicating water at the screened interval before pumping was 

likely similar to water drawn into the well from the aquifer 

during pumping. After the probes were removed and returned 

to the well, there were observed differences in dissolved oxygen (DO) in all wells, and in pH and SC in Well 2B. Values 

for DO returned to approximately prepurge values in less than 

one day. It took approximately 5 days for SC in Well 2B to 

return to values similar to those observed before pumping, and 

pH in Well 2B remained consistently smaller than prepurge 

values for 7 days before stabilizing at approximately prepurge 

values. Removing and returning the probe may have caused 

a disturbance affecting water quality similar to deploying a 

grab sampler; however, properties returned to prepurge, or 

approximately prepurge, and fairly consistent values within 7 

days or fewer indicating the calculated 7-day period should be 

sufficient to attain the desired equilibrium conditions.

Comparison of Nutrient Concentrations between 

Pump and Grab Samples

Concentrations between pump and grab samples were 

similar for analyzed nutrient species (ammonia [as nitrogen, 

hereafter referred to as ammonia], nitrite plus nitrate [as 

nitrogen, hereafter referred to as nitrite plus nitrate], nitrite [as 

nitrogen, hereafter referred to as nitrite], and orthophosphate 

[as phosphate, hereafter referred to as orthophosphate]) and 

varied by less than 0.3 mg/L (table 3). Method Mean RPDs 

for ammonia and orthophosphate were greater than 10 percent 

(table 2); however the variability between methods was less 

than the variability between historical duplicate samples 

(RMSE Differences were negative), and there were no significant differences determined (p-values greater than 0.05; 

table 2).
8 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

Table 1. Concentrations of quality assurance and environmental samples collected within the Independence well field during 2013 

with absolute and relative percent differences near Independence, Missouri.

[RPD, relative percent difference; mg/L, milligram per liter; --, no data; <, less than and left-censored data; μg/L, microgram per liter]

Constituent 

(unit)

Well 1B; Station 

identification number 

390920094243502

Well 10A; Station identification number 

390951094234501

Well 20B; Station identification number 

391023094235602

Blank Environmental 

pump

Duplicate 

pump

Absolute 

difference RPD Environmental grab

Duplicate 

grab

Absolute 

difference RPD

Nutrients

Ammonia, as nitrogen (mg/L) -- 0.123 0.117 0.006 4.99 0.092 0.728 0.637 155

Nitrite plus Nitrate, as nitrogen 

(mg/L)

-- <0.04 <0.04 -- -- <0.04 <0.04 -- --

Nitrite, as nitrogen (mg/L) -- <0.001 0.001 -- -- 0.003 0.002 0.001 31.4

Orthophosphate, as phosphate 

(mg/L)

-- 0.083 0.147 0.063 55.2 0.679 0.059 0.620 168

Major ions

Calcium (mg/L) -- 81.9 83.6 1.70 2.06 165 176 10.7 6.29

Magnesium (mg/L) -- 22.9 22.7 0.180 0.79 26.0 34.7 8.71 28.7

Sodium (mg/L) -- 52.4 52.5 0.130 0.25 24.8 10.5 14.3 81.1

Silica (mg/L) -- 17.2 17.3 0.140 0.81 28.7 32.4 3.73 12.20

Chloride (mg/L) -- 23.2 22.6 0.615 2.68 9.32 6.83 2.49 30.9

Fluoride (mg/L) -- 0.402 0.402 0.000 0.00 0.170 0.165 0.005 2.99

Sulfate (mg/L) -- 172 167 4.98 2.94 43.9 52.4 8.51 17.7

Aluminum (μg/L) -- <2.2 <2.2 -- -- 4.29 2.73 1.56 44.4

Barium (μg/L) -- 329 328 0.900 0.274 328 518 190 44.8

Beryllium (μg/L) -- <0.006 <0.006 -- -- 0.027 <0.02 -- --

Cadmium (μg/L) -- <0.016 <0.016 -- -- <0.03 <0.03 -- --

Chromium (μg/L) -- 0.594 0.439 0.156 30.1 0.311 <0.3 -- --

Cobalt (μg/L) -- 0.058 0.059 0.001 1.89 0.114 0.096 0.017 16.5

Copper (μg/L) -- <0.8 <0.8 -- -- <0.8 <0.8 -- --

Iron (μg/L) -- 4,140 4,170 30.0 0.72 5,080 18,100 13,020 112

Lead (μg/L) -- <0.025 <0.025 -- -- <0.04 <0.04 -- --

Lithium (μg/L) -- 27.0 28.3 1.23 4.45 28.3 34.6 6.39 20.3

Manganese (μg/L) -- 187 190 2.40 1.27 2,273 2,165 108 4.87

Molybdenum (μg/L) -- 2.901 2.967 0.066 2.25 <0.05 0.758 -- --

Nickel (μg/L) -- 0.782 0.808 0.026 3.32 1.10 0.899 0.205 20.5

Silver (μg/L) -- <0.005 <0.005 -- -- <0.02 <0.02 -- --

Strontium (μg/L) -- 580 572 7.90 1.37 828 851 23.5 2.80

Thallium (μg/L) -- <0.010 <0.010 -- -- <0.03 <0.03 -- --

Vanadium (μg/L) -- <0.080 <0.08 -- -- 0.235 0.086 0.149 92.6

Zinc (μg/L) -- <1.4 <1.4 -- -- 9.684 22.770 13.1 80.6

Antimony (μg/L) -- <0.027 <0.027 -- -- 0.229 0.055 0.174 122.40

Arsenic (μg/L) -- 0.721 0.742 0.021 2.84 4.07 6.52 2.45 46.4

Boron (μg/L) -- 65.6 69.0 3.39 5.04 105 112 6.70 6.17

Selenium (μg/L) -- 0.078 0.065 0.013 17.5 0.066 0.054 0.012 19.8

Uranium (μg/L) -- 0.159 0.158 0.001 0.820 0.049 0.136 0.087 94.17

Fuel compounds

Benzene (μg/L) <0.1 <0.1 <0.1 -- -- -- -- -- --

Ethylbenzene (μg/L) <0.1 <0.1 <0.1 -- -- -- -- -- --

tert-Butyl methyl ether (μg/L) <0.2 <0.2 <0.2 -- -- -- -- -- --

m- plus p-Xylene (μg/L) <0.2 <0.2 <0.2 -- -- -- -- -- --

o-Xylene (μg/L) <0.1 <0.1 <0.1 -- -- -- -- -- --

Toluene (μg/L) <0.1 <0.1 <0.1 -- -- -- -- -- --

Xylene (μg/L) <0.2 <0.2 <0.2 -- -- -- -- -- --
Results of Concentration Comparison between Pump and Grab Samples 9

Table 2. Summary statistics for selected constituents comparing purge and pump samples with grab samples and environmental 

pump samples with historical duplicate pump samples from the Independence well field during 2013 and current (2015) drinking water 

standards near Independence, Missouri.

[RPD, relative percent difference; Method Mean RPD, mean relative percent difference between purge and pump and grab sample concentrations; Historical 

Duplicate Mean RPD, mean relative percent difference between historical duplicate samples and environmental pump samples; RMSE, root mean square error; 

Method RMSE, root mean square error between purge and pump and grab sample concentrations; Historical Duplicate RMSE, root mean square error between 

historical duplicate samples and environmental pump samples; Wilcoxon test, Wilcoxon signed-rank test; Wilcoxon p-value, Wilcoxon signed-rank test p-value; 

FMCL, federal maximum contaminant level; %, percent; mg/L, milligrams per liter; --, no data; μg/L, micrograms per liter]

Constituent

RPD RMSE Wilcoxon 

test

Drinking Water 

Standards1

Method 

Mean 

RPD

Historical 

Duplicate 

Mean RPD

RPD 

Difference

Method 

RMSE

Historical 

Duplicate 

RMSE

RMSE 

Difference

Wilcoxon 

p-value FMCL

National 

secondary 

regulation

Nutrients % (mg/L) -- (mg/L)

Ammonia, as nitrogen 15.4 5.99 9.40 0.100 0.348 -0.248 0.10 -- --

Orthophosphate, as 

phosphate

47.7 51.9 -4.21 0.084 0.132 -0.048 0.97 -- --

Major ions % (mg/L) -- (mg/L)

Calcium 2.25 1.98 0.271 4.0 5.1 -1.1 0.83 -- --

Magnesium 1.88 1.19 0.692 0.889 0.751 0.138 0.70 -- --

Sodium 4.19 1.49 2.7 1.3 0.644 0.656 0.03 -- --

Silica 2.85 2.20 0.651 1.0 1.7 -0.7 0.83 -- --

Chloride 3.39 0.91 2.48 1.3 0.354 0.946 0.17 -- 250

Fluoride 8.34 3.05 5.29 0.033 0.011 0.022 0.01 4 --

Sulfate 21.8 0.708 21.0 10.3 1.9 8.4 0.37 -- 250

Trace elements % (μg/L) -- (μg/L)

Barium 32.3 4.92 27.4 158 38.4 120 0.00 2,000 --

Cobalt 19.6 7.33 12.3 2.0 0.1 1.9 0.02 -- --

Iron 26.4 2.21 24.2 2,910 322 2,590 0.08 -- 300

Lithium 5.39 2.54 2.85 2.7 0.8 1.9 0.28 -- --

Manganese 19.3 1.02 18.3 404 17.0 387 0.00 -- 50

Molybdenum 18.9 3.29 15.6 0.257 0.057 0.200 0.08 -- --

Nickel 8.40 5.51 2.89 1.5 0.3 1.2 0.58 -- --

Strontium 3.35 1.17 2.18 50.3 26.4 23.9 0.97 -- --

Arsenic 37.7 6.35 31.4 8.6 0.5 8.1 0.17 10 --

Boron 19.4 5.46 13.9 23.9 18.5 5.4 0.00 -- --

Uranium 18.9 1.50 17.4 0.060 0.009 0.051 1.00 0.03 --

1

U.S. Environmental Protection Agency (2009).
10 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

Figure 2. Concentrations of selected water-quality properties within three selected wells in the Independence well field before, during, and after pumping.

8/21/13

00:00

8/23/13

00:00

8/25/13

00:00

8/27/13

00:00

8/29/13

00:00

970960950940930920910900

1,000

990980

0

2.01.81.61.41.21.00.80.60.40.26.0

970960950940930920910900

1,000

990980

8.07.87.67.47.27.06.86.66.46.2

8/20/13

12:00

8/22/13

12:00

8/24/13

12:00

8/26/13

12:00

8/28/13

12:00

0

2.01.81.61.41.21.00.80.60.40.26.0

970960950940930920910900

1,000

990980

8.07.87.67.47.27.06.86.66.46.2

8/20/13

12:00

8/22/13

12:00

8/24/13

12:00

8/26/13

12:00

8/28/13

12:00

0

2.01.81.61.41.21.00.80.60.40.2

pH, in standard units

8.07.87.67.47.27.06.86.66.46.26.0

Dissolved oxygen, in milligrams per liter

Specific conductance, 

in microsiemens per centimeter

at 25 degrees Celsius

Period of 

pumping

Period of 

pumping

Period of 

pumping

Property concentrationProperty concentrationProperty concentration

Property concentrationProperty concentration

Property concentration

Property concentrationProperty concentrationProperty concentration

Date and time

Well 10A Well 28A Well 2BWell 10A Well 28A Well 2BWell 10A Well 28A Well 2B
Results of Concentration Comparison between Pump and Grab Samples 11

Table 3. Groundwater nutrient and major ion concentrations using pump and grab sampling method from the Independence well field 

and calculated absolute and relative percent differences near Independence, Missouri.

[mg/L, milligrams per liter; Grab, grab sampling method; <, less than and left-censored data; Pump, purge and pump sampling method; --, no data; RPD, relative 

percent difference; E, concentrations reported below the laboratory reporting level and above the long-term method detection limit]

Type of sample

Nutrients (mg/L) Major ions (mg/L)

Ammonia, 

as 

nitrogen

Nitrite plus 

nitrate, as 

nitrogen 

Nitrite, 

as 

nitrogen

Orthophosphate, 

as phosphate Calcium Magnesium Sodium Silica Chloride Fluoride Sulfate

Well 1B, station identification number 390920094243502

Grab 2.96 <0.04 0.003 0.09 187 45.4 20.5 36.6 11.8 0.13 59.3

Pump 3.21 <0.04 0.002 0.17 194 47.6 22.0 38.5 11.7 0.13 28.0

Absolute difference 0.254 -- 0.001 0.079 6.50 2.20 1.49 1.90 0.03 0.00 31.30

RPD 8.23 -- 37.80 59.25 3.41 4.74 7.03 5.06 0.22 0.00 71.67

Well 2A, station identification number 390923094242001

Grab 2.15 <0.04 0.006 0.38 127 34.6 39.3 39.6 22.9 0.30 26.9

Pump 2.12 <0.04 0.006 0.52 122 33.0 38.6 38.5 25.0 0.29 36.4

Absolute difference 0.030 -- 0.000 0.141 5.30 1.59 0.740 1.09 2.11 0.013 9.45

RPD 1.42 -- 7.18 31.22 4.25 4.71 1.90 2.79 8.80 4.38 29.85

Well 2B; station identification number 390923094242002

Grab 1.42 <0.04 0.003 0.60 113 27.3 53.4 34.1 28.9 0.33 74.6

Pump 1.55 <0.04 0.004 0.58 112 28.0 53.0 33.3 28.8 0.32 70.0

Absolute difference 0.132 -- 0.001 0.022 1.80 0.650 0.41 0.860 0.065 0.011 4.58

RPD 8.88 -- 19.56 3.68 1.60 2.35 0.77 2.55 0.23 3.41 6.34

Well 5B; station identification number 390924094234503

Grab 0.34 <0.04 <0.001 0.03 184 31.0 63.2 25.8 104.5 0.19 149.7

Pump 0.32 <0.04 0.003 0.20 176 30.8 61.3 26.1 101.0 0.20 142.3

Absolute difference 0.020 -- -- 0.169 7.80 0.184 1.93 0.260 3.49 0.007 7.36

RPD 6.18 -- -- 148.07 4.33 0.60 3.10 1.00 3.40 3.58 5.04

Well 9A; station identification number 390948094234001

Grab 0.15 <0.04 0.003 0.10 104 21.5 43.1 23.3 23.9 0.37 126.3

Pump 0.19 <0.04 <0.001 0.06 106 21.6 41.2 22.9 22.8 0.33 124.1

Absolute difference 0.036 -- -- 0.044 1.40 0.180 1.93 0.390 1.11 0.046 2.28

RPD 20.94 -- -- 54.31 1.33 0.84 4.58 1.69 4.74 13.22 1.82

Well 10A; station identification number 390951094234501

Grab 0.076 <0.04 E0.002 0.168 79.26 23.1 52.8 17.1 24.0 0.409 175

Pump 0.123 <0.04 <0.001 0.083 81.87 22.9 52.4 17.2 23.2 0.402 172

Absolute difference 0.047 -- -- 0.085 2.61 0.260 0.42 0.100 0.716 0.007 2.50

RPD 47.49 -- -- 67.74 3.24 1.13 0.80 0.58 3.03 1.73 1.44

Well 11C; station identification number 390945094233003

Grab 0.28 <0.04 <0.001 0.02 142 20.9 38.2 21.4 20.0 0.32 92.6

Pump 0.19 <0.04 <0.001 0.01 141 20.8 36.6 22.6 19.9 0.25 94.7

Absolute difference 0.092 -- -- 0.006 0.800 0.170 1.61 1.21 0.051 0.067 2.09

RPD 39.16 -- -- 38.12 0.57 0.82 4.30 5.49 0.26 23.63 2.23

Well 18B; station identification number 391014094235702

Grab 0.49 <0.04 0.002 0.20 60 12.6 60.0 15.4 20.9 0.45 141.1

Pump 0.58 <0.04 <0.001 0.09 60 12.7 58.7 15.0 20.8 0.44 140.0

Absolute difference 0.084 -- -- 0.111 0.060 0.160 1.27 0.380 0.017 0.013 1.15

RPD 15.78 -- -- 78.31 0.10 1.26 2.14 2.50 0.08 2.92 0.82

Well 19A; station identification number 391018094234401

Grab 0.26 <0.04 E0.001 0.17 65 16.5 55.8 17.1 19.4 0.44 149.2

Pump 0.27 E0.07 <0.001 0.14 66 16.7 54.6 16.2 20.0 0.41 149.9

Absolute difference 0.011 -- -- 0.024 0.900 0.230 1.13 0.900 0.671 0.027 0.748

RPD 4.00 -- -- 15.23 1.37 1.39 2.05 5.42 3.40 6.41 0.50

Well 27A; station identification number 391015094245802

Grab 0.66 <0.04 0.003 0.19 145 36.9 12.2 34.3 8.3 0.22 0.2

Pump 0.71 <0.04 E0.002 0.17 147 36.6 11.0 34.4 7.9 0.21 0.7

Absolute difference 0.058 -- 0.001 0.026 1.80 0.330 1.22 0.070 0.43 0.011 0.469

RPD 8.49 -- 46.45 14.75 1.24 0.898 10.49 0.204 5.30 5.20 111.27

Well 28A; station identification number 391029094240202

Grab 0.72 <0.04 0.003 0.16 160 38.3 15.9 32.9 8.8 0.26 19.7

Pump 0.79 <0.04 0.003 0.18 154 37.6 14.6 34.3 8.2 0.20 21.4

Absolute difference 0.066 -- 0.000 0.024 5.20 0.730 1.37 1.38 0.670 0.063 1.71

RPD 8.78 -- 1.68 14.19 3.31 1.92 8.99 4.10 7.87 27.21 8.32
12 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

RMSE differences and Method Mean RPDs were not calculated for nitrite plus nitrate or orthophosphate because these 

datasets were greater than 10 percent left-censored. All nitrite 

plus nitrate data were left-censored except one pump value 

which was below the laboratory reporting level and above the 

long-term method detection limit. Nitrite absolute differences 

did not exceed 0.002 mg/L (table 3). Variability between pump 

and grab samples of analyzed nutrients may have minimal 

effect on the ability to monitor temporal changes and potential 

groundwater contamination threats.

Comparison of Major Ion Concentrations 

between Pump and Grab Samples

Major ions (calcium, magnesium, sodium, silica, chloride, fluoride, and sulfate) had no censored data (table 3), 

their Method Mean RPDs were less than 10, and their RMSE 

Differences were negative or minimal (less than 1 mg/L) 

with the exception of sulfate (table 2). Although sulfate had a 

larger Method Mean RPD (21.8) and RMSE Difference (8.4) 

than other major ions, its concentrations between sampling 

methods were not significantly different (p-value = 0.37; 

table 2). Statistically significant differences were determined 

between pump and grab samples for sodium (p-value = 0.03) 

and fluoride (p-value = 0.01; table 2). Sodium, fluoride, 

and sulfate concentrations plot closely to their 1:1 line and 

their slopes and R2 values are close to 1 (fig. 3), indicating a 

strong association between pump and grab samples based on 

bivariate plots and simple linear regressions. Grab sample 

concentrations were generally larger than pump concentrations and may be because of using a larger pore sized filter 

for grab samples. The Federal Maximum Contaminant Level 

(FMCL) for sulfate is 250 mg/L (U.S. Environmental Protection Agency, 2009), and the Method RMSE is 10.3 mg/L. The 

FMCL for fluoride is 4.0 mg/L (U.S. Environmental Protection 

Agency, 2009), and the Method RMSE was 0.033 mg/L. This 

indicates that the error between sulfate and fluoride pump and 

grab sample concentrations is substantially less than the level 

at which these ions need to be monitored. There is no FMCL 

for sodium (U.S. Environmental Protection Agency, 2009). 

If the Method RMSE is substantially less than the level at 

which the contaminant needs to be monitored (for example an 

established contaminant level) then the differences between 

sampling methods may not affect the ability to monitor for this 

constituent.

Method RMSEs, which indicate the amount of variation 

between pump and grab samples, is substantially less than 

the fluoride and sulfate FMCLs. Variability between pump 

and grab samples of analyzed major ions may have minimal 

effect on the ability to monitor temporal changes and potential groundwater contamination threats; however application 

objectives need to be considered in determining acceptable 

variation amounts. 

Comparison of Trace Element Concentrations 

between Pump and Grab Samples

Differences in Method RMSE and Method Mean RPDs 

were calculated for 11 trace element constituents (out of 

23 trace element constituents analyzed; table 2; table 4). 

Method Mean RPDs were greater than 10 percent for most 

analyzed trace elements (barium, cobalt, iron, manganese, 

molybdenum, arsenic, boron, and uranium). Lithium, nickel, 

and strontium had less than 10 percent Method Mean RPDs. 

Historical Duplicate Mean RPDs were less than 10 percent 

for analyzed trace elements (table 2). All analyzed trace element Method RMSEs were greater than Historical Duplicate 

RMSEs (table 2). Barium, cobalt, manganese, and boron had 

concentrations that were significantly different (Wilcoxon test 

p-value less than or equal to 0.05) between sampling methods 

(table 2). 

Barium, molybdenum, boron, and uranium method 

concentrations plotted close to the 1:1 line on bivariate plots 

and had slopes and R2 values that were close to 1.0, indicating a close association between pump and grab samples based 

on bivariate plots and simple linear regressions (fig. 3). Grab 

sample concentrations were generally larger than pump concentrations for these elements and may be because of using a 

larger pore sized filter for grab samples. Bivariate and simple 

linear regressions indicate cobalt, iron, manganese, and arsenic 

pump and grab sample relations were less direct, although 

cobalt and iron R2 values were close to 1.0 (fig. 3). Manganese 

had a slope that was close to 1 (0.91), but had a low R2 value 

of 0.57, likely due to two sampling method outliers (fig. 3). 

The cobalt method concentration relation was skewed by a 

single sample data pair—when this outlier is removed, the 

RMSE difference is negative (indicating smaller, more minimal differences between pump and grab samples than between 

environmental and duplicate samples) and the Method Mean 

RPD is less than 10 percent. However, this was the only data 

pair for cobalt having a concentration greater than 1 μg/L, 

which may indicate the relation between pump and grab 

sample concentrations is not strong at higher concentrations. 

The Method RMSE is an indication of the mean difference between the pump and grab samples or the error between 

the two methods. If the Method RMSE is substantially less 

than the level at which the contaminant needs to be monitored 

(for example, an established contaminant level) then the differences between sampling methods may not affect the ability to 

monitor for this element. The FMCL for barium is 2,000 μg/L 

(U.S. Environmental Protection Agency, 2009), and the 

Method RMSE concentrations was 158 μg/L (table 2), suggesting the ability to detect and monitor for this element may 

not be affected by the use of grab samplers. Method RMSEs 

were greater than drinking water standards for iron, manganese, and uranium: the national secondary drinking water 

regulation for iron is 300 μg/L and the iron Method RMSE 
Results of Concentration Comparison between Pump and Grab Samples 13 Figure 3. Comparison of concentrations of selected compounds between samples collected using the purge and pump and samples collected using the grab-sampling method, with environmental and duplicate purge and pump samples.

0 50 350 300 150 100 250 200 400

Sulfate

0

50

400350300250200150100

0 50 350 300 150 100 250 200 400

y=0.98x+4.77

R2=0.97

1:1 Line

Sodium

70 60 0 10 50 40 30 20

0

70605040302010 y=1.01x+0.44

R2=1.0

1:1 Line

Fluoride

0

0.05

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0

0.450.400.350.300.250.200.150.100.05

y=x+0.02

R2=0.94

1:1 Line

0

200

1,400

1,200

1,000

800

600

400

1,600

1,500

Barium

0

800600400200

1,6001,5001,4001,2001,000

y=0.99x+149.49

R2=0.96

1:1 Line

Molybdenum

4.5 4.0 0 0.5 3.5 3.0 2.5 2.0 1.5 1.0 0

4.54.03.53.02.52.01.51.00.5

1:1 Line

y=1.03x+0.04

R2=0.97

Manganese

0

800400

2,4002,0001,6001,200

0

1,200

800

400

1,600

2,000

2,400

y=0.91x+267.07

R2=0.57

1:1 Line

Iron

0

5,000

35,000

30,000

25,000

20,000

15,000

10,000

40,000

0

5,000

40,00035,00030,00025,00020,00015,00010,000 y=1.15x-700.37

R2=0.95

1:1 Line

Cobalt

0 1 7 6 5 4 3 2 9 8 10

9876543210

10

y=4.18x-0.30

R2

1:1 Line

=1.0

Arsenic

50

504540353025201510

45 50 0 5 35 30 25 20 15 10 40

y=1.45x+0.23

R2=0.82

1:1 Line

Boron

0

50

400350300250200150100

y=0.92x+27.22

R2=0.92

1:1 Line

Uranium

0

4.03.53.02.52.01.51.00.5

4.0 0 0.5 3.5 3.0 2.5 2.0 1.5 1.0

y=1.11x+0.01

R2=1.0

1:1 Line

Environmental grab or historical 

pump duplicate or grab duplicate

concentration (milligrams per liter)

Environmental grab or historical 

pump duplicate or grab duplicate

concentration (micrograms per liter)

Environmental grab or historical 

pump duplicate or grab duplicate

concentration (micrograms per liter)

Environmental pump or

environmental grab concentration

(micrograms per liter)

Environmental pump or

environmental grab concentration

(micrograms per liter)

Environmental pump or

environmental grab concentration

(micrograms per liter)

Environmental pump or

environmental grab concentration

(micrograms per liter)

Environmental pump or

environmental grab concentration

(milligrams per liter)

Environmental pump or

environmental grab concentration

(milligrams per liter)

Environmental pump or

environmental grab concentration

(milligrams per liter)

y=0.98x+4.77

R2=0.97

Environmental pump/environmental 

grab samples

Environmental pump/historical pump 

EXPLANATION

duplicate samples 

Environmental grab/grab duplicate 

samples 

Equation of linear line of best for 

environmental pump (x) and environmental grab (y) samples

Coefficient of determination for 

environmental pump/environmental grab samples
14 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

was 2,910 μg/L; the national secondary drinking water regulation for manganese is 50 μg/L and the manganese Method 

RMSE was 404 μg/L; the FMCL for uranium is 0.03 μg/L and 

the uranium Method RMSE is 0.060 μg/L (table 2; U.S. Environmental Protection Agency, 2009). As such, the degree of 

error between pump and grab samples is greater than the level 

to which these elements have established guidelines. Iron and 

manganese concentrations, however, were determined above 

their national secondary drinking water regulations at all wells 

with both sampling methods. All instances where uranium 

concentrations were above its FMCL in pump samples the 

grab sample concentrations were also above its FMCL and 

vice versa except for samples collected in well 18B. Arsenic Method RMSE (8.6 μg/L; table 2) is close to its FMCL 

(10 μg/L; U.S. Environmental Protection Agency, 2009). 

Cobalt, lithium, molybdenum, nickel, strontium, and boron do 

not have FMCLs or national secondary drinking water regulations (U.S. Environmental Protection Agency, 2009). 

Summary statistics were not calculated for 12 trace elements (aluminum, beryllium, cadmium, chromium, copper, 

lead, silver, thallium, vanadium, zinc, antimony, and selenium) 

that had datasets with greater than 10 percent left-censored 

data. These constituents had 106 data pairs with left-censored 

data and 84 percent of pairs were either both left-censored 

data or contained one left-censored value and one estimated 

(table 4). There were 26 data pairs without left-censored 

data, and RPDs between uncensored pump and uncensored 

grab sample concentrations ranged from 0 to 169 (table 4). 

Estimated concentrations are below the laboratory reporting level and above the long-term (typically 6-12 months) 

method detection limit (Childress, 1999) and were considered 

to be within a reasonable amount of difference from the leftcensored data. Uncensored data paired with left-censored data 

were close to censoring levels (less than or equal to 0.01 μg/L) 

except for chromium and zinc. Chromium and zinc had larger 

differences between uncensored and left-censored data than 

other elements that contained left-censored data. There were 

six chromium data pairs containing one left-censored datum 

and in all pairs grab sample concentrations were less than the 

method detection limit and pump sample concentrations were 

above. It is not possible to determine if the higher concentrations of chromium in the pump samples were due to the 

introduction of chromium during pump procedures or from 

a reduced ability to detect chromium with the grab sample 

procedures and analyses. However, chromium pump concentrations paired with left-censored grab concentrations were 

small and, the greatest detected chromium concentration was 

0.639 μg/L (table 4). The FMCL for chromium is 100 μg/L 

(U.S. Environmental Protection Agency, 2009). Differences 

between pump and grab sample chromium concentrations 

were substantially smaller than established standards and these 

differences may not affect the ability to monitor chromium at 

levels close to the FMCL. 

There were large differences in pump and grab sample 

zinc concentrations. Zinc concentrations in all pump samples 

were less than the method detection limit, except for one 

sample below the laboratory reporting level and above the 

long-term method detection limit. All grab sample zinc concentrations were greater than the method detection limit and 

the smallest concentration was 21.7 μg/L (table 4). Analysis of 

several zinc blank samples suggests the syringe or filter may 

be a source of zinc contamination. The zinc blank sample that 

was collected using deionized water exposed to the grab sampler and filtered with a syringe and a 0.70-μm syringe filter 

had a reported zinc concentration of 13.1 μg/L and was higher 

than the unfiltered blank sample and the blank sample filtered 

with standard tubing and a 0.45-μm capsule filter that were 

collected from the same water. Although, grab sample zinc 

concentrations were larger than pump sample concentrations 

the largest grab sample zinc concentration was 62.3 μg/L, 

which is substantially smaller than its FMCL of 5,000 μg/L 

(table 4; U.S. Environmental Protection Agency, 2009).

Variations of analyzed trace elements between pump and 

grab samples could reduce the ability to monitor temporal 

changes and potential groundwater contamination threats. It 

is unknown if concentration differences between the two sampling methods are due to the different sampling equipment, 

different sampling procedures, or from the use of different filters with different pore sizes. Tracking temporal zinc changes 

within the Independence well field with samples collected 

using zinc contaminated equipment could be problematic. The 

degree of precision necessary for monitoring potential groundwater threats and application objectives need to be considered 

when determining acceptable variation amounts. 

Comparison of Fuel Compound Concentrations 

between Pump and Grab Samples

The concentrations of seven fuel compounds (benzene, 

ethylbenzene, tert-butyl methyl ether, m- plus p-xylene, 

o-xylene, toluene, and xylene) were analyzed, and there were 

no concentrations above the minimum detection limit (Childress, 1999) for either sampling method. It is not possible 

to determine if fuel compounds would be detected at similar 

concentrations. 
Results of Concentration Comparison between Pump and Grab Samples 15

Table 4. Groundwater trace element concentrations using purge and pump and grab sampling method from the Independence 

well field and calculated absolute and relative percent differences near Independence, Missouri.

[μg/L, micrograms per liter; Grab, grab sampling method; <, less than and left-censored data; Pump, purge and pump sampling method; --, no data; 

RPD, relative percent difference; E, concentrations reported below the laboratory reporting level and above the long-term method detection limit]

Type of sample Trace elements (μg/L)

Aluminum Barium Beryllium Cadmium Chromium Cobalt Copper Iron Lead Lithium Manganese Molybdenum

Well 1B, station identification number 390920094243502

Grab <6.6 490 <0.018 <0.048 <0.21 E0.110 <2.4 38,210 <0.075 21.1 1,713 0.484

Pump <2.2 452 0.015 <0.016 <0.07 0.110 <0.8 32,920 <0.025 21.1 1,709 0.227

Absolute difference1 -- 37.9 -- -- -- 0.000 -- 5,290 -- 0.065 4.30 0.257

RPD1 -- 8.05 -- -- -- 0.000 -- 14.9 -- 0.308 0.251 72.2

Well 2A, station identification number 390923094242001

Grab <6.6 710 <0.06 <0.09 <0.9 E0.220 <2.4 30,560 <0.12 16.2 1,348 0.784

Pump <6.6 422 <0.06 <0.09 <0.9 E0.190 <2.4 25,080 <0.12 16.4 1,321 0.809

Absolute difference1 -- 289 -- -- -- 0.030 -- 5,480 -- 0.168 27.3 0.026

RPD1 -- 51.0 -- -- -- 14.6 -- 19.7 -- 1.03 2.05 3.20

Well 2B; station identification number 390923094242002

Grab 5.75 775 <0.02 <0.03 <0.3 0.138 <0.8 8,110 <0.04 26.0 701 0.964

Pump E2.7 595 <0.02 <0.03 <0.3 0.124 <0.8 13,330 <0.04 25.6 692 1.514

Absolute difference1 3.05 180 -- -- -- 0.014 -- 5,220 -- 0.420 8.90 0.550

RPD1 72.2 26.3 -- -- -- 10.4 -- 48.7 -- 1.63 1.28 44.4

Well 5B; station identification number 390924094234503

Grab <2.2 550 0.012 E0.017 <0.07 0.12 <0.8 9,947 <0.025 28.9 1,489 0.867

Pump <2.2 413 E0.011 <0.016 0.260 0.12 <0.8 10,770 <0.025 27.4 885 0.731

Absolute difference1 -- 137 0.001 -- -- 0.003 -- 823 -- 1.49 604 0.135

RPD1 -- 28.4 12.0 -- -- 2.70 -- 7.95 -- 5.29 50.9 16.9

Well 9A; station identification number 390948094234001

Grab <2.2 443 <0.006 <0.016 <0.07 0.08 <0.8 7,204 E0.025 27.8 385 1.530

Pump <2.2 268 <0.006 <0.016 0.353 0.08 <0.8 5,256 <0.025 30.5 383 1.482

Absolute difference1 -- 175 -- -- -- 0.001 -- 1,948 -- 2.66 1.70 0.048

RPD1 -- 49.1 -- -- -- 0.637 -- 31.3 -- 9.13 0.443 3.19

Well 10A; station identification number 390951094234501

Grab E2.77 480 E0.012 E0.020 <0.07 0.070 <0.8 5,100 <0.025 31.2 189 3.04

Pump <2.2 329 <0.006 <0.016 0.594 0.058 <0.8 4,140 <0.025 27.0 187 2.90

Absolute difference1 -- 152 -- -- -- 0.012 -- 960 -- 4.15 1.60 0.141

RPD1 -- 37.5 -- -- -- 18.7 -- 20.8 -- 14.3 0.851 4.75

Well 11C; station identification number 390945094233003

Grab <2.2 194 <0.006 0.121 <0.07 8.60 <0.8 1,019 <0.025 35.7 1,744 1.298

Pump <2.2 100 <0.006 0.222 0.639 2.12 <0.8 310 <0.025 34.3 561 0.905

Absolute difference1 -- 94.5 -- 0.101 -- 6.47 -- 709 -- 1.35 1180 0.393

RPD1 -- 64.2 -- 58.8 -- 121 -- 107 -- 3.86 103 35.7

Well 18B; station identification number 391014094235702

Grab E3.0 497 <0.006 <0.016 <0.07 E0.04 <0.8 6,522 <0.025 26.1 894 4.025

Pump <2.2 329 <0.006 <0.016 0.281 E0.04 <0.8 5,635 <0.025 26.7 766 3.683

Absolute difference1 -- 168 -- -- -- 0.000 -- 887 -- 0.610 128 0.342

RPD1 -- 40.6 -- -- -- 0.000 -- 14.6 -- 2.31 15.4 8.87

Well 19A; station identification number 391018094234401

Grab E2.7 520 <0.006 <0.016 <0.07 0.06 <0.8 5,493 <0.025 28.3 288 4.111

Pump <2.2 397 <0.006 <0.016 0.253 E0.04 <0.8 4,615 <0.025 28.1 222 3.934

Absolute difference1 -- 123 -- -- -- 0.016 -- 878 -- 0.200 66.1 0.177

RPD1 -- 26.9 -- -- -- 33.2 -- 17.4 -- 0.709 25.9 4.40

Well 27A; station identification number 391015094245802

Grab E2.6 1,002 <0.006 <0.016 E0.08 0.09 <0.8 12,940 <0.025 35.1 1,158 0.728

Pump <2.2 890 <0.006 <0.016 0.220 0.08 <0.8 12,390 <0.025 35.1 1,092 0.717

Absolute difference1 -- 112 -- -- 0.140 0.004 -- 550 -- 0.070 66.0 0.011

RPD1 -- 11.8 -- -- 93.3 4.61 -- 4.34 -- 0.20 5.87 1.47

Well 28A; station identification number 391029094240202

Grab 6.5 1,308 0.019 E0.019 E0.1 0.12 <0.8 13,690 E0.035 37.6 813 0.985

Pump 74.3 1,165 E0.01 <0.016 0.166 0.13 <0.8 13,180 0.201 30.6 761 0.870

Absolute difference1 67.9 143 0.009 -- 0.066 0.012 -- 510 0.166 7.01 52.5 0.115

RPD1 168 11.6 60.1 -- 49.7 9.84 -- 3.80 141 20.6 6.67 12.3
16 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

Table 4. Groundwater trace element concentrations using purge and pump and grab sampling method from the Independence 

well field and calculated absolute and relative percent differences near Independence, Missouri.—Continued

[μg/L, micrograms per liter; Grab, grab sampling method; <, less than and left-censored data; Pump, purge and pump sampling method; --, no data; 

RPD, relative percent difference; E, concentrations reported below the laboratory reporting level and above the long-term method detection limit]

Type of sample Trace elements (μg/L)

Nickel Silver Strontium Thallium Vanadium Zinc Antimony Arsenic Boron Selenium Uranium

Well 1B, station identification number 390920094243502

Grab 0.900 <0.015 1,077 <0.030 <0.240 25.4 <0.081 37.350 250.4 <0.09 0.124

Pump 0.796 <0.005 1,133 <0.010 <0.080 <1.4 <0.027 14.230 234.2 0.102 0.082

Absolute difference1 0.104 -- 55.7 -- -- -- -- 23.1 16.2 -- 0.041

RPD1 12.3 -- 5.04 -- -- -- -- 89.6 6.69 -- 40.1

Well 2A, station identification number 390923094242001

Grab E0.837 <0.06 989 <0.090 E0.298 24.6 <0.081 46.410 124.1 <0.15 <0.042

Pump E0.834 <0.06 903 <0.090 E0.264 <6.0 <0.081 31.260 119.6 <0.15 <0.042

Absolute difference1 0.003 -- 86.1 -- 0.034 -- -- 15.2 4.4 -- --

RPD1 0.359 -- 9.10 -- 12.1 -- -- 39.0 3.64 -- --

Well 2B; station identification number 390923094242002

Grab 0.863 <0.02 763 <0.030 <0.080 21.7 <0.027 9.621 116.4 E0.076 0.036

Pump 0.806 <0.02 774 <0.030 <0.080 <2.0 <0.027 16.850 108.9 E0.09 0.056

Absolute difference1 0.056 -- 10.7 -- -- -- -- 7.23 7.50 0.014 0.0

RPD1 6.76 -- 1.39 -- -- -- -- 54.6 6.66 16.9 44.3

Well 5B; station identification number 390924094234503

Grab 1.302 <0.005 696 <0.010 E0.106 51.1 E0.032 0.130 191.3 0.061 0.364

Pump 1.394 <0.005 686 <0.010 E0.084 <1.4 E0.033 0.112 183.2 E0.046 0.306

Absolute difference1 0.092 -- 10.4 -- 0.022 -- 0.001 0.019 8.10 0.015 0.059

RPD1 6.82 -- 1.51 -- 23.2 -- 3.08 15.5 4.33 28.4 17.5

Well 9A; station identification number 390948094234001

Grab 0.822 <0.005 626 <0.010 <0.080 62.3 <0.027 0.495 90.7 E0.051 0.175

Pump 0.914 <0.005 618 <0.010 <0.080 <1.4 <0.027 0.676 79.2 E0.058 0.202

Absolute difference1 0.091 -- 7.80 -- -- -- -- 0.182 11.5 0.007 0.027

RPD1 10.5 -- 1.25 -- -- -- -- 31.1 13.5 12.8 14.5

Well 10A; station identification number 390951094234501

Grab 0.792 <0.005 559 <0.010 E0.087 39.1 <0.027 0.975 97.8 0.087 0.154

Pump 0.782 <0.005 580 <0.010 <0.08 <1.4 <0.027 0.721 65.6 0.078 0.159

Absolute difference1 0.010 -- 21.2 -- -- -- -- 0.254 32.2 0.009 0.005

RPD1 1.26 -- 3.72 -- -- -- -- 30.0 39.5 10.8 3.26

Well 11C; station identification number 390945094233003

Grab 13.860 <0.005 710 0.051 0.582 33.1 0.131 1.859 92.7 0.166 1.873

Pump 8.875 <0.005 707 0.055 0.281 E2.8 0.105 0.829 76.9 0.610 1.695

Absolute difference1 4.99 -- 2.80 0.003 0.301 30.3 0.026 1.03 15.8 0.445 0.178

RPD1 43.9 -- 0.395 6.22 69.9 169 22.1 76.6 18.7 115 9.98

Well 18B; station identification number 391014094235702

Grab 0.546 <0.005 427 <0.010 <0.080 54.4 <0.027 0.240 69.5 0.086 0.025

Pump 0.559 <0.005 424 <0.010 <0.080 <1.4 <0.027 0.257 52.7 0.068 0.032

Absolute difference1 0.013 -- 2.30 -- -- -- -- 0.017 16.8 0.018 0.007

RPD1 2.39 -- 0.541 -- -- -- -- 6.80 27.5 23.9 23.1

Well 19A; station identification number 391018094234401

Grab 0.612 <0.005 470 <0.010 E0.093 44.5 <0.027 0.464 82.8 0.090 0.020

Pump 0.596 <0.005 461 <0.010 E0.086 <1.4 <0.027 0.355 63.9 0.087 0.024

Absolute difference1 0.015 -- 8.40 -- 0.007 -- -- 0.109 18.9 0.003 0.004

RPD1 2.52 -- 1.81 -- 7.82 -- -- 26.6 25.8 3.74 19.3

Well 27A; station identification number 391015094245802

Grab 0.795 <0.005 1,114 <0.010 <0.080 24.0 <0.027 E0.048 101.9 E0.046 0.068

Pump 0.821 <0.005 1,103 <0.010 <0.080 <1.4 <0.027 E0.043 92.0 E0.051 0.067

Absolute difference1 0.026 -- 11.0 -- -- -- -- 0.005 9.89 0.005 0.001

RPD1 3.22 -- 0.99 -- -- -- -- 11.0 10.2 10.3 0.74

Well 28A; station identification number 391029094240202

Grab 0.982 E0.006 1,084 E0.012 0.185 58.6 E0.028 0.255 138.1 0.071 0.083

Pump 0.958 <0.005 1,212 <0.010 0.244 <1.4 <0.027 0.180 77.1 E0.054 0.119

Absolute difference1 0.024 -- 128 -- 0.060 -- -- 0.075 61.0 0.017 0.035

RPD1 2.48 -- 11.1 -- 27.8 -- -- 34.4 56.7 27.8 34.9

1

 Calculated using raw values, which may result in greater accuracy than results calculated with presented values.
Summary and Conclusions 17

Summary and Conclusions

The U.S. Geological Survey, in cooperation with the City 

of Independence, Missouri, Water Department, has historically 

collected water-quality samples using the purge and pump 

method (hereafter referred to as pump method) to identify 

potential contamination in groundwater supply wells within 

the Independence well field. Pump methods require purging 

the well before sampling by removing large volumes of water. 

The formation water that is drawn into the well is sampled. 

No-purge grab sampling methods (hereafter referred to as 

grab method) collect water directly from a section of the well 

screen interval without well purging or the use of a pump. 

High hydraulic conductivity is necessary to maintain the same 

chemical and physical properties between the surrounding 

aquifer and the water at the sampled well section. If the necessary hydrogeologic conditions exist and grab sample results 

are comparable to the pump method, grab samplers may 

reduce time, labor, overall cost, and allow remote wells to be 

sampled more easily. 

Independence provides approximately 250,000 residents 

and surrounding communities with water from supply wells 

completed within the Missouri River alluvial aquifer. Several nonpoint and point contamination sources for potential 

contamination were identified within the contributing recharge 

area. These contamination threats increase the need to monitor 

the quality of groundwater supplied by Independence. 

This study was designed to compare constituent concentrations between samples collected within the Independence 

well field using the pump method and the grab method. To 

quantify concentration differences between sample methods 

beyond what was expected, the variability between methods 

was compared to the variability between environmental and 

historical duplicate samples using root mean square error 

(hereafter referred to as RMSE) and relative percent difference (hereafter referred to as RPD). The difference between 

pump and grab sample RMSE and environmental and historical RMSE was calculated (hereafter referred to as RMSE 

Difference). The mean RPD is the average of all RPDs for 

a given constituent between environmental and historical 

duplicate sample concentrations and pump and grab sample 

concentrations (hereafter referred to Method Mean RPD). A 

Wilcoxon signed-rank test was used to determine if the median 

of the differences between the pump and grab samples was 

statistically different than zero. A significance level of 0.05 

(σ = 0.05) was used. Bivariate plots and simple linear regression techniques were used for selected constituents to compare 

pump and grab sample concentrations. Constituent concentrations were less than the method detection limit (left-censored) 

in several samples. Datasets that contained a large amount of 

left-censored data were evaluated qualitatively.

RPDs between environmental grab and duplicate grab 

samples were greater than 10 percent for 80 percent of the 

constituents. Duplicate grab samples were collected by 

tethering two grab samplers together, because the amount of 

water collected by each grab sampler is close to the amount 

necessary for analysis. The screened interval lengths of 

monitoring wells within the Independence well field is not 

conducive to collecting multiple grab samples by tethering 

samplers. Quality assurance duplicate samples are included in 

the groundwater monitoring plan for the Independence well 

field. The inability to collect required duplicate quality assurance samples may limit the use of grab samplers. However, 

limiting the number of analyzed constituents per sample event 

or reducing the volume required for sample analyses by the 

laboratory such that sufficient volume for both an environmental and quality assurance sample can be obtained from one 

grab sample may allow grab sampler use.

Concentrations between pump and grab samples were 

similar for analyzed nutrient species (ammonia as nitrogen, 

nitrite plus nitrate as nitrogen, nitrite, and orthophosphate), 

the variability between methods was less than the variability between historical duplicate samples, and there were no 

significant differences determined. Variability between pump 

and grab samples of analyzed nutrients may have minimal 

effect on the ability to monitor temporal changes and potential 

groundwater contamination threats.

Major ion Method Mean RPDs were less than 10 percent 

and major ion RMSE Differences were negative or minimal 

(less than 1 milligram per liter [mg/L]) with the exception of 

sulfate. Statistically significant differences were determined 

between methods for sodium and fluoride. Sulfate, sodium, 

and fluoride concentrations plot closely to their 1:1 line and 

their slopes and coefficient of determination (R2) values are 

close to 1, indicating a strong association between pump and 

grab samples based on bivariate plots and simple linear regressions. Variability between pump and grab samples of analyzed 

major ions may have minimal effect on the ability to monitor 

temporal changes and potential groundwater contamination 

threats.

Differences in Method RMSE and Method Mean RPDs 

were calculated for 11 trace element constituents (out of 

23 trace element constituents analyzed). Method Mean RPDs 

were greater than 10 percent for most analyzed trace elements 

(barium, cobalt, iron, manganese, molybdenum, arsenic, 

boron, and uranium). Barium, cobalt, manganese, and boron 

had concentrations that were significantly different between 

sampling methods. Barium, molybdenum, boron, and uranium 

method concentrations plotted close to the 1:1 line and had 

slopes and R2 values that were close to 1.0, indicating a close 

association between pump and grab samples based on bivariate plots and simple linear regressions. Grab sample concentrations were generally larger than pump concentrations for 

these elements and may be because of using a larger pore sized 

filter for grab samples. Bivariate and simple linear regressions 

indicate cobalt, iron, manganese, and arsenic pump and grab 

sample relations were less direct. 

Summary statistics were not calculated for the 12 trace 

elements that had datasets with greater than 10 percent leftcensored data. These constituents had 106 data pairs with 

left-censored data and 84 percent of pairs were either both leftcensored or contained one left-censored value and one below 
18 Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013

the laboratory reporting level and above the long-term method 

detection limit. There were 26 data pairs without left-censored 

data, and RPDs between uncensored pump and uncensored 

grab sample concentrations ranged from 0 to 169. Uncensored 

data paired with left-censored data for these 12 elements 

were close to censoring levels except for chromium and zinc. 

Analysis of several zinc blank samples suggests the syringe or 

filter may be a source of zinc contamination. Variations of analyzed trace elements between pump and grab samples could 

reduce the ability to monitor temporal changes and potential 

groundwater contamination threats. The degree of precision 

necessary for monitoring potential groundwater threats and 

application objectives need to be considered when determining 

acceptable variation amounts. 

The concentrations of seven fuel compounds (benzene, 

ethylbenzene, tert-butyl methyl ether, m- plus p-xylene, 

o-xylene, toluene, and xylene) were analyzed, and there were 

no concentrations above the minimum detection limit for 

either sampling method. It is not possible to determine if fuel 

compounds would be detected at similar concentrations. 

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Publishing support provided by: 

Rolla Publishing Service Center

For more information concerning this publication, contact: 

Director, Missouri Water Science Center 

U.S. Geological Survey 

1400 Independence Road, MS-100 

Rolla, MO 65401 

(573) 308-3667

Or visit the Missouri Water Science Center Web site at: 

http://mo.water.usgs.gov/

Krempa—Comparison of Constituents in the Missouri River using Two Sampling Methods, near Independence, Missouri, 2013—SIR 2015–5144

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155144