Cover. Selected hydrogeologic settings (Coastal Lowlands, and parts of the Middle and Inner Coastal Plain) in the Northern Atlantic 

Coastal Plain.
Natural and Human Influences on 

Water Quality in a Shallow Regional 

Unconsolidated Aquifer, Northern 

Atlantic Coastal Plain

By Scott W. Ator 

Scientific Investigations Report 2008–5190

U.S. Department of the Interior

U.S. Geological Survey

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: 2008

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Suggested citation:

Ator, S.W., 2008, Natural and human influences on water quality in a shallow regional unconsolidated aquifer, 

Northern Atlantic Coastal Plain: U.S. Geological Survey Scientific Investigations Report 2008–5190, 19 p.

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iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that helps to 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 

Nation’s water resources is critical to ensuring long-term availability of water that is safe for drinking and 

recreation and is suitable for industry, irrigation, and fish and wildlife. Population growth and increasing 

demands for water make the availability of that water, 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, State, and local information needs and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of our Nation’s streams and ground water? How are 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 and established a baseline understanding of water-quality conditions in 51 of the Nation’s river 

basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments of waterquality conditions and trends. These regional assessments are based on major river basins and principal 

aquifers, which encompass larger regions of the country than the Study Units. Regional assessments 

extend the findings in the Study Units by filling critical gaps in characterizing the quality of surface water 

and ground water, and by determining status and trends at sites that have been consistently monitored 

for more than a decade. In addition, the regional assessments continue to build an understanding of how 

natural features and human activities affect water quality. Many of the regional assessments employ 

modeling and other scientific tools, developed on the basis of data collected at individual sites, to help 

extend knowledge of water quality to unmonitored, yet comparable areas within the regions. The models 

thereby enhance the value of our existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various resource-management scenarios and in predicting how 

our actions, such as reducing or managing nonpoint and point sources of contamination, land conversion, 

and altering flow and (or) pumping regimes, are likely to affect water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of information on 

pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic ecology; 

and continuing national topical studies on the fate of agricultural chemicals, effects of urbanization on 

stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on 

stream ecosystems, and transport of contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to address 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 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, regional, interstate, Tribal, and local—as well as 

nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

 Matthew C. Larsen

Associate Director for Water

v

Contents

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

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

Purpose and Scope ..............................................................................................................................1

Study Area – The Northern Atlantic Coastal Plain...................................................................................2

Methods...........................................................................................................................................................3

Compilation of Ground-Water-Quality Data......................................................................................3

Compilation of Geographic Data ........................................................................................................4

Data Analysis.........................................................................................................................................4

Natural and Human Influences on Ground-Water Quality......................................................................6

Major Ions and Nitrate.........................................................................................................................6

Pesticides.............................................................................................................................................11

Volatile Organic Compounds.............................................................................................................13

Summary and Conclusions.........................................................................................................................14

Acknowledgments.......................................................................................................................................15

References Cited..........................................................................................................................................15

Figures

1. Map showing sampled wells and selected hydrogeologic settings in the Northern 

Atlantic Coastal Plain...................................................................................................................2

2-5. Graphs showing—

2. Distribution of dissolved-oxygen concentrations in samples with complete 

major-ion data among three hydrogeologic subregions, Northern Atlantic 

Coastal Plain.........................................................................................................................7

3. Summary of major-ion chemistry in samples with complete major-ion data, 

Northern Atlantic Coastal Plain.........................................................................................8

4. Estimated probability of nitrate concentration exceeding 0.4 milligrams per 

liter (as nitrogen) in samples from three hydrogeologic subregions, Northern 

Atlantic Coastal Plain........................................................................................................12

5. Relation of deethylatrazine concentration to ground-water redox condition 

(as represented by Principal Component 2) in samples from the Coastal 

Lowlands (Subregion 1)....................................................................................................13

Tables

1. Summary of selected hydrogeologic settings in the Northern Atlantic Coastal 

Plain and distribution of wells from which compiled ground-water-quality data 

were collected ..............................................................................................................................5

2. Summary of selected characteristics of ground-water-quality data compiled for 

the Northern Atlantic Coastal Plain...........................................................................................6

3. Loadings on the first three principal components from unrotated principal 

components analysis ...................................................................................................................9

4. Summary of correlations (Spearman’s rho) among selected water-quality and 

geographic variables in three hydrogeologic subregions...................................................10

5. Summary of logistic regression models of ground-water quality ......................................11
vi

Conversion Factors

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

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

°F=(1.8×°C)+32

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 either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).


Abstract 

Data collected from more than 400 wells in the surficial 

unconfined aquifer in the Northern Atlantic Coastal Plain 

(New York through North Carolina) were compiled and 

analyzed to improve understanding of multiple natural and 

human influences on water quality in such shallow regional 

aquifers. Geochemical patterns were identified and described 

through principal components analysis on major ions, and correlation and logistic regression were used to relate observed 

concentrations of nitrate and selected pesticide compounds 

(atrazine, metolachlor, simazine, and deethylatrazine, an atrazine degradate) and volatile organic compounds (chloroform, 

1,1,1-trichloroethane, tetrachlorethene, and methyl tert-butyl 

ether) to likely influences, such as observed geochemical 

patterns, land use, hydrogeology, and soils. Variability in 

major-ion concentrations is primarily related to ionic strength 

and redox condition. Concentrations of nitrate, pesticides, and 

volatile organic compounds are related to natural conditions, 

as well as the distribution of likely sources reflected in land 

use. Nitrate is most common in aerobic ground water and in 

relatively well-drained areas, for example; concentrations 

greater than 0.4 milligrams per liter may result from a variety 

of human activities, although concentrations greater than 3 

milligrams per liter are more likely in agricultural areas. Atrazine, deethylatrazine, and metolachlor also are related to geochemical patterns, likely because ground-water geochemistry 

reflects hydrogeologic and soil conditions affecting pesticide 

transport to the water table. Results demonstrate the value of 

geochemical information along with the distribution of sources 

and other influences to understanding the regional occurrence 

of selected compounds in ground water. Such influences are 

not unique to the Northern Atlantic Coastal Plain, and thus 

observations and interpretations are relevant to broader areas.

Introduction

Synthetic organic pesticides and volatile organic compounds (VOCs) have been detected in ground water in many 

areas of the United States (Barbash and Resek, 1996; Barbash 

and others, 1999; Moran and others, 2002; Squillace and 

others, 2002; Gilliom and others, 2006; Zogorski and others, 

2006). Ground water in many areas also contains nitrate at 

concentrations exceeding natural levels (Nolan and Stoner, 

2000; Squillace and others, 2002). Although pesticides, VOCs, 

and nitrogen compounds have contributed significantly to 

agricultural productivity, industry, public health, transportation, and other applications, the occurrence of these compounds in ground water at certain concentrations can affect 

its potability (Toccalino and others, 2004), and the ecology of 

streams, estuaries, and other receiving waters. Understanding 

the interacting effects of land-use practices, soil and geologic 

conditions, and other natural and human factors on the occurrence and distribution of pesticides, VOCs, and nutrients in 

ground water at different scales can contribute to the safe and 

effective use of these compounds.

Purpose and Scope

Natural and human influences on the occurrence and 

distribution of nitrate and selected pesticides and VOCs in 

shallow [generally less than 10 m (meters) deep], unconfined 

ground water at the regional scale in an unconsolidated aquifer 

are described and discussed in this report. Regional analysis 

and interpretation of water quality are intended as a compromise between local studies (which may be difficult to extrapolate to wider areas) and national studies (in which detailed 

understanding may be limited by great variability in potential 

influences). Water-quality data collected from 1991 through 

2003 from more than 400 wells in the surficial aquifer in the 

Northern Atlantic Coastal Plain in parts of North Carolina, 

Virginia, Maryland, Delaware, New Jersey, and New York 

(fig. 1) were compiled and analyzed. Geochemical patterns 

were identified and described through principal components 

analysis (PCA) on major ions and were considered along 

with other likely influences (such as land use and hydrogeologic and soil conditions) in statistical approaches (such as 

correlation and logistic regression) to understand regional 

ground-water quality, including pesticides and VOCs. Results 

are interpreted and discussed to illustrate important sources 

of selected compounds to ground water in different settings, 

geochemical, hydrogeologic, and soil conditions that affect 

Natural and Human Influences on Water Quality in a 

Shallow Regional Unconsolidated Aquifer, Northern 

Atlantic Coastal Plain

By Scott W. Ator
2 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

the distribution of such compounds, and the importance of 

geochemical indicators (such as major ions) along with other 

data for understanding the regional occurrence of chemicals in 

shallow ground water.

Study Area—The Northern Atlantic 

Coastal Plain

The Northern Atlantic Coastal Plain includes 116,000 km2

(square kilometers) of the Coastal Plain Physiographic Province (Fenneman, 1928) in the eastern United States (fig. 1). 

Land use and land cover in the Coastal Plain are diverse, 

including relatively natural forests and wetlands, as well as 

agriculture (including row crops and livestock production) and 

densely populated urban areas, particularly around Washington, D.C., and to the north. The Coastal Plain is underlain by 

a heterogeneous wedge of primarily unconsolidated deposits 

that thickens from 0 m at the Fall Zone to several thousand 

meters along the Atlantic Coast, and forms an alternating 

series of aquifers and confining units (Meng and Harsh, 1988; 

Smolensky and others, 1989; Zapecza, 1989; Vroblesky and 

Fleck, 1991; Trapp, 1992; Winner and Coble, 1996). Permeable surficial and near-surface deposits comprising the unconfined aquifer are generally flat-lying and unconsolidated, but 

vary considerably in age, texture, origin, and thickness over 

the region (Trapp, 1992; Ator and others, 2005). Although 

some unconfined ground water supplies recharge to underlying confined aquifers, most discharges directly to nearby surface waters (Leahy and Martin, 1993). Ground water generally 

moves through the unconfined aquifer from the water table to 

local discharge areas within a few decades, although residence 

times are considerably greater along longer, deeper flow paths 

in areas where the unconfined aquifer is particularly thick 

(Buxton and Modica, 1992; Böhlke and Denver, 1995; McFarland, 1995; Speiran, 1996; Szabo and others, 1996; Kauffman and others, 2001). Ground-water discharge as base flow 

contributes most of the streamflow in the Northern Atlantic 

Coastal Plain (Leahy and Martin, 1993), and a considerable 

load of nutrients to downstream estuaries and other receiving 

waters (Bachman and Phillips, 1996; Bachman and others, 

1998).

Water quality in the unconfined aquifer has been studied 

at local to sub-regional scales at various locations within the 

Coastal Plain in recent decades. Investigations at such scales 

can support particularly detailed analyses of ground-water 

quality, but are often necessarily restricted to a limited range 

of hydrogeologic and land-use settings, and results may be 

difficult to interpret over wider areas. Although broad regional 

patterns related to natural geochemical processes are apparent 

in the water quality in confined Coastal Plain aquifers, water 

quality in the unconfined aquifer is typically related primarily to local conditions (particularly land use), and is therefore 

more spatially variable (Knobel and others, 1998). Natural 

ground water in the well-weathered siliciclastic sediments 

typical of many parts of the surficial aquifer is commonly relatively dilute, and may be significantly affected by chemical 

applications (Denver, 1989; Knobel and others, 1998; Ator and 

others, 2000). Effects of such applications may be less pronounced in other areas where natural sources of soluble ions in 

soils or shallow sediments are more common. The occurrence 

of nitrate attributed to human sources in parts of the aquifer 

has been reported since at least the early 1970s (Cushing 

and others, 1973; Bachman, 1984; Denver, 1989; Böhlke 

Figure 1. Sampled wells and selected hydrogeologic settings in 

the Northern Atlantic Coastal Plain.
Methods 3

and Denver, 1995; Eckhardt and Stackelberg, 1995; Bolton, 

1996; Spruill and others, 1997; Stackelberg and others, 1997; 

Clawges and others, 1999; Shedlock and others, 1999; Denver 

and others, 2004). Nitrate concentrations are generally highest 

in areas with permeable sediments and well-drained soils; 

nitrate may be removed from ground water by denitrification in anoxic areas such as poorly drained coastal or riparian 

wetlands (Phillips and others, 1993; Böhlke and Denver, 1995; 

Tesoriero and others, 2005). Ground water affected by agriculture, septic effluent, or road salting also may contain elevated 

concentrations of calcium, magnesium, sodium, potassium, or 

chloride (Denver, 1989), although some of these major ions 

may also be derived from natural sources, such as the dissolution of soluble minerals in soils or aquifer sediments (Tesoriero and others, 2004). Pesticide concentrations are typically 

less than 1 μg/L (microgram per liter) in the aquifer, and are 

greatest near areas of agricultural or urban use, and where 

movement into and through the ground-water system is not 

impeded by poorly drained or organic-rich soils or aquifer sediments (Koterba and others, 1993; Eckhardt and Stackelberg, 

1995; Bolton, 1996; Stackelberg and others, 1997; Clawges 

and others, 1999; Stackelberg and others, 2000; Tesoriero and 

others, 2004). VOCs are most commonly detected in unconfined ground water near sources in urban areas (Eckhardt and 

Stackelberg, 1995; Stackelberg and others, 1997; Stackelberg 

and others, 2000; Ferrari, 2002). Concentrations are generally 

low (less than 1 μg/L), but can vary substantially and may be 

considerably higher near source areas.

Methods

Data collected over a multi-year period were compiled 

and analyzed to describe factors affecting the occurrence 

and distribution of nitrate and selected pesticides and VOCs 

in shallow unconfined ground water at the regional scale in 

the Northern Atlantic Coastal Plain. Observed ground-water 

quality was compared to land use, soils, and other potential 

explanatory variables to provide insight into the interacting 

natural and human influences on shallow ground water in the 

area at the regional scale.

Compilation of Ground-Water-Quality Data

Data were compiled to support a regional interpretation 

of shallow, unconfined ground-water quality. Available data 

collected by the U.S. Geological Survey (USGS) and cooperating agencies from the early 1990s through September 

2003 in the study area were evaluated for inclusion in light 

of the project objectives. In light of the regional scale of the 

interpretation, only data collected as part of regional projects 

or programs (covering areas generally larger than a single 

county) were included. In addition, only data from wells that 

are unused (such as observation wells) or relatively low-producing (such as domestic wells) were targeted for inclusion. 

Data from wells used primarily for irrigation, public supply, 

or other potentially large withdrawals were avoided; such 

wells may draw ground water from relatively large contributing areas, and therefore complicate efforts to relate observed 

ground-water quality to specific land uses or other geographic 

variables. Data from wells installed to monitor known or 

suspected contamination from specific point sources were also 

avoided.

Ground-water-quality data for a large part of the Northern 

Atlantic Coastal Plain were retrieved from USGS databases 

from August through November 2004 and compiled to a 

single database. Many of these data were collected as part of 

the National Water-Quality Assessment (NAWQA) Program 

(Gilliom and others, 1995; Lapham and others, 1995; Spruill 

and others, 1998; Ayers and others, 2001; Denver and others, 

2004; Debrewer and others, 2007); additional data were compiled from USGS cooperative-study projects (Bolton, 1996; 

Cartwright and others, 1998; Phillips and others, 1999; Serfes 

and others, 2006). Laboratory analyses for NAWQA and cooperative projects were conducted at the USGS National WaterQuality Laboratory (NWQL) in Denver, Colorado. Wells in 

the unconfined aquifer that were sampled for these projects are 

generally shallow. Because of the particularly thick unsaturated zone and unconfined aquifer in parts of Long Island, 

New York, compiled data from that area were restricted to 

samples collected from wells completed within approximately 

15 m of the measured water table. Where measured water 

levels were unavailable, Long Island wells were included if 

less than 15 m deep or completed above sea level. Wells in 

the resulting overall dataset are generally less than 10 m deep 

(median, 9 m); the water table in these wells (where reported) 

was generally less than 5 m below land surface. Although 

included data were collected over a span of more than 12 years 

(March 1991 through September 2003), all data were considered comparable for the purposes of regional interpretation. 

Statistically significant temporal trends in ground-water nitrate 

and pesticide concentrations over near-decadal time periods 

have been detected recently in some areas (for example, Bexfield, 2008; Debrewer and others, 2008), although temporal 

changes in concentrations in individual wells are typically 

relatively small compared to spatial differences among wells.

Specific compounds were selected for data analysis on 

the basis of data availability and (particularly for pesticides 

and VOCs) detection frequency. Pesticide data were limited to 

concentrations determined in filtered water samples with a single laboratory method in common use by NAWQA and other 

programs (Zaugg and others, 1995). Atrazine, deethylatrazine 

(DEA, an atrazine degradate), metolachlor, and simazine were 

the most commonly detected pesticide compounds in the compiled data and were therefore selected for data analysis. VOC 

data were restricted to samples collected after the most commonly used method for NAWQA VOC analyses in unfiltered 

samples (Connor and others, 1998) was fully implemented at 

the NWQL (October 1, 1996) to avoid potential complications 

in interpreting previous data with typically higher reporting 

levels. VOC data were further limited to samples that were 
4 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

analyzed for each of four compounds that were selected 

for data analysis: chloroform, 1,1,1-trichloroethane (TCA), 

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

Nitrate data were limited to reported concentrations of nitrite 

plus nitrate as nitrogen (referred to hereinafter as “nitrate”) in 

filtered water samples. For analyses and discussion of majorion chemistry, data were restricted to samples with reported 

specific conductance, pH, and concentrations of dissolved 

oxygen, calcium, magnesium, sodium, potassium, iron, 

manganese, chloride, sulfate, nitrate, bicarbonate, and silica. 

Reported concentrations of cations and bicarbonate in unfiltered samples from some wells were assumed to be comparable to concentrations in filtered (“dissolved”) samples from 

the remainder of the study area. Bicarbonate concentrations 

were reported from incremental field titrations; where such 

values were missing, bicarbonate concentrations computed 

from reported acid-neutralizing capacity (Hem, 1985) in fixedendpoint laboratory titrations were assumed to be comparable.

Compiled data were screened to limit effects of spatial 

bias on the results of regional analyses. Although some wells 

were sampled multiple times, only the most recent available 

data of each type (pesticide, VOC, nitrate, or major-ion) from 

each well were retained for analysis. These data were typically 

from the same sample. The geographic distribution of wells 

also was reviewed to maximize sample independence and to 

minimize spatial autocorrelation among wells in the compiled 

dataset (Barringer and others, 1990). Among wells located 

within 1,000 m of one another, data from only one well were 

retained for analysis. Wells were selected from among such 

clustered wells on the basis of (in decreasing order of importance) most available pesticide, VOC, nitrate, or major-ion 

data; shallowest; and most recent pesticide data.

Compilation of Geographic Data

Geographic data were compiled for comparison to 

observed ground-water quality. Land-cover data at 30-m 

resolution from the early 1990s (U.S. Geological Survey, 

1999) were compiled to represent land uses and associated 

potential human effects on ground water in the study area. 

Potential natural influences on the quality of shallow unconfined ground water in the Coastal Plain include soil composition and hydrogeologic conditions. Schwarz and Alexander 

(1995) computed layer-thickness-weighted averages of 

selected hydrologically important soil characteristics from 

soils data available for the United States. A framework summarizing surficial and near-surface hydrogeologic conditions 

for the Mid-Atlantic Coastal Plain (Ator and others, 2005) was 

extended to include Long Island, New York for comparison to 

measured ground-water quality (fig. 1; table 1).

Proportions of various land uses and area-weighted 

averages of selected soil properties were computed for areas 

within 500 m of wells in the compiled dataset. Such relatively 

local land-use and soil conditions were selected on the 

assumption that ground water contributing to the primarily 

shallow, relatively low-pumpage wells in the dataset is derived 

mostly from relatively small areas near the well. Ground-water 

modeling coupled with particle tracking indicated that ground 

water contributing to selected compiled NAWQA wells in 

New Jersey is derived from within 500 m of the wells (Stackelberg and others, 2000). Greene and others (2005) found that 

logistic regression models relating concentrations of nitrate in 

ground water to selected geographic variables were strongest 

for 1,500-m radii, although their modeling covered a larger 

area of the Mid-Atlantic Region and included wells completed 

in fractured bedrock. Also, nitrate can be particularly stable in 

ground water, and might be expected to move greater distances 

along flow paths than other compounds such as pesticides or 

VOCs.

Data Analysis

Concentrations of major ions were evaluated to illustrate 

general patterns in major-ion geochemistry in samples of shallow unconfined ground water of the Coastal Plain. Trilinear 

diagrams (Helsel and Hirsch, 1992) were constructed to illustrate major cations and anions in various water types, and PCA 

(Hamilton, 1992) was conducted to summarize and identify 

underlying patterns in major-ion ground-water geochemistry. 

Censored data were assumed to represent zero in the construction of trilinear diagrams. PCA was run on ranks of data values 

to accommodate censored data. PCA has been used previously 

to interpret ground-water chemistry at various scales, often 

through interpretation of scatterplots of principal components 

(for example, Nolan, 1999; Stetzenbach and others, 2001; 

Cruz and Amaral, 2004). Dewhurst and others (2003) related 

principal components of ground-water chemistry in an urban 

area to toxicity.

Concentrations of nitrate and selected pesticides and 

VOCs were summarized and compared to the geochemical 

conditions [major-ion principal components (PCs)] and among 

different land uses, soils, and hydrogeologic conditions to 

evaluate the relative importance of various natural and human 

influences on regional ground-water quality. Geochemical PCs 

also were compared among these geographic settings. Relations between continuous variables were evaluated through 

examination of scatterplots and rank-transform correlation 

coefficients (Spearman’s rho). Rank-based correlation coefficients evaluate monotonic relations and can be useful even 

when data are severely censored (Helsel and Hirsch, 1992). 

Rank-transform analysis-of-variance (ANOVA) and Tukey 

tests (α=0.05) (Helsel and Hirsch, 1992) were used to compare 

the distribution of continuous variables among hydrogeologic 

subregions. 

Binary logistic regression (Helsel and Hirsch, 1992) was 

used to further illustrate land-use, soil, hydrogeologic, and 

geochemical factors related to the occurrence of selected pesticides, VOCs, and nitrate in ground water. Logistic-regression 

models for nitrate describe factors affecting the probability 

of nitrate occurring in ground water at concentrations greater 
Methods 5

than 0.4 mg/L (milligrams per liter), a previous estimate of 

the maximum concentration in natural ground water in part 

of the Coastal Plain (Hamilton and others, 1993), and greater 

than 3 mg/L, a concentration likely indicative of human influences. Other models describe factors affecting the probability 

of detecting atrazine or DEA, metolachlor, or one of the four 

VOCs in ground water, using sensitive analytical methods 

typical of NAWQA (at several nanograms per liter for the 

pesticide compounds, or a few tenths of a microgram per liter 

for the VOCs). Quantified values of pesticide compounds at 

any level were considered detections for the purpose of the 

pesticide models; such values reported by the NWQL indicate 

that the presence of the compound is confirmed, even if the 

concentration may be estimated (see below) (Gilliom and others, 2006). Explanatory variables were selected for each model 

using a stepwise procedure (α=0.05). Candidate variables 

included hydrogeologic subregion, well depth, geochemical 

PCs, dissolved oxygen, specific conductance, and selected 

land-use (urban, agriculture, forest, wetland) and soil (clay, 

organic matter, permeability, hydrologic group, drainage, 

slope) characteristics. Dissolved oxygen was included as a 

binary class variable indicating concentrations less than, or 

greater than or equal to, 1 mg/L. Because nitrate was used 

to define the geochemical PCs, these PCs were excluded 

from consideration for the nitrate models, and all available 

dissolved-oxygen and specific-conductance data corresponding to nitrate observations (including those without a complete 

set of major-ion data) were included. Logit plots were examined for each model, and the sensitivity of the list of selected 

explanatory variables in each model to unusually influential 

or poorly fitted points was checked by rerunning the stepwise 

selection without such points to verify that each such selected 

variable remained significant.

Relations between ground-water quality and potential 

natural and human influences were determined for three 

surficial hydrogeologic subregions in the Coastal Plain — the 

Coastal Lowlands, the Middle Coastal Plain – Sands with 

Overlying Gravels (hereinafter referred to as “the Middle 

Coastal Plain”), and the Inner Coastal Plain – Dissected Outcrop Belt (hereinafter referred to as “the Inner Coastal Plain”) 

(table 1). These three subregions account for the vast majority 

of compiled ground-water-quality data, and represent a variety 

Table 1. Summary of selected hydrogeologic settings in the Northern Atlantic Coastal Plain and 

distribution of wells from which compiled ground-water-quality data were collected. 

[n, number of wells in compiled dataset (total=409); Tot, proportion of land use in entire subregion (in percent); Med, median 

percent land use within 500 meters of wells in compiled dataset; IQR, interquartile range (75th percentile minus 25th percentile) of percent land use within 500 meters of wells in compiled dataset; Agric, agriculture; <, less than]

Hydrogeologic 

Subregion1 n

Land-Use Distribution 

[Tot / Med (IQR)] Summary Description

Forest Agriculture Urban

1: Coastal Lowlands 63 19 / 12 (23) 27 / 52 (52) 6 / 1 (19) Predominantly flat and low-lying 

with consequent poor natural 

drainage. Streams are typically 

sluggish and wetlands with 

organic-rich soils are common. 

Water table is relatively shallow and ground water may be 

poorly oxidized.

4: Middle Coastal 

Plain, Sands 

with Overlying 

Gravels

160 38 / 17 (29) 39 / 30 (69) 7 / 5 (64) Coarse surficial sediments and 

moderate relief promote 

infiltration and drainage of the 

land surface, and oxidation of 

shallow ground water.

6: Inner Coastal 

Plain, Dissected 

Outcrop Belt

144 42 / 6 (15) 22 / <1 (18) 28 / 85 (68) Topographic relief promotes 

runoff. Surficial sediment 

texture and oxidation of ground 

water is variable; some units 

are coarse and permeable 

while others are fine-grained. 

Includes glacial sediments on 

Long Island.

1

 Modified from Smolensky and others, 1989; Ator and others, 2005.
6 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

of distinct hydrogeologic settings in the area. Because relatively few VOC data were available from the Coastal Lowlands, VOC analyses were further limited to two subregions. 

Geochemical PCs were considered comparable to pesticide or 

VOC data for use in regression or other analyses when based 

on concentrations measured in the same sample. Reported estimated values (such as below long-term method detection levels) were included as quantities in data analyses to maximize 

the scientific information in the data, although particularly low 

reported concentrations are less certain than higher concentrations, and the chance of false negatives is greater (L.M. 

Bexfield, U.S. Geological Survey, written commun., 2005; 

Gilliom and others, 2006; Stackelberg and others, 2006). Data 

were censored to a common level for each numeric variable, 

and censored data were set to a common lower level before 

ranks were computed for use in statistical analyses. Rare pesticide, VOC, or nitrate observations censored at unusually high 

concentrations were assumed to represent poor measurements 

due to sample interference or some other type of measurement 

issue (Gilliom and others, 2006), and were excluded from 

analyses. Although procedures for reporting censored pesticide 

values were revised at the NWQL in October 2000 (Oblinger 

Childress and others, 1999), censored pesticide data for the 

entire period of study were considered equivalent (Gilliom and 

others, 2006), and such censored concentrations were assumed 

to be less than the lowest quantified values when computing 

ranks for statistical tests (L.M. Bexfield, U.S. Geological 

Survey, written commun., 2005). Effects of including such 

particularly low pesticide concentrations as quantities in data 

analyses on results and data interpretation are minimal; results 

of analyses of pesticide data using this approach and using 

data recensored up to maximum long-term method detection 

levels for the study period (see table 2) were very similar.

Natural and Human Influences on 

Ground-Water Quality

Ground-water quality in the unconfined aquifer in the 

Northern Atlantic Coastal Plain is related to natural and human 

influences in local areas. Land use, soil, and hydrogeologic 

conditions near (within 500 m of) sampled wells impart 

observable effects on shallow ground water. Concentrations of 

major ions and nitrate are particularly variable in the Coastal 

Plain; this variability among compiled data can be explained 

by a limited number of geochemical factors related to land 

use, soils, and hydrogeologic conditions. Observed pesticide, 

VOC, and nitrate concentrations are also related to geochemical conditions, as well as land use, soils, and hydrogeology.

Major Ions and Nitrate

Water in much of the surficial aquifer in the Coastal Plain 

is well-oxidized and reflects the influence of soluble ions from 

natural or human sources at or near the land surface. Although 

ground water is dilute in some areas, the median specific conductance among 272 samples with complete major-ion data 

was 247 μS/cm (microsiemens per centimeter), considerably 

higher than would be expected in many areas from precipitation and mineral dissolution alone (Denver, 1989). Similarly, 

reported pH in these samples ranged from 3.7 to 8.5, but the 

median of 5.4 likely reflects the buffering of precipitation 

with ions from natural or human sources (Denver, 1989). 

Dissolved oxygen is abundant in many areas (sample median, 

3.8 mg/L), although concentrations were less than 1 mg/L in 

samples from 75 of 272 wells. The generally lower dissolvedoxygen concentrations observed in the Coastal Lowlands than 

in the Middle or Inner Coastal Plain (fig. 2) reflects the poor 

drainage, organic-rich soils, and relatively reducing conditions 

common in that hydrogeologic setting (fig. 1; table 1). 

The major-ion composition of unconfined ground water 

in the Coastal Plain varies from place to place, reflecting 

associated spatial variability in natural and human sources of 

major cations and anions (fig. 3). Dominant cations are typically calcium or sodium in most of the 272 samples; dominant 

anions typically vary from bicarbonate to chloride and nitrate. 

Magnesium and sulfate are generally minor components of 

Table 2. Summary of selected characteristics of groundwater-quality data compiled for the Northern Atlantic Coastal 

Plain. 

[n, number of samples; mg/L, milligrams per liter; μg/L, micrograms 

per liter; DEA, deethylatrazine; TCA, 1,1,1-trichloroethane; PCE, 

tetrachlorethene; MTBE, methyl tert-butyl ether; <, less than]

Compound 

(units) n Maximum 90thPercentile Median

Nitrate (mg/L 

as N) 352 55.5 11.9 2.0

Atrazine (μg/L) 346 4.2 0.017 1

<0.004

DEA (μg/L) 351 0.480 0.014 1

<0.003

Metolachlor 

(μg/L) 349 3.97 0.014 1

<0.006

Simazine (μg/L) 346 4.71 0.014 1

<0.006

Chloroform 

(μg/L) 203 26.3 1.14 <0.1

TCA (μg/L) 203 19.9 <0.1 <0.1

PCE (μg/L) 202 805 0.27 <0.1

MTBE (μg/L) 203 116 0.7 <0.2

1

 Censored percentiles for pesticides are listed as less than the maximum 

long-term method detection level for the study period (Gilliom and others, 

2006).
Natural and Human Influences on Ground-Water Quality 7

sampled unconfined ground water in the Coastal Plain (fig. 3). 

Although ground-water chemistry may vary with land use or 

other influences in each hydrogeologic setting, major-ion composition among compiled samples is relatively consistent in 

the Middle Coastal Plain, where major anions are commonly 

limited primarily to chloride and nitrate. In contrast, major-ion 

composition in the Coastal Lowlands is particularly variable; 

ground water in this subregion varies from a calcium-bicarbonate type to the calcium-sodium-chloride-nitrate type more 

typical of the other subregions (fig. 3).

Nitrate concentrations reflect human inputs of nitrogen in 

many areas of the Northern Atlantic Coastal Plain. Although 

reported nitrate concentrations were less than 0.06 mg/L in 

more than one quarter of the wells in the compiled dataset, 

concentrations vary considerably among different areas of 

the Coastal Plain, and nitrate is a dominant anion in some 

samples. Nitrate concentrations as high as 55.5 mg/L were 

reported; the median was about 2 mg/L (table 2). Although 

concentrations of nitrate attributable to natural sources are 

uncertain and likely vary spatially, most previous estimates 

for areas of the Coastal Plain are less than 1 mg/L (Hamilton 

and others, 1993; Stackelberg and others, 2000). Nitrate may 

affect the potability of untreated ground water in some areas of 

the surficial aquifer; reported nitrate concentrations in samples 

from 52 of 352 wells were greater than 10 mg/L, the current 

Federal standard for drinking water (U.S. Environmental 

Protection Agency, 2004). 

Variability in major-ion composition in unconfined 

ground water in the Coastal Plain is related primarily to a 

limited number of factors. Specific conductance, pH, and concentrations of dissolved oxygen, calcium, magnesium, sodium, 

potassium, iron, manganese, chloride, sulfate, nitrate, bicarbonate, and silica are generally correlated with one another, 

and PCA indicates that three components explain more than 

70 percent of the variability in these observations in the 

compiled dataset (table 3). The first component (PC1) explains 

40 percent of the variability in major-ion composition, and 

apparently represents an indicator of overall ionic strength. 

Strong positive loadings for PC1 are greatest for specific conductance, and also include most of the major cations (calcium, 

magnesium, and sodium) and anions (chloride and bicarbonate) measured in Coastal Plain ground water. The second 

principal component (PC2), which explains 22 percent of the 

variability in major-ion composition, is apparently an indicator 

of redox condition. PC2 has relatively strong positive loadings 

for nitrate and dissolved oxygen (indicative of oxygen-rich 

ground water), and relatively strong negative loadings for iron 

(which is most soluble under reducing conditions). The third 

principal component (PC3) explains only 9 percent of the 

variability in major-ion composition, and may be an indicator 

of different sources of soluble ions to ground water. Calcium, 

bicarbonate, and pH (suggestive of the dissolution of lime, 

shell material, or other carbonate minerals) have relatively 

strong positive loadings for PC3. Ions with relatively strong 

negative loadings for PC3 include manganese, sodium, chloride, and sulfate (table 3).

The major-ion composition of unconfined ground water is 

related to different land-use and soil factors in different areas 

of the Coastal Plain. Major ions contributing to the overall 

ionic strength of ground water are likely derived from different sources important to water quality in different settings. 

Ionic strength (PC1) is positively correlated with agriculture 

near (within 500 m of) sampled wells in the Middle Coastal 

Plain, and with urban land near sampled wells in the Inner 

Coastal Plain (table 4). Ionic strength is not related to urban or 

agricultural land use in the Coastal Lowlands, where PC1 is 

more strongly related to soil conditions (table 4). The positive 

correlation of ionic strength with soil clay content (and related 

negative correlation with permeability) apparent in all three 

hydrogeologic subregions may be indicative of natural sources 

of soluble major ions, but may also be related to correlations 

between land use and soil conditions in some areas. Clay 

content is positively correlated with agriculture among wells 

in the Middle Coastal Plain, and with urban land use among 

wells in the Inner Coastal Plain. Ionic strength is not related 

to agricultural or urban land use in the Coastal Lowlands, 

Figure 2. Distribution of dissolved-oxygen concentrations in 

samples with complete major-ion data among three hydrogeologic 

subregions, Northern Atlantic Coastal Plain. [Listed p-value and 

letters in plot reflect results of rank-transform ANOVA and Tukey 

tests. Groups with the same letter are not significantly different.]
8 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

Figure 3. Summary of major-ion chemistry in samples with complete major-ion data, Northern 

Atlantic Coastal Plain.
Natural and Human Influences on Ground-Water Quality 9

however, and the positive correlation between PC1 and clay 

content suggests ions contributing to ionic strength may be 

derived from natural sources in relatively clay-rich soils or 

associated impermeable or poorly weathered sediments in that 

subregion. Such conditions also may contribute soluble ions to 

ground water where they occur in the other subregions.

Ground-water redox condition and other geochemical 

properties also vary among different land-use, soil, and hydrogeologic settings in the Coastal Plain. Redox condition (PC2) 

is correlated most strongly to land use and soil properties 

among sampled wells in the Middle Coastal Plain, possibly 

because PC2 is most variable in this subregion. Ground water 

in this subregion is typically more reducing (and contains 

correspondingly less nitrate) in more poorly drained forested 

or wetland areas with greater organic matter and lower slopes 

(table 4). In contrast, ground water is more oxic (with more 

nitrate) in more agricultural areas of the Middle Coastal Plain, 

which also typically contain more clay-rich and impermeable 

soils. In the Coastal Lowlands, reducing conditions in ground 

water are similarly associated with poorly drained, organicrich soils near sampled wells (table 4). Ion source (PC3) is 

most strongly related to land use and soil conditions in the 

Inner Coastal Plain, where negative values of PC3 (related to 

manganese, sodium, chloride, and sulfate) are most common 

in more poorly drained, organic-rich wetlands. Correlations 

also suggest calcium-bicarbonate waters are more common 

at greater depths in the Coastal Lowlands and Inner Coastal 

Plain, and are weakly related to agriculture in the Middle 

Coastal Plain (table 4). 

The occurrence of nitrate at different concentrations in 

unconfined ground water is related to similar hydrogeologic 

and geochemical conditions, but possibly different sources, 

in different parts of the Coastal Plain. Results of correlations 

of nitrate concentrations to land use and soil conditions in 

the Coastal Lowlands and Middle Coastal Plain are similar 

to those for redox condition (PC2) (table 4), as might be 

expected given that PC2 has a high positive loading for nitrate 

(table 3). These results suggest that nitrate is related to natural 

redox conditions related to soils and (presumably) hydrogeology in these subregions, and to primarily agricultural sources, 

at least in the Middle Coastal Plain. Comparisons of nitrate 

to land use and soil characteristics in the Inner Coastal Plain, 

however, demonstrate some (generally weak) correlations 

that were not apparent for redox condition (PC2). The negative correlation of nitrate with natural forest and wetland, and 

the lack of a significant correlation with either specifically 

urban or agricultural land use, suggest that nitrate may be 

derived from a variety of urban or agricultural activities in this 

subregion. Nitrate concentrations in sampled wells in the Inner 

Coastal Plain also increase with decreasing soil organic matter 

and permeability, but increasing drainage (table 4), conditions 

typical of less forested or wetland areas near sampled wells in 

that subregion. 

Regression models (table 5) further illustrate natural 

and human factors important to the occurrence of nitrate in 

sampled ground water. Nitrate is significantly more likely 

to exceed both 0.4 mg/L and 3 mg/L where ground water 

contains at least 1 mg/L of dissolved oxygen (table 5). Nitrate 

is also less likely to exceed estimated natural levels (0.4 mg/L) 

in the Coastal Lowlands than in the Middle or Inner Coastal 

Plain (fig. 4), as might be expected in light of the hydrogeologic and reducing conditions typical of the Coastal Lowlands 

(table 1, fig. 2). Similarly, nitrate concentrations greater than 3 

mg/L are more likely in areas with better soil drainage. Nitrate 

also is more likely to exceed both 0.4 mg/L and 3 mg/L in less 

forested areas, although the likelihood of nitrate concentrations greater than 3 mg/L is also specifically related to the 

occurrence of agriculture. The negative specific conductance 

term in the 0.4-mg/L model suggests that sources of nitrate at 

this concentration are different from possible sources of high 

conductance or ionic strength (such as road salt).

Table 3. Loadings1 on the first three principal components 

from unrotated principal components analysis.

 [Loadings with absolute value greater than 0.60 are shown in bold; loadings with absolute value less than than 0.25 are omitted. PC; principal 

component]

Principal component 

(interpretation)

PC1

(ionic 

strength)

PC2

(redox 

condition)

PC3

(source 

of ions)

Proportion of variance 

explained 0.40 0.22 0.09

Specific conductance 0.90 0.31

Calcium 0.82 0.25 0.28

Magnesium 0.77 0.47

Sodium 0.76 -0.29

Chloride 0.75 0.36 -0.28

Bicarbonate 0.76 -0.36 0.46

pH 0.69 -0.41 0.52

Silica 0.68 -0.32

Nitrate 0.84

Dissolved oxygen -0.38 0.70

Iron 0.50 -0.68

Manganese 0.44 -0.30 -0.53

Sulfate 0.46 0.47 -0.26

Potassium 0.39 0.48

1

Loadings quantify relations between principal components and original 

variables (Hamilton, 1992).
10 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

Table 4. Summary of correlations (Spearman’s rho) among selected water-quality and geographic variables in 

three hydrogeologic subregions. 

[Results are shown only where p<0.05. Shaded cells indicate p<0.01. +, rho positive; -, rho negative; nc, not computed; PC, principal 

component (PC1, ionic strength; PC2, redox condition; PC3, source of ions); DEA, deethylatrazine; MTBE, methyl tert-butyl ether; 

TCA, 1,1,1-trichloroethane; PCE, tetrachlorethene; <, less than]

Compound 

or Principal 

Component1

Geographic or other potential explanatory characteristic

Land use Soils Other

ForestWetlandUrbanAgricultureClay ContentOrganic ContentPermeabilityHydrologic Group2Drainage3SlopeWell DepthPC1PC2PC3

Subregion 1 – Coastal Lowlands

PC1 - + - + - nc nc nc

PC2 - - - nc nc nc

PC3 + nc nc nc

Nitrate4 - - - nc nc nc

Atrazine - +

DEA - - - +

Subregion 4 – Middle Coastal Plain, Sands with Overlying Gravels

PC1 - + + - + nc nc nc

PC2 - - + + - - - + nc nc nc

PC3 + - nc nc nc

Nitrate4 - - + + - - - + + nc nc nc

Atrazine - - + + - - - + + +

DEA - - + + - - + + + +

Metolachlor - - + + - + +

Simazine - - + +

Chloroform - - + -

MTBE +

Subregion 6 – Inner Coastal Plain, Dissected Outcrop Belt

PC1 - - + - + - - + - - nc nc nc

PC2 nc nc nc

PC3 - - - + - + nc nc nc

Nitrate4 - - - - - nc nc nc

Atrazine - - -

DEA -

Metolachlor - + - + + - -

Simazine -

Chloroform - - + - + - - + - + + +

TCA - - + +

PCE - - + - + - - + - + + +

MTBE - + - + - - + - +

1

Compounds are omitted in subregions where n<20 or more than 90 percent of data are censored.

2

Lower values indicate higher infiltration rates.

3

Values range from 1 (excessive drainage) to 7 (very poor drainage).

4

Results of correlations of nitrate with PCs are not shown because nitrate was used in principal components analysis (see table 3).
Natural and Human Influences on Ground-Water Quality 11

Pesticides

Atrazine and DEA were the most commonly detected 

pesticide compounds in samples from the unconfined aquifer. 

These compounds were detected in 30 and 31 percent, respectively, of the approximately 350 samples that were analyzed 

for pesticides. Metolachlor and simazine were detected in 23 

and 18 percent of samples, respectively. Concentrations rarely 

exceeded 1 μg/L, and were generally well below 0.1 μg/L 

(table 2). No other pesticides were detected in at least 15 percent of samples, and only three others (prometon, p,p’ DDE, 

and dieldrin) were detected in as many as 5 percent. Although 

individual compounds are infrequently detected, at least one of 

the four most commonly detected compounds was detectable 

in nearly half (46 percent) of the 338 samples for which data 

for each were available. Where present in unconfined ground 

water, pesticide compounds typically occur in mixtures. More 

than two-thirds (68 percent) of samples with detectable concentrations of atrazine, DEA, metolachlor, or simazine contained more than one of these compounds. The most common 

mixtures contained atrazine and its degradate, DEA, followed 

by metolachlor and simazine. Although pesticide concentrations are typically low, isolated higher concentrations may 

affect the potability of untreated ground water in some areas. 

Table 5. Summary of logistic regression models of ground-water quality. 

[c, concordance; mg/L, milligrams per liter; >, greater than; DEA, deethylatrazine; PC, principal component 

(PC1, ionic strength; PC2, redox condition; PC3, source of ions); VOC, volatile organic compound]

Response variable

Response 

profile

(yes/no)

Explanatory variables1

(sign of parameter) c

Hosmer / 

Lemeshow 

Goodness 

of fit (p)

P (nitrate > 0.4 mg/L as N) 169 / 96 Percent forest (-) 0.91 0.12

Specific conductance (-)

Dissolved oxygen2

Hydrogeologic subregion3

P (nitrate > 3 mg/L as N) 122 / 143 Percent forest (-) 0.84 0.35

Soil drainage (-)

Percent agriculture (+)

Dissolved oxygen2

P (atrazine or DEA detected) 82 / 125 PC2 (+) 0.86 0.47

Percent agriculture (+)

PC1 (-)

PC3 (+)

P (metolachlor detected) 42 / 167 PC2 (+) 0.86 0.21

Percent agriculture (+)

PC1 (-)

P (VOC4) 66 / 75 Percent urban (+) 0.84 0.51

Percent wetland (-)

Percent forest (+)

1

Continuous variables are listed in decreasing order of the absolute value of the standardized estimate, 

followed by class variables.

2

Estimated probability is greater where dissolved-oxygen concentration is greater than or equal to 1 mg/L.

3

Estimated probability is greater in the Middle Coastal Plain, Sands with Overlying Gravels (subregion 4), and 

the Inner Coastal Plain, Dissected Outcrop Belt (subregion 6), than in the Coastal Lowlands (subregion 1).

4

Probability that chloroform, tetrachlorethene (PCE), 1,1,1-trichlorethane (TCA), or methyl tert-butyl ether 

(MTBE) is detected above common censoring level.
12 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

Concentrations were greater than Federal drinking-water 

standards (U.S. Environmental Protection Agency, 2004) for 

atrazine in one sample and for simazine in two samples; such 

standards do not exist for metolachlor, DEA, or mixtures of 

compounds.

Concentrations of selected pesticides in ground water 

in different parts of the Coastal Plain are related to application patterns reflected in local land use. In the Middle and 

Inner Coastal Plain, concentrations of atrazine, DEA, and (or) 

metolachlor generally increase with increasing proportion of 

agriculture near sampled wells (table 4). Logistic models indicate that the occurrence of these compounds in ground water is 

also more likely in areas with a greater proportion of agriculture (table 5), as might be expected given the agricultural uses 

of atrazine and metolachlor (Gilliom and others, 2006). Simazine is positively correlated with urban land in the Middle 

Coastal Plain, and is not significantly correlated with land 

use in the Inner Coastal Plain. Atrazine, DEA, metolachlor, 

and simazine were infrequently detected in samples from the 

Coastal Lowlands, and atrazine and DEA are not significantly 

correlated with land use in that subregion (table 4).

Pesticide concentrations in unconfined Coastal Plain 

ground water are related to natural soil and geochemical 

conditions, as well as land use. Organic matter may retard the 

transport of pesticides to ground water in some areas. Concentrations of atrazine, DEA, and (or) metolachlor decrease with 

increasing soil organic matter in the Coastal Lowlands and 

Middle Coastal Plain (table 4). A negative correlation between 

agriculture and soil organic matter near sampled wells may at 

least partly explain this relation in the Middle Coastal Plain. 

Similarly, the positive correlation between metolachlor concentration and organic matter in the Inner Coastal Plain may 

be related to a positive correlation between organic matter and 

agriculture among wells in that subregion. Organic matter is 

not related to agriculture among wells in the Coastal Lowlands, however, and the observed correlation between organic 

matter and DEA concentration suggests this compound may 

be sorbed onto organic compounds in the soils (Barbash and 

Resek, 1996), at least in that subregion. DEA concentrations 

also increase with increasing drainage in samples from the 

Coastal Lowlands, even though agriculture is more common 

in poorly drained areas. Atrazine concentrations also increase 

with increasing soil drainage near sampled wells in the Middle 

Coastal Plain, and metolachlor and simazine concentrations 

increase with decreasing soil clay content in the Inner Coastal 

Plain (table 4). Apparent correlations between pesticide 

concentrations and other soil variables in these subregions are 

likely related to similar correlations with the occurrence of 

agriculture.

Pesticide concentrations are also related to ground-water 

geochemistry in some areas of the Coastal Plain. Geochemical conditions reflected in PCs were the only significant 

terms other than agricultural land use in the pesticide logistic regression models (table 5). Pesticide concentrations are 

generally greater in more oxic ground water. Correlations 

between pesticide concentrations and ground-water redox 

condition (PC2) are positive and significant among samples in 

the Middle Coastal Plain, and even in the Coastal Lowlands, 

where pesticides were infrequently detected (table 4, fig. 5). 

Atrazine (or DEA) and metolachlor are also more likely to 

occur in oxic ground water than in areas with lower PC2 

(table 5). These compounds are also more likely to be found 

in ground water with lower ionic strength (PC1) (table 5), and 

significant correlations between pesticide concentrations and 

ionic strength are negative in the Coastal Lowlands and Inner 

Coastal Plain (table 4). As discussed earlier, major ions contributing to ionic strength are likely derived (at least in part) 

from agricultural sources in samples from the Middle Coastal 

Plain, but from other sources in those from the Coastal Lowlands and Inner Coastal Plain. Although DEA concentrations 

are positively correlated with ionic strength in the Middle 

Coastal Plain (table 4), ground-water samples from this 

subregion were generally more dilute (lower ionic strength, 

ANOVA p<0.0001) than in the other subregions. The presence 

of atrazine (or DEA) is also more likely in areas with calciumbicarbonate ground water (higher PC3) (table 5), and atrazine 

and DEA concentrations are positively correlated with PC3 in 

the Middle Coastal Plain (table 4).

Figure 4. Estimated probability of nitrate concentration 

exceeding 0.4 milligrams per liter (as nitrogen) in samples from 

three hydrogeologic subregions, Northern Atlantic Coastal Plain.
Natural and Human Influences on Ground-Water Quality 13

Volatile Organic Compounds

VOC data were compiled for approximately 200 wells 

on the Delmarva Peninsula and to the north in New Jersey and 

New York (table 2). Samples from about half (48 percent) of 

these wells contained at least 0.1 μg/L of chloroform, PCE, or 

TCA, or 0.2 μg/L of MTBE, and 19 percent contained more 

than one of these compounds at these concentrations. Chloroform was the most commonly detected individual compound 

at these concentrations, although only 35 percent of samples 

contained at least 0.1 μg/L. The frequencies of occurrence of 

PCE and TCA at 0.1 μg/L or greater were less than 15 percent. 

Although VOC concentrations were typically low in sampled 

ground water, locally higher concentrations may affect the 

potability of untreated ground water in some areas. One 

sample contained MTBE at a concentration greater than 70 

μg/L, a drinking-water standard in New Jersey (Toccalino and 

others, 2004), and seven samples contained PCE at concentrations exceeding 5 μg/L, a Federal drinking-water standard 

(U.S. Environmental Protection Agency, 2004). 

VOCs in sampled unconfined ground water in the Coastal 

Plain occur most commonly near urban areas. As might be 

expected, considering the sources and uses of VOCs (Moran 

and others, 2002), correlations with reported concentrations of 

chloroform, MTBE, and PCE (where significant) are generally 

positive with urban land use and negative with other land uses 

(table 4). VOC occurrence is also more likely in urban areas 

and less likely near wetlands in the Middle and Inner Coastal 

Plain (table 5). Measured concentrations of TCA rarely 

exceeded 0.1 μg/L in any subregion, and correlations between 

concentrations of this compound and land use are generally 

insignificant (table 4).

Although human sources of VOCs to ground water 

are likely important, chloroform may also be derived from 

natural sources (Moran and others, 2002; McCulloch, 2003; 

Ivahnenko and Barbash, 2004). In the presence of the positive urban and negative wetland terms, logistic regression also 

detected a smaller (based on the standardized parameter estimate) but significant positive relation between forest and the 

occurrence of VOCs (table 5). This may reflect natural sources 

of chloroform in a particular hydrogeologic setting; forest was 

not selected as a significant term in a similar model for which 

chloroform was omitted from the definition of the dependent 

variable. Although only 17 of 141 wells included in the VOC 

model (table 5) are in predominantly (greater than 50 percent 

within 500 m) forested areas, chloroform was detected in 

samples from six of these wells in the Middle Coastal Plain in 

New Jersey. The chloroform in ground water at these sites may 

have been from human sources; chloroform has been detected 

in septic-tank effluent (Moran and others, 2002), and three of 

these samples also contained detectable PCE or pesticides. 

The specific conductance of each of these six samples was 

less than 70 μS/cm, however, which suggests minimal human 

influence. Chloroform may form naturally from biomass burning or from organic matter degradation in acidic forest soils 

(Moran and others, 2002; McCulloch, 2003; Ivahnenko and 

Barbash, 2004). Acidic soils and forest fires are particularly 

common in much of the forest of the New Jersey Coastal Plain 

(New Jersey Pinelands Commission, 2006).

VOC occurrence in Coastal Plain ground-water samples 

is also related to soil and geochemical conditions, although 

the relative importance of these natural conditions and land 

use may be difficult to determine from compiled data. Concentrations of chloroform, MTBE, and PCE in samples from 

the Inner Coastal Plain generally increase with decreasing soil 

organic matter, permeability, and infiltration, but increasing 

clay content and drainage (table 4). Concentrations of VOCs 

are also generally greater at greater depths in the Inner Coastal 

Plain. Although the fate and transport of VOCs in ground 

water may be affected by natural hydrogeologic conditions 

represented by these variables, the significance of these correlations may also be related to the distribution of land uses. 

Among wells in the compiled dataset, urban land use is also 

positively correlated with drainage and depth in the Inner 

Coastal Plain, and negatively correlated with soil organic 

matter, permeability, and infiltration. The interpretation of 

significant correlations between VOC concentrations and 

geochemical indicators may be similarly complicated by the 

relations between geochemistry and land use. Concentrations 

of selected VOCs are positively correlated with ionic strength 

(PC1) in the Inner Coastal Plain and negatively correlated with 

ionic strength in the Middle Coastal Plain (table 4), although 

ionic strength is also related to urban land use in the Inner 

Coastal Plain, and agriculture in the Middle Coastal Plain 

(see above). PCE concentrations increase with increasingly 

oxic conditions (indicated by PC2) in samples from the Inner 

Coastal Plain (table 4). This relation may be more meaningful 

than those with ionic strength; redox condition is not related to 

Figure 5. Relation of deethylatrazine concentration to groundwater redox condition (as represented by Principal Component 2) 

in samples from the Coastal Lowlands (Subregion 1).
14 Natural and Human Influences on Water Quality in a Shallow Regional Unconsolidated Aquifer, Northern Atlantic Coastal Plain

land use among sampled wells in the Inner Coastal Plain (table 

4), and PCE is more stable in ground water under oxic conditions (Zogorski and others, 2006). Similarly, chloroform and 

TCA concentrations increase with increasing calcium-bicarbonate type waters (indicated by PC3) in the Inner Coastal 

Plain. PC3 and urban land use are not correlated in the Inner 

Coastal Plain, although both are correlated with well depth.

Summary and Conclusions

The quality of shallow unconfined ground water is related 

to the availability of chemicals from natural or human sources 

at or near the land surface, as well as to natural hydrogeologic 

and geochemical conditions that affect the transport and fate 

of these chemicals in the environment. Data collected recently 

(1991-2003) from several hundred wells in the Northern 

Atlantic Coastal Plain were analyzed to illustrate various influences on ground-water quality at the regional scale. Variability 

in observed major-ion composition is related primarily to ionic 

strength and redox condition. Patterns in major-ion chemistry 

are related to human influences, but also reflect natural geochemical and hydrogeologic conditions important to overall 

ground-water quality. Selected pesticide compounds [atrazine, metolachlor, simazine, and deethylatrazine (DEA), an 

atrazine degradate] and volatile organic compounds (VOCs) 

[chloroform, 1,1,1-trichloroethane (TCA), tetrachloroethene 

(PCE), and methyl tert-butyl ether (MTBE)] were occasionally detected, although concentrations rarely exceeded 0.2 

micrograms per liter. Nitrate was also commonly detected in 

ground-water samples, typically at concentrations exceeding 

likely natural levels. Concentrations of nitrate, pesticides, and 

VOCs in ground water are related to chemical use in agricultural or urban areas, as well as to natural conditions that affect 

their movement to ground water in different settings.

Observed spatial variability in ground-water quality 

reflects major sources of different chemical compounds to 

ground water in different areas. Surficial aquifer sediments in 

much of the Coastal Plain are well-weathered and siliciclastic, 

and ground water is naturally relatively dilute. Major ions in 

ground water in such areas are likely derived primarily from 

human sources, such as agriculture in the Middle Coastal 

Plain, or urban sources in the Inner Coastal Plain. Major ions 

reflected in ionic strength may also be derived from natural 

sources where available in younger, more poorly weathered 

sediment, however, such as in parts of the Coastal Lowlands. 

Calcium-bicarbonate waters reflected in positive values of 

Principal Component 3 (PC3), for example, may be related to 

natural sources at depth in the Coastal Lowlands, but derived 

from agricultural lime applications in the Middle Coastal 

Plain. Nitrate at relatively low concentrations [greater than 

0.4 milligrams per liter (mg/L)] may be derived from a variety 

of human sources, although higher concentrations (greater 

than 3 mg/L) are more common in agricultural areas. Sources 

of synthetic compounds, such as pesticides and VOCs, are 

related to patterns of human use, including agriculture for 

atrazine, DEA, and metolachlor, and urban uses for VOCs and 

(possibly) simazine. Chloroform may also form naturally in 

some settings. 

Understanding the distribution of chemical compounds in 

ground water requires consideration of natural geochemical, 

hydrogeologic, and soil conditions, as well as the distribution 

of sources reflected in land use. The importance of dissolved 

oxygen to the occurrence and transport of nitrate in ground 

water is well understood and documented, and is reflected 

in the greater predicted likelihood of nitrate exceeding 0.4 

mg/L or 3 mg/L in relatively oxic ground water in the Coastal 

Plain. Nitrate is also more likely in ground water in relatively 

well-drained areas, such as the Middle and Inner Coastal 

Plain. Pesticides are also more likely in relatively oxic ground 

water, probably because well-drained, organic-poor soils 

and sediments that promote oxygen-rich ground water also 

typically favor the transport of soluble pesticides to the water 

table. The importance of ionic strength [(reflected in Principal Component 1 (PC1)] to pesticide occurrence suggested 

by the logistic models may also reflect that pesticides may 

be transported relatively easily in areas (such as the Middle 

Coastal Plain) with well-drained, well-weathered sediments 

and, therefore, relatively dilute ground water. Similar hydrogeologic and geochemical conditions may affect the transport 

and transformation of VOCs in ground water, although these 

relations were generally not apparent in the compiled data, 

possibly because VOC data were abundant in a less diverse 

group of settings. Although soil conditions are also commonly 

related to the occurrence of nitrate, pesticides, or VOCs in 

ground water, the detection of such relations in the compiled 

dataset may have been complicated by the correlation of soil 

conditions with other potential explanatory variables (notably 

land use). A more thorough and complete understanding of 

interacting land-use, soil, hydrogeologic, and geochemical 

factors affecting ground-water quality would likely require a 

particularly well-designed study, rather than a compilation of 

available data.

A combination of geochemical information with available 

geographic data can be useful for understanding and predicting the occurrence of certain chemical compounds in ground 

water. Geochemical conditions reflected in PCs or dissolvedoxygen concentrations were significant predictors of nitrate 

and pesticide occurrence in the compiled dataset. Where available, such data may provide more insight into the occurrence 

of these compounds than land use, soil conditions, or other 

geographic variables alone. Even limited measurements (such 

as specific conductance and dissolved-oxygen concentrations) 

may thus provide a useful and cost-effective summary of geochemical conditions for understanding ground-water quality.

A review of major-ion chemistry may be particularly 

useful for understanding the occurrence of selected chemical 

compounds in ground water. Principal components of majorion chemistry reflecting geochemical conditions such as ionic 

strength and redox condition were more predictive of the 

occurrence of pesticides in sampled ground water than corre-
References Cited 15

sponding individual measurements (specific conductance and 

dissolved-oxygen concentration, respectively) alone. 

Geochemical conditions reflected in PCs were also the only 

factors other than agricultural land use that were selected for 

inclusion in the pesticide logistic regression models (table 5). 

The apparent utility of these variables is likely a result of the 

summary of natural and human influences on ground water 

that is reflected in concentrations of major ions. Dissolved 

oxygen alone may reflect natural conditions in ground water 

conducive to pesticide transport, for example, whereas 

dissolved oxygen along with other ions such as nitrate may 

reflect such natural conditions as well as potential sources 

(such as agriculture). Major-ion chemistry may thus identify 

ground water that has been affected by certain sources, rather 

than ground water that is merely vulnerable to such sources 

(D.W. Bolton, Maryland Geological Survey, written commun., 

2003). Concentrations of major ions may be particularly 

informative in areas such as the Coastal Plain, where natural 

ground water is relatively dilute, but may be less so in other 

areas (such as carbonate aquifers), where ground-water chemistry is dominated by natural sources. 

Information about interacting influences on ground-water 

quality provides a conceptual basis for predicting groundwater quality in wider areas. Analyses described herein were 

designed and intended to expand the understanding of natural 

and human influences on regional ground-water quality, and 

not to provide quantitative predictions for unsampled areas. 

Also, although data to support these analyses were compiled 

from a broad range of land-use, hydrogeologic, and other 

settings, these data do not represent the entire range of conditions in the Coastal Plain or other areas. Insights provided by 

these and similar analyses at different scales, however, support 

a conceptual model of ground-water quality that might be 

expected in wider areas. Natural ground water, for example, 

is likely relatively dilute and acidic in permeable, well-weathered sediments, but may be more concentrated in younger 

or poorly drained sediments with more abundant soluble 

minerals. Nitrate would be expected only in aerobic ground 

water most likely in well-drained areas, and may be derived 

from a variety of human sources, although relatively high concentrations might be expected in areas of agricultural nutrient 

applications. Synthetic compounds such as pesticides and most 

VOCs would be expected primarily downgradient from usage 

areas, and where hydrogeologic and geochemical conditions 

promote their movement and stability in ground water. 

Insights on environmental and human factors affecting 

ground-water quality can be applied for a number of uses. 

Such considerations may constitute an important first step in 

many environmental applications, such as land-use and waterdevelopment planning, restoration of streams or other receiving waters, establishment of local or regional water-quality 

standards, or the design of management practices. Information 

about the co-occurrence of different compounds in certain 

settings may also be useful for the design of similar environmental studies in the future.

Acknowledgments

This work is a product of the National Water-Quality 

Assessment (NAWQA) Program of the U.S. Geological 

Survey (USGS). The assistance of many individuals is 

greatly appreciated. In particular, general ideas and advice on 

data analysis and interpretation were provided by Judith M. 

Denver, Paul E. Stackelberg, Stephen A. Terracciano, and A. 

Jim Tesoriero of the USGS. Anne C. Neale (U.S. Environmental Protection Agency) provided patient assistance with 

logistic regression, and Michael J. Brayton, Dennis R. Helsel, 

and Jeffrey D. Martin (USGS) also provided valuable suggestions or assistance with data analysis. Thoughtful comments on this manuscript were provided by Terri L. Arnold, 

Tracy C. Hancock, and Paul E. Stackelberg of the USGS. 

Mark R. Nardi and Betzaida Reyes of the USGS assisted with 

geographic analysis, and Linda M. Debrewer, Jacob Gibs, 

Kathleen L. Hibbs, Christopher E. Schubert, and Erik L. Staub 

of the USGS and David W. Bolton of the Maryland Geological 

Survey assisted with data compilation.

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Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

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