Factors Affecting Spatial and Temporal 

Variability in Nutrient and Pesticide 

Concentrations in the Surficial Aquifer 

on the Delmarva Peninsula

By Linda M. Debrewer, Scott W. Ator, and Judith M. Denver

Scientific Investigations Report 2005–5257

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Debrewer, L.M., Ator, S.W., and Denver, J.M., 2007, Factors affecting spatial and temporal variability in nutrient and 

pesticide concentrations in the surficial aquifer on the Delmarva Peninsula: U.S. Geological Survey Scientific Investigations Report 2005–5257, 44 p.
iii

Foreword

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

management of water, biological, energy, and mineral resources. Information on the quality of the 

Nation’s water resources is of critical interest to the USGS because it is so integrally linked to the 

long-term availability of water that is clean and safe for drinking and recreation and that is suitable 

for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing 

demands for the multiple water uses make water availability, now measured in terms of quantity and 

quality, even more critical to the long-term sustainability of our communities and ecosystems.

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

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

and policy. Shaped by and coordinated with ongoing efforts of other Federal, State, and local agencies, the NAWQA Program is designed to answer: What is the condition of our Nation’s streams 

and ground water? How are the conditions changing over time? How do natural features and human 

activities affect the quality of streams and ground water, and where are those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide science-based insights for current and emerging 

water issues. NAWQA results can contribute to informed decisions that result in practical and effective water-resource management and strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. Collectively, 

these Study Units account for more than 60 percent of the overall water use and population served 

by public water supply, and are representative of the Nation’s major hydrologic landscapes, priority 

ecological resources, and agricultural, urban, and natural sources of contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and 

analysis. The assessments thereby build local knowledge about water-quality issues and trends in a 

particular stream or aquifer while providing an understanding of how and why water quality varies 

regionally and nationally. The consistent, multi-scale approach helps to determine if certain types of 

water-quality issues are isolated or pervasive, and allows direct comparisons of how human activities and natural processes affect water quality and ecological health in the Nation’s diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile 

organic compounds, trace metals, and aquatic ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings.

The USGS places high value on the communication and dissemination of credible, timely, and relevant science so that the most recent and available knowledge about water resources can be applied 

in management and policy decisions. We hope this NAWQA publication will provide you the needed 

insights and information to meet your needs, and thereby foster increased awareness and involvement in the protection and restoration of our Nation’s waters.

The NAWQA Program recognizes that a national assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated 

understanding of watersheds and for cost-effective management, regulation, and conservation of our 

Nation’s water resources. The Program, therefore, depends extensively on the advice, cooperation, 

and information from other Federal, State, interstate, Tribal, and local agencies, non-government 

organizations, industry, academia, and other stakeholder groups. The assistance and suggestions of 

all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water

Contents

Foreword ........................................................................................................................................................iii

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

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

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

Description of Study Area ...................................................................................................................4

Hydrogeologic Setting ................................................................................................................4

Land Use and Water Use............................................................................................................5

Ground-Water Chemistry............................................................................................................5

Methods of Study...........................................................................................................................................6

Network Design.....................................................................................................................................6

Sample Collection and Analysis.........................................................................................................7

Quality Control.......................................................................................................................................7

Age Dating..............................................................................................................................................8

Data Analysis.........................................................................................................................................9

Factors Affecting Spatial and Temporal Variability..................................................................................9

Regional Variability.............................................................................................................................10

Domestic-Well Network ...........................................................................................................10

Agricultural-Well Network .......................................................................................................13

Nutrients in the Surficial Aquifer .....................................................................................................14

Nitrogen.......................................................................................................................................14

Nitrate .........................................................................................................................................15

Trends in Nitrate Concentrations ............................................................................................16

Ammonia and Organic Nitrogen..............................................................................................18

Phosphorus.................................................................................................................................19

Pesticides in the Surficial Aquifer ...................................................................................................19

Trends in Pesticide Detections................................................................................................27

Summary and Conclusions.........................................................................................................................29

Acknowledgments.......................................................................................................................................30

References Cited..........................................................................................................................................30

Appendix A. Site Information for Wells Sampled in the Surficial Aquifer on the 

Delmarva Peninsula ............................................................................................................35

Appendix B. Estimated Uncertainty in Reported Concentrations of Selected Major Ions, 

Nutrients, and Pesticides in the Surficial Aquifer on the Delmarva Peninsula ........37

Appendix C. Pesticides and Pesticide Degradates Analyzed in Samples From the 

Surficial Aquifer in 2001......................................................................................................40

Appendix D. Pesticides Analyzed in Samples From the Surficial Aquifer in 1988...........................44
vi

Figures

1–2. Maps showing—

1. Location of the Delmarva Peninsula and study area.....................................................3

2. Land use and land cover on the Delmarva Peninsula and location of wells 

sampled in the study ...........................................................................................................5

3–5. Graphs showing—

3. Poultry production on the Delmarva Peninsula, 1970–2002 .........................................6

4. Estimated nutrient inputs from inorganic fertilizer and manure on the 

Delmarva Peninsula, 1940–2000 ......................................................................................13

5. Relation of domestic-well principal component 1 (DOMPC1) to apparent 

recharge date in the deeper part of the surficial aquifer on the 

Delmarva Peninsula ..........................................................................................................13

6. Pie chart showing estimated nitrogen sources for the Chesapeake Bay drainage 

area on the Delmarva Peninsula..............................................................................................14

7–8. Graphs showing —

7. Relation of nitrate concentration in ground water to sampled well depth 

in the surficial aquifer on the Delmarva Peninsula......................................................15

8. Distribution of nitrate concentrations in different oxidation-reduction (redox) 

environments on the Delmarva Peninsula.....................................................................16

9. Map showing soil drainage and areal distribution of nitrate concentrations in 

the surficial aquifer on the Delmarva Peninsula...................................................................17

10–18. Graphs showing—

10. Distribution of measured nitrate concentrations in wells from oxic 

environments sampled in 1998 and 2001 in the domestic-well network and 

the agricultural-well network on the Delmarva Peninsula.........................................18

11. Distribution of ammonia plus organic nitrogen concentrations in areas of 

different soil drainage and dissolved oxygen concentrations in the shallow 

surficial aquifer on the Delmarva Peninsula.................................................................18

12. Relation of ammonia plus organic nitrogen concentrations in ground water 

to sampled well depth in the shallow agricultural-well network on the 

Delmarva Peninsula ..........................................................................................................19

13. Estimated agricultural pesticide use on the Delmarva Peninsula, 1997 ..................20

14. Occurrence of pesticides and pesticide degradates in the surficial aquifer 

on the Delmarva Peninsula ..............................................................................................22

15. Distribution of selected pesticides and pesticide degradates in the surficial 

aquifer on the Delmarva Peninsula ................................................................................25

16. Distribution of selected pesticides in areas with different soil drainage in 

the agricultural-well network on the Delmarva Peninsula.........................................27

17. Relation of total metolachlor concentrations to sampled well depth in the 

domestic-well network on the Delmarva Peninsula....................................................28

18. Relation of total atrazine concentrations to apparent recharge date in the 

surficial aquifer on the Delmarva Peninsula.................................................................28
vii

Tables

1. Metolachlor concentrations in environmental samples immediately preceding 

and following field blanks in 2001 and 2002 on the Delmarva Peninsula ............................8

2. Chemistry of wet atmospheric deposition at Wye River, Maryland, 1994–2002.................9

3. Summary statistics for selected physical properties and major ions and elements 

in ground water of the surficial aquifer on the Delmarva Peninsula.................................10

4. Loadings on the first three components from unrotated principal-components 

analysis for ground water representing domestic drinking-water supply in the 

surficial aquifer of the Delmarva Peninsula and correlation of each principal 

component with selected well information............................................................................11

5. Loadings on the first three components from unrotated principal-components 

analysis for shallow ground water beneath agricultural areas of the Delmarva 

Peninsula and correlation of each principal component with selected well 

information...................................................................................................................................12

6. Summary statistics for selected nutrients in the surficial aquifer on the 

Delmarva Peninsula ...................................................................................................................15

7. Relation of nitrate concentrations in the surficial aquifer on the Delmarva 

Peninsula to potential explanatory factors ............................................................................16

8. Relation of ammonia plus organic nitrogen concentrations in the surficial 

aquifer on the Delmarva Peninsula to potential explanatory factors ................................19

9. Description of commonly used and frequently detected pesticide compounds 

on the Delmarva Peninsula .......................................................................................................21

10. Summary of analytical results for selected pesticides and degradates in 

samples from the surficial aquifer on the Delmarva Peninsula in 2001.............................23

11. Physical and chemical characteristics of commonly used pesticides on 

the Delmarva Peninsula.............................................................................................................25

12. Correlation of total atrazine and total metolachlor for shallow ground water 

beneath agricultural areas and deeper ground water representing domestic 

supply on the Delmarva Peninsula ..........................................................................................26

13. Pesticide detection frequency in the areal-network wells sampled in 1988 

on the Delmarva Peninsula .......................................................................................................28
viii

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) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per second (ft3/s) 8.02 gallon per minute (gal/min)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

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

 °F=(1.8×°C)+32

Vertical datum: In this report, vertical coordinate information is referenced to the National 

Geodetic Vertical Datum of 1929 (NGVD 29) and (where noted) to the North American Datum of 

1983 (NAD 83). 

Altitude, as used in this report, refers to distance above or below sea level.

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).

Conversion Factors and Vertical Datum
Abstract

Water quality in the unconfined, unconsolidated surficial aquifer on the Delmarva Peninsula is influenced by the 

availability of soluble ions from natural and human sources, 

and by geochemical factors that affect the mobility and fate 

of these ions within the aquifer. Ground-water samples were 

collected from 60 wells completed in the surficial aquifer of 

the peninsula in 2001 and analyzed for major ions, nutrients, 

and selected pesticides and degradation products. Analytical results were compared to similar data from a subset of 

sampled wells in 1988, as well as to land use, soils, geology, 

depth, and other potential explanatory variables to demonstrate 

the effects of natural and human factors on water quality in 

the unconfined surficial aquifer. This study was conducted 

as part of the National Water-Quality Assessment Program 

of the U.S. Geological Survey, which is designed (in part) to 

describe the status and trends in ground-water quality and to 

provide an understanding of natural and human factors that 

affect ground-water chemistry in different parts of the United 

States. Results of this study may be useful for water-resources 

managers tasked with addressing water-quality issues of local 

and regional importance because the surficial aquifer on the 

Delmarva Peninsula is a major source of water for domestic 

and public supply and provides the majority of flow in 

local streams.

Human impacts are apparent in ground-water quality 

throughout the surficial aquifer. The surficial aquifer on the 

Delmarva Peninsula is generally sandy and very permeable 

with well-oxygenated ground water. Dissolved constituents 

found throughout various depths of the unconfined aquifer are 

likely derived from the predominantly agricultural practices 

on the peninsula, although effects of road salt, mineral 

dissolution, and other natural and human influences are 

also apparent in some areas. Nitrate occurred at concentrations exceeding natural levels in many areas, and commonly 

exceeded 10 milligrams per liter (as nitrogen). In addition to 

land use in the aquifer recharge area, concentrations of nitrate 

in ground water are related to regional patterns in soil drainage that affect underlying aquifer redox conditions. Over the 

peninsula, nitrate concentrations are not related to recharge 

date of the water, but are positively correlated with depth in 

shallow wells screened beneath agricultural areas. Nitrate concentrations increased in oxic areas (dissolved oxygen greater 

than 1 milligram per liter) of the deeper part of the surficial 

aquifer used for domestic supply by an average of about 2 milligrams per liter between 1988 and 2001, although no changes 

were apparent in shallower parts of the aquifer over that same 

period. Water in the surficial aquifer generally flows from 

land-surface recharge to surface-water discharge areas in less 

than 30 years. As a result, the entire flow system in the surficial aquifer has likely been affected by human activities on and 

near the land surface over the past several decades.

Pesticide compounds occurred widely at low levels 

throughout the surficial aquifer. The most commonly used 

herbicides (metolachlor, alachlor, and atrazine) were the most 

commonly detected. These pesticides primarily occurred in 

ground water in the form of degradation products. The widespread occurrence of these and other pesticide compounds 

reflects their abundant use as well as chemical properties and 

aquifer characteristics that allow their movement into ground 

water. Mixtures of pesticides are common. Most samples 

contained at least 3 different compounds; several samples 

contained as many as 11. Pesticide concentrations in the 

surficial aquifer are relatively high beneath recharge areas 

with well-drained soils in the shallow part of the aquifer and in 

oxic environments throughout the surficial aquifer. Concentrations are generally below existing drinking-water standards, 

although standards are not available for all of the pesticide 

compounds detected. Temporal trends in pesticide concentrations are difficult to determine from available data. Although 

pesticides were more frequently detected in 2001 than in 1988, 

this increased frequency of detection is likely due to improved 

sampling and analytical methods that report detections at 

lower levels, and may not be indicative of actual changes in 

pesticide concentrations in the surficial aquifer since 1988.

Factors Affecting Spatial and Temporal Variability in 

Nutrient and Pesticide Concentrations in the 

Surficial Aquifer on the Delmarva Peninsula

By Linda M. Debrewer, Scott W. Ator, and Judith M. Denver
Introduction

Water-resources managers need a thorough understanding 

of the natural and human factors affecting the spatial and 

temporal variability in water quality to address water-resource 

issues of regional and local importance. Water from the 

unconfined surficial aquifer on the Delmarva Peninsula 

(fig. 1) is widely used for domestic and public supplies. 

Ground water also provides a significant part of the annual 

flow in local streams. The presence of pesticides, nutrients, 

and other anthropogenic chemicals in the aquifer has been 

well documented for the past several decades. Sources of 

these compounds from agriculture and other human activities 

are widespread on the peninsula. The well-drained soils and 

permeable well-weathered sediments of the surficial aquifer, 

typical of many areas of the peninsula, promote the movement 

of pesticides from the land surface into shallow ground water. 

Once these chemicals enter the surficial aquifer, they can 

travel along flow paths to deeper parts of the aquifer used for 

drinking water, and, eventually to discharge areas of 

local streams.

Dissolved nutrients (primarily nitrate) are widespread in 

the surficial aquifer on the Delmarva Peninsula. Miller (1972) 

reported nitrate concentrations in parts of the surficial aquifer 

in Delaware approaching 10 mg/L1 (milligrams per liter) as 

early as 1971; 10 mg/L is the Maximum Contaminant Level 

(MCL) for drinking water established by the U.S. Environmental Protection Agency (U.S. Environmental Protection 

Agency, 2004). Nitrate is primarily attributed to agricultural 

applications of nutrients through fertilizer and manure and to 

wastewater disposal (Miller, 1972; Bachman, 1984a; Denver, 

1986; Andres, 1986; Hamilton and others, 1993). Nitrogen is 

often applied as ammonia or in organic forms, but it is rapidly 

converted to nitrate in the well-oxidized water typical of much 

of the surficial aquifer. In agricultural areas, nitrate is commonly the dominant anion in water of the surficial aquifer 

(Hamilton and others, 1993). Nitrate concentrations are typically highest beneath areas with well-drained soils near urban 

or agricultural land and are lower or absent beneath areas with 

poor drainage, regardless of land use (Bachman, 1984a; Hamilton and others, 1993). Elevated nitrate concentrations are not 

limited to shallow depths within the surficial aquifer; concentrations greater than 10 mg/L have been measured in water 

near the base of the aquifer, at depths approaching 100 ft (feet) 

(Denver, 1989). Although phosphorus is also an important 

nutrient and component of fertilizers and human and animal 

waste, it is relatively insoluble, and is rarely detected in the 

surficial aquifer (Denver, 1986; Hamilton and others, 1993).

Water in some parts of the surficial aquifer on the 

Delmarva Peninsula contains dissolved pesticides as well 

as nutrient compounds. Herbicides commonly used on corn 

and soybeans, such as alachlor, atrazine, metolachlor, and 

1

 Concentrations of nitrogen species are presented in this report as equivalent concentrations of elemental nitrogen. Nitrate concentrations also include 

any measurable nitrite.

simazine, have been detected in ground water of the surficial 

aquifer since the 1980s (William F. Ritter, University of 

Delaware, written commun.,1987; Denver, 1993; Koterba and 

others, 1993; Ator and Ferrari, 1997; Blaier and Baxter, 2000; 

Ferrari, 2002). Samples of ground water often contain multiple 

pesticides; however, their individual concentrations rarely 

exceed 1 μg/L (microgram per liter). Although herbicides are 

detectable in ground water throughout the surficial aquifer, 

concentrations are generally greatest in shallow ground 

water (within 10 meters of the water table) in or near areas 

of application to corn, soybeans, and similar crops (Denver, 

1993; Koterba and others, 1993; Blaier and Baxter, 2000). 

Concentrations are also typically greatest in areas where 

soils are permeable, well-drained, and contain little organic 

matter, properties that allow the movement of herbicides from 

the land surface to the water table (Denver, 1993; Koterba 

and others, 1993; Blaier and Baxter, 2000). Insecticides 

(carbaryl, carbofuran, dieldrin, lindane, and propoxur) and 

less commonly used herbicides (butylate, diphenamid, eptam, 

hexazinone, pebulate, and propachlor) have also been detected 

in ground water of the surficial aquifer, generally at much 

lower concentrations than the major-use herbicides (Koterba 

and others, 1993; Ferrari, 2002).

Nitrate and pesticides in ground water may cause health 

problems when consumed in drinking water. The surficial 

aquifer on the Delmarva Peninsula is widely used for domestic 

and public water supply. Drinking water containing nitrate at 

concentrations greater than 10 mg/L has the potential to cause 

methemaglobanemia, a life-threatening illness in infants. This 

is an issue of concern on the peninsula as many domestic 

wells are not routinely tested for water quality. Drinking-water 

standards have also been established for several pesticides. 

Existing standards are seldom exceeded in ground water; however, the potential additive or synergistic effects of exposure 

to multiple pesticide compounds are largely unknown (Denver 

and others, 2004).

Nitrate and pesticides in ground water of the surficial 

aquifer may also contribute to ecological problems on the 

Delmarva Peninsula. Ground-water discharge contributes 

more than half of the flow in streams on the Delmarva 

Peninsula (Cushing and others, 1973); the majority of this 

discharge is from the surficial aquifer (Bachman and Phillips, 

1996). Excessive nitrate in surface water may lead to eutrophic 

conditions that are unhealthy for some aquatic biota and 

certain pesticides may be toxic to some aquatic organisms 

(Phillips and Caughron, 1997). Although hydrogeologic 

and soil conditions in some areas of the peninsula promote 

denitrification or may retard pesticide transport, about half of 

the first-order (headwater) streams on the peninsula contain 

more than 3 mg/L of nitrate during spring base flow, and 

more than 20 percent contain metolachlor at concentrations 

exceeding 0.1 μg/L (Böhlke and Denver, 1995; Speiran, 1996; 

Denver and others, 2004). Bachman and Phillips (1996) 

estimated that 40 percent of the nitrogen entering Chesapeake 

Bay from the Delmarva Peninsula is derived from groundwater discharge to streams.

 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Figure 1. Location of the Delmarva Peninsula and study area.

Introduction 
Management strategies intended to mitigate anthropogenic effects on ground-water quality are complicated 

by the variable, and often unknown, traveltime required 

for water to move completely through the surficial aquifer. 

Estimates of the age of ground water in the surficial aquifer 

on the peninsula typically range from 10–20 years, although 

water recharging the aquifer along topographic divides may 

require as much as 30–35 years to reach discharge areas in 

streams (Dunkle and others, 1993; Böhlke and Denver, 1995; 

Speiran, 1996; Ferrari, 2002; Lindsey and others, 2003). 

Therefore, several decades may be needed for the effects of 

management strategies designed to lower the concentrations 

of contaminants in the surficial aquifer to be fully realized 

in the deeper part of the aquifer used for drinking water and 

in Delmarva streams. A ground-water flow model was used 

to estimate the effects of ground-water residence times on 

hypothetical management strategies for nitrate in a similar 

hydrogeologic setting in the Coastal Plain of southern New 

Jersey (Kauffman and others, 2001). The model suggests that 

nitrate concentrations in both streams and ground water would 

continue to increase for several years or decades after nitrate 

inputs at the land surface were reduced or fixed at a constant 

level. Even if inputs from human activities were immediately 

eliminated, decades would be required for concentrations 

in streams and ground water to decrease to natural levels 

(Kauffman and others, 2001).

Purpose and Scope

This study was conducted as part of the National WaterQuality Assessment (NAWQA) Program (Gilliom and others, 

1995) of the U.S. Geological Survey (USGS), which is 

designed (in part) to describe the status and trends in groundwater quality and to provide an understanding of natural and 

human factors that affect ground-water chemistry in different 

parts of the United States (see Foreword). Natural and human 

factors affecting spatial and temporal variability in the quality 

of ground water (particularly concentrations of nutrients and 

pesticides) in the surficial aquifer on the Delmarva Peninsula 

are described and discussed in this report. Because agricultural 

land use is dominant on the peninsula, this study focuses on 

the effects of agriculture on shallow ground-water quality. 

Results are based on analyses of samples collected from two 

networks of wells screened in the surficial aquifer—relatively 

shallow wells screened beneath agricultural areas (median 

depth of 22 ft bls, below land surface), and typically deeper 

wells screened in the part of the aquifer often used for 

domestic drinking-water supply (median depth of 45 ft bls). 

Major-ion chemistry is summarized, along with the occurrence 

and distribution of nutrients and pesticides, and the apparent 

relation between water quality and land use, geology, and 

other potential explanatory factors. Concentrations of selected 

chemical constituents measured in 2001 are compared to 

concentrations measured in 1988 in the same set of wells to 

identify and quantify significant trends in ground-water quality 

over the intervening years.

Description of Study Area

The Delmarva Peninsula lies entirely within the Atlantic Coastal Plain Physiographic Province and includes most 

of Delaware and parts of Maryland and Virginia east of the 

Chesapeake Bay (fig. 1). The peninsula covers about 

6,000 square miles and is bounded to the north by the Fall 

Line and the Piedmont Physiographic Province; to the west 

by the Chesapeake Bay; and to the east by the Atlantic Ocean 

and Delaware Bay. The peninsula consists of a flat to gently 

sloping broad central upland flanked by low plains that slope 

toward the coasts. The highest elevation of the central uplands 

of the peninsula is about 80 ft, with a maximum elevation of 

about 500 ft in the northernmost part of the peninsula. Tidal 

wetlands fringe the coastal areas along the Chesapeake and 

Delaware Bays. Barrier beaches, inland bays, tidal lagoons, 

and marshes characterize the Atlantic coastline. Surface drainage from two-thirds of the peninsula contributes to the Chesapeake Bay and the remaining areas drain to the Delaware Bay 

or other tidal estuaries along the Atlantic Coast.

Hydrogeologic Setting

The peninsula is underlain by a wedge of unconsolidated sediment that thickens to the south and east from 0 ft at 

the Fall Line to as much as 8,000 ft along the Atlantic Coast 

(Cushing and others, 1973). Aquifer sediments range in age 

from Cretaceous to Holocene, and are primarily quartz sand, 

clay, silt, gravel, and variable amounts of shells. Organic material is present in swamps and wetlands and in some younger 

geologic formations. The surficial aquifer has been referred 

to as the Quaternary aquifer (Cushing and others, 1973), the 

Pleistocene aquifer (Andres, 1986), and the Columbia aquifer 

(Bachman, 1984b). The sandy deposits of the surficial unconfined aquifer cover 90 percent of the study area; sediments are 

highly permeable across most of the peninsula and range from 

20 to over 100 ft in thickness (Hamilton and others, 1993). 

The hydrogeologic setting ranges from well-drained regions 

where the surficial aquifer is thick and streams are incised to 

poorly drained regions where the surficial aquifer is thin or 

absent, topography is flat, stream channels may incise into 

underlying confined aquifers, and ditching practices are common to promote drainage of agricultural fields. The surficial 

aquifer may become locally confined by the Walston Silt and 

Omar Formations in the central part of the peninsula where 

water-table conditions are shallowest and streams are not 

deeply incised in the sediments (Bachman, 1984b; Hamilton 

and others, 1991). The surficial aquifer is underlain by a series 

of confined aquifers described by Cushing and others (1973) 

and Harsh and Laczniak (1990). Most of the confined aquifers 

crop out along tidal streams or subcrop below the surficial 

aquifer (Hamilton and others, 1991).

Most of the land surface on the Delmarva Peninsula is a 

ground-water recharge area. About 14 inches of water per year 

(32 percent of the annual average precipitation) recharges the 

surficial aquifer, primarily by infiltration of rainfall or snow-

 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
melt (Johnston, 1973). Recharge mostly occurs when evapotranspiration is at a minimum and vegetation is dormant from 

late fall to early spring. Depth of the water table in the surficial 

aquifer typically ranges from 0 to 20 ft, with depths of 30 ft or 

more in well-drained areas, and can vary by several feet due 

to seasonal changes in precipitation and evapotranspiration 

(Hamilton and others, 1993). The longest ground-water flow 

paths originate at topographic divides. Ground water flows 

from upland recharge areas toward discharge areas in streams, 

freshwater ponds and wetlands, tidal marshes, bays, and the 

Atlantic Ocean. The lengths of ground-water flow paths range 

from less than a few hundred feet to a few miles, depending on 

regional and local drainage conditions.

Land Use and Water Use

Land use is predominantly agricultural throughout 

the Delmarva Peninsula (fig. 2). Crops are mainly soybean 

and corn grown for poultry feed. These crops are grown in 

annual rotation with winter wheat and other small grains 

(U.S. Department of Agriculture, 1991). The most significant 

agribusiness is the poultry industry, which has increased 

over the past several decades to production levels of about 

600 million broiler chickens per year on the Delmarva 

Peninsula (fig. 3, Delmarva Poultry Industry, Inc., 2003). 

The greenhouse and nursery industry has expanded on the 

peninsula since the early 1980s (American Farmland Trust, 

2003). Dairy farms and sod farms also are common, and fruits 

and vegetables are grown for local and regional markets. Food 

processing and other small industries are located in smaller 

towns and cities scattered throughout the peninsula. Although 

agriculture remains the predominant land use, suburban 

development is encroaching on agricultural areas (American 

Farmland Trust, 2003). Urban and industrial development is 

greatest in northern New Castle County, Delaware; urban and 

suburban development is also great along the Atlantic Coast in 

Delaware and Maryland.

The surficial aquifer is the primary source of water for 

domestic and agricultural use on the Delmarva Peninsula 

(Denver and others, 2004). The surficial aquifer recharges 

underlying confined aquifers, is the dominant source of base 

flow to streams, and, where thick enough, is used for public 

drinking-water supply. In areas where the surficial aquifer 

is relatively thin, as in the northern part of the peninsula, 

confined aquifers are major sources for public-water supply 

(Shedlock and others, 1999).

Ground-Water Chemistry

Under natural conditions, the chemistry of water in the 

surficial aquifer is controlled by the chemical properties of 

rainfall and mineral dissolution, oxidation-reduction (redox) 

reactions, and residence time in the ground-water flow system. 

It can also be affected by saltwater intrusion near the coasts. 

The surficial aquifer is composed primarily of relatively 

Figure 2. Land use and land cover on the Delmarva Peninsula 

and location of wells sampled in the study. (Land use from 

Volgelmann and others, 2001.)

Introduction 
Specific conductance is elevated above natural 

background levels in waters influenced by human sources of 

chemicals, such as agriculture, septic-system discharge, and 

road salt. Because of the dilute nature of natural waters, these 

human sources can create a new chemical signature in ground 

water. In water influenced by agriculture, the predominant land 

use on the peninsula, calcium and magnesium are commonly 

present from dolomitic lime, potassium and chloride from 

potash fertilizer, and nitrate from inorganic fertilizer and 

manure; the median specific conductance is 170 μS/cm 

(Denver, 1989; Hamilton and others, 1993). Water in parts 

of the surficial aquifer affected by agriculture is generally a 

calcium-magnesium-nitrate-type (Hamilton and others, 1993). 

Bicarbonate concentrations (and pH) are generally lower in 

agricultural areas than under natural conditions. Ground water 

affected by septic-system effluent commonly has elevated 

concentrations of sodium, chloride, and(or) nitrate (Denver, 

1989). Road salt is another potential source of sodium and 

chloride. Road salt may also contribute sodium and chloride to 

ground water in rural and urban areas (Delaware Department 

of Natural Resources and Environmental Control, 2005). 

Because the same major ions may be elevated in waters 

from different sources, it is important to consider land use in 

understanding the sources of chemical contamination in water 

in the surficial aquifer.

Methods of Study

Two networks of randomly located wells were sampled 

in 2001 as part of a regional assessment of ground-water 

quality in the surficial aquifer on the Delmarva Peninsula. The 

networks were derived from an earlier network established in 

1988 during the initial pilot phase of the NAWQA Program, 

the “areal” network described in Koterba and others (1990) 

and Shedlock and others (1993). Selected areal-network 

wells and additional wells were combined to create networks 

representative of different parts of the surficial aquifer. Water 

in these wells was analyzed to identify and quantify apparent 

spatial and temporal trends in the quality of ground water 

from the shallow unconfined aquifer on the peninsula, and to 

relate these trends to land use, hydrogeology, or other potential 

explanatory factors.

Network Design

The areal network was designed and sampled to assess 

water quality throughout the surficial aquifer of the Delmarva 

Peninsula (Koterba and others, 1990). To minimize bias with 

respect to sources of contamination, areal-network wells were 

selected using grid-based random sampling described in Scott 

(1990) and Alley (1993). The study unit was divided into 12.5- 

by 12.5-minute grid cells, which were aggregated to create 

35 polygons of equal area. One site was randomly selected in 

each polygon, and suitable nearby wells were located through 

Figure 3. Poultry production on the Delmarva Peninsula, 

1970–2002. Poultry production increased from 1970 to 1995, and 

then declined slightly. (Data from Delmarva Poultry Industry, 

Inc., 2003.)

insoluble sand and gravel with low organic-matter content, 

resulting in relatively dilute and acidic ground water with low 

natural concentrations of nitrate. In a previous study of the 

surficial aquifer, water defined as natural had a median value 

of 5.8 for pH, 115 μS/cm (microsiemens per centimeter) for 

specific conductance, and 0.1 mg/L for the concentration of 

nitrate (Hamilton and others, 1993). Ground water with natural 

chemistry is typically limited in the surficial aquifer to that 

which recharges through forested areas, where the effects of 

anthropogenic chemicals are minimal.

Four major natural water types have been identified 

in the surficial aquifer of the Delmarva Peninsula (Hamilton and others, 1993). The sodium-bicarbonate water type 

reflects ground-water transport through sediments consisting 

mostly of weather-resistant quartz sand. This water type is 

the most dilute, with a median specific conductance of about 

60 μS/cm (Denver, 1989). The other three natural water types 

are: calcium-bicarbonate water, which forms when ground 

water is in contact with shell material or other marine deposits in the sediments; sodium-chloride water, which is usually 

associated with saltwater intrusion and can occur near tidal 

areas; and calcium-sodium-sulfate water in areas dominated 

by fine-grained sediments with organic matter. Parts of the 

surficial aquifer with natural water quality also commonly 

have elevated concentrations of dissolved iron and bicarbonate 

as many areas with forested land use also contain wetlands and 

conditions that result in low or no dissolved oxygen in ground 

water. The specific conductance of natural waters is greater in 

these water types and where dissolved oxygen is absent than in 

the sodium-bicarbonate water.

 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
a search of state and county records. Areal-network wells were 

originally sampled from 1988 through 1990. In this report, 

these samples are referred to hereafter as “areal” or “1988” 

samples.

The areal network included pairs of deeper and shallower wells. Existing domestic and supply wells were selected 

wherever possible to represent the deeper part of the surficial 

aquifer used for drinking water. Deep wells were installed 

where existing wells were not suitable for sampling (Hardy 

and others, 1989; Koterba and others, 1995). Shallow wells 

screened within a few feet of the water table were installed 

within one-quarter mile of each deeper well to represent the 

part of the aquifer likely to be most vulnerable to surficial 

contamination from the land use immediately surrounding the 

well, which, in most cases, was predominantly agricultural.

Wells from the areal network were used as the basis for 

developing two 29-well networks in the surficial aquifer 

(fig. 2; Appendix A). The “agricultural-well network” (median 

depth, 22 ft bls) was designed to be representative of shallow 

ground water beneath agricultural areas. This network includes 

16 of the shallow wells from the areal network, as well as 13 

that were installed or selected to replace areal network wells 

that were unsuitable for sampling (Lapham and others, 1995). 

All replacement wells were randomly selected within the same 

polygon as the original wells. Two additional shallow wells 

were also included in forested areas where anthropogenic 

effects on ground water are likely minimal; samples from 

these wells will be used to document reference conditions. 

A deeper (median depth, 45 ft bls) “domestic-well network” 

represents the part of the surficial aquifer commonly used for 

domestic drinking-water supply. As these wells are deeper, 

they represent a mixture of water from a broader area and from 

a wider range of land uses. Twenty-three of the deeper wells 

had been previously sampled in the areal network.

Agriculture, followed by forest and wetlands, is the 

dominant land use near wells in both networks. Land use 

and land cover were documented for the area within approximately 1,640 ft of each well (Appendix A). These data were 

derived from the USGS National Land Cover Dataset (NLCD) 

(Vogelmann and others, 2001). The NLCD was compiled from 

Landsat thematic mapper imagery from 1992 with 98-ft spatial 

resolution, and was coded using a modification of the Anderson land-use and land-cover classification system (Anderson 

and others, 1976). Although the resolution is somewhat coarse, 

the data were used to illustrate general patterns of land use 

surrounding the wells sampled for this study and the distribution of land use on the Delmarva Peninsula.

Sample Collection and Analysis

Ground-water samples were collected from 60 wells 

in both networks in 2001 and processed using protocols 

designed to obtain samples representative of the surficial 

aquifer, minimizing sample contamination and measurement 

variability (Hardy and others, 1989; Jones, 1987; Koterba 

and others, 1995). Wells were purged to remove standing 

water in the casing (generally three well volumes) before 

samples were collected. Purging continued until dissolved 

oxygen (±0.3 mg/L), pH (±0.1 units), specific conductance 

(±3 percent), water temperature (±0.2oC, degrees Celsius), and 

turbidity (±10 percent) stabilized. Water-quality samples were 

collected through Teflon tubing inside a sampling chamber. A 

subset of samples intended for dissolved analyses was passed 

through a 0.45-μm (micrometer) capsule filter (for inorganic 

constituents) or a 0.7-μm baked glass fiber filter (for pesticides 

and metabolites); selected trace metals and major-ion samples 

were preserved with nitric acid to a pH below 2. Bottles were 

chilled to maintain a temperature of 4oC during shipment to 

the laboratory.

Samples were analyzed for concentrations of inorganic 

ions and compounds (major ions and trace metals), selected 

pesticides and pesticide degradates, and nutrients. Bicarbonate 

concentrations were calculated on the basis of field titrations 

for alkalinity (Radtke and others, 1998). Most chemical 

analyses were conducted at the USGS National Water-Quality 

Laboratory in Denver, Colorado (Fishman, 1993; Furlong 

and others, 2001; Garbarino, 1999; Zaugg and others, 1995). 

Analyses for the concentration of gaseous sulfur hexafluoride 

(SF6) were done at the USGS Reston Chlorofluorocarbon 

Laboratory in Reston, Virginia (Busenberg and Plummer, 

2000), and analyses for pesticide degradates were done at 

the USGS Organic Geochemistry Research Laboratory in 

Lawrence, Kansas (Zimmerman and others, 2000; Lee and 

others, 2001a,b).

Quality Control

Quality-control samples were collected to provide 

an estimate of sample contamination, analytical matrix 

effects, and measurement variability associated with the 

data-collection process (Koterba and others, 1995). Blanks, 

replicates, and spike samples were collected, preserved, 

and analyzed using the same methods as those used for the 

environmental samples. Field blanks were collected to ensure 

that equipment was adequately cleaned to minimize potential 

cross-contamination and to ensure that sample collection 

and processing did not result in contamination. Sequential 

replicate samples were collected immediately following the 

environmental samples using the same equipment. These 

samples aid in determining the variability of the chemical 

analyses and the consistency of sampling techniques. Selected 

replicates were fortified with known quantities of pesticides 

to determine bias as a result of matrix interference and to 

estimate method recoveries.

Field blanks indicate that reported concentrations may 

contain minimal bias due to sample contamination during 

collection, processing, or shipment (Appendix B). No nutrient 

compounds were detected in field blanks at concentrations 

exceeding laboratory reporting levels. Of pesticides detected in 

environmental samples, only metolachlor was detected in field 

Methods of Study 
blanks; concentrations in field blanks were as high as 

0.0228 μg/L. Metolachlor concentrations were above this 

level in only 4 of 60 environmental samples, although the 

lack of detectable metolachlor in environmental samples 

immediately preceding and following apparently contaminated 

blank samples suggests that the contamination problem 

may be limited and not systematic (table 1). Field blanks 

suggest that reported concentrations of major ions and trace 

elements may contain bias from sample contamination, but 

field blank concentrations are insignificant when compared 

to typical concentrations in environmental samples. Replicate 

analyses suggest minimal variability in laboratory and 

sampling technique (Appendix B). Estimated uncertainties in 

reported concentrations exceeded 10 percent only for nitrogen 

ammonia, phosphorus, atrazine degradation products, and 

metolachlor. Estimated uncertainties in the pesticides and 

degradates are based on one or two replicate sets. Median 

standard deviations in concentrations for replicate sets were 

generally low. Median recoveries for spiked pesticides ranged 

from 46 percent to 155 percent (Appendix B). Reported 

concentrations for only 3 of the 36 pesticide and degradate 

compounds detected in the environmental samples may 

substantially differ from true environmental concentrations; 

carbaryl and carbofuran recoveries are biased high and 

deethylatrazine recoveries are biased low. These compounds 

have historically poor recoveries with the analytical method 

and have been documented in Zaugg and others (1995).

Age Dating

Recharge dates for ground-water samples were estimated 

on the basis of measured concentrations of SF6 (Busenberg 

and Plummer, 2000) (Appendix A). SF6 is a colorless, odorless 

stable gas primarily of anthropogenic origin with a small contribution from natural sources. The SF6 method can be used to 

date ground water that is in equilibrium with atmospheric SF6

during recharge and is not subsequently exposed to significant 

SF6 from other sources. Industrial production of SF6 began in 

Table 1. Metolachlor concentrations in environmental samples immediately preceding and 

following field blanks in 2001 and 2002 on the Delmarva Peninsula.

[Well names detailed in Appendix A; bold indicates the date blank sample was collected and concentration 

of metolachlor in the blank sample; E, estimated; <, less than]

Well name Sampling date Concentration Sample type Interpretation

Ec42-15 7/5/2001 0.6030

7/11/2001 0.0228 Blank

Le24-11 7/11/2001 E 0.0033 Possible contamination.

KE Be 59 7/12/2001 E 0.0028 Possible contamination.

Gc14-03 7/17/2001 < 0.013 End of possible contamination.

Gc14-03 7/17/2001 < 0.013

7/18/2001 0.0046 Blank

KE Be 46 7/18/2001 < 0.013 No contamination from blank.

Ri22-03 9/5/2001 E 0.0075

9/6/2001 < 0.013 Blank No contamination detected.

WO Fc 46 9/6/2001 < 0.013

CO Dd 74 11/6/2001 < 0.013

11/7/2001 0.0033 Blank

TA Be 83 11/7/2001 < 0.013 No contamination from blank.

Ib32-08 12/17/2002 < 0.013

12/19/2002 < 0.013 Blank No contamination detected.

Ng45-02 12/19/2002 E 0.0049

 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
1953 and rates of increase in the atmosphere have been documented since 1978 (Maiss and Brenninkmeijer, 1998). Once 

released into the environment, SF6 has a long atmospheric 

half-life of about 3,200 years. Due to its rapidly increasing 

mixing ratio (7 percent per year), low solubility in water, high 

soil stability, and resistance to biodegradation and degradation under highly reducing conditions, SF6

 is a good tracer for 

age-dating young water (since 1970). Age-dating uncertainty 

is higher for water recharged since 1970 (±3 years) than for 

water recharged more recently in 1980 or 1990 (less than 1 

and less than 0.5 years, respectively) (Busenberg and Plummer, 2000). Wells are screened over several feet of aquifer 

sediment; therefore, recharge date represents the average age 

of water withdrawn from a well.

Data Analysis

Analytical techniques were selected to identify spatial 

and temporal trends in the quality of unconfined ground water 

on the Delmarva Peninsula. Principal-components analysis 

(PCA, see inset) (Hamilton, 1992) was used to identify 

underlying patterns in pH, specific conductance, dissolved 

oxygen, dissolved silica and the concentrations of major ions 

(calcium, magnesium, sodium, potassium, chloride, sulfate, 

iron, and bicarbonate), and nutrients (nitrate and ammonia 

plus organic nitrogen) in ground water in 2001, and to provide 

continuous variables (principal components) summarizing 

those patterns. Data were censored to a common level for 

each compound (several compounds had multiple laboratory 

reporting levels) and censored values were set to one-half of 

the reporting level prior to PCA.

Water-quality variables were compared to land use, 

hydrogeology, and other potentially related environmental 

variables using Spearman’s rank correlation and ranktransform analysis-of-variance (ANOVA) techniques (Helsel 

and Hirsch, 1992; Conover, 1999). All statistical tests were 

evaluated at the 95-percent confidence level (p=0.05). Data 

were censored to a common level for each compound and 

censored values were set to zero prior to correlation or 

ANOVA analyses.

Factors Affecting Spatial and 

Temporal Variability

The quality of water in the surficial aquifer on the 

Delmarva Peninsula is influenced by the availability of soluble 

ions from natural and human sources, and by geochemical 

factors that affect the mobility and fate of these ions within 

the aquifer. Natural precipitation on the peninsula is generally 

acidic and very dilute (table 2). Ground water in the surficial 

aquifer typically has a higher pH and specific conductance 

than rain, although it is still relatively dilute and acidic 

(table 3).

Geochemical transformations within the aquifer are 

largely influenced by the availability of dissolved oxygen. 

Ground water in the surficial aquifer is typically welloxygenated (oxic, dissolved oxygen concentrations greater 

than 1 mg/L). In some poorly drained areas of the peninsula, 

anoxic (dissolved oxygen concentrations less than 1 mg/L) 

ground water is sometimes present. More than half of the 

samples from each network contained greater than 3 mg/L 

of dissolved oxygen (table 3). Iron, naturally occurring as a 

grain coating on aquifer sediments where dissolved oxygen 

is present, is desorbed from aquifer sediments and iron 

concentrations are typically elevated along with bicarbonate in 

ground water with low dissolved oxygen (Denver, 1986).

Table 2. Chemistry of wet atmospheric deposition at Wye River, 

Maryland, 1994–2002.

[μS/cm, microsiemens per centimeter; mg/L, milligrams per liter]

Chemical constituent Precipitation-weighted 

mean concentration

Specific conductance 22.2 μS/cm

pH 4.35

Calcium 0.06 mg/L

Magnesium 0.026 mg/L

Potassium 0.018 mg/L

Sodium 0.18 mg/L

Ammonia as NH4 0.18 mg/L

Nitrate as N 0.28 mg/L

Chloride 0.36 mg/L

Sulfate 1.74 mg/L

Source: National Atmospheric Deposition Program, 2003a.

Factors Affecting Spatial and Temporal Variability 
Regional Variability

PCA illustrates the regional influence of agriculture 

on water quality in the surficial aquifer of the Delmarva 

Peninsula as well as the local availability of certain ions from 

saltwater intrusion or shell material. More than 70 percent of 

the variability in ground-water quality throughout the aquifer 

(represented by the domestic-well network) is explained by 

three principal components that suggest that agriculture is 

the most important influence on ground-water quality on 

a regional basis, although natural sources of ions are also 

important, particularly in non-agricultural areas (table 4). 

More than 75 percent of the variability in shallow groundwater chemistry beneath agricultural areas (represented by the 

agricultural-well network) is similarly related to the relative 

importance of natural and human influences, as well as the 

intensity or nature of agricultural practices (table 5).

Domestic-Well Network

Agriculture is the primary regional influence on groundwater quality in the surficial aquifer on the Delmarva Peninsula. The first domestic-well network principal component 

(DOMPC1), which explains 38 percent of the variability in 

ground-water quality, represents a distinction between ground 

water affected by anthropogenic (primarily agricultural) 

influences and relatively natural ground water (table 4). Dissolved components with relatively high positive loadings for 

DOMPC1 that are commonly associated with agricultural 

influences include magnesium, nitrate, potassium, chloride, 

and dissolved oxygen. Nitrate is common in oxic ground water 

in agricultural areas; the relatively high positive loadings for 

potassium and chloride suggest that nitrate may be derived 

from inorganic fertilizer or manure applications. Also, the 

relatively weak loading for calcium (which has other potential 

Table 3. Summary statistics for selected physical properties and major ions and elements in ground water of the surficial aquifer on 

the Delmarva Peninsula.

[μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; <, less than]

Constituent

(units)

Median 

in natural 

ground water 

(77 samples)1

Agricultural-well network

(29 samples)2

Domestic-well network

(29 samples)3

Minimum Median Maximum Minimum Median Maximum

Physical properties

pH (standard units) 5.76 3.9 5.3 6.8 4.3 5 7.6

Specific conductance (μS/cm) 115 34 243 2,140 22 195 611

Dissolved oxygen (mg/L) 0.02 3.3 8.6 0.05 3.7 9.5

Major ions or elements

Bicarbonate (mg/L)4 31.7 1 11 218 1 6 223

Calcium (mg/L) 4.8 2.2 12.1 103.3 0.9 10.2 77.6

Chloride (mg/L) 9.1 <0.2 22.9 599.6 6.2 14.2 96.7

Iron (μg/L) 1,800 <10 11.12 19,569 <10 28.7 14,001

Magnesium (mg/L) 1.7 0.70 7.2 33.9 0.2 4.2 13.6

Nitrate (mg/L as N) 0.1 <0.06 5.4 37.5 <0.05 5.5 27.2

Potassium (mg/L) 1.1 0.3 2.02 23.1 0.7 1.9 6.3

Silica (mg/L) 24 4.7 12.4 65.6 6.6 16.5 46.7

Sodium (mg/L) 9.4 2.41 9.21 269.09 2.2 9.1 82

Sulfate (mg/L) 6.6 <0.18 19.3 180.4 <0.1 5.8 59.2

1

 1976 to 1990 (Hamilton and others, 1993).

2

 Network of shallow wells (median depth of 22 feet below land surface) in predominantly agricultural areas.

3

 Network of deep wells (median depth of 45 feet below land surface) screened in the part of the surficial aquifer used for domestic supply.

4

 Summary statistics for the concentration of bicarbonate in 25 agricultural-network wells and 28 domestic-network wells.

10 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Table 4. Loadings on the first three components from unrotated principal-components analysis1

 for ground water 

representing domestic drinking-water supply in the surficial aquifer of the Delmarva Peninsula and correlation of 

each principal component with selected well information.

[DOMPC, domestic-well network principal component; Rho, Spearman’s rho (Helsel and Hirsch, 1992); oxic, oxidizing conditions 

(dissolved oxygen greater than 1 milligram per liter); NA, not applicable; <, less than]

Component DOMPC1 DOMPC2 DOMPC3 Communality 

estimates

Magnesium 0.7394 0.6024

Nitrate 0.7255 0.5167 0.7935

Potassium 0.7174 0.3252 0.6593

Chloride 0.7172 0.4295 -0.3800 0.8432

Dissolved oxygen 0.6481 -0.3600 0.4366 0.7402

Silica -0.7290 0.6080

pH -0.7213 0.5483 0.8255

Iron -0.6772 0.4929

Bicarbonate -0.6023 0.7009 0.9002

Calcium 0.8470 0.3718 0.8617

Specific conductance 0.4944 0.8498 0.9796

Sodium 0.4462 0.3682 -0.6212 0.7207

Sulfate 0.5498 0.3692 -0.4772 0.6664

Ammonia plus organic nitrogen -0.4496 0.3172

Variance explained 

(percent) 0.38 0.22 0.11

Correlation with 

well information2

Rho 

(p-value)

Rho 

(p-value)

Rho 

(p-value)

Depth -0.526 (0.004) -0.304 (0.116) 0.055 (0.781)

Apparent recharge date 0.468 (0.012) -0.134 (0.495) -0.024 (0.904)

Agriculture 0.414 (0.028) -0.118 (0.549) 0.316 (0.102)

Forest or wetlands -0.541 (0.003) 0.119 (0.547) -0.168 (0.393)

Urban 0.353 (0.065) 0.187 (0.340) -0.215 (0.271)

Comparison among areas 

with different:2

Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Redox conditions3 <0.001 (oxic) 0.196 (NA) 0.046 (oxic)

Soil drainage4 0.063 (NA) 0.745 (NA) 0.745 (NA)

1

 Loadings greater than 0.60 are in bold; loadings less than 0.30 are omitted.

2

 Results in bold where p<0.05. Relation to land use indicates correlation with percentage land use within 1,640 feet of each well.

3

 Represents oxidation-reduction conditions in the surficial aquifer.

4

 Represents well-drained and poorly drained soil conditions.

Factors Affecting Spatial and Temporal Variability 11
Table 5. Loadings on the first three components from unrotated principal-components analysis1

 for shallow 

ground water beneath agricultural areas of the Delmarva Peninsula and correlation of each principal component 

with selected well information.

[AGPC, agricultural-well network principal component; Rho, Spearman’s rho (Helsel and Hirsch, 1992); oxic, oxidizing conditions 

(dissolved oxygen greater than 1 milligram per liter); well, well-drained; NA, not applicable; <, less than]

Component AGPC1 AGPC2 AGPC3 Communality 

estimates

Specific conductance 0.9519 0.9808

Sodium 0.9482 0.9152

Chloride 0.8778 0.3069 0.8763

Magnesium 0.7041 0.4401 0.3843 0.8371

Bicarbonate 0.6076 0.3816 -0.5237 0.7890

Calcium 0.6782 0.6638 0.9219

Ammonia plus organic nitrogen 0.6240 -0.6256 0.7807

Iron 0.5483 -0.7707 0.9155

Silica 0.4060 -0.7724 0.7943

Sulfate 0.5703 -0.5702 0.7358

Dissolved oxygen 0.5781 0.6986 0.8421

Nitrate -0.4018 0.3838 0.7180 0.8243

pH 0.4029 0.4500 -0.4310 0.5505

Potassium 0.0436

Variance explained 

(percent) 0.39 0.25 0.13

Correlation with

well information2

Rho 

(p-value)

Rho 

(p-value)

Rho 

(p-value)

Depth -0.387 (0.051) 0.264 (0.193) 0.408 (0.038)

Apparent recharge date -0.176 (0.391) 0.244 (0.229) 0.055 (0.788)

Agriculture -0.076 (0.714) 0.220 (0.281) 0.435 (0.026)

Forest or wetlands 0.027 (0.896) -0.163 (0.426) -0.421 (0.032)

Urban 0.057 (0.784) -0.109 (0.596) -0.172 (0.400)

Comparison among areas

with different:2

Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Redox conditions3 0.609 (NA) 0.029 (oxic) 0.002 (oxic)

Soil drainage4 0.959 (NA) 0.006 (well) 0.090 (NA)

1

 Loadings greater than 0.60 are in bold; loadings less than 0.30 are omitted.

2

 Results in bold where p<0.05. Relation to land use indicates correlation with percentage land use within 1,640 feet of each well.

3

 Represents oxidation-reduction conditions in the surficial aquifer.

4

 Represents well-drained and poorly drained soil conditions.

12  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
sources in water such as shell material) and high loading for 

magnesium (which has very limited natural sources in the 

surficial aquifer and is primarily derived from dolomitic lime) 

on DOMPC1 suggests that agricultural lime applications may 

be a potential source for the magnesium. The agricultural 

signature of DOMPC1 is similar to the chemical signature of 

agriculturally influenced ground water described in Hamilton 

and others (1993). Silica, bicarbonate, and iron, which are 

common major constituents in natural, particularly anoxic, 

ground water on the peninsula, have relatively strong negative loadings for DOMPC1 (table 4). In addition, DOMPC1 

is positively correlated with agricultural intensity and apparent recharge dates; agricultural influences are more apparent 

(DOMPC1 is higher) in younger and shallower ground water 

(table 4). Estimated trends in fertilizer application (fig. 4) and 

the growth of the poultry industry (fig. 3) on the Delmarva 

Peninsula are reflected in temporal trends in DOMPC1 

(fig. 5). Agricultural influences reflected in DOMPC1 generally increase with estimated recharge date until around the late 

1980s; significant trends through the 1990s are less apparent 

(fig. 5). DOMPC1 also reflects the importance of redox 

conditions on the transport of chemicals to ground water. 

Agricultural influences reflected in DOMPC1 are significantly 

greater in oxic ground water than in ground water containing 

less than 1 mg/L of dissolved oxygen (table 4).

The second and third principal components from the 

domestic-well network (DOMPC2 and DOMPC3, respectively) apparently represent different natural influences on 

ground-water quality. DOMPC2 explains 22 percent of the 

variability in ground-water quality, and has relatively high 

positive loadings for calcium and bicarbonate (table 4). The 

relatively weak loading for magnesium suggests that natural 

shell material in aquifer sediments may be a potential source 

of these ions, and the strong loading for specific conductance 

suggests this material may be an important influence on water 

quality, where present. Similarly, DOMPC3 has relatively 

strong negative loadings for sodium, chloride, and sulfate 

(table 4), which suggest possible saltwater influence. The 

relatively strong positive loading on DOMPC3 for nitrate 

suggests possible human influence, although values of neither 

DOMPC2 nor DOMPC3 are related to land use (table 4). 

DOMPC2 and DOMPC3 are also not related to depth, apparent age, or soil drainage (table 4).

Agricultural-Well Network

The primary factor affecting shallow ground-water quality within agricultural areas of the Delmarva Peninsula is overall ionic strength. The first agricultural-well network principal 

component (AGPC1) explains 39 percent of the variability in 

ground-water samples from the shallow aquifer, and has high 

positive loadings for specific conductance, as well as many 

of the most frequently detected major cations and anions 

(table 5); several samples have high ammonia plus organic 

nitrogen values that are likely the result of intensive manure 

application near the wells (based on field observations). The 

weaker negative loading for nitrate and the lack of a significant correlation of AGPC1 with depth, apparent age, soil 

Figure 4. Estimated nutrient inputs from inorganic fertilizer 

and manure on the Delmarva Peninsula, 1940–2000. (Data from 

Alexander and Smith, 1990; Battaglin and Goolsby, 1994; David 

Lorenz, U.S. Geological Survey, written commun., 2002.)

Figure 5. Relation of domestic-well principal component 1 

(DOMPC1) to apparent recharge date in the deeper part of the 

surficial aquifer on the Delmarva Peninsula.

Factors Affecting Spatial and Temporal Variability 13
drainage, redox conditions, or land use suggest that many of 

these ions may also be derived from natural sources. As in 

the deeper parts of the aquifer sampled for the domestic-well 

network, the calcium and bicarbonate represented by AGPC1 

may be derived from the dissolution of shell material in aquifer sediments; the sodium and chloride may be derived from 

saline intrusion (Hamilton and others, 1991).

About a quarter (25 percent) of the variability in shallow ground-water quality within agricultural areas is related 

to the redox condition. The second agricultural-well network 

principal component (AGPC2) has relatively strong positive 

loadings for calcium and dissolved oxygen, and relatively 

strong negative loadings for silica, iron, ammonia plus organic 

nitrogen, and sulfate (table 5), which are typical of anoxic 

ground water in poorly drained areas. Silica, iron, and sulfate 

may be derived from dissolution of minerals in estuarine or 

marine sediments in anoxic areas. Ammonia and organic nitrogen may be derived from natural or human sources, although 

these compounds would likely be converted to nitrate in the 

presence of dissolved oxygen. As might be expected, values of 

AGPC2 are highest in oxic ground water and in well-drained 

areas (table 5). AGPC2 is not significantly correlated with land 

use, apparent age, or depth.

The third factor affecting shallow ground-water quality 

within agricultural areas (AGPC3) of the Delmarva Peninsula may be related to the nature or intensity of agriculture. 

AGPC3 has relatively strong positive loadings for nitrate and 

dissolved oxygen, and negative loadings for pH and bicarbonate (table 5). AGPC3 is significantly correlated with agriculture, and apparently not related to urban land use (table 5). 

Although agricultural areas were specifically targeted in the 

design of the shallow well network, the areas surrounding the 

wells represent a range of settings, from less than 10 percent 

to greater than 90 percent agricultural; the other major land 

uses surrounding the wells are forest and wetlands. AGPC3 

may represent a range of agricultural practices that affect the 

transport of nitrate to the surficial aquifer, as well as this range 

in agricultural intensity.

Nutrients in the Surficial Aquifer

Sources of nutrients in the ground-water system on the 

Delmarva Peninsula are both anthropogenic and natural. The 

major anthropogenic sources of nitrogen and phosphorus on 

the peninsula are inorganic fertilizer and manure applications (fig. 6). Inputs from nitrogen fertilizer applications have 

increased since the 1940s (fig. 4). Although data show that 

nutrient inputs from manure have decreased recently, they 

still contribute significantly to total nutrient inputs. Inputs of 

phosphorus from fertilizer and manure also have generally 

decreased in recent years (fig. 4). Other sources of phosphorus 

have decreased as well, such as the use of phosphate detergents and the amount of phosphorus discharged from upgraded 

wastewater-treatment plants (Fuhrer and others, 1999). Atmospheric deposition contributes an estimated 15 million tons of 

nitrogen to Delmarva, which is less than 4 percent of the total 

estimated nitrogen input (National Atmospheric Deposition 

Program, 2003a,b). Septic systems and other point sources 

make up only a small percentage of the total input of nutrients 

on the peninsula (Brakebill and Preston, 1999).

Nitrogen

Nitrogen is applied to agricultural fields in an ammonium 

form in inorganic fertilizer, or in ammonium and organic 

forms in manure. In areas that are well-drained, most of the 

nitrogen present is oxidized to nitrite then rapidly to nitrate 

(nitrification), which is readily soluble. Excess nitrogen not 

taken up by plants can be leached from the soil zone into 

shallow ground water with infiltrating rain. Due to the rapid 

conversion of nitrite to nitrate, nitrite concentrations are usually negligible and nitrate is the major form of nitrogen in 

ground water on the peninsula (table 6). Ammonium that is 

not oxidized is readily adsorbed onto soil particles and generally has a low concentration in ground water. In poorly drained 

areas where nitrification does not occur, ammonium or organic 

nitrogen not held in the soil zone may be transported to ground 

water and are the dominant nitrogen species.

Figure 6. Estimated nitrogen sources for the Chesapeake 

Bay drainage area on the Delmarva Peninsula. (Data from 

Brakebill and Preston, 1999.)

14  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Table 6. Summary statistics for selected nutrients in the surficial aquifer on the Delmarva Peninsula.

[mg/L, milligrams per liter; E, estimated; <, less than; N, nitrogen; P, phosphorus]

Constituent 

(mg/L)

Agricultural-well network Domestic-well network 

Minimum Median Maximum Minimum Median Maximum

Nitrate as N1 <0.06 5.4 37.5 <0.05 5.5 27.2

Nitrite as N <0.006 0.007 0.02 <0.006 <0.006 E0.043

Ammonia plus organic nitrogen as N <0.10 <0.10 0.67 <0.10 <0.10 0.53

Ammonia as N <0.04 <0.04 0.48 <0.04 <0.04 E0.49

Orthophosphate as P <0.02 <0.02 0.03 <0.02 <0.02 0.18

1

 Includes nitrite.

Figure 7. Relation of nitrate concentration in ground water 

to sampled well depth in the surficial aquifer on the Delmarva 

Peninsula.

Nitrate

Nitrate is widespread throughout the surficial aquifer on 

the Delmarva Peninsula. Nitrate concentrations rarely exceed 

0.4 mg/L under natural conditions in ground water on the 

peninsula (Hamilton and others, 1993); concentrations greater 

than the natural background level are likely impacted by 

anthropogenic activity. In 2001, concentrations exceeded this 

level in 20 of 29 wells in both the shallow agricultural-well 

network and the deeper domestic-well network (fig. 7). Nitrate 

was not detected in the two predominantly forested reference 

wells.

Nitrate is the predominant nutrient detected in the 

surficial aquifer; median concentrations are similar in both 

the agricultural-well network and the domestic-well network 

(table 6). The median concentration was 5.4 mg/L in the 29 

shallow wells beneath agricultural areas and 5.5 mg/L in the 

29 wells screened in the deeper part of the surficial aquifer 

used for domestic water supply (fig. 8). An additional study of 

water from 30 public-supply wells in Delaware also showed 

a similar median nitrate concentration of 5.2 mg/L at greater 

depths in the surficial aquifer (median depth of 85 ft bls) 

(Ferrari, 2002). The presence of elevated nitrate concentrations at all depths of the surficial aquifer might be expected, 

considering that most of the water recharged the aquifer during 

the last 25 years (Appendix A), which is within the timeframe 

of consistently high nitrogen application rates on the peninsula 

(fig. 4).

Concentrations of nitrate were above the MCL for 

drinking water (10 mg/L; U.S. Environmental Protection 

Agency, 2004) in about one-third of the samples from both the 

shallow agricultural-network wells and the deeper domesticnetwork wells (fig. 7). Overall, the highest concentrations 

tended to be at the shallower depths. This pattern was also 

seen in a previous study on the Delmarva Peninsula in which 

the effects of agriculture on ground-water quality were greatest 

at less than 40 ft bls (Hamilton and others, 1993).

Land use and associated activities are the leading factors 

controlling water quality and the distribution of nitrate in 

the surficial aquifer on the peninsula. Nitrate concentrations 

are positively correlated with the percentage of agricultural 

land use in both the shallow and deeper parts of the surficial 

aquifer and negatively correlated with the percentage of forest 

and wetlands (table 7). In the shallower part of the surficial 

aquifer, represented by the agricultural-well network, nitrate 

concentrations also are positively correlated with depth. 

Elevated nitrate is present beneath other land-use settings and 

at all depths throughout the surficial aquifer; water recharged 

in upgradient agricultural areas moves horizontally along 

ground-water flow paths beneath forested and residential areas 

toward discharge areas (Denver, 1993).

Factors Affecting Spatial and Temporal Variability 15
Table 7. Relation of nitrate concentrations in the surficial 

aquifer on the Delmarva Peninsula to potential explanatory 

factors. 

[Analysis based on 29 samples; Rho, Spearman’s rho (Helsel and Hirsch, 

1992); <, less than]

Nitrate

Well network

Agricultural Domestic

Correlation with 

well information1

Rho 

(p-value)

Rho 

(p-value)

Depth 0.479 (0.009) -0.247 (0.197)

Apparent recharge date 0.256 (0.180) 0.235 (0.220)

Agriculture 0.502 (0.006) 0.416 (0.025)

Forest or wetlands -0.490 (0.007) -0.515 (0.004)

Urban -0.042 (0.830) 0.166 (0.389)

1

 Results in bold where p<0.05. Relation to land use indicates correlation 

with percentage land use within 1,640 feet of each well.

Median nitrate concentrations are significantly higher in 

oxic environments than in anoxic environments (p=0.0043 in 

the agricultural-well network and p=0.0004 in the domesticwell network) (fig. 8). The median concentration of nitrate is 

above 10 mg/L (the drinking-water standard) in the presence 

of dissolved oxygen in the part of the surficial aquifer used 

for domestic supply. In poorly drained anoxic areas, nitrate is 

unstable and generally denitrifies to nitrogen gas; therefore, 

nitrate concentrations are low and ammonia and organic 

nitrogen are the most common forms of nitrogen. Water 

withdrawn from wells may intercept both oxic and anoxic 

waters in different layers of aquifer sediments. As a result, 

some water samples with low dissolved oxygen concentrations 

may have high nitrate, or some waters with high dissolved 

oxygen concentrations may also have dissolved iron because 

of mixing of waters with different geochemical characteristics 

that are near each other in the aquifer.

Regionally, concentrations of nitrate in ground water 

are related to patterns in soil drainage that affect underlying 

aquifer redox conditions. The median nitrate concentration 

is significantly higher in the agricultural-well network below 

areas with well-drained soils where nitrate is stable and readily 

transported to the surficial aquifer than beneath areas with 

more poorly drained soils (fig. 9); the median concentration 

in well-drained soils (10.7 mg/L) is above the MCL. Overall, 

nitrate concentrations tend to be higher in the central uplands 

region of the peninsula where soils are generally well drained, 

agriculture is prevalent, and fertilizer use is greater than in 

poorly drained areas (fig. 9). Shedlock and others (1999) also 

found that median concentrations were significantly lower in 

the lowland and surficial confined regions along the fringe of 

the peninsula where soils are poorly drained and there is less 

agricultural land use. Local variability in soil drainage may 

explain unusually high nitrate values in poorly drained areas.

Trends in Nitrate Concentrations

Nitrate concentrations increased in oxic areas of the 

deeper part of the surficial aquifer used for domestic supply 

between 1988 and 2001 (fig. 10). Nitrate concentrations in 11 

wells increased by an average of about 2 mg/L. This increase 

could reflect the higher amounts of nitrogen loading due to the 

regional increase in nitrogen fertilizer applications from the 

mid-to-late 1970s (represented in the areal network sampling) 

to the early 1990s (represented in the current resampling 

event). The median nitrate concentration was above the 

drinking water standard in oxic areas sampled in 2001 

(11.4 mg/L). Although the sample size is low (a total of eight 

paired samples in oxic areas), the data suggest there is no 

Figure 8. Distribution of nitrate concentrations in different 

oxidation-reduction (redox) environments on the Delmarva 

Peninsula.

16  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Figure 9. Soil drainage and areal distribution of nitrate concentrations in the surficial aquifer on the Delmarva Peninsula. 

Well drained includes well-drained uplands and inner Coastal Plain. Poorly drained includes poorly drained lowland, surficial 

confined region, poorly drained uplands, fine-grained lowland, and other regions (modified from Shedlock and others, 1999).

Factors Affecting Spatial and Temporal Variability 17
significant change in nitrate concentrations over time in the 

shallower ground water underlying agricultural areas (fig. 10). 

Differences in nitrate concentrations over time are small and 

have increased in some wells and decreased in others, which 

may reflect more recent local variations in nitrogen use for 

corn and soybean crops. Nitrate concentrations are consistently low in anoxic areas throughout the surficial aquifer.

Ammonia and Organic Nitrogen

Concentrations of ammonia and organic nitrogen are 

relatively low in the surficial aquifer (median less than 0.1, the 

laboratory reporting level) (table 6), although ammonia and 

organic nitrogen are the dominant forms of nitrogen in poorly 

drained, anoxic environments. In the shallow agricultural-well 

network, concentrations are higher in poorly drained, anoxic 

environments (p=0.012 and p=0.001, respectively) (fig. 11). 

Ammonia and organic nitrogen concentrations are negatively 

correlated with depth in the shallower part of the aquifer 

(table 8, fig. 12). This pattern may reflect the relatively 

shallow water table in poorly drained anoxic areas, where 

ammonia and organic nitrogen are more likely to be present 

than in well-drained oxygenated areas.

Figure 10. Distribution of measured nitrate concentrations in 

wells from oxic environments (dissolved oxygen greater than 

1 milligram per liter) sampled in 1998 and 2001 in the domesticwell network and the agricultural-well network on the 

Delmarva Peninsula. Although median nitrate concentrations 

increased by about 2 milligrams per liter in the deeper 

domestic-well network, there was no significant change in the 

shallower agricultural-well network.

Figure 11. Distribution of ammonia plus organic nitrogen 

concentrations in areas of different soil drainage and 

dissolved oxygen concentrations in the shallow surficial 

aquifer on the Delmarva Peninsula. Ammonia plus organic 

nitrogen concentrations are greater in samples from poorly 

drained, oxygen-poor environments.

18  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Phosphorus

Phosphorus concentrations are relatively low throughout 

the surficial aquifer on the peninsula; median concentrations 

are below the laboratory reporting level (0.02 mg/L) in the 

shallow agricultural-network wells and in the domestic-network wells (table 6). Concentrations exceed 0.1 mg/L in only 

three wells in the deeper network; dissolved oxygen is low or 

absent in samples from these wells.

Pesticides in the Surficial Aquifer

Pesticide compounds are used for agricultural and nonagricultural purposes on the Delmarva Peninsula. Agricultural 

uses of pesticides are predominantly for weed and insect 

control. Non-agricultural uses include turf maintenance on 

residential, commercial, and recreational areas; insect control 

for residential and commercial structures; and vegetative clearing along transportation and utility rights-of-way (Barbash and 

Resek, 1996).

In 1997, 151 different pesticide compounds were applied 

to crops on the Delmarva Peninsula, representing over 

3.8 million pounds of active ingredient (Gail Thelin, U.S. 

Geological Survey, written commun., 2002). Over 70 percent 

of the pesticide compounds were herbicides used for crop 

production. Metolachlor, atrazine, glyphosate, and alachlor 

are the four most widely used agricultural herbicides; methyl 

bromide is widely used as a soil fumigant on the Virginia 

part of the Delmarva Peninsula (fig. 13, table 9). All of the 

pesticides commonly used on the peninsula were not included 

in chemical analyses done for this study (Appendix C).

Pesticide compounds were detected at low levels 

throughout the surficial aquifer on the Delmarva Peninsula. 

Detectable concentrations of pesticides or their degradates 

(see inset, next page) were found in 93 percent of the 

shallow agricultural wells and in over 83 percent of the 

deeper domestic wells; no pesticide residue was detected 

in the two reference wells (fig. 14, table 10). Metolachlor 

and atrazine, which have been used for over 25 years, were 

the most commonly detected parent pesticide compounds 

in the surficial aquifer. Several relatively new herbicides 

(flumetsulam, imazaquin, imazethapyr, and glyphosate) also 

were detected, but less frequently. Degradates of several 

pesticides, including metolachlor, alachlor, and atrazine, were 

among the most frequently detected compounds (table 10). 

Widespread detection of pesticide compounds reflects their 

abundant use on the peninsula as well as chemical properties 

(moderate to high water solubility; persistence) and the aquifer 

characteristics (good soil drainage) that promote movement 

into ground water (table 11). Concentrations were typically 

low (below 1 μg/L) and were always below established 

drinking-water standards and health advisory levels (HALs). 

Such guidelines have been established for only 14 of the 36 

compounds detected, however (table 10).

Table 8. Relation of ammonia plus organic nitrogen concentrations in the surficial aquifer on the Delmarva Peninsula to 

potential explanatory factors.

[Analysis based on 29 samples; Rho, Spearman’s rho (Helsel and Hirsch, 

1992); <, less than]

Nitrate

Well network

Agricultural Domestic

Correlation with 

well information1

Rho 

(p-value)

Rho 

(p-value)

Depth -0.502 (0.006) -0.0003 (0.999)

Apparent recharge date 0.073 (0.707) -0.305 (0.108)

Agriculture -0.393 (0.035) -0.191 (0.322)

Forest or wetlands 0.351 (0.062) 0.179 (0.352)

Urban 0.023 (0.906) -0.171 (0.377)

1

 Results in bold where p<0.05. Relation to land use indicates correlation 

with percentage land use within 1,640 feet of each well.

Figure 12. Relation of ammonia plus organic nitrogen 

concentrations in ground water to sampled well depth in 

the shallow agricultural-well network on the Delmarva 

Peninsula.

Factors Affecting Spatial and Temporal Variability 19
Of the other commonly applied pesticides, glyphosate 

was rarely detected and methyl bromide was not detected in 

the surficial aquifer (only samples from the domestic-well 

network were tested for methyl bromide). Low detection 

frequencies for glyphosate might be expected as glyphosate 

has only been used since 1986 and chemical properties 

indicate that it strongly adsorbs to most soils and suspended 

organic and mineral matter in the aquifer (table 9). Methyl 

bromide is rarely detected in ground water across the Nation; 

it was only detected in 2 wells out of over 20,000 sampled 

between 1971 and 1991 (Barbash and Resek, 1996).

Metolachlor, alachlor, and atrazine compounds primarily 

occurred in ground water in the form of degradation products. 

Degradates of these pesticides were typically detected more 

frequently and at higher concentrations than their parent 

compounds in both the agricultural-well network and the 

domestic-well network (table 10, fig. 15). The median 

concentration of the ethanesulfonic acid (ESA) degradation 

product for metolachlor and alachlor was at least one order of 

magnitude higher then their parent compounds in both well 

networks. Alachlor oxanilic acid (OA), 2,6-Diethylaniline, and 

metolachlor OA also were detected but much less frequently. 

Alachlor and metolachlor are typical of many pesticides in that 

they tend to adsorb onto organic matter and quickly degrade 

in the soil zone, restricting movement of the parent compound 

into ground water. ESA and OA degradates are more soluble 

than the parent compounds and more likely to move into 

ground water (Phillips and others, 1999). Acetochlor ESA 

was also detected in both well networks, although its parent 

compound, acetochlor, was not; acetechlor has only been in 

use since 1994 (Meister, 2000). A relatively low amount of 

acetochlor was applied to corn on the Delmarva Peninsula in 

Figure 13. Estimated 

agricultural pesticide use on 

the Delmarva Peninsula, 1997. 

(Data from Gail Thelin, U.S. 

Geological Survey, written 

commun., 2002.)

20 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Table 9. Description of commonly used and frequently detected pesticide compounds on the Delmarva Peninsula.

[USEPA, U.S. Environmental Protection Agency; ESA, ethanesulfonic acid; OA, oxanilic acid]

Compound Use Properties Common degradates Trends1

Metolachlor First registered for use in the United States in 1976; 

broad-spectrum herbicide; used for weed control primarily in agricultural areas, on lawn and turf, ornamental plants, trees, shrubs and vines, rightsof-way, fencerows and hedgerows, and in forestry (USEPA, 1995)

Moderately persistent in soil, moderately soluble 

and highly persistent in water, and degrades mainly by microbial activity (Extension Toxicity Network, 1996a)

Metolachlor ESA and 

metolachlor OA

Between 1992 and 1997, 

use increased nationally by 13 percent.

Atrazine First registered for use in the United States in 1958 

(USEPA, 2003); restricted use in United States as of 1990 (Briggs, 1992); selective herbicide; used for weed control

Highly persistent in soils with low clay or organic 

matter; moderately to highly mobile in soils; moderately soluble in water; and degrades by chemical hydrolysis and microbial activity (Extension Toxicity Network, 1996b)

Deethylatrazine (DEA), 

deisopropylatrazine (DIA), deethyldeisopropylatrazine, and hydroxyatrazine

Between 1992 and 1997, 

use increased nationally by 3 percent.

Glyphosate First registered for use in the United States in 1986; 

broad-spectrum, non-selective herbicide; used on a wide range of agricultural crops and non-crop areas where total vegetation control is needed (USEPA, 1993)

Moderately persistent in soils; strongly adsorbs to 

most soils, resulting in a low runoff potential; strongly adsorbed to suspended organic and mineral matter in water (Extension Toxicity Network, 1996c)

Aminomethylphosphonic 

acid (AMPA)

Between 1992 and 1997, 

use increased nationally by 107 percent.

Alachlor First registered for use in the United States in 1969; 

highly restricted since 1988 and has been slowly replaced with acetochlor since 1994; selective systemic herbicide; used to control annual grasses and broadleaf weeds in corn and soybeans (USEPA, 1998; Extension Toxicity Network, 1996d)

Low persistence in soil; moderately mobile 

in sandy and silty soils, highly mobile and moderately persistent in water; breaks down rapidly in natural water; the degradation rate is slower in anoxic waters; degrades by microbial activity (USEPA, 1998; Extension Toxicity Network, 1996d)

Alachlor ESA, 2,6-Diethylaniline, and alachlor OA Between 1992 and 1997, use decreased nationally by 71 percent.

Methyl bromide Production will be eliminated by 2005; used as a soil 

fumigant to control nematodes, fungi, pathogens, insects, and weeds (Carpenter and others, 2000, USEPA, 2000)

Moderately persistent in soils; poorly sorbed 

by soils; slightly soluble in water (Extension Toxicity Network, 1996e)

Bromide ion Between 1992 and 1997, 

use decreased nationally by 26 percent.

1 Gianessi and Silvers, 2000.

Factors Affecting Spatial and Temporal Variability  21
Figure 14. Occurrence of pesticides and pesticide degradates (shown in italics and parentheses) in the 

surficial aquifer on the Delmarva Peninsula. (Analytical reporting levels vary among compounds and are 

shown in Appendix C.)

22  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Table 10. Summary of analytical results for selected pesticides and degradates in samples from the surficial aquifer on the 

Delmarva Peninsula in 2001.—Continued

[n, number of samples; μg/L, micrograms per liter; %, percent; MCL, maximum contaminant level; HAL, health advisory level; E, estimated; <, less than]

Compound1 Use

Detection 

limit1

(μg/L)

n

Percent 

detection 

(%)

Concentration 

(μg/L)

Drinkingwater 

standard2

(μg/L)

MCL or 

(HAL)

Median Maximum

Agricultural-well network

Metolachlor ESA Herbicide degradate (metolachlor) 0.05 29 86 1.34 35

Alachlor ESA Herbicide degradate (alachlor) 0.05 29 69 0.17 5.54

Deethylatrazine Herbicide degradate (atrazine) 0.006 29 48 <0.006 E0.413

Atrazine Herbicide 0.007 29 48 <0.007 0.657 3

Metolachlor OA Herbicide degradate (metolachlor) 0.05 29 48 <0.05 19

Metolachlor Herbicide 0.013 29 45 <0.013 2.43 (100)

Deethyldeisopropylatrazine Herbicide degradate (atrazine) 0.01 26 21 <0.01 E0.5758

Deisopropylatrazine Herbicide degradate (atrazine, cyanazine, 

simazine)

0.044 26 21 <0.044 E0.0844

Bromacil Herbicide 0.033 26 18 <0.033 E2.5242 (90)

Alachlor OA Herbicide degradate (alachlor) 0.05 29 17 <0.05 3.48

Hydroxyatrazine Herbicide degradate (atrazine) 0.008 26 15 <0.008 E0.0971

Flumetsulam Herbicide 0.011 26 14 <0.011 E0.075

Prometon Herbicide 0.015 29 13 <0.015 0.0206 (100)

Imazaquin Herbicide 0.016 26 11 <0.016 E0.0975

Imazethapyr Herbicide 0.017 26 11 <0.017 E0.0609

Acetochlor ESA Herbicide degradate (acetochlor) 0.05 29 10 <0.05 0.88

Imidacloprid Insecticide 0.0068 26 7 <0.0068 0.2236

Glyphosate Herbicide 0.1 28 7 <0.1 0.16 700

Metalaxyl Fungicide 0.02 26 7 <0.02 0.0208

Propoxur Insecticide 0.008 26 7 <0.008 0.0103

Dimethenamid ESA Herbicide degradate (dimethenamid) 0.05 29 7 <0.05 0.1

2,6-Diethylaniline Herbicide degradate (alachlor) 0.006 28 7 <0.006 <0.006

Simazine Herbicide 0.011 29 6 <0.011 <0.011 4

Methomyl Insecticide 0.0044 26 4 <0.0044 E0.2485 (200)

Metsulfuron methyl Herbicide 0.025 26 4 <0.025 <0.025

Linuron Herbicide 0.035 29 3 <0.035 <0.035

Carbofuran Insecticide/nomatocide 0.02 29 3 <0.02 <0.02 40

Bentazon Herbicide 0.011 26 4 <0.011 <0.011 (200)

Benomyl Fungicide 0.0038 26 4 <0.0038 0.0671

Flufenacet OA Herbicide degradate (flufenacet) 0.05 29 3 <0.05 0.07

Metribuzin Herbicide 0.006 29 3 <0.006 0.763 (200)

Alachlor Herbicide 0.0045 29 3 <0.0045 0.0407 2

Factors Affecting Spatial and Temporal Variability  23
Table 10. Summary of analytical results for selected pesticides and degradates in samples from the surficial aquifer on the 

Delmarva Peninsula in 2001.—Continued

[n, number of samples; μg/L, micrograms per liter; %, percent; MCL, maximum contaminant level; HAL, health advisory level; E, estimated; <, less than]

Compound1 Use

Detection 

limit1

(μg/L)

n

Percent 

detection 

(%)

Concentration 

(μg/L)

Drinkingwater 

standard2

(μg/L)

MCL or 

(HAL)

Median Maximum

Domestic-well network

Metolachlor ESA Herbicide degradate (metolachlor) 0.05 29 69 0.12 15

Alachlor ESA Herbicide degradate (alachlor) 0.05 29 69 0.16 4.82

Deethylatrazine Herbicide degradate (atrazine) 0.006 29 41 <0.006 E0.186

Metolachlor OA Herbicide degradate (metolachlor) 0.05 29 34 <0.05 4.85

Atrazine Herbicide 0.007 29 31 <0.007 0.163 3

Deisopropylatrazine Herbicide degradate (atrazine, cyanazine, 

simazine)

0.044 27 22 <0.044 E0.0941

Metolachlor Herbicide 0.013 29 21 <0.013 0.735 (100)

Deethyldeisopropylatrazine Herbicide degradate (atrazine) 0.01 27 19 <0.01 E0.1556

Flumetsulam Herbicide 0.011 27 19 <0.011 E0.0257

Bromacil Herbicide 0.033 27 11 <0.033 E0.2149 (90)

Hydroxyatrazine Herbicide degradate (atrazine) 0.008 27 11 <0.008 E0.057

Imazaquin Herbicide 0.016 27 11 <0.016 E0.019

Glyphosate Herbicide 0.1 29 10 <0.1 0.14 700

Imazethapyr Herbicide 0.017 27 7 <0.017 E0.1557

Propoxur Insecticide 0.008 27 7 <0.008 0.041

Alachlor OA Herbicide degradate (alachlor) 0.05 29 7 <0.05 2.2

Simazine Herbicide 0.011 29 7 <0.011 0.0497 4

Dieldrin Insecticide 0.0048 29 7 <0.0048 0.0052

Bentazon Herbicide 0.011 27 4 <0.011 E0.0155 (200)

Dinoseb Herbicide 0.012 27 4 <0.012 <0.012 7

Metalaxyl Fungicide 0.02 27 4 <0.02 <0.02

Carbaryl Insecticide 0.041 29 3 <0.041 <0.041 (700)

Benomyl Fungicide 0.0038 27 4 <0.0038 0.0065

Acetochlor ESA Herbicide degradate (acetochlor) 0.05 29 3 <0.05 1.1

Alachlor Herbicide 0.0024 29 3 <0.0024 0.223 2

Metribuzin Herbicide 0.006 29 3 <0.006 0.119 (200)

Butylate Herbicide 0.002 29 3 <0.002 0.0032 (400)

1 Italics represent pesticide degradation products.

2

 U.S. Environmental Protection Agency, 2004; not all compounds have an MCL or HAL.

24  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Table 11. Physical and chemical characteristics of commonly 

used pesticides on the Delmarva Peninsula.

[Values for persistence from Rao and Hornsby, 1989, and CambridgeSoft 

Corporation, 2004; Values for solubility and sorption from CambridgeSoft 

Corporation, 2004, and U.S. Department of Agriculture, 1995; T1/2, half life; 

ppm, parts per million; oC, degrees Celsius; Koc, adsorption to soil organic 

matter; mL/g, milliliters per gram]

Compound Persistence

(T1/2, days)

Solubility 

in water

(ppm at 25oC)

Sorption

(Koc, mL/g)

Metolachlor 15–90 530a 200

Alachlor 15–70 240 170

Atrazine 60–365 33 100

Glyphosate 47 12,000 24,000

Methyl bromide 55 13,400 22

a

 20oC.

Figure 15. Distribution of selected pesticides and pesticide 

degradates in the surficial aquifer on the Delmarva Peninsula. 

Degradates generally occur at higher concentrations than their 

parent compounds.

1997 (fig. 13). Acetochlor use is increasing, however, as it is 

being used to replace alachlor. Atrazine was detected almost 

as frequently and at similar concentrations as deethylatrazine 

in both well networks (table 10, fig. 15). Additional 

degradation products of atrazine (deethyldeisopropylatrazine, 

deisopropylatrazine, and hydroxyatrazine) also were detected 

in at least 15 percent of the agricultural-network wells and at 

least 11 percent of the domestic-network wells. A combination 

of relatively high mobility and chemical stability in the soil 

zone allows atrazine to move to, and persist in, the groundwater system longer than metolachlor or alachlor, which tend 

to break down more completely to degradates before reaching 

the ground-water system (table 11) (Denver and others, 2004). 

Atrazine likely degrades into other unmeasured compounds 

as well.

Although the parent compounds were not analyzed at 

the laboratory, degradation products of dimethenamid and 

flufenacet, herbicides used on corn and soybean crops, also 

were detected in the shallower agricultural-well network 

(table 10, fig. 14). Dimethenamid has been in use in the 

United States since 1993 and was the 16th most commonly 

used pesticide on the peninsula in 1997; flufenacet has only 

been in use in the United States since 1998, and use data are 

not available (Scribner and others, 2004).

Mixtures of pesticide compounds are commonly found 

in individual water samples throughout the surficial aquifer. 

Pesticides are frequently used in combination on crops for 

better control (U.S. Department of Agriculture, 1999). In 

addition, because of crop rotation, different chemicals are used 

in sequential years. Most wells sampled in the agriculturalwell network (90 percent) and the domestic-well network 

(69 percent) contained 3 or more compounds; as many as 11 

compounds were detected in several wells. The combined 

toxicity or synergistic effects of multiple pesticide compounds 

in a sample are currently unknown.

Concentrations of commonly used herbicides are related 

to agricultural land use and aquifer redox conditions on the 

Delmarva Peninsula. Analysis of total concentrations (sum of 

parent and degradates) of atrazine and metolachlor indicates 

that although pesticide and degradate compounds are detected 

in wells surrounded by all land-use settings throughout the 

peninsula, pesticide concentrations are positively correlated 

with the percentage of agricultural land use and are present 

at significantly higher concentrations in oxic environments 

(table 12). Pesticide concentrations are influenced by the 

drainage characteristics of overlying soil at shallow depths 

Factors Affecting Spatial and Temporal Variability  25
Table 12. Correlation of total atrazine and total metolachlor for shallow ground water beneath agricultural 

areas and deeper ground water representing domestic supply on the Delmarva Peninsula.

[Total concentrations are parent compound plus degradation products in molar units; Rho, Spearman’s rho (Helsel and Hirsch, 

1992); oxic, oxidizing conditions (dissolved oxygen greater than 1 milligram per liter); well, well-drained; NA, not applicable; 

<, less than]

Atrazine

Well network

Agricultural Domestic

Correlation with well information1 Rho

(p-value)

Rho

(p-value)

Depth 0.334 (0.077) -0.214 (0.266)

Apparent recharge date 0.415 (0.025) 0.380 (0.042)

Agriculture 0.705 (<.001) 0.554 (0.002)

Forest or wetlands -0.596 (0.001) -0.564 (0.001)

Urban 0.022 (0.909) 0.129 (0.504)

Comparison among areas with different:1 Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Redox conditions2 0.005 (oxic) 0.001 (oxic)

Soil drainage3 0.002 (well) 0.065 (NA)

Metolachlor

Well network

Agricultural Domestic

Correlation with well information1 Rho 

(p-value)

Rho 

(p-value)

Depth 0.308 (0.104) -0.475 (0.009)

Apparent recharge date 0.076 (0.693) 0.317 (0.094)

Agriculture 0.689 (<.001) 0.545 (0.002)

Forest or wetlands -0.634 (0.001) -0.485 (0.008)

Urban 0.098 (0.614) 0.123 (0.526)

Comparison among areas with different:1 Rank-sum p-value 

(High group)

Rank-sum p-value 

(High group)

Redox conditions2 0.001 (oxic) 0.026 (oxic)

Soil drainage3 0.015 (well) 0.503 (NA)

1

 Results in bold where p<0.05. Relation to land use indicates correlation with percentage land use within 1,640 feet of 

each well.

2

 Represents oxidation-reduction conditions in the surficial aquifer.

3

 Represents well-drained and poorly drained soil conditions.

26  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Figure 16. Distribution of selected pesticides in areas 

with different soil drainage in the agricultural-well 

network on the Delmarva Peninsula. Concentrations tend 

to be higher in well-drained soils. (Total atrazine refers 

to the sum of atrazine, deethylatrazine, hydroxyatrazine, 

deisopropylatrazine, and deethyldeisopropylatrazine. Total 

metolachlor refers to the sum of metolachlor, metolachlor 

ESA, and metolachlor OA. Molar sums were converted to 

micrograms per liter of parent compound.)

in the surficial aquifer (represented by the agricultural-well 

network); total concentrations tend to be higher in welldrained areas (fig. 16, table 12). Despite the fact that both 

metolachlor and atrazine compounds occurred at all depths, 

total metolachlor did not occur below 60 ft in the domesticwell network (fig. 17). Total atrazine concentrations tend to 

be higher in younger water throughout the surficial aquifer 

(fig. 18).

Trends in Pesticide Detections

Temporal trends in pesticide concentrations from 1988 

to 2001 in the surficial aquifer are difficult to determine due 

to changes in sampling and analytical procedures during the 

intervening years. Analytical procedures greatly improved 

during the 1990s, and method reporting levels decreased by 

as much as two orders of magnitude for some compounds 

(Wershaw and others, 1987; Zaugg and others, 1995; Furlong 

and others, 2001). Also, only whole-water (unfiltered) samples 

were analyzed prior to the early 1990s, and the suite of compounds was relatively small. These factors, along with inconsistent usage from year to year, make analysis of environmental trends difficult. Five pesticides were detected from 21 

compounds analyzed in 1988: atrazine, metolachlor, alachlor, 

cyanazine, and dicamba (table 13, Appendix D). Alachlor 

and cyanazine were not detected in the same wells in 2001, 

in spite of lower laboratory reporting levels; dicamba also 

was not detected in the same wells in 2001, but the reporting 

level increased slightly. The wells that contained atrazine and 

metolachlor in 1988 also contained these compounds in 2001. 

Increased frequency of detection of pesticide compounds in 

the more recent samples is more likely due to better method 

sensitivities than to increases in pesticide concentrations over 

time. Pesticide concentrations remained low throughout the 

surficial aquifer.

Factors Affecting Spatial and Temporal Variability  27
Table 13. Pesticide detection frequency in the areal-network wells sampled in 1988 on the 

Delmarva Peninsula.

[μg/L, micrograms per liter; %, percent; MCL, maximum contaminant level; HAL, health advisory level]

Compound 

(unfiltered) Use Detection limit (μg/L)

Percent 

detection

(%)

Maximum 

concentration

(μg/L)

Drinking-water 

standard1

(μg/L)

MCL or (HAL)

Agricultural-well network

Atrazine Herbicide 0.05 20 0.8 3

Metolachlor Herbicide 0.15 13 0.8 (100)

Alachlor Herbicide 0.15 7 2.0 2

Cyanazine Herbicide 0.05 7 0.10

Domestic-well network

Dicamba Herbicide 0.01 4 0.01

1

 U.S. Environmental Protection Agency, 2004; not all compounds have an MCL or HAL.

Figure 17. Relation of total metolachlor concentrations 

to sampled well depth in the domestic-well network on the 

Delmarva Peninsula.

Figure 18. Relation of total atrazine concentrations to apparent 

recharge date in the surficial aquifer on the Delmarva Peninsula. 

Concentrations tend to be higher in more recently recharged 

ground water.

28  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
and atrazine and their degradates were the most commonly 

detected pesticide compounds. For pesticides with degradate 

analyses, degradation products typically were detected more 

frequently and at higher concentrations than parent compounds 

throughout the surficial aquifer. Pesticide mixtures also were 

common throughout the aquifer and most wells contained 

three or more compounds. As many as 11 different pesticide 

compounds were detected in several wells, representing the 

mix of chemicals used on the land surface.

In addition to chemical properties and aquifer 

characteristics, factors affecting spatial trends in groundwater quality also include natural geochemical conditions 

and land-use practices. Redox conditions and soil drainage 

affect the occurrence of nitrate and pesticides in the surficial 

aquifer. Geochemical transformations within the aquifer 

are largely controlled by the availability of dissolved 

oxygen. Nitrate concentrations were typically higher in oxic 

environments where nitrate is stable; nitrate is unstable and 

denitrifies in poorly drained, anoxic environments where 

ammonia and organic nitrogen become the dominant forms of 

nitrogen. As most of the surficial aquifer is under oxidizing 

conditions, little nitrate is removed along ground-water 

flow paths in the surficial aquifer. Concentrations of nitrate 

beneath nonagricultural land-use settings may be the result 

of upgradient agriculturally influenced ground water moving 

along deeper flow paths toward discharge areas. Pesticide 

concentrations in the surficial aquifer also tend to be higher 

beneath recharge areas with well-drained soils in the shallow 

part of the aquifer and in oxic environments throughout the 

surficial aquifer. Concentrations of nitrate and pesticides were 

significantly higher in the central uplands of the peninsula, 

which have generally well-drained soils and oxidizing aquifer 

conditions, and where most of the agricultural land is located.

Implementation of agricultural management practices 

over the past few decades may explain why there is no 

increase in nitrate concentrations in the shallow part of the 

aquifer underlying agricultural areas. Decreases in nitrate 

concentrations resulting from improved nutrient management 

practices may not be apparent in the deeper part of the 

surficial aquifer for years or decades due to the relatively 

slow movement of ground water. Results indicate that nitrate 

concentrations in oxic areas have increased approximately 

2 milligrams per liter over the past decade in the deeper 

surficial aquifer; in 2001, the median nitrate concentration in 

this part of the aquifer was above the Maximum Contaminant 

Level of 10 milligrams per liter.

Temporal trends in pesticide detections are difficult 

to determine from the available data. Wells that contained 

atrazine and metolachlor in 1988 also contained these 

compounds in 2001. Overall, pesticides were more frequently 

detected in 2001 than in 1988; however, the increased 

frequency of detection in the more recent samples is likely due 

to lower detection levels rather than to increases in pesticide 

concentrations over time.

Degraded ground-water quality may have considerable 

implications for water supply and for the health of Delmarva 

Summary and Conclusions

Human influences are apparent in shallow ground water 

underlying agricultural areas as well as in deeper water used 

for domestic supply in the unconsolidated surficial aquifer 

on the Delmarva Peninsula. Dissolved constituents found 

throughout the unconfined aquifer are likely derived from 

the predominantly agricultural practices on the peninsula. In 

water affected by agriculture, calcium and magnesium were 

present from the application of lime, potassium and chloride 

from potash fertilizer, and nitrate from inorganic fertilizer 

and manure. Septic-system effluent may contribute sodium, 

chloride, and nitrate to ground water, although the part of 

the aquifer affected by septic-system effluent is likely very 

small relative to that affected by agriculture because of the 

differences in the magnitude of residential and agricultural 

land use. Sodium and chloride from road salt were also 

present in some water samples. Principal-components analysis 

suggests that in the deeper parts of the surficial aquifer used 

for domestic drinking water, the major sources of ions in 

ground water are anthropogenic and are primarily related 

to agricultural land use. Ions from other sources, including 

the dissolution of shells and iron minerals in the sediments 

and seawater intrusion, also are important, particularly in 

nonagricultural areas and along the coasts. Even though ions 

from natural sources dominate in some areas, only 5 percent 

of the ground-water samples collected for this study could 

be classified as natural and did not show some degree of 

anthropogenic influence.

Nitrate was widespread throughout ground water in 

the surficial aquifer on the Delmarva Peninsula and was 

the predominant nutrient detected; median concentrations 

were similar in both the agricultural-well network and the 

domestic-well network. The statistical distribution of nitrate 

concentrations in the surficial aquifer was nearly identical 

in the regional agricultural- and domestic-well networks as 

well as in the deeper Delaware public-supply well network 

(medians from 5.2 to 5.5 milligrams per liter). Nitrate 

was detected above the Maximum Contaminant Level 

(10 milligrams per liter) in about one-third of the samples 

from both the shallower agricultural-well network and the 

deeper domestic-well network. Concentrations of nitrate in 

ground water are not related to recharge date of the water, 

indicating that the entire flow system in the surficial aquifer 

has potentially been affected by human activities on and near 

the land surface. This is anticipated because nutrient inputs 

have been relatively high over the 35-year period during which 

most water currently in the surficial aquifer was recharged.

Pesticide compounds were detected at low levels 

throughout the surficial aquifer. Pesticides were present 

in 85 percent of the ground-water samples collected from 

the surficial aquifer for this study. Widespread detection 

reflects the abundant use of pesticide compounds as well as 

chemical properties and aquifer characteristics that allow 

their movement into ground water. Metolachlor, alachlor, 

Summary and Conclusions  29
Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 

1976, A land use and land cover classification system for 

use with remote sensor data: U.S. Geological Survey 

Professional Paper 964, 41 p.

Andres, A.S., 1986, Stratigraphy and depositional history of 

the post-Choptank Chesapeake Group: Delaware Geological Survey Report of Investigations 42, 39 p.

Ator, S.W., and Ferrari, M.J., 1997, Nitrate and selected 

pesticides in ground water of the Mid-Atlantic Region: 

U.S. Geological Survey Water-Resources Investigations 

Report 97–4139, 8 p.

Bachman, L.J., 1984a, Nitrate in the Columbia aquifer, central 

Delmarva Peninsula, Maryland: U.S. Geological Survey 

Water-Resources Investigations Report 84–4322, 32 p.

Bachman, L.J., 1984b, The Columbia aquifer of the eastern 

shore of Maryland, Part 1: Hydrogeology: Maryland Geological Survey Report of Investigations 40, 34 p.

Bachman, L.J., and Phillips, P.J., 1996, Hydrologic landscapes 

on the Delmarva Peninsula: Part 2: Estimates of baseflow nitrogen to Chesapeake Bay: Journal of the American 

Water Resources Association, v. 32, no. 4, p. 779–791.

Barbash, J.E., and Resek, E.A., 1996, Pesticides in ground 

water: Distribution, trends, and governing factors: Chelsea, 

Michigan, Ann Arbor Press, Pesticides in the Hydrologic 

System series, v. 2, 588 p.

Battaglin, W.A., and Goolsby, D.A., 1994, Spatial data in Geographic Information System format on agricultural chemical 

use, land use, and cropping practices in the United States: 

U.S. Geological Survey Water-Resources Investigations 

Report 94–4176, 87 p.

Blaier, S.C., and Baxter, S.J., 2000, The occurrence and 

distribution of several agricultural pesticides in Delaware’s 

shallow ground water: Delaware Geological Survey Report 

of Investigations No. 61, 23 p.

Böhlke, J.K., and Denver, J.M., 1995, Combined use of 

groundwater dating, chemical, and isotopic analyses to 

resolve the history and fate of nitrate contamination in two 

agricultural watersheds, Atlantic Coastal Plain, Maryland: 

Water Resources Research, v. 31, no. 9, p. 2,319–2,339.

Brakebill, J.W., and Preston, S.D., 1999, Digital data used to 

relate nutrient inputs to water quality in the Chesapeake 

Bay Watershed, Version 1.0: U.S. Geological Survey 

Open-File Report 99–60, available at http://md.water.usgs.

gov/publications/ofr-99-60/

Briggs, S.A., 1992, Basic guide to pesticides—Their characteristics and hazards: Washington, D.C., Taylor & Francis, 

283 p.

streams and adjacent estuaries. Nitrate concentrations above 

the Maximum Contaminant Level in parts of the aquifer 

used for domestic-water supply may be a concern as private 

domestic wells are not commonly subject to water-quality 

testing. Streams on the peninsula receive over half of 

their flow from ground-water discharge and the chemical 

composition of stream base flow on the Delmarva Peninsula is 

similar to that of shallow ground water. The dominant source 

of nitrogen discharged to the Chesapeake Bay and its tidal 

tributaries is from ground water. Excessive levels of nutrients 

(such as the nitrate contributed from ground water) in streams 

can lead to eutrophication (blooms of algae and other plants) 

and hypoxia (areas of low dissolved oxygen), resulting in 

fishkills and loss of other living resources. Pesticides also 

are common in parts of the aquifer used for domestic supply. 

Although pesticide detections are almost always lower 

than existing standards for drinking water, many pesticide 

compounds, including most degradation products, have as yet 

unknown health implications. In addition, the additive and 

synergistic effects of ingesting multiple pesticide compounds 

are unknown. Pesticides also have been measured in stream 

base flow; these and other organic chemicals in stream water 

also are of concern because of their potential toxic effects on 

aquatic biota.

Acknowledgments

The authors wish to extend a special thanks to the 

cooperative landowners of the Delmarva Peninsula who 

allowed access to their property and domestic wells for drilling 

and sampling. Deborah Bringman, Julie Matlaga, and David 

Hudson of the USGS collected much of the data presented in 

this report. The authors offer their appreciation to the technical 

reviewers, Charles Crawford and Cherie Miller of the USGS.

References Cited

Alexander, R.B., and Smith, R.A., 1990, County-level estimates of nitrogen and phosphorus fertilizer use in the 

United States, 1945 to 1985: U.S. Geological Survey 

Open-File Report 90–130, 12 p.

Alley, W.M., 1993, Ground-water-quality surveys, in Alley, 

W.M., ed., Regional ground-water quality: New York, 

Van Nostrand Reinhold, p. 63–85.

American Farmland Trust, 2003, Agricultural situation and 

trends of the Delmarva Peninsula—Summary Report, 

accessed November 3, 2004, at http://www.farmland.org/

mid%5Fatlantic/Delmarva%20Peninsula% 20Agricultural

%20Trends%20Summary.pdf

30 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Busenberg, Eurybiades, and Plummer, L.N., 2000, Dating 

young ground water with sulfur hexafluoride: Natural 

and anthropogenic sources of sulfur hexafluoride: Water 

Resources Research, v. 36, no. 10, p. 3,011–3,030.

CambridgeSoft Corporation, 2004, Characteristics of 

pesticides, maintained and archived by the CambridgeSoft 

Corporation, accessed October 22, 2004, at http://www3.

bae.ncsu.edu/info1/courses/bae573/models/gleams/wwwdocs/tabp2.txt

Carpenter, Janet, Gianessi, Leonard, and Lynch, Lori, 2000, 

The economic impact of the scheduled U.S. phaseout of 

methyl bromide: National Center for Food and 

Agricultural Policy, accessed November 3, 2004, at 

http://www.ncfap.org/reports/pesticides/methyl%20bromide/

methylbromide.htm

Conover, W.J., 1999, Practical nonparametric statistics, 3d. 

ed.: New York, John Wiley and Sons, 577 p.

Cushing, E.M., Kantrowitz, I.H., and Taylor, K.R., 1973, 

Water resources of the Delmarva Peninsula: U.S. Geological Survey Professional Paper 822, 58 p.

Delaware Department of Natural Resources and Environmental Control, 2005, Managing urban runoff: Delaware 

Department of Natural Resources and Environmental Control, Delaware Coastal Programs, accessed June 6, 2005, 

at http://www.dnrec.state.de.us/dnrec2000/Divisions/Soil/

dcmp/cdurban.htm

Delmarva Poultry Industry, Inc., 2003, Delmarva soybeans & 

corn production and broiler chicken use, accessed April 14, 

2004, at http://dpichicken.com/download/SOYBEANS.DOC

Denver, J.M., 1986, Hydrogeology and geochemistry of the 

unconfined aquifer, west-central and southwestern Delaware: Delaware Geological Survey Report of Investigations 

No. 41, 100 p.

Denver, J.M., 1989, Effects of agricultural practices and 

septic-system effluents on the quality of water in the unconfined aquifer in parts of Eastern Sussex County, Delaware: 

Delaware Geological Survey Report of Investigations 

No. 45, 66 p.

Denver, J.M., 1993, Herbicides in shallow ground water at two 

agricultural sites in Delaware: Delaware Geological Survey 

Report of Investigations No. 51, 28 p.

Denver, J.M., Ator, S.W., Debrewer, L.M., Ferrari, M.J., 

Barbaro, J.R., Hancock, T.C., Brayton, M.J., and Nardi, 

M.R., 2004, Water quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001: U.S. Geological 

Survey Circular 1228, 26 p.

Dunkle, S.A., Plummer, L.N., Busenberg, Eurybiades, 

Phillips, P.J., Denver, J.M., Hamilton, P.A., Michel, R.L., 

and Coplen, T.B., 1993, Chlorofluorocarbons (CCl3F and 

CCl2F2) as dating tools and hydrologic tracers in shallow 

ground water of the Delmarva Peninsula, Atlantic Coastal 

Plain, United States: Water Resources Research, v. 29, 

no. 12, p. 3,837–3,860.

Extension Toxicity Network, 1996a, Pesticide information 

profiles—metolachlor: maintained and archived by 

Oregon State University, accessed October 10, 2002, at 

http://extoxnet.orst.edu/pips/metolach.htm

Extension Toxicity Network, 1996b, Pesticide information 

profiles—atrazine: maintained and archived by 

Oregon State University, accessed October 21, 2002, at 

http://extoxnet.orst.edu/pips/atrazine.htm

Extension Toxicity Network, 1996c, Pesticide information 

profiles—glyphosate: maintained and archived by 

Oregon State University, accessed October 22, 2002, at 

http://extoxnet.orst.edu/pips/glyphosa.htm

Extension Toxicity Network, 1996d, Pesticide information 

profiles—alachlor: maintained and archived by 

Oregon State University, accessed October 21, 2002, at 

http://extoxnet.orst.edu/pips/alachlor.htm

Extension Toxicity Network, 1996e, Pesticide information 

profiles—methyl bromide, bromomethane: maintained and 

archived by Oregon State University, accessed November 3, 

2004, at http://extoxnet.orst.edu/pips/methylbr.htm

Ferrari, M.J., 2002, Occurrence and distribution of selected 

contaminants in public drinking-water supplies in the surficial aquifer in Delaware: U.S. Geological Survey Open-File 

Report 01–327, 62 p.

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey OpenFile Report 93–125, 217 p.

Fuhrer, G.J., Gilliom, R.J., Hamilton, P.A., Morace, J.L., 

Nowell, L.H., Rinella, J.F., Stoner, J.D., and Wentz, D.A., 

1999, The quality of our Nation’s waters—Nutrients and 

pesticides: U.S. Geological Survey Circular 1225, 82 p.

Furlong, E.T., Anderson, B.D., Werner, S.L., Soliven, P.P., 

Coffey, L.J., and Burkhardt, M.R., 2001, Methods of 

analysis by the U.S. Geological Survey National Water 

Quality Laboratory—Determination of pesticides in water 

by graphitized carbon-based solid phase extraction and 

high-performance liquid chromatography/mass spectrometry: U.S. Geological Survey Water-Resources Investigations 

Report 01–4134, 73 p.

References Cited  31
Jones, B.E., 1987, Quality-control manual of the U.S. Geological Survey’s National Water Quality Laboratory: 

U.S. Geological Survey Open-File Report 87–457, 36 p.

Kauffman, L.J., Baehr, A.L., Ayers, M.A., and Stackelberg, 

P.E., 2001, Effects of land use and travel time on the distribution of nitrate in the Kirkwood-Cohansey aquifer system 

in Southern New Jersey: U.S. Geological Survey WaterResources Investigations Report 01–4117, 49 p.

Koterba, M.T., Banks, W.S.L., and Shedlock, R.J., 1993, Pesticides in shallow groundwater in the Delmarva Peninsula: 

Journal of Environmental Quality, v. 22, no. 3, p. 500–518.

Koterba, M.T., Shedlock, R.J., Bachman, L.J., and Phillips, 

P.J., 1990, Regional and targeted groundwater quality 

networks in the Delmarva Peninsula, in Nash, R.G., and 

Leslie, A.R., eds., Groundwater residue sampling design: 

Washington, D.C., American Chemical Society Symposium 

Series, no. 465, chap. 6, p. 110–138.

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

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program—Collection and documentation of water-quality samples and related 

data: U.S. Geological Survey Open-File Report 95–399, 

113 p.

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

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program—Selection, installation, and documentation of wells, and collection 

of related data: U.S. Geological Survey Open-File Report 

95–398, 69 p.

Lee, E.A., Kish, J.L., Zimmerman, L.R., and Thurman, E.M., 

2001a, Methods of analysis by the U.S. Geological Survey 

Organic Geochemistry Research Group—Update and additions to the determination of chloroacetanilide herbicide 

degradation compounds in water using high-performance 

liquid chromatography/mass spectrometry: U.S. Geological 

Survey Open-File Report 01–10, 17 p.

Lee, E.A., Strahan, A.P., and Thurman, E.M., 2001b, Methods 

of analysis by the U.S. Geological Survey Organic Geochemistry Research Group—Determination of glyphosate, 

aminomethylphosphonic acid, and glufosinate in water 

using online solid-phase extraction and high-performance 

liquid chromatography/mass spectromentry: U.S. Geological Survey Open-File Report 01–454, 13 p.

Lindsey, B.D., Phillips, S.W., Donnelly, C.A., Speiran, G.K., 

Plummer, L.N., Böhlke, J.K., Focazio, M.J., Burton, W.C., 

and Busenberg, Eurybiades, 2003, Residence times and 

nitrate transport in ground water discharging to streams in 

the Chesapeake Bay watershed: U.S. Geological Survey 

Water-Resources Investigations Report 03–4035, 201 p.

Garbarino, J.R., 1999, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of dissolved arsenic, boron, lithium, selenium, 

strontium, thallium, and vanadium using inductively 

coupled plasma-mass spectrometry: U.S. Geological 

Survey Open-File Report 99–093, 31 p.

Gianessi, L.P., and Silvers, C.S., 2000, Trends in crop pesticide use: Comparing 1992 and 1997: National Center 

for Food and Agricultural Policy Report, November 2000, 

Office of Pest Management Policy, U.S. Department of 

Agriculture—Agricultural Research Service Cooperative 

Agreement 58-0790-7-039, 165 p.

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

the National Water-Quality Assessment Program—

Occurrence and distribution of water-quality conditions: 

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

Hamilton, L.C., 1992, Regression with graphics—A second 

course in applied statistics: Belmont, California, Duxbury 

Press, 363 p.

Hamilton, P.A., Denver, J.M., Phillips, P.J., and Shedlock, R.J., 

1993, Water-quality assessment of the Delmarva Peninsula, 

Delaware, Maryland, and Virginia—Effects of agricultural 

activities on, and distribution of, nitrate and other inorganic 

constituents in the surficial aquifer: U.S. Geological Survey 

Open-File Report 93–40, 87 p.

Hamilton, P.A., Shedlock, R.J., and Phillips, P.J., 1991, Waterquality assessment of the Delmarva Peninsula, Delaware, 

Maryland, and Virginia—Analysis of available groundwater-quality data through 1987: U.S. Geological Survey 

Water-Supply Paper 2355-B, 65 p.

Hardy, M.A., Leahy, P.A., and Alley, W.M., 1989, Well installation and documentation, and ground-water sampling 

protocols for the pilot National Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 

89–396, 36 p.

Harsh, J.F., and Laczniak, R.J., 1990, Conceptualization and 

analysis of the ground-water flow system in the Coastal 

Plain of Virginia and adjoining parts of Maryland and 

North Carolina: U.S. Geological Survey Professional Paper 

1404-F, 100 p.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Amsterdam, Elsevier Science Publishers, 

522 p.

Johnston, R.H., 1973, Hydrogeology of the Columbia (Pleistocene) deposits of Delaware: An appraisal of the regional 

water-table aquifer: Delaware Geological Survey Bulletin 

No. 14, 78 p.

32  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Maiss, Manfred, and Brenninkmeijer, C.A.M., 1998, Atmospheric SF6

: Trends, sources, and prospects: Environmental 

Science & Technology, v. 32, no. 20, p. 3,077–3,086.

Meister, R.T., ed., 2000, Farm chemicals handbook 2000: 

Willoughby, Ohio, Meister Publishing Co., v. 86, 931 p.

Miller, J.C., 1972, Nitrate contamination of the water-table 

aquifer in Delaware: Delaware Geological Survey Report 

of Investigations No. 20, 36 p.

National Atmospheric Deposition Program, 2003a, 1997 data 

from National Atmospheric Deposition Program/National 

Trends Network monitoring location MD13, Wye, Maryland, accessed April 24, 2003, at http://nadp.sws.uiuc.

edu/sites/siteinfo.asp?net=NTN&id=MD13.

National Atmospheric Deposition Program, 2003b, Estimated 

inorganic nitrogen deposition from nitrate and ammonium, 

1997: National Atmospheric Deposition Program/National 

Trends Network isopleth map, accessed April 24, 2003, at 

http://nadp.sws.uiuc.edu/isopleths/maps1997/ndep.gif.

Phillips, P.J., Eckhardt, D.A., Terracciano, S.A., and Rosenmann, Larry, 1999, Pesticides and their metabolites in wells 

of Suffolk County, New York, 1998: U.S. Geological Survey Water-Resources Investigations Report 99–4095, 12 p.

Phillips, S.W., and Caughron, W.R., 1997, Overview of the 

U.S. Geological Survey Chesapeake Bay Ecosystem Program: U.S. Geological Survey Fact Sheet FS-124-97, 4 p.

Radtke, D.B., Wilde, F.D., Davis, J.V., and Popowski, T.J., 

1998, National field manual for collection of water-quality data—Alkalinity and acid neutralizing capacity: U.S. 

Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A6.6, 33 p.

Rao, P.S.C., and Hornsby, A.G., 1989, Behavior of pesticides 

in soils and water: University of Florida, Institute of Food 

Agriculture, Soil Science Fact Sheet SL—40 (revised), 7 p.

Scott, J.C., 1990, Computerized stratified random site-selection approaches for design of ground-water quality sampling network: U.S. Geological Survey Water-Resources 

Investigations Report 90–4101, 109 p.

Scribner, E.A., Dietze, J.E., and Thurman, E.M., 2004, Acetamide herbicides and their degradation products in ground 

water and surface water of the United States—1993–2003: 

U.S. Geological Survey Data Series 88, 252 p.

Shedlock, R.J., Denver, J.M., Hayes, M.A., Hamilton, P.A., 

Koterba, M.T., Bachman, L.J., Phillips, P.J., and Banks, 

W.S.L., 1999, Water-quality assessment of the Delmarva 

Peninsula, Delaware, Maryland, and Virginia: Results of 

investigations, 1987–91: U.S. Geological Survey WaterSupply Paper 2355-A, 41 p.

Shedlock, R.J., Hamilton, P.A., Denver, J.M., and Phillips, P.J., 

1993, Multiscale approach to regional ground-water-quality 

assessment of the Delmarva Peninsula, in Alley, W.M., ed., 

Regional ground-water quality: New York, Van Nostrand 

Reinhold, p. 563–587.

Speiran, G.K., 1996, Geohydrology and geochemistry near 

coastal ground-water-discharge areas of the Eastern Shore, 

Virginia: U.S. Geological Survey Water-Supply Paper 

2479, 73 p.

U.S. Department of Agriculture, 1991, Acreage report: 

U.S. Department of Agriculture, National Agricultural 

State Board, Report Cr-Pr-2-2, 21 p.

U.S. Department of Agriculture, 1995, Pesticide properties—glyphosate: maintained by the Agricultural Research 

Service, accessed October 22, 2002, at http://www.arsusda.

gov/acsl/services/ppdb/textfiles/GLYPHOSATE

U.S. Department of Agriculture, 1999, Crop profiles for corn 

(sweet) in Maryland: U.S. Department of Agriculture, 

Office of Pest Management Policy, accessed October 21, 

2002, at http://pestdata.ncsu.edu/cropprofiles/docs/Mdcornsweet.html

U.S. Environmental Protection Agency, 1993, R.E.D. Facts—

Glyphosate: U.S. Environmental Protection Agency, Office 

of Prevention, Pesticides and Toxic Substances (7508W), 

EPA-738-F-93-011, 7 p.

U.S. Environmental Protection Agency, 1995, R.E.D. Facts—

Metolachlor: U.S. Environmental Protection Agency, 

Office of Prevention, Pesticides and Toxic Substances 

(7508W), EPA-738-F-95-007, 14 p.

U.S. Environmental Protection Agency, 1998, R.E.D. Facts—

Alachlor: U.S. Environmental Protection Agency, Office 

of Prevention, Pesticides and Toxic Substances (7508C), 

EPA-738-F-98-018, 12 p.

U.S. Environmental Protection Agency, 2000, Protection 

of stratospheric ozone: Incorporation of Clean Air Act 

Amendments for reductions in class I, group VI controlled 

substances: Federal Register—November 28, 2000, 

vol. 65, no. 229 Rules and Regulations, p. 70795–70804, 

accessed November 3, 2004, at http://www.epa.gov/fedrgstr/

EPA-AIR/2000/November/Day-28/a30109.htm

U.S. Environmental Protection Agency, 2003, Atrazine Interim 

Reregistration Eligibility Decision (IRED) Q&A’s—January 2003: U.S. Environmental Protection Agency, accessed 

April 28, 2004, at http://www.epa.gov/pesticides/factsheets/

atrazine.htm

References Cited  33
Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, 

K.M., 1995, Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—Determination 

of pesticides in water by C-18 solid-phase extraction and 

capillary-column gas chromatography/mass spectrometry 

with selected-ion monitoring: U.S. Geological Survey 

Open-File Report 95–181, 60 p.

Zimmerman, L.R., Hostetler, K.A., and Thurman, E.M., 2000, 

Methods of analysis of the U.S. Geological Survey Organic 

Geochemistry Research Group—Determination of chloroacetanilide herbicide metabolites in water using high-performance liquid chromatography-diode array detection and 

high-performance liquid chromatography/mass spectrometry: U.S. Geological Survey Open-File Report 00–182, 

28 p.

U.S. Environmental Protection Agency, 2004, 2004 Edition 

of the drinking water standards and health advisories: U.S. 

Environmental Protection Agency, Office of Groundwater 

and Drinking Water, accessed April 28, 2004, at http://www.

epa.gov/waterscience/drinking/standards/dwstandards.pdf

Vogelmann, J.E., Howard, S.M., Yang, L., Larson, C.R., 

Wylie, B.K., and Van Driel, J.N., 2001, Completion of the 

1990s National Land Cover Data Set for the conterminous 

United States from Landsat thematic mapper data and 

ancillary data sources: Photogrammetric Engineering and 

Remote Sensing, no. 67, p. 650–652.

Wershaw, R.L., Fishman, M.J., Grabbe, R.R., and Lowe, 

L.E., eds., 1987, Methods for the determination of organic 

substances in water and fluvial sediments: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 5, chap. A3, 80 p.

34  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Appendix A. Site information for wells sampled in the surficial aquifer on the Delmarva Peninsula.—Continued

[Wells in bold are trend wells sampled both in 1988 and 2001; bls, below land surface]

Well name Well depth

(ft bls) Use of well Recharge date

Land use1

Agriculture Forest-wetland Open water Urban2

Agricultural-well network

(median depth 22 feet below land surface)

Ec42-15 38 Domestic 1996.5 93 1 0 7

KE Bd 42 27 Observation 1988.0 92 7 0 0

KE Be 59 26.5 Observation 1994.2 97 0 2 1

Gc14-04 34 Observation 1996.5 74 26 0 0

QA Db 45 35 Domestic 1983.5 28 18 33 22

QA Df 55 26 Observation 1991.0 77 22 0 1

Ib32-08 14 Observation 1995.5 77 15 0 8

Jc52-04 9 Observation 1995.0 47 53 0 0

TA Cc 53 26.5 Observation 1990.5 73 16 0 11

CO Db 79 22 Observation 1993.0 89 11 0 0

CO De 16 17 Observation 1988.5 74 26 0 0

Le24-11 13.5 Observation 1994.5 97 3 0 0

DO Cb 8 15 Observation 1980.5 29 37 29 4

CO Ec 36 22 Observation 1993.5 51 27 0 22

Oc21-03 23 Observation 1998.5 61 39 0 0

Oe44-01 18.5 Observation 2000.0 58 42 0 0

Ng45-02 22 Observation 1998.5 66 34 0 0

Ri22-10 23 Observation 1981.5 66 34 0 0

DO Cf 36 16 Observation 1999.5 72 4 0 24

Qc22-04 29 Observation 1994.0 69 31 0 0

WI Ch 51 20 Observation 1991.5 43 57 0 0

Rf24-08 19 Observation 1987.5 79 20 0 1

SO Be 114 19 Observation 1987.5 79 19 0 2

WO Cc 3 21 Observation 1992.0 1 99 0 0

WO Cg 78 13 Observation 1994.5 35 59 4 2

WO Fe 1 24 Observation 1991.0 39 60 0 0

66L 11 15 Observation 1989.0 36 64 0 0

64J 28 13 Observation 1996.0 37 37 0 27

64J 30 22 Observation 1997.0 47 32 1 20

Reference-well network

Gb51-07 20 Observation 1995.0 17 76 1 6

Of12-05 13 Observation 2000.0 2 98 0 0

Appendix A  35
Appendix A. Site information for wells sampled in the surficial aquifer on the Delmarva Peninsula.—Continued

[Wells in bold are trend wells sampled both in 1988 and 2001; bls, below land surface]

Well name Well depth

(ft bls) Use of well Recharge date

Land use1

Agriculture Forest-wetland Open water Urban2

Domestic-well network

(median depth 45 feet below land surface)

Cd31-19 75 Observation 1998.0 3 6 3 75

CE Ce 86 85 Domestic 1991.0 60 40 0 0

KE Be 46 50 Domestic 1991.0 98 0 1 1

Gc14-03 80 Domestic 1994.5 29 62 9 0

QA Db 40 35 Domestic 1980.5 10 11 59 20

QA Df 54 42 Domestic 1989.0 76 23 0 1

Ib32-05 30 Observation 1978.5 84 14 0 2

Jc55-03 30 Domestic 1994.0 80 12 0 8

TA Be 83 37 Domestic 1991.5 79 13 0 8

CO Dd 74 15 Observation 1999.0 85 15 0 0

CO De 15 33 Observation 1995.5 73 27 0 0

Le35-11 48 Domestic 1993.0 50 28 0 22

CO Ec 25 28 Observation 1992.0 50 30 0 20

Oc21-02 45 Observation 1992.0 57 43 0 0

Oe44-02 90 Irrigation 1984.5 58 42 0 0

Nh44-08 100 Domestic 1983.5 42 57 0 0

Ri22-03 50 Stock 1974.5 69 31 0 0

DO Ch 1 41 Domestic 1990.0 78 22 0 0

22DR 60 Domestic 1987.5 81 19 0 0

WI Ch 50 70 Domestic 1981.5 46 54 0 0

Rf14-02 56 Domestic 1989.5 84 16 0 0

SO De 44 43 Observation 1991.0 49 50 0 0

SO Be 115 51 Observation 1997.5 78 20 0 2

WO Cc 2 55 Observation 1992.0 1 99 0 0

WO Cg 76 90 Observation 1977.0 37 56 4 2

WO Fc 46 43 Observation 1976.5 39 60 0 0

66L 12 28 Observation 1990.0 35 65 0 0

64J 29 40 Observation 1995.0 38 36 0 26

64J 31 30 Observation 1992.0 46 32 2 20

1

 Represents land use within 1,640 feet radius of well, based on data described in Vogelmann and others, 2001. Due to rounding, land-use percentage 

may not add to 100 percent.

2

 Land use includes residential, commercial, and industrial.

36  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Appendix B. Estimated uncertainty in reported concentrations of selected major ions, nutrients, and pesticides in the surficial 

aquifer on the Delmarva Peninsula.—Continued

[BN, number of field blanks; BND, number of blanks in which compound was detected; BMAX, maximum reported concentration in field blanks; 

SN, number of field spikes; SMED, median recovery from field spikes; %, percent; RN, number of replicate sets; RNAGREE, number of replicate sets within 

which compound was consistently detected or not detected; RUNC, median estimated uncertainty in reported concentrations; NC, not computed because 

no replicate sets had consistent detected concentrations; ESA, ethanesulfonic acid; OA, oxanilic acid; --, not applicable]

Compound or ion1

Contamination bias Sampling and analytical uncertainty

BN BND BMAX SN SMED% RN RNAGREE RUNC%1

Major ions

(mg/L except where noted)

Calcium 6 5 0.46 -- -- 9 9 0.69

Chloride 6 2 0.30 -- -- 9 9 1.22

Iron (μg/L) 6 3 28 -- -- 9 8 0.40

Magnesium 6 3 0.01 -- -- 9 9 0.98

Potassium 6 1 0.10 -- -- 9 9 1.07

Silica 6 2 7.32 -- -- 9 9 0.68

Sodium 6 3 1.77 -- -- 9 9 1.68

Sulfate 6 1 0.18 -- -- 9 9 1.05

Nutrients

(mg/L)

Nitrogen ammonia 6 0 -- -- -- 9 9 10.50a

Nitrogen, nitrite 6 0 -- -- -- 9 7 NC

Nitrogen ammonia + organic 6 0 -- -- -- 9 8 5.89

Nitrate as nitrogen 6 4 0.06 -- -- 9 9 1.82

Phosphorus, ortho 6 0 -- -- -- 9 8 22.81b

Carbon, organic 7 4 0.39 -- -- 6 6 2.61

Pesticides

(μg/L)

2,4-D 5 1 0.14 -- -- 5 5 NC

2,4-D methyl ester 5 0 -- -- -- 5 5 NC

2,4-DB FLTRD 5 0 -- -- -- 5 5 NC

2,6-Diethylaniline 5 0 -- 8 99 6 6 NC

Acetochlor 5 0 -- 8 111 6 6 NC

Acetochlor ESA 1 0 -- -- -- 2 2 NC

Acetochlor OA 1 0 -- -- -- 2 2 NC

Acifluorfen 5 0 -- -- -- 5 5 NC

Alachlor 5 0 -- 8 113 6 6 NC

Alachlor OA 1 0 -- -- -- 2 2 NC

Alachlor ESA 1 0 -- -- -- 2 2 0.00

Aldicarb 5 0 -- -- -- 5 5 NC

Aldicarb sulfone 5 0 -- -- -- 5 5 NC

Aldicarb sulfoxide 5 0 -- -- -- 5 5 NC

Alpha BHC 5 0 -- 8 99 6 6 NC

Atrazine 5 0 -- 8 101 6 6 1.78

Bendiocarb 5 0 -- -- -- 5 5 NC

Benfluralin 5 0 -- 8 84 6 6 NC

Benomyl 5 0 -- -- -- 5 5 NC

Bensulfuron-methyl 5 0 -- -- -- 5 5 NC

Appendix B  37
Appendix B. Estimated uncertainty in reported concentrations of selected major ions, nutrients, and pesticides in the surficial 

aquifer on the Delmarva Peninsula.—Continued

[BN, number of field blanks; BND, number of blanks in which compound was detected; BMAX, maximum reported concentration in field blanks; 

SN, number of field spikes; SMED, median recovery from field spikes; %, percent; RN, number of replicate sets; RNAGREE, number of replicate sets within 

which compound was consistently detected or not detected; RUNC, median estimated uncertainty in reported concentrations; NC, not computed because 

no replicate sets had consistent detected concentrations; ESA, ethanesulfonic acid; OA, oxanilic acid; --, not applicable]

Compound or ion1

Contamination bias Sampling and analytical uncertainty

BN BND BMAX SN SMED% RN RNAGREE RUNC%1

Pesticides

(μg/L)

Bentazon 5 0 -- -- -- 5 5 NC

Bromacil 5 0 -- -- -- 5 5 NC

Bromoxynil 5 0 -- -- -- 5 5 NC

Butylate 5 0 -- 8 107 6 6 NC

Carbaryl 5 0 -- 8 155 6 6 NC

Carbofuran 5 0 -- 8 140 6 6 NC

Chlorpyrifos 5 0 -- 8 100 6 6 NC

Cyanazine 5 0 -- 8 104 6 6 NC

Cycloate 5 0 -- -- -- 5 5 NC

Deethylatrazine 5 0 -- 8 46 6 6 8.14

Deethyldeisopropylatrazine 5 0 -- -- -- 5 5 26.87b

Deisopropylatrazine 5 0 -- -- -- 5 5 21.56a

Diazinon 5 0 -- 8 101 6 6 NC

Dicamba 5 0 -- -- -- 4 4 NC

Dieldrin 5 0 -- 8 109 6 6 NC

Dimethenamid ESA 1 0 -- -- -- 2 2 NC

Dimethnamid OA 1 0 -- -- -- 2 2 NC

Dinoseb 5 0 -- -- -- 5 5 NC

Diphenamid 5 0 -- -- -- 5 5 NC

Diuron 5 0 -- -- -- 5 5 NC

Fenuron 5 0 -- -- -- 5 5 NC

Flufenacet OA 1 0 -- -- -- 2 2 NC

Flufenacet ESA 1 0 -- -- -- 2 2 NC

Flumetsulam 5 0 -- -- -- 5 5 NC

Fluometuron 5 0 -- -- -- 5 5 NC

Fonofos 5 0 -- 8 100 6 6 NC

Hydroxyatrazine 5 0 -- -- -- 5 5 55.13a

Imazaquin 5 0 -- -- -- 5 5 NC

Imazethapyr 5 0 -- -- -- 5 5 9.08

Imidacloprid 5 0 -- -- -- 5 5 NC

Lindane 5 0 -- 8 100 6 6 NC

Linuron 5 0 -- 8 121 6 6 NC

Malathion 5 0 -- 8 109 6 6 NC

MCPA 5 0 -- -- -- 5 5 NC

MCPB 5 0 -- -- -- 5 5 NC

38  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Appendix B. Estimated uncertainty in reported concentrations of selected major ions, nutrients, and pesticides in the surficial 

aquifer on the Delmarva Peninsula.—Continued

[BN, number of field blanks; BND, number of blanks in which compound was detected; BMAX, maximum reported concentration in field blanks; 

SN, number of field spikes; SMED, median recovery from field spikes; %, percent; RN, number of replicate sets; RNAGREE, number of replicate sets within 

which compound was consistently detected or not detected; RUNC, median estimated uncertainty in reported concentrations; NC, not computed because 

no replicate sets had consistent detected concentrations; ESA, ethanesulfonic acid; OA, oxanilic acid; --, not applicable]

Compound or ion1

Contamination bias Sampling and analytical uncertainty

BN BND BMAX SN SMED% RN RNAGREE RUNC%1

Pesticides

(μg/L)

Metalaxyl 5 0 -- -- -- 5 5 NC

Metolachlor ESA 1 0 -- -- -- 2 2 7.57

Metolachlor OA 1 0 -- -- -- 2 2 NC

Metolachlor 5 3 0.02 8 106 6 6 48.31a

Metribuzin 5 0 -- 8 90 6 6 NC

Metsulfuron-methyl 5 0 -- -- -- 5 5 NC

p,p’-DDE 5 0 -- 8 76 6 6 NC

Parathion 5 0 -- 8 108 6 6 NC

Pebulate 5 0 -- 8 108 6 6 NC

Pendimethalin 5 0 -- 8 104 6 6 NC

Cis-permethrin 5 0 -- 8 67 6 6 NC

Phorate 5 0 -- 8 79 6 6 NC

Picloram 5 0 -- -- -- 5 5 NC

Prometon 5 0 -- 8 91 6 6 4.81

Propachlor 5 0 -- 8 133 6 6 NC

Propham 5 0 -- -- -- 5 5 NC

Propoxur 5 0 -- -- -- 5 5 NC

Simazine 5 0 -- 8 87 6 6 NC

Tebuthiuron 5 0 -- 8 134 6 6 NC

Terbacil 5 0 -- 8 103 6 6 NC

Terbufos 5 0 -- 8 78 6 6 NC

1

 Estimated uncertainty is the median of the relative standard deviation of reported concentrations for replicate sets in which the compound was 

detected in all replicates. The relative standard deviation is the standard deviation divided by the mean.

a

 Estimated uncertainty above 10 percent based on one set of replicates.

b

 Estimated uncertainty above 10 percent based on two sets of replicates.

Appendix B  39
Appendix C. Pesticides and pesticide degradates analyzed in samples from the surficial aquifer on 

the Delmarva Peninsula in 2001.—Continued

[Degradation products are in italics; CAS, Chemical Abstracts Service; μg/L, micrograms per liter; ESA, ethanesulfonic 

acid; OA, oxanilic acid; --, no CAS registry number available]

Organic analyte CAS registry number® Analytical method reporting level 

(μg/L)

2,4-D 94-75-7 0.021

2,4-D methyl ester 1928-38-7 0.0086

2,4-DB 94-82-6 0.016

2,6-Diethylaniline 579-66-8 0.006

2-Hydroxyatrazine 2163-68-0 0.008

3(4-Chlorophenyl)-1-methyl urea 5352-88-5 0.024

3-Hydroxycarbofuran 16655-82-6 0.0058

3-Ketocarbofuran 16709-30-1 1.5

Acetochlor 34256-82-1 0.006

Acetochlor ESA -- 0.05

Acetochlor OA -- 0.05

Acifluorfen 50594-66-6 0.0066

Alachlor 15972-60-8 0.0045

Alachlor ESA -- 0.05

Alachlor OA -- 0.05

Aldicarb 116-06-3 0.04

Aldicarb sulfone 1646-88-4 0.02

Aldicarb sulfoxide 1646-87-3 0.0082

alpha-HCH 319-84-6 0.0046

AMPA 1066-51-9 0.1

Atrazine 1912-24-9 0.007

Azinphos-methyl 86-50-0 0.05

Bendiocarb 22781-23-3 0.025

Benfluralin 1861-40-1 0.01

Benomyl 17804-35-2 0.0038

Bensulfuron-methyl 83055-99-6 0.015

Bentazon 25057-89-0 0.011

Bromacil 314-40-9 0.033

Bromoxynil 1689-84-5 0.017

Butylate 2008-41-5 0.002

Caffeine 58-08-2 0.0096

Carbaryl 63-25-2 0.028

Carbofuran 1563-66-2 0.0056

Chloramben, methyl ester 7286-84-2 0.018

Chlorimuron-ethyl 90982-32-4 0.0096

40 Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Appendix C. Pesticides and pesticide degradates analyzed in samples from the surficial aquifer on 

the Delmarva Peninsula in 2001.—Continued

[Degradation products are in italics; CAS, Chemical Abstracts Service; μg/L, micrograms per liter; ESA, ethanesulfonic 

acid; OA, oxanilic acid; --, no CAS registry number available]

Organic analyte CAS registry number® Analytical method reporting level 

(μg/L)

Chlorothalonil 1897-45-6 0.035

Chlorpyrifos 2921-88-2 0.005

cis-Permethrin 54774-45-7 0.006

Clopyralid 1702-17-6 0.013

Cyanazine 21725-46-2 0.018

Cycloate 1134-23-2 0.013

Dacthal 1861-32-1 0.003

Dacthal monoacid 887-54-7 0.011

Deethylatrazine 6190-65-4 0.006

Deethyldeisopropylatrazine 3397-62-4 0.01

Deisopropylatrazine 1007-28-9 0.044

Desulfinylfipronil -- 0.004

Desulfinylfipronil amide -- 0.009

Diazinon 333-41-5 0.005

Dicamba 1918-00-9 0.012

Dichlorprop 120-36-5 0.013

Dieldrin 60-57-1 0.0048

Dimethenamid ESA -- 0.05

Dimethenamid OA -- 0.05

Dinoseb 88-85-7 0.012

Diphenamid 957-51-7 0.026

Disulfoton 298-04-4 0.021

Diuron 330-54-1 0.015

EPTC 759-94-4 0.002

Ethalfluralin 55283-68-6 0.009

Ethoprophos 13194-48-4 0.005

Fenuron 101-42-8 0.031

Fipronil 120068-37-3 0.007

Fipronil sulfide 120067-83-6 0.005

Fipronil sulfone 120068-36-2 0.005

Flufenacet ESA -- 0.05

Flufenacet OA -- 0.05

Flumetsulam 98967-40-9 0.011

Fluometuron 2164-17-2 0.031

Fonofos 944-22-9 0.0027

Appendix C  41
Appendix C. Pesticides and pesticide degradates analyzed in samples from the surficial aquifer on 

the Delmarva Peninsula in 2001.—Continued

[Degradation products are in italics; CAS, Chemical Abstracts Service; μg/L, micrograms per liter; ESA, ethanesulfonic 

acid; OA, oxanilic acid; --, no CAS registry number available]

Organic analyte CAS registry number® Analytical method reporting level 

(μg/L)

Glufosinate 77182-82-2 0.1

Glyphosate 1071-83-6 0.1

Imazaquin 81335-37-7 0.016

Imazethapyr 81335-77-5 0.017

Imidacloprid 138261-41-3 0.0068

Lindane 58-89-9 0.004

Linuron 330-55-2 0.014

Malathion 121-75-5 0.027

MCPA 94-74-6 0.016

MCPB 94-81-5 0.015

Metalaxyl 57837-19-1 0.02

Methiocarb 2032-65-7 0.008

Methomyl 16752-77-5 0.0044

Metolachlor 51218-45-2 0.013

Metolachlor ESA -- 0.05

Metolachlor OA -- 0.05

Metribuzin 21087-64-9 0.006

Metsulfuron methyl 74223-64-6 0.025

Molinate 2212-67-1 0.0016

Napropamide 15299-99-7 0.007

Neburon 555-37-3 0.012

Nicosulfuron 111991-09-4 0.013

Norflurazon 27314-13-2 0.016

Oryzalin 19044-88-3 0.017

Oxamyl 23135-22-0 0.012

p,p’-DDE 72-55-9 0.0025

Parathion 56-38-2 0.01

Parathion-methyl 298-00-0 0.006

Pebulate 1114-71-2 0.0041

Pendimethalin 40487-42-1 0.022

Phorate 298-02-2 0.011

Picloram 1918-02-1 0.019

Prometon 1610-18-0 0.015

Propachlor 1918-16-7 0.01

Propanil 709-98-8 0.011

42  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Appendix C. Pesticides and pesticide degradates analyzed in samples from the surficial aquifer on 

the Delmarva Peninsula in 2001.—Continued

[Degradation products are in italics; CAS, Chemical Abstracts Service; μg/L, micrograms per liter; ESA, ethanesulfonic 

acid; OA, oxanilic acid; --, no CAS registry number available]

Organic analyte CAS registry number® Analytical method reporting level 

(μg/L)

Propargite 2312-35-8 0.023

Propham 122-42-9 0.0096

Propiconazole 60207-90-1 0.021

Propoxur 114-26-1 0.008

Propyzamide 23950-58-5 0.0041

Siduron 1982-49-6 0.016

Simazine 122-34-9 0.005

Sulfometuron-methyl 74222-97-2 0.0088

Tebuthiuron 34014-18-1 0.016

Terbacil 5902-51-2 0.0098

Terbufos 13071-79-9 0.017

Thiobencarb 28249-77-6 0.0048

Tri-allate 2303-17-5 0.0023

Tribenuron-methyl 101200-48-0 0.0088

Triclopyr 55335-06-3 0.022

Trifluralin 1582-09-8 0.009

Appendix C  43
Appendix D. Pesticides analyzed in samples from the surficial 

aquifer on the Delmarva Peninsula in 1988.

[μg/L, microgram per liter; CAS, Chemical Abstracts Service]

Organic analyte CAS registry 

number®

Analytical method 

reporting level

(μg/L)

2,4,5-T 93-76-5 0.01

2,4-D 94-75-7 0.01

Dichlorprop 120-36-5 0.01

Alachlor 15972-60-8 0.1

Ametryn 834-12-8 0.1

Atrazine 1912-24-9 0.1

Carbaryl 63-25-2 0.5

Cyanazine 21725-46-2 0.1

Dicamba 1918-00-9 0.01

Methomyl 16752-77-5 0.05

Metolachlor 51218-45-2 0.1

Metribuzin 21087-64-9 0.1

Picloram 1918-02-1 0.01

Prometon 1610-18-0 0.1

Prometryn 7287-19-6 0.1

Propazine 139-40-2 0.1

Propham 122-42-9 0.05

Silvex 93-72-1 0.01

Simazine 122-34-9 0.1

Simetryn 1014-70-6 0.1

Trifluralin 1582-09-8 0.1

44  Factors Affecting Variability in Nutrient and Pesticide Concentrations in the Surficial Aquifer on the Delmarva Peninsula
Prepared by Publishing Service Centers 3 and 1.

Edited by Valerie M. Gaine.

Graphics by Timothy W. Auer.

Layout by Ann Marie Squillacci.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, MD 21237

or visit our Web site at:

http://md.water.usgs.gov