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 waterresource 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
This Page Intentionally Left Blank
v

CONTENTS

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

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

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

Purpose and scope .........................................................................................................................................................2

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

The Delmarva Peninsula ........................................................................................................................................................3

The upper Pocomoke River ...........................................................................................................................................3

Chesterville Branch .......................................................................................................................................................5

Data collection and analysis...................................................................................................................................................5

Data collection...............................................................................................................................................................7

Data analysis................................................................................................................................................................12

Water chemistry in the upper Pocomoke River and Chesterville Branch Watersheds.........................................................12

Ground water in the upper Pocomoke River Watershed .............................................................................................12

Ground water in the Chesterville Branch Watershed ..................................................................................................13

Stream chemistry in the upper Pocomoke River and Chesterville Branch..................................................................14

Hydrologic and geochemical controls on the transport of pesticides and nutrients.............................................................19

The upper Pocomoke River .........................................................................................................................................19

Chesterville Branch .....................................................................................................................................................26

Summary and conclusions....................................................................................................................................................30

References cited ...................................................................................................................................................................31

Figures

 1–2. Maps showing:

1. The location of the Delmarva Peninsula, and Chesterville Branch and upper

Pocomoke River Watersheds ..................................................................................................................4

2. Land use and land cover in the upper Pocomoke River Watershed and vicinity and 

line of section A–A′ ................................................................................................................................6

3. Hydrogeologic cross section A–A′ in the vicinity of the upper Pocomoke River Watershed ..............................7

4. Map showing land use and land cover in the Chesterville Branch Watershed and vicinity and 

line of section B–B ′ ........................................................................................................................................8

5. Hydrogeologic cross section B–B ′ through the Chesterville Branch Watershed and vicinity..............................9

6–26. Graphs showing:

6. Summary of stream and ground-water chemistry in the Chesterville Branch Watershed.........................14

7. Summary of stream and ground-water chemistry in the upper Pocomoke River Watershed....................15

8. Concentrations of selected pesticides and metabolites in the upper Pocomoke River, 1999–2001 ..........18

9. Concentrations of selected pesticides and metabolites in Chesterville Branch, 1999–2001.....................19

10. Relation of the second principal component of stream chemistry (POCOPC2) to

streamflow in the upper Pocomoke River, 1999–2001 .........................................................................20

11. Relation of phosphorus and suspended-sediment concentrations to streamflow in the

upper Pocomoke River, 1999–2001 ......................................................................................................21

12. Relation of the first principal component of stream chemistry (POCOPC1) to

streamflow in the upper Pocomoke River, 1999–2001 .........................................................................22
vi

Figures—Continued

6–26. Graphs showing—Continued

13. Relation of alachlor and selected metabolites to streamflow in the

upper Pocomoke River, 1999–2001 ......................................................................................................22

14. Relation of metolachlor and selected metabolites to streamflow in the

upper Pocomoke River, 1999–2001 ......................................................................................................22

15. Seasonal variability in the first principal component of stream chemistry

(POCOPC1) in the upper Pocomoke River, 1999–2001 .......................................................................24

16. Seasonal variability in bicarbonate concentrations in the upper Pocomoke River, 1999–2001................24

17. Seasonal variability in concentrations of atrazine and metolachlor in the

upper Pocomoke River, 1999–2001 ......................................................................................................24

18. Relation of instantaneous loads of selected pesticides to streamflow in the

upper Pocomoke River, 1999–2001 ......................................................................................................26

19. Relation of the first principal component of stream chemistry (CHESPC1)

to streamflow in Chesterville Branch, 1997–2001 ................................................................................26

20. Seasonal variability in the second principal component of stream chemistry

(CHESPC2) in Chesterville Branch, 1997–2001 ..................................................................................27

21. Relation of phosphorus and suspended-sediment concentrations to streamflow

in Chesterville Branch, 1996–2001 .......................................................................................................28

22. Seasonal variability in the first principal component of stream chemistry

(CHESPC1) in Chesterville Branch, 1997–2001 ..................................................................................28

23. Seasonal variability in atrazine and metolachlor concentrations in

Chesterville Branch, 1999–2001 ...........................................................................................................29

24. Relation of concentrations of selected pesticides to streamflow in

Chesterville Branch, 1999–2001 ...........................................................................................................29

25. Relation of instantaneous loads of selected pesticides to streamflow in

Chesterville Branch, 1999–2001 ...........................................................................................................29

26. Concentrations of pronamide in Chesterville Branch, 1999–2001............................................................30

Tables

1. Estimated uncertainty in reported concentrations of major ions, suspended sediment,

nutrients, and pesticides in Chesterville Branch near Crumpton, Maryland (1996–2001)

and the upper Pocomoke River near Willards, Maryland (1999–2001)............................................................10

2. Summary of analytical results for samples collected from the upper Pocomoke River (1999–2001)

and Chesterville Branch (1996–2001) and analyzed for nutrients or pesticide compounds .............................16

3. Loadings on the first two components from unrotated principal components analysis on

major-ion and nutrient data collected from the upper Pocomoke River, 1999–2001........................................20

4. Summary of streamflow characteristics for the upper Pocomoke River near Willards, Maryland

and Chesterville Branch near Crumpton, Maryland, July 1996 through September 2001................................21

5. Results of comparisons of concentrations of selected pesticides and relative proportions of

parent and daughter compounds in the upper Pocomoke River and Chesterville Branch to

streamflow and among different seasons, 1999–2001.......................................................................................23

6. Results of comparisons of stream chemistry in the upper Pocomoke River (1999–2001)

and Chesterville Branch (1996-2001) to streamflow and among different seasons..........................................25

7. Loadings on the first two components from unrotated principal components analysis on

major-ion and nutrient data collected from Chesterville Branch, 1997–2001 ..................................................27
vii

Conversion Factors and Vertical Datum

_____________________________________________________________________________________________________________________________________

Multiply By To obtain

_____________________________________________________________________________________________________________________________________

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2 ) 259.0 hectare

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) 448 gallon per minute (gal/min)

Mass

Pound, avoirdupois (lb) 0.4536 kilogram (kg)

Load

microgram per second (μg/s) 0.0036 gram per hour (g/h)

_____________________________________________________________________________________________________________________________________

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

°

F = (1.8 x °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 (NAD83).

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).

.
This Page Intentionally Left Blank
Abstract 1

Hydrologic and Geochemical Controls on Pesticide and Nutrient 

Transport to Two Streams on the Delmarva Peninsula

By Scott W. Ator, Judith M. Denver, and Michael J. Brayton

Abstract

Pesticides and nutrients move from application 

areas through ground water and surface runoff to 

streams on the Delmarva Peninsula. The relative 

importance of different transport media to the 

movement of these compounds in different watersheds is related to locally variable hydrologic and 

geochemical conditions among areas of regionally 

similar land use, geology, and soils. Consideration 

of such local variability is important to land-management efforts or future environmental investigations on the Peninsula.

Chemical analyses of samples collected over 

a multiyear period from two streams on the

Delmarva Peninsula were analyzed along with 

similar available analyses of ground water to document the occurrence of pesticides and nutrients, 

and illustrate important processes controlling their 

movement through watersheds to streams. The 

upper Pocomoke River and Chesterville Branch 

drain predominantly agricultural watersheds typical of the Delmarva Peninsula. Chesterville 

Branch drains a watershed of moderate relief, 

good drainage, and a permeable surficial aquifer 

that ranges in thickness from about 15 to 25 

meters. The upper Pocomoke River Watershed, 

however, is extremely flat with poorly drained 

soils and abundant artificial drainage. Influences 

on the chemistry of water in each stream were 

determined from seasonal patterns in the concentrations of selected constituents from 1996 through 

2001, and relations with streamflow.

Nutrients and pesticides are detectable throughout the year in the upper Pocomoke River and 

Chesterville Branch. Water in both streams is

typically dilute, slightly acidic, and well oxygenated, and nitrate and phosphorus concentrations 

generally exceed estimated natural levels. Pesticide concentrations are generally low, although 

concentrations of selected metabolites commonly 

exceed 1 microgram per liter, particularly in

Chesterville Branch. Nitrate and metabolites of 

pesticide compounds are apparently transported to 

Chesterville Branch preferentially through ground 

water in the surficial aquifer, although selected 

pesticide parent compounds and less soluble nutrients move primarily in surface runoff. Conversely, the relative proportion of discharge from 

surficial and partially confined aquifers is the most 

important factor controlling the chemistry of water 

in the upper Pocomoke River. Surface runoff in 

the larger and predominantly flat upper Pocomoke 

River Watershed is apparently limited to particularly significant precipitation events. Transport of 

pesticides in surface runoff becomes important in 

both watersheds during such events. Instantaneous loads of pesticides in streams typically 

stabilize or continue to increase with increasing 

flow even after runoff begins, although in-stream 

concentrations may decrease due to dilution.

Introduction

Synthetic organic pesticides are present in streams and 

aquatic ecosystems in many parts of the United States and 

the world (Larson and others, 1997; Fuhrer and others, 1999; 

Nowell and others, 1999; Capel and others, 2001). Many 

streams also contain nutrients (including nitrogen and phosphorus compounds) at concentrations exceeding natural 

levels (Fuhrer and others, 1999). Although pesticide and 

nutrient applications are useful for many purposes, excessive 

levels of these compounds in the environment may cause a 

variety of adverse ecological or human-health effects. An 

understanding of the hydrologic, geochemical, and other 

factors affecting the transport of pesticides and nutrients 

from areas of application to streams and other surface-water 

bodies is fundamental to the safe and effective use of these 

chemicals.

The occurrence and distribution of pesticides and nutrients in streams are related to physical and chemical transport 
2 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

and transformation processes as well as application rates 

(Larson and others, 1997; Capel and others, 2001). Pesticides and nutrient compounds are applied for a variety of 

agricultural, commercial, and domestic purposes (Puckett, 

1995; Aspelin, 1997). These compounds can move from 

areas of application to streams in several ways, including 

direct discharge from point sources, direct fallout in precipitation or from spraying, overland flow in storm runoff, or 

through ground water or the unsaturated zone. The time of 

travel from the original source to a stream is a function of the 

dominant physical transport mechanisms and potential transformation processes that may alter the original compound. 

During transport, nutrients and pesticides can be transformed 

to other related compounds through physical, chemical, or 

biological processes (Larson and others, 1997; Capel and 

others, 2001).

The movement of nutrients and pesticides in the environment is related to the mobility and persistence of each individual compound, as well as local hydrologic, geologic, 

atmospheric, and soil conditions. Although relatively soluble compounds such as nitrate and metolachlor can move in 

streams and ground water, less soluble chemicals such as 

phosphorus often attach to soil particles and may only move 

in overland runoff or streams during periods of relatively 

high flow (Fuhrer and others, 1999). Airborne transport may 

be considerable for volatile compounds or those that are dispersed during application (Puckett, 1995; Majewski and 

others, 1998). Streams are particularly vulnerable to contamination as they may carry surface runoff from urban and 

agricultural areas as well as ground-water discharge (Larson 

and others, 1997). The persistence of nutrients in water 

depends largely on biologic uptake, chemical transformations, and sedimentation. Nitrate, for example, is extremely 

soluble but may be converted to nitrous oxide or nitrogen gas 

through denitrification by anaerobic bacteria. Pesticides, by 

design, are not incorporated as easily into non-target living 

plants and animals. Although some pesticides can remain in 

the environment for years, many newer compounds are 

designed to decompose more rapidly (Fuhrer and others, 

1999). Newer pesticides are generally more soluble and 

therefore more mobile and less likely to adhere to soil and 

sediment (Larson and others, 1997).

Although nitrogen and phosphorus are essential for 

healthy plant and animal populations, excessive nutrients 

can be damaging to aquatic ecosystems. Algal blooms 

caused by surplus nitrogen or phosphorus may block sunlight from the water column and disrupt photosynthesis in 

submerged aquatic plants. The biochemical oxygen demand 

(BOD) from the subsequent death of the algae may consume 

dissolved oxygen in the stream and cause eutrophication 

(Fuhrer and others, 1999). In 1999, the Chesapeake Bay was 

listed as “impaired” under the Clean Water Act due to harmful effects of excess nutrients and sediment (Phillips, 2002).

 Pesticides can also cause unintended harmful effects on 

aquatic ecosystems and human health. Possible humanhealth effects from overexposure to pesticides include 

cancer, reproductive and nervous-system disorders, and 

acute toxicity (Fuhrer and others, 1999). Recent studies 

have shown that low concentrations of herbicides may disrupt endocrine and hormonal activity in certain amphibians 

and other aquatic species (Hayes and others, 2002). It is also 

possible for some pesticides to bioaccumulate in fish and 

other organisms, and to move along the food chain and thus 

disturb the ecosystem at a larger scale (Nowell and 

others, 1999).

Purpose and Scope

Hydrologic and geochemical factors affecting the transport of pesticides and nutrient compounds to two streams 

in different environmental settings on the Delmarva 

Peninsula are described in this report. Major-ion and nutrient concentrations in samples collected from the nontidal 

upper Pocomoke River and Chesterville Branch in eastern 

Maryland (fig. 1) were analyzed to identify major factors 

affecting the variability of water chemistry in each stream. 

Stream chemistry was also compared to the known chemistry of ground water in upstream aquifers to determine the 

dominant sources of flow to each stream in different seasons 

and under different flow conditions. Periods of significant 

surface runoff were determined from streamflow records and 

concentrations of suspended sediment and relatively insoluble compounds. Concentrations of selected herbicides, 

herbicide metabolites, and nutrient compounds in these 

streams were examined to identify natural factors affecting 

their transport through watersheds in different environmental 

settings on the Peninsula. Implications of these natural 

hydrologic and geochemical factors for agricultural and 

other land-management practices are also discussed.

Analyses presented in this report are based largely on 

data collected from the mid–1990s through 2001. Specific 

conductance, pH, and concentrations of dissolved oxygen, 

silica, calcium, magnesium, sodium, potassium, chloride, 

sulfate, bicarbonate, iron, and manganese (hereafter collectively referred to as “major ions”) were measured along 

with suspended sediment and selected total and dissolved 

nitrogen and phosphorus species in samples collected at 

one site above the head of tide between 1996 and 2001 in 

Chesterville Branch and 1999 and 2001 in the upper 

Pocomoke River in eastern Maryland. Pesticide analyses are 

based on samples collected between 1999 and 2001 from 

each stream.

Acknowledgments

This report is a product of the National Water-Quality 

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

Survey (USGS) (Gilliom and others, 1995), which is 

intended (in part) to provide an understanding of the natural 

and human processes affecting stream chemistry in different 

parts of the United States (see Foreword). Deborah 

Bringman, David Brower, Catherine Bozek, and Julie 

Matlaga collected most of the field samples. Technical 

reviews of this report were provided by A. Scott Andres of 

the Delaware Geological Survey and John McCoy of the 

Maryland Department of Natural Resources. Valerie Gaine 

of the USGS provided the editorial review; Mark Nardi, 
The Delmarva Peninsula 3

Betzaida Reyes, and Timothy Auer of the USGS provided 

help with illustrations, and Donna Knight of the USGS prepared the report layout. Thanks are also due to several property owners who provided access to sampling sites, without 

whom this work would have been impossible.

The Delmarva Peninsula

The Delmarva Peninsula is located within the Coastal 

Plain Physiographic Province and includes most of the 

State of Delaware, and parts of Maryland and Virginia east 

of the Chesapeake Bay (fig. 1). The Peninsula is characterized by flat and gently rolling topography. The drainage network includes a series of streams that originate in the central 

upland and become tidal within 16 km (kilometers) downstream (Shedlock and others, 1999). The largest nontidal 

watershed is less than 300 km2 (square kilometers), and most 

watersheds are less than 26 km2 in area. Land use on the 

Peninsula is predominantly rural; only about 7 percent of the 

land is urbanized (Denver and others, 2004 ). About half 

(48 percent) of the Peninsula is used for agriculture; much of 

the agricultural land is used to grow corn and soybeans for 

poultry feed. More than 600 million broiler chickens were 

produced on the Peninsula in 2000 (Garbarino and others, 

2003).

The Delmarva Peninsula is underlain by a wedge of 

unconsolidated sediments that includes a series of confined 

aquifers and associated confining beds overlain by an extensive surficial aquifer that is unconfined over most of the 

Peninsula. The surficial aquifer is an important source of 

water supply, recharge to underlying confined aquifers, and 

base flow to streams. It is composed mostly of quartz and 

other minerals that are generally resistant to weathering 

(Cushing and others, 1973). The surficial aquifer is generally sandy at the surface in the central part of the Peninsula 

where it is composed mostly of fluvial and marginal marine 

sediments, and finer grained near the coast in areas dominated by tidal marsh and wetland sediments.

Patterns in sedimentation and geomorphology on the 

Delmarva Peninsula have resulted in several different hydrogeologic settings. These settings have been delineated on 

the basis of geomorphology and the thickness and texture of 

surficial sediments and underlying confining beds at several 

different scales by different methods (Shedlock and others, 

1993; Bachman and others, 1998; Ator and others, in press). 

The general hydrogeologic conditions described by each 

method are similar, and range from well-drained settings 

with incised stream channels and surficial aquifers as thick 

as tens of meters, to poorly drained settings with flat topography and ditched stream channels overlying surficial sands. 

In the northern and central part of the Peninsula, stream 

channels may incise through the surficial aquifer into underlying confined aquifers and confining beds. On the flanks of 

the Peninsula, the surficial aquifer is thin or absent, topography is very flat, and stream channels and ditches are commonly tidal.

The typically sandy unconfined surficial aquifer on the 

Delmarva Peninsula is vulnerable to anthropogenic 

contamination from a variety of sources, including septicsystem discharges and applications of fertilizer, pesticides, 

lime, and manure. Hamilton and others (1993) found concentrations of nitrate1 above 3 mg/L (milligrams per liter) in 

unconfined ground water from most areas of the Peninsula; 

the highest concentrations were beneath agricultural areas. 

Nitrate concentrations have increased since the late 1980s at 

moderate depths in well-oxygenated parts of the surficial 

aquifer, although concentrations remain very low in poorly 

oxygenated ground water (Denver and others, 2004 ). Very 

low concentrations of herbicides (below Federal drinkingwater standards), primarily those associated with corn and 

soybean crops, have been measured in shallow ground water 

near agricultural fields, as well as in nontidal streams during 

base-flow conditions (Koterba and others, 1993; Denver and 

others, 2004).

The Upper Pocomoke River

The upper Pocomoke River drains a watershed of 

approximately 155 km2 above the USGS stream gage near 

Willards, Maryland (fig. 2). Elevations in the watershed 

range from about 8 to 24 m (meters) above sea level, and the 

topography is generally flat with occasional sand dunes. 

Soils are generally moderately permeable but poorly drained 

(U.S. Department of Agriculture, 1970) and the water table 

is shallow (generally less than 2 m below land surface during 

wet periods). Tributaries are low gradient with sluggish flow 

and are typically channelized. Ditches to promote drainage 

of agricultural fields are common; the Pocomoke River 

Watershed (including areas downstream of Willards, 

Maryland) contains an estimated 1,930 km of artificial ditching (Bricker and others, 2003). 

Land use in the upper Pocomoke River Watershed is 

45 percent agricultural with interspersed forest and swamps 

(Vogelmann and others, 1998). Corn, soybeans, and small 

grains are the predominant crops, and poultry farms are 

located throughout the watershed. In 1997, more than 

320 million broiler chickens were produced in Sussex 

County, Delaware and Wicomico and Worcester Counties, 

Maryland (the three counties in the watershed) (U.S. Department of Agriculture, 1999). Manure generated from the 

intensive poultry production is typically applied to nearby 

agricultural fields; nutrient application rates in the area are 

among the highest in the Chesapeake Bay Watershed 

(Brakebill and Preston, 1999). Recent estimates of annual 

nitrogen and phosphorus applications in the form of manure 

for parts of the Pocomoke River Watershed are greater than 

6 million pounds for nitrogen and 1.5 million pounds for 

phosphorus (Brakebill and Preston, 1999). An additional 

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

measurable nitrite.
4 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula
Data Collection and Analy sis 5

estimated 2 million pounds of nitrogen and 1.5 million 

pounds of phosphorus are applied annually in the form of 

commercial fertilizers. The most commonly used agricultural pesticides in Wicomico and Worcester Counties (which 

contain the Maryland part of the upper Pocomoke River 

Watershed) include alachlor, atrazine, glyphosate, metolachlor, oxyfluorfen, and simazine (Maryland Department of 

Agriculture, 1999).

The surficial aquifer underlying the upper Pocomoke 

River Watershed is complex and heterogeneous (fig. 3) 

(Owens and Denny, 1978, 1979; Ramsey and Schenck, 

1990; Andres and Howard, 2000, 2002). It includes the 

Beaverdam Sand (mostly sand with thin beds of gravel at its 

base), overlain by 3 to 6 m of discontinuous clay, silt, and 

some peat of the Walston Silt and Omar Formation, which 

form confining units where present. These deposits are in 

turn overlain by 3 to 6 m of the Parsonsburg Sand, an eolian 

sandy unit interspersed with clay and silt. Organic-rich 

loamy sand and silt are present in shallow parts of the

Parsonsburg Sand (Owens and Denny, 1978, 1979). Surficial sediments in the primarily swampy northeastern part of 

the watershed include fine sand, silt, clayey silt, and organic 

matter of the Cypress Swamp Formation (Andres and 

Howard, 2000). 

The aquifer configuration and the extensive network of 

drainage ditches result in complex patterns of ground-water 

flow that change seasonally as the water table rises and falls. 

Flow paths are generally highly localized and short in the 

Parsonsburg Sand, and longer in the Beaverdam Sand 

(Patrick Phillips, U.S. Geological Survey, written commun., 

1991; Hamilton and others, 1993). When the water table is 

high, short flow paths to drainage ditches comprise an 

important part of streamflow. During drier periods, when the 

ditches are dry or flow is stagnant, longer flow paths in the 

Parsonsburg Sand and the Beaverdam Sand maintain base 

flow in the Pocomoke River and its major tributaries. 

Ground water is generally 35 to 50 years old in the 

Beaverdam Sand, but less than 15 years old in the 

Parsonsburg Sand (Dunkle and others, 1993).

Chesterville Branch

Chesterville Branch drains an approximately 15-km2 

watershed and flows directly into the tidal part of the 

Chester River (fig. 4). Elevations range from less than 6 to 

greater than 27 m above sea level, with broad, gently sloped 

uplands and deeply incised stream channels. Soils are typically moderately well-drained to well-drained on the 

uplands, with moderately poorly drained soils in narrow 

flood plains. The water table ranges from near the land surface in the stream channels to about 6 m below land surface 

on upland watershed divides.

Land use in the Chesterville Branch Watershed is about 

93 percent agricultural, of which about 21 percent is cultivation of plant nursery stock. The remaining agricultural land 

is used for corn, soybean, and small grain production. Forested areas comprise the remaining 7 percent and occur primarily in narrow bands in riparian zones (Vogelmann and 

others, 1998). Although nutrient applications to agricultural 

areas of the Chesterville Branch Watershed are unknown, 

application rates for the larger Chester River Watershed are 

generally much lower than for parts of the Pocomoke River 

Watershed. Estimated annual applications of nitrogen and 

phosphorus from manure are less than 0.5 and 0.2 million 

pounds (respectively), and estimated applications of commercial fertilizer total 0.5 to 2 million pounds of nitrogen, 

and 0.2 to 0.5 million pounds of phosphorus (Brakebill and 

Preston, 1999). The pesticides most commonly used for 

agriculture in Kent County, Maryland (which includes the 

Chesterville Branch Watershed) include acetochlor, atrazine, 

glyphosate, metolachlor, and simazine (Maryland Department of Agriculture, 1999).

The surficial aquifer underlying the Chesterville Branch 

Watershed includes deeply weathered sands and gravels of 

the Pensauken Formation (the Columbia aquifer) and the 

underlying weathered glauconitic sands of the upper Aquia 

Formation (the Aquia aquifer) (fig. 5) (Hamilton and others, 

1993; Bachman and others, 2002). The surficial aquifer may 

also contain fine to medium marine sand of the Old Church 

Formation (fig. 5) (Hansen, 1992), although these sediments 

are typically very thin within the Chesterville Branch Watershed (Bachman and others, 2002). The lower part of the 

Aquia Formation contains a low permeability silt-clay that 

forms a 6- to 10-m confining layer (Bachman and others, 

2002). The uppermost confined aquifer beneath the confining layer is the Hornerstown aquifer, which includes glauconitic sand in the upper part of the Hornerstown Formation 

(Hansen, 1992) (also known as the Hornerstown Sand, 

Owens and Minard, 1962). The surficial aquifer ranges in 

thickness from about 15 m at the northern edge of the watershed to about 25 m near Crumpton, Maryland (Bachman and 

others, 2002).

The surficial aquifer is unconfined throughout the watershed. Ground water in the aquifer ranges in age from several 

years near the water-table surface to greater than 30 years 

near the base of the aquifer in ground-water discharge areas 

(Dunkle and others, 1993). Chesterville Branch is incised 

into the surficial aquifer and perennial throughout the watershed. Water in the confined Hornerstown aquifer is consistently greater than 40 years old (Bachman and others, 2002). 

Data Collection and Analysis

Water samples were collected to characterize the chemical composition of selected streams on the Delmarva Peninsula and to describe the variability of stream chemistry 

during different seasons and flow conditions. Data analyses 

were selected to identify major factors affecting the variability of water chemistry in each stream, including major transport processes in each watershed, and dominant sources of 

flow to each stream in different seasons and under different 

flow conditions. 
6 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula
Data Collection and Analy sis 7

Data Collection

Water-quality data were collected periodically by the 

USGS from the late 1990s through September 2001 from 

the upper Pocomoke River near Willards, Maryland, and 

Chesterville Branch near Crumpton, Maryland. One 

sampling location was selected on each stream near an existing stream gage above the head of tide. The frequency of 

sampling at each stream varied slightly over time depending 

on the objectives and design of participating projects and 

programs. Routine sampling of the upper Pocomoke River 

began in June 1999 as part of the NAWQA Program. Sampling was conducted approximately monthly until March 

2001; subsequent samples were collected twice a month. 

Relatively intensive sampling of Chesterville Branch for 

nutrients and (less frequently) suspended sediment and 

major ions began in July 1996 as part of the River Input 

Monitoring project, a cooperative effort between the USGS 

and the State of Maryland to quantify nutrient loads to 

Chesapeake Bay (Sprague and others, 2000). Samples were 

typically collected at least monthly; additional samples 

were collected during selected high-flow events to support 

calibration of empirical load models (Smith and others, 

1997; Yochum, 2000). Additional samples were collected 

at Chesterville Branch between February 1999 and 

September 2001 as part of the NAWQA Program and the 

Landscape Indicators for Pesticides Study (LIPS), a cooperative effort between the USGS and the U.S. Environmental 

Protection Agency to develop empirical models predictive of 

pesticide occurrence in streams (Pitchford and others, 2000). 

Approximately two samples per month were collected from 

Chesterville Branch through October 1999 as part of these 

programs; subsequent sampling was conducted monthly. 

NAWQA and LIPS samples from both streams were analyzed for selected pesticides and pesticide metabolites, as 

well as nutrients and major ions.

Stream samples were collected, processed, and analyzed 

using procedures designed to minimize contamination

while ensuring results representative of sampled conditions 

(Shelton, 1994). Specific conductance, dissolved oxygen, 

and pH were measured directly in the stream (Wilde and 

Radtke, 1998). Instantaneous streamflow (discharge) during 

sample collection was also measured in the stream (Rantz 

and others, 1982) or noted on existing stream gages at each 

site (U.S. Geological Survey, 1950–2001). Depth-integrated 

water samples were collected at equal-width intervals along 

a cross section of the stream perpendicular to flow and 

combined into a single composite. Subsamples for analysis 

of dissolved constituents were passed through a 0.45-μm 
8 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula
Data Collection and Analy sis 9

(micrometer) cellulose-nitrate capsule filter (for inorganic 

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

and metabolites). Subsamples intended for analysis of 

selected nutrients and cations were preserved with sulfuric 

acid and nitric acid, respectively. Bicarbonate concentrations were computed on the basis of alkalinity titrations conducted in the field (Wilde and Radtke, 1998). Suspended 

sediment concentrations were measured as described in 

Sholar and Shreve (1998). All other subsamples were 

chilled to 4 degrees Celsius and shipped overnight to the 

USGS Organic Geochemistry Research Laboratory in 

Lawrence, Kansas (for analyses of selected pesticide metabolites; Zimmerman and others, 2000; Lee and others, 2001) 

or the USGS National Water-Quality Laboratory in Denver, 

Colorado (for all other analyses; Zaugg and others, 1995). A 

mild detergent solution and deionized water were used to 

clean all reusable sampling equipment between sites (Wilde 

and others, 1998).

Field blanks, split-replicates, and (for pesticides) field 

spikes (Mueller and others, 1997) were collected and analyzed to estimate bias and variability in reported analyte concentrations (table 1). Analyses of field blanks suggest that 

reported concentrations contain minimal bias due to contamination during sample collection, processing, or shipment. 

Among pesticides, only metolachlor was detected in field 

blanks, and concentrations were invariably less than the laboratory reporting level of 0.013 μg/L (micrograms per liter). 

Concentrations of major ions and nutrients in field blanks 

were similarly insignificant when compared to routine laboratory reporting levels. Median spike recoveries for pesticides were generally near 100 percent; reported recoveries 

for only 8 of 47 compounds were less than 70 percent or 

greater than 130 percent. Reported concentrations for these 

compounds may substantially differ from true environmental 

concentrations. Sample matrix effects or other reasons for 

poor analytical recovery are well documented for some of 

these compounds (Zaugg and others, 1995). Variability in 

sampling and analytical results estimated on the basis of 

replicate analyses is also generally minimal, particularly for 

major ions and nutrients. Estimated variability in reported 

pesticide concentrations is typically greater (often more than 

10 percent), although these estimates were generated from 

only two to four analyses (table 1). A review of a much 

larger set of replicate samples collected from around the 

United States as part of the NAWQA Program (Martin, 

2002) may be more indicative of the reliability of reported 

pesticide concentrations. Further information on variability 

in reported concentrations of pesticides and selected metabo-
10 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

 

Table 1. Estimated uncertainty in reported concentrations of major ions, suspended sediment, 

nutrients, and pesticides in Chesterville Branch near Crumpton, Maryland 

(1996–2001) and the upper Pocomoke River near Willards, Maryland (1999–2001)

[BN, number of field blanks; BND, number of blanks in which compound was detected; BMAX, maximum reported concentration in

field blanks in which compound was detected; SN, number of field spikes; SMED , median recovery from field spikes; %, percent;

SIQR, interquartile range of recovery from field spikes; RN, number of replicate sets; RNAGREE, number of replicate sets within which 

compound was consistently detected or not detected; RUNC, estimated uncertainty in reported concentrations; NC, not computed because 

no replicate sets had consistent detected concentrations; N, nitrogen; ESA, ethanesulfonic acid; OA, oxanilic acid; MET, metabolite]

Compound or Ion

Contamination Bias Sampling and Analytical Uncertainty

 BN BND BMAX 1 SN

SMED

 (%)

SIQR

 (%)

 RN RNAGREE

 RUNC2

 (%)

Calcium 7 4 0.03 0 - - 9 9 0.3

Magnesium 7 0 - 0 - - 9 9 0.1

Sodium 7 2 0.05 0 - - 9 9 0.5

Potassium 7 0 - 0 - - 9 9 0.8

Silica 11 3 0.14 0 - - 13 13 0.6

Sulfate 7 0 - 0 - - 9 9 0.4

Chloride 7 0 - 0 - - 9 9 1.2

Suspended Sediment 2 0 - 0 - - 13 13 5.7

Iron 7 1 6.6 0 - - 9 9 5.4

Manganese 11 0 - 0 - - 10 10 0.3

Ammonia plus Organic N 13 2 0.06 0 - - 17 17 2.6

Ammonia plus Organic N, total 13 3 0.16 0 - - 17 17 8.1

Nitrate 13 3 0.03 0 - - 17 17 1.2

Phosphorus 13 3 0.002 0 - - 17 17 8.6

Phosphorus, total 13 3 0.001 0 - - 17 15 1.6

Acetochlor 5 0 - 7 113 10 4 4 6.3

Acetochlor ESA (MET) 2 0 - 0 - - 2 2 34.6

Acetochlor OA (MET) 2 0 - 0 - - 2 2 17.7

Alachlor 5 0 - 7 119 13 4 4 2.4

Alachlor ESA (MET) 2 0 - 0 - - 2 2 4.8

Alachlor OA (MET) 2 0 - 0 - - 2 2 5.5

2, 6-diethylaniline (MET) 5 0 - 7 90 13 4 4 NC

Alpha BHC 5 0 - 7 92 29 4 4 NC

Atrazine 5 0 - 7 103 8 4 4 1.3

Desethylatrazine (MET) 5 0 - 7 73 22 4 4 1.7

Benfluralin 5 0 - 7 78 29 4 4 NC

Butylate 5 0 - 7 103 8 4 4 NC

Carbaryl 5 0 - 7 169 193 4 4 NC

Carbofuran 5 0 - 7 178 203 4 4 21.1

Chlorpyrifos 5 0 - 7 100 21 4 4 6.3

Cyanazine 5 0 - 7 126 23 4 4 NC

DCPA 5 0 - 7 101 15 4 4 NC

Diazinon 5 0 - 7 104 21 4 4 NC

Dieldrin 5 0 - 7 108 32 4 4 NC

Disulfoton 5 0 - 7 62 35 4 4 NC

EPTC 5 0 - 7 105 6 4 4 NC

Ethalfluralin 5 0 - 7 90 36 4 4 NC

Ethoprop 5 0 - 7 107 18 4 4 NC

Fonofos 5 0 - 7 104 26 4 4 NC
Data Collection and Analy sis 11

 

Table 1. Estimated uncertainty in reported concentrations of major ions, suspended sediment, 

nutrients, and pesticides in Chesterville Branch near Crumpton, Maryland 

(1996–2001) and the upper Pocomoke River near Willards, Maryland (1999–2001)

—Continued

Compound or Ion

Contamination Bias Sampling and Analytical Uncertainty

 BN BND BMAX1 SN

SMED

 (%)

SIQR

 (%)

 RN RNAGREE

 RUNC2

 (%)

Lindane 5 0 - 7 97 26 4 4 NC

Linuron 5 0 - 7 135 45 4 4 NC

Malathion 5 0 - 7 127 17 4 4 NC

Methyl-azinphos (MET) 5 0 - 7 201 40 4 4 NC

Metolachlor 5 3 0.011 7 114 22 4 4 1.1

Metolachlor ESA (MET) 2 0 - 0 - - 2 2 6.4

Metolachlor OA (MET) 2 0 - 0 - - 2 2 7.3

Metribuzin 5 0 - 7 108 22 4 4 NC

Molinate 5 0 - 7 108 11 4 4 NC

Napropamide 5 0 - 7 121 23 4 4 NC

p, p’-DDE (MET) 5 0 - 7 59 13 4 4 NC

Parathion 5 0 - 7 108 46 4 4 NC

Methyl-parathion (MET) 5 0 - 7 102 37 4 4 NC

Pebulate 5 0 - 7 105 6 4 4 NC

Pendimethalin 5 0 - 7 107 25 4 4 NC

Permethrin 5 0 - 7 53 12 4 4 NC

Phorate 5 0 - 7 79 28 4 4 NC

Prometon 5 0 - 7 97 10 4 4 1.5

Pronamide 5 0 - 7 105 16 4 4 9.6

Propachlor 5 0 - 7 128 13 4 4 NC

Propanil 5 0 - 7 112 18 4 4 NC

Propargite 5 0 - 7 127 20 4 4 NC

Simazine 5 0 - 7 109 28 4 4 0.9

Tebuthiuron 5 0 - 7 139 23 4 4 NC

Terbacil 5 0 - 7 125 52 4 4 NC

Terbufos 5 0 - 7 84 21 4 4 NC

Thiobencarb 5 0 - 7 106 12 4 4 NC

Triallate 5 0 - 7 104 7 4 4 NC

Trifluralin 5 0 - 7 84 31 4 4 NC

1 In milligrams per liter or (for iron and metolachlor) micrograms per liter.

2 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.
12 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

lites is available in Zaugg and others (1995), Zimmerman 

and others (2000), and Lee and others (2001).

Ancillary data were compiled to support the analysis and 

interpretation of observed water chemistry in Chesterville 

Branch and the upper Pocomoke River. Ground-water 

chemistry in surficial and near-surface aquifers within or 

near each watershed has been measured at various times 

since the late 1980s (see below). Base-flow chemistry in 

selected tributaries of each stream was measured during late 

winter and spring 2000 as part of the LIPS project (Pitchford 

and others, 2000).

Data Analysis

Exploratory and nonparametric data-analysis techniques 

were selected to identify significant sources of water to 

Chesterville Branch and the upper Pocomoke River during 

different seasons and under different flow conditions, and to 

relate those sources to observed pesticide and nutrient concentrations. All statistical tests were evaluated at the

95-percent confidence level (α =0.05). Correlation among 

continuous variables was evaluated through the use of scatter 

plots and correlation (Spearman’s rho) matrices (Helsel and 

Hirsch, 1992; Conover, 1999). Rank-sum tests (large-sample approximation with correction for ties; Conover, 1999) 

were used to compare water quality in each stream during 

different seasons; samples were assigned to relatively “wet” 

(December through May) or “dry” (June through November) 

seasons on the basis of long-term streamflow records 

(U.S. Geological Survey, 1950–2001). Rank-transform analysis of variance (ANOVA) tests and rank-sum tests were also 

used to compare pesticide and metabolite concentrations 

within each stream. Data were censored to a common level 

for each compound and censored data were set to zero before 

ranks were computed for correlation, rank-sum, or ANOVA 

tests (Helsel and Hirsch, 1992). Trilinear diagrams (Piper, 

1944) were used to compare stream chemistry during different seasons to the chemistry of tributaries during late winter 

and spring base flow, and to that of ground water in different 

aquifers in the contributing watersheds.

Principal-components analysis (PCA) (Hamilton, 1992; 

Helsel and Hirsch, 1992) was used to identify underlying 

patterns in the concentrations of major ions and nutrients in 

each stream. Concentrations of different major ions and 

nutrient species are typically well correlated with each other 

and thus good candidates for PCA. Data were censored to a 

common level, and rare censored values were set to zero to 

separate them from values detected at the reporting level. 

Concentrations were ranked prior to analysis. PCA has been 

previously used to interpret stream chemistry in the Potomac 

River Watershed (Miller and others, 1997) and the 

Chesapeake Bay Watershed (Liu and others, 2000).

Available historical streamflow data (U.S. Geological 

Survey, 1950–2001) were used to estimate long-term average relative contributions of ground-water discharge and surface runoff to the upper Pocomoke River and Chesterville 

Branch. The proportion of base flow contributing to mean 

daily streamflow for each day in each stream was estimated 

on the basis of the rise and recession of streamflow in 

response to precipitation. The estimates were computed 

using the HYSEP computer program with the local-minimum method (Sloto and Crouse, 1996). Estimates computed 

using such techniques may be questionable for short time 

periods, but may be useful for comparing hydrologic conditions among similar watersheds.

Water Chemistry in the Upper Pocomoke River 

and Chesterville Branch Watersheds

The chemistry of water in the upper Pocomoke River and 

Chesterville Branch reflects the predominance of groundwater discharge from well-weathered siliciclastic surficial 

sediments and agriculture in their contributing watersheds. 

Concentrations and trends in stream chemistry are related to 

natural aquifer conditions and agricultural practices that 

affect ground-water chemistry in the contributing watersheds. Discussion of the water chemistry in the upper 

Pocomoke River and Chesterville Branch during the period 

of this study (1996–2001) begins with a review of previous 

work on ground-water chemistry in each watershed, including possible chemical changes that may occur as ground 

water discharges to each stream and its tributaries.

Hydrologic studies began in both watersheds as part of 

the NAWQA program in 1987 (Hamilton and others, 1993). 

In the upper Pocomoke River Watershed, a local-scale study 

area was established near an existing USGS stream gage on 

the Pocomoke River near Willards, Maryland. A similar 

study was established including the Chesterville Branch and 

adjacent Morgan Creek Watersheds near Locust Grove, 

Maryland. Boreholes were drilled within each watershed to 

provide information on aquifer characteristics, and wells 

were installed to allow for collection of ground-water samples for chemical analyses. Flow measurements were made 

and stream samples were also collected periodically. These 

data were analyzed in conjunction with soil, geologic, and 

land-use data to describe the effects of agriculture on 

ground- and surface-water quality in these watersheds.

Research has continued within and near both watersheds 

as part of NAWQA and other programs of the USGS 

(Dunkle and others, 1993; Reilly and others, 1994; Böhlke 

and Denver, 1995; Phillips and Donnelly, 2003), the

U.S. Environmental Protection Agency (Hantush and 

Marino, 2001; Bachman and others, 2002), and State and 

local agencies (Sprague and others, 2000). These studies 

have contributed to the understanding of the processes controlling the movement of water and chemicals in these watersheds through the interpretation of geologic data, isotopic 

tracer studies, and dating and modeling of ground-water 

flow and chemistry.

Ground Water in the Upper Pocomoke River Watershed

Shallow ground-water chemistry in the upper Pocomoke 

River Watershed is affected by geologic conditions and 

agricultural applications of fertilizer, lime, and manure. The 

natural chemistry of ground water in the Parsonsburg Sand is 
Water Chemistry in the Upper Pocomoke River and Chesterville Branch Watersheds 13

a calcium-sodium-sulfate type (Hamilton and others, 1993). 

Concentrations of silica and other ions from dissolution of 

silicate minerals are relatively low because of the short 

residence time of water in the Parsonsburg Sand. Natural 

sources of organic matter in sediments are probably a major 

source of sulfate in ground water (Puckett, 1987). Organic 

matter and fine-grained sediments in the Parsonsburg Sand 

result in variable oxidation conditions in shallow ground 

water, which affect the transport and transformation of nutrients.

Agricultural applications of inorganic fertilizer, lime, and 

poultry manure affect water chemistry in the Parsonsburg 

Sand. Nitrate concentrations range from less than 1 mg/L to 

greater than 60 mg/L in ground water (Hamilton and others, 

1993; Phillips and Donnelly, 2003). The highest concentrations are associated with samples from shallow wells in areas 

of manure application. Many of the samples with the lowest 

concentrations contain excess (nonatmospheric) dissolved 

nitrogen gas, which indicates possible nitrate removal 

through denitrification. Concentrations of phosphorus and 

dissolved ammonium and organic nitrogen are highest in 

ground water in anoxic parts of the Parsonsburg Sand. 

Chloride, calcium, and magnesium from agricultural sources 

also contribute to the major-ion chemistry of shallow ground 

water (Patrick Phillips, U.S. Geological Survey, written 

commun., 1991).

Pesticides commonly used on corn and soybean crops 

and their metabolites are present at low levels in ground 

water in the Parsonsburg Sand. Atrazine, acetochlor, 

alachlor, metolachlor, simazine, prometon, and carbofuran 

have occasionally been detected, usually at concentrations 

below 0.1 μg/L (U.S. Geological Survey, 1950–2001). 

Selected pesticide metabolites have also been detected, often 

at concentrations higher than those of corresponding parent 

compounds.

Ground water in confined parts of the Beaverdam Sand is 

generally anoxic, with high concentrations of silica, bicarbonate, and iron (Patrick Phillips, U.S. Geological Survey, 

written commun., 1991; Hamilton and others, 1993). The 

typically low concentrations of chloride, nitrate, and nitrogen gas indicate that ground water has been largely unaffected by agricultural chemicals. Concentrations of 

orthophosphate and ammonium are generally higher in the 

Beaverdam Sand than in some parts of the Parsonsburg Sand 

(U.S. Geological Survey, 1950–2001), and are likely derived 

from natural sources in the organic sediments of the overlying confining layer. Some parts of the Beaverdam Sand are 

hydraulically connected with the overlying Parsonsburg 

Sand where the confining bed is thin or absent (fig. 3). In 

these areas, ground-water chemistry is a mixture of the two 

water types (Patrick Phillips, U.S. Geological Survey, written commun., 1991). Pesticides and other agricultural chemicals have not typically been detected in water from the 

Beaverdam Sand, except in areas in hydraulic connection 

with the Parsonsburg Sand (Hamilton and others, 1993;

U.S. Geological Survey, 1950–2001).

Surface-water chemistry in the upper Pocomoke River 

Watershed is affected to varying degrees by discharge of 

ground water from the Parsonsburg and Beaverdam Sands. 

In general, as stream order increases from the ditched headwaters to the main river channel, increasing concentrations 

of sodium and silica suggest an increase in the proportion of 

flow coming from the deeper, older flow paths in the Beaverdam Sand (Patrick Phillips, U.S. Geological Survey, written 

commun., 1991). Conversely, nitrate concentrations and 

concentrations of other chemicals from agricultural sources 

are generally highest in the headwater ditches, and decrease 

downstream.

Ground Water in the Chesterville Branch Watershed

Water chemistry in the surficial aquifer reflects the influence of agriculture in the Chesterville Branch Watershed. 

Predominant dissolved ions include calcium, magnesium, 

and nitrate, which are typical of agricultural areas in sandy 

Coastal Plain sediments on the Delmarva Peninsula 

(Hamilton and others, 1993). Effects of agricultural activities are apparent in all ground-water samples collected from 

the surficial aquifer in the watershed, reflecting the fact that 

fertilizer and lime applications predate the early 1970s, the 

oldest recharge dates of most surficial ground water 

(Hamilton and others, 1993). Ground water in the surficial 

aquifer is aerobic and nitrate is stable to the base of the aquifer (Hamilton and others, 1993). Concentrations of nitrate 

are as high as 15 mg/L, with a median near 10 mg/L. There 

is evidence of denitrification in water from the deepest wells 

screened near the interface between the surficial aquifer and 

the confining bed at the base of the Aquia Formation 

(Böhlke and Denver, 1995). This may be caused by upward 

discharge of water from the confined Hornerstown aquifer to 

Chesterville Branch (Bachman and others, 2002).

Pesticides that are commonly used on corn, soybean, and 

nursery crops and their metabolites have been detected in 

ground water from the Chesterville Branch Watershed. The 

most commonly detected compounds are atrazine, simazine, 

metolachlor and their metabolites. Concentrations of parent 

compounds are generally below 1 μg/L; metabolites are generally detected at levels that are an order of magnitude or 

more greater than parent compounds. Several other herbicides and insecticides are less frequently detected. Pesticides are also detected in surface water under base-flow 

conditions (Shedlock and others, 1999).

In the Chesterville Branch Watershed, concentrations of 

nitrate and other ions from agricultural sources in ground 

water generally decrease with depth, along flow paths, and 

with increasing age. This trend is related to changes in fertilizer application rates since around the 1940s, rather than

denitrification (Böhlke and Denver, 1995). Much of the 

nitrate in ground water is discharged directly to surface 

water through sandy streambed sediments. Although most 

of Chesterville Branch has a forested riparian zone (fig. 4), 

most of the ground water discharging to streams moves 

along deeper flow paths and bypasses the shallow sediments 

of the riparian zone where nitrate may be removed from the 

system. Aerobic ground-water samples have been collected 
14 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

beneath streambeds in aquifer discharge areas (Böhlke and 

Denver, 1995). 

Concentrations of nitrate and other ions in surface 

water during base flow are similar to those in ground 

water. Nitrate concentrations in Chesterville Branch near 

Crumpton, Maryland, in the early 1990s ranged from about 

9 to 10 mg/L during base flow, and decreased during periods 

of surface runoff (Böhlke and Denver, 1995). Base-flow 

concentrations of nitrate are highest upstream where younger 

flow paths with higher concentrations of nitrate discharge, 

and generally decrease downstream as the proportion of 

older water from deeper flow paths increases (Böhlke and 

Denver, 1995). Similar nitrogen isotope ratios in ground 

water and surface water indicate that the nitrate discharging 

to Chesterville Branch is relatively unaltered by denitrification (Böhlke and Denver, 1995). Some loss of nitrate is 

likely in surface water from biotic uptake and reaction with 

organic matter.

Stream Chemistry in the Upper Pocomoke River and 

Chesterville Branch

Water in the upper Pocomoke River and Chesterville 

Branch is typically dilute and slightly acidic. The median 

pH and specific conductance measured during sampling 

from the late 1990s through 2001 in the upper Pocomoke 

River were 5.9 and 120 μS/cm (microsiemens per centimeter), respectively; median pH and specific conductance in 

Chesterville Branch were 6.5 and 159 μS/cm, respectively. 

Both streams are also generally well-oxygenated; measured 

dissolved oxygen levels exceeded 4 mg/L during every sampling trip to each stream. Aqueous chemistry in Chesterville 

Branch is generally dominated by dissolved chloride, nitrate, 
Water Chemistry in the Upper Pocomoke River and Chesterville Branch Watersheds 15

bicarbonate, and calcium, with lesser amounts of sodium, 

potassium, and magnesium (fig. 6). Aqueous chemistry 

in the upper Pocomoke River is similar, but more variable 

(fig. 7). Sulfate is a more significant anion in the upper 

Pocomoke River than in Chesterville Branch, and measured 

iron concentrations were as high as 2.3 mg/L. The maximum concentration of dissolved iron among samples from 

Chesterville Branch was 0.3 mg/L.

Nutrients were detectable in nearly every sample collected from both streams (table 2). Nitrate, ammonia plus 

organic nitrogen, and phosphorus were detectable in all 

32 samples collected from the upper Pocomoke River and in 

over 90 percent of samples collected from Chesterville 

Branch. Among nitrogen compounds, nitrate typically 

occurred at higher concentrations than more reduced species 

in both streams, although concentrations of ammonia and/or 

organic nitrogen may be higher during certain seasons or 

under certain flow conditions. Median nitrate and total 

phosphorus concentrations in both streams exceeded 0.09 

and 0.02 mg/L, respectively (table 2)—the estimated median 

concentrations in natural streams of the United States (Clark 

and others, 2000). Most of the phosphorus transport probably occurs in the particulate phase; median total phosphorus 

concentrations were three to four times higher than those of 

dissolved phosphorus in both streams. A similar review of 

median concentrations of total and dissolved ammonia plus 

organic nitrogen suggests that slightly less than half of the 

in-stream transport of those species occurs in the particulate 

phase in both streams (table 2).
16 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula Table 2. Summary of analytical results for samples collected from the upper Pocomoke River (1999–2001) and Chesterville Branch (1996–2001) and analyzed for nutrients or pesticide compounds [Compounds or ions detected in at least 50 percent of samples from either stream are shown in bold. N, nitrogen; ESA, ethanesulfonic acid; OA, oxanilic acid; MRL, most common minimum reporting level; n, number of samples; Freq, detection frequency in percent; Med, median concentration; Max, maximum concentration; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than] Compound or Ion Units MRL Pocomoke River near Willards, Maryland Chesterville Branch near Crumpton, Maryland n Freq Med Max n Freq Med Max Ammonia plus Organic N mg/L as N 0.2 32 100 0.65 1.84 130 96 0.35 1.63 Ammonia plus Organic N, total mg/L as N 0.1 32 100 0.81 2.78 129 98 0.54 2.50 Nitrate mg/L as N 0.05 32 100 1.96 4.95 128 100 5.31 8.57 Phosphorus mg/L 0.01 32 100 0.03 0.72 130 89 0.02 0.76 Phosphorus, total mg/L 0.05 32 100 0.09 1.22 129 95 0.08 1.28 Acetochlor

μg/L 0.002 32 34 <0.004 0.073 40 3 <0.005 0.023

Acetochlor ESA μg/L 0.05 21 95 0.55 1.33 36 3 <0.05 0.34Acetochlor OA μg/L 0.05 21 100 0.22 1.18 36 0 <0.05 <0.20Alachlor μg/L 0.002 32 75 <0.010 0.014 40 100 0.047 0.482Alachlor ESA μg/L 0.05 21 100 0.91 1.38 36 100 2.19 4.65Alachlor OA μg/L 0.05 21 95 0.19 0.50 36 100 0.66 4.142,6 diethylaniline

μg/L 0.003 32 0 <0.003 <0.003 40 3 <0.003 <0.003

Alpha-BHC

μg/L 0.002 32 0 <0.005 <0.005 40 0 <0.002 <0.005

Atrazine μg/L 0.007 32 94 0.020 0.541 40 100 0.039 4.10Desethylatrazine μg/L 0.006 32 78 0.006 0.219 40 100 0.115 0.215Benfluralin

μg/L 0.002 32 0 <0.010 <0.010 40 0 <0.002 <0.010

Butylate

μg/L 0.002 32 0 <0.002 <0.002 40 0 <0.002 <0.002

Carbaryl

μg/L 0.003 32 9 <0.041 0.186 40 30 <0.003 0.064

Carbofuran

μg/L 0.02 32 3 <0.020 <0.030 40 40 <0.060 0.103

Chlorpyrifos

μg/L 0.005 32 0 <0.005 <0.005 40 30 <0.004 0.014

Cyanazine

μg/L 0.004 32 3 <0.018 <0.018 40 8 <0.004 <0.018

DCPA

μg/L 0.002 32 9 <0.003 <0.003 40 0 <0.002 <0.003

Diazinon

μg/L 0.002 32 6 <0.005 0.031 40 13 <0.002 0.005

Dieldrin

μg/L 0.001 32 0 <0.005 <0.005 40 0 <0.001 <0.005

Disulfoton

μg/L 0.017 32 0 <0.021 <0.021 40 0 <0.017 <0.021

EPTC

μg/L 0.002 32 3 <0.002 0.010 40 3 <0.002 0.002

Ethalfluralin 

μg/L 0.004 32 0 <0.009 <0.009 40 0 <0.004 <0.009

Ethoprop

μg/L 0.003 32 0 <0.005 <0.005 40 0 <0.003 <0.005

Fonofos

μg/L 0.003 32 0 <0.003 <0.003 40 0 <0.003 <0.003
Water Chemistry in the Upper Pocomoke River and Chesterville Branch Watersheds 17

Table 2. Summary of analytical results for samples collected from the upper Pocomoke River (1999–2001) and 

 Chesterville Branch (1996–2001) and analyzed for nutrients or pesticide compounds—Continued

Compound or Ion Units MRL Pocomoke River near Willards, Maryland Chesterville Branch near Crumpton, Maryland

 n Freq Med Max n Freq Med Max

Lindane

μg/L 0.004 32 0 <0.004 <0.007 40 0 <0.004 <0.004

Linuron

μg/L 0.002 32 0 <0.035 <0.035 40 8 <0.002 0.036

Malathion

μg/L 0.005 32 0 <0.027 <0.027 40 5 <0.005 0.034

Methyl-azinphos

μg/L 0.001 32 0 <0.050 <0.050 40 0 <0.001 <0.050

Metolachlor μg/L 0.013 32 97 0.061 0.617 40 100 0.395 5.39Metolachlor ESA μg/L 0.05 21 100 5.09 8.42 36 100 2.41 4.72Metolachlor OA μg/L 0.05 21 100 1.88 3.05 36 100 0.79 2.36Metribuzin

μg/L 0.004 32 6 <0.006 <0.010 40 3 <0.004 0.062

Molinate

μg/L 0.004 32 0 <0.002 <0.005 40 0 <0.004 <0.004

Napropamide

μg/L 0.003 32 0 <0.007 <0.007 40 3 <0.003 0.053

p, p’ DDE

μg/L 0.006 32 0 <0.003 <0.006 40 0 <0.006 <0.006

Parathion

μg/L 0.004 32 0 <0.007 <0.007 40 0 <0.004 <0.007

Methyl-parathion

μg/L 0.006 32 0 <0.006 <0.006 40 0 <0.006 <0.006

Pebulate

μg/L 0.004 32 0 <0.002 <0.004 40 0 <0.004 <0.004

Pendimethalin

μg/L 0.004 32 0 <0.010 <0.010 40 10 <0.004 0.018

Permethrin

μg/L 0.005 32 0 <0.006 <0.006 40 0 <0.005 <0.006

Phorate

μg/L 0.002 32 0 <0.011 <0.011 40 0 <0.002 <0.011

Prometon μg/L 0.018 32 75 <0.015 0.065 40 3 <0.018 <0.018Pronamide μg/L 0.004 32 0 <0.004 <0.004 40 78 0.016 0.286Propachlor

μg/L 0.007 32 0 <0.010 <0.010 40 0 <0.007 <0.010

Propanil

μg/L 0.004 32 0 <0.011 <0.011 40 0 <0.004 <0.011

Propargite

μg/L 0.013 32 0 <0.023 <0.023 40 0 <0.013 <0.023

Simazine μg/L 0.011 32 66 <0.011 0.213 40 100 0.375 5.67Tebuthiuron

μg/L 0.01 32 0 <0.016 <0.016 40 0 <0.010 <0.016

Terbacil

μg/L 0.007 32 0 <0.034 <0.034 40 0 <0.007 <0.034

Terbufos

μg/L 0.013 32 0 <0.017 <0.017 40 0 <0.013 <0.017

Thiobencarb

μg/L 0.002 32 0 <0.005 <0.005 40 0 <0.002 <0.005

Triallate

μg/L 0.001 32 0 <0.002 <0.002 40 0 <0.001 <0.002

Trifluralin

μg/L 0.002 32 0 <0.009 <0.009 40 13 <0.002 <0.009
18 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

Selected pesticides are present year-round in both the 

upper Pocomoke River and Chesterville Branch. Although 

concentrations of parent compounds rarely exceeded 1 μg/L, 

metabolite concentrations were commonly higher (table 2). 

Of the 53 pesticides and metabolite compounds for which 

stream samples were analyzed, 28 were detected in at least 

one sample from either stream. The most frequently 

detected pesticide compounds were all herbicides or herbicide metabolites (table 2), as might be expected considering 

that applications of herbicides generally far exceed those of 

insecticides or fungicides in agricultural areas (Aspelin, 

1997). Every sample collected from Chesterville Branch 

contained detectable concentrations of two triazine herbicides (atrazine and simazine) and two acetanilide herbicides 

(alachlor and metolachlor), as well as selected metabolites of 

each (table 2). Ethanesulfonic (ESA) and oxanilic acid (OA) 

metabolites of acetanilide herbicides were also detected in 

every sample from the upper Pocomoke River, although parent compounds were detected slightly less frequently and at 

lower concentrations than were the metabolites.

The majority of selected pesticides occur in the upper 

Pocomoke River and Chesterville Branch in the form of 

metabolites. Acetochlor, alachlor, and metolachlor occurred 

in the upper Pocomoke River from 1999 through 2001 

almost exclusively in the form of metabolites, particularly 

ESA (fig. 8). Concentrations of metabolites of atrazine, 

metolachlor, and alachlor far exceeded those of their parent 

compounds in Chesterville Branch as well (fig. 9). Pesticide 

transformations through metabolism or other processes 

typically occur in the soil zone in areas of application. A 

review of data from across the United States revealed that 

less than 2 percent of applied alachlor, atrazine, or metolachlor typically reaches stream networks, but little further 

in-stream losses of these compounds occur (Capel and 

Larson, 2001; Capel and others, 2001). Atrazine concentrations in the upper Pocomoke River were greater than those of 

desethylatrazine (fig. 8), which may indicate that soil conditions in the upper Pocomoke River Watershed do not favor 

the metabolism of atrazine, or favor the formation of hydroxyatrazine or some other metabolite. Although the formation or movement of OA metabolites may be favored over 

that of ESA metabolites in coarse-grained soils (at least for 

metolachlor; Phillips and others, 1999), concentrations of 

metolachlor ESA and alachlor ESA were generally much 

higher than those of OA metabolites or parent compounds in 

both the upper Pocomoke River (fig. 8) and Chesterville 

Branch (fig. 9). 
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 19

Hydrologic and Geochemical Controls on the 

Transport of Pesticides and Nutrients

Dominant pathways for the transport of agricultural 

chemicals to the upper Pocomoke River and Chesterville 

Branch are related to geologic, topographic, and soil conditions in each watershed and vary mainly with the weather. 

The distinctive chemistry of ground water derived from 

within different geologic formations in each watershed provides an opportunity to distinguish the relative contribution 

of these sources to streamflow as they change seasonally and 

over individual storm events.

Two principal components explain nearly two-thirds of 

the variability in water chemistry over the sampling period in 

each stream. In the upper Pocomoke River, the relative volume of discharge from two ground-water sources is apparently the dominant factor affecting stream chemistry. 

Surface runoff is evidently less important in the upper 

Pocomoke River Watershed, but explains nearly half of the 

variability in the chemistry of Chesterville Branch.

The Upper Pocomoke River

Ground water is a significant hydrologic chemical transport pathway to streams in the upper Pocomoke River Watershed. The first principal component of stream chemistry in 

the upper Pocomoke River (POCOPC1), which explains 

nearly half (44 percent) of the overall variability in majorion and nutrient concentrations in the river, apparently represents a distinction between two ground-water sources (table 

3). POCOPC1 correlates directly with calcium, magnesium, 

chloride, sulfate, and nitrate (as well as specific conductance), which are typical of agricultural influences observed 

in ground water from the unconfined Parsonsburg Sand. 

Inverse correlations with POCOPC1 are strongest for iron 

and bicarbonate, which are typical of ground water in the 

Beaverdam Sand.

Contributions of particulates and other relatively insoluble materials from surface runoff are evidently of secondary 

importance to the chemistry of water in the upper Pocomoke 

River. The second principal component of stream chemistry 

(POCOPC2, table 3) is strongly correlated with streamflow 

(fig. 10), and apparently represents an indicator of flow 

regime. The relatively strong negative loadings of 

POCOPC2 with phosphorus and ammonia plus organic 

nitrogen, as well as (to a lesser extent) potassium and manganese, suggests negative values of POCOPC2 are indicative 

of a high-flow regime. Phosphorus is relatively insoluble 

and often bound to sediment; potassium and ammonium 

(which constitutes most of the ammonia at pH typical of the 

upper Pocomoke River) may also be adsorbed onto mineral 

surfaces (Hem, 1985). Organic nitrogen in surface runoff 

may be indicative of surface applications of poultry manure. 

The relatively strong direct correlation of POCOPC2 with 

silica and sodium suggests base flow, especially from the 

Beaverdam Sand.

As is the case in many areas, the mobility of particulate 
20 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

 

matter and relatively insoluble chemical species through the 

upper Pocomoke River Watershed is dependent on relatively 

high streamflow. In the predominantly flat upper Pocomoke 

River Watershed, however, direct runoff of precipitation 

along the land surface contributes significantly to streamflow in the river only during particularly large or intense 

storm events. Concentrations of total phosphorus and suspended sediment were consistently low when streamflow 

was less than 100 ft3/s (cubic feet per second), but increased 

dramatically above that level (fig. 11). Total flow of

100 ft3/s in the Pocomoke River near Willards, Maryland, 

may be an indicator of when overland runoff becomes significant in the watershed. Bank erosion or resuspension of bed 

load may also contribute to elevated phosphorus or sediment 

concentrations in the river during periods of high flow.

Periods of significant overland flow in the upper 

Pocomoke River Watershed are relatively infrequent. 

POCOPC2 represents only 25 percent of the variability in 

stream chemistry from 1999 through 2001 (table 3). Hourly 

streamflow measurements exceeding 100 ft3/s occurred only 

about 20 percent of the time between July 1996 and 

September 2001, and HYSEP estimates indicate that over 

70 percent of flow in the upper Pocomoke River is generated 

from ground-water discharge (table 4). 

Table 3. Loadings on the first two components from unrotated principal components analysis

 on major-ion and nutrient data collected from the upper Pocomoke River, 1999–2001

[Loadings with absolute value greater than 0.70 are shown in bold; those with absolute value less than 0.40 are omitted]

Compound, ion, or 

physical characteristic

Component 1

(POCOPC1)

Component 2

(POCOPC2)

Communality

estimates

Percent of overall variance 44 25

Specific conductance 0.8553 0.7353

Sulfate 0.9148 0.8660

Calcium 0.9278 0.9193

Magnesium 0.9389 0.8908

Nitrate 0.8893 0.8675

Chloride 0.7127 0.4807 0.7390

Silica 0.8841 0.8627

Sodium 0.8236 0.7801

Ammonia plus organic nitrogen 0.4009 -0.8183 0.8302

Phosphorus, total -0.4629 -0.7652 0.7998

Bicarbonate -0.6662 0.5887

Iron -0.6304 0.4212

Manganese 0.6351 -0.5045 0.6579

Potassium 0.6282 -0.5690 0.7183

pH 0.0606

Dissolved oxygen 0.4196 0.3265
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 21

Unconfined ground water in the Parsonsburg Sand represents an important conduit through which soluble agricultural or other chemicals are transported through the upper 

Pocomoke River Watershed. Concentrations of these chemicals may be diluted in the river by discharge from the confined Beaverdam Sand, however, particularly during dry 

periods. The relative contribution of water from the 

Parsonsburg Sand to that from the Beaverdam Sand 

increases with increasing streamflow (fig. 12). This relation 

is apparently related to base flow, however, and is not apparent above 100 ft3/s where surface runoff may be significant. 

POCOPC1 is more strongly correlated with streamflow 

below 100 ft3/s (rho = 0.749, p <0.0001, n =22) than over the 

entire range of flow conditions (rho = 0.479, p = 0.0075, 

n = 30) (fig. 12). The relative discharge from the two 

ground-water sources is probably controlled by the level of 

the water table in the Parsonsburg Sand. The higher water 

table during wet periods would induce greater discharge 

from this aquifer to the upper Pocomoke River and its tributaries. These tributaries are primarily artificial ditches, however, and may contain little or no water during dry periods. 

Ground-water discharge from the Beaverdam Sand, which is 

probably fairly consistent over different seasons, would have 

its greatest influence on stream chemistry during drier 

periods.

The occurrence of pesticides in the upper Pocomoke 

River similarly reflects the importance of transport through 

shallow ground water in the Parsonsburg Sand. Where significant, correlations between pesticide concentrations and 

streamflow are generally negative (table 5). Ground-water 

 

Table 4. Summary of streamflow characteristics for the upper Pocomoke River near Willards, Maryland 

and Chesterville Branch near Crumpton, Maryland, July 1996 through September 2001

[ km2, square kilometers; ft3/s, cubic feet per second]

Stream

Drainage

area

(km2 )

Median

estimated

percent

monthly 

base flow 1

Estimated

threshold

for runoff

events 2

(ft 3/s)

Percentage

of time

exceeding

threshold 3

Number

of runoff 

events 3

Mean

duration

of runoff

events 4

(hours)

Pocomoke River near Willards, MD 155 72.2 100 20.8 72 133

Chesterville Branch near Crumpton, MD 15 78.8 10 21.6 140 71

1 Estimated from HYSEP, Sloto and Crouse, 1996. Estimates for Chesterville Branch include August 1996 through September 2001.

2 Estimated on the basis of stream chemistry.

3 Estimated on the basis of hourly streamflow measurements, neglecting missing data.

4 Computed from TE * TT/NE, where TE = percentage of time exceeding threshold (as a proportion), TT = 46,032, the total number of hours between July 1, 

1996 and September 30, 2001, and NE = total number of runoff events.
22 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

discharge probably accounts for the entire mass of alachlor 

in the river; alachlor was rarely detected when streamflow 

exceeded 100 ft3/s (fig. 13). Although concentrations of acetochlor ESA, metolachlor ESA, and metolachlor OA 

increase with increasing streamflow (table 5), this is probably indicative of ground-water discharge from the Parsonsburg Sand rather than rainfall or surface runoff. Correlations 

of both metolachlor metabolites with streamflow are stronger below 100 ft3/s than over the entire range of sampled 

conditions (fig. 14). 

Seasonal variability in major-ion and nutrient chemistry 

in the upper Pocomoke River reflects the seasonal hydrologic variability and resulting changes in the relative contributions from the Parsonsburg and Beaverdam Sands. 

POCOPC1 was significantly higher ( p=0.0252) between 

December and May than during the drier period, suggesting 

a greater relative contribution of flow from the Parsonsburg 

Sand than from the Beaverdam Sand (table 3, fig. 15). Individually, concentrations of nitrate, ammonia plus organic 

nitrogen, sulfate, and manganese were also significantly 

higher during the wetter period, but concentrations of iron, 
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 23

Table 5. Results of comparisons of concentrations of selected pesticides and relative proportions of parent and daughter compounds in the upper 

Pocomoke River and Chesterville Branch to streamflow and among different seasons, 1999–2001

[Results are omitted in cases where compound was detected in less than 50 percent of samples, total sample size is less than 20, or (for seasonal comparisons) the number of samples in either season is 

less than 10. Results in bold are significant at alpha =0.05; n, number of samples in correlation of streamflow and concentration; rho, Spearman’s rho; p value, p value from Spearman’s test for 

correlation; nWET, number of samples collected during the “wet” season (December–May); nDRY, number of samples collected during the “dry” season (June–November)]

Chemical variable

Pocomoke River near Willards, Maryland Chesterville Branch near Crumpton, Maryland

Streamflow 1 Season Streamflow 1 Season

 n rho p value nWET nDRY p value Higher season n rho p value nWET nDRY p value Higher season

Atrazine 32 0.0089 0.9615 13 19 0.1671 40 0.3529 0.0255 20 20 0.2448Desethylatrazine 32 0.1573 0.3898 13 19 0.0408 DRY 40 -0.4788 0.0018 20 20 0.6948Percent Atrazine 30 -0.3932 0.0316 12 18 0.4707 40 0.5937 0.0001 20 20 0.2674Acetochlor 32 0.0500 0.7859 13 19 0.4549Acetochlor ESA 21 0.5367 0.0121 10 11 1.0000Acetochlor OA 21 0.2761 0.2257 10 11 0.6460Percent Acetochlor 2 21 -0.2923 0.1986 10 11 0.6678Percent Acetochlor ESA 2 21 0.3085 0.1736 10 11 0.3242Percent Acetochlor OA2 21 -0.2319 0.3118 10 11 0.0910Alachlor 32 -0.3940 0.0257 13 19 0.4400 40 0.2483 0.1223 20 20 0.0119 WETAlachlor ESA 21 -0.8156 0.0001 10 11 0.0670 36 0.0903 0.6004 19 17 0.5057Alachlor OA 21 -0.8238 0.0001 10 11 0.1805 36 0.2977 0.0778 19 17 0.8741Ratio of Alachlor ESA/ Alachlor 3 36 -0.2057 0.2287 19 17 0.0513Ratio of Alachlor OA/ Alachlor 3 36 0.0656 0.7038 19 17 0.0638Percent Alachlor2 21 -0.1323 0.5676 10 11 0.5720 36 0.1359 0.4295 19 17 0.0900Percent Alachlor ESA 2 21 0.0890 0.7013 10 11 0.8327 36 -0.3973 0.0164 19 17 0.8121Percent Alachlor OA 2 21 -0.1046 0.6519 10 11 0.6221 36 0.2358 0.1663 19 17 0.3665Metolachlor 32 0.2195 0.2275 13 19 0.6045 40 0.4638 0.0026 20 20 0.3301Metolachlor ESA 21 0.6550 0.0013 10 11 0.0782 36 0.1581 0.3571 19 17 0.2674Metolachlor OA 21 0.4463 0.0426 10 11 0.8327 36 0.5120 0.0014 19 17 0.8740Ratio of Metolachlor ESA/ Metolachlor 3 20 0.2272 0.3355 36 -0.3643 0.0290 19 17 0.6920Ratio of Metolachlor OA/ Metolachlor 3 20 0.0707 0.7671 36 -0.1865 0.2761 19 17 0.1885Percent Metolachlor2 21 -0.3027 0.1823 10 11 0.0910 36 0.3385 0.0435 19 17 0.4957Percent Metolachlor ESA 2 21 0.2884 0.2048 10 11 0.0411 WET 36 -0.3938 0.0175 19 17 0.8121Percent Metolachlor OA 2 21 -0.2105 0.3598 10 11 0.0411 DRY 36 0.2180 0.2016 19 17 0.0379 DRYPrometon 32 -0.0227 0.9020 13 19 0.0087 DRYPronamide 40 0.1461 0.3683 20 20 0.0001 WETSimazine 32 0.1217 0.5071 13 19 0.3282 40 0.3301 0.0375 20 20 0.71501 Instantaneous streamflow measured at the time of sample collection.2 Percentages were computed on a molar basis, only in cases where both parent and daughter compounds were measured and at least one was detected. Censored data were assumed to equal zero.3 Ratios were computed on a molar basis, only in cases where both parent and daughter compounds were detected.
24 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

bicarbonate, and silica were significantly lower (table 6). 

Nitrate is indicative of discharge from the Parsonsburg Sand; 

sulfate and ammonia during the wetter months may also be 

derived from Parsonsburg discharge, or from organic matter 

in ditches and other upper Pocomoke tributaries. Iron and 

bicarbonate represent the influence of discharge of ground 

water from the Beaverdam Sand during the drier months. 

The increase in dissolved iron and the sharp increase in 

bicarbonate concentrations in the upper Pocomoke River in 

the late spring (fig. 16) may also be the result of the dissolution of amorphous ferric hydroxide in ditch sediments as 

flow decreases and conditions in the ditches become stagnant and anoxic (Denver, 1986, 1989). A similar seasonal 

pattern is apparent in figure 7; samples from the upper 

Pocomoke River during the wetter months were consistently 

similar to water from the Parsonsburg Sand, but samples 

from the drier months represented more of a variable mixture 

from both aquifers, and the effects of stagnant ditch conditions. 

Seasonal variability in pesticide concentrations in the 

upper Pocomoke River is related to application patterns. 

Concentrations of atrazine and metolachlor were generally 

highest in the river during the growing season (fig. 17). The 

relatively rapid response of pesticide concentrations in the 

upper Pocomoke River to pesticide application in the watershed is likely indicative of drainage from the Parsonsburg 

Sand through artificial ditching, as well as pesticide transport during relatively unusual periods of surface runoff. 

Instantaneous pesticide loads increase with increasing 

streamflow throughout the entire range of sampled condi- 
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 25

Table 6. Results of comparisons of stream chemistry in the upper Pocomoke River (1999–2001) and Chesterville Branch (1996–2001) to 

streamflow and among different seasons

[Results in bold are significant at alpha = 0.05. Results are omitted in cases where total sample size is less than 20 or (for seasonal comparisons) the number of samples in either season is less than 10. 

n , number of samples in correlation of streamflow and concentration; rho, Spearman’s rho; p value, p value from Spearman’s test for correlation; nWET, number of samples collected during the “wet” 

season (December–May); nDRY, number of samples collected during the “dry” season (June-November)]

Chemical or physical variable

Pocomoke River near Willards, Maryland Chesterville Branch near Crumpton, Maryland

Streamflow1 Season Streamflow 1 Season

n rho p value nWET nDRY p value Higher Season n rho p value nWET nDRY p value Higher Season

Specific conductance 31 0.325 0.0743 13 18 0.2885 124 -0.701 0.0001 67 59 0.7599Calcium 32 0.268 0.1380 13 19 0.1987 51 -0.750 0.0001 25 27 0.5395Magnesium 32 0.385 0.0296 13 19 0.1732 51 -0.442 0.0012 25 27 0.3646Sodium 32 -0.542 0.0014 13 19 0.4543 51 -0.596 0.0001 25 27 0.0490 WETPotassium 32 0.751 0.0001 13 19 0.3374 51 0.421 0.0021 25 27 0.0034 DRYSilica 32 -0.874 0.0001 13 19 0.0148 DRY 74 -0.693 0.0001 39 37 0.4512Sulfate 32 0.334 0.0620 13 19 0.0274 WET 51 0.563 0.0001 25 27 0.1753Chloride 32 0.009 0.9619 13 19 0.3001 51 -0.547 0.0001 25 27 0.2375Bicarbonate 31 -0.710 0.0001 13 18 0.0026 DRY 89 -0.607 0.0001 46 44 0.0001 DRYSuspended sediment 86 0.727 0.0001 46 42 0.0930Iron 32 -0.343 0.0543 13 19 0.0183 DRY 51 0.512 0.0001 25 27 0.0013 WETManganese 32 0.652 0.0001 13 19 0.0026 WET 52 0.294 0.0346 25 28 0.0001 WETAmmonia plus organic nitrogen 32 0.822 0.0001 13 19 0.0316 WET 128 0.630 0.0001 67 63 0.0556Nitrate 32 0.742 0.0001 13 19 0.0023 WET 126 -0.665 0.0001 66 62 0.0285 WETPhosphorus 32 0.207 0.2551 13 19 0.4314 128 0.747 0.0001 67 63 0.2853Phosphorus, total 32 0.269 0.1369 13 19 0.4315 127 0.776 0.0001 67 62 0.1785

1 Instanteous streamflow measured at the time of sample collection.
26 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

tions in the upper Pocomoke River (fig. 18). Increasing pesticide loads with increasing streamflow below 100 ft3/s 

(fig. 18) are likely indicative of the increasing relative 

importance of discharge from the Parsonsburg Sand. Once 

surface runoff becomes significant, however, additional 

amounts of some pesticides are carried in surface runoff to 

the stream (fig. 18). 

Chesterville Branch

Both ground water and overland runoff represent significant pathways for chemical transport to Chesterville Branch 

and its tributaries. Overland runoff may be a more important 

transport mechanism in the Chesterville Branch Watershed 

than in the upper Pocomoke River Watershed due to the 

greater topographic relief and/or the smaller watershed area. 

Ground-water transport through the permeable, unconfined 

Columbia and Aquia aquifers, however, is also important for 

soluble chemical species. Although stream chemistry in 

Chesterville Branch is also affected by discharge from the 

confined Hornerstown aquifer, these effects may be minimal. 

Even the smallest tributaries draining the relatively thick 

unconfined aquifer are typically perennial.

The distinction between base flow and surface runoff is 

apparently of primary importance to the chemistry of water 

in Chesterville Branch. The first principal component of 

stream chemistry (CHESPC1) is related to streamflow

(fig. 19) and explains nearly half (47 percent) of the variability in major ion and nutrient concentrations (table 7). The 

pattern of correlations with CHESPC1 appears to represent a 

distinction between agricultural chemicals that are relatively 

soluble and those that are not. Strongly positive values of 

CHESPC1 associated with low streamflow are correlated 

with relatively soluble ions typical of ground water in the 

surficial unconfined aquifer, including calcium, sodium, 

nitrate, and magnesium. Chemical species with relatively 

strong negative correlations with CHESPC1 include phosphorus, ammonia plus organic nitrogen, sulfate, and potassium (table 7). These chemicals are commonly transported 

in overland runoff, often attached to sediment particles. 

The second principal component (CHESPC2) represents 

seasonal variability in the chemistry of water in Chesterville 

Branch. Positive values of CHESPC2 are correlated with 

manganese, iron, and dissolved oxygen (table 7) and occur 

almost exclusively during the relatively wet season from 

December through May (fig. 20). Negative values of 

CHESPC2 are correlated with potassium and bicarbonate 

and occur only during the typically drier period from June 

through November. Iron is relatively insoluble in oxic conditions (Hem, 1985). Although iron concentrations were 

generally low in Chesterville Branch, the occurrence of relatively high concentrations of iron and dissolved oxygen in 

the stream during the wet season suggests some type of 

chemical disequilibrium. The relatively high concentrations 

of bicarbonate associated with strongly negative values of 

CHESPC2 suggests that discharge from the Hornerstown 
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 27

 

aquifer may be important when the water table is low during 

the dry season. 

Surface runoff in the Chesterville Branch Watershed 

probably becomes significant when total streamflow near 

Crumpton reaches about 10 ft3/s. CHESPC1 is strongly correlated with streamflow; strongly negative values indicative 

of overland runoff occur only at streamflow above 10 ft3/s 

(fig. 19). Suspended sediment and total phosphorus 

occurred at fairly low and consistent concentrations below 

this level, but were mobilized more significantly at flows 

greater than 10 ft3/s (fig. 21). HYSEP estimates suggest that 

about 79 percent of flow in Chesterville Branch is derived 

from ground-water discharge (table 4). Flow records at 

Chesterville Branch, however, date only to 1996, and include 

several years of severe drought (U.S. Geological Survey, 

1950–2001). Although estimated long-term net contributions from base flow are similar to those in the upper 

Pocomoke River, periods of high flow occurred only about 

half as often in the upper Pocomoke River Watershed 

between July 1996 and September 2001, but lasted an average of about twice as long (table 4). Although this pattern is 

likely due at least in part to the difference in drainage area 

between the two streams, differences in hydrogeology and 

topography between the two watersheds are probably also 

important.

The transport of relatively soluble ions from the land 

surface to streams in the Chesterville Branch Watershed 

apparently occurs mainly through the permeable unconfined 

Table 7. Loadings on the first two components from unrotated principal components analysis on 

major-ion and nutrient data collected from Chesterville Branch, 1997–2001

[Loadings with absolute value greater than 0.70 are shown in bold; those with absolute value less than 0.40 are omitted]

Compound, ion, or

physical characteristic

Component 1

(CHESPC1)

Component 2

(CHESPC2)

Communality 

estimates

Percent of overall variance 47 15

Specific conductance 0.8483 0.7321

Sodium 0.8270 0.7277

Calcium 0.8775 0.7711

Magnesium 0.7399 0.5811

Nitrate 0.7670 0.7214

Ammonia plus organic nitrogen -0.8188 0.7123

Phosphorus, total -0.9271 0.8748

Potassium -0.7094 -0.4056 0.6677

Sulfate -0.7180 0.6553

Iron -0.4503 0.6897 0.6783

Manganese 0.8108 0.7362

Dissolved oxygen 0.6755 0.5598

Bicarbonate 0.5741 -0.4938 0.5734

Chloride 0.6775 0.4938

Silica 0.6423 0.4141

pH 0.0184
28 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

aquifer system. The chemistry of water in Chesterville 

Branch is evidently minimally affected by discharge from 

deeper confined aquifers, however, and concentrations of 

these ions during base flow are not greatly diluted by mixing 

in the stream. Nitrate, calcium, and magnesium, typical 

of ground water affected by agriculture, occurred in 

Chesterville Branch at relatively high concentrations during 

all seasons (fig. 22); variability is primarily related to flow 

regime (fig. 19 and table 6). Alachlor, atrazine, metolachlor, 

simazine, and selected metabolites of each were also detectable throughout the year in Chesterville Branch (fig. 23, 

table 2). 

The transport of pesticides and metabolites to 

Chesterville Branch occurs through both surface runoff and 

ground-water discharge. Both parent compounds and metabolites were detectable throughout the year and during all 

sampled flow regimes (table 2). Although soluble pesticide 

metabolites may be transported to Chesterville Branch primarily through ground water, parent compounds likely move 

predominantly in surface runoff. Significant correlations 

with streamflow over the entire range of sampled conditions 

are generally positive for parent compounds (including atrazine, metolachlor, and simazine), and negative for most 

metabolites (table 5). The variability in transport implied by 

these correlations may be related to solubility or persistence 

in the soil. ESA and OA metabolites are much more soluble 

than their parent compounds, and runoff conditions may 

transport parent compounds to streams before they are converted to metabolites (Phillips and others, 1999). Groundwater transport is apparently particularly important for ESA 

metabolites; relative proportions of these metabolites of 

metolachlor and alachlor to OA metabolites and parent compounds decreased with increasing streamflow. OA metabolites may be less soluble than ESA metabolites. Although 

concentrations of most OA metabolites are not related to 

streamflow, concentrations of metolachlor OA increased 

with increasing flow (table 5). 

The movement of pesticides to Chesterville Branch during wet periods is related to their use and persistence in the 

watershed as well as to flow conditions. The dependence of 

pesticide transport on streamflow is probably more complex 

than is suggested by simple correlations of concentrations 

with streamflow. Concentrations of selected parent compounds are typically highest near the initiation of surface 

runoff at around 10 ft3/s, but may decrease or stabilize as 

flows continue to increase (fig. 24). Instantaneous loads of 

metolachlor and atrazine, however, apparently stabilize or 

continue to increase as streamflow in Chesterville Branch 

increases above 10 ft3/s (fig. 25). The apparent decrease in 

concentrations of these compounds during high flow is likely 

the result of simple dilution. 

Although pesticide concentrations typically peak during 

the growing season in Chesterville Branch, seasonal variability in the overall stream chemistry is generally minimal. 
Hydrologic and Geochemical Controls on the Transport of Pesticides and Nutrients 29

Seasonal patterns in pesticide concentrations are probably 

related to similar patterns in application. From 1999 through 

2001, the highest concentrations of metolachlor typically 

occurred in March or April, but concentrations of atrazine 

typically peaked later during the growing season (fig. 23). 

Pronamide concentrations were generally highest early in the 

year (fig. 26). Although metolachlor and pronamide are 

often used in pre-emergence applications, atrazine may be 

used for selective weed control throughout the growing season (Farm Chemical Handbook, 1999). Concentrations of 

nutrients and most other chemical species are related most 

significantly to streamflow and are fairly consistent throughout the year (tables 5–6). The unconfined aquifer is relatively thick (as much as 25 m) in the Chesterville Branch 

Watershed (fig. 5), and the typically perennial headwater 

streams probably carry discharge from this surficial aquifer 

throughout the year (Bachman and others, 2002). Conversely, discharge from underlying confined aquifers is probably limited by the relatively small size of the Chesterville 

Branch Watershed, and is likely significant only during 

extremely dry periods. 
30 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

Summary and Conclusions

Water chemistry in the upper Pocomoke River and 

Chesterville Branch reflects the influence of agriculture and 

the predominance of permeable siliciclastic surficial sediments in both watersheds. Samples were collected periodically from the late 1990s through 2001 from each stream and 

analyzed for major ions, nutrients, and selected pesticides 

and pesticide metabolites as part of the National Water-Quality Assessment and similar programs of the U.S. Geological 

Survey. Variability in the water chemistry in each stream 

was analyzed to identify important transport processes in different hydrogeologic settings during different seasons and 

flow conditions. Excessive nutrients, pesticides, and other 

agricultural chemicals in streams may be harmful to aquatic 

biota, and understanding hydrologic and geochemical processes that affect their transport through watersheds can be 

useful for the safe and effective use of these compounds.

The typically dilute, slightly acidic, and well-oxygenated 

water in the upper Pocomoke River and Chesterville Branch 

contains dissolved nitrogen and phosphorus compounds at 

detectable levels during all seasons and all flow conditions, 

and concentrations are typically higher than would be 

expected under natural conditions. Nitrate generally occurs 

in higher concentrations than ammonia or organic nitrogen, 

although concentrations of reduced nitrogen species may be 

higher during periods of significant surface runoff. Most of 

the phosphorus transported in the upper Pocomoke River and 

Chesterville Branch is in the particulate phase.

A review of major ion and nutrient concentrations in the 

upper Pocomoke River and Chesterville Branch demonstrates the importance of variable topography, geology, and 

hydrology to stream chemistry in each watershed. In the 

upper Pocomoke River Watershed, runoff of precipitation 

over the land surface is limited by the extremely flat topography. Only 25 percent of the variability in measured stream 

chemistry from 1999 through 2001 is attributable to the distinction between surface runoff and base flow, and a review 

of long-term records suggests that 72 percent of the flow in 

the upper Pocomoke River is derived from ground-water discharge. Significant surface runoff in the upper Pocomoke 

River Watershed probably occurs when flow in the 

Pocomoke River near Willards, Maryland, reaches about 100 

cubic feet per second, although this threshold likely varies 

seasonally. The primary hydrologic influence on stream 

chemistry in the upper Pocomoke River is the relative proportion of flow contributed from two ground-water sources. 

Nearly half (44 percent) of the variability in major ion and 

nutrient concentrations in the river is related to a distinction 

between ground-water discharge from the Parsonsburg and 

Beaverdam Sands. Concentrations of nitrate, calcium, magnesium, and other ions typical of agricultural effects in 

ground water of the Parsonsburg Sand are generally higher 

in the upper Pocomoke River when the water table is highest 

during the winter and spring; concentrations of iron and 

bicarbonate typical of ground water in the Beaverdam Sand 

are typically highest in the river during the drier season from 

June through November.

Surface runoff is more influential on water chemistry in 

Chesterville Branch than in the upper Pocomoke River. 

Nearly half (47 percent) of the variability in concentrations 

of major ions and nutrients in Chesterville Branch is attributable to the distinction between runoff and base flow. Significant surface runoff in the Chesterville Branch Watershed 

probably begins when total flow in Chesterville Branch near 

Crumpton, Maryland, reaches about 10 cubic feet per 

second. Contributions to Chesterville Branch from the 

Hornerstown aquifer and other confined aquifers are probably limited by the relatively small size of the Chesterville 

Branch Watershed and the thickness of the surficial aquifer. 

Most of the first-order tributaries draining the Columbia and 

Aquia aquifers in the Chesterville Branch Watershed are 

perennial.

Selected herbicides and herbicide metabolites are detectable throughout the year in both the upper Pocomoke River 

and Chesterville Branch, although concentrations of most 

compounds rarely exceed 1 microgram per liter. Analyses 

for pesticide metabolites may be particularly important in 

any efforts to compute total pesticide loads or estimate exposures to humans or aquatic ecosystems; metabolite concentrations generally far exceed those of parent compounds in 

both watersheds. Metabolites of acetochlor, alachlor, and 
Summary and Conclusions and References Cited 31

metolachlor were detected in every sample collected from 

the upper Pocomoke River, although parent compounds were 

detected less frequently. Atrazine, alachlor, metolachlor, 

simazine, and selected metabolites of each were also 

detected in every sample collected from Chesterville Branch. 

High detection frequencies for parent compounds as well as 

metabolites in Chesterville Branch probably reflect the 

importance of transport through both surface runoff and 

ground water in that watershed. Relatively low concentrations of desethylatrazine in the upper Pocomoke River suggest that sandy but poorly drained soils typical of the 

watershed may not favor metabolism of atrazine, or may 

favor the formation of hydroxyatrazine or some other metabolite.

Pesticide concentrations are typically highest in both 

watersheds during application periods. The relatively rapid 

transport from areas of application to streams suggested by 

this pattern may be related to shallow ground-water flow as 

well as surface runoff, particularly in the upper Pocomoke 

River Watershed, where artificial ditches intercept shallow 

flow paths. In the Chesterville Branch Watershed, conversely, parent compounds are probably transported primarily through surface runoff, but metabolites (particularly 

ESA metabolites) move primarily through ground water. 

Both parent compounds and metabolites may be transported 

to either stream during significant runoff events, however. 

Although in-stream concentrations of pesticides may 

decrease due to dilution, instantaneous loads of pesticides in 

both streams typically stabilize or continue to increase with 

increasing flow after runoff begins. Most compounds are 

apparently not completely flushed out of the watershed during the early stages of surface runoff. The continued transport of pesticides after the initiation of surface runoff may be 

related to the variable length of overland flow paths, and to 

the amount of pesticides in watershed soils, particularly following application periods.

Consideration of local conditions is important to agricultural and other land or water management in the Delmarva 

Peninsula and the wider Atlantic Coastal Plain. Although 

regional patterns and similarities are evident in hydrologic 

and geochemical conditions across the Peninsula, a review 

of water chemistry in the upper Pocomoke River and 

Chesterville Branch illustrates considerable local variability, 

an understanding of which may be necessary for effective 

resource management. Nutrient and pesticide transport 

occurs through both surface runoff and ground water in both 

watersheds. Airborne transport can also be significant for 

both types of compounds. A consideration of the relative 

importance of different transport pathways is necessary for 

mitigation and other management efforts designed for 

specific watersheds.

An understanding of local hydrology is also important to 

environmental studies in any watershed. Streamflow in the 

upper Pocomoke River is generated from a variety of 

sources, including at least two aquifers with very different 

ground-water quality. Investigating nutrient dynamics or 

seasonal patterns in geochemistry, aquatic ecology, or other 

environmental characteristics in such streams can be complicated by seasonally variable inputs from these sources, as 

well as biologic uptake and other factors. Analyses of major 

ions in streams can be extremely useful for interpreting 

watershed hydrology for environmental studies.

Ground water represents a significant reservoir of dissolved nutrients and pesticides on the Delmarva Peninsula, 

and likely carries a considerable load of such chemicals to 

the Chesapeake Bay and other receiving waters. Although 

confined aquifers may contain little, if any, nutrients, pesticides, or other agricultural chemicals, previous studies have 

documented the occurrence and persistence of these compounds in surficial aquifers on the Delmarva Peninsula. The 

occurrence of these compounds at detectable levels in the 

upper Pocomoke River and Chesterville Branch during baseflow conditions throughout the year further demonstrates 

that many are successfully transported in ground water 

through the riparian and hyporheic zone to streams. The 

slow movement typical of ground water and the resulting 

lag time between land application and delivery to streams 

further complicate water-resources management and mitigation strategies.

References Cited

Andres, A.S., and Howard, C.S., 2000, The Cypress 

Swamp Formation, Delaware: Delaware Geological 

Survey Report of Investigations 62, 14 p.

_____ 2002, Results of hydrogeologic studies of the Cypress 

Swamp Formation, Delaware: Delaware Geological 

Survey Report of Investigations 64, 16 p.

Aspelin, A.L., 1997, Pesticide industry sales and usage— 

1994 and 1995 market estimates: U.S. Environmental 

Protection Agency Report 733-R-97-002, 35 p.

Ator, S.W., Denver, J.M., Krantz, D.E., Newell, W.L., and 

Martucci, S.K., in press, A surficial hydrogeologic 

framework for the Mid-Atlantic Coastal Plain: 

U.S. Geological Survey Professional Paper 1680.

Bachman, L.J., Krantz, D.E., and Böhlke, J.K., 2002, 

Hydrogeologic framework, ground-water geochemistry, 

and assessment of nitrogen yield from base flow in two 

agricultural watersheds, Kent County, Maryland: 

U.S. Environmental Protection Agency Report EPA/600/

R-02/008, 79 p.

Bachman, L.J., Lindsey, B.D., Brakebill, J.W., and 

Powars, D.S., 1998, Ground-water discharge and baseflow nitrate loads of nontidal streams, and their relation 

to a hydrogeomorphic classification of the Chesapeake 

Bay watershed, Middle Atlantic Coast: U.S. Geological 

Survey Water-Resources Investigations Report 98–4059, 

71 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, 
32 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

 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/

report.html.

Bricker, O.P., Newell, W.L., and Simon, N.N., 2003, Bog 

iron formation in the Nassawango Watershed, Maryland: 

U.S. Geological Survey Open-File Report 03–346, 

Online only, available at http://pubs.usgs.gov/of/2003/

of03-346.

Capel, P.D., and Larson, S.J., 2001, Effect of scale on the 

behavior of atrazine in surface waters: Environmental 

Science and Technology, v. 35, no. 4, p. 648–657.

Capel, P.D., Larson, S.J., and Winterstein, T.A., 2001, 

The behaviour of 39 pesticides in surface waters as a 

function of scale: Hydrologic Processes, v. 15, 

p. 1,251–1,269.

Clark, G.M., Mueller, D.K., and Mast, M.A., 2000, 

Nutrient concentrations and yields in undeveloped 

stream basins of the United States: Journal of the 

American Water Resources Association, v. 36, no. 4,

p. 849–860.

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

3d ed.: New York, John Wiley and Sons, Inc., 584 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.

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

unconfined aquifer in west-central and southwestern 

Delaware: Delaware Geological Survey Report of 

Investigations 41, 100 p.

_____ 1989, Effects of agricultural practices and septicsystem effluent on the quality of water in the unconfined 

aquifer in parts of eastern Sussex County, Delaware: 

Delaware Geological Survey Report of Investigations 45, 

66 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, 30 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, n. 12, p. 3,837–3,860.

Farm Chemical Handbook, 1999: Willoughby, Ohio, 

Meister Publishing Company, 1,029 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.

Garbarino, J.R., Bednar, A.J., Rutherford, D.W., Beyer, 

R.S., and Wershaw, R.L., 2003, Environmental fate of 

roxarsone in poultry litter, Part 1, Degradation of 

roxarsone during composting: Environmental Science 

and Technology, v. 37, no. 8, p. 1,509–1,514.

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.

Hansen, H.J., 1992, Stratigraphy of upper Cretaceous and 

Tertiary sediments in a core hole drilled near 

Chesterville, Kent County, Maryland: Maryland 

Geological Survey Open-File Report 93–02–7, 38 p.

Hantush, M.M., and Marino, M.A., 2001, Analytical 

modeling of the influence of denitrifying sediments on 

nitrate transport in aquifers with sloping beds: Water 

Resources Research, v. 37, n. 12, p. 3,177–3,192.

Hayes, T., Haston, K., Tsui, M., Hoang, A., Haeffele, C., 

and Vonk, A., 2002, Feminization of male frogs in the 

wild: Nature, v. 419, p. 895–896.

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

water resources: Amsterdam, Elsevier Publishers, 522 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 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, 

p. 500–518.

Larson, S.J., Capel, P.D., and Majewski, M.S., 1997, 

Pesticides in surface waters—Distribution, trends, and 

governing factors, v. 3 of Gilliom, R.J., ed., Pesticides in 

the hydrologic system: Chelsea, Michigan, Ann Arbor 

Press, 373 p.

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

E.M., 2001, 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 highperformance liquid chromatography/mass spectrometry: 

U.S. Geological Survey Open-File Report 01–10, 17 p.

Liu, Z.J., Weller, D.E., Correll, D.L., and Jordan, T.E., 

2000, Effects of land cover and geology on stream 

chemistry in watersheds of Chesapeake Bay: Journal of 

the American Water Resources Association, v. 36, no. 6, 

p. 1,349–1,365.
References Cited 33

Majewski, M.S., Foreman, W.T., Goolsby, D.A., and 

Nakagaki, N., 1998, Airborne pesticide residues along 

the Mississippi River: Environmental Science and 

Technology, v. 32, p. 3,689–3,698.

Martin, J.D., 2002, Variability of pesticide detections and 

concentrations in field replicate water samples collected 

for the National Water-Quality Assessment Program: 

U.S. Geological Survey Water-Resources Investigations 

Report 01-4178, 84 p.

Maryland Department of Agriculture, 1999, Maryland 

pesticide statistics for 1997: Maryland Department of 

Agriculture Report MDA-265-99, 56 p.

Maryland Department of Natural Resources, 1996, 

Digital orthophoto quarter quadrangle: Annapolis, 

Maryland, scale 1;12,000.

Miller, C.V., Denis, J.M., Ator, S.W., and Brakebill, J.W., 

1997, Nutrients in streams during baseflow in selected 

environmental settings of the Potomac River Basin: 

Journal of the American Water Resources Association, 

v. 33, no. 6, p. 1,155–1,171.

Mueller, D.K., Martin, J.D., and Lopes, T.J., 1997, 

Quality-control design for surface-water sampling in 

the National Water-Quality Assessment Program: 

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

Nowell, L.H., Capel, P.D., and Dileanis, P.D., 1999, 

Pesticides in stream sediment and aquatic biota— 

Distribution, trends, and governing factors, v. 4 of 

Gilliom, R.J., ed., Pesticides in the hydrologic system: 

Chelsea, Michigan, Ann Arbor Press, 1,001 p.

Owens, J.P., and Denny, C.S., 1978, Geologic map of 

Worcester County: Maryland Geological Survey, 

1 sheet, scale 1:62,500.

_____ 1979, Geologic map of Wicomico County: 

Maryland Geological Survey, 1 sheet, scale 1:62,500.

Owens, J.P., and Minard, J.P., 1962, Pre-Quaternary 

geology of the Columbus Quadrangle, New Jersey: 

U.S. Geological Survey Geologic Quadrangle 

Map GQ-160, 1 sheet, scale 1:24,000.

Phillips, P.J., Wall, G.R., Thurman, E.M., Eckhardt, 

D.A., and Van Hoesen, J., 1999, Metolachlor and its 

metabolites in tile drain and stream runoff in the 

Canajoharie Creek Watershed: Environmental Science 

and Technology, v. 33, p. 3,531–3,537.

Phillips, S.W., ed., 2002, The U.S. Geological Survey and 

the Chesapeake Bay—The role of science in 

environmental restoration: U.S. Geological Survey 

Circular 1220, 32 p.

Phillips, S.W., and Donnelly, C.A., 2003, Upper Pocomoke 

Watershed, p. 50–70, in 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.

Piper, A.M., 1944, A graphical procedure in the 

geochemical interpretation of water analyses: EOS, 

Transactions of the American Geophysical Union, v. 25, 

p. 914–923.

Pitchford, A.M., Denver, J.M., Olson, A.R., Ator, S.W., 

Cormier, S., Nash, M.S., and Mehaffey, M.H., 2000, 

Testing landscape indicators for stream condition related 

to pesticides and nutrients—Landscape indicators for 

pesticides study for Mid-Atlantic coastal streams

(LIPS-MACS): U.S. Environmental Protection Agency 

EPA-600-R-00-087, 84 p.

Puckett, L.J., 1987, The influence of forest canopies on the 

chemical quality of water and the hydrologic cycle, 

in Averett, R.C., and McKnight, D.M., eds. Chemical 

quality of water and the hydrologic cycle: Chelsea, 

Michigan, Lewis Publishers, p. 3–22.

_____ 1995, Identifying the major sources of nutrient water 

pollution: Environmental Science and Technology, v. 29, 

no. 9, p. 408A–414A.

Ramsey, K.W., and Schenck, W.S., 1990, Geologic map of 

southern Delaware: Delaware Geological Survey 

Open-File Report 32, 1 sheet, scale 1:100,000.

Rantz, S.E. and others, 1982, Measurement and 

computation of streamflow: U.S. Geological Survey 

Water-Supply Paper 2175, 915 p.

Reilly, T.E., Plummer, L.N., Phillips, P.J., and Busenberg, 

Eurybiades, 1994, The use of simulation and multiple 

environmental tracers to quantify ground water flow in a 

shallow aquifer: Water Resources Research, v. 30, n. 2, 

p. 421–433.

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, 1987–1991: U.S. Geological 

Survey Water-Supply 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.

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

processing stream-water samples for the National WaterQuality Assessment Program: U.S. Geological Survey 

Open-File Report 94–455, 42 p.

Sholar, C.J., and Shreve, E.A., 1988, Quality-assurance 

plan for the analysis of fluvial sediment by the 

Northeastern Region, Kentucky District Sediment 

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

98–384, 20 p.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP—A 

computer program for streamflow hydrograph separation 

and analysis: U.S. Geological Survey Water-Resources 

Investigations Report 96–4040, 46 p.

Smith, R.A., Schwarz, G.E., and Alexander, R.B., 1997, 

Regional interpretation of water-quality monitoring data: 

Water Resources Research v. 33, no. 12, p. 2,781–2,798.
34 Hydrologic and Geochemical Controls on Pesticide and Nutrient Transport to Two Streams on the Delmarva Peninsula

Sprague, L.A., Langland, M.J., Yochum, S.E., Edwards, 

R.E., Blomquist, J.D., Phillips, S.W., Shenk, G.W., and 

Preston, S.D., 2000, Factors affecting nutrient trends in 

major rivers of the Chesapeake Bay Watershed: U.S. 

 Geological Survey Water-Resources Investigations 

Report 00–4218, 98 p.

U.S. Department of Agriculture, 1970, Soil survey of 

Wicomico County, Maryland: Soil Conservation 

Service, 90 p.

_____ 1999, 1997 Census of agriculture, vol. 2, Ranking 

of states and counties, available at http://

www.nass.usda.gov/census/.

U.S. Geological Survey, 1950–2001, Water resources data, 

Maryland and Delaware, water years 1950–2001: 

U.S. Geological Survey Water-Data Reports 

MD-DE-50–MD-DE-01, [variously paged].

Vogelmann, J.E., Sohl, T.L., Campbell, P.V., and Shaw, 

D.M., 1998, Regional land-cover characterization using 

Landsat and ancillary sources: Environmental 

Monitoring and Assessment, v. 51, p. 415–428.

Wilde, F.D., and Radtke, D.B., 1998, Field measurements: 

National field manual for the collection of water-quality 

data: U.S. Geological Survey Techniques for WaterResources Investigations, book 9, chap. A6, 177 p.

Wilde, F.D., Radtke, D.B., Gibs, J., and Iwatsubo, R.T., 

1998, Cleaning of equipment for water sampling: 

National field manual for the collection of water-quality 

data: U.S. Geological Survey Techniques for WaterResources Investigations, book 9, chap. A3, 70 p.

Yochum, S.E., 2000, A revised load estimation procedure 

for the Susquehanna, Potomac, Patuxent, and Choptank 

Rivers: U.S. Geological Survey Water-Resources 

Investigations Report 00–4156, 49 p.

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

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

U.S. Geological Survey National Water-Quality 

Laboratory—Determination of pesticides in water by 

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

chromatography/mass spectrometry with selected ion 

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

95–181, 49 p.

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.