Introduction

Pesticides are synthetic organic chemicals used to control weeds, 

insects, and other pests. Nationally, about 

75 percent of all pesticide use is agricultural, although pesticides are also used 

for commercial, industrial, transportation, public-health, and other applications 

(Kiely and others, 2004). 

The occurrence and distribution of 

pesticides in ground water is affected by 

land use and pesticide application, as well 

as chemical and physical properties of 

different pesticides, and natural factors 

that affect their fate and movement in the 

environment (Barbash and Resek, 1996). 

Pesticides that are detected most frequently are those that are used most frequently, are relatively water soluble, and 

(or) have the greatest mobility and (or) 

persistence in the environment (Gilliom 

and others, 2006). The mobility of a 

pesticide is related to its water solubility 

and partitioning among different environmental media, such 

as air, water, soil, and biota. 

Some pesticides are highly 

volatile and likely to disperse 

into the air, for example, while 

others may sorb strongly to soil 

organic matter and accumulate 

in the soil or sediment rather 

than in water. Persistence refers 

to the tendency of a compound 

to remain in its original chemical form in the environment. 

Chemicals with low persistence 

rapidly transform to other 

compounds, referred to as 

degradates. These transformations occur through both abiotic 

and biotic processes. Abiotic 

processes involve photochemical or chemical transformations that require sunlight or 

naturally occurring chemicals 

in the soils and water. Biotic 

Abstract

Selected pesticides are detectable at low levels (generally 

less than 0.1 microgram per liter) in unconfined ground water 

in many parts of the Maryland Coastal Plain. Samples were recently collected (2001–04) from 47 wells in the Coastal Plain and 

analyzed for selected pesticides and degradate compounds 

(products of pesticide degradation). Most pesticide degradation 

occurs in the soil zone before infiltration to the water table, and 

degradates of selected pesticides were commonly detected in 

ground water, often at higher concentrations than their respective parent compounds. Pesticides and their degradates often 

occur in ground water in mixtures of multiple compounds, reflecting similar patterns in usage. All measured concentrations 

in ground water were below established standards for drinking 

water, and nearly all were below other health-based guidelines. 

Although drinking-water standards and guidelines are typically 

much higher than observed concentrations in ground water, 

they do not exist for many detected compounds (particularly 

degradates), or for mixtures of multiple compounds. 

The distribution of observed pesticide compounds reflects 

known usage patterns, as well as chemical properties and 

environmental factors that affect the fate and transport of these 

compounds in the environment. Many commonly used pesticides, such as glyphosate, pendimethalin, and 2,4-D were not 

detected in ground water, likely because they were sorbed onto 

organic matter or degraded in the soil zone. Others that are 

more soluble and (or) persistent, like atrazine, metolachlor, and 

several of their degradates, were commonly detected in ground 

water where they have been used. Atrazine, for example, an 

herbicide used primarily on corn, was most commonly detected 

in ground water on the Eastern Shore (where agriculture is 

common), particularly where soils are well drained. Conversely, 

dieldrin, an insecticide previously used heavily for termite 

control, was detected only on the Western Shore, where urban 

land is more common. Use of dieldrin was suspended in 1987, 

but this compound is relatively persistent in the environment, 

and several decades are typically required for ground water to 

move completely through the surficial aquifer.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS 2006–3119

2006

Location of the Maryland Coastal Plain.
processes involve transformations by animals, plants, or microbes. Most pesticide 

transformation occurs in the soil zone 

where these processes are most active 

(Barbash and Resek, 1996). Pesticides 

are complex molecules and a wide variety 

of degradates may form from one parent 

compound. The degradates may have 

similar or different chemical properties 

that affect their mobility and transformation in the environment. Over time they 

will break down into simple molecules 

like carbon dioxide and water.

Pesticides applied to the land surface 

or incorporated into soil can be carried 

as dissolved compounds in water from 

rainfall or irrigation that travels through 

the soil and unsaturated zone to ground 

water. Pesticide transport into ground 

water is most likely in well-drained areas 

with highly permeable sandy soils and 

aquifer sediments (Helling and Gish, 

1986). In such areas, water moves to 

the water table relatively quickly and 

easily, and soils and sediments often 

contain little organic matter that may 

sorb pesticides and restrict their movement to ground water. Pesticides are not 

commonly transported into confined 

aquifers because of fine-grained confining-bed sediments that impede downward 

transport of dissolved pesticides or other 

contaminants. In addition, decades or 

centuries are often required for water to 

move from the land surface to confined 

aquifers, and much of the water currently 

in confined aquifers infiltrated the land 

surface prior to the widespread use of 

pesticides in recent decades.

Pesticides in ground water may 

cause ecological and human-health 

effects. Many pesticides are toxic 

or known or suspected carcinogens 

(Toccalino and others, 2004; U.S. 

Environmental Protection Agency, 2004), 

and the occurrence of pesticides may 

affect the use of ground water for human 

consumption. This may be of particular 

concern in areas with limited alternative 

water supplies, or where supplies (such 

as domestic wells) are not routinely 

monitored. Ground water also contributes 

the majority of water to streams in many 

areas, and may be the only source of 

water during dry periods. Pesticides in 

ground water may thus contribute to the 

overall pesticide load in streams and 

downstream estuaries.

The occurrence and 

distribution of selected pesticides and pesticide degradates in shallow unconfined 

ground water of the Maryland 

Coastal Plain (fig. 1) are 

described and discussed 

in this report. Potential 

implications of observed 

pesticides for water supply, 

stream ecology, and waterresources management also 

are discussed. Concentrations 

of pesticides and degradates 

measured in samples collected recently (2001–04) from 

47 wells in Maryland are 

included. These wells were 

sampled as part of three wider 

regional studies for the U.S. 

Geological Survey National 

Water-Quality Assessment 

(NAWQA) Program (Gilliom 

and others, 1995). Sampled 

wells in Maryland represent 

ground water in primarily 

agricultural areas on the 

Eastern Shore, and along 

the Fall Line on the Western 

Shore, which has more 

variable land use and is 

typically more urban (fig. 1). No data 

are available for the southern part of 

the Western Shore, although that area is 

predominantly forested and has relatively 

limited unconfined ground water.

The Maryland Coastal Plain

The Coastal Plain includes the part 

of Maryland south and east of the Fall 

Line (fig. 1). This area is underlain by 

unconsolidated sand, gravel, silt, clay, 

and shells (Vroblesky and Fleck, 1991), 

and is geologically distinct from areas to 

the north and west, where consolidated 

rocks occur at or near the land surface. 

Unconsolidated sediments of the Coastal 

Plain form a wedge of aquifers and 

confining units that thickens from 0 ft 

(feet) along the Fall Line to greater than 

8,000 ft along the Atlantic Coast. Most of 

the ground water used for drinking water 

on the Western Shore and much of the 

Eastern Shore comes from confined aquifers. Although these aquifers are confined 

at depth, along the Fall Line where they 

subcrop or outcrop at the land surface 

they are under water-table conditions and 

2 Pesticides in Ground Water of the Maryland Coastal Plain

Figure 1. Land use, location of sampled wells, and summary of pesticides in ground water (2001–04) in 

the Maryland Coastal Plain.
part of the unconfined aquifer system. On 

the Eastern Shore, the confined aquifers 

are overlain by an extensive unconfined 

surficial aquifer, which is a major source 

of drinking water for small towns and 

individual homes.

Ground-water quality is determined 

by sediment type, natural geochemical 

conditions, and — particularly where an 

aquifer is unconfined — infiltration of 

chemicals from human sources. In confined aquifers, water chemistry reflects 

the mineralogy of the sediments, such as 

having high concentrations of calcium 

and bicarbonate from dissolution of shell 

material. Confined ground water also is 

generally suboxic or anaerobic (with little 

or no dissolved oxygen). There are typically no chemicals present from human 

sources because the confining layer 

provides protection from the influence 

of overlying land use. Water quality also 

is affected by mineral dissolution in the 

unconfined surficial aquifer system. Over 

much of the Maryland Coastal Plain, the 

surficial aquifer is susceptible to infiltration of chemicals from human sources 

because the sediments are permeable and 

the water table is relatively shallow. Most 

of the sediments of the surficial aquifer 

are composed of relatively insoluble 

quartz sands, and water chemistry is 

naturally dilute; chemicals leached from 

agricultural and urban sources can locally dominate ground-water chemistry 

(Shedlock and others, 1999). 

The average annual rainfall in 

Maryland is about 43 inches (Wheeler, 

2003), about a third of which infiltrates 

through the soil zone and recharges the 

unconfined surficial aquifer. The resulting 

water chemistry in the surficial aquifer 

reflects the spatially variable land use 

in aquifer recharge areas and the susceptibility of different areas to leaching 

of pesticides and other chemicals to 

ground water. Once infiltrating water 

and any associated chemicals enter the 

surficial aquifer, they are carried along 

ground-water flow paths to discharge 

areas in local streams and coastal waters. 

A small amount of water also moves 

downgradient into underlying confined 

aquifers. Most of the water in the surficial 

aquifer is less than 50 years old, and in 

many places, the entire aquifer has been 

impacted by chemicals from modern 

land uses (Debrewer and others, in press; 

Focazio and others, 1998).

Pesticide use in the Maryland 

Coastal Plain reflects variable land use 

in different areas. Of the hundreds of 

pesticides produced, few are widely used 

(Maryland Department of Agriculture, 

2002). The most commonly used pesticides in the Maryland Coastal Plain are 

herbicides used to control weeds in corn, 

soybean, and small grain crops (table 1). 

These include glyphosate, metolachlor, 

and atrazine, which are used most heavily 

on the Eastern Shore where agriculture 

is the dominant land use (fig. 1). The 

herbicide 2,4-D and the insecticides 

chlorpyrifos and diazinon, conversely, 

are used for both urban and agricultural 

purposes. These compounds are ap­plied 

Pesticides in Ground Water of the Maryland Coastal Plain 3
more equally on the Eastern Shore and 

the Western Shore, which has more 

variable land use, and includes heavily 

urbanized areas along the Fall Line (fig. 

1). Pesticide use has changed over time 

as new compounds are introduced and 

others are discontinued. Acetochlor was 

introduced in the mid-1990s to replace 

alachlor, for example; alachlor use has 

greatly declined since then (Maryland 

Department of Agriculture, 1996, 1999, 

2002). In addition, use of several insecticides, including diazinon, chlorpyrifos, 

and dieldrin, was recently banned or 

greatly reduced (U.S. Environmental 

Protection Agency, 2000).

Measuring Pesticides in 

Ground Water

Water-quality data described in 

this report were collected from 47 

wells that are part of three regional 

NAWQA networks that include parts of 

the Maryland Coastal Plain (table 2). 

Regional NAWQA ground-water surveys 

are designed to document and explain (to 

the extent possible) the occurrence and 

distribution of selected chemical compounds (such as pesticides) in ground 

water in particular types of settings 

(Gilliom and others, 2006). Land-use 

studies are designed to evaluate the 

quality of relatively shallow, recently 

recharged ground water in a particular 

land-use setting, whereas major-aquifer 

studies are designed to evaluate ground 

water throughout an aquifer. Regional 

networks are established for each type of 

study, typically including 20 to 30 wells 

that are areally distributed throughout 

the targeted setting. Regional NAWQA 

ground-water networks that include parts 

of the Maryland Coastal Plain include an 

agricultural land-use study and a majoraquifer study in the surficial aquifer on 

the Delmarva Peninsula (Debrewer and 

others, in press), and a major-aquifer 

study in the unconfined aquifer along the 

Fall Line from the Washington, D.C. area 

through New Jersey (table 2).

Water samples were recently collected (2001 through 2004) from each well 

in the regional networks and analyzed for 

selected chemical compounds, including 

selected pesticides and degradates. 

Sample collection, processing, and analysis for the Eastern Shore networks are described in Debrewer and others (in press); 

similar protocols were used for wells 

on the Western Shore. Although some 

network wells are used for water supply, 

samples were collected prior to any 

filtering or treatment, and are therefore 

representative of the available groundwater resource rather than necessarily of 

drinking water. Samples were analyzed 

for a broad (though not complete) suite of 

commonly used pesticides and degradates (table 3); these compounds were 

measured in filtered water samples using 

analytical methods designed to measure 

particularly low concentrations, around 

0.005 mg/L (micrograms per liter) for 

4 Pesticides in Ground Water of the Maryland Coastal Plain
some compounds. Although these concentrations may be far below established 

water-quality standards, such information 

can be particularly useful for detecting 

trends in pesticide concentrations, and for 

understanding natural and human influences affecting pesticide occurrence in 

ground water. 

Pesticides in Ground Water 

of the Maryland Coastal Plain

A variety of pesticide compounds 

are detectable at typically low concentrations in unconfined ground water of the 

Maryland Coastal Plain. Of the more than 

100 pesticides and degradate compounds 

for which samples were analyzed (table 

3), 29 were detected at least once (fig. 2). 

Concentrations were generally less than 

0.1 mg/L for most compounds. No samples contained any pesticide compounds 

at concentrations exceeding established 

Federal drinking-water standards, 

although such standards exist for only 4 

of the 29 compounds detected (see inset, 

page 6). Non-regulatory health-based 

guidelines for an additional nine detected 

compounds also were rarely exceeded, 

although three samples contained dieldrin 

at concentrations exceeding 0.002 mg/L. 

Most (21 of 29) of the detected compounds are herbicides typically used for 

agriculture (such as atrazine and metolachlor) or their degradates, reflecting 

the importance of agricultural herbicides 

among pesticides used in Maryland 

(table 1). The dominance of agricultural 

herbicides among pesticides detected also 

reflects the prevalence of agriculture in 

sampled areas, however (fig. 1, table 2), 

as well as the dominance of herbicides 

among target analytes (table 3). Only 

4 of the 29 detected compounds are 

insecticides; no insecticide was detected 

in more than 10 percent of the samples 

for which it was analyzed (fig. 2).

Concentrations of selected herbicide 

degradates often substantially exceed 

concentrations of their respective parent 

compounds in ground water (Debrewer 

and others, in press). Selected degradates 

of atrazine, metolachlor, and alachlor 

were among the most frequently detected 

compounds in ground water of the 

Maryland Coastal Plain (fig. 2), even 

though many metolachlor and alachlor 

degradates were measured in samples 

from only 30 of 47 wells (those from the 

Eastern Shore networks). Metolachlor 

ESA (ethanesulfonic acid), metolachlor 

OA (oxanilic acid), and alachlor ESA 

were each detected in more than 40 

percent of the 30 samples for which they 

were analyzed, and measured concentrations were typically greater than or equal 

to 0.1 mg/L (fig. 2). The median metolachlor ESA concentration among these 

30 samples was 0.59 mg/L. By contrast, 

metolachlor and alachlor concentrations 

were each greater than or equal to 0.1 

mg/L in only 1 of these same 30 samples. 

The relative magnitude of concentrations of selected degradates and parent 

compounds in Coastal Plain ground water 

reflects the fact that most pesticide degradation occurs in the soil zone, before infiltration reaches the water table (Barbash 

and Resek, 1996). Also, ESA and OA 

degradates of metolachlor and alachor 

are more soluble than their respective 

parent compounds (Phillips and others, 

1999). The similar detection frequencies 

of atrazine and deethylatrazine relative to 

those of metolachlor and its degradates 

(fig. 2) likely reflect the fact that atrazine 

degrades primarily to hydroxyatrazine, 

which is less mobile and more likely to 

sorb to soils (Barbash and Resek, 1996). 

Observed concentrations of metolachlor and alachlor degradates similarly 

Pesticides in Ground Water of the Maryland Coastal Plain 5

Figure 2. Detection frequency and maximum measured concentration for 29 pesticides 

and degradates detected in ground water in the Maryland Coastal Plain, 2001–04 

[E, estimated].
exceeded concentrations of parent compounds in two streams on the Eastern 

Shore, which receive most of their flow 

from ground-water discharge (Ator and 

others, 2005). Pesticide degradates also 

have been frequently detected in ground 

water and streams in other parts of the 

Nation (Gilliom and others, 2006).

When present, pesticides commonly 

occur in mixtures in ground water. Of 

the 47 wells from which samples were 

analyzed, 32 (68 percent) contained 

detectable concentrations of at least 1 

pesticide compound or degradate (fig. 

1). Only 2 of these 32 samples contained 

only 1 detectable compound, however, 

and both of these were samples from the 

Western Shore, which were analyzed for 

a relatively limited number of analytes 

(table 3). On the Eastern Shore, 24 of 

30 samples (80 percent) contained at 

least 1 detectable pesticide or degradate 

compound, and 16 (53 percent) contained 

at least 5 detectable compounds. One 

sample contained 11 different compounds. Multiple compounds also were 

detected in about half (19) of 37 groundwater samples with detectable pesticides 

collected from 1988 through 1990 on the 

Delmarva Peninsula (Koterba and others, 

1993). Similarly, about half (47 percent) 

of shallow wells in agricultural areas 

and 37 percent of such wells in urban 

areas across the Nation sampled recently 

(1992–2001) by NAWQA contained 

two or more detectable pesticides or 

degradates (Gilliom and others, 2006). 

The prevalence of pesticide mixtures in 

ground water reflects similar patterns 

in usage; multiple compounds are often 

applied to the same area (often in mixed 

formulations) for greater pest control. In 

addition, many pesticides have similar 

chemical properties that control their fate 

and movement in the environment.

Pesticides are detectable in many 

parts of the Maryland Coastal Plain, 

although the occurrence of different 

compounds generally reflects known 

usage patterns. The 32 wells from which 

samples with detectable pesticides or 

degradates were collected are located in 

many areas of the Coastal Plain, primarily near likely sources in agricultural or 

urban areas (fig. 1). Atrazine or deethylatrazine, for example, was detected in 

18 of the 30 samples (60 percent) from 

the Eastern Shore (where agriculture is 

common and atrazine use is consequently 

relatively high), but in only 2 of 17 wells 

on the Western Shore (fig. 3). Debrewer 

and others (in press) noted that atra­zine 

and metolachlor concentrations in 

ground water on the Delmarva Peninsula 

(including the 30 Eastern Shore samples 

described in this report) increase with 

an increasing proportion of agriculture 

near sampled wells. Dieldrin, an insecticide that was primarily used for termite 

control, however, was only detected in 

3 of 47 wells, each in relatively urban 

areas on the Western Shore. Seven of the 

eight samples from the Western Shore 

with detectable pesticide or degradate 

compounds contained dieldrin and (or) 

an herbicide typically used in urban areas 

(prometon, simazine, or tebuthiuron), 

although five of these samples also 

contained atrazine or metolachlor. The 

15 wells with no detectable pesticides are 

mostly located in relatively forested areas 

(such as parts of Cecil County), or where 

6 Pesticides in Ground Water of the Maryland Coastal Plain

Ground Water and Drinking Water

Available standards and guidelines for drinking water provide some context 

for understanding observed pesticide concentrations in ground water. Maximum 

Contaminant Levels (MCLs) are enforceable standards for the maximum level of a 

contaminant in drinking water (U.S. Environmental Protection Agency, 2004). For 

unregulated compounds (those with no established MCL or other standard), nonenforceable Health-Based Screening Levels (HBSLs) have been estimated on the 

basis of U.S. Environmental Protection Agency (USEPA) methodologies and toxicity 

data for comparison to water quality (Toccalino and others, 2003, 2004). Both MCLs 

and HBSLs apply to concentrations in drinking water, rather than untreated (“raw”) 

ground water described in this report, however. Also, single samples from each well 

have limited utility for estimating potential exposure over time. Interpreting observed 

ground-water quality in a human-health context is also complicated by the lack of 

MCLs or HBSLs for many compounds or for mixtures of compounds, and the HBSL for 

one detected compound (dieldrin) is lower than the current laboratory reporting limit 

(Toccalino and others, 2004).

Figure 3. Atrazine usage in the Maryland Coastal Plain and detection of atrazine and (or)

deethylatrazine in ground water (2001–04).
poor drainage or organic soils limit pesticide movement to ground water (such as 

parts of the lower Eastern Shore) (fig. 1).

The occurrence of pesticides in 

ground water is related to chemical 

properties and environmental factors that 

affect their fate and transport, as well as 

usage patterns. Pesticides such as atrazine, metolachlor, and their degradates 

that are relatively commonly detected in 

ground water are generally more water 

soluble and (or) persistent than other 

compounds, which may be used more frequently. Glyphosate, pendimethalin, 

2,4-D, and chlorpyrifos, conversely, are 

widely used but have chemical properties that limit their movement to ground 

water. Glyphosate, for example, is more 

soluble in water than atrazine, but less 

persistent and more likely to sorb onto 

organic matter in soils or sediments 

(Debrewer and others, in press). Soluble 

compounds are less likely to accumulate 

in soil, sediment, or wildlife, but more 

likely to occur in ground water or 

streams. Pesticide occurrence in ground 

water is also related to environmental 

factors that affect the movement of these 

compounds from the land surface to the 

water table. Atrazine concentrations in 

shallow ground water in agricultural areas 

of the Delmarva Peninsula are greater in 

relatively well-drained areas (such as the 

central Eastern Shore) than in areas with 

more poorly drained soils (such as parts 

of the lower Eastern Shore) that may 

limit the movement of pesticides to the 

water table (fig. 3) (Koterba and others, 

1993; Debrewer and others, in press). 

Atrazine concentrations also are greater 

in the surficial aquifer on the Delmarva 

Peninsula in more recently recharged 

ground water, as might be expected if 

older ground water entered the aquifer 

in previous years when less atrazine 

was used (Koterba and others, 1993; 

Debrewer and others, in press). The importance of ground-water residence time 

on the occurrence of pesticides is further 

demonstrated by the detection of dieldrin 

in three samples. Its use was discontinued 

in 1987.

Summary and Discussion

Selected pesticides and degradate 

compounds are detectable in surficial, 

unconfined ground water in many areas 

of the Maryland Coastal Plain, although 

measured concentrations were generally less than 0.1 mg/L, and no existing 

Federal drinking-water standards were 

exceeded. As noted previously in other 

areas, pesticides and degradates typically 

occur in ground water in mixtures of 

multiple compounds, and concentrations 

of degradates of selected compounds 

generally far exceed those of parent 

compounds. Spatial patterns in pesticide 

occurrence in ground water of the 

Maryland Coastal Plain generally reflect 

known usage patterns, as well as chemical properties and environmental factors 

that affect the fate and transport of these 

compounds in the environment.

Implications of observed pesticide 

occurrence on the potability of drinking 

water from the surficial aquifer in the 

Maryland Coastal Plain are uncertain 

on the basis of available information. 

Pesticides are present at relatively low 

levels in parts of the aquifer. Debrewer 

and others (in press) noted similar 

patterns of occurrence in the entire 

Delmarva Peninsula, and pesticides also 

have been detected in deeper parts of the 

aquifer more typically used for public 

supplies in Delaware (Ferrari, 2002; 

Denver and others, 2004). Although measured pesticide concentrations as part of 

these studies were generally below MCLs 

or HBSLs, such standards or guidelines 

are not available for many compounds, 

particularly the degradates. In addition, 

little information about possible additive 

or synergistic effects of multiple compounds is available. 

Pesticides in ground water may 

have ecological impacts on streams in 

the Maryland Coastal Plain and downstream estuaries. Ground-water discharge 

provides the majority of flow to streams 

in many parts of the Maryland Coastal 

Plain, and is often the only source of 

water during dry periods. Much of 

this discharge (particularly for smaller 

streams) is generated from the surficial 

aquifer. Dissolved pesticides and chemicals in ground water may move into 

streams with ground-water discharge, 

and contribute to the overall load of 

these compounds in streams and downstream estuaries such as Chesapeake Bay. 

Pesticides have been detected in streams 

of the Delmarva Peninsula during baseflow periods, when streamflow is mainly 

derived from ground-water discharge 

(Denver and others, 2004; Ator and 

others, 2005).

Pesticides in Ground Water of the Maryland Coastal Plain 7

Management practices designed to 

protect aquifers from chemical contamination are complicated by the unique 

nature of the ground-water resource. 

Infiltration to ground water occurs over 

broad areas of the land surface, particularly in well-drained areas most suitable 

for cultivation or other development. 

Limiting pesticide applications in groundwater recharge areas may therefore be 

particularly difficult. Traditional management practices such as forested riparian 

buffers, contour tillage, or grassed swales 

may be effective at limiting overland 

movement of sediment, nutrients, pesticides, and other chemicals to streams, but 

may increase infiltration of stormwater, 

and divert soluble chemicals to ground 

water. Water-resources management also 

is complicated by the particularly slow 

movement of ground water and resulting 

long residence times in aquifers. Many 

decades may be required before implications of management practices at the land 

surface are fully realized throughout the 

extent of the surficial aquifer (Kaufmann 

and others, 2001). 

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Report 99–4095, 12 p.

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M.A., Hamilton, P.A., Koterba, M.J., 

Bachman, L.J., Phillips, P.J., and 

Banks, W.S.L., 1999, Water-quality 

assessment of the Delmarva Peninsula, 

Delaware, Maryland, and Virginia 

— Results of investigations, 1987–91: 

U.S. Geological Survey Water-Supply 

Paper 2355–A, 41 p.

Toccalino, P.L., Norman, J.E., Phillips, 

R.H., Kauffman, L.J., Stackelberg, 

P.E., Nowell, L.H., Krietzman, S.J., 

and Post, G.B., 2004, Application 

of health-based screening levels to 

ground-water quality data in a statescale pilot effort: U.S. Geological 

Survey Scientific Investigations Report 

2004–5174, 64 p.

Toccalino, Patricia, Nowell, Lisa, Wilber, 

William, Zogorski, John, Donohue, 

Joyce, Eiden, Catherine, Krietzman, 

Sandra, and Post, Gloria, 2003,

Development of health-based screening levels for use in state or localscale water-quality assessments: U.S. 

Geological Survey Water-Resources 

Investigations Report 03–4054, 22 p. 

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2000, Chlorpyrifos revised risk assessment and agreement with registrants, 

accessed August 1, 2006 at http://

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2004, 2004 Edition of the drinking 

water standards and health advisories: 

U.S. Environmental Protection Agency 

Report EPA 822–R–04–005, 12 p.

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land-cover dataset, edition 1, accessed 

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USGS Fact Sheet 

FS 2006–3119

Prepared by USGS Publishing Service Center 3.

Edited by Valerie M. Gaine.

Graphics and design by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, MD 21237

or visit our Web site at:

http://md.water.usgs.gov

Also contact: 

Maryland Department of Agriculture

Pesticide Regulation Section

50 Harry S. Truman Parkway

Annapolis, MD 21401

410-841-5710

or visit the MDA Web site at:

http://www.mda.state.md.us