NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

New Cumberland, Pennsylvania

2001

SUMMARY OF AND FACTORS AFFECTING PESTICIDE CONCENTRATIONS IN STREAMS

AND SHALLOW WELLS OF THE LOWER SUSQUEHANNA RIVER BASIN,

PENNSYLVANIA AND MARYLAND, 1993-95

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 01-4012

by Robert A. Hainly, Tammy M. Zimmerman, Connie A. Loper, and Bruce D. Lindsey
ii

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information Copies of this report may be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

215 Limekiln Road Box 25286

New Cumberland, Pennsylvania 17070-2424 Denver, Colorado 80225-0286

Information regarding the National Water-Quality Assessment (NAWQA) Program is available on the

Internet via the World Wide Web. You may connect to the NAWQA Home Page using the Universal

Resource Locator (URL) at:

http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM III

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely

scientific information that helps enhance and protect the overall quality of life, and facilitates

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

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

linked to the long-term availability of water that is clean and safe for drinking and recreation and that

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

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

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

ecosystems.

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

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

and policy. Shaped by and coordinated with ongoing efforts of other Federal, State, and local

agencies, the NAWQA Program is designed to answer: What is the condition of our Nation’s

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

and human activities affect the quality of streams and ground water, and where are those effects

most pronounced? By combining information on water chemistry, physical characteristics, stream

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

and emerging water issues. NAWQA results can contribute to informed decisions that result in

practical and effective water-resource management and strategies that protect and restore water

quality.

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

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

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

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

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

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

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

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

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

water-quality issues are isolated or pervasive, and allows direct comparisons of how human

activities and natural processes affect water quality and ecological health in the Nation’s diverse

geographic and environmental settings. Comprehensive assessments on pesticides, nutrients,

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

through comparative analysis of the Study-Unit findings.

The USGS places high value on the communication and dissemination of credible, timely, and

relevant science so that the most recent and available knowledge about water resources can be

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

the needed insights and information to meet your needs, and thereby foster increased awareness

and involvement in the protection and restoration of our Nation’s waters.

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

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

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

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

cooperation, and information from other Federal, State, interstate, Tribal, and local agencies, nongovernment organizations, industry, academia, and other stakeholder groups. The assistance and

suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD
IV NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

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

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

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

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Previous investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Description of study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Climate and hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Hydrogeologic settings and environmental subunits . . . . . . . . . . . . . . . . . . . . 9

Pesticide use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Study methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Data collection and analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Statistical analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Quality-assurance procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Quality-assurance results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Surrogates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Blanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Replicates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Summary of detected pesticides and measured concentrations . . . . . . . . . . . . . . . 27

Detection frequency of measured pesticides . . . . . . . . . . . . . . . . . . . . . . . . . 27

Concentrations of selected pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Factors affecting pesticide concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Conceptual models for major factors affecting pesticide concentrations . . . . 37

Factors affecting seasonal concentration patterns in streams . . . . . . . . . . . . 37

Seasonality of applications and climate . . . . . . . . . . . . . . . . . . . . . . . . 38

Ground-water retention and discharge. . . . . . . . . . . . . . . . . . . . . . . . . 45

Factors affecting areal concentration patterns in wells and streams . . . . . . . 46

Areal application patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Areal application patterns in agricultural areas . . . . . . . . . . . . . 46

Areal application patterns in nonagricultural areas . . . . . . . . . . 53

Method of pesticide application, pesticide persistence and

leaching potential, and infiltration capacity of soils . . . . . . . . . . . . . 57

Ground-water retention and discharge. . . . . . . . . . . . . . . . . . . . . . . . . 61

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Conclusions from analysis of concentration data . . . . . . . . . . . . . . . . . . . . . . 66

Conclusions from analysis of seasonal patterns . . . . . . . . . . . . . . . . . . . . . . 66

Conclusions from analysis of areal patterns. . . . . . . . . . . . . . . . . . . . . . . . . . 67

References cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Appendix—Methodology used to characterize streamflow condition . . . . . . . . . . . . 75

Development of a time-interval estimate for return to base-flow

conditions following a storm event. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Application of estimated time interval and determination of stormaffected samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Consideration of anthropogenic influences on the stream hydrograph . . . . . 75

Consideration of observed streamflow conditions . . . . . . . . . . . . . . . . . . . . . 75

CONTENTS

Page
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM V

Figures 1-4. Maps showing:

1. Major physical features and generalized land use in the

Lower Susquehanna River Basin study unit,

Pennsylvania and Maryland. . . . . . . . . . . . . . . . . . . . . . . . . 8

2. Subunits within the Lower Susquehanna River Basin

study unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3. The Lower Susquehanna River Basin study unit,

counties, major environmental subunits, and location

of long-term monitoring basins and stream sites where

pesticide samples were collected from 1993 to 1995. . . . . 16

4. The Lower Susquehanna River Basin study unit,

counties, major environmental subunits, and location

of shallow ground-water wells where pesticide samples

were collected from 1993 to 1995 . . . . . . . . . . . . . . . . . . . 18

 5-9. Graphs showing:

5. Monthly precipitation and cumulative percentage of

corn planted in the East Mahantango Creek Basin

and base-flow herbicide concentrations and daily

mean streamflow for East Mahantango Creek,

1993-94.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6. Monthly precipitation and cumulative percentage of

corn planted in the Mill Creek Basin and base-flow

herbicide concentrations and daily mean streamflow

for Mill Creek, 1993-94. . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7. Monthly precipitation and cumulative percentage of

corn planted in the Cedar Run Basin and base-flow

herbicide concentrations and daily mean streamflow

for Cedar Run, 1993-95. . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8. Streamflow and concentrations of diazinon, chlorpyrifos,

and atrazine measured in Cedar Run at Eberlys Mill, Pa.,

before, during, and after selected storms in May through

July 1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

9. Streamflow and concentrations of diazinon, chlorpyrifos,

and atrazine measured in Bachman Run at Annville, Pa.,

before, during, and after selected storms in May through

July 1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

ILLUSTRATIONS

Page
VI NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Figure 10-11. Maps showing:

10. Categorized average annual atrazine agricultural

applications, 1990-94, and categorized atrazine

concentrations measured in water from streams

(subunit synoptic surveys) and ground water

(shallow-well synoptic surveys), 1993-95, in the

Lower Susquehanna River Basin study unit . . . . . . . . . . . 49

11. Patterns of average annual pendimethalin agricultural

applications, 1990-94, and pendimethalin

concentrations measured in water from streams

(subunit synoptic surveys) and ground water

(shallow-well synoptic surveys), 1993-95,

in the Lower Susquehanna River Basin study unit. . . . . . . 51

12. Graphs showing estimated annual use, 1990-94, for agricultural

purposes and the median base-flow concentrations of five

selected pesticides observed in water from streams in

three environmental subunits of the Lower Susquehanna

River Basin study unit where long-term monitoring stream

sites were located . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

13. Maps showing categorized average annual simazine agricultural

applications, 1990-94, counties with high residential land

use, and categorized simazine concentrations measured

in water from streams (subunit synoptic surveys) and

ground water (shallow-well synoptic surveys), 1993-95,

in the Lower Susquehanna River Basin study unit . . . . . . . . . . . . . 56

14-15. Graphs showing:

14. Infiltration-capacity ratings, leaching potential, and

frequency of detection for selected pesticides,

1993-95, within various hydrogeologic settings

of the Lower Susquehanna River Basin study unit . . . . . . 60

15. Atrazine and simazine concentrations in wells and

streams during base-flow synoptics, 1993-95,

in selected environmental subunits, Lower

Susquehanna River Basin . . . . . . . . . . . . . . . . . . . . . . . . . 65

ILLUSTRATIONS—CONTINUED

Page
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM VII

Table 1. Description of environmental subunits where pesticide

samples were collected in 1993-95 in the Lower

Susquehanna River Basin study unit, Pennsylvania and

Maryland. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Pesticides for which ground- and surface-water samples were

analyzed in the Lower Susquehanna River Basin study unit

from 1993 to 1995 and information on water-quality standards

and pesticide use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3. Summary of characteristics of pesticides selected for detailed

analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4. Statistical summary of surrogate recoveries (in percent) from

National Water Quality Laboratory reagent water and from

environmental samples collected from the Lower

Susquehanna River Basin study unit . . . . . . . . . . . . . . . . . . . . . . . 25

5. Pesticides (listed alphabetically by pesticide type), the Method

Detection Limit for the pesticide, the rank of its use for

agricultural purposes in the study unit, frequency of detection

in ground and surface water, and median and maximum

measured concentrations, Lower Susquehanna River Basin

study unit, 1993-95 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

6. Pesticides commonly used in Pennsylvania, pesticides analyzed,

and number of detections in water from streams during stormaffected and base-flow conditions and water from ground-water

wells, Lower Susquehanna River Basin study unit, 1993-95 . . . . . 31

7. Statistical summary of atrazine, simazine, pendimethalin,

chlorpyrifos, and diazinon concentrations in water samples

collected within the Lower Susquehanna River Basin study

unit, 1993-95. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

8. Average annual amounts of selected pesticides used for

agricultural purposes and rank, by county, in the Lower

Susquehanna River Basin study unit, 1990-94. . . . . . . . . . . . . . . . 47

9. Cropland and suburban residential land use within the Lower

Susquehanna River Basin study unit, by county. . . . . . . . . . . . . . . 54

10. Summary of detection frequency in water from streams (baseflow synoptic surveys) and ground water (shallow-well synoptic

surveys) for selected pesticides for various environmental

subunits within the Lower Susquehanna River Basin study

unit, 1993-95. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

TABLES

Page
VIII NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square mile (mi2) 2.590 square kilometer

Mass

pound, avoirdupois (lb) 0.4536 kilogram

Application Rate

pounds per acre per year 1.121 kilograms per

[(lb/acre)/yr] hectare per year

Temperature

degree Fahrenheit (°F) °C = 5/9 × (°F − 32) degree Celsius

Other Abbreviations:

L, liter

μg/L, micrograms per liter

μL, microliter

μm, micrometer

mm, millimeter

lb/yr, pounds per year

Chemical concentrations used in this report are given in micrograms per

liter (μg/L), a metric unit. Micrograms per liter is a unit expressing the

concentration of chemical constituents or compounds in solution as weight

(micrograms) of solute per unit volume (liter) of water. For concentrations

less than 7,000 μg/L, the numerical value is the same as for

concentrations in parts per billion.

CONVERSION FACTORS AND ABBREVIATIONS

Multiply by To obtain
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 1

SUMMARY OF AND FACTORS AFFECTING PESTICIDE

CONCENTRATIONS IN STREAMS AND SHALLOW WELLS

OF THE LOWER SUSQUEHANNA RIVER BASIN,

PENNSYLVANIA AND MARYLAND, 1993-95

by Robert A. Hainly, Tammy M. Zimmerman,

Connie A. Loper, and Bruce D. Lindsey

ABSTRACT This report presents the detection frequency of 83 analyzed pesticides, describes the

concentrations of those pesticides measured in water from streams and shallow wells,

and presents conceptual models of the major factors affecting seasonal and areal

patterns of pesticide concentrations in water from streams and shallow wells in the

Lower Susquehanna River Basin. Seasonal and areal patterns of pesticide

concentrations were observed in 577 samples and nearly 40,000 pesticide analyses

collected from 155 stream sites and 169 shallow wells from 1993 to 1995. For this

study, shallow wells were defined as those generally less than 200 feet deep.

The most commonly detected pesticides were agricultural herbicides—atrazine,

metolachlor, simazine, prometon, alachlor, and cyanazine. Atrazine and metolachlor

are the two most-used agricultural pesticides in the Lower Susquehanna River Basin.

Atrazine was detected in 92 percent of all the samples and in 98 percent of the stream

samples. Metolachlor was detected in 83 percent of all the samples and in 95 percent

of the stream samples. Nearly half of all the analyzed pesticides were not detected in

any sample. Of the 45 pesticides that were detected at least once, the median

concentrations of 39 of the pesticides were less than the detection limit for the

individual compounds, indicating that for at least 50 percent of the samples collected,

those pesticides were not detected. Only 10 (less than 0.025 percent) of the

measured concentrations exceeded any established drinking-water standards;

25 concentrations exceeded 2 μg/L (micrograms per liter) and 55 concentrations

exceeded 1 μg/L. None of the elevated concentrations were measured in samples

collected from streams that are used for public drinking-water supplies, and 8 of the 10

were measured in storm-affected samples.

The timing and rate of agricultural pesticide applications affect the seasonal and areal

concentration patterns of atrazine, simazine, chlorpyrifos, and diazinon observed in

water from wells and streams in the Lower Susquehanna River Basin. Average annual

pesticide use for agricultural purposes and nonagricultural pesticide use indicators

were used to explain seasonal and areal patterns. Elevated concentrations of some

pesticides in streams during base-flow and storm-affected conditions were related to

the seasonality of agricultural-use applications and local climate conditions.

Agricultural-use patterns affected areal concentration patterns for the high-use

pesticides, but indicators of nonagricultural use were needed to explain concentration

patterns of pesticides with smaller amounts used for agricultural purposes.

Bedrock type influences the movement and discharge of ground water, which in turn

affects concentration patterns of pesticides. The ratio of atrazine concentrations in

stream base flow to concentrations in shallow wells varied among the different general

rock types found in the Lower Susquehanna River Basin. Median concentrations of

atrazine in well water and stream base flow tended to be similar in individual areas

underlain by carbonate bedrock, indicating the connectivity of water in streams and
2 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

shallow wells in these areas. In areas underlain by noncarbonate bedrock, median

concentrations of atrazine tended to be significantly higher in stream base flow than in

well water. This suggests a deep ground-water system that delivers water to shallow

wells and a near-surficial system that supplies base-flow water to streams. In addition

to the presence or absence of carbonate bedrock, pesticide leaching potential and

persistence, soil infiltration capacity, and agricultural land use affected areal patterns

in detection frequency and concentration differences between samples collected from

streams during base-flow conditions and shallow wells.

INTRODUCTION

The National Water-Quality Assessment (NAWQA) Program is a nationwide, longterm U.S. Geological Survey (USGS) study to assess the quality of our Nation’s

waters. The full-scale NAWQA Program was implemented over a 6-year period in

51 separate study units. The study units are river basins or aquifer systems that range

from about 1,200 to 50,000 mi2 and include about 60 to 70 percent of the Nation’s

water use. The design of the NAWQA Program, as described by Gilliom and others

(1995), includes four interrelated components: 1) retrospective analysis,

2) occurrence and distribution assessment, 3) trend and change assessment, and

4) case studies of sources, transport, fate, and effects. The goals of two of these

components—the assessment of occurrence and distribution and case studies in

relation to pesticides in the water column—are addressed in this report.

Studies began in 1991 in the Lower Susquehanna River Basin NAWQA study unit,

hereafter termed “the study unit.” The bulk of the water-quality sampling was

completed during 1993-95, the intensive phase. Following this phase, interpretive

reports were completed and a low intensity water-quality sampling program was

conducted from 1997-2000. Topics for national synthesis and retrospective analysis

include pesticides, volatile organic pesticides, nutrients, and suspended sediment.

The investigation of pesticides, the focus of this report, began with compilation and

analysis of available data. Pesticide use for agricultural purposes in the study unit

ranks high nationally (Gianessi and Puffer, 1991). In a few counties in the southern

part of the study unit, application rates of pesticides are among the highest in the

Nation (Barbash and Resek, 1996), making the effects of pesticide usage on the

environment and human health a topic of concern.

Purpose and Scope

This report presents the detection frequencies of all analyzed pesticides, describes

the concentrations measured in water from streams and shallow wells, and presents

conceptual models of the major factors affecting seasonal and areal patterns of

pesticide concentrations in water from wells and streams in the Lower Susquehanna

River Basin.

The report is limited to an overview of pesticide concentrations measured in water

from shallow wells (generally less than 200 ft deep) and streams in the study unit

during the 1993-95 water years (the 12-month period October 1 to September 30

designated by the calendar year in which it ends) and a detailed analysis of five

selected pesticides. Concentrations of 46 selected pesticides were analyzed in all

samples. Concentrations of an additional 37 pesticides were measured in slightly

more than half of the samples. A total of nearly 40,000 pesticide concentrations

measured in 577 samples obtained from 169 ground-water wells and 155 stream

water-quality sites are summarized.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 3

Acknowledgments

The authors acknowledge significant efforts by the Lower Susquehanna River Basin

NAWQA team in the collection of data for this report. In addition, thanks are given to

the report review and production team for constructive comments that improved the

quality and presentation of the study results and the document.

PREVIOUS INVESTIGATIONS

Numerous studies have been conducted to describe the occurrence of pesticides in

water from wells and streams in the study unit. From 1970 to 1990, 10 ground-water

studies, 8 surface-water studies, and 2 studies that focused on both ground and

surface waters were done. Atrazine was frequently detected in ground-water and/or

surface-water samples in many of these studies. Concentrations of atrazine and other

detected pesticides rarely exceeded maximum contaminant levels for drinking water

supplies. Ground-water studies were done at national, basin-wide, and local scales

and will be summarized briefly in the paragraphs to follow. Ranges of concentrations

for frequently detected pesticides are given when possible, but detection limits among

the studies may have varied.

Two national well-water surveys included wells located in the study unit. Both studies

determined the occurrence of a broad suite of pesticides in water from nearly

1,500 wells nationwide (Holden and others, 1992; Klein and others, 1993; U.S.

Environmental Protection Agency, 1990a, 1990b). The work of Holden and others

(1992) and Klein and others (1993) summarized one national survey in which the

detection frequency of the pesticide alachlor was the primary focus. Less than

1 percent of the estimated six million existing private domestic wells had detectable

levels of alachlor (Holden and others, 1992). The other national well-water survey

included analyses of 126 pesticides and pesticide degradates. Atrazine and DCPA

acid metabolites were most commonly found; maximum concentrations were 7.0 and

2.4 μg/L, respectively. Pesticide concentrations were generally low and rarely

exceeded any established U.S. Environmental Protection Agency (USEPA) drinkingwater guidelines (U.S. Environmental Protection Agency, 1996). Both studies were

designed to provide information on frequency of detection and concentration of

pesticides in wells on a nationwide basis; therefore, it was not possible to extract

specific information on wells found in the Lower Susquehanna River Basin.

Several well-water studies also were completed from 1970 to 1990 in the study unit.

These studies were performed in areas where the predominant land use was

agriculture. A well-water study by Fishel and Lietman (1986) determined the

effectiveness of agricultural best-management practices on herbicide and nitratenitrogen concentrations. Water was collected in the heavily farmed, 188-mi2

Conestoga River Basin from 42 wells and 1 spring in an area of the Piedmont

Physiographic Province underlain by carbonate bedrock. High herbicide

concentrations (atrazine, 3 μg/L; simazine, 3.4 μg/L; alachlor, 3.0 μg/L) were

measured and were associated with agricultural practices. Another study performed

by Hippe and others (1994) analyzed concentrations of triazine and chloroacetamide

herbicides and nutrients in ground water from wells and springs in an agricultural area

in southcentral Pennsylvania near Carlisle. This study area was in the Great Valley

Section of the Ridge and Valley Physiographic Province underlain by carbonate

bedrock. Herbicides were detected, but none of the samples had concentrations that

exceeded the USEPA Maximum Contaminant Level’s (MCL’s). For example, atrazine

concentrations ranged from 0.1 to 0.2 μg/L—much less than the USEPA MCL for

atrazine (3 μg/L).
4 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Two studies were completed in intensely agricultural areas of the Mahantango Creek

watershed. Both studies analyzed waters from about 20 wells to determine the

occurrence of 9 different pesticides (alachlor, atrazine, carbofuran, chlorpyrifos,

cyanazine, fonofos, metolachlor, simazine, and terbufos) in the Appalachian Mountain

Section of the Ridge and Valley Physiographic Province underlain by sandstone and

shale bedrock (Pionke and others, 1988; Pionke and Glotfelty, 1989). The study by

Pionke and others (1988) also analyzed waters for 2,4-D and dicamba, but due to

analytical problems, these two pesticides were not included in the study by Pionke and

Glotfelty (1989). Atrazine was the pesticide most commonly detected in both studies

but was measured at concentrations mostly less than 0.1 μg/L.

A ground-water study by Harrison and others (1995) analyzed waters from 189 private

wells in rural corn-producing regions of Pennsylvania. Water samples were analyzed

to determine the occurrence of 11 pesticides (alachlor, atrazine, carbofuran,

chlorpyrifos, cyanazine, 2,4-D, dicamba, metolachlor, pendimethalin, simazine, and

terbufos) in several of Pennsylvania’s physiographic provinces. Atrazine and 2,4-D

were the most prevalent pesticides detected in the samples. None of the detections

exceeded the USEPA drinking-water guidelines except for water from one well that

was used only for irrigation purposes. Another ground-water study by Becher (1996)

analyzed waters from a network of wells and springs for a broad suite of constituents

including seven herbicides (alachlor, atrazine, cyanazine, metolachlor, propazine,

simazine, and toxaphene). Alachlor, atrazine, and simazine were the only herbicides

detected in samples. Atrazine was detected most frequently; concentrations were

generally less than 0.31 μg/L. The study was an assessment of wells in 10 valleys and

7 counties in Pennsylvania in the Cambro-Ordovician carbonate rocks of the

Appalachian Mountain Section of the Ridge and Valley Physiographic Province.

Studies were conducted on major tributaries to the Chesapeake Bay. An assessment

of the three major Chesapeake Bay tributaries—the Susquehanna, Potomac, and

James Rivers—was completed by Lang (1982). Waters from the Susquehanna River

at Conowingo Dam in Conowingo, Md., were analyzed for a seasonal characterization

of organochlorine and organophosphorus insecticides and chlorophenoxy-acid

herbicides. Atrazine and 2,4-D were the pesticides most frequently detected.

Minimum concentrations reported as zero were below the minimum detection limit for

that pesticide. Concentrations ranged from 0 to 1.2 μg/L for atrazine and 0 to 0.3 μg/L

for 2,4-D.

Fishel (1984) collected data on various water-quality characteristics of the

Susquehanna River at Harrisburg, Pa., to determine the relative contribution of

constituent loads in the Susquehanna River to the Chesapeake Bay. Waters were

analyzed for a suite of selected herbicides and insecticides. The herbicides atrazine

and 2,4-D were detected most frequently; atrazine concentrations ranged from 0 to

3.4 μg/L, and 2,4-D ranged from 0 to 0.41 μg/L.

A study by Breen and others (1995) assessed the presence of triazine herbicide

concentrations in the Susquehanna River and selected tributaries. Samples were

collected from 43 sites that represented four major regions of corn production in the

Lower Susquehanna River Basin. Triazine herbicides were detected in 39 of 43 sites;

concentrations ranged from 0.1 to 1.0 μg/L. Triazine herbicides were more likely to be

found in streams of the Piedmont Physiographic Province than in the Ridge and Valley

Physiographic Province. A study by Hainly and Kahn (1996) evaluated the occurrence

and transport of agricultural herbicides in the Susquehanna and Potomac Rivers.

A total of 43 samples was collected in June 1994, and of the 47 pesticides included in

the analysis, atrazine, metolachlor, simazine, alachlor, and cyanazine were the most
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 5

commonly detected. Atrazine, metolachlor, and simazine ranged from 0 to 10 μg/L,

0 to 10 μg/L, and 0 to 1.4 μg/L, respectively. Herbicide concentrations were usually

considerably less than the USEPA MCL’s. Land use was a key factor affecting

herbicide concentrations. The highest concentrations were in areas where large

amounts of corn were planted.

An assessment of pesticide occurrence in small streams by Truhlar and Reed (1975)

was conducted to determine the magnitude of pesticide contamination in four basins

with drainage areas ranging from 1.26 to 46.2 mi2 with differing land uses (forested,

general-farming, residential, and orchard-farming) in Pennsylvania. Another objective

of the study was to determine whether or not the pesticides were present in amounts

harmful to human or aquatic life. The study area included two basins within the study

unit—Bixler Run in western Perry County and a tributary of Spring Creek in Dauphin

County. Waters were analyzed to determine concentrations of chlorinated

hydrocarbon insecticides and chlorophenoxy-acid herbicides. The insecticides DDT

and dieldrin were detected most frequently. No pesticide concentrations exceeded

established health standards.

Studies were conducted in basins within the Piedmont Physiographic Province where

the predominant land use is agriculture. Two of these studies were completed in the

Pequea Creek Basin (Ward, 1987; Ward and Eckhardt, 1979). Stream-water samples

were analyzed to determine concentrations of organochlorine and organophosphate

insecticides and chlorophenoxy-acid and triazine herbicides. The pesticides detected

most frequently in the basin were atrazine, 2,4-D, dieldrin, DDT, heptachlor epoxide,

and lindane. Atrazine concentrations ranged from 0 to 24 μg/L, and 2,4-D ranged from

0 to 1.2 μg/L. A similar study by Lietman and others (1983) determined the effects of

land use on the water quality of receiving streams. Data were collected to quantify

nonpoint-source loadings from four specific subbasins: 1) forest, 2) cornfield, 3) rural

residential, and 4) pasture. Waters were analyzed for triazine herbicides and alachlor,

a chloroacetamide herbicide. Atrazine, simazine, and prometon were detected

frequently in the base-flow samples. Reported atrazine concentrations ranged from

0 to 3.9 μg/L during base flow. Lietman and Hall (1991) report atrazine concentrations

ranging from 0 to 200 μg/L during storms. The highest concentration was in a

composite storm sample collected from the cornfield site after herbicide application.

Two studies analyzed for pesticides in ground and surface waters and are most like

the NAWQA Lower Susquehanna River Basin study. Lietman and Hall (1991)

analyzed for triazine herbicides and the chloroacetamide herbicides, alachlor and

metolachlor, in the intensely farmed Pequea Creek and Conestoga River watersheds

in Lancaster County, Pa. The study was part of the Chesapeake Bay and Rural Clean

Water Programs. Atrazine was the herbicide most frequently detected in water

samples; atrazine concentrations ranged from 0 to 200 μg/L. Detections were

commonly reported for simazine, cyanazine, alachlor, and metolachlor. The U.S.

Department of Agriculture (1992) studied the Conestoga Headwaters during a

Pennsylvania Rural Clean Water Program project. The project evaluated the severity

of nonpoint-source pollutants on water quality. Best-management practices were

implemented on the basis of the findings. Waters were analyzed for a broad suite of

constituents including seven herbicides (atrazine, simazine, cyanazine, dieldrin,

alachlor, metolachlor, and propazine). In the Regional Study area, atrazine, alachlor,

and metolachlor were detected most frequently, and more than 35 percent of the

28 wells sampled in agricultural areas underlain by carbonate bedrock had detectable

atrazine concentrations (greater than or equal to 0.2 μg/L). Atrazine and simazine

concentrations in surface-water samples generally ranged from 0 to 1.0 μg/L and 0 to

1.3 μg/L, respectively.
6 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

The review of previous studies showed that geographical and technical gaps in

pesticide data existed. The Lower Susquehanna River Basin NAWQA study helps to

fill in those gaps and adds to the work of past studies by looking at spatial and

temporal distributions of pesticides analyzed. In addition, numbers of detections in

surface and ground water are reported for a broader spectrum of pesticides than was

previously examined for the entire basin.

Initially, Breen and others (1991) described water-quality objectives for the Lower

Susquehanna River Basin NAWQA Program in which the key issues included

1) contamination of surface and ground water by pesticides used in agricultural and

urban areas and by other organic chemicals, and 2) the extent of and processes

involved in ground-water contamination in limestone karst and in highly fractured-rock

aquifers. The NAWQA study by Hainly and Kahn (1996) furthered the understanding

of these key issues by measuring pesticide concentrations of surface-water sites in

agricultural areas and by determining streamflow yields in carbonate systems. The

NAWQA study by Breen and others (1995) identified triazine herbicide concentrations

during base-flow conditions on the Susquehanna River and selected tributaries. The

Lower Susquehanna River Basin NAWQA study, the study on which this report is

based, is the first basin-wide well and streamwater assessment of pesticide

concentrations in agricultural, urban, and forested areas. The study revisits some

settings studied by others and examines water from wells and streams in the five

major hydrogeologic settings, determined by physiology and bedrock type, in the

study unit. These settings were further subdivided by incorporating land-use activity.

Nationwide consistency in sampling and analytical methods used by the NAWQA

Program will allow the use of these pesticide data in a national study of pesticide

occurrence and will determine the magnitude of pesticide concentrations detected in

streams and shallow wells, nationally.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 7

DESCRIPTION OF STUDY AREA

The Susquehanna River drains about 27,000 mi2 in New York, Pennsylvania, and

Maryland. About 80 percent of the Susquehanna’s watershed lies within

Pennsylvania. From its headwaters near Cooperstown, N.Y., the Susquehanna River

flows 447 mi to the Chesapeake Bay. In terms of total discharge at the mouth and its

drainage area, the Susquehanna is the largest river on the eastern seaboard of the

United States, the 18th largest in the United States (Kammerer, 1987), and is the

largest tributary to the Chesapeake Bay, providing about one-half of the total

freshwater to the Bay.

The study unit consists of the lower 9,200 mi2 of the basin from where the West

Branch and main stem of the Susquehanna River join near Sunbury, Pa., downstream

to the Chesapeake Bay at Havre de Grace, Md. (fig. 1). The study unit also contains

an area of about 150 mi2 that includes parts of the Northeast Creek and Elk River

Basins located upstream from the Fall Line. The Fall Line is an approximate boundary

defined by the contact between the Piedmont and Coastal Plain Physiographic

Provinces. Northeast Creek and Elk River drain directly into Chesapeake Bay. In this

report, the term “Lower Susquehanna River Basin” (the study unit) is meant to include

this small area that drains directly to the Bay. A detailed description of the study area

and the hydrogeologic settings and environmental subunits can be found in Risser

and Siwiec (1996).

Climate and Hydrology

Mean annual air temperature in the Lower Susquehanna River Basin is near 50°F.

Mean daily air temperatures vary widely throughout the year, from the low 20’s in

January to the mid 70’s in July. The mean annual precipitation is about 40 in. The

areas that generally receive the most precipitation are the western edge and the

southeastern part of the basin, primarily because of the mountain ridges and the

Atlantic Ocean, respectively.

Runoff in the study unit, as indicated by the flow measured at the streamflow-gaging

site on the Susquehanna River at Conowingo, Md., near the mouth, averaged about

20.5 in. annually for the period 1968-95 (James and others, 1995). This amount of

runoff is slightly more than half of the average annual precipitation in the study unit.

Natural streamflows vary seasonally or monthly. In general, streamflows are highest

during March and April. About 60 percent of the annual streamflow is during the

period February to May. Evapotranspiration during the growing-season months of July,

August, and September reduces streamflow to its smallest amounts of the year.

During these months, streamflow is primarily from ground-water sources. According to

analyses of 24 streamflow-gaging sites in the study unit, 55 to 88 percent of annual

streamflow is from ground water.

Climate and hydrology affect pesticide concentrations in water from wells and streams

by determining the length of time that agricultural activities occur each year (the

growing season) and the crops that grow best in that climate. This directly affects the

amount and type of pesticide applied to cropland. The variability of runoff rates in the

study unit and the seasonality of climate and hydrology affect the volume of water and

concentrations in the streams. The seasonality of ground-water contributions to

streamflow also affects in-stream pesticide concentrations.

The growing season covers periods of normally high and low streamflow and extends

from April through September. It ranges from 160 days in the northern areas to

200 days in the south. On the average, slightly more than one-half of the annual
8 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Figure 1. Major physical features and generalized land use in the Lower Susquehanna River Basin study

unit, Pennsylvania and Maryland.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 9

precipitation is received during the nongrowing season (October through March),

allowing much of this quantity to be available for recharge to ground water. A major

part of the precipitation that falls during the growing season is during the early months

of the growing season (April through June)a time when pesticide applications for

agricultural purposes are most common. This provides a mechanism for localized

atmospheric transport of spray clouds and removal of recently applied pesticides from

the soil surface and subsequent transport to streams by runoff.

The climate of the study unit is controlled by a prevailing westerly circulation of air and

the proximity of the basin to the Atlantic Ocean—the source for most precipitation in all

but the western part of the study unit. The western part of the study unit has a humid

continental climate characterized by large seasonal temperature variations and the

eastern part has a more coastal-type climate characterized by moderated

temperatures and precipitation amounts somewhat greater than the other parts of the

basin (U.S. Geological Survey, 1991). Majewski and Capel (1995) report that high

agricultural use of pesticides in the Midwestern and Mid-Atlantic parts of the United

States may be one source for pesticides detected in rain and air. Common air

circulation patterns may facilitate atmospheric transport of pesticides to the study unit.

Hydrogeologic Settings and Environmental Subunits

Hydrogeologic setting includes the environmental factors of physiography, bedrock

type, and soils. Differences within the study unit for these factors are generally defined

by major physiographic provinces or province sections and bedrock type. The study

unit contains parts of five physiographic provinces: the Appalachian Plateau, Ridge

and Valley, Blue Ridge, New England, and Piedmont (Berg and others, 1989).

Together, the Ridge and Valley and Piedmont Physiographic Provinces account for

97 percent of the study unit. The Ridge and Valley Physiographic Province accounts

for the northwestern two-thirds of the study unit and is characterized by valleys and

long, narrow, and relatively steep ridges that trend southwest to northeast. The Great

Valley Section of the Ridge and Valley Physiographic Province is located along the

eastern edge of the province and in the central part of the study unit. It features a fairly

wide and relatively flat valley floor. The Piedmont Physiographic Province covers the

southeastern third of the study unit and is characterized by gently rolling hills.

Physiography affects water quality and pesticide concentrations in the study unit by

controlling the route that precipitation must follow through the flow system on its way

to a discharge point on the Susquehanna River. Physiography also determines the

amount of time the precipitation is in contact with materials such as soil, rock, and

vegetative cover that affect the chemical content of surface and ground waters.

Bedrock in the study unit has a complex structure and is diverse in type (Berg and

others, 1980). Metamorphic and igneous crystalline rocks (for example, schist, gneiss,

gabbro, and quartzite) crop out in the southeastern part of the basin. Carbonate rocks

(limestone and dolomite) crop out predominantly in two east-west trending bands near

the southcentral part of the basin, in a southwest to northeast band in the

northwestern part of the study unit, and in thin ribbons in the center of the basin. The

remaining area of the study unit is underlain by rocks consisting of sandstone, shale,

and siltstone, hereafter termed sandstone and shale.

Bedrock type plays a major role in controlling the chemical composition of ground

water and stream base flow. Ground water with elevated dissolved solids, hardness,

and pH are the result of weathering of carbonate rocks. The median hardness of

ground-water samples from carbonate rocks is about three times the median for

sandstone and shale rocks and about six times the median for crystalline rocks (Taylor
10 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

and Werkheiser, 1984). In addition, because of fractures common in carbonate rocks,

aquifers in carbonate bedrock areas tend to have a rapid hydrologic response and are

well-connected to the surface hydrologic system. The fractured rock also allows for the

fairly rapid movement of dissolved constituents, such as some pesticides, to the

aquifer.

Soils cover the bedrock throughout most of the study unit and were formed by physical

and chemical weathering of bedrock. Soils formed by chemical weathering generally

exhibit the chemical characteristics of the parent rock type and can be categorized

into major groups by the type of parent material. The location of the soil groups is

generally inferred by the location of each bedrock type within the study unit. Soil

groups based on their capacity for infiltration in the study unit include the influences of

the parent material but also include the characteristics of slope and thickness. Soils

weathered from carbonate bedrock are generally considered to have an excellent

infiltration capacity; those from crystalline rocks are considered to have good

capacities, and those from sandstone and shale rocks are labeled as having good to

poor capacities.

Soil-infiltration capacity affects the movement of precipitation, irrigation water, and

soluble pesticides to ground water. Soil type also affects the availability of organic

material and clay in the soil, which, in turn, influences transport processes in the soil.

Chemical processes such as dissolution and adsorption are major factors in the

transport and fate of pesticides. The location of rich agricultural soils, formed by the

weathering of carbonate rocks, is also an important factor to consider. The location of

this soil type encourages the use of the land for intense agricultural purposes and

determines areas where pesticides might be applied and, potentially, areas of high

agricultural pesticide use.

In the Lower Susquehanna River Basin, five major hydrogeologic settings, determined

by physiography and bedrock type, are present: 1) crystalline rocks in the Piedmont

and Blue Ridge Physiographic Provinces, 2) carbonate rocks in the Piedmont

Physiographic Province, 3) sandstone and shale rocks in the Piedmont Physiographic

Province, 4) carbonate rocks in the Ridge and Valley Physiographic Province, and

5) sandstone and shale rocks in the Ridge and Valley Physiographic Province.

Land use is an important environmental factor in describing pesticide use for both

agricultural and nonagricultural purposes. In the study unit, land use is evenly divided

between 47 percent agriculture and 47 percent forest, based on 1970’s data (Risser

and Siwiec, 1996). However, local variations in this ratio exist. In the southern half of

the basin (the Piedmont Physiographic Province and the Great Valley Section of the

Ridge and Valley Physiographic Province) agriculture dominates (fig. 1). Nearly threequarters of the land in the southern half of the basin is used for agricultural purposes.

In the northern half of the basin (the Appalachian Mountain Section of the Ridge and

Valley Physiographic Province) about two-thirds of the land is forested and one-third is

used for agricultural purposes. Because the majority of pesticide use in the study unit

is for agricultural purposes, the location and density of agricultural land use is an

important factor to consider when describing factors affecting pesticide concentrations

in the environment.

In comparison, about one-third of the Chesapeake Bay Basin has herbaceous

agricultural land cover and slightly more than half of the basin is covered by forests

(Langland and others, 1995). Most of the remainder of the Bay Basin is covered by

urban land uses and water. In the Chesapeake Bay Basin, most agricultural land use

is in the Piedmont Physiographic Province, the Great Valley Section of the Ridge and

Valley Physiographic Province, and the eastern shore area of Maryland in the Coastal
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 11

Plain Province. In the Potomac River Basin, the next largest basin in the Bay

watershed, the intensity of agriculture is similar to the study unit only in the Piedmont

Physiographic Province (Fisher, 1995).

More recent studies in selected areas of the Lower Susquehanna River Basin indicate

land use is changing. Since the 1970’s, 5-10 percent of the cropland in the Piedmont

Physiographic Province and the Great Valley Section of the Ridge and Valley

Physiographic Province has been converted to urban land uses (Petersen and others,

1992), which indicates a possible trend in pesticide use and type in these areas from

agricultural to homeowner and commercial purposes.

In order to study the land-use diversity and hydrogeologic settings within the study

unit, environmental subunits, hereafter termed subunits, were established. The

subunits combine the physical and hydrogeologic factors of physiography and

topography, bedrock type, and predominant land-use activity. This classification

method produced more subunits than could be studied in this phase of the NAWQA

Program. Along with input from a local group of interested environmental and waterresource managers, the NAWQA staff selected seven subunits that would address

high-priority water-quality issues. The selected subunits allowed an assessment of

water-quality conditions in agricultural areas underlain by carbonate bedrock and

comparisons between agricultural, urban, and forested land uses in different

hydrogeologic settings.
12 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Water samples were collected in the following subunits: (1) agricultural areas

underlain by crystalline bedrock in the Piedmont Physiographic Province,

(2) agricultural areas underlain by carbonate bedrock in the Piedmont Physiographic

Province, (3) agricultural areas underlain by carbonate bedrock in the Great Valley

Section of the Ridge and Valley Physiographic Province, (4) urban areas underlain by

carbonate bedrock in the Great Valley Section of the Ridge and Valley Physiographic

Province, (5) agricultural areas underlain by carbonate bedrock in the Appalachian

Mountain Section of the Ridge and Valley Physiographic Province, (6) agricultural

areas underlain by sandstone and shale in the Appalachian Mountain Section of the

Ridge and Valley Physiographic Province nearest to the Susquehanna River, and

(7) forested areas underlain by sandstone and shale in the Appalachian Mountain

Section of the Ridge and Valley Physiographic Province (fig. 2 and table 1).

Table 1. Description of environmental subunits where pesticide samples were collected in

1993-95 in the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland

Environmental subunit

Physiographic

province

(section)

Topographic

setting

Dominant

bedrock type

Dominant

land use

Percentage

of study unit

Piedmont crystalline

agricultural

Piedmont Hilltop and

hillside

Igneous and

metamorphic

Agriculture 9.8

Piedmont carbonate

agricultural

Piedmont Valley Limestone and

dolomite

Agriculture 4.7

Great Valley carbonate

agricultural

Ridge & Valley

(Great Valley)

Valley Limestone and

dolomite

Agriculture 3.0

Great Valley carbonate

urban

Ridge & Valley

(Great Valley)

Valley Limestone and

dolomite

Urban .6

Appalachian Mountain

carbonate agricultural

Ridge & Valley

(Appalachian

Mountain)

Valley Limestone and

dolomite

Agriculture 4.6

Appalachian Mountain

sandstone and shale

agricultural

Ridge & Valley

(Appalachian

Mountain)

Valley and

hillside

Sandstone,

siltstone,

and shale

Agriculture 6.3

Appalachian Mountain

sandstone and shale

forested

Ridge & Valley

(Appalachian

Mountain)

Valley and

hillside

Sandstone,

siltstone,

and shale

Forest 34.2
NATIONAL

WATER-QUALITYASSESSMENTPROGRAM 13 Figure 2. Subunits within the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland.
14 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

PESTICIDE USE

Herbicides are used extensively in the study unit for the control of numerous weeds

that affect the growth of corn, alfalfa, soybeans, and small grain crops. Weed-control

programs are designed to control the growth and spread of weeds so that they will not

adversely affect the use and the value of cropland, reduce the yield or the quality of

crops, or increase the costs of production and harvest (Hippe and others, 1994). Most

herbicide applications in the study unit are to row crops, especially corn and

soybeans. Pasture and unplanted cropland receive very little herbicide treatment

(Anderson and Gianessi, 1995). Row crops, and some cover crops, commonly receive

one or more herbicide applications. Cover crops, such as forage, and small-grain

crops, like alfalfa, barley, wheat, and oats, receive fewer herbicide applications

because they leave little room for weed growth and do not require applications for

weed control. Herbicide combinations are sometimes used for more effective weed

control. Multiple applications are common and generally include some combination of

pre-plant applications of selective or nonselective herbicides and pre-emergent and

post-emergent applications of selective herbicides (Hippe and others, 1994).

Additionally, nonselective burndown herbicides are sprayed on fields after small grain

crop cuttings or along with no-till operations.

Little information is available concerning herbicide use for nonagricultural purposes.

For this reason, determining the direct effect of nonagricultural applications on water

quality is difficult to define and is generally only implied. Herbicides applied for these

purposes will be discussed in a qualitative manner only. Nonagricultural uses of

herbicides include weed control on golf courses, public lands, and homeowner lawns

and vegetation-clearing along railways, highways, and transmission lines throughout

the growing season. Weed-control application frequency for nonagricultural purposes

ranges from routine application by commercial operations to sporadic application on

an as-needed basis by private homeowners.

Estimates of herbicide use for agricultural and nonagricultural purposes indicate

greater than 99 percent of all herbicide use is for agricultural purposes (Barbash and

Resek, 1996, p. 175). In addition, agricultural pesticide use is considerably more

extensive, areally, than nonagricultural use. Even though data are not available to

estimate the herbicide amounts used for nonagricultural purposes in the study unit, it

should not be considered insignificant. A compilation of national pesticide-use data by

Barbash and Resek (1996) indicate commercial sod operations, lawn-care services,

and golf courses are three of the top five pesticide uses. Nationally, about 35 percent

of the available turf has almost 6 lb of active ingredient applied per acre on an average

annual basis (Barbash and Resek, 1996, p. 109). In comparison, average application

rates for atrazine on corn crops in the study unit range from 0.5 to 1.5 lb of active

ingredient per acre per year (Anderson and Gianessi, 1995).

Insecticides and fungicides are applied on a routine basis only where recurring

problems are known. Generally, such pesticides are applied to remedy sporadic

problems and may be applied anytime throughout the growing season before the crop

matures. This type of application program also is used to control insects and fungi

associated with turf damage on homeowner lawns and golf courses and to control

insects in structures. Insecticides like chlorpyrifos and diazinon are applied more

commonly for home and garden use than for agricultural use. Nonagricultural uses are

estimated to account for 50 to 70 percent of the chlorpyrifos and 90 to 98 percent of

the diazinon used in the United States (Barbash and Resek, 1996, pp. 172-175). Most

agricultural use of insecticides and fungicides is applied to orchard crops. Almost all

fruit and vegetable crops in the study unit receive some kind of insecticide treatment

(Anderson and Gianessi, 1995).
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 15

STUDY METHODS

The selection of streams and shallow wells for the collection of water-quality samples

for pesticide analysis was directed by the design of the various water-quality studies

being conducted as part of the NAWQA Program. The issues addressed by the waterquality studies ranged from local scale (the effects of agricultural practices on

pesticides in ground-water wells and streams) to national scale (the occurrence,

distribution, and concentrations of pesticides in ground-water wells and streams). The

objectives of the stream water-quality studies were addressed by designing and

conducting both long-term monitoring (up to 3 years) and synoptic studies. The

ground-water synoptic studies focused on specific land-use types or areas underlain

by similar bedrock types. Sampled wells were generally less than 200 ft deep and less

than 20 years old. For a more detailed description of the design of water-quality

studies, site-selection procedures, the implementation of the studies, and site and

basin characteristics, see Siwiec and others (1997).

Water-quality studies conducted by the NAWQA Program in the Lower Susquehanna

River Basin study unit may be broadly categorized into four major types: long-term

monitoring of streams, basinwide synoptic studies of streams, subunit synoptic

studies of ground water and streams, and focused synoptic studies of streams.

Synoptic studies were completed over periods of 1 day to 2 weeks. Basinwide

synoptic studies designed to cover a broad geographical area, subunit synoptic

studies conducted in areas of homogeneous physiography and bedrock type and, in

most cases, land use or land cover, and synoptic studies focused on a small

geographic area or a particular water-quality issue were used to collect pesticide

samples from streams.

Long-term monitoring consisted of the collection of samples at fixed intervals at seven

streams, each representing one of seven major environmental subunits in the Lower

Susquehanna River Basin (fig. 3). Sample-collection frequency ranged from weekly to

monthly depending on the season and the stream. From four of these seven streams,

water samples were collected routinely for pesticide analysis. At these streams, the

number of water samples collected over a 2-3-year period ranged from nearly 50 to

slightly more than 100. Less than five water samples for pesticide analysis were

collected from each of the three remaining streams during the same period. Most

fixed-interval water samples were collected during base-flow conditions, but some

samples were collected during storm-affected flow conditions (see Appendix for

explanation). For two of the streams, water samples for pesticide analysis were

collected over the hydrograph of several storms during the early growing season of

1995. A total of 250 pesticide samples was collected at the long-term monitoring sites

from 1993 to 1995 (Siwiec and others, 1997, table 2, p. 12).

A total of 22 synoptic studies were conducted in the study unit for the analysis of

pesticides in ground water and streams from 1993 to 1995 (Siwiec and others, 1997).

One basinwide synoptic study of streams was conducted in 1993. That study was an

analysis of triazine herbicides at 47 stream sites located throughout the study unit.

Subunit and focused synoptic studies also were conducted for the analysis of

pesticides in streams (fig. 3). Six subunit synoptic studies in streams were conducted

in the study unit; the number of sites sampled in each subunit ranged from 10 to

17 sites. Focused synoptic studies were conducted for the analysis of pesticides in

streams; eight focused synoptic studies were conducted in the study unit, and the

number of sites sampled in each study ranged from 5 to 19 sites. Subunit synoptic

studies were conducted for pesticide analysis in ground water. Seven subunit synoptic

studies were conducted; the number of wells sampled in each subunit generally
16

NATIONALWATER-QUALITYASSESSMENTPROGRAM Figure 3. The Lower Susquehanna River Basin study unit, counties, major environmental subunits, and location of long-term monit

stream sites where pesticide samples were collected from 1993 to 1995.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 17

ranged from 20 to 30. Water samples were collected for pesticide analysis from

169 wells (fig. 4). In all synoptic studies, wells and streams were sampled one time

only.

Data Collection and Analysis

The process for selecting target analytes for pesticide analyses in the NAWQA

Program involved consideration of several factors (Robert J. Gilliom, U.S. Geological

Survey, written commun., 1994). The primary factors were (1) pesticides with annual

applications greater than 8,000 lb of active ingredient, (2) pesticides previously and

currently analyzed in various monitoring and survey programs by other agencies such

as USEPA and National Oceanic and Atmospheric Administration (NOAA),

(3) pesticides with USEPA Drinking Water Regulations and Health Advisories,

(4) pesticides on the USEPA Priority Pollutant List, (5) results of published reports in

which pesticides were reported in ground and surface water, (6) pesticides and

metabolites that have a high potential for leaching into ground water, (7) pesticides

that are highly toxic to mammals or aquatic life but have a low national use, (8) input

from NAWQA study units on pesticides of major concern in the study units, and

(9) new pesticides that are replacing discontinued pesticides. The pesticides analyzed

for in this study, other selected information, and estimated annual-use data for the

study unit and for the Commonwealth of Pennsylvania are listed in table 2. In addition,

seven pesticides applied in the study unit but not analyzed in the water-quality

samples were included in the table because of their relatively high use in the study

unit (>20,000 lb/yr). Agricultural pesticide-use estimates for the Lower Susquehanna

River Basin (table 2) were obtained from Anderson and Gianessi (1995). Estimates for

statewide agricultural pesticide use were obtained from Gianessi and Puffer (1991,

1992a, 1992b).

The discussion of concentrations measured in environmental and quality-assurance

samples collected from streams and wells will be focused on five pesticides—atrazine,

simazine, pendimethalin, chlorpyrifos, and diazinon. These pesticides were selected

to represent a range of amounts used for agricultural purposes and a range of

detection frequencies in wells and streams. Other factors considered were toxicity, as

indicated by the existence of health-advisory guidelines, and susceptibility to leaching

(table 3). Atrazine and simazine are representative of the triazine herbicide group and

pendimethalin is a dinitroaniline herbicide. Chlorpyrifos and diazinon are

organophosphate insecticides.

Measured pesticide concentrations were compared to the drinking-water MCL’s or

maximum contaminant level goals (MCLG) but were not evaluated from a regulatory

standpoint. For drinking water, an MCL is exceeded when the average concentration

over a pre-defined period of time is higher than the MCL. Because typically only one

sample was collected per site and the samples were not from public drinking-water

supplies, it is not possible to determine if the MCL was exceeded from a regulatory

standpoint. Drinking-water standards were used for comparison because there are no

ambient water-quality criteria or suggested guidelines for the protection of aquatic

organisms available for these pesticides.

Each pesticide analyzed by the USGS National Water Quality Laboratory (NWQL) in

Arvada, Colo., for this study has a corresponding method detection limit (MDL), which

is the minimum concentration of a substance that can be identified, measured, and

reported with 99-percent confidence that the analyte concentration is greater than

zero (Timme, 1995, p. 92). Nondetected concentrations are assigned a value of less

than (<) the MDL and can be any concentration less than the MDL, including zero

cccccc
18

NATIONALWATER-QUALITYASSESSMENTPROGRAM Figure 4. The Lower Susquehanna River Basin study unit, counties, major environmental subunits, and location of shallow ground-w

pesticide samples were collected from 1993 to 1995.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 19

Table 2. Pesticides for which ground- and surface-water samples were analyzed in the Lower

Susquehanna River Basin study unit from 1993 to 1995 and information on water-quality

standards and pesticide use

[Shaded pesticides were analyzed by the gas chromatography/mass spectrometry method

(Sandstrom and others, 1992; Zaugg and others, 1995); unshaded pesticides were analyzed by

the high performance liquid chromatography method (Werner and others, 1996); Parameter

code, 5-digit number used in the U.S. Geological Survey computerized data system, NWIS, to

uniquely identify a specific constituent; Italicized pesticides indicate those pesticides that were

not analyzed in this study but commonly are used in Pennsylvania; *, Restricted-use pesticide;

NWIS, National Water Information System; LSR, Lower Susquehanna River Basin; MCL,

Maximum Contaminant Level; MCLG, Maximum Contaminant Level Goal; --, not established;

μg/L, micrograms per liter; lb/yr, pounds per year]

Pesticide

NWIS

parameter

code

Selected

trade name(s)

MCL or

MCLG1

(μg/L)

Amount applied for

agricultural use

(lb/yr)

LSR Basin2

(1990-1994)

Statewide-Pa.

(1988)

Herbicides3

Acetochlor 49260 Harness, Surpass -- -- --

Acifluorfen 49315 Blazer, Scepter --/zero -- 13,300

Alachlor* 46342 Lasso, Alanox 2 127,000 514,000

Atrazine* 39632 AAtrex, Trac 3 417,000 1,610,000

Atrazine, desethyl 04040 -- -- --

Benfluralin 82673 Balan, Benefin, Bonalin -- -- --

Bentazon 38711 Basagran, Forte --/20 324 13,000

Bromacil 04029 Hyvar, Uragan -- -- --

Bromoxynil 49311 Buctril, Brominex -- 19,100 4630

Butylate 04028 Sutan+, Genate Plus -- -- 121,000

Chloramben 49307 Amiben, Vegiben -- 1,730 9,560

Clopyralid 49305 Lontrel, Stinger -- -- --

Cyanazine* 04041 Bladex, Fortrol --/1 98,500 450,000

DCPA* 82682 Dacthal -- 2,160 11,200

Dicamba, mono-acid- 38442 Banvel, Metambane -- 21,300 112,000

Dichlobenil 49303 Barrier, Casoron -- -- 2,380

Dichlorprop, mono-acid- 49302 Weedone, Corasil -- -- --

Diethylanaline 82660 -- -- --

Dinoseb6 49301 Premerge, DNBP 7 -- --

Diuron 49300 Karmex, Direx -- 2,740 28,900

EPTC 82668 Eptam, Alirox -- 895 202,000

Ethalfluralin 82663 Sonalan, Curbit -- 441 --

Fenuron 49297 -- -- --

Fluometuron 38811 Cotoran, Meturon -- -- --

Glyphosate 39941 Roundup, Rodeo 700 21,300 526,000

Linuron* 82666 Lorox, Linex -- 45,300 60,100

MCPA, mono-acid- 38482 Chiptox, Weedar -- 10,200 36,900

MCPB, mono-acid- 38487 Thistrol, Tropotox -- <10 514

Metolachlor 39415 Dual, Pennant -- 470,000 1,410,000

Metribuzin 82630 Lexone, Sencor -- 9,660 12,600

Molinate 82671 Ordram, Molinam -- -- --

1-Naphthol 49295 -- -- --

Napropamide 82684 Devrinol, Naproguard -- 2,110 15,100

Neburon 49294 Neburex, Propuron -- -- --

Norflurazon 49293 Predict, Zorial -- 505 6,520
20 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Herbicides—Continued

Oryzalin 49292 Surflan, Snapshot -- 483 2,000

Paraquat -- Cyclone, Gramoxone -- 28,900 91,400

Pebulate 82669 Tillam -- 4,570 17,800

Pendimethalin 82683 Prowl, Squadron -- 137,000 257,000

Picloram, mono-acid-* 49291 Grazon, Tordon 500 -- --

Prometon 04037 Pramitol -- -- --

Pronamide* 82676 Kerb, Propyzamid -- 2,350 --

Propachlor 04024 Prolex, Ramrod -- <10 2,200

Propham 49236 Chem-Hoe, Birgin -- -- --

Silvex, mono-acid-6 39762 2,4,5-TP, Fenoprop 50 -- --

Simazine 04035 Princep, Aquazine 4 10,700 106,000

Tebuthiuron 82670 Spike, Tebusan -- -- --

Terbacil 82665 Sinbar -- 5,750 43,390

Thiobencarb* 82681 Bolero, Saturn -- -- --

Triallate6 82678 Far-Go, Avadex BW -- -- --

Triclopyr, mono-acid- 49235 Grandstand, Turflon -- -- --

Trifluralin 82661 Herbiflan, Treflan -- 9,780 42,580

2,4-D, mono-acid- 39732 Miracle, Dacamine 70 30,500 144,000

2,4-DB, mono-acid- 38746 Butyrac -- 24,500 84,000

2,4,5-T, mono-acid-6 39742 Weedar -- -- --

Insecticides5

Aldicarb* 49312 Temik, Sanacarb 7 -- --

Aldicarb Sulfone 49313 Standak, Aldoxycarb 7 -- --

Aldicarb Sulfoxide 49314 7 -- --

Azinphos, Methyl* 82686 Acifon, Guthion -- 23,600 87,700

Carbaryl 82680 Sevin, Slam -- 10,500 56,300

Carbofuran* 82674 Furadan, Carbodan 40 23,600 189,000

Carbofuran, 3-Hydroxy- 49308 -- -- --

Chlorpyrifos 38933 Dursban, Scout -- 94,600 614,000

Cryolite -- Kryocide, Prokil -- 21,600 92,000

DDE, p,p 34653 -- -- --

Diazinon 39572 Basudin, Knox-Out -- 2,460 19,200

Dieldrin6 39381 Panoram D-31 -- -- --

Disulfoton* 82677 Disyston, Disultex -- -- 2,600

DNOC6 49299 Elgetol, Trifocide -- -- --

Esfenvalerate 49298 Asana XL, Sumi-alpha -- 1,170 6,030

Table 2. Pesticides for which ground- and surface-water samples were analyzed in the Lower

Susquehanna River Basin study unit from 1993 to 1995 and information on water-quality

standards and pesticide use—Continued

[Shaded pesticides were analyzed by the gas chromatography/mass spectrometry method

(Sandstrom and others, 1992; Zaugg and others, 1995); unshaded pesticides were analyzed by

the high performance liquid chromatography method (Werner and others, 1996); Parameter

code, 5-digit number used in the U.S. Geological Survey computerized data system, NWIS, to

uniquely identify a specific constituent; Italicized pesticides indicate those pesticides that were

not analyzed in this study but commonly are used in Pennsylvania; *, Restricted-use pesticide;

NWIS, National Water Information System; LSR, Lower Susquehanna River Basin; MCL,

Maximum Contaminant Level; MCLG, Maximum Contaminant Level Goal; --, not established;

μg/L, micrograms per liter; lb/yr, pounds per year]

Pesticide

NWIS

parameter

code

Selected

trade name(s)

MCL or

MCLG1

(μg/L)

Amount applied for

agricultural use

(lb/yr)

LSR Basin2

(1990-1994)

Statewide-Pa.

(1988)
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 21

Insecticides—Continued

Ethoprop* 82672 Mocap, Ethoprophos -- -- --

Fonofos* 04095 Capfos, Dyfonate -- 17,600 151,000

HCH, alpha6 34253 (none) -- -- --

HCH, gamma* 39341 Lindane, Lintox 0.2 -- --

Malathion 39532 Cythion, Maltox -- 24,800 104,000

Methiocarb 38501 Draza, Mesurol -- -- --

Methomyl 49296 Lannate, Lanox -- 13,700 54,600

Methyl Bromide 34413 Meth-O-Gas, Celfume -- 64,200 --

Oxamyl* 38866 Vydate L 200 4,620 16,200

Parathion, Ethyl6 39542 Parathion, Panthion -- 22 --

Parathion, Methyl* 82667 Penncap-M, Paraton -- 10,600 22,700

Permethrin, cis 82687 Ambush, Pounce -- 7,200 52,200

Phorate* 82664 Thimet, Granutox -- 2,680 48,100

Propargite6 82685 Comite, Ornamite -- 3,220 26,700

Propoxur 38538 Baygon, Suncide -- -- --

Terbufos* 82675 Counter, Pilarfox -- 20,100 99,600

Fungicide7

Captan 39640 Orthocide, Merpan -- 160,000 288,000

Chlorothalonil 49306 Bravo, Daconil 2787 -- -- 92,000

Mancozeb -- Manzate, Aimcozeb -- 52,700 326,000

Ziram 81827 Pomarsol Z, Mezene -- 25,800 107,000

1 U.S. Environmental Protection Agency, 1996.

2 Anderson and Gianessi, 1995.

3 Gianessi and Puffer, 1991.

4 Possible discrepancy in the way the two studies determined the percentage of crop treated.

5 Gianessi and Puffer, 1992a.

6 Not registered for use in Pennsylvania.

7 Gianessi and Puffer, 1992b.

Table 2. Pesticides for which ground- and surface-water samples were analyzed in the Lower

Susquehanna River Basin study unit from 1993 to 1995 and information on water-quality

standards and pesticide use—Continued

[Shaded pesticides were analyzed by the gas chromatography/mass spectrometry method

(Sandstrom and others, 1992; Zaugg and others, 1995); unshaded pesticides were analyzed by

the high performance liquid chromatography method (Werner and others, 1996); Parameter

code, 5-digit number used in the U.S. Geological Survey computerized data system, NWIS, to

uniquely identify a specific constituent; Italicized pesticides indicate those pesticides that were

not analyzed in this study but commonly are used in Pennsylvania; *, Restricted-use pesticide;

NWIS, National Water Information System; LSR, Lower Susquehanna River Basin; MCL,

Maximum Contaminant Level; MCLG, Maximum Contaminant Level Goal; --, not established;

μg/L, micrograms per liter; lb/yr, pounds per year]

Pesticide

NWIS

parameter

code

Selected

trade name(s)

MCL or

MCLG1

(μg/L)

Amount applied for

agricultural use

(lb/yr)

LSR Basin2

(1990-1994)

Statewide-Pa.

(1988)
22 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

(censored data). Methods used to account for censored data are described in each

section that dealt with censored data.

Streamwater and ground-water samples for pesticide analysis were collected and

processed in accordance with methods described by Shelton (1994) and by Koterba

and others (1995), respectively. Samples were collected from wadeable streams with

a US-DH-81 sampler and glass or Teflon bottles using the Equal-Width-Increment

(EWI) and depth-integrated methods. Samples from unwadeable streams were

collected in 3-L Teflon bottles with a US-D-77 TM sampler suspended from a bridge.

In addition to the equipment-cleaning protocols described in Shelton (1994) and

Koterba and others (1995), all sampling and processing equipment was rinsed with

native water before sample collection. Sample splitting was conducted with a Teflon

cone (decaport) splitter (Capel and others, 1995). Water samples for pesticide

analysis were filtered using 142-mm diameter, baked, glass-fiber filters with 0.7-μm

pore size on an aluminum-plate filter stand and collected in 1-L, baked and ambercolored glass bottles. A Teflon diaphragm pump and Teflon tubing were used to force

the water sample through the filter membrane. Water samples were collected at each

site for analysis of 46 pesticides by gas chromatography/mass spectrometry (GCMS)

(Sandstrom and others, 1992; Zaugg and others, 1995). All ground-water and some

streamwater samples were analyzed for an additional 37 pesticides by highperformance liquid chromatography (HPLC) (Werner and others, 1996) (table 2).

For surface-water synoptic surveys, the methods used for sample collection and

processing were similar to those used for samples collected at the long-term

monitoring sites and were in accordance to methods provided by Shelton (1994).

During synoptic surveys, samples were generally collected using hand samplers and

the EWI and depth-integrated methods unless the streams were too narrow or depths

were too shallow. In this case, a sample was collected without the use of a sampler

and from the center of flow. Laboratory methods and analyses were identical to those

used for long-term monitoring site streamwater samples, with the exception that no

water samples collected for pesticide analyses were analyzed by HPLC for the

37 additional pesticides. A more detailed description of the methods used for the

Table 3. Summary of characteristics of pesticides selected for detailed analysis

[MCL, Maximum Contaminant Level; HAL, Health Advisory Limit; NR, not rated]

Pesticide

Characteristics considered

Generalized

agricultural-use

category1

Frequency of

detection in

streams2

Frequency of

detection

in wells2

Health advisory

exists?, type

Leaching

potential3

Herbicides

Atrazine High High High Yes, MCL High

Pendimethalin High Low Low No NR

Simazine Low High Medium Yes, MCL Medium

Insecticides

Chlorpyrifos High Low Low Yes, HAL Low

Diazinon Low Low Low Yes, HAL Low

1 Arbitrarily selected, High = >90,000 pounds per year (1990-1994 average);

Low = <12,000 pounds per year (1990-1994 average) (Anderson and Gianessi, 1995). 2 Arbitrarily selected, High = >90 percent; Low = <20 percent.

3 Hippe and Hall, 1996, pages 44-45.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 23

collection and processing of surface-water samples in the Lower Susquehanna River

Basin can be found in Siwiec and others (1997). Shelton (1994) provides specific

guidance on surface-water sample collection for the NAWQA Program.

Ground-water samples were collected in a sampling chamber from a Teflon sampling

hose connected to an outside spigot, pressure tank, or directly to a sampling pump. All

sampling lines and connections between the faucet and the sampling chamber were

Teflon or stainless steel. All bottles were filled inside the sampling chamber to

minimize the potential for contamination by dust or other atmospheric contaminants.

Powderless latex gloves were worn during sampling. As with the surface-water

samples, ground-water samples for pesticide analysis were filtered using 0.7-μm pore

size, 142-mm diameter, baked, glass-fiber filters. Water samples were collected at

each site for analysis by GCMS and HPLC. Refer to Siwiec and others (1997) for a

more detailed description of the methods used for the collection and processing of

ground-water samples.

All water samples for pesticide analysis were kept chilled and stored away from

sunlight until they were processed. The samples for analysis by GCMS were filtered

through a solid-phase extraction (SPE) cartridge in the Pennsylvania District

Laboratory following the guidelines in Shelton (1994) and Manning and others (1994).

The SPE cartridge was sent chilled to the NWQL for elution and analysis of pesticides.

Samples for analysis by HPLC were shipped chilled in 1-L, baked and amber-colored

glass bottles to the NWQL, which did the SPE. All laboratory analyses were

performed at the NWQL.

Water samples collected during ground water and stream-synoptic surveys were

characterized as base-flow condition samples. Synoptic surveys were designed to

represent non-recharge or base-flow conditions and were conducted only when

ground-water discharge dominated the supply of flow to the region’s streams. Baseflow or storm-affected stream conditions were designated for water samples collected

from streams. Analysis of regional streamflow was based on evaluations of

hydrographs from streams near the sampled basins. Hydrographs of water levels in

wells were used to verify that ground-water samples were being collected during nonrecharge periods.

To characterize water samples collected from streams at the long-term monitoring

sites, base-flow samples were those collected a sufficient amount of time after a

significant storm event so that the flow in the stream was reasonably stable and

predominantly supplied by ground-water discharge. A more detailed description of the

algorithm used is given in the Appendix.

Statistical Analysis Methods

All statistical analysis methods used in this report are described in Helsel and Hirsch

(1992). Statistical Analysis Systems (SAS Institute Inc., 1990) software was used to

complete the statistical tests. Other than summary statistics (mean, median, standard

deviation, and percentiles), the only other statistical tests and methods employed in

this analysis were contingency tables using the Chi-square statistic and the KruskalWallis test. The Chi-square statistic was used to determine if the difference between

the number of detections and nondetections of two data sets was statistically

significant. The Kruskal-Wallis test was used to determine if differences between the

medians of two data sets were statistically significant.
24 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Quality-Assurance Procedures

Three types of quality-assurance data are available for analysis—surrogate pesticide

concentrations and recoveries, spiked and unspiked replicates, and equipment and

field blanks. Surrogates are organic pesticides injected into all filtered water samples

prior to extraction by the SPE cartridge to provide quality control by monitoring for

matrix effects and gross sample-processing errors. Surrogates are expected to

behave similarly chemically and physically to target analytes in terms of SPE recovery

and are not expected to occur in the environment (Timme, 1995; Werner and others,

1996).

A field blank is analyte-free water carried through the entire sample collection, field

processing, preservation, transportation, and laboratory handling process as an

environmental sample and is used to identify possible contamination introduced

during data collection. An equipment blank is similar to a field blank, but equipment

blanks are not exposed to field conditions and are normally done in the laboratory.

Blanks are acceptable if the amount of analyte in the blank water is less than the MDL

(Sandstrom, 1994).

Sequential-replicate samples were collected immediately following the collection of

the environmental samples using the same type of equipment and collection methods.

Sample replicates are submitted as quality-control samples to determine the precision

and random error of the NWQL analyses, assuming negligible change in water

chemistry between samples. A spike is the addition of a known quantity of one or

more pesticides of interest to the sample prior to analysis, which yields data on the

results (accuracy) that can be expected from a suite of similar samples when used

with a synthetic matrix. The spike is used to verify analytical method performance by

recovery of analytes in that synthetic matrix (Timme, 1995).

Quality-assurance samples were collected at a ratio of about one for every five

environmental samples. For surface-water samples, the ratio of environmental sample

replicates (spiked or unspiked) to field blanks was about 2:1. All ground-water

replicate samples were spiked. Blank or replicate samples were collected at

10 percent of all sites.

Replicates or spiked replicates were collected immediately following collection of the

environmental samples. Spiked replicates for pesticides had 100 μL of a solution

added that was prepared specifically for the GCMS and HPLC analytes by the NWQL.

Concentrations of unspiked replicate samples were used to analyze the precision of

NWQL methods and consistency of sampling techniques (assuming the water

chemistry remained unchanged between subsequent replicate samples), and

concentrations of spiked replicate samples were used to analyze the accuracy of

NWQL methods.

A ground-water equipment blank was conducted at the beginning of the first year of

sampling; all subsequent ground-water blanks were field blanks conducted to assess

the equipment decontamination procedures and contamination introduced at the

sampling site. Surface-water blanks, on the other hand, were collected in the field

during the first year of sampling, after which equipment blanks were conducted in the

laboratory. Following the collection and processing of surface- and ground-water

environmental samples, the equipment was cleaned and set up for the processing of

samples using pesticide-grade organic-free water. The processing of blank-water

samples followed the same method as that used for the environmental samples.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 25

Organic-free and pre-analyzed inorganic-free water were used for blank samples for

surface-water samples. Pesticide-grade organic-free water was used for all groundwater blank samples.

QUALITY-ASSURANCE RESULTS

Quality-control procedures for the Lower Susquehanna River Basin NAWQA study

included the use of surrogates and the collection of sample blanks (field and

equipment) and sample replicates. According to Horowitz and others (1994), qualitycontrol samples must be collected for any sampling and analysis program because

without quality-control information, the quality of the data can not be evaluated or

qualified.

Surrogates

Surrogate pesticide recoveries were compared to NWQL reagent water recoveries for

the HPLC surrogate, BDMC, and the three GCMS surrogates, isotopically-marked

diazinon-d10 and alpha-HCH-d6, and tebuthylazine (table 4). Surrogate recoveries

outside the range of the NWQL mean recoveries indicate the recovery performances

were affected by the environmental matrix of the sample. All the mean values for the

GCMS surrogates for surface-water storm samples, surface-water base-flow samples,

and ground-water samples were higher than the NWQL mean reagent water

recoveries (table 4). Surrogate recoveries for environmental study-unit samples

performed equivalently to the NWQL mean reagant water recoveries. This indicates

that the results for GCMS analyses were reliable. The HPLC surrogate, BDMC, had a

mean recovery for all samples that was consistently lower than the NWQL mean

reagent water recovery of 81 percent (table 4). According to Werner and others

(1996), BDMC has not performed as expected, consequently the ability to infer

performance for an individual sample has been limited.

Table 4. Statistical summary of surrogate recoveries (in percent) from National Water Quality

Laboratory reagent water and from environmental samples collected from the Lower

Susquehanna River Basin study unit

[Max, maximum; Min, minimum]

Surrogate

National Water

Quality

Laboratory

mean reagent

water recovery

(percent)

Surrogate recoveries from environmental samples (percent)

Surface water

storm-affected

(n = 109)

Surface water

base flow

(n = 299)

Ground water

(n = 169)

Max Mean Min Max Mean Min Max Mean Min

BDMC 1 81 118 59 2 189 49 0 276 71 0

Diazinon-d10 2 88 166 114 56 182 108 47 163 91 0

alpha-HCH-d6 2 90 152 98 72 150 96 60 172 92 66

Tebuthylazine 2 100 285 114 88 173 111 72 141 105 80

1 Pirkey and Horodyski, 1994.

2 Zaugg and others, 1995.
26 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Blanks

Approximately 95 percent of the equipment and field blanks collected as part of this

study contained no measurable concentrations. Three blanks contained trace

amounts of several pesticides at or near the respective minimum detection limits.

Atrazine was detected in all of these blanks in the 0.003 - 0.004-μg/L range. Desethyl

atrazine (estimated 0.005 μg/L), metolachlor (0.006 μg/L), trifluralin and benefluralin

(both 0.002 μg/L), triallate (0.001 μg/L), and pronamide (0.013 μg/L) also were

detected in one blank. Therefore, pesticide concentrations below these detected

concentrations will be qualified as potentially due to contamination from an unknown

source and cannot, with assurance, be attributed to sampled concentrations.

Overall, it is considered highly unlikely that cross contamination occurred between

samples because of the high percentage of blank samples collected following the

processing of a sample with measurable concentrations and equipment cleaning in

which no pesticides were detected. An evaluation of 1992-95 NAWQA pesticide blank

data shows the number of pesticide detections in blanks from the Lower Susquehanna

River study unit was similar to detections from other NAWQA study units across the

United States (Martin and others, 1999).

Replicates

Ninety-eight percent of replicate concentration measurements for atrazine, simazine,

pendimethalin, chlorpyrifos, and diazinon were within two standard deviations of the

environmental concentration measurements. About 85 to 95 percent of the replicated

pairs for pendimethalin, chlorpyrifos, and diazinon had concentrations less than the

detection limit; therefore, these data do not give a great deal of quantitative

information about the precision of the analyses. Concentrations measured in

80 percent of the replicate samples for atrazine and 67 percent of the replicate

samples for simazine were within 10 percent of the concentrations measured in the

environmental samples. No temporal trends in precision of replicate concentration

measurements were observed.

The mean recoveries for atrazine, simazine, chlorpyrifos, and diazinon in spiked

replicate samples were between 94 and 100 percent; the mean recovery for

pendimethalin in the same group of spiked-replicates was 74 percent. Recoveries

were generally within two standard deviations of the mean. Diazinon, for example, had

a mean recovery of 95 percent and a standard deviation of 12 percent. About

95 percent of the recoveries for diazinon were within the range of two standard

deviations (71 and 119 percent).
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 27

SUMMARY OF DETECTED PESTICIDES AND MEASURED

CONCENTRATIONS

The results of an analysis of detection frequency for all pesticide data and a detailed

analysis of the concentrations of five selected pesticides will be discussed in the

following sections. Tables are provided that compare the agricultural-use ranking for

the pesticide to the detection frequency and the median and maximum

concentrations.

Detection Frequency of Measured Pesticides

The detection frequency was determined for each pesticide analyzed in all water

samples from wells and streams. Pesticides were grouped (table 5) alphabetically by

pesticide type: herbicide, insecticide, or fungicide. Included were 53 herbicides,

29 insecticides, and 1 fungicide. Several pesticides have median concentrations less

than the MDL (nondetects), indicating low numbers of detections.

Forty-six percent of the herbicides, insecticides, and fungicides analyzed were not

detected in any of the 577 samples. Herbicides were detected most frequently, and

about 60 percent of all herbicides analyzed were detected at least once.

Approximately 45 percent of all insecticides analyzed were detected at least once.

The only fungicide analyzed in the study was chlorothalonil, and it was not detected in

any of the samples. Of the 83 pesticides analyzed, 55 are registered for use in

Pennsylvania. The remainder are either classified as restricted-use pesticides or are

not registered for use in Pennsylvania. Of the 55 pesticides registered for use in

Pennsylvania, 23 were not detected in any samples collected within the study unit.

Only 7 of the 19 pesticides analyzed that were classified as restricted-use pesticides

were not detected in any samples collected within the study unit.

Agricultural-use rankings were compared to detection frequency. The five pesticides

with the highest agricultural-use rankings in the study unit from 1990 to 1994 were

atrazine, metolachlor, captan, pendimethalin, and alachlor (Anderson and Gianessi,

1995) (table 5). The detection frequencies of the herbicides atrazine and metolachlor

were 91 and 83 percent, respectively. Captan, a fungicide, was not included in the

standard suite of pesticides analyzed for this study. The detection frequencies of the

herbicides pendimethalin and alachlor were 13 and 37 percent, respectively.

The most commonly detected pesticides were agricultural herbicides—atrazine,

metolachlor, simazine, prometon, alachlor, and cyanazine. Atrazine and desethyl

atrazine, a metabolite of atrazine, were detected more frequently (91 and 92 percent,

respectively) than any other pesticides analyzed. Desethyl atrazine recovery

performances were poor and because of this, all concentrations of desethyl atrazine

were marked as ‘estimated’ values to qualify the results. Because of the national

importance of desethyl atrazine, the pesticide remains on the suite of pesticides

analyzed (Zaugg and others, 1995) in spite of its poor recovery. Metolachlor was the

second most frequently detected compound. It was detected in 478 of the

577 samples (83 percent) (table 5). All of the herbicides detected most frequently are

used for weed control, and with the exception of prometon, they are used primarily for

weed control on corn and grain crops.
28 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Table 5. Pesticides (listed alphabetically by pesticide type), the Method Detection Limit for the

pesticide, the rank of its use for agricultural purposes in the study unit, frequency of detection in

ground and surface water, and median and maximum measured concentrations, Lower

Susquehanna River Basin study unit, 1993-95

[Italicized pesticides indicate those pesticides that were not analyzed in this study;

*, Restricted-use pesticide; MDL, statistically determined method detection limit in micrograms

per liter; μg/L, micrograms per liter; --, not available; <, less than]

Pesticide MDL

(μg/L)

Agricultural

use

rank1

Number

of

samples

Detection frequency

(percent)

Concentration

(μg/L)

0 20 40 60 80 100 Median Maximum

Herbicides

Acetochlor 0.002 314 <0.002 1.5

Acifluorfen .035 304 <.035 .64

Alachlor* .002 5 577 <.002 3.4

Atrazine* .001 2 577 .120 12

Atrazine, Desethyl .002 577 .094 1.0

Benfluralin .002 577 <.002 .039

Bentazon .014 47 304 <.014 .88

Bromacil .035 304 <.035 .12

Bromoxynil .035 22 304 -- --

Butylate .002 577 <.002 .016

Chloramben .011 304 -- --

Clopyralid .050 304 -- --

Cyanazine* .004 6 577 <.004 3.9

DCPA* .002 40 577 <.002 .72

Dicamba, mono-acid- .035 19 304 <.035 .21

Dichlobenil .020 304 <.020 .21

Dichlorprop, monoacid-.032 304 -- --

Diethlyanaline .003 577 <.003 .01

Dinoseb2 .035 304 -- --

Diuron .020 36 304 <.020 .64

EPTC .002 43 577 <.002 .21

Ethalfluralin .004 46 577 -- --

Fenuron .013 304 -- --

Fluometuron .035 304 -- --

Glyphosate -- 20 0 -- --

Linuron* .002 11 577 <.002 .54

MCPA, mono-acid- .050 28 304 <.050 .1

MCPB, mono-acid- .035 304 -- --

Metolachlor .002 1 577 .032 11

Metribuzin .004 30 577 <.004 .15

Molinate .004 577 -- --

1-Naphthol .007 304 -- --

Napropamide .003 41 577 <.003 .07

Neburon .015 304 -- --

Norflurazon .024 44 304 -- --

Oryzalin .019 45 304 <.019 .05

Paraquat -- 13 0 -- --

Pebulate .004 34 577 <.004 .053

Pendimethalin .004 4 577 <.004 .24

Picloram, mono-acid-* .050 293 -- --
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 29

Herbicides—Continued

Prometon 0.018 577 0.022 1.5

Pronamide* .003 39 577 .013 (one

detection)

Propachlor .007 577 <.007 .036

Propham .035 304 -- --

Silvex, monoacid-2

.021 304 -- --

Simazine .005 25 577 .031 7.6

Tebuthiuron .010 577 <.010 .12

Terbacil .007 32 561 <.007 .34

Thiobencarb* .002 577 -- --

Triallate2 .001 577 -- --

Triclopyr, mono-acid- .050 304 -- --

Trifluralin .002 29 577 <.002 .028

2,4-D, mono-acid- .035 9 304 <.035 1.4

2,4-DB, mono-acid- .035 304 -- --

2,4,5-T, mono-acid-2 .035 304 -- --

Insecticides

Aldicarb* .016 304 -- --

Aldicarb Sulfone .016 301 -- --

Aldicarb Sulfoxide .021 301 -- --

Azinphos, Methyl* .001 16 561 <.001 .41

Carbaryl .003 27 577 <.003 .65

Carbofuran* .003 18 577 <.003 .48

Carbofuran, 3-

Hydroxy-

.014 304 -- --

Chlorpyrifos .004 7 577 <.004 .09

Cryolite -- 17 0 -- --

DDE, p,p .006 577 <.006 .005

Diazinon .002 38 577 <.002 .06

Dieldrin2 .001 577 <.001 .019

Disulfoton* .017 577 -- --

DNOC2 .035 304 -- --

Esfenvalerate .019 42 304 -- --

Ethoprop* .003 577 <.003 .052

Fonofos* .003 23 577 <.003 .015

HCH, alpha2 .002 577 -- --

HCH, gamma* .004 577 -- --

Malathion .005 15 577 <.005 .13

Table 5. Pesticides (listed alphabetically by pesticide type), the Method Detection Limit for the

pesticide, the rank of its use for agricultural purposes in the study unit, frequency of detection in

ground and surface water, and median and maximum measured concentrations, Lower

Susquehanna River Basin study unit, 1993-95—Continued

[Italicized pesticides indicate those pesticides that were not analyzed in this study;

*, Restricted-use pesticide; MDL, statistically determined method detection limit in micrograms

per liter; μg/L, micrograms per liter; --, not available; <, less than]

Pesticide MDL

(μg/L)

Agricultural

use

rank1

Number

of

samples

Detection frequency

(percent)

Concentration

(μg/L)

0 20 40 60 80 100 Median Maximum
30 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Pesticide detections in well and stream samples were compared for 83 pesticides

(table 6). Approximately 53 percent of all herbicides and 17 percent of all insecticides

analyzed were detected in well-water samples and approximately 51 percent of all

herbicides and 48 percent of all insecticides analyzed were detected in stream

samples. Base-flow samples accounted for nearly 75 percent of the stream samples,

and approximately 45 percent of all herbicides and 41 percent of all insecticides

analyzed were detected in base-flow samples. Storm-affected samples account for

the remainder of the stream samples, and approximately 51 percent of all herbicides

and 41 percent of insecticides analyzed were detected in storm-affected samples. The

percentage of pesticide detections was higher in stream samples for 25 of the 28 total

pesticides detected in both ground and surface waters. For example, the herbicide

atrazine was detected in 98 percent of the stream samples but in only 75 percent of

the well-water samples.

Insecticides—Continued

Methiocarb 0.026 304 -- --

Methomyl .017 24 301 <0.017 0.19

Methyl Bromide3 .2 8 0 -- --

Oxamyl* .018 33 301 -- --

Parathion, Ethyl2 .004 48 577 -- --

Parathion, Methyl* .006 26 577 <.006 .051

Permethrin, cis .005 31 577 -- --

Phorate* .002 37 577 -- --

Propargite2 .013 35 577 -- --

Propoxur .035 297 -- --

Terbufos* .013 21 577 .030 (one

detection)

Fungicides

Captan -- 3 0 -- --

Chlorothalonil .035 302 -- --

Mancozeb -- 10 0 -- --

Ziram -- 14 0 -- --

1 This ranking was determined using the information in table 2 (p. 19)—amount applied in the study

unit basin (1990-94). 2 Not registered for use in Pennsylvania.

3 This pesticide was analyzed as a volatile organic compound and was not analyzed as a pesticide

for this study.

Table 5. Pesticides (listed alphabetically by pesticide type), the Method Detection Limit for the

pesticide, the rank of its use for agricultural purposes in the study unit, frequency of detection in

ground and surface water, and median and maximum measured concentrations, Lower

Susquehanna River Basin study unit, 1993-95—Continued

[Italicized pesticides indicate those pesticides that were not analyzed in this study;

*, Restricted-use pesticide; MDL, statistically determined method detection limit in micrograms

per liter; μg/L, micrograms per liter; --, not available; <, less than]

Pesticide MDL

(μg/L)

Agricultural

use

rank1

Number

of

samples

Detection frequency

(percent)

Concentration

(μg/L)

0 20 40 60 80 100 Median Maximum
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 31

Table 6. Pesticides commonly used in Pennsylvania, pesticides analyzed, and number of

detections in water from streams during storm-affected and base-flow conditions and water

from ground-water wells, Lower Susquehanna River Basin study unit, 1993-95

[Italicized pesticides indicate those pesticides that were not analyzed in this study but are

commonly used in Pennsylvania; *, Restricted-use pesticide; --, not available]

Pesticide

Storm-affected stream

samples

Base-flow stream

samples

Ground-water

samples

Number of

samples

Number of

detections

Number of

samples

Number of

detections

Number of

samples

Number of

detections

Herbicides

Acetochlor 72 34 142 10 100 0

Acifluorfen 40 0 96 1 168 1

Alachlor* 109 81 299 116 169 18

Atrazine* 109 109 299 292 169 126

Atrazine, Desethyl 109 109 299 289 169 131

Benfluralin 109 13 299 0 169 1

Bentazon 40 0 96 0 168 4

Bromacil 40 0 96 0 168 1

Bromoxynil 40 0 96 0 168 0

Butylate 109 2 299 2 169 1

Chloramben 40 0 96 0 168 0

Clopyralid 40 0 96 0 168 0

Cyanazine* 109 52 299 102 169 11

DCPA* 109 52 299 35 169 0

Dicamba, mono-acid- 40 1 96 0 168 1

Dichlobenil 40 0 96 0 168 1

Dichlorprop, mono-acid- 40 0 96 0 168 0

Diethlyanaline 109 1 299 1 169 1

Dinoseb1 40 0 96 0 168 0

Diuron 40 4 96 5 168 6

EPTC 109 10 299 3 169 2

Ethalfluralin 109 0 299 0 169 0

Fenuron 40 0 96 0 168 0

Fluometuron 40 0 96 0 168 0

Glyphosate -- -- -- -- -- --

Linuron* 109 14 299 9 169 1

MCPA, mono-acid- 40 1 96 1 168 0

MCPB, mono-acid- 40 0 96 0 168 0

Metolachlor 109 109 299 279 169 90

Metribuzin 109 17 299 11 169 1

Molinate 109 0 299 0 169 0

1-Naphthol 40 0 96 0 168 0

Napropamide 109 7 299 5 169 1

Neburon 40 0 96 0 168 0

Norflurazon 40 0 96 0 168 0

Oryzalin 40 0 96 0 168 1

Paraquat -- -- -- -- -- --

Pebulate 109 0 299 1 169 2

Pendimethalin 109 49 299 25 169 1

Picloram, mono-acid-* 40 0 96 0 157 0
32 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Herbicides—Continued

Prometon 109 94 299 236 169 71

Pronamide* 109 0 299 0 169 1

Propachlor 109 4 299 2 169 0

Propham 40 0 96 0 168 0

Silvex, mono-acid-1 40 0 96 0 168 0

Simazine 109 107 299 272 169 83

Tebuthiuron 109 50 299 84 169 12

Terbacil 104 2 288 8 169 1

Thiobencarb* 109 0 299 0 169 0

Triallate1 109 0 299 0 169 0

Triclopyr, mono-acid- 40 0 96 0 168 0

Trifluralin 109 18 299 0 169 1

2,4-D, mono-acid- 40 9 96 5 168 1

2,4-DB, mono-acid- 40 0 96 0 168 0

2,4,5-T, mono-acid-1 40 0 96 0 168 0

Insecticides

Aldicarb* 40 0 96 0 168 0

Aldicarb Sulfone 40 0 96 0 165 0

Aldicarb Sulfoxide 40 0 96 0 165 0

Azinphos, Methyl* 104 7 288 18 169 0

Carbaryl 109 52 299 21 169 7

Carbofuran* 109 11 299 19 169 4

Carbofuran, 3-Hydroxy- 40 0 96 0 168 0

Chlorpyrifos 109 41 299 24 169 0

Cryolite -- -- -- -- -- --

DDE, p,p 109 2 299 2 169 4

Diazinon 109 28 299 13 169 2

Dieldrin1 109 7 299 14 169 8

Disulfoton* 109 1 299 0 169 0

DNOC1 40 0 96 0 168 0

Esfenvalerate 40 0 96 0 168 0

Ethoprop* 109 2 299 1 169 0

Fonofos* 109 5 299 4 169 0

HCH, alpha1 109 0 299 0 169 0

HCH, gamma* 109 0 299 0 169 0

Malathion 109 15 299 5 169 0

Methiocarb 40 0 96 0 168 0

Methomyl 40 1 96 0 165 0

Methyl Bromide -- -- -- -- -- --

Oxamyl* 40 0 96 0 165 0

Parathion, Ethyl1 109 0 299 0 169 0

Table 6. Pesticides commonly used in Pennsylvania, pesticides analyzed, and number of

detections in water from streams during storm-affected and base-flow conditions and water

from ground-water wells, Lower Susquehanna River Basin study unit, 1993-95—Continued

[Italicized pesticides indicate those pesticides that were not analyzed in this study but are

commonly used in Pennsylvania; *, Restricted-use pesticide; --, not available]

Pesticide

Storm-affected stream

samples

Base-flow stream

samples

Ground-water

samples

Number of

samples

Number of

detections

Number of

samples

Number of

detections

Number of

samples

Number of

detections
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 33

In October 1994, the herbicide acetochlor was added to the list of pesticides analyzed

by the GCMS method. It is a selective preemergent herbicide used on corn to control

weeds and grasses and has restricted registration for use in the United States to

replace some of the more widely used corn herbicides such as atrazine, metolachlor,

and alachlor. Acetochlor’s restricted registration is contingent on concentration limits

that have been set for stream and ground-water samples. These limits can not be

exceeded or registration will be revoked (Lindley and others, 1996). In the Lower

Susquehanna River Basin study unit, 314 samples were analyzed for acetochlor, and

of those, 44 (14 percent) had measurable concentrations (table 5). Storm-affected

stream samples had the highest number of detections; acetochlor was detected in 34

of 72 samples (47 percent).

Concentrations of Selected Pesticides

The discussion of concentrations of pesticides measured in streams and wells will be

focused on five compounds—atrazine, simazine, pendimethalin, chlorpyrifos, and

diazinon. For data-analysis purposes, concentrations designated as less than the

MDL were assigned a concentration of one-half the MDL (table 7). Measured

concentrations were compared to MCL’s established by the USEPA. Of the five

selected pesticides, MCL’s have been established only for atrazine and simazine

(table 7). The data for this study are listed in the USGS annual water-data reports

(Durlin and Schaffstall, 1994; 1996; 1997).

Of the nearly 40,000 pesticide concentrations measured in water samples from

streams and shallow wells, only 7 measurements (less than 0.018 percent) exceeded

any established MCL or MCLG for drinking water. All of the concentrations that

exceeded the drinking-water standards were in samples from streams not used as

drinking-water supplies, and five of the seven exceedances were in samples collected

during three storm events during June, when agricultural applications on row crops

Insecticides—Continued

Parathion, Methyl* 109 1 299 1 169 0

Permethrin, cis 109 0 299 0 169 0

Phorate* 109 0 299 0 169 0

Propargite1 109 0 299 0 169 0

Propoxur 38 0 93 0 166 0

Terbufos* 109 0 299 1 169 0

Fungicides

Captan -- -- -- -- -- --

Chlorothalonil 38 0 96 0 168 0

Mancozeb -- -- -- -- -- --

Ziram -- -- -- -- -- --

1 Not registered for use in Pennsylvania.

Table 6. Pesticides commonly used in Pennsylvania, pesticides analyzed, and number of

detections in water from streams during storm-affected and base-flow conditions and water

from ground-water wells, Lower Susquehanna River Basin study unit, 1993-95—Continued

[Italicized pesticides indicate those pesticides that were not analyzed in this study but are

commonly used in Pennsylvania; *, Restricted-use pesticide; --, not available]

Pesticide

Storm-affected stream

samples

Base-flow stream

samples

Ground-water

samples

Number of

samples

Number of

detections

Number of

samples

Number of

detections

Number of

samples

Number of

detections
34

NATIONALWATER-QUALITYASSESSMENTPROGRAM

Table 7. Statistical summary of atrazine, simazine, pendimethalin, chlorpyrifos, and diazinon concentrations in water samples

collected within the Lower Susquehanna River Basin study unit,1993-95

[μg/L, micrograms per liter; MCL, Maximum Contaminant Level; MDL, Method Detection Limit; --, not available;

n, number of samples; Percentiles, percentage of samples in which pesticide concentrations were less than or equal to thoseshown (in μg/L); P10, tenth percentile; P25, twenty-fifth percentile; P75, seventy-fifth percentile; P90, ninetieth percentile]

Pesticide MCL (μg/L) MDL (μg/L)

Pesticide concentration, in μg/L

Storm-affected stream water samples

(n=109)

Base-flow stream water samples

(n=299)

Ground-wa

(n=

P10 P25 Median P75 P90 P10 P25 Median P75 P90 P10 P25 Me

Atrazine 3.0 0.001 0.064 0.085 0.16 0.34 1.01 0.04 0.093 0.13 0.21 0.31 <0.001 <0.001 0.

Simazine 4.0 .005 .016 .028 .044 .10 .81 <.005 .018 .041 .076 .28 <.005 <.005 <.

Pendimethalin -- .004 <.004 <.004 <.004 .019 .045 <.004 <.004 <.004 <.004 <.004 <.004 <.004 <.

Chlorpyrifos -- .004 <.004 <.004 <.004 .008 .016 <.004 <.004 <.004 <.004 <.004 <.004 <.004 <.Diazinon -- .002 <.002 <.002 <.002 .005 .018 <.002 <.002 <.002 <.002 <.002 <.002 <.002 <.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 35

are common. Concentrations of four agricultural herbicides—alachlor, atrazine,

cyanazine, and simazine—exceeded the standards. Concentrations during storm

events are elevated due primarily to surface-runoff contributions and are expected to

be short-lived. Simazine concentrations in samples from two streams exceeded the

MCL during base-flow conditions.

A comparison of the median concentrations of the five selected pesticides shows

concentrations in storm-affected stream samples are higher than concentrations in

ground-water and stream base-flow samples (table 7). Simazine, for example, has a

median concentration of 0.044 μg/L in storm-affected stream samples—this means

that 50 percent of the storm-affected concentrations were above 0.044 μg/L and

50 percent of the concentrations were below 0.044 μg/L. The median concentrations

of simazine for base-flow stream samples (0.041 μg/L) and ground-water samples

(<0.005 μg/L) were lower than those of the storm-affected stream samples (table 7).

Again using simazine as an example and looking at the ninetieth (P90) percentiles,

storm-affected stream samples had a concentration of 0.81 μg/L—this means that

10 percent of the concentrations were higher than 0.81 μg/L. Concentrations at the

ninetieth percentile for base-flow stream samples (0.28 μg/L) and ground-water

samples (0.051 μg/L) were again lower than that of the storm-affected stream

samples (table 7).

Differences between median concentrations may not be statistically significant due to

the variability of concentrations within each data subset and the variability of the

number of samples between data subsets. The Kruskal-Wallis test (Helsel and Hirsch,

1992, p. 159-163) was used to compare storm-affected and base-flow and base-flow

and ground-water median concentrations and determine statistical significance for

atrazine and simazine. The test indicated statistically significant differences in median

concentrations for the storm-affected to base-flow and base-flow to ground-water

comparisons for both pesticides. A confidence level of 95 percent (ρ ≤ 0.05) was used

to determine statistical significance.

The comparison of pesticide detection frequency and concentration data (tables 5, 6,

and 7) indicated that not only does atrazine have the highest detection frequency of

the pesticides analyzed, but it consistently has the highest concentrations in both

ground and surface waters. This result was found despite the fact that, on an annual

basis, more metolachlor than atrazine is applied to agricultural land (table 2) in the

Lower Susquehanna River Basin. Atrazine is second only to metolachlor as the mostused herbicide for agricultural purposes in the study unit (table 5). Approximately

417,000 lb and 470,000 lb of active ingredient of atrazine and metolachlor are applied

annually. Determining the reason for this discrepancy was not within the scope of this

study.

Atrazine was detected in 98 percent of stream samples and 75 percent of the wellwater samples. Of the samples in which there were detections of atrazine, 50 percent

had concentrations between 0.093 and 0.21 μg/L for the base-flow stream samples

and between <0.001 and 0.19 μg/L for the ground-water samples. These

concentration ranges correspond to the twenty-fifth (P25) and seventy-fifth (P75)

percentiles (table 7).

Another of the selected pesticides, the insecticide diazinon, had a median

concentration of <0.002 μg/L for all samples (table 7), a comparatively low median

concentration in relation to atrazine. Diazinon was detected in only 13 of 299

(4 percent) samples of stream base flow and in 2 of 169 (1 percent) ground-water
36 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

samples. For agricultural purposes in the study unit, diazinon is a low-use pesticide

with approximately 2,460 lb of active ingredient applied annually (table 2) and was

ranked 38 out of 48 pesticides in terms of annual usage (table 5).

The Kruskal-Wallis test could not be used to evaluate differences in median

concentrations for pendimethalin, chlorpyrifos, and diazinon because of the large

number of samples that had concentrations below the detection limit. For each of

these pesticides, contingency tables were used to evaluate if the differences were

statistically significant between the number of samples above the detection limit and

the number of samples below the detection limit for the following comparisons: stormaffected to base-flow, base-flow to ground-water, and storm-affected to ground-water

(table 6). Comparison of data subsets indicate statistically significant differences

(ρ ≤ 0.05) between numbers of samples above and below the detection limit for all

comparisons except the diazinon base-flow to ground-water comparison.

Maximum pesticide concentrations in the study unit were measured in storm-affected

stream samples. Samples collected during one storm in particular, during the second

week of June 1994, provided most of the higher concentrations. Storm-affected

stream samples were generally collected during the growing season. Base-flow

stream samples were collected throughout the year, whereas ground-water samples

were collected only during the summer months of June through August.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 37

FACTORS AFFECTING PESTICIDE CONCENTRATIONS

Pesticide data-collection studies implemented by the Lower Susquehanna River Basin

NAWQA study unit were designed, among other things, to provide data to describe

areal and temporal variations in pesticide concentrations in ground water and

streams. Concentration data from three herbicides—atrazine, simazine, and

pendimethalin—and two insecticides—chlorpyrifos and diazinon—will be used to

display observed patterns in concentrations. This group of compounds was selected

to represent various types of uses, ranges of amounts used for agricultural purposes,

and ranges of detection frequencies in water from streams and wells in the study unit.

Conceptual models that guide the discussion of the major factors affecting seasonal

and areal patterns of pesticide concentrations are described.

Conceptual Models for Major Factors Affecting Pesticide Concentrations

Two factors observed to have a major influence on seasonal patterns of pesticide

concentrations in the four streams where data are available are 1) seasonality of

applications and climate, and 2) ground-water retention and discharge to streams.

The conceptual model for seasonal patterns relates annual and short-term patterns of

storm-runoff concentrations observed in streams to the delivery of applied pesticides

and the subsequent discharge to streams by surface runoff and ground water. The

timing of periods of elevated concentrations in streams will be related to the timing of

applications to bare soils and foliage, to the method of transport to the stream, and to

the traveltime of water as it passes through the hydrogeologic system.

The conceptual model for areal patterns of pesticide concentrations in streams and

shallow wells in the Lower Susquehanna River Basin includes the following factors:

1) areal-application patterns (agricultural and nonagricultural) and initial availability of

the pesticide for transport; 2) the pesticides’ environmental persistence, affinity for

dissolution in water or adsorption to sediment particles, and the infiltration capacity of

the soils to which the pesticides are applied; and 3) ground-water retention and

discharge to the streams. Many of these factors originate from differences in bedrock

type (which affects the percentage of agricultural/nonagricultural land use, infiltration

capacity of the soils, and to a limited extent, the ground-water retention and discharge

to streams) and differences in the pesticides’ chemical properties (which affects the

affinity for dissolution and the rate of transformation resulting in degradation).

Factors Affecting Seasonal Concentration Patterns in Streams

Multi-year pesticide-concentration data are available for the description of seasonal

patterns. Samples were collected multiple times at three long-term monitoring sites on

streams in Pennsylvania: East Mahantango Creek in Schuylkill County (47 samples),

Cedar Run in Cumberland County (104 samples), and Mill Creek in Lancaster County

(47 samples). At an additional site (Bachman Run in Lebanon County), pesticideconcentration data are available from 53 samples collected over 1 year; these data will

be used to describe short-term effects during the application season. Data collected

from the streams represent the water quality of four environmental subunits in the

study unit. Three of the sites drain basins in which agriculture is the predominant land

use. Cedar Run drains a predominantly urban area. The basin upstream of the East

Mahantango Creek site is in the Appalachian Mountain Section of the Ridge and

Valley Physiographic Province and is underlain by sandstone and shale. The three

remaining streams drain basins underlain by carbonate rock. Two basins, Cedar Run

and Bachman Run, are located in the Great Valley Section of the Ridge and Valley

Physiographic Province, and the third, Mill Creek, is located in the Lowlands Section

of the Piedmont Physiographic Province (fig. 3). More detailed information about the
38 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

environmental setting of these subunits can be found in Risser and Siwiec (1996).

More detailed information about basin characteristics, sample-collection frequency,

and the types of field and laboratory measurements made at each of the sites can be

found in Siwiec and others (1997).

Seasonality of Applications and Climate

Because herbicides are applied shortly before planting or shortly before the

emergence of crops, crop-planting information can be used to approximate when

herbicides are applied in any given year. This assumption is supported by Barbash

and Resek (1996, p. 107) and, for corn, by Crawford (1995). Barbash and Resek

suggest crop acreage is a reliable predictor of pesticide use, on a local scale.

Crawford showed a relation between percentage of corn planted and the timing of

annual peak atrazine concentrations in the White River, Ind. In this report, the

progressive amount of acres planted in corn is used to estimate pre-planting, preemergent, and post-emergent herbicide-application periods.

The recorded stages of corn crop planting and growth provide estimates of time

periods when burndown, pre-emergent, and post-emergent applications may occur.

Statewide average 5-day-interval corn planting and growth schedules for 1993 and

1994 are available from the Pennsylvania Department of Agriculture (1996). During

these 2 years, the planting of corn generally began by April 25, 10-20 percent of the

corn was planted by May 5, and about 85 percent of the total crop was planted by

June 5 (figs. 5-7). By mid-July, corn has generally reached a height at which

applications cannot be made without crop damage. These stages of planting and

growth imply burndown herbicide applications during the month of April, pre-emergent

applications during the month of May, and post-emergent applications during the

month of June. The three herbicides displayed in figures 5-7 (atrazine, simazine, and

pendimethalin) are generally used for pre- and post-emergent applications. Atrazine is

applied on soil and foliar surfaces. Simazine and pendimethalin are generally applied

to soil surfaces only, and simazine is sometimes applied with other herbicides for

burndown purposes. Generally, the herbicides applied during a particular growing

season would not be expected to be seen in the environment before April of that

season. Concentrations measured in wells and streams prior to April are probably the

result of applications during prior years.

Concentrations of atrazine and simazine measured in samples collected during baseflow conditions from three streams at long-term monitoring sites exhibit seasonal

patterns (figs. 5-7). Little variation in concentration was measured from September

through April. This period coincides with the nongrowing season and the

nonapplication period for the three herbicides. A general increase in concentrations of

atrazine and simazine in the two streams draining agricultural basins, East

Mahantango Creek (fig. 5) and Mill Creek (fig. 6), is observed from May through

August, the planting, growing, and herbicide-application seasons. In contrast, Cedar

Run (fig. 7) is a predominantly urban basin with some agricultural area in the

headwaters of the basin. Variations in stream-water herbicide concentrations that may

result from agricultural activity in headwater areas of the basin may be masked by the

quantity and quality of flow contributed to Cedar Run by the downstream urban and

commercial areas.

While the seasonality of applications appears to play a major role in the seasonal

elevation of selected pesticide concentrations in streams, the effects are enhanced by

the concurrent seasonality of the climate in the northeastern United States. The longterm averages for precipitation in Pennsylvania show a 60-percent increase from

March (low) to July (high) (National Oceanic and Atmospheric Administration, 1995).
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 39

0

10

5

PRECIPITATION,

IN INCHES

SELINSGROVE, PA. CLIMATOLOGICAL STATION

APR MAY JUNE JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

1993 1994

1

5,000

10

100

1,000

DAILY MEAN

STREAMFLOW,

IN CUBIC FEET

EAST MAHANTANGO CREEK AT KLINGERSTOWN, PA.

0

0.05

0.01

0.02

0.03

0.04

CONCENTRATION,

IN MICROGRAMS

PER LITER

0

1.0

0.5

CONCENTRATION,

IN MICROGRAMS

 PER LITER

0

100

50

CUMULATIVE

PERCENTAGE OF

CORN PLANTED

0

1.0

0.5

CONCENTRATION,

IN MICROGRAMS

PER LITER

ATRAZINE

SIMAZINE

PENDIMETHALIN

Figure 5. Monthly precipitation and cumulative percentage of corn planted in the East Mahantango Creek Basin

and base-flow herbicide concentrations and daily mean streamflow for East Mahantango Creek, 1993-94.

EXPLANATION MEASURED

CONCENTRATION

CONCENTRATION LESS

THAN METHOD

PER SECOND

DETECTION LIMIT

APR MAY JUNE JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

1993 1994

STATEWIDE

ANNUAL

SCHUYLKILL COUNTY
40 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

10

0

5

PRECIPITATION,

IN INCHES

LANCASTER, PA. CLIMATOLOGICAL STATION

APR MAY JUNE JULY APR SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

1993 1994

1

5,000

10

100

1,000

DAILY MEAN

STREAMFLOW,

IN CUBIC FEET

MILL CREEK NEAR LYNDON, PA.

0

6

2

4

CONCENTRATION,

IN MICROGRAMS

PER LITER

0

100

50

CUMULATIVE

PERCENTAGE OF

CORN PLANTED

0.6

0.2

0.4

CONCENTRATION,

IN MICROGRAMS

PER LITER

ATRAZINE

0.05

0

0.01

0.02

0.03

0.04

CONCENTRATION,

IN MICROGRAMS

PER LITER

SIMAZINE

PENDIMETHALIN

Figure 6. Monthly precipitation and cumulative percentage of corn planted in the Mill Creek Basin and baseflow herbicide concentrations and daily mean streamflow for Mill Creek, 1993-94.

MEASURED EXPLANATION

CONCENTRATION

PER SECOND

CONCENTRATION LESS

THAN METHOD

DETECTION LIMIT

APR MAY JUNE JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

1993 1994

STATEWIDE

ANNUAL

LANCASTER COUNTY
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 41

0

10

5

PRECIPITATION,

IN INCHES

MIDDLETOWN, PA. CLIMATOLOGICAL STATION

1993 1994 1995

1

500

2

5

10

20

50

100

200

DAILY MEAN

STREAMFLOW,

IN CUBIC FEET

CEDAR RUN AT EBERLYS MILL, PA.

0

100

50

CUMULATIVE

PERCENTAGE OF

CORN PLANTED

0

0.050

0.025

CONCENTRATION,

IN MICROGRAMS

PER LITER

0

0.10

0.05

CONCENTRATION,

IN MICROGRAMS

PER LITER

MEASURED EXPLANATION

CONCENTRATION

0

1.2

0.25

CONCENTRATION,

IN MICROGRAMS

PER LITER

(1.1 μg/L) ATRAZINE

SIMAZINE

PENDIMETHALIN

DATA FOR 1995

NOT AVAILABLE

Figure 7. Monthly precipitation and cumulative percentage of corn planted in the Cedar Run Basin and baseflow herbicide concentrations and daily mean streamflow for Cedar Run, 1993-95. PER SECOND

CONCENTRATION LESS

THAN METHOD

DETECTION LIMIT

1993 1994 1995

ANNUAL

STATEWIDE

CUMBERLAND COUNTY

AMJ J ASOND J FMAMJ J ASOND J FMAMJ J AS

AMJ J ASOND J FMAMJ J ASOND J FMAMJ J AS

1.0
42 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Evapotranspiration is also seasonally low in the spring, which increases the potential

for precipitation to recharge ground water. Therefore, soluble pesticides applied during

the spring have a higher potential to enter the ground water and eventually discharge

to the stream through base flow (figs. 5-7). Compared to long-term average monthly

precipitation, precipitation in the spring of 1993 was abnormally low. Studies

examining fluxes in concentrations of selected pesticides (including atrazine) in the

Susquehanna River above the Fall Line to the northern Chesapeake Bay, also found

that the greatest pesticide concentrations coincided with the months of field

application (Foster and Lippa, 1996; G.D. Foster, K.A. Lippa, and C.V. Miller, written

commun, 1998).

Applied pesticides also can be volatilized or become attached to dust particles and be

redeposited during precipitation. Although data of this type were not collected during

this investigation, a study that examined samples collected in 23 states from 1990 to

1991 as part of the National Atmospheric Depositional Program/National Trends

Network found that about one-third of the samples contained detectable

concentrations of triazine and/or acetanilide herbidices (Stamer and others, 1998).

Atrazine and alachlor were detected most frequently. Pesticide concentrations in

precipitation varied seasonally and corresponded to pesticide-application periods;

however, the overall contributions by this delivery method were relatively small

(Stamer and others, 1998).

The effect of seasonal stormflow on herbicide transport is shown in figures 8 and 9.

During the stormflows sampled in May and June from Cedar Run and Bachman Run,

general increases in atrazine concentrations were observed along the rising limb of

the hydrograph; the concentration occasionally peaked after the peak in streamflow

had passed. This implies that atrazine applied from April through June (application

months) is available and is being flushed into the streams during storms. The storm in

July shows a decrease in atrazine concentrations along the rising limb of the stream

hydrographs—a time when increases were noted during events earlier in the season.

Except for the one concentration measured in Bachman Run near the peak of the July

1995 storm, this pattern of decreasing storm concentrations in July is consistent. It

implies a reduction from the supply of atrazine available during the months of

application.

Studies in other basins in the United States indicate similar patterns in stream

concentrations during storms. Crawford (1995) reported atrazine concentrations in the

White River at Hazelton, Ind., from 1991 through 1995 were highest during the

planting period and the occurrence of peak concentrations was not necessarily related

to peak streamflow. In the White River, the concentrations typically were highest

during the first one or two runoff periods after application.

A short-term reduction in the atrazine supply also can occur during the application

season. The hydrograph of the June 11-14, 1995, storm (fig. 8) shows two successive

storms within about 12 hours. The first, with a considerably lower streamflow peak,

produced the higher peak concentrations of atrazine of the two storms. This type of

transport is common for limited-supply constituents like atrazine.

Concentrations of the insecticides diazinon and chlorpyrifos (figs. 8 and 9) behaved in

a manner similar to herbicides during the rising limb of storms but differed significantly

during base-flow conditions between storms. Organophosphate insecticides such as

these have a relatively high capacity for adsorption to sediment particles. Triazine

herbicides like atrazine have a relatively low adsorption potential and have a higher

potential to dissolve and be transported in the water, as opposed to by the water

through attachment to sediment. Concentrations of diazinon and chlorpyrifos
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 43 2400 2400 2400 2400 10 11 12 May 1995

1

200

25

102050

100

STREAMFLOW, IN

CUBIC FEET PER SECOND

0.0010.0500.0050.0100.0150.0400.0010.0250.0050.0100.0150.020

2400 2400 2400 2400

11 12 13 14

June 1995

2400 2400 2400 

5 6 

July 1995

DIAZINON 

CHLORPYRIFOS

DIAZINON

CHLORPYRIFOS CHLO

CONCENTRATION, IN MILLIGRAMS PER LITER

Figure 8. Streamflow and concentrations of diazinon, chlorpyrifos, and atrazine measured in Cedar Run at Eberlys Mill, Pa., beforeselected storms in May through July 1995. [Samples identified as triangles are nearest samples collected before or after storms at b

(5/4/95 @ 1445) (5/16/95 @ 1115)

2400

(5/30/95 @1335)

(.047 μg/L)

(5/30/95 @1335) (5/4/95 @ 1445) (5/16/95 @ 1115) (7

(7

(7

0.040.200.060.080.100.120.140.160.18 ATRAZINE

(5/4/95 @ 1445) (5/16/95 @ 1115)

ATRAZINE

(5/30/95 @1335)

EXPLANATION

MEASURED

CONCENTRATION

CONCENTRATION

LESS THAN METHODDETECTION LIMIT

MEASURED CONCENTRATIONS

OF SAMPLES COLLECTED BEFORE OR

AFTER STORMS AT BASE-FLOW CONDITIONS

CONCENTRATION LESS THAN METHOD

DETECTION LIMIT FOR SAMPLES COLLECTEDBEFORE OR AFTER STORMS AT BASE-FLOW 

PERIOD OF PEAK FLOW BREAK IN RECORD SHOWN
44 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 2400 240

5 

July 

2400 2400 2400 2400 2400 11 12 13 14 June 1995 ATRAZINE CHLORPYRIFOS DIAZINON 

0.001 0.009 0.003 0.005 0.007 0.001 0.009 0.003 0.005 0.007 0.1 0.7 0.3 0.5 CHLORPYRIFOS DIAZINON ATRAZINE 2400 2400 2400 2400 10 11 12 May 1995

2

12

468

10

STREAMFLOW, IN

CUBIC FEET PER

Figure 9. Streamflow and concentrations of diazinon, chlorpyrifos, and atrazine measured in Bachman Run at Annville, Pa., beforeselected storms in May through July 1995. [Samples identified as triangles are nearest samples collected before or after storms at b

SECOND

CONCENTRATION, IN MILLIGRAMS PER LITER(5/16/95 @ 0900) (5/4/95 @ 1145) (5/4/95 @ 1145) (5/16/95 @ 0900)

(5/4/95 @ 1145) (5/16/95 @ 0900)

(7/1/95 @ 1030)

(7/1/95 @ 1030)

(7/1/95 @ 1030)

EXPLANATION

MEASURED

CONCENTRATION

CONCENTRATION

LESS THAN METHODDETECTION LIMIT

MEASURED CONCENTRATIONS

OF SAMPLES COLLECTED BEFORE OR

AFTER STORMS AT BASE-FLOW CONDITIONS

CONCENTRATION LESS THAN METHOD

DETECTION LIMIT FOR SAMPLES COLLBEFORE OR AFTER STORMS AT BASE-F

PERIOD OF PEAK FLOW BREAK IN RECORD SHOWN
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 45

exceeded the reporting limit only when overland runoff was a significant component of

the streamflow. The introduction of these adsorptive pesticides to streams appears to

be dependent on the energy supplied by overland runoff to dislodge and transport

sediment particles. During all three storm periods shown on figures 8 and 9, not just

those early in the season, measurable insecticides concentrations were observed.

This is probably because insecticides are applied to control pests throughout the

growing seasona pattern unlike the seasonal method used for herbicides applied to

corn. Most insecticide applications in the study unit are probably for nonagricultural

purposes.

Ground-Water Retention and Discharge

The important role of ground water in the retention of pesticides and their eventual

discharge to streams is shown in figures 5-7. Although concentrations varied between

the three streams, a relatively constant year-round baseline atrazine concentration

was measured in base-flow samples in each stream. In two of the three streams, a

similar pattern was observed for simazine. Pendimethalin, a herbicide with a higher

potential for adsorption and a lower potential for dissolution than either atrazine or

simazine, was rarely measured in base-flow samples.

Measurable in-stream concentrations of herbicides throughout the year in study-unit

streams indicate aquifers are constantly transporting a small amount of contaminant

to the stream with a reservoir-like effect. In reservoirs, the effect is the result of the

mixing of inflow within the impounded pool of water and the discharge of the mixed

water through an outlet at a fixed location. The extent of mixing in the reservoir is

dependent on several factors, but primarily on residence time. The physical properties

of the soils, bedrock, and deep aquifers appear to have a tendency to attenuate the

effects of seasonal applications of herbicides and provide a relatively constant

baseline release of measurable concentrations to streams from a deep ground-water

source replenished during each application season. More shallow ground water is

believed to provide the seasonal pulses observed in base flow of streams in the study

unit.

The concept of a ground-water reservoir that discharges to stream base flow is

supported by other studies. Barbash and Resek (1996) indicate seasonal patterns

also have been observed in shallow ground water in the mid-continent of the United

States. Generally, low median pesticide concentrations and detection frequencies are

maintained throughout the winter months and reach peak concentrations during late

spring and early summer. Studies by Goodman (1991) in South Dakota, Libra and

others (1993) in Iowa, and Risch (1994) in Indiana also found frequencies of pesticide

detection in shallow ground water are significantly lower during the winter than those

observed in the spring and summer application periods.

A study by Hippe and others (1994) measured herbicide concentrations in samples

collected from springs near Carlisle, Pa., that flow into Conodoguinet Creek, a

tributary to the Susquehanna River. The study was conducted from May 1990 through

May 1991. The area is underlain by carbonate bedrock and is in the Great Valley

Section of the Ridge and Valley Physiographic Province. They found atrazine

concentrations in the spring discharges varied little throughout the year. This setting is

not unlike the basin characteristics for the Cedar Run Basin (fig. 6) where similar

results were observed. The concentrations measured by Hippe appeared to be

independent of streamflow and indicated the discharge of a well-mixed reservoir of

ground water. Simazine was detected in only 4 of 101 samples collected from the

springs sampled in the Conodoguinet Creek Basin, at a minimum reporting level of

0.1 μg/L.
46 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Stamer and Zelt (1994) report a similar release mechanism of pesticides in samples

collected from a surface-water reservoir in the Lower Kansas River Basin, and

Squillace and Thurman (1992) suggest this type of herbicide transport in the Cedar

River in Iowa. Pesticide movement through the hydrologic system is assumed to be

slow (on the order of months to years). The approximate age of the water discharged

to these streams is 1-10 years, but the application year of the discharged pesticides is

unknown. Because of mixing, the discharged herbicides are probably from multiple

years of application.

Herbicide and insecticide concentration patterns observed in the Lower Susquehanna

River Basin were similar to those observed in streams in the Chesapeake Bay

watershed (U.S. Environmental Protection Agency, 1994). Organonitrogen herbicide

and organochlorine and organophosphorus insecticide concentrations in stream water

that discharged from three major tributaries directly into Chesapeake Bay were

measured at the Fall Line, the contact between the Piedmont and Atlantic Coastal

Plain Physiographic Provinces. Organonitrogen herbicide concentrations, primarily

atrazine, peaked in May or June and dropped to lower levels during the winter months.

Organophosphorus insecticides were rarely detected, and organochlorine insecticides

did not exhibit the seasonal pattern of herbicides.

Factors Affecting Areal Concentration Patterns in Wells and Streams

The relatively constant and year-round release of soluble herbicides to streams

draining areas affected by agricultural land use and the intermittent delivery of

relatively non-soluble insecticides to streams described in the previous section are

probably the combined result of several factors. Areal patterns of pesticide

concentrations and the factors that appear to affect them will be discussed with

respect to the following topics: 1) areal application patterns—in agricultural and

nonagricultural areas, 2) pesticide persistence and solubility and the infiltration

capacity of soils, and 3) ground-water retention and discharge.

Areal Application Patterns

Areal application patterns and detection frequencies of pesticides may vary

significantly depending on the percentage of land in agricultural or nonagricultural use

in the basin. As would be expected, patterns of pesticide use and detection of

pesticides for agricultural purposes reflect patterns of agricultural land use in the study

unit (fig. 1). Use of nonagricultural pesticides is usually limited to turf management,

roadside vegetation clearing, and residential and garden insect control.

Areal application patterns in agricultural areas

Areal application patterns of selected pesticides for agricultural purposes in the study

unit are based on county-level data supplied by Anderson and Gianessi (1995).

Applications of the five selected pesticides in Lancaster, York, Cumberland, Adams,

and Lebanon Counties, all located in the southern part of the study unit, account for a

majority of the annual amount of the pesticides used for agricultural purposes in the

study unit (table 8). Lancaster County, an area of intense agriculture, has the highest

amounts applied for three of the five pesticides. Applications in Adams County

accounted for the highest amounts of the two remaining pesticides. York County

receives the second highest amounts for all five pesticides.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 47

Table 8. Average annual amounts of selected pesticides used for agricultural purposes and rank, by county, in the Lower

Susquehanna River Basin study unit, 1990-94

[Source: Anderson and Gianessi (1995); No data are reported for Cambria, Columbia, and Somerset Counties in

Pennsylvania and Baltimore and Carroll Counties in Maryland because less than 5 percent of their respective areas occupy

the Lower Susquehanna River Basin; No data are reported for Chester County because pesticide use is reported as

negligible in the part of the county within the Lower Susquehanna River Basin; Amount shown for each county has been

adjusted by the percentage of the county in the Lower Susquehanna River Basin; LSR, Lower Susquehanna River Basin;

lb/yr, average annual amount used in pounds; ---, no known use]

County, state

Percentage

of county

in LSR

Atrazine Simazine Pendimethalin Chlorpyrifos Diazinon

Amount

used

(lb/yr)

Rank

Amount

used

(lb/yr)

Rank

Amount

used

(lb/yr)

Rank

Amount

used

(lb/yr)

Rank

Amount

used

(lb/yr)

Rank

Adams, Pa. 52 9,680 14 3,170 1 3,870 12 5,140 4 1,230 1

Bedford, Pa. 72 10,100 13 324 9 3,310 14 4,030 7 99 6

Berks, Pa. 11 6,400 20 110 18 1,970 18 1,200 20 20 15

Blair, Pa. 100 10,300 12 292 10 10,000 3 4,520 5 93 8

Cecil, Md. 34 8,710 17 637 5 136 22 3,150 13 3 18

Centre, Pa. 27 6,490 19 55 20 2,740 15 1,390 19 1 19

Cumberland, Pa. 100 27,900 3 546 6 8,230 4 6,070 3 142 5

Dauphin, Pa. 100 17,700 6 210 14 6,000 7 3,180 12 23 13

Franklin, Pa. 22 9,660 15 495 7 2,670 16 2,450 17 165 3

Fulton, Pa. 34 1,920 22 16 22 788 20 551 22 --- ---

Harford, Md. 37 9,220 16 882 4 155 21 3,330 10 1 20

Huntingdon, Pa. 100 12,600 10 108 19 5,370 8 3,560 9 --- ---

Juniata, Pa. 100 10,800 11 245 11 4,950 9 2,850 16 50 10

Lancaster, Pa. 100 107,000 1 1,120 3 33,600 1 22,900 1 154 4

Lebanon, Pa. 85 23,000 4 227 12 7,210 5 4,090 6 20 14

Mifflin, Pa. 100 13,100 9 162 16 3,500 13 3,060 14 25 12

Northumberland, Pa. 62 19,100 5 217 13 6,430 6 2,950 15 39 11

Perry, Pa. 100 16,400 8 166 15 4,860 11 3,320 11 15 16

Schuylkill, Pa. 41 7,470 18 129 17 2,170 17 1,400 18 64 9

Snyder, Pa. 100 16,700 7 406 8 4,910 10 3,640 8 97 7

Union, Pa. 28 4,200 21 44 21 1,330 19 880 21 5 17

York, Pa. 100 68,400 2 1,120 2 22,300 2 11,000 2 220 2

LSR TOTALS1 416,850 10,681 136,499 94,661 2,466

1 Column totals may not be equal to sum of pesticide use for all counties in LSR Basin shown in table 2 due to rounding.
48 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Atrazine use is closely tied to agriculture. Risser and Siwiec (1996) reported that

agriculture is the dominant land use in Lancaster, York, Cumberland, Dauphin, and

Lebanon Counties—the five southern counties of the Lower Susquehanna River

Basin study unit. Sixty percent of the study unit’s entire agricultural land use is located

in this five-county area that occupies only 35 percent of the study-unit drainage area.

Estimated average annual applications of atrazine for agricultural purposes in these

five counties comprise about 58 percent of the total applied in the study unit. A similar

pattern of use was observed for another pesticide applied mostly for agricultural

purposes—pendimethalin.

Agricultural-use application patterns in the study unit were determined by first ranking

the average annual amount applied in each county for each of the five selected

pesticides. The top 25 percent of the counties, by ranked use, were designated as

high-use areas. The bottom 25 percent of the counties, by ranked use, were

designated as low-use areas. The remainder of the counties were designated as

medium-use areas. Each pesticide was treated separately in determining the county

ranks.

Shallow-well synoptic surveys (169 samples) and stream-subunit synoptic surveys

(94 samples) were conducted, in part, to determine areal patterns of concentrations.

Pesticide concentration patterns were determined in a manner similar to that used for

application patterns. Sample measurements collected during subunit surveys

including nondetected concentrations were ranked separately. For the multiple-sample

long-term monitoring sites, only the single samples collected during the stream

synoptic surveys were used in this analysis.

Using this overall ranking for each pesticide, the concentrations were divided into

three groups. The top 25 percent of the measured concentrations were designated as

high concentrations; the lower 25 percent as low concentrations. The remainder of the

concentrations were designated as medium. If the number of detected concentrations

for a particular pesticide was small compared to the total number of samples, the

measured concentrations were grouped into only two categories—detected and

nondetected concentrations. The designations of concentrations in the low, medium,

and high categories are simply an arbitrary categorization of the measured

concentrations and were determined on the basis of quartile limits. A high designation

should not be considered in relation to any existing or proposed health standard and

does not imply a risk to human or aquatic ecosytem health.

The pattern of atrazine concentrations detected in these surveys conducted from

1993 to 1995 resembles the pattern of atrazine applications for agricultural purposes

(fig. 10). If no relation existed between atrazine application patterns and atrazine

concentrations measured in streams and ground water, the areal pattern of samples

with high concentrations would be expected to match the distribution of all samples

among the atrazine-use areas. The pattern found appears to support a high-use, highconcentration relation. Ten percent of the samples were collected in the low-use

areas, and less than 5 percent of the high concentrations were measured in the lowuse areas. In contrast, about 60 percent of the samples were collected in high-use

areas, and about 75 percent of the concentrations in the high category were

measured in the high-use areas. When compared to pendimethalin, atrazine’s high

solubility in water and low adsorption potential also may explain the frequency and

range of detection for atrazine in surface- and ground-water samples.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 49

Figure 10. Categorized average annual atrazine agricultural applications, 1990-94, and categorized atrazine

concentrations measured in water from streams (subunit synoptic surveys) and ground water (shallow-well synoptic

surveys), 1993-95, in the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland.
50 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

The high number of nondetected pendimethalin concentrations did not allow a

descriptive display of areal concentration patterns or a relation to be established with

application patterns (fig. 11). Only three samples (one in a shallow well and two in

streams) had detectable concentrations. As indicated during the discussion of factors

affecting seasonal patterns of pendimethalin, the low number of detections may be

affected by its relatively low solubility in water and relatively high affinity for adsorption

to soil.

Statistical analysis of the categorized use and concentration data for atrazine and

simazine using contingency tables and the Chi-square statistic indicates the level of

combined well and stream concentrations of atrazine and simazine and the level of

use of each herbicide for agricultural purposes are significantly related at a 95-percent

confidence level (ρ ≤ 0.05). Analyzed separately, well and stream concentrations also

are significantly related to use.

Pesticide-use estimates and stream median base-flow pesticide concentrations are

shown for five selected pesticides in three environmental subunits of the study unit in

figure 12. Pesticide concentrations measured at the long-term stream-monitoring sites

and basins were selected to represent those concentrations typically found in the

individual subunits. Estimates of agricultural pesticide use in three subunits

representing selected environmental settings of the study unit generally were based

on pesticide use in the county in which each of the long-term stream monitoring

basins are located. The exception to this latter rule was Lancaster County, where a

more regionalized approach was used.

Averaged Lancaster and York County pesticide-use data are used to represent the

agricultural areas underlain by carbonate bedrock in the Piedmont Physiographic

Province (PD-CAR) and the Mill Creek Basin. For most agricultural pesticides,

Lancaster County has the greatest use, by far, of an individual pesticide in the study

unit. A better estimate of use for the subunit that includes parts of Lancaster County is

the average pesticide use in the Lancaster and York County region. York County also

has high amounts of agricultural land use, but the intensity of agriculture is less and is

more typical of the two subunits that include parts of Lancaster and York Counties.

Lebanon County pesticide-use data represent agricultural areas underlain by

carbonate bedrock in the Great Valley Section of the Ridge and Valley Physiographic

Province (GV-CAR) and the Bachman Run Basin. Schuylkill County data represent

agricultural areas underlain by sandstone and shale bedrock in the Appalachian

Mountain Section of the Ridge and Valley Physiographic Province (AM-SIL) and the

East Mahantango Creek Basin. Data collected by Pionke and others (1988) indicate

pesticide-use estimates for the East Mahantango Creek Basin and Schuylkill County

may have been underestimated by Anderson and Gianessi (1995). Because all

available pesticide-use data are based on use for agricultural purposes, no estimates

are made for pesticide use in the urban area.

Using the areas within the subunit boundaries (fig. 2) and the data presented in table

8 and supplied by Anderson and Gianessi (1995), agricultural pesticide-use areal

patterns are apparent (fig. 12). When compared to the Great Valley (GV-CAR) and

Appalachian Mountain (AM-SIL) agricultural subunits, the counties within the

agricultural areas underlain by carbonate bedrock in the Piedmont Physiographic

Province (PD-CAR) subunit have the largest masses of atrazine, simazine,

chlorpyrifos, and pendimethalin used on a per-acre-of-agricultural-land basis (fig. 12).

The agricultural areas underlain by sandstone and shale bedrock in the AM-SIL

subunit are where the least amount of pesticides are applied annually for four of the

five pesticides. Generally, the subunits underlain by carbonate bedrock are the areas
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 51

Figure 11. Patterns of average annual pendimethalin agricultural applications, 1990-94, and pendimethalin

concentrations measured in water from streams (subunit synoptic surveys) and ground water (shallow-well

synoptic surveys), 1993-95, in the Lower Susquehanna River Basin study unit.
52 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Figure 12. Estimated annual use, 1990-94, for agricultural purposes and the median base-flow concentrations

of five selected pesticides observed in water from streams in three environmental subunits of the Lower

Susquehanna River Basin study unit where long-term monitoring stream sites were located.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 53

where the most agriculture is located and the most agricultural pesticides are used.

This relation of agricultural chemical use, agricultural land use, and carbonate bedrock

in the Chesapeake Bay watershed has been reported by others (Hainly and Kahn,

1996; Fisher, 1995).

The relation between pesticide use and concentrations measured in the environment

is sometimes used to develop a mass balance for the pesticide. For the selected

pesticides, defining the relation between agricultural pesticide use in a subunit and the

magnitude of pesticide concentrations measured in the hydrologic environment is a

difficult task. Using concentrations of atrazine and simazine measured in streams in

three selected subunits (fig. 12), it appears that a general relation may exist between

stream concentrations and elevated use in agricultural areas underlain by carbonate

bedrock in the Piedmont Physiographic Province and the Great Valley Section of the

Ridge and Valley Physiographic Province. Because of a low detection frequency, the

determination of a relation between annual use and stream concentration was not

possible for pendimethalin, chlorpyrifos, and diazinon. The ability to measure streamwater concentrations of these pesticides may be affected by their relatively low

solubility in water and relatively high affinity for adsorption to soil and may not be

related to annual application amounts for agricultural purposes.

Areal application patterns in nonagricultural areas

Although pesticides are widely and, in some areas of the study unit, heavily used for

agricultural purposes, pesticides also are applied in numerous areas for

nonagricultural purposes. Some common nonagricultural herbicide uses include turf

management on residential and commercial lawns and golf courses, and vegetationclearing along railways, highways, and transmission lines. Insecticides commonly are

used for pest control in homes and commercial buildings and in private and public

gardens. Pesticide amounts used for nonagricultural purposes in areas smaller than

nationwide are poorly documented. Barbash and Resek (1996, p. 115) report that, in

1991, pesticide sales to professional applicators and consumers nationwide were

slightly more than 20 percent of the sales in the agricultural market. Additionally,

Barbash and Resek describe the results of a 1989-90 USEPA study that estimated

73 percent of households, nationwide, used some type of pesticide during 1990.

Little data on nonagricultural pesticide use in the study unit is available. In this report,

potential magnitude of pesticide use for nonagricultural purposes in the study unit will

be indicated by the percentage of residential land use, excluding urban areas, in the

counties within the study unit. The density of residential land use will be used to

estimate relative areal patterns in pesticide use for turf management and insect

control. A relation between the density of residential areas and the occurrence of

insecticides in the environment in a developed area of New York has been

demonstrated by Eckhardt and others (1989).

The county-based summary of residential land use (table 9) indicates the highest

uses of herbicides for turf management on residential lawns may be in the southern

part of the study unit comprising York, Lancaster, Cumberland, Dauphin, and Lebanon

Counties. About 70 percent of the residential land use in the study unit is in this fivecounty area (table 9). In addition, about 80 percent of the public golf courses identified

by the Pennsylvania Atlas and Gazetteer (1990) in the study unit are located in this

five-county area. Turf management and insect control on golf courses are potentially

other uses that may affect pesticide concentrations in water from wells and streams.

Insecticides applied to control pests in homes and commercial buildings also may be

concentrated in the same five-county area. The areas of residential land use were

determined by combining the information available from digital datasets of land use
54 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Table 9. Cropland and suburban residential land use within the Lower Susquehanna

River Basin study unit, by county

[Source for land-use data: Cropland, U.S. Environmental Protection Agency (1990a);

Residential area, Mitchell and others (1977), U.S. Department of Commerce (1991), Hitt

(1994); No data are reported for Cambria, Columbia, and Somerset Counties in Pa. and

Baltimore and Carroll Counties in Md. because less than 5 percent of their respective

areas occupy the Lower Susquehanna River Basin; LSR, Lower Susquehanna River

Basin; mi2, square miles]

County, state

Percentage

of county

in LSR

Cropland area within

county and LSR

Residential area within

county and LSR

(mi2) (percent) (mi2) (percent)

Adams, Pa. 52 140 52 5.2 1.9

Bedford, Pa. 72 164 22 6.2 .9

Berks, Pa. 11 43 45 .8 .9

Blair, Pa. 100 90 17 26.7 5.1

Cecil, Md. 34 32 26 2.5 1.8

Centre, Pa. 27 39 13 1.6 .6

Chester, Pa. 12 29 32 4.2 4.5

Cumberland, Pa. 100 217 40 37.6 6.9

Dauphin, Pa. 100 152 29 47.6 9.0

Franklin, Pa. 22 64 37 1.4 .9

Fulton, Pa. 34 33 22 .3 .2

Harford, Md. 37 40 24 1.4 .7

Huntingdon, Pa. 100 148 17 6.6 .8

Juniata, Pa. 100 102 26 2.1 .5

Lancaster, Pa. 100 552 58 77.2 8.1

Lebanon, Pa. 85 139 45 15.4 4.9

Mifflin, Pa. 100 81 20 9.6 2.3

Northumberland, Pa. 62 129 45 8.8 3.0

Perry, Pa. 100 125 22 5.4 1.0

Schuylkill, Pa. 41 61 19 10.5 3.2

Snyder, Pa. 100 114 35 4.7 1.4

Union, Pa. 28 27 31 1.2 1.4

York, Pa. 100 410 45 59.3 6.5
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 55

(Mitchell and others, 1977) and 1990 population (U.S. Department of Commerce,

1991). The method for combining the datasets was developed by staff of the NAWQA

Program (Hitt, 1994).

Identifying agricultural or nonagricultural uses as the dominant pesticide source in the

hydrologic environment is difficult in the study unit because most of the agriculture

(fig. 1, table 9), agricultural pesticide use (table 8), residential areas (table 9), and

public golf courses are located in a five-county area in the southern part of the study

unit. For instance, simazine is used for agricultural purposes (fig. 13, table 8) but is

used more extensively on fruits and vegetables than on grain crops. Thirty percent of

the total simazine used in the study unit is applied in the five southern counties.

Another 30 percent of the simazine used in the study unit on an annual basis is

applied in Adams County, where many orchards are located. Simazine also is used for

weed control in residential areas along railways, highways, and transmission lines.

The multiple uses for simazine, and other pesticides, make it difficult to determine

their dominant source in the environment.

Areal patterns of simazine concentration in water from streams and shallow wells

(fig. 13) most likely reflect the combined effects of agricultural and nonagricultural

uses. Almost all the detected concentrations in the designated high range were in the

five-county area in the southern part of the Lower Susquehanna River Basin study

unit. Agricultural uses alone do not account for all the higher concentrations detected

because samples from streams and wells in Dauphin, Lebanon, and Blair Counties

also had concentrations in the higher range, but they are considered to be in the

medium-use category for agricultural purposes. Activities associated with the high

amount of residential area in these three counties may account for the high

concentrations. Most of the high concentrations were measured in water samples

collected from streams and shallow wells in locally intensive agricultural areas.

Another pesticide with concentrations that may be affected by nonagricultural use is

diazinon. Common household uses include insect control on fruits, vegetables, and

ornamentals, grub and nematode control in turf soils, and control of cockroaches,

silverfish, and ants. Majewski and Capel (1995) report outdoor applications of

diazinon by homeowners in 1990 were about 30 times greater than the amount used

for agricultural purposes. Crawford (1996) found significantly higher concentrations of

diazinon in streams in urban areas of the White River Basin, Ind., than in agricultural

areas. Very few measurable diazinon detections were found in the Lower

Susquehanna River Basin base-flow samples, and statistical analysis using

contingency tables shows no statistically significant differences in diazinon detections

between the urban basin in the study unit (Cedar Run) and the agricultural basins (Mill

Creek, Bachman Run, and East Mahantango Creek). When the data sets were

expanded to include storm-affected samples, contingency tables showed statistically

significant differences between the number of diazinon detections in samples

collected at Cedar Run and those collected at Bachman Run and East Mahantango

Creek and also between the expanded data set from Cedar Run and the base-flow

samples from Cedar Run. On the basis of this analysis, a significantly higher number

of detections were measured in the urban basin due to storm-affected samples when

compared to samples from two of the agricultural basins or from the urban basin

during base-flow conditions.

The areal simazine and diazinon detection patterns in areas where agricultural

pesticide use is not high indicate pesticide applications for agricultural purposes alone

may not fully explain the areal patterns in pesticide concentrations observed in water
56 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Figure 13. Categorized average annual simazine agricultural applications, 1990-94, counties with high

residential land use, and categorized simazine concentrations measured in water from streams (subunit

synoptic surveys) and ground water (shallow-well synoptic surveys), 1993-95, in the Lower Susquehanna

River Basin study unit.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 57

from streams and shallow wells. The density of residential land use and golf courses

are factors that add to the explanation of areal concentration and detection patterns

for these two pesticides.

Method of Pesticide Application, Pesticide Persistence and Leaching Potential, and

Infiltration Capacity of Soils

The method of pesticide application, pesticide persistence and leaching potential, and

infiltration capacity of soils are three factors that affect the fate of a pesticide after

application. These factors, along with the previously described seasonal application

and climate patterns, play a large part in determining whether an applied pesticide will

be transported to ground water and the fraction of the stream pesticide load that will

be contributed by a subsurface component.

Three application methods commonly used in the study unit are applications to

foliage, applications to the land surface, and incorporation into the soil. Application

methods of the five selected pesticides generally differ by the type of control.

Herbicides (atrazine, simazine, and pendimethalin) are generally applied to the soil

surface but sometimes to weed foliage. Insecticides (chlorpyrifos and diazinon)

generally use a foliar application method but are sometimes incorporated into the soil.

Those applied to the soil surface would be expected to be more readily available for

transport to the subsurface. Another factor that could affect the pesticide transport

through the system is the organic-carbon content of the soils. This factor will not be

discussed, however, because the organic-carbon content of soils does not vary greatly

within the areas studied (Knox and Moody, 1991).

Data collected in this study were not designed to address pesticide persistence.

Information from previous investigations of pesticide persistence in the environment

are presented to describe the relative degree of availability for leaching and infiltration

of atrazine, simazine, pendimethalin, chlorpyrifos, and diazinon. The discussion will be

limited to processes initiated by agricultural-use applications of pesticides. Areal

patterns of infiltration rates, based on the hydrogeologic framework within the study

unit, will be related to detection-frequency differences in streams and shallow wells to

further explain areal patterns in pesticide concentrations.

A pesticide’s persistence (normally associated with “half-life”) in the environment is

determined by several factors and is generally measured by its rates of volatilization,

evaporation, and degradation. Pesticide degradation is a general term used to include

the complex transformation and degradation processes of biotransformation, chemical

hydrolysis, photolysis, and oxidation-reduction reactions. Most simulations of pesticide

fate and transport use the combined losses of evaporation and volatilization and the

losses due to degradation to determine the amount available for leaching into ground

water or surface transport.

The pesticide leaching potential is affected by several factors, including persistence

and solubility. Hippe and Hall (1996) used physical and chemical properties of

pesticides described by Wauchope and others (1992) to simulate the fate and

transport of four of the five selected pesticides in a carbonate-rock terrain in the Great

Valley Section of the Ridge and Valley Physiographic Province in the study unit. The

fate and transport of atrazine, simazine, chlorpyrifos, and diazinon were simulated

over a 2-year period of near-average annual climatic conditions. Of the factors

included in the sensitivity analysis, affinity for adsorption to carbon in the soil and

degradation rate had the greatest effects on fate and transport. Two other chemical

properties, vapor density and water solubility, were also moderately influential. On the

basis of Hippe and Hall’s analysis of the examined pesticides relative to each other,
58 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

atrazine has a large leaching potential, simazine has a medium leaching potential,

and the insecticides, chlorpyrifos and diazinon, have small leaching potentials. The

properties of pendimethalin also are described by Wauchope and others (1992).

Values given for water solubility and sorption coefficients indicate pendimethalin is

more likely to behave in a manner similar to chlorpyrifos and diazinon and will

probably have a small leaching potential.

For the purposes of this discussion, a simplistic model is used that assumes the

amount of pesticide that reaches ground water is proportional to the amount available

for leaching, the physical properties of the pesticide that affect the leaching potential

(i.e. pesticide persistence, solubility in water, and affinity for adsorption to soil), and

the infiltration rate of the soils to which it is applied. After application and the combined

effects of surface processes on the amount of material available for leaching, the

movement of pesticides below the land surface is governed primarily by infiltration and

the flow of water through the saturated zones. During a normal year and even during

slightly less than normal years, precipitation amounts in the study unit are sufficient for

crop production. Because of this, irrigation is not a common practice in the study unit

and the major source of water applied to the land surface is considered to be

precipitation.

Armbruster (1976a) developed an infiltration index for the Susquehanna River Basin

using basin characteristics that included physical, climatic, soil, and geologic features.

Armbruster (1976b) also published a map providing an infiltration-capacity rating for

regions of the Susquehanna River Basin. The infiltration-capacity ratings are based on

analyses completed by the U.S. Soil Conservation Service for the Susquehanna River

Basin Coordinating Committee (1970).

Topography, soils, and infiltration capacities vary throughout the Lower Susquehanna

River Basin study unit. The Appalachian Mountain Section of the Ridge and Valley

Physiographic Province accounts for the northwestern two-thirds of the study unit and

is characterized by long, narrow forested ridges and agricultural valleys that trend

southwest to northeast (fig. 2). The Appalachian Mountain carbonate agricultural

subunit and the Appalachian Mountain sandstone and shale agricultural and forested

subunits were studied in this Section. The topography of the Appalachian Mountain

carbonate subunit is generally flat with wider valleys than those underlain by

sandstone and shale, and the carbonate bedrock exhibits typical karst features such

as sinkholes and internal drainage. The soils in this subunit have excellent infiltration

capacities. The Appalachian Mountain sandstone and shale subunits have valleys that

are narrower and steeper than the carbonate valleys due to the more resistant

sandstone and shale bedrock and ridges. The soils in Appalachian Mountain

sandstone and shale subunits have infiltration capacities that range from good to poor.

The Great Valley Section (fig. 3) borders the eastern edge of the Appalachian

Mountain Section of the Ridge and Valley Physiographic Province. The area is a broad

10-15 mi wide valley and is unique in the province. The Great Valley carbonate

agricultural and urban subunits were studied in this Section. The topography in the

Great Valley subunits is predominantly flat, and the underlying carbonate bedrock also

exhibits typical karst features. Soils generally have excellent infiltration capacities

except in urban areas where parking lots, buildings, and paved roads reduce

infiltration and increase runoff (Lindsey and others, 1997).

The Piedmont Physiographic Province (fig. 2) covers the southeastern third of the

study unit. This area is characterized by low, rolling hills and broad valleys. The

Piedmont carbonate and crystalline agricultural subunits were studied in this Province.

The topography in the Piedmont carbonate subunit is generally flat due to the
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 59

underlying carbonate bedrock, and flow through the weathered rock can move rapidly

through the system. The soil infiltration capacity in this subunit is excellent. The

Piedmont crystalline subunit, which is in the southern part of the Piedmont

Physiographic Province, is characterized by low rolling hills with underlying bedrock

that includes igneous and metamorphic rocks. The soil infiltration capacity in the

Piedmont crystalline subunit is good.

Infiltration-capacity ratings are incorporated into figure 14 along with the general

hydrogeologic setting of the regions defined by Armbruster (1976b). Detection

frequencies of atrazine, simazine, pendimethalin, chlorpyrifos, and diazinon for

samples from shallow wells and from streams during all flow conditions from each of

the hydrogeologic settings were determined. The pesticides were grouped into the

following categories, on the basis of leaching potentials described by Hippe and Hall

(1996) and Wauchope and others (1992):

Small leaching potentialpendimethalin, chlorpyrifos, diazinon,

Medium leaching potentialsimazine,

Large leaching potentialatrazine.

Pesticide detection frequencies in streams are presented to indicate the relative

availability of each pesticide to the hydrologic environment in each hydrogeologic

setting. To simplify the graphical presentation and to show the relative differences due

to leaching potential, pendimethalin was selected to represent the pesticides with

small leaching potentials (fig. 14). Differences or similarities in detection frequencies

between water from streams and shallow wells for each of the pesticides are used to

indicate the effect of leaching potential and soil infiltration capacity on the delivery of

the pesticide to an aquifer.

It appears the probability of an applied pesticide being detected in streams is more

related to leaching potential than infiltration capacity of the soils. For water from

shallow wells, the probability of a pesticide being detected is related to the pesticide’s

leaching potential and the infiltration capacity of the soils in the area where the

pesticides are applied. Hydrogeologic settings underlain by carbonate bedrock

generally have the highest infiltration capacities. Pesticides, such as atrazine, with a

high leaching potential are detected almost as frequently in well water as in streams in

these settings. In carbonate settings, pesticides with a smaller leaching potential, such

as pendimethalin, tend to have lower detection frequencies in shallow well water and

streams than those with larger leaching potentials. Also, in each hydrogeologic

setting, the magnitude of the difference between the detection frequencies of wells

and streams has a tendency to increase as leaching potential gets smaller.

In noncarbonate settings, for a pesticide such as atrazine with a large leaching

potential, the detection frequencies in streams and shallow wells are not similar. The

detection frequency in water from wells is much lower than the detection frequency in

water from streams. This is most likely related to the reduced infiltration capacity of

noncarbonate settings in relation to carbonate settings. Pesticides with smaller

leaching potentials applied in noncarbonate settings tend to have the lowest detection

frequencies in wells and streams and the detection frequency of pesticides in well

water is generally lower than the frequency in stream samples.

Detection frequency in shallow wells decreases along the vertical axis from carbonate

to crystalline to sandstone and shale bedrock type (from excellent to poor infiltration

capacity). Detection frequencies in streams increase along the horizontal axis from

small to large leaching potential. Pesticides with a large leaching potential are
60 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Figure 14. Infiltration-capacity ratings, leaching potential, and frequency of detection for selected pesticides,

1993-95, within various hydrogeologic settings of the Lower Susquehanna River Basin study unit. [Leaching

potential groups: small—pendimethalin, chlorpyrifos, diazinon; medium—simazine; large—atrazine.]
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 61

expected to be detected most frequently—especially in areas underlain by carbonate

bedrock. Conversely, the lowest pesticide detection frequency in streams and shallow

wells would be expected in an area with sandstone and shale bedrock type and a low

soil infiltration capacity where pesticides with a small leaching potential are applied.

Statistical analysis of the categorized infiltration capacity and leaching-potential

ratings in relation to the detection-frequency differences for water from streams and

shallow wells using contingency tables and the Chi-square statistic indicates

differences in well water and stream water detection frequencies are significantly

related at a 99-percent confidence level (ρ ≤ 0.01).

Ground-Water Retention and Discharge

For the purposes of this discussion, ground-water retention is defined as the capacity

of an aquifer to attenuate the delivery of a pesticide once it is delivered through some

mechanism from the surface. In this sense, the aquifer is treated as a reservoir of

dissolved pesticides. The delivery of the pesticides and the magnitude of their

concentration is dependent on the hydrologic properties of the aquifer. The dominant

transport mechanism to streams is considered to be ground-water and spring

discharge. A water sample collected from a stream during base-flow conditions is

considered to have one or both of these two sources of discharge as its dominant

source.

Based on synoptic sampling data collected for this study, differences in the probability

of a particular pesticide being detected in water from a shallow well or from a stream

during base-flow conditions in selected subunits are shown in table 10. The summary

is similar to the data presented in figure 14 except that land use is not used to

subdivide the hydrogeologic settings presented in figure 14 into environmental

subunits, and figure 14 includes data from storm samples.

For atrazine, a soluble herbicide used primarily for agricultural purposes, bedrock type

appears to explain areal patterns in detection frequency. Atrazine is generally

detected at a higher frequency in shallow ground water in carbonate bedrock settings

that in noncarbonate settings. For water from streams during base-flow conditions,

bedrock type alone does not explain areal patterns in atrazine detection frequency

among the subunits. The addition of land use as a factor, and more specifically the

dominance of nonforested land use in a subunit, better describes areal patterns of

detection frequencies in water collected from streams. The detection frequency of

atrazine in streams during base-flow conditions and shallow wells is most similar in

carbonate bedrock-agricultural subunits. Subunits with either sandstone and shale

bedrock or those dominated by forested land use exhibited the most difference in

detection frequencies between water from streams during base-flow conditions and

shallow wells. The similarity of detection frequencies in wells and streams in

carbonate areas, especially in agricultural areas, indicates a fairly well-connected

hydrologic system. Atrazine detection frequency of water discharged to streams from

ground water during base-flow conditions is a reasonable representation of detection

frequencies in ground water in carbonate settings.
62 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Table 10. Summary of detection frequency in water from streams (base-flow synoptic surveys) and ground water

(shallow-well synoptic surveys) for selected pesticides for various environmental subunits within the Lower

Susquehanna River Basin study unit, 1993-95

[Well water, analyses of water from shallow wells during synoptic surveys; stream water, analyses of water from

streams during synoptic base-flow conditions]

Environmental subunit1

Pesticide detection frequency (percent)

Atrazine Simazine Pendimethalin Chlorpyrifos Diazinon

Well

water

Stream

water

Well

water

Stream

water

Well

water

Stream

water

Well

water

Stream

water

Well

water

Stream

water

Piedmont crystalline agriculture

(22 well-water samples, 33 streamwater samples)

82 100 9 97 0 0 0 0 0 3

Piedmont carbonate agriculture

(30 well-water samples, 31 streamwater samples)

93 100 50 97 3 0 0 3 0 13

Great Valley carbonate agricultural

(30 well-water samples, 16 streamwater samples)

100 100 97 94 0 6 0 25 3 0

Great Valley carbonate urban

(20 well-water samples, 24 streamwater samples)

90 100 80 96 0 0 0 12 5 8

Appalachian Mountain carbonate

agricultural

(30 well-water samples, 26 streamwater samples)

100 100 67 92 0 4 0 4 0 0

Appalachian Mountain sandstone

and shale agricultural

(22 well-water samples, 2 streamwater samples)

4 100 0 50 0 0 0 0 0 0

Appalachian Mountain sandstone

and shale forested

(7 well-water samples, 16 streamwater samples)

0 62 0 25 0 0 0 0 0 0

1 See table 1 for a more detailed description of environmental subunits.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 63

Simazine, a herbicide with a lower leaching potential than atrazine, exhibited areal

detection frequency patterns similar to atrazine although the magnitude of differences

between water from streams and shallow wells is generally greater. Detection

frequencies slightly lower than those determined for atrazine were measured in

stream waters and markedly lower frequencies were measured in shallow well water

in all the subunits except the agricultural areas underlain by carbonate bedrock in the

Great Valley Section of the Ridge and Valley Physiographic Province and the

agricultural areas underlain by sandstone and shale bedrock in the Appalachian

Mountain Section of the Ridge and Valley Physiographic Province. A slightly higher

detection frequency was measured in well water than in streams in the agricultural

areas underlain by carbonate bedrock in the Great Valley Section of the Ridge and

Valley Physiographic Province. On the basis of previous analyses in this report, this

result is not expected and may be due to the small sample size for streams. In the

agricultural areas underlain by sandstone and shale bedrock in the Appalachian

Mountain Section of the Ridge and Valley Physiographic Province, no measurable

simazine concentrations were detected in water from wells.

The number of detections of the three remaining pesticides—pendimethalin,

chlorpyrifos, and diazinon—were too few in waters from streams during base-flow

conditions and shallow wells to support any specific conclusions. The data in table 10

tend to indicate a generalized pattern of higher detection frequencies in samples from

base-flow streamwater than in samples from shallow wells in carbonate areas.

The general conclusion of this analysis is that the degree of presence or absence of a

pesticide in water from streams during base-flow conditions and wells can be

determined fairly well and indirectly by sampling stream water during base-flow

conditions or well water in subunits where carbonate bedrock or agricultural land use

dominates. This conclusion does not apply to any subunit underlain by sandstone and

shale bedrock. The uncertainty of the prediction of presence or absence of a pesticide

ranges from high for a soluble pesticide in a carbonate bedrock-agricultural land use

subunit to low for a relatively insoluble pesticide in a sandstone and shale bedrockforested land use subunit.

Combining the data and conclusions provided by figure 14 and table 10 suggests an

areal pattern in the study unit determined primarily by bedrock type and secondarily

by land use. Land use appears to be a better indicator of initial availability and

detection frequency in streams than as an indicator of detection frequency differences

between shallow well water and streams. Pesticides applied in carbonate bedrock

areas that have a high leaching potential are easily transported through the soils to

the aquifer, and the occurrence of pesticides in water discharged to streams is

proportional to their occurrence in shallow well water.

Lower proportions of the applied pesticides reach ground water and are discharged to

streams in a crystalline bedrock system. A moderate amount of degradation is

indicated along transport paths from the surface to the aquifer and from the aquifer to

the stream. Of the three major bedrock types in the study unit, sandstone and shale

bedrock permit the slowest flow through the system and, therefore, provide the

greatest chance for degradation due to time of travel. In this case, very little of the

available pesticide reaches the deepest ground-water aquifers and attenuated

amounts are seen in stream base flow originating from shallow ground-water zones.

In addition to determining the relative occurrence of pesticides in wells and streams

during base-flow conditions in selected subunits, the relatively high frequency of

detections for atrazine and simazine provides a concentration data set with sufficient

sample size to allow inferences about the relation between concentrations in shallow
64 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

wells and the water discharged to streams from ground water in selected subunits.

The limited number of detections in wells and streams in the Appalachian Mountainsandstone and shale bedrock-forested subunit does not allow this more detailed

analysis. It should be noted, though, that measured concentration levels in streams

and shallow wells in the subunit are equivalent (fig. 15).

Relative differences in median atrazine concentrations determined from stream and

ground-water samples did not exhibit any areal patterns. In addition, when the relative

differences were compared among subunits characterized by bedrock type or land

use, no patterns were observed. Two of the four subunits underlain by carbonate

bedrock showed higher median concentrations in streams than in wells and two

showed lower medians. Two of four subunits where agricultural land use dominates

showed higher median concentrations in streams and two showed lower medians than

those determined from wells (fig. 15). The magnitude of the differences between

median concentrations in wells and streams was generally greatest in nonagricultural

or noncarbonate subunits. The similarity of median concentrations in carbonatebedrock subunits indicates the connectivity of water in streams and shallow wells in

these subunits. The range of measured concentrations between the tenth and

ninetieth percentiles was also generally greatest in agricultural-carbonate subunits.

This implies greater variability in the sources and source concentrations available to

aquifers and streams in these subunits.

In the five subunits with sufficient data to analyze, a general areal pattern of higher

median concentrations in streams during base-flow conditions than in shallow well

water was observed for simazine, regardless of bedrock type or the dominance of

agricultural land use in the subunit (fig. 15). This reflects the earlier general

observation that less of the available simazine is transported to ground water and also

indicates that simazine concentrations in streams are enhanced by some other

source. Possible sources could be surficial ones located upstream or others located

along the pathway from the aquifer to the stream. The data collected during this study

do not allow the identification of these additional simazine sources.

A statistical analysis of the median atrazine concentrations determined for wells and

streams in each of the subunits was performed using the Kruskal-Wallis test (Helsel

and Hirsch, 1992, p. 159). The test indicated statistically significant differences at a

confidence level of 95 percent (ρ ≤ 0.05) existed between the median atrazine

concentrations in shallow well water and base-flow stream water in the urban areas

underlain by carbonate bedrock in the Great Valley Section of the Ridge and Valley

Physiographic Province, in agricultural areas underlain by crystalline bedrock in the

Piedmont Physiographic Province, and in agricultural areas underlain by carbonate

bedrock in the Piedmont Physiographic Province. Streams in all three subunits had

higher median concentrations during base-flow conditions than shallow wells.

The median atrazine concentrations of streams and shallow wells in two agricultural

areas underlain by carbonate bedrock in the Appalachian Mountain and Great Valley

Sections located in the Ridge and Valley Physiographic Provinces were not

significantly different. No significant difference in the medians infers that the system

delivering the pesticides to streams has relatively little effect on the magnitude of the

concentration in the stream and that the water in the aquifer is chemically similar, as

far as atrazine is concerned, to the water in the stream.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 65

AM-SIL-F

7

16

20

24

22

33

30

26

30

16

30

31

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

ATRAZINE CONCENTRATION, IN MICROGRAMS PER LITER

Figure 15. Atrazine and simazine concentrations in wells and streams during base-flow synoptics, 1993-95, in selected

environmental subunits, Lower Susquehanna River Basin.

EXPLANATION

90TH

75TH

25TH

10TH

MEDIAN (50TH)

22 NUMBER OF SAMPLES

SUBUNIT

= Piedmont crystalline agricultural

= Piedmont carbonate agricultural

= Great Valley carbonate agricultural

= Great Valley carbonate urban

= Appalachian Mountain carbonate

SIMAZINE CONCENTRATION, IN MICROGRAMS PER LITER

AM-SIL-F

GV-CAR-U

GV-CAR-U

PD-CRY-A

PD-CRY-A

AM-CAR-A

AM-CAR-A

GV-CAR-A

GV-CAR-A

PD-CAR-A

PD-CAR-A

ENVIRONMENTAL SUBUNITS AND WATER SOURCE

GW

SW

Water from wells

Water from streams

WATER SOURCE

7 16

20

24

22

33

30

26

30

16

30

31

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

agricultural

= Appalachian Mountain sandstone

and shale forested

PD-CRY-A

PD-CAR-A

GV-CAR-A

GV-CAR-U

AM-CAR-A

AM-SIL-F

PERCENTILE- Percentage of analyses equal to

or less than indicated values.

METHOD DETECTION LIMIT (MDL)
66 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

SUMMARY AND CONCLUSIONS

Pesticides in water from wells and streams are an important water-quality issue in the

Lower Susquehanna River Basin study unit of the National Water-Quality Assessment

Program. Intensive agriculture in parts of the study unit and relatively high amounts of

pesticides applied in agricultural areas introduce thousands of pounds of pesticides to

the environment each year. Nonagricultural uses of pesticides for turf management

and insect control are also a source of these compounds. As a part of the National

Water-Quality Assessment Program, concentrations of 53 herbicides, 29 insecticides,

and 1 fungicide were measured in water samples from streams and shallow wells

located in the Lower Susquehanna River Basin in Pennsylvania and Maryland.

Average annual pesticide application amounts for agricultural purposes in the study

unit were determined from crop-specific pesticide application rates and average

annual county-based crop acreages. The five pesticides with the highest amounts

used for agricultural purposes from 1990 to 1994 were metolachlor, atrazine, captan,

pendimethalin, and alachlor.

From 1993 through 1995, a total of 577 samples were collected from 169 shallow

wells and 155 stream sites. Samples were collected on a fixed interval over a 1- to

3-year period at four of the stream sites. At two of these four stream sites, samples

also were collected over the hydrograph of selected storms.

Conclusions from Analysis of Concentration Data

• Nearly 40,000 concentrations of 83 different pesticide compounds were determined

• Thirty-eight of the 83 compounds were not detected in any of the 577 samples

• About 60 percent of all herbicides analyzed were detected at least once

• Approximately 45 percent of all insecticides analyzed were detected at least once

• The most commonly detected pesticides in the study unitatrazine, metolachlor,

simazine, prometon, alachlor, and cyanazineare agricultural herbicides primarily

used on corn and grain crops

• Atrazine was detected in 98 percent of the stream samples and 75 percent of the

well samples

• Approximately 49 percent of all herbicides and 13 percent of all insecticides

analyzed in the study unit were detected in well samples

• Approximately 42 percent of all herbicides and 37 percent of all insecticides

analyzed were detected in stream samples

• Of the nearly 40,000 pesticide concentrations measured in water from streams and

shallow wells, only 7 measurements exceeded any established maximum

contaminant level for drinking water (atrazine, simazine, and alachlor);

55 measurements exceeded 1 μg/L, and 24 measurements exceeded 2 μg/L

Conclusions from Analysis of Seasonal Patterns

• Seasonal patterns of atrazine, simazine, pendimethalin, chlorpyrifos, and diazinon

concentrations were observed in water from streams during base-flow conditions
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 67

• Elevated concentrations in water from streams during base-flow and storm-affected

conditions were related to seasonal applications of pesticides for agricultural

purposes

• The availability of pesticides for runoff during the application season in a series of

storms was related to how recently a previous storm had occurred and not

necessarily to the peak discharge of the storm

• Ground-water retention and discharge of pesticides was indicated by a year-round

discharge of measurable concentrations of soluble herbicides

• In some cases, relatively constant and year-round concentrations of pesticides and

attenuation of the seasonal pulse of elevated concentrations were observed in

stream base flow

Conclusions from Analysis of Areal Patterns

• Agricultural-use patterns described many of the areal concentration patterns for the

high-use compounds

• Indicators of nonagricultural use such as the location of suburban residential areas

were needed to explain concentration or detection frequency patterns for some

pesticides such as diazinon and simazine

• The density of residential land use and golf courses may be factors that add to the

explanation of areal concentration and detection patterns for simazine and diazinon

• Differences in median concentrations among environmental subunits were related to

three coincident factors: agricultural land use, basins underlain by carbonate

bedrock, and agricultural pesticide-use patterns

• Ratings of soil infiltration capacity of various hydrologic settings and the persistence

of a compound explained some differences in detection-frequency areal patterns in

water from streams and wells

• Relatively soluble herbicides, such as atrazine, applied in areas covered by soils

formed by the weathering of carbonate bedrock were detected at similar frequencies

in water from streams and shallow wells

• Water samples from shallow wells in areas underlain by sandstone and shale

provided relatively few detections for compounds with a large leaching potential and

no detections for compounds with a smaller leaching potential

• Areal patterns of ground-water retention time and the discharge of an attenuated

concentration of a pesticide to streams from ground water were best explained by

bedrock type and land use

• Water samples from streams in three of the five environmental subunits comprising

the study area had significantly higher atrazine concentrations during base-flow

conditions than those concentrations measured in water samples from shallow wells

• Two of the three environmental subunits underlain by carbonate bedrock where

agricultural land use dominates had no statistically significant difference in median

atrazine concentrations measured in water from shallow wells and streams during

base-flow conditions
68 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

The data collected in or available to this study did not allow an extensive interpretation

of the effect of several factors that influence pesticide concentrations in water from

wells and streams. Estimates of agricultural pesticide use are currently based on a

summary of application rates and cropping practices developed on a county basis.

Agricultural-use estimates based on more-refined stream basin boundaries would

enhance the analyses presented in this report. In addition, estimates of pesticide use

for nonagricultural purposes would be a valuable complement to the agricultural-use

estimates. Additional data collection to further describe concurrent seasonal

variations in streams and, especially in shallow wells, would provide invaluable

assistance to developing a conceptual model for pesticide transport in streams and

aquifers. Flow-path studies and additional samples collected from streams during a

storm would aid in the verification of the conceptual model. Finally, the accumulation

of data and application of a pesticide fate and transport simulation in other

environmental settings within the Lower Susquehanna River Basin would provide a

valuable tool for water-resource managers.
REFERENCES CITED

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 69

Anderson, J.E., and Gianessi, L.P., 1995,

Agricultural pesticide use in the Lower

Susquehanna River Basin, A pilot

study of major hydrologic basins of the

nation: The National Center for Food

and Agricultural Policy, 26 p.,

2 appendixes.

Armbruster, J.T., 1976a, An infiltration

index useful in estimating low-flow

characteristics of drainage basins:

U.S. Geological Survey Journal of

Research, v. 4, no. 5, p 533-538.

_____1976b, Technical manual for

estimating low-flow frequency

characteristics of streams in the

Susquehanna River Basin:

U.S. Geological Survey WaterResources Investigations Report 76-

51, 51 p.

Barbash, J.E., and Resek, E.A., 1996,

Pesticides in ground water—

distribution, trends, and governing

factors, in Gilliom, R.J., ed., Pesticides

in the hydrologic system: Chelsea

Mich., Ann Arbor Press, Inc., v. 2,

588 p.

Becher, A.E., 1996, Ground-water

resources of Cambrian and Ordovician

carbonate rocks in the Valley and

Ridge Physiographic Province of

Pennsylvania: U.S. Geological

Survey Open-File Report 90-109,

134 p.

Berg, T.M., Barnes, J.H., Sevon, W.D.,

Skema, V.W., Wilshusen, J.P., and

Yannacci, D.W., 1989, Physiographic

provinces of Pennsylvania:

Pennsylvania Geological Survey, 4th

ser., Map 13 (color), scale

1:2,000,000, 8.5 x 11 inches.

Berg, T.M., Edmunds, W.E., Geyer, A.R.,

Glover, A.D., Hoskins, D.M.,

Maclachlan, D.B., Root, S.I., Sevon,

W.D., and Socolow, A.A., comps.,

1980, Geologic map of Pennsylvania:

Pennsylvania Geological Survey, 4th

ser., scale 1:250,000, 2 sheets.

Breen, K.J., Gavin, A.J., and Schnabel,

R.R., 1995, Occurrence and yields of

triazine herbicides in the Susquehanna River and tributaries during

base-flow conditions in the Lower

Susquehanna River Basin, Pennsylvania and Maryland, June 1993, in Hill,

Paula, and Nelson, Steve, eds.,

Toward a Sustainable Watershed —

The Chesapeake Experiment,

Proceedings of the 1994 Chesapeake

Research Conference, Norfolk, Va.,

June 1-3, 1994: Edgewater, Md.,

Chesapeake Research Consortium

Publication No. 149, p. 312-328.

Breen, K.J., Hainly, R.A., and Hoffman,

S.A., 1991, National Water-Quality

Assessment Program—The Lower

Susquehanna River Basin:

U.S. Geological Survey Open-File

Report 91-168, 2 p.

Capel, P.D., Nacionales, F.C., and

Larson, S.J., 1995, Precision of a

splitting device for water samples:

U.S. Geological Survey Open-File

Report 95-293, 6 p.

Crawford, C.G., 1995, Occurrence of

pesticides in the White River, Indiana,

1991-95: U.S. Geological Survey Fact

Sheet 233-95, 4 p.

_____1996, Influence of natural and

human factors on pesticide

concentrations in surface waters of the

White River Basin, Indiana: U.S.

Geological Survey Fact Sheet 119-96,

4 p.

Durlin, R.R., and Schaffstall, W.P., 1994,

Water resources data, Pennsylvania,

water year 1993, vol. 2, Susquehanna

and Potomac River Basins: U.S.

Geological Survey Water-Data Report

PA-93-2, 361 p.

_____1996, Water resources data,

Pennsylvania, water year 1994, vol. 2,

Susquehanna and Potomac River

Basins: U.S. Geological Survey

Water-Data Report PA-94-2, 418 p.
REFERENCES CITED—CONTINUED

70 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

_____1997, Water resources data,

Pennsylvania, water year 1995, vol. 2,

Susquehanna and Potomac River

Basins: U.S. Geological Survey

Water-Data Report PA-95-2, 518 p.

Eckhardt, D.A., Flipse, W.J., and

Oaksford, E.T., 1989, Relation

between land use and ground-water

quality in the upper glacial aquifer in

Nassau and Suffolk Counties, Long

Island, New York: U.S. Geological

Survey Water-Resources Investigations Report 86-4142, 35 p.

Fishel, D.K., 1984, Water-quality and

chemical loads of the Susquehanna

River at Harrisburg, Pennsylvania,

April 1980 to March 1981: U.S.

Geological Survey Water-Resources

Investigations Report 83-4164, 90 p.

Fishel, D.K., and Lietman, P.L., 1986,

Occurrence of nitrate and herbicides in

ground water in the Upper Conestoga

River Basin, Pennsylvania: U.S.

Geological Survey Water-Resources

Investigations Report 84-4202, 8 p.

Fisher, G.T., 1995, Selected herbicides

in major streams in the Potomac River

Basin upstream from Washington,

D.C.: U.S. Geological Survey Fact

Sheet 107-95, 4 p.

Foster, G.D., and Lippa, K.A., 1996,

Fluvial loadings of selected

organonitrogen and organophosphorus pesticides to Chesapeake

Bay: Journal of Agricultural Food

Chemistry, v. 44, no. 8, p. 2447-2454.

Gianessi, L.P., and Puffer, C.A., 1991,

Herbicide use in the United States:

Washington, D.C., Resources for the

Future, December 1990 (revised April

1991), 128 p.

_____1992a, Insecticide use in

U.S. crop production: Washington,

D.C., Resources for the Future

[variously paged].

_____1992b, Fungicide use in U.S. crop

production: Washington, D.C.,

Resources for the Future [variously

paged].

Gilliom, R.J., Alley, W.M., and Gurtz,

M.E., 1995, Design of the National

Water-Quality Assessment Program—

Occurrence and distribution of waterquality conditions: U.S. Geological

Survey Circular 1112, 33 p.

Goodman, J., ed., 1991, Ten-year

report—Oakwood Lakes-Poinsett rural

clean water program 1981-1991:

Pierre, S. Dak., Oakwood LakesPoinsett Rural Clean Water Program,

variously paged.

Hainly, R.A., and Kahn, J.M., 1996,

Factors affecting herbicide yields in the

Chesapeake Bay Watershed, June

1994: Water Resources Bulletin,

v. 32, no. 5, p. 965-984.

Harrison, S.A., Bogus, E.R., Baumer,

G.M., Mumma, R.O., Petersen, G.W.,

and Hock, W.K., 1995, Pesticide

concentrations in rural wells in cornproducing regions of Pennsylvania:

University Park, Pa., Pennsylvania

State University publication, 21 p.

Helsel, D.R., and Hirsch, R.M., 1992,

Statistical methods in water

resources: New York, Elsevier

Science Publishing Company, Inc.,

522 p.

Hippe, D.J., and Hall, D.W., 1996,

Hydrogeologic setting and simulation

of pesticide fate and transport in the

unsaturated zone of a regolithmantled, carbonate-rock terrain near

Newville, Pennsylvania:

U.S. Geological Survey WaterResources Investigations Report 96-

4062, 56 p.

Hippe, D.J., Witt, E.C., III, and

Giovannitti, R.M., 1994, Hydrogeology,

herbicides, and nutrients in ground

water and springs, and relation of

water quality to land use and

agricultural practices near Carlisle,

Pennsylvania: U.S. Geological

Survey Water-Resources

Investigations Report 93-4172, 66 p.
REFERENCES CITED—CONTINUED

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 71

Hitt, K.J., 1994, Refining 1970’s land-use

data with 1990 population data to

indicate new residential development:

U.S. Geological Survey WaterResources Investigations Report 94-

4250, 15 p.

Holden, L.R., Graham, J.A., Whitmore,

R.W., Alexander, W.J., Pratt, R.W.,

Liddle, S.K., and Piper, L.L., 1992,

Results of the national alachlor well

water survey: Environmental Science

and Technology, v. 26, p. 935-943.

Horowitz, A.J., Demas, C.R., Fitzgerald,

K.K., Miller, T.L., and Rickert, D.A.,

1994, U.S. Geological Survey protocol

for the collection and processing of

surface-water samples for the

subsequent determination of inorganic

constituents in filtered water:

U.S. Geological Survey Open-File

Report 94-539, 59 p.

James, R.W., Simmons, R.H., and

Helinsky, B.M., 1995, Water resources

data, Maryland and Delaware, water

year 1995, vol. 1, Surface-water data:

U.S. Geological Survey Water-Data

Report MD-DE-95-1, 330 p.

Kammerer, J.C., 1987, Largest rivers in

the United States: U.S. Geological

Survey Open-File Report 87-242, 2 p.

Klein, A.J., Graham, J.A., Holden, L.R.,

Whitmore, R.W., and Alexander, W.J.,

1993, Results of the national alachlor

well water survey: Eos Trans. AGU,

74(16), Spring Meet. Suppl., S129,

1993.

Knox, E.G., and Moody, D.W., 1991,

Influence of hydrology, soil properties,

and agricultural land use on nitrogen in

ground water, in Follet, R.F., Keeney,

D.R., and Cruse, R.M., eds., Managing

nitrogen for ground water quality and

farm profitability: Madison, Wis., Soil

Science Society of America, p. 35.

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

W.W., 1995, Ground-water data

collection protocols and procedures for

the National Water-Quality

Assessment Program—Collection and

documentation of water-quality

samples and related data: U.S.

Geological Survey Open-File Report

95-399, 113 p.

Lang, D.J., 1982, Water quality of the

three major tributaries to the

Chesapeake Bay, the Susquehanna,

Potomac, and James Rivers, January

1979-April 1981: U.S. Geological

Survey Water-Resources

Investigations 82-32, 64 p.

Langland, M.J., Lietman, P.L., and

Hoffman, S.A., 1995, Synthesis of

nutrient and sediment data for

watersheds within the Chesapeake

Bay Drainage Basin: U.S. Geological

Survey Water-Resources

Investigations Report 95-4233, 121 p.

Libra, R.D., Hallberg, G.R., Rex, K.D.,

Kross, B.C., Seigley, L.S., Culp, M.A.,

Johnson, J.K., Nicholson, H.F.,

Berberich, S.L., and Cherryholmes,

K.L., 1993, The Iowa state-wide rural

water-well survey—June 1991, repeat

sampling of the 10% subset: Iowa

Department of Natural Resources

Technical Information Series 26, 30 p.

Lietman, P.L., and Hall, D.W., 1991,

Herbicides in surface and ground

water of agricultural, carbonate

valleys, Lancaster County,

Pennsylvania: Proceedings of the

Third National Research Conference

on Pesticides, November 8-9, 1990,

Virginia Polytechnic Institute and State

University, 56 p.

Lietman, P.L., Ward, J.R., and Behrendt,

T.E., 1983, Effects of specific land

uses on nonpoint sources of

suspended sediment, nutrients, and

herbicides—Pequea Creek Basin,

Pennsylvania 1979-80:

U.S. Geological Survey WaterResources Investigations Report 83-

4113, 88 p.
REFERENCES CITED—CONTINUED

72 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Lindley, C.E., Stewart, J.T., and

Sandstrom, M.W., 1996, Determination of low concentration of acetochlor

in water by automated solid-phase

extraction and gas chromatography

with mass-selective detection:

Journal of AOAC International, v. 79,

no. 4, p. 962-966.

Lindsey, B.E., Loper, C.A., and Hainly,

R.A., 1997, Nitrate in ground water

and stream base flow in the Lower

Susquehanna River Basin, Pennsylvania and Maryland: U.S. Geological

Survey Open-File Report 97-4146,

66 p.

Majewski, M.S., and Capel, P.D., 1995,

Pesticides in the atmosphere—

distribution, trends, and governing

factors, in Gilliom, Robert J., ed.,

Pesticides in the hydrologic

system: Chelsea, Mich., Ann Arbor

Press, Inc., v. 1, 214 p.

Manning, T.K., Smith, K.E., Wood, C.D.,

and Williams, J.B., 1994, Pesticidesampling equipment, samplecollection and processing procedures,

and water-quality data at Chicod

Creek, North Carolina, 1992:

U.S. Geological Survey Open-File

Report 94-50, 35 p.

Martin, J.D., Gilliom, R.J., and Schertz,

T.L. 1999, Summary and evaluation of

pesticides in field blanks collected for

the National Water-Quality

Assessment Program, 1992-95: U.S.

Geological Survey Open-File Report

98-412, 102 p.

Mitchell, W.B., Guptill, S.C., Anderson,

K.E., Fegeas, R.G., and Hallam, C.A.,

1977, GIRAS—a geographic

information retrieval and analysis

system for handling land use and land

cover data: U.S. Geological Survey

Professional Paper 1059, 16 p.

National Oceanic and Atmospheric

Administration, 1995, Climatological

data, 1994 annual summary,

Pennsylvania: v. 99, no. 13, p. 23.

Pennsylvania Atlas and Gazetteer (3d

ed.), 1990: Freeport, Maine,

DeLorme Mapping Company, 96 p.

Pennsylvania Department of Agriculture,

1996, Statistical summary and annual

report: Pennsylvania Agricultural

Statistics Service, Publication No.

PASS-117, 123 p.

Petersen, G.W., Baumer, G.M.,

Downing, R.H., and Twark, R.D., 1992,

Prediction of Pennsylvania’s land use

for the years 2000 and 2020: The

Pennsylvania State University

Environmental Resources Research

Institute Report ER9204, 34 p.

Pionke, H.B., and Glotfelty, D.E., 1989,

Nature and extent of groundwater

contamination by pesticides in an

agricultural watershed: Water

Research, v. 23, no. 8, p. 1,031-1,037.

Pionke, H.B., Glotfelty, D.E., Lucas, A.D.,

and Urban, J.B., 1988, Pesticide

contamination of groundwaters in the

Mahantango Creek watershed:

Journal of Environmental Quality,

v. 17, no. 1, p. 76-84.

Pirkey, K.D., and Horodyski, S.S., 1994,

Water quality control charts for the

National Water-Quality Assessment

Program: U.S. Geological Survey

National Water Quality Laboratory

Report.

Risch, M.R., 1994, A summary of

pesticides in ground-water data

collected by government agencies in

Indiana, December 1985 to April

1991: U.S. Geological Survey OpenFile Report 93-133, 30 p.

Risser, D.W., and Siwiec, S.F., 1996,

Environmental setting of the Lower

Susquehanna River Basin: U.S.

Geological Survey Water-Resources

Investigations Report 94-4245, 70 p.,

23 supp. maps, 3 plates.
REFERENCES CITED—CONTINUED

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 73

Sandstrom, M.W., Wydoski, D.S.,

Schroeder, M.P., Zamboni, J.L., and

Foreman, W.T., 1992, Methods of

analysis by the U.S. Geological Survey

National Water Quality Laboratory—

Determination of organonitrogen

herbicides in water by solid-phase

extraction and capillary-column gas

chromatography/mass spectrometry

with selected-ion monitoring:

U.S. Geological Survey Open-File

Report 91-519.

Sandstrom, M.W., 1994, Description and

use by districts of Laboratory QC

sample information in organic

determinations: National Water

Quality Laboratory Technical

Memorandum 94-07, 7 p.

SAS Institute Inc., 1990, SAS

procedures guide, version 6 (3rd ed.):

Cary, N.C., SAS Institute Inc., 705 p.

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

collecting and processing streamwater samples for the National WaterQuality Assessment Program: U. S.

Geological Survey Open-File Report

94-455, 42 p.

Siwiec, S.F., Hainly, R.A., Lindsey, B.D.,

Bilger, M.D., and Brightbill, R.A., 1997,

Water-quality assessment of the

Lower Susquehanna River Basin,

Pennsylvania and Maryland—Design

and implementation of water-quality

studies, 1992-95: U.S. Geological

Survey Water-Resources Investigations Report 97-583, 121 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.

Squillace, P.J., and Thurman, E.M.,

1992, Herbicide transport in rivers—

Importance of hydrology and

geochemistry in non-point source

contamination: Environmental

Science and Technology, v. 26, p. 538-

545.

Stamer, J.K., Goolsby, D.A., and

Thurman, E.M, 1998, Herbicides in

rainfall across the Midwestern and

Northeastern United States, 1990-91:

U.S. Geological Survey Fact Sheet

181-97, 4 p.

Stamer, J.K., and Zelt, R.B., 1994,

Organonitrogen herbicides in the lower

Kansas River basin: Journal

American Water Works Association,

v. 86, no. 1, p. 93-104.

Susquehanna River Basin Study

Coordinating Committee, 1970,

Susquehanna River Basin study,

Appendix D, Hydrology:

Susquehanna River Basin Study

Coordinating Committee, 391 p.

Taylor, L.E., and Werkheiser, W.H.,

1984, Ground-water resources of the

Lower Susquehanna River Basin,

Pennsylvania: Pennsylvania

Geological Survey, 4th ser., Water

Resources Report 57, 130 p.

Timme, P.J., 1995, National water quality

laboratory 1995 services catalog:

U.S. Geological Survey Open-File

Report 95-352, 120 p.

Truhlar, J.F., and Reed, L.A., 1975,

Occurrence of pesticide residues in

four streams draining different landuse areas in Pennsylvania:

U.S. Geological Survey WaterResources Investigations 6-75, 23 p.

U.S. Department of Agriculture,

Agricultural Stabilization and

Conservation Service, 1992,

Conestoga Headwaters Project,

Pennsylvania, Rural Clean Water

Program, 10-Year Report, 1981-1991:

329 p.

U.S. Department of Commerce, 1991,

1990 municipal population—

Pennsylvania: U.S. Department of

Commerce, Bureau of the Census,

Pennsylvania State Data Center

Report 90-01-91, 68 p.
REFERENCES CITED—CONTINUED

74 NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

U.S. Environmental Protection Agency,

1990a, Land use for the Chesapeake

Bay watershed model: U.S. Environmental Protection Agency Publication

No. CBP/TRS 39/90, 49 p.

_____1990b, National survey of

pesticides in drinking water wells,

Phase I report: Office of Pesticides

and Toxic Substances, EPA

570/9.90.015, 98 p.

_____1994, Chesapeake Bay Fall Line

Toxics Monitoring Program—Final

Report: 154 p.

_____1996, Drinking water regulations

and health advisories: Washington,

D.C., EPA 822-B-96-002, 18 p.

U.S. Geological Survey, 1991, National

water summary, 1988-89—Floods and

droughts: U.S. Geological Survey

Water-Supply Paper 2375, 591 p.

Viessman, W., Jr., Knapp, J.W., Lewis,

G.L., Harbaugh, T.E., 1977,

Introduction to hydrology (2d ed.):

New York, Harper and Row, 704 p.

Ward, J.R., 1987, Surface-water quality

in Pequea Creek basin, Pennsylvania,

1977-79: U.S. Geological Survey

Water-Resources Investigations

Report 85-4250, 66 p.

Ward, J.R., and Eckhardt, D.A., 1979,

Nonpoint-source discharges in Pequea

Creek basin, Pennsylvania, 1977:

U.S. Geological Survey WaterResources Investigations Report 79-

88, 110 p.

Wauchope, R.D., Buttler, T.M., Hornsby,

A.G., Augustijn-Beckers, P.W.M., and

Burt, J.P., 1992, The SCS/ARS/CES

pesticide properties database for

environmental decision-making:

Reviews of Environmental

Contamination and Toxicology, v. 123,

p. 1-164.

Werner, S.L., Burkhardt, M.R., and

DeRusseau, S.N., 1996, Methods of

analysis by the U.S. Geological Survey

National Water-Quality Laboratory—

Determination of pesticides in water by

Carbopak-B solid-phase extraction

and high-performance liquid

chromatography: U.S. Geological

Survey Open-File Report 96-216, 42 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 capillarycolumn gas chromatography/mass

spectrometry with selected-ion

monitoring: U.S. Geological Survey

Open-File Report 95-181, 49 p.
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 75

APPENDIX—METHODOLOGY USED TO CHARACTERIZE STREAMFLOW

CONDITION

The following methods were used to characterize the streamflow condition

represented by samples collected from streams at long-term monitoring sites.

Development of a time-interval estimate for return to base-flow conditions following a

storm event

A commonly accepted method used for hydrograph separation,

N=A0.2, where N = time interval, in days and A = drainage area, in mi2, (1)

was used to determine the time interval from the peak of the storm through the falling

limb of the hydrograph when ground-water discharge was not the predominant

contributor of flow (Viessman and others, 1977). A review of stream hydrographs at

the long-term monitoring sites to verify the estimated time intervals indicated this

method produced reasonable estimates for six of the seven streams. Examination of

hydrographs from East Mahantango Creek generally revealed a slower recession and

longer time interval than the estimated value. For this reason, 24 hours was added to

the estimated time interval for East Mahantango Creek. Samples collected outside the

time interval estimated for each site were considered base-flow samples. Stream

hydrographs and precipitation records from nearby sites were used to determine the

occurrence and approximate time of a storm peak for samples collected before stagerecording equipment was installed at the long-term monitoring sites or during periods

of equipment malfunction.

Application of estimated time interval and determination of storm-affected samples

For days when elevated streamflow was determined to be from a natural event, the

fixed interval method of the HYSEP hydrograph separation program (Sloto and

Crouse, 1996) was used to separate streamflow into two components, surface runoff

(stormflow) and base flow. To negate the elimination of samples collected shortly after

relatively minor precipitation events, a day of elevated streamflow was only considered

a storm-affected day when the total streamflow for the day was 30 percent greater

than the base-flow component for the day. Therefore, only samples collected within

the estimated time interval from a storm peak that met or exceeded the 1.3 ratio of

total flow to base flow were considered storm-affected samples and excluded from the

base-flow sample set.

Consideration of anthropogenic influences on the stream hydrograph

If there was evidence that an observed change in streamflow was due to the

anthropogenic release or storage of water and not a runoff event, samples collected

within the estimated time interval were considered base-flow samples. This exception

only applies to three samples collected from Mill Creek during two periods of elevated

stage recorded at the long-term monitoring site. Continuous specific-conductance

record at the site was used to confirm the lack of change to water quality during the

periods of elevated stage and flow.

Consideration of observed streamflow conditions

A sample collected as a result of either the fixed-interval sampling program or the

storm-sampling program that was collected on the rising limb or a rapidly descending

falling limb of a hydrograph was considered storm-affected. At the time of collection for

these samples, observed streamflow conditions indicated overland runoff was a

significant component of streamflow. This rule was applied regardless of whether the

storm event produced a total streamflow greater than 30 percent of the computed

base-flow component.