U.S. Department of the Interior

U.S. Geological Survey

Hydrogeology and Water Quality of the Pepacton 

Reservoir Watershed in Southeastern New York

Part 1. Concentrations of Pesticides and their Degradates in Stream Baseflow,

2000-01. 

In cooperation with

NEW YORK STATE DEPARTMENT OF ENVIRONMENTAL CONSERVATION

Water-Resources Investigations Report - 03-4137
This page has been left blank intentionally.
Hydrogeology and Water Quality of the 

Pepacton Reservoir Watershed 

in Southeastern New York

Part 1. Concentrations of pesticides 

and their degradates in stream baseflow, 

2000-2001

By Patrick J. Phillips and Paul Heisig

In cooperation with 

New York State Department of Environmental Conservation

Water-Resources Investigations Report 03-4137

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

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reproduce any copyrighted materials contained within this report.
iii

Contents

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

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

Purpose and Scope ............................................................................................................................. 3

Geologic Setting ................................................................................................................................... 4

Methods ......................................................................................................................................................... 4

Sample Collection ................................................................................................................................ 4

Analytical Methods and Quality Control .......................................................................................... 4

Statistical and Computational Methods of Analysis ...................................................................... 5

Concentrations of Pesticides and Selected Degradates ....................................................................... 6

Pepacton Baseflow Data in Relation to Ground-Water Data from Similar RowCrop Areas ............................................................................................................................... 6

SAM Ratios ........................................................................................................................................... 6

Relation of Pesticide Concentrations to Corn-Crop History ......................................................... 7

Seasonal Variability in Pesticide and Pesticide Degradate Concentrations ........................... 11

Summary ..................................................................................................................................................... 12

Figures

1. Baseflow-sampling sites in the Pepacton Reservoir watershed, December 2000 through 

November 2001 ......................................................................................................................................... 2

2. Statistical summary of atrazine, metolachlor, deethylatrazine, and metolachlor ESA 

concentrations in baseflow samples from Pepacton Reservoir watershed in December 

2000 and July 2001 and in ground-water samples from monitoring wells and public-supply 

wells in row-crop areas of upstate New York. Summary includes concentrations reported 

below method detection limit .................................................................................................................... 8

3. SAM ratios for Pepacton baseflow samples, shallow ground water from public-supply 

wells in upstate row-crop areas, and Canajoharie Creek samples, 1997-98 ................................. 9

4. SAM ratios (ratio of metolachlor ESA concentration to metolachlor concentration) 

for samples from ANGLH site, December 2000 through November 2002 ....................................... 9

5. Concentrations of atrazine, metolachlor, deethylatrazine, and metolachlor ESA

in December 2000 and July 2001 baseflow samples from streams in Pepacton Reservoir 

watershed, by watershed corn-crop history ..................................................................................... 10

6. A. Concentration of atrazine, deethylatrazine, and metolachlor ESA in samples collected 

from ANGLH site, December 2000-November 2001, B. discharge of East Branch of the 

Delaware River at Roxbury, and date when samples collected at ANGLH, C. monthly mean 

discharge at Tremper Kill near Andes expressed as percentage of mean monthly discharge 

for 1937-2001 .....................................................................................................................................................11

Tables

1. Sites sampled for pesticide analysis in the Pepacton Reservoir watersheds, Delaware 

County, N.Y., December 2000 through November 2001 .................................................................... 3
iv

2. Method detection and laboratory reporting limits for pesticides and pesticide degradates 

for which base-flow samples were measured from Pepacton Reservoir watershed, 

Delaware County, N.Y., 2000-01 ............................................................................................................. 5

3. Summary of pesticide analyses of base-flow samples from Pepacton Reservoir watershed, 

Delaware County, N.Y., 2000-01 .............................................................................................................. 7
Abstract 

Baseflow samples were collected from 20 small streams 

in the Pepacton Reservoir watershed in Delaware County, 

N.Y., from December 2000 through November 2001 as part 

of an investigation to define the occurrence of pesticides in 

shallow ground water in watersheds containing either a recent 

(2001) corn crop, a previous (1993-94) corn crop , or no 

history of row-crop cultivation. Baseflow water quality was 

assumed to represent the chemical quality of shallow ground 

water within the drainage area above each sampling site. 

Baseflow samples were analyzed for 57 pesticides and 

pesticide degradates. Three herbicides (atrazine, metolachlor 

and simazine) and three herbicide degradates (alachlor ESA 

[ethanesulfonic acid], deethylatrazine, and metolachlor 

ESA) were detected, but no concentrations exceeded any 

Federal or State water-quality criteria, and the maximum 

concentrations of all compounds except metolachlor ESA 

were less than 0.10 microgram per liter. The most frequently 

detected compounds (atrazine, metolachlor, deethylatrazine 

and metolachlor ESA) are either those typically used on corn 

crops, or those whose parent compounds are commonly used 

on corn crops and have been detected in streams that drain 

row-crop settings elsewhere in New York State. The pesticide 

and pesticide-degradate concentrations in baseflow samples 

collected in December 2000 and July 2001 samples generally 

corresponded to the amount of cornfield acreage in each 

watershed in 2001. 

The types of pesticides detected, and their median 

concentrations, were similar to those noted in two previous 

ground-water studies in row-crop areas elsewhere in upstate 

New York. Also the SAM ratios (ratio of metolachlor ESA 

concentration to metolachlor concentration) for the Pepacton 

samples were similar to those for ground-water samples from 

other agricultural settings in upstate New York, but were 

significantly higher than that for stormflow and baseflow 

samples collected in 1997-98 from Canajoharie Creek, 

an upstate stream that drains row-crop farmland. These 

comparisons confirm that the baseflow samples were derived 

from, and were representative of, ground water in their 

respective watersheds. Late-summer decreases in atrazine and 

deethylatrazine concentrations at a site where corn was grown 

in 2001 may have resulted from the seasonally dry conditions 

and the accompanying decrease in ground-water discharge 

from the upper-most part of the surficial aquifer system to 

streams. The lack of a similar decrease in metolachlor ESA 

concentrations during this period may reflect the transport of 

metolachlor ESA to deeper parts of the surficial aquifer that 

continued to discharge to streams during the dry period.

Introduction

The Pepacton Reservoir watershed (372 mi2 [mile] fig. 1) 

is in the western part of the Catskill Mountains of central New 

York and is part of the Delaware Reservoir system, which 

supplies about 50 percent of New York City’s drinking water. 

The watershed, which is located in Delaware County, is 69 

percent forested, but some areas contain substantial amounts 

of agricultural land (dairy farming and row crops). The recent 

detection of agricultural herbicides and their degradates in 

the Pepacton Reservoir (Phillips and others, 2000) has caused 

concern as to whether pesticides used in agricultural parts of 

the watershed are affecting the water quality of the reservoir. 

In 2000, the U.S. Geological Survey (USGS), in cooperation 

with the New York State Department of Environmental 

Conservation, began a 2-year study to evaluate shallow 

ground-water quality throughout the Pepacton watershed in 

an effort to identify the effects of agriculture on the quality of 

water entering the Pepacton Reservoir (fig. 1).

Past research on pesticides in ground waters in 

agricultural areas in New York State (Eckhardt and others, 

2001; Phillips and others, 2000) has indicated that herbicides 

that are commonly used on corn are present in shallow ground 

water, although usually at concentrations well below Federal 

and State water-quality criteria. Two such herbicides that are 

commonly used on corn—atrazine and metolachlor—and their 

Hydrogeology and Water Quality of the Pepacton 

Reservoir Watershed in Southeastern New York

Part 1. Concentrations of pesticides and their degradates 

in stream baseflow, 2000-2001

by Patrick J. Phillips and Paul Heisig
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Figure 1. Baseflow-sampling sites in the Pepacton Reservoir watershed, December 2000 through November 2001. 

(Additional data on these sites are given in table 1.)

2 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 1
degradates deethylatrazine and metolachlor ethanesulfonic 

acid (ESA), respectively, which form by the breakdown of 

the parent compound, are frequently detected in ground water 

beneath fields or corn and other row crops. 

Purpose and Scope

This report (1) describes the sampling approach, and the 

methods of chemical and statistical analysis, (2) compares 

concentrations of selected pesticides in Pepacton baseflow 

samples with those in ground-water samples from other 

row-crop areas in upstate New York, (3) use the ratios of 

the concentration of metolachlor ESA to that of metolachlor 

(its parent compound) to confirm that the Pepacton baseflow samples reflect ground-water quality, (4) relates the 

occurrence and concentrations of pesticides and degradates 

in baseflow samples to the history of corn cultivation (2001, 

1993-94, or neither) within the respective watersheds, and (5) 

evaluates the seasonal changes in pesticide concentrations and 

relates them to the effects of drought conditions.

Baseflow was used as a surrogate indicator of shallow 

ground-water quality because ground water sustains stream 

discharge during dry periods—either from springs or from 

seepage directly into stream channels. Baseflow accounts for 

most of the water that enters the New York City reservoirs 

during periods of little or no rain and represents 60 to 70 

percent of total annual stream discharge. 

Baseflow samples were collected at 20 sites (table 1) 

representing small watersheds of less than 2 mi2 within the 

Pepacton watershed for chemical analysis to relate land use 

to pesticide presence in shallow ground water. Samples were 

collected in December 2000 and July 2001. Three types of 

watersheds (defined based on the extent of agricultural use) 

were selected for comparison—those in which corn was 

grown during 1993-94 and in 2001, those in which corn 

was grown during 1993-94 but not in 2001, and those with 

no history of corn grown in either 1993-94 or 2001. From 

five to seven additional samples were collected at each of 

three sites (one from each watershed type) between December 

2000 and November 2001.

Introduction 3

Table 1. Sites sampled for pesticide analysis in the Pepacton Reservoir watersheds, Delaware County, N.Y., December 2001 

through November 2002. 

[All sites were sampled once except as indicated; additional sampling (in parentheses) at three sites occurred from June through

November 2001. mi2, square miles. Locations are shown in fig. 1; no,number; USGS, U. S. Geological Survey.] 

a Corn-crop estimates for 1993-94 and housing density are based on New York City Department of Environmental Protection (1999).

Corn-crop estimates for 2001 based on photointerpretation and field checking. 

b “Active” denotes sites with a corn crop in 2001; “inactive” denotes sites with corn crop observed in 1993-94 but not in 2001, 

and “No Corn” denotes sites with no evidence of corn crop in 1993-94 or 2001 

Site

abbreviation

USGS

Station no. Site name

Drainage 

area

(mi2)

Housing

density

(per mi2))a

Percentage of 

watershed used 

for corn cropa Corn-crop 

classificationb 1993-94 2001

Sites Sampled in December 2000 and July 2001

ANAND 01414840 Gladstone Hollow Brook near Andes 5.8 13.3 2.8 1.2 Active

ANCBH 01414860 Campbell Hollow Brook .9 2.2 .54 0 Inactive

ANGLH (9 times) 01414835 Gladstone Hollow Brook north of Andes 2.6 6.6 3.5 .89 Active 

ARTIL 01414290 Mill Brook Tributary 2.0 1.5 0 0 No Corn

FLRDK 01413346 Red Kill 5.5 11.6 .23 0 Inactive

MRBYB 01413990 Bryants Brook 4.4 9.1 1.1 0 Inactive

MRSNH (9 times) 01413930 Sanford Hollow Brook 2.5 23.2 .16 0 Inactive

MRWRH 01413920 Winter Hollow Brook 3.5 22.5 1.8 1.5 Active 

MRWVH 01413950 Weaver Hollow Brook 3.1 10.2 .03 0 Inactive

RXBTK 0141309740 Batavia Kill 9.0 13.2 .2 .17 Active 

RXVGA 0141309730 Tributary To Batavia Kill 1. 7 20.4 .88 0 Inactive

SEHMB 01414280 Mill Brook 1.4 0 0 0 No Corn

SERDH 01413276 Rider Hollow Brook near Mapledale 4.1 5.7 0 0 No Corn

WKJSH 01413303 Vly Creek above Halcott Center 3.3 4.0 .86 .36 Active 

WKVLY 01413306 Vly Creek near Halcott Center 10.2 8.8 .47 .12 Active

Sites Sampled July 2001 but not December 2000

DNCTH 01416870 Cat Hollow Brook 3.7 9.3 0 0 No Corn 

DNPP2 01416895 Unnamed tributary to Pepacton Reservoir .38 5.3 0 0 No Corn

FLLRK 01413343 Little Red Kill .79 29.1 0 0 No Corn 

RXMGH 01413085 Montgomery Hollow 3. 7 10.1 .01 0 Inactive

SEMNH (7 times) 01413274 Rider Hollow Brook East of Mapledale 1.7 0 0 0 No Corn

Table 1. Sites sampled for pesticide analysis in the Pepacton Reservoir watersheds, Delaware County, N.Y., December 2000 

through November 2001. 

[All sites were sampled once except as indicated; additional sampling (in parentheses) at three sites occurred from June through 

November 2001. mi 2 , square miles. Locations are shown in fig. 1; no,number; USGS, U. S. Geological Survey.] 
Geologic Setting

The Pepacton watershed is underlain by sandstone, 

siltstone, shale, and conglomerate of Devonian age and is 

blanketed by glacial-drift deposits of variable thickness. 

Bedrock dips gently to the southwest with low, broad folds 

that trend northeast-southwest (Woodward, 1957; Soren, 

1963). Ground-water flow in bedrock is primarily through 

fractures. The most common unconsolidated deposit in the 

uplands is till, an unsorted mixture deposited by glaciers and 

primarily derived from local bedrock. Till thickness within the 

watershed ranges from zero to more than 200 ft (feet) (Fluhr, 

1953). The thickest deposits generally are near the base of 

south-facing hills that were in the lee of regional ice advance 

during glaciation (Coates, 1966). The predominant soil 

textures developed on till are silty and sandy loam (Ozsvath, 

1985). All but the shallowest upland soils that developed 

on till are underlain by a compact hardpan subsoil of low 

permeability (Lounsbury and others, 1930). The floors of 

small tributary valleys typically are covered by uncompacted, 

poorly sorted sediment (with some minor sorted zones) that 

melted out of stagnant ice (Ozsvath, 1985). Till in the valleybottom areas gradually transitions to stratified sediments in 

the down-valley direction, and stratification and sorting of 

valley-bottom deposits tend to be proportional to valley size 

(Ozsvath, 1985). 

Methods

Baseflow conditions were defined as those preceded 

by at least 3 days without substantial rain. Samples were not 

collected during the winter snowpack period nor during spring 

periods with above-freezing daily high temperatures to avoid 

potential meltwater contributions to baseflows. None of the 

20 sites had a continuous stream discharge gage; therefore, 

the smallest gaged watershed (13.5 mi2) within the Pepacton 

Reservoir watershed—the East Branch Delaware River at 

Roxbury (fig. 1)—was used to represent baseflow conditions 

and seasonal changes in baseflow discharge. The Roxbury 

station, although useful for representing discharge conditions 

for the 2-year study period, was not useful for comparisons 

with long-term average flow conditions because its short 

period of record beginning in 2000; therefore, discharge 

records for the Tremper Kill near Andes station (fig. 1) were 

used to relate flow conditions during the study period to 

long-term flow conditions. The Tremper Kill site represents a 

33.2-mi2 drainage area, and its period of record is from 1937 

through 2001. 

Baseflow samples were collected at 15 of the 20 sites 

during December 2000 and July 2001 (fig. 1). July was 

selected because it represents the 1- or 2-month period that 

closely follows the spring pesticide application, and December 

was chosen as representative of baseflow conditions several 

months after the spring application. The other five sites (table 

1) were added in July 2001 to increase the representation of 

watersheds with little or no history of pesticide application. 

Additional baseflow samples were collected from December 

2000 through November 2001 to investigate the seasonal 

variability in pesticide concentrations; these samples were 

collected nine times at the ANGLH and MRSNH sites and 

seven times at the SEMNH site (table 1) . The ANGLH site 

was considered to represent the drainage area with the greatest 

pesticide use; the SEMNH represented the drainage area 

with no pesticide use, and the MRSNH site represented the 

drainage area with an intermediate amount. 

Estimates of corn-crop history in the watershed above 

each site were available for two periods— 1993-94 and 

2001; estimates of corn crop are not available for the period 

1995-2000 . Values for 1993-94 were estimated by the New 

York City Department of Environmental Protection (New 

York City Department of Environmental Protection, 1996, 

1999) from Landsat Thematic Mapper scenes acquired in 

1992 and 1993; these estimates were ground truthed through 

field visits in 1993 and 1994 and interpretation of aerial 

photography. Values for 2001 were based on aerial photos 

and field visits by the USGS in the late summer of that year. 

The aerial photographs were used to delineate fields, and 

each field was visited in August or September to verify that 

corn was grown. After field checks, the amount of watershed 

represented by corn crop was calculated for each watershed 

(table 1); the small watershed size and the small amount of 

cornfield acreage in the study area allowed accurate estimation 

of corn-crop acreage from the aerial photographs and field 

verification. Sites with corn grown in 2001 are referred to here 

as “active”; those with a corn crop reported for 1993-94 but 

not observed in 20001 are referred to as “inactive,” and those 

with no corn grown either in 1993-94 or 2001 are designated 

as “no corn.” All of the sites at which corn was grown in 2001 

were found to have had corn growth in 1993-94. 

Sample Collection

Samples were collected in accordance with methods 

described in Shelton (1994). The small width (generally less 

than 20 ft wide) and shallow depth of the streams (generally 

less than 1 ft deep) required collection from midchannel with 

an open bottle. Equal-width-increment samples were collected 

at the two largest watersheds (sites WKVLY, RXBTK; table 

1) during December 2000. Stream discharge at sampling sites 

was measured only during December 2000 and July 2001. 

Analytical Methods and Quality Control

Samples were analyzed through two USGS analytical 

procedures: (1) a gas-chromatography/mass-spectrometry 

method (referred to as schedule 2010), which was used for 

47 pesticides and degradates at the USGS National Water 

Quality Laboratory in Denver, Colo. (Zaugg and others, 1995; 

table 2a), and (2) a liquid-chromatography/mass-spectrometry 

4 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 1
method (referred to as the Chloroacetanilide Herbicide 

Degradate [CHD] method), which was used for 10 degradates 

of chloroacetanilide herbicides at the USGS Organic Research 

Laboratory in Lawrence, Kans. (Lee and others, 2001) (table 

2b). The detection limits of the schedule 2010 and CHD 

methods ranged from 0.001 μg/L to 0.1 μg/L (table 2a, 2b). 

This range is much lower than that obtained by analytical 

methods typically used in public-water-supply-monitoring 

programs and provides much higher rates of detection than 

would be possible with the less sensitive analytical methods. 

Both methods used here require a filtered sample extracted on 

a solid-phase cartridge; thus, these concentrations represent 

the dissolved phase of pesticides and pesticide degradates. 

Blank and replicate samples were collected as qualitycontrol samples. Three blanks were collected for each 

analytical method; no target analytes were detected. Twelve 

replicates were collected for each analytical method; these 

were found to contain the 16 compounds that were also 

detected in field samples. Differences between the field and 

replicate concentrations in these 16 comparisons ranged 

from 0 to 20 percent; the difference for most compounds was 

less than 10 percent. Two compounds were detected in one 

replicate sample, but not in the corresponding field sample; the 

concentration of both compounds were at the detection limit. 

Statistical and Computational Methods 

of Analysis

Median concentrations were compared among sites and 

seasons through Kruskal-Wallace and Tukey nonparametric 

analyses of variance tests. Nonparametric statistical techniques 

were used because they are more appropriate than parametric 

techniques for censored data (values below detection limits) 

and for data that are not normally distributed (Helsel and 

Hirsch, 1992). 

SAM ratios (concentration of metolachlor ESA divided 

by concentration of metolachlor) were calculated for all 

sites. Previous investigations in New York have shown that 

ground water has higher SAM ratios (higher concentrations 

of metolachlor ESA than of metolachlor) than surface water 

because metolachlor ESA is transported from land surface 

to ground water more readily than metolachlor, and because 

metolachlor is washed into streams by storm runoff more 

readily than metolachlor ESA (Phillips and others, 1999a, b). 

SAM ratios obtained in these studies indicate that the baseflow 

samples reflect ground-water quality and did not contain 

surface runoff. SAM ratios were calculated through methods 

described by Phillips and others (1999a, 1999b). Calculation 

of the SAM ratio for samples in which metolachlor ESA 

Methods 5

 Table 2. Method detection and laboratory reporting limits for pesticides and pesticide degradates for which base-flow samples were 

measured from Pepacton Reservoir watershed, Delaware County, N.Y., 2000-01.

[Detection- and reporting-limit concentrations are in micrograms per liter. Asterisk denotes degradation product.] 

a GCMS, gas chromatography and mass spectrometry

LCMS, liquid chromatography and mass spectrometry

 ESA , ethanasulfonic acid

OA, oxanilic acid

Pesticide Detection Limit Pesticide Detection Limit Pesticide Detection Limit

Pesticide Detection Limit 

A. Method detection limits for Schedule 2010 GCMS a method of U.S. Geological Survey National Water Quality Laboratory, Denver, Colo

Acetochlor 0.0041 Deethylatrazine * 0.006 Metolachlor 0.013 Propyzamide 0.0041)

Alachlor 0.0024 Diazinon 0.005 Metribuzin 0.006 Propachlor 0.010)

alpha-HCH 0.0046 Dieldrin 0.0048 Molinate 0.0016 Propanil 0.011)

Atrazine 0.007 Disulfoton 0.021 Napropamide 0.007 Propargite 0.023)

Benfluralin 0.010 EPTC 0.002 p,p’-DDE * 0.0025 Simazine 0.011)

Butylate 0.002 Ethalfluralin 0.009 Parathion 0.007 Tebuthiuron 0.016

Carbaryl 0.041 Ethopropos 0.005 Parathion-methyl 0.006 Terbacil 0.034

Carbofuran 0.020 Fonofos 0.0027 Pebulate 0.0016 Terbufos 0.017

Chlorpyrifos 0.005 Lindane 0.004 Pendimethalin 0.010 Terbuthylazine 0.1

Cyanazine 0.018 Linuron 0.0035 cis-Permethrin 0.006 Thiobencarb 0.0048

Dacthal 0.003 Malathion 0.027 Phorate 0.011 Tri-allate 0.0023

2,6-Diethylaniline* 0.0017 Methyl azinphos 0.05 Prometon 0.015 Trifluarlin 0.009

B. Laboratory reporting limits for chloroacetanilide herbicide degradate method by the LCMS1 method of U.S. Geological Survey 

Organic Research Laboratory, Lawrence, Kansas 

Acetachlor ESA* 0.05 Dimethenamid ESA*0.05 Metolachlor OA* 0.05

Acetachlor OA* 0.05 Dimethenamid OA* 0.05 Metolachlor ESA* 0.05 

Alachlor ESA* 0.05 Flufenacet ESA* 0.05

Alachlor OA* 0.05 Flufenacet OA* 0.05

Table 2. Method detection and laboratory reporting limits for pesticides and pesticide degradates for which base-flow samples were 

measured from Pepacton Reservoir watershed, Delaware County, N.Y., 2000-01. 

[Detection- and reporting-limit concentrations are in micrograms per liter. Asterisk denotes degradation product.] 
and metolachlor concentrations were measurable was 

straightforward, and all samples in which metolachlor was 

detected also contained metolachlor ESA. SAM ratios for 

the 11 samples in which metolachlor ESA was measurable, 

but metolachlor concentrations were below the detection 

limit, were calculated as the metolachlor ESA concentration 

divided by 0.001 μg/L, which is lower than the lowest reported 

metolachlor concentration (0.0013 μg/L). This method was 

used in similar studies by Phillips and others (1999a, b). 

Concentrations of Pesticides and 

Selected Degradates

Of the 57 pesticides and pesticide degradates for which 

samples were analyzed, 6 were detected – 3 herbicides 

(atrazine, metolachlor and simazine) and 3 herbicide 

degradates (alachlor ESA, deethylatrazine, and metolachlor 

ESA). No insecticide or insecticide degradates were detected. 

Maximum concentrations of detected compounds 

ranged from 0.006 μg/L (simazine) to 0.42 μg/L (metolachlor 

ESA). Maximum concentrations of all compounds except 

metolachlor ESA were less than 0.10 μg/L (table 3). 

Maximum concentrations of alachlor ESA, atrazine, and 

deethylatrazine were between 0.01 μg/L and 0.1 μg/L, and 

maximum concentrations of metolachlor and simazine 

were below 0.007 μg/L. The two most frequently detected 

herbicides were atrazine and metolachlor, which are applied 

on corn; the two most frequently detected herbicide degradates 

were deethylatrazine (a degradate of atrazine) and metolachlor 

ESA (a degradate of metolachlor). No concentrations 

exceeded any Federal or State water-quality criteria. 

All compounds (or their parent compounds) detected in 

this study except simazine are used primarily on corn crops 

and, thus, are derived from agricultural applications (simazine 

is used in row-crop and nonagricultural settings; see Hoffman 

and others, 2000). The restriction of sampling to baseflow 

conditions presumably yielded concentrations that were much 

lower than the annual maximum concentrations, which tend to 

occur during stormflows after the late-spring or early-summer 

pesticide application. 

Pepacton Baseflow Data in Relation to GroundWater Data from Similar Row-Crop Areas

A comparison of the pesticide concentrations in the 

Pepacton baseflow samples with those in samples from (1) 

16 monitoring wells in row-crop settings elsewhere in upstate 

New York (Phillips and others, 2000), and (2) 32 publicwater-supply wells in agricultural settings in upstate New 

York (Eckhardt and others, 2000) indicates strong similarity. 

All monitoring wells and all but one of the public-supply were 

finished in highly permeable sand and gravel aquifers. 

The most commonly detected pesticides in all three 

studies were atrazine, metolachlor, and deethylatrazine; these 

compounds were detected in more than 25 percent of the 

samples from the public-supply wells and in more than half of 

the samples from the monitoring wells. Metolachlor ESA was 

not measured in the monitoring-well study, but was detected 

in more than 25 percent of the samples from the public-supply 

wells. Comparison of atrazine, metolachlor, deethylatrazine, 

and metolachlor ESA concentrations in the Pepacton baseflow 

samples (separated into December 2000 and July 2001 groups) 

with those measured in the monitoring- and public-supply 

wells in row-crop areas indicate no significant differences 

(fig. 2).

The similarity between the assemblages of herbicides and 

herbicide degradates detected in the Pepacton study and those 

in the public-supply and monitoring-well studies indicates that 

the quality of baseflow in the Pepacton watershed is typical of 

that in ground water underlying row-crop farmland in upstate 

New York. 

SAM Ratios

The lack of significant difference between the SAM 

ratios for the Pepacton baseflow samples (79) and those of the 

ground-water samples from upstate public-supply wells (350) 

(fig. 3) confirms that the baseflow samples reflect the local 

shallow-ground-water quality. In contrast, median SAM ratios 

for the Pepacton baseflow and public-supply-well samples 

were significantly greater than those for samples collected 

from Canajoharie Creek (in a crop-growing area of eastcentral New York) in June and July 1997-98 (9.1) and from 

August and September of 1997 and 1998 (4.9; fig 3). 

Surface-water samples from Canajoharie Creek had 

higher concentrations of metolachlor than of metolachlor 

ESA and, thus, lower SAM ratios than ground-water samples 

from similar upstate row-crop settings, because metolachlor 

is readily transported by surface runoff to streams, where its 

high concentrations can result in low SAM ratios, particularly 

during stormflows. (The Canajoharie Creek data represent 

stormflow as well as baseflow). The higher SAM ratios in 

ground water result from the formation of metolachlor ESA 

within the soil as a metolachlor degradate, and its rapid 

migration to the water table because it is less easily bound to 

soils than metolachlor (Phillips and others, 1999a, 199b). Most 

SAM ratios for the Pepacton baseflow samples and the publicsupply well samples were greater than 40, whereas most SAM 

ratios for Canajoharie Creek samples were below 40.

Further evidence that the Pepacton baseflow samples 

reflect shallow-ground-water quality is given by the timeseries data from the ANGLH site (fig. 4). SAM ratios for 

ANGLH samples ranged from 70 to 150, and those for the 

June and July samples were similar to those for samples 

collected in other months. In contrast, median SAM ratios 

for Canajoharie Creek samples collected in June and July 

were significantly lower than those for samples collected 

6 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 1
during other months (fig. 3). The low SAM ratios for the June 

and July samples from Canajoharie Creek are attributed to 

elevated concentrations of metolachlor at the time of sampling 

– just after pesticide application in June, when the washoff of 

metolachlor during storms causes the highest concentrations 

of metolachlor (and the lowest SAM ratios) (Phillips and 

others, 1999b). The lack of substantially lower SAM ratios 

for ANGLH baseflow samples in June and July confirms that 

these samples consisted of ground water and were not affected 

by surface runoff.

Relation of Pesticide Concentrations to 

Corn-Crop History

Comparison of atrazine, metolachlor, deethylatrazine, 

and metolachlor ESA concentrations among sampling sites 

with corn-crop acreage and history indicates that the highest 

Concentrations of Pesticides and Selected Degradates 7

Table 3. Summary of pesticide analyses of base-flow samples from Pepacton Reservoir watershed, Delaware County, N.Y., 2000-01 

[ μg/L, micrograms per liter. A dash (--) indicates no water-quality criteria available. Site locations are shown in fig. 1.]

a Includes reported concentrations below method detection limit.

b Analytical method described by Zaugg and others, 1995 

c Analytical method described by Lee and others, 2001 

d Canadian or Canadian Interim standards for the protection of aquatic life (Canadian Council of Resource and Environment Ministers, 1997; 

Environment Canada, 1999);

e New York State Surface Water Standard (New York State Department of Health, 1998);

f Great Lakes standard for the protection of aquatic life (International Joint Commission Canada and United States, 1977). 

Sampling location and 

common pesticide 

name

Method

detection limit 

(μg/L)

Percentage of 

samples with 

a detectiona

Percentage of samples with 

concentration exceeding 

indicated value Water-quality 

criterion 

(μg/L)

Maximum 

concentration 

(μg/L)

Site or month

with highest 

0.01 μg/L 0.05 μg/L concentration

A. All 15 sites in Pepacton Reservoir watershed (one sample per site)

December 2000 

Herbicides

Atrazine b 0.007 53 0 0 1.8 f 0.0065 MRWRH

Metolachlor b .013 33 0 0 7.5 d .0051 MRWRH

Herbicide degradates

Alachlor ESA c .05 13 13 13 50 e 0.07 ANAND

Deethylatrazine b .006 53 0 0 -- .009 MRWRH

Metolachlor ESA c .05 40 40 40 50 e .20 WKJSH

July 2001 - samples collected from the five additional sites at this time (DNCTH, DNPP2, FLLRK, RXMGH, 

and SEMNH) contained no targeted compounds and were not used in statistical summaries.

Herbicides

Atrazine b 0.007 47 6.7 6.7 1.8 f 0.0586 MRWRH

Metolachlor b .013 13 0 0 7.5 d .0059 WKJSH

Herbicide degradates

Deethylatrazine b .006 47 6.7 0 -- .011 WKJSH

Metolachlor ESA c .05 40 40 40 50 e .42 WKJSH

B. Sites that were sampled nine times from December 2000 through November 20001

MRSNH (Sanford Hollow Brook) 

Herbicide

Simazineb 0.011 11 0 0 .5 d 0.006 October 2001

Herbicide degradate 

Deethylatrazineb .006 11 0 0 -- .0009 December 2000

 ANGLH (Gladstone Hollow Brook north of Andes) 

Herbicide

Atrazineb 0.007 67 11 0 1.8 e 0.011 June 2001

Herbicide degradates

Alachlor ESA c .05 56 56 56 50 d .09 June 2001

Deethylatrazineb .006 67 0 0 -- .0052 June 2001

Metolachlor ESA c .05 89 89 89 50 d .15 August 2001

Table 3 . Summary of pesticide analyses of base-flow samples from Pepacton Reservoir watershed, Delaware County, N.Y., 2000-01 

[μg/L, micrograms per liter. A dash (--) indicates no water-quality criteria available. Site locations are shown in fig. 1.] 
concentrations of pesticides and pesticide degradates were 

at “active” sites – those within watersheds with recently 

grown corn (2001). Few pesticides or pesticide degradates 

were detected in samples from “inactive sites” (corn grown 

in 1993-94 but not in 2001), and no pesticides or degradates 

were detected in samples from “no-corn” sites (no history of 

corn crop during 1993-94 or 2001). Median concentrations of 

atrazine, deethylatrazine, and metolachlor ESA were higher 

among samples from active sites than inactive sites or no-corn 

sites (fig. 5) and did not differ significantly between inactive 

and no-corn sites. More than two-thirds of the samples from 

the six active sites contained measurable amounts of these 

three compounds, whereas only a third (or fewer) of the 

inactive sites contained these three compounds. No pesticide 

or pesticide degradate was detected in any of the samples from 

the no-corn sites. 

 Median concentrations of metolachlor did not differ 

significantly among samples from the three types of sites, 

probably because the active-site samples showed relatively 

few metolachlor detections – four detections among the six 

active-site samples collected in December 2000 and two 

among the six active-site samples collected in July 2001 

(fig 5). The active-site samples showed more frequent 

detections of the other three commonly detected pesticide or 

pesticide degradate compounds –five or six detections in the 

six samples collected during each period. Metolachlor was 

infrequently detected in samples from inactive sites – only one 

detection among the December 2000 samples, and none in the 

July 2001 samples.

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Figure 2. Statistical summary of 

atrazine, metolachlor, deethylatrazine, 

and metolachlor ESA concentrations 

in baseflow samples from Pepacton 

Reservoir watershed in December 2000 

and July 2001 and in ground-water 

samples from monitoring wells and 

public-supply wells in row-crop areas 

of upstate New York. Summary includes 

concentrations reported below method 

detection limit. (Monitoring-well results 

are discussed in Phillips and others, 2000; 

public-supply-well results are discussed 

in Eckhardt and others, 2000.) 

8 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 1
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Figure 3. SAM ratios for Pepacton baseflow samples, shallow ground water from public-supply 

wells in upstate row-crop areas, and Canajoharie Creek samples, 1997-98. (Public-supply-well 

data from Eckhardt and others, 2001, Canajoharie Creek data from Phillips and others, 1999a,b) SAM RATIO

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2000 2001

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Figure 4. SAM ratios (ratio of metolachlor ESA concentration to 

metolachlor concentration) for samples from ANGLH site, December 

2000 through November 2002. (Site location is shown in fig. 1; SAM 

ratio not computed for May 2001 sample because metolachlor ESA 

was not detected.)

Concentrations of Pesticides and Selected Degradates 9
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Figure 5. Concentrations of atrazine, metolachlor, deethylatrazine, and metolachlor ESA in December 2000 

and July 2001 baseflow samples from streams in Pepacton Reservoir watershed, by watershed corn-crop 

history (active, inactive, or none). Summary includes concentrations reported below method detection limit.

10 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 1
Seasonal Variability in Pesticide and Pesticide 

Degradate Concentrations

Seven or more samples were collected at three sites 

(ANGLH, MRSNH, and SEMNH) from December 2000 

through November 2001. No pesticides were detected in 

samples from SEMNH, which represents a 

watershed that is nearly 100 percent forested 

and contained no corn crop in either 1993-94 or 

2001. The samples from the inactive MRSNH 

site contained two pesticides (table 3B), with 

maximum concentrations less than 0.01 μg/L. 

The samples from the active ANGLH site (table 

3B) contained four compounds, with three of 

these compounds (atrazine, deethylatrazine, and 

metolachlor ESA) were detected in more than 

60 percent of the samples from this site, and 

three compounds (atrazine, alachlor ESA, and 

metolachlor ESA) exceeding 0.01 μg/L.

The decrease in the concentrations of atrazine 

and deethylatrazine during the Summer at the 

ANGLH site may reflect the dry conditions of that 

season. The concentrations of those compounds 

peaked in July at ANGLH, probably in response 

to the atrazine application earlier that spring, but 

from July through October, the concentrations 

decreased, and no atrazine was detected after 

September (fig. 6). The dry conditions of that 

period are indicated by discharge records from 

neighboring streams and precipitation data from 

Arkville (fig. 1). Baseflow discharge at the East 

Branch Delaware River site for sampling days 

between December 2000 through May 2001 

ranged from 10 to 17 ft3/s (cubic feet per second), 

and were highest in early May, after a period of 

snowmelt, whereas discharge on the sampling 

dates from August through November was less 

than 3.5 ft3/s (fig. 6). Precipitation from April 

through November 2001 was 18 percent lower than 

normal (National Climatic Data Center, 2002). A 

comparison of monthly discharge (expressed as 

percentage of the mean monthly flow) for Tremper 

Kill near Andes (the smallest gaged watershed in 

the Pepacton watershed with a long-term discharge 

record) from July through December 2001 (fig. 

6) indicates that monthly mean discharges after 

June 2001 were less than 50 percent of the longterm mean monthly discharge. Conditions were 

especially dry from August through December 

2001, when discharges did not exceed 30 percent 

of the long-term mean monthly discharge (fig 6). 

The decrease in atrazine and deethylatrazine 

concentrations in ANGLH samples from the 

summer through the fall of 2001 reflects the 

gradual decrease in ground-water discharge from 

shallow-most part of the surficial aquifer system 

as the drought continued into the fall. The lack of a similar 

decrease in metolachlor ESA concentrations during this 

period is consistent with the tendency of metolachlor ESA to 

migrate more readily than metolachlor to the water table and 

therefore to be discharged from deeper parts of the aquifer to 

streams. The short study duration makes this explanation of 

the difference between the late-summer concentration changes 

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Figure 6. A. Concentration of atrazine, deethylatrazine, and metolachlor 

ESA in samples collected from ANGLH site, December 2000-November 

2001, B. discharge of East Branch of the Delaware River at Roxbury, and 

date when samples collected at ANGLH, C. monthly mean discharge 

at Tremper Kill near Andes expressed as percentage of mean monthly 

discharge for 1937-2001. (Locations are shown in fig. 1). 

Concentrations of Pesticides and Selected Degradates 11
of atrazine and those of metolachlor ESA difficult to confirm, 

but if the shallow-most aquifer zone is the major source 

of atrazine and deethylatrazine, the decrease in discharge 

from this zone could account for the autumn decline in their 

concentrations. The continued discharge of metolachlor ESAbearing water from deeper parts of the aquifer during this 

period (fig. 6) would maintain fairly constant concentrations 

in baseflow. In other words, the atrazine and deethylatrazine 

contributions from shallow ground water decrease during 

periods of low flow, while the metolachlor ESA contributions 

from deeper ground water continue. 

Summary 

Small streams in the Pepacton Reservoir watershed were 

sampled between December 2000 and November 2001 as part 

of a cooperative project between the USGS and New York 

State Department of Environmental Conservation to assess 

the occurrence of pesticides in shallow ground water with 

differing amounts of corn cultivation. Baseflow samples from 

20 sites in the Pepacton watershed, a part of the Delaware 

Reservoir system that supplies about 50 percent of New York 

City’s water drinking water, were analyzed for pesticides. Of 

the 57 pesticides and pesticide degradates for which samples 

were analyzed, only 6 were detected – 3 herbicides (atrazine, 

metolachlor and simazine) and 3 herbicide degradates 

(alachlor ESA, deethylatrazine, and metolachlor ESA). No 

insecticide or insecticide degradates were detected. Maximum 

concentrations of all compounds except metolachlor ESA 

were less than 0.10 μg/L, and no concentrations exceeded any 

Federal or State water-quality criteria. The pesticides detected 

in this study are generally used on corn and, thus, represent the 

effect of agricultural applications. 

A comparison of the pesticide concentrations in the 

Pepacton baseflow samples with those of the most frequently 

detected pesticides in ground-water samples from monitoring 

wells and public-supply wells in row-crop areas of upstate 

New York indicated that the most commonly detected 

pesticides in all three studies were atrazine, metolachlor, and 

deethylatrazine. A fourth compound (metolachlor ESA) was 

detected in many of the Pepacton baseflow samples and in 

the public-supply samples. The similarity of concentrations 

among the three studies indicates that the concentrations in the 

Pepacton samples are typical of those in ground water in other 

agricultural areas of upstate New York. 

SAM ratios (ratio of metolachlor ESA concentration 

to metolachlor concentrations) for the Pepacton baseflow 

samples were similar to SAM ratios for the supply-well 

samples, but were significantly higher than the median SAM 

ratio for a stream in upstate New York that drains a row-crop 

area; this confirms that the Pepacton baseflow samples were 

derived from ground water and did not contain surface runoff. 

Metolachlor is transported in surface runoff more readily than 

to ground water, in part because it is sorbed to soil. Thus, 

surface waters have lower SAM ratios than ground water. 

Concentrations of three of the most frequently detected 

pesticides and pesticide degradates were related to current 

distribution of cornfields. The highest median concentrations 

of atrazine, deethylatrazine, and metolachlor ESA in baseflow 

samples collected at 15 sites in December 2000 and 20 sites 

in July 2001 were from watersheds in which corn was grown 

in 2001; the median concentration of these compounds for 

samples from watersheds with corn grown in 1993-94 but not 

in 2001 was statistically similar to that for watersheds with no 

history of corn crop during or after 1993-94.

More than 60 percent of the nine samples collected 

at the active ANGLH site (corn crop gown in 1993-94 and 

2001) contained atrazine, deethylatrazine, and metolachlor 

ESA. Decreases in the baseflow concentration of atrazine 

and deethylatrazine at this site during the summer of 2001 

may reflect the decline of ground-water discharge from the 

shallow-most part of the surficial aquifer in response to a 

drought. The lack of a similar decrease in metolachlor ESA 

concentrations during this period may reflect the transport of 

metolachlor ESA into deeper parts of the surficial aquifer that 

continued to discharge to streams throughout the fall.

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