Changes in Nutrient and Pesticide 

Concentrations in Urban and Agricultural Areas 

of the South Platte River Basin, Colorado, 

Wyoming, and Nebraska, 1994–2000

By Lori A. Sprague and Adrienne I. Greve

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4270

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Denver, Colorado

2003
District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046

For additional information write to:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

Copies of this report can be purchased 

from:

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225
CONTENTS III

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

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

Why are we concerned about nutrients and pesticides? ........................................................................................................ 3

How do nutrients and pesticides reach rivers? ...................................................................................................................... 4

How have nutrient concentrations changed in the South Platte River at Denver and near Kersey from 

1994 through 2000? ....................................................................................................................................................... 6

How have pesticide concentrations changed in the South Platte River at Denver and near Kersey from 

1994 through 2000? ....................................................................................................................................................... 8

What can be done to further reduce the concentrations of nutrients and pesticides in the South Platte River?.................... 11

References.............................................................................................................................................................................. 12

FIGURES

1. Map showing land use in the South Platte River Basin ............................................................................................ 2

2. Schematic of nutrient and pesticide movement in the hydrologic cycle................................................................... 4

3. Graph of nitrogen and phosphorus concentrations in the South Platte River at Denver and near 

Kersey, Colorado, from 1994 through 2000.............................................................................................................. 7

4. Graph of pesticide concentrations in the South Platte River at Denver and near Kersey, Colorado, 

in 1994, 1997, 1998, 1999, and 2000........................................................................................................................ 10

5. Graph showing streamflow in the South Platte River at Denver and near Kersey, Colorado, 

from 1994 through 2000............................................................................................................................................ 11

TABLES

1. The eight most common pesticides and pesticide degradation products detected during the study in the 

South Platte River at Denver and near Kersey, Colorado, from 1994 through 2000 .............................................. 5

2. Trends in streamflow and concentrations of selected constituents in the South Platte River at Denver and 

near Kersey, Colorado, from 1994 through 2000.................................................................................................... 8

CONTENTS
1

Changes in Nutrient and Pesticide Concentrations in 

Urban and Agricultural Areas of the South Platte 

River Basin, Colorado, Wyoming, and Nebraska, 

1994–2000

By Lori A. Sprague and Adrienne I. Greve

Abstract

As part of the National Water-Quality 

Assessment (NAWQA) Program, the U.S. 

Geological Survey (USGS) monitored two sites 

on the main-stem South Platte River— an urban 

site in Denver and a mixed urban/agricultural site 

near Kersey—to determine changes in nutrient 

and pesticide concentrations from 1994 through 

2000. Concentrations of nitrate, nitrite, ammonia, 

and orthophosphorus decreased at the Denver site 

during the study period, likely due to an increase 

in instream dilution of wastewater-treatment plant 

(WWTP) discharge and upgrades at the WWTPs. 

In contrast, only concentrations of orthophosphorus decreased at the Kersey site; agricultural 

inputs between Denver and Kersey may have 

offset the observed decreases in other nutrients 

upstream. During the extreme low-flow conditions in 1994, when there was relatively little 

snowmelt to dilute instream pesticide concentrations, total median pesticide concentrations at 

both sites were the highest of the study period. 

During the less extreme conditions in 1997 

through 2000, greater amounts of snowmelt likely 

led to lower total median pesticide concentrations 

at both sites. Because pesticide-use data are not 

available, the contribution of changes in the 

amount and type of pesticides applied on the land 

to changes in the concentration of pesticides in 

the river is not known but likely was substantial. 

In general, insecticides predominated at the 

Denver site, whereas herbicides predominated at 

the Kersey site.

INTRODUCTION

The South Platte River originates in the Rocky 

Mountains of central Colorado at the Continental 

Divide and flows about 450 miles northeast across the 

Great Plains to its confluence with the North Platte 

River at North Platte, Nebraska (fig.1). More than 

2.8 million people currently (2002) live in the Front 

Range urban corridor along the base of the Rocky 

Mountains in the South Platte River Basin; the population increased by about 670,000 people from 1990 

through 2000 (U.S. Census Bureau, 2000). On the 

plains downstream from the Denver metropolitan area, 

population densities are small and the land is used 

primarily for agriculture and livestock production.

The geologic and land-use diversity has had a 

profound effect on water-quality conditions within the 

basin. In 1991, the U.S. Geological Survey (USGS) 

began a study of the South Platte River Basin as part 

of the National Water-Quality Assessment (NAWQA) 

Program. The goals of the NAWQA Program are to 

assess the quality of surface water, ground water, and 

aquatic ecosystems throughout the Nation and to 

understand the primary natural and human factors 

affecting the quality of these resources. To help 

address water-quality issues of concern in the South 

Platte River Basin, the USGS monitored two sites on 

the main-stem South Platte River from 1994 through 

2000 to determine long-term changes in nutrient and 

pesticide concentrations in urban and mixed 

urban/agricultural areas. 
2

Denver

Boulder

Greeley

Cheyenne

Sterling

Julesburg

Fort Morgan

North 

Fort Collins

Kersey

Henderson

South Platte

River

South

Platte

River

Clear Creek

Boulder

Creek

St. Vrain

Creek

Box Elder Creek

Plum

Creek

COLORADO

WYOMING NEBRASKA

Creek

Cherry

Big

Thompson

River

Lonetree Creek

Crow

Creek

Lodgepole Creek

North Fork

Kiowa

Creek

Bijou Creek

Creek Beaver

0 50 100 MILES0 50 100 KILOMETERS

EXPLANATION

WaterUrbanOtherForestRangelandAgriculturalKersey site watershedDenver site watershedMonitoring station

Base from the U.S. Geological Survey,U.S. Census Bureau digital data, 1974 to 1992,1:100,000 to 1:500,000 Albers Equal Area projectionStandard parallels 29°30N and 49°30N, Central meridian 104°WLatitude of projection origin 23°W

COLORADO

WYOMING

NEBRA

S

ChatfieldReservoir

North Fork

Cache 

Cache la Poudre

laPoudre

River

105°

104°

103°

102°

10

41°

40°

106°

39°

Figure 1. Land use in the South Platte River Basin varies from forested areas in the mountains, to urban areas along the Front Range and to agricultural areas in the plains. Different land uses can lead to different water-quality conditions throughout the basin.
3

The monitoring site on the South Platte River at 

Denver (USGS station number 06714000) is located 

downstream from the confluence of Cherry Creek and 

the South Platte River in downtown Denver (fig. 1). 

Most of the water at the site originates downstream 

from Chatfield Reservoir, where the land use is 

predominantly urban. Streamflow is derived primarily 

from urban base flow (wastewater-treatment-plant 

discharges and ground-water inflows), urban storm 

runoff, and reservoir releases (Litke and Kimbrough, 

1998). From 1994 through 2000, substantial commercial and residential development in this already urbanized area increased the impervious surface area and 

drainage-pipe discharge to the river. This site is representative of urban land use.

Downstream from Denver, the South Platte 

River flows through predominantly agricultural land 

toward Kersey. The monitoring site on the South Platte 

River near Kersey (USGS station number 06754000) 

is located near the confluence of Lonetree Creek and 

the South Platte River (fig. 1). Streamflow at this site 

is derived, in varying degrees, from upstream wastewater-treatment-plant discharges, irrigation-return 

flow, snowmelt runoff, and urban runoff (Litke and 

Kimbrough, 1998). Because of the combination of 

sources, this site is representative of urban and agricultural land uses.

Why are we concerned about nutrients 

and pesticides?

Although nutrients such as nitrogen and phosphorus are necessary for plant and animal life, in 

excessive quantities they can accelerate the growth of 

aquatic plants and cause algal blooms (Wetzel, 1983). 

Algal blooms and excessive aquatic-plant growth may 

result in unsuitable habitat conditions for aquatic 

animals and waterfowl and can interfere with recreational activities such as fishing, swimming, and 

boating. The subsequent decay and decomposition of 

algae and other aquatic plants can cause odor and taste 

problems in drinking-water supplies and can consume 

dissolved oxygen, which can adversely affect fish and 

other aquatic life. 

Pesticides used to kill weeds, insects, and other 

pests can have harmful effects on humans and the 

environment. Potential health effects on humans and 

aquatic life because of overexposure to certain pesticides include cancer and nervous-system disorders. 

Recent studies suggest that some pesticides can 

disrupt endocrine systems and affect reproduction by 

interfering with natural hormones (Fuhrer and others, 

1999). Certain pesticides with low solubility in water 

and high resistance to degradation can accumulate in 

riverbed sediments and fatty tissue in fish and can 

persist in the environment for long periods of time. 

Often, contamination occurs as a mixture of pesticides, and little is known about the toxicity of these 

mixtures.

Most of the water at the monitoring site on the 

South Platte River at Denver originates 

downstream from Chatfield Reservoir, where 

the land use is primarily urban.

Water at the monitoring site on the South Platte 

River near Kersey is derived from a combination 

of urban and agricultural sources.
4

How do nutrients and pesticides reach 

rivers?

Nutrients are delivered to the South Platte River 

from point and nonpoint sources (fig. 2). Point sources 

of nutrients can be attributed to a single location, such 

as a wastewater discharge pipe. Nonpoint sources of 

nutrients are widely distributed over an area, such as 

storm runoff from agricultural fields and urban areas 

like lawns and golf courses, atmospheric deposition, 

and ground-water discharge to the river. Approximately 67 percent of the total wastewater discharge to 

the South Platte River occurs upstream from Henderson, Colo., where the river flows through the Denver 

metropolitan area; another 28 percent of the wastewater discharge occurs between Henderson and 

Kersey (Litke, 1996). Downstream from Kersey, the 

influence of wastewater discharges decreases, and 

water is derived primarily from nonpoint agricultural 

sources in the plains.

Pesticides are delivered to the South Platte 

River largely from nonpoint sources (fig. 2). Potential 

sources include storm runoff from agricultural fields 

and urban areas, atmospheric deposition, and groundwater discharge to the river. Because of different pestcontrol needs in agricultural compared to urban areas, 

different pesticides are applied and in different 

amounts and combinations in the two areas. For 

example, atrazine primarily is used as a preemergent 

herbicide on corn, whereas diazinon commonly is used 

as a household insecticide (table 1). The amount of 

pesticide available to be carried in storm runoff to 

streams is greatest just after application, before the 

pesticide degrades in the environment or is assimilated 

by plants. Because the timing of application differs

Figure 2. Nutrients and pesticides can reach the South Platte River from point sources, such as wastewater-treatment-plant 

discharge pipes, and from nonpoint sources, such as storm runoff from agricultural fields and suburban lawns.

Dry 

deposition

Precipitation

Industrial

emissions

Infiltration

Ground-water Infiltration

discharge to stream

Discharge to 

stream

In-stream

process

Runoff es

Runoff

Runoff

Wind erosion 

spray drift

SOIL

AQUIFER (WATER-BEARING 

ROCK OR SOIL) NUTRIENTS

PESTICIDES
5

among pesticides, they often reach surface water 

during different months of the year. In addition, certain 

pesticides may appear in surface water for months 

after application because they are more resistant to 

degradation or because they are transported through 

ground water.

Some efforts have been made to reduce the 

transport of nutrients and pesticides to rivers. To 

control point-source pollution, many wastewatertreatment plants have implemented new technologies 

that more effectively remove nitrogen and phosphorus 

from wastewater. In addition, since 1970, 27 States 

and the District of Columbia have enacted mandatory 

bans on the use of phosphate detergents (Litke, 1999). 

The States in the South Platte River Basin do not have 

mandatory bans, but the detergent industry voluntarily 

phased out the use of phosphorus in domestic laundry 

detergent throughout the Nation by about 1994. 

To control nonpoint-source pollution, many best 

management practices (BMPs) have been implemented to minimize concentrations of nutrients and 

pesticides reaching the river. Nonstructural BMPs, 

designed in part to minimize the application of fertilizers and pesticides, include educational programs, 

low-impact development that maintains predevelopment hydrology, and the creation of new pesticides 

that may have less of an effect on the environment. 

Structural BMPs, designed to reduce surface runoff 

of nutrients and pesticides to rivers, include sediment 

fences, constructed wetlands, and stormwaterretention ponds. With some structural BMPs, however, 

nutrient- or pesticide-rich runoff that is diverted away 

from rivers can infiltrate to ground water, which can 

eventually discharge to a river. Because ground water 

moves slowly, it may take many years to assess the 

effectiveness of the BMPs.

Treated wastewater discharge to the South 

Platte River downstream from Denver is a 

source of nutrients.

Runoff from agricultural fields treated with 

pesticides and fertilizer is a source of nutrients 

and pesticides.

Table 1. The eight most common pesticides and pesticide 

degradation products detected during the study in the South 

Platte River at Denver and near Kersey, Colorado

[n/a, not applicable; H, herbicide; I, insecticide; D, degradation product]

Pesticide Type Common 

trade name Common use

Atrazine H Aatrex Weed control primarily 

on corn crops.

Carbaryl I Sevin, 

Chipco

Insect control primarily 

on lawns, pets, livestock, and rangeland.

DCPA H Dacthal Weed control primarily 

on onion crops.

Deethylatrazine D n/a n/a

Diazinon I Basudin, 

Gardentox

Insect control primarily 

in households.

Metolachlor H Dual Weed control primarily 

on corn crops.

Prometon H Pramitol Weed control on bareground, noncropped 

areas.

Tebuthiuron H Spike, 

Graslan

Weed control on bareground, noncropped 

areas.
6

How have nutrient concentrations 

changed in the South Platte River at 

Denver and near Kersey from 1994 

through 2000?

Water samples for nutrient analyses were 

collected from the South Platte River at Denver and 

near Kersey approximately monthly from 1994 

through 2000. Nitrogen exists in natural waters as one 

of several different forms depending on the source and 

the environmental conditions. Common forms include 

nitrite, nitrate, ammonia, and organic nitrogen. 

Because nitrogen is readily converted from one form 

to another in a stream, identifying sources of nitrogen 

at a single monitoring station is difficult. However, 

nitrogen from specific sources enters streams in characteristic forms. Sources of ammonia and organic 

nitrogen include the decay of organic material such as 

plant material and animal wastes and urban and industrial disposal of sewage and organic waste. Large 

amounts of ammonia and organic nitrogen also are 

applied in agricultural and urban areas as fertilizer. 

Ammonia and organic nitrogen can be transformed by 

bacteria into nitrite and nitrate, the more mobile forms 

of nitrogen. Nitrate derived from agricultural fertilizer, 

animal waste, or decaying plant material can more 

readily infiltrate ground water, which in turn can 

discharge to streams. Nitrogen oxides discharged to 

the atmosphere by plants and the burning of fossil 

fuels are transformed to nitrate that is present in 

precipitation (Wetzel, 1983). 

Like nitrogen, phosphorus is present in natural 

waters in several different forms, primarily as orthophosphorus and organic phosphorus. Sources of orthophosphorus include weathering of natural soils and 

rocks and wastewater-treatment-plant discharge. 

Orthophosphorus also is applied in agricultural and 

urban areas as fertilizer; this form of phosphorus binds 

strongly to soil and is not very mobile on its own. 

Erosion of soil, however, may lead to high concentrations of sediment-associated phosphorus in streams. 

Orthophosphorus can be taken up by plants and 

animals and ultimately transformed into organic phosphorus. Wastewater-treatment-plant discharge also is a 

source of organic phosphorus. 

Concentrations of the different forms of 

nitrogen and phosphorus at the two monitoring 

stations from 1994 through 2000 are shown in figure 

3. During the study period, the median concentration 

of total nitrogen at the Denver site was 4.8 milligrams 

per liter (mg/L), most of which was composed of 

nitrate. The median concentration of total phosphorus 

was 0.56 mg/L, most of which was composed of 

orthophosphorus. The median concentrations of total 

nitrogen and total phosphorus were higher at the 

Kersey site: 7.4 mg/L for nitrogen and 0.75 mg/L for 

phosphorus. As with the Denver site, most of the 

nitrogen at the Kersey site was in the form of nitrate, 

and most of the phosphorus was in the form of orthophosphorus, but there also was slightly more ammonia 

at the Kersey site than at the Denver site. The higher 

nutrient concentrations at the Kersey site may have 

resulted from wastewater-treatment-plant discharges 

and agricultural fertilizer application between Denver 

and Kersey. At both sites, median total phosphorus 

concentrations exceeded the limit of 0.1 mg/L recommended by the U.S. Environmental Protection Agency 

(USEPA) for the prevention of nuisance plant growth 

in streams (U.S. Environmental Protection Agency, 

1986). During the study period, measured nitrate 

concentrations at the Denver site twice exceeded the 

maximum contaminant level of 10 mg/L established 

by the USEPA for drinking water (U.S. Environmental 

Protection Agency, 1986).

Trends are overall increases or decreases in a 

measurement such as concentration during a specified 

period. Trends in nutrient concentrations can be 

affected by BMPs or wastewater-treatment-plant 

upgrades, which are designed to achieve downward 

trends. In many cases, concentration trends also are 

affected by changes in streamflow. If a point source, 

such as discharge from a wastewater-treatment plant, 

is the dominant nutrient source to a stream, dilution

Nutrients in the South Platte River at an 

urban site near Denver and at a mixed 

urban/agricultural site near Kersey are 

derived from different sources, contributing to different nutrient concentrations 

at the two sites. Median concentrations of 

nitrogen and phosphorus in the river near 

Kersey were higher than concentrations 

near Denver during the study period.
7

from an increase in streamflow may cause a downward trend in nutrient concentrations. In contrast, if a 

nonpoint source, such as runoff from agricultural land, 

is the dominant nutrient source, increased streamflow 

from an increase in stormwater runoff may cause an 

upward trend in nutrient concentrations. Sometimes, a 

decrease in nutrient concentrations that results from a 

BMP can be offset by an increase in concentrations 

that results from higher streamflow.

Trends in nutrient concentrations in the South 

Platte River from 1994 through 2000 were obtained by 

using the Seasonal-Kendall trend test (Hirsch and 

Slack, 1984; Stevens and Sprague, 2003). These trends 

account for all factors that affect instream nutrient 

concentrations, including streamflow and BMPs. To 

help determine the influence of streamflow on trends 

in nutrient concentrations, trends in streamflow also 

were determined by using the Seasonal-Kendall trend 

test. 

Concentrations of nitrate, nitrite, ammonia, and 

orthophosphorus decreased at the Denver site from 

1994 through 2000 (table 2). Streamflow increased 

during this period, and the concentration decreases 

likely resulted from the corresponding instream dilution of nutrients being discharged from wastewatertreatment plants, the largest source of nutrients in the 

river at this location (Litke, 1996). Upgrades to 

the treatment plants designed to reduce nutrient

Figure 3. Median nitrogen and phosphorus concentrations in the South Platte River were higher in the mixed urban/agricultural area near Kersey than in the urban area at Denver from 1994 through 2000. The higher concentrations at the 

Kersey site may have been the result of wastewater-treatment-plant discharge and agricultural fertilizer application that 

occurred in the basin between Denver and Kersey.

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

CONCENTRATION, IN 

MILLIGRAMS PER LITER 

AS NITROGEN

TOTAL 

NITROGEN

NITRATE

TOTAL ORGANIC 

NITROGEN

AMMONIA

NITRITE

TOTAL 

NITROGEN

NITRATE

TOTAL ORGANIC 

NITROGEN

AMMONIA

NITRITE

South Platte River near Kersey

0

0.5

1.0

1.5

2.0

CONCENTRATION, IN 

MILLIGRAMS PER LITER 

AS PHOSPHORUS

0

0.5

1.0

1.5

2.0

CONCENTRATION, IN 

MILLIGRAMS PER LITER 

AS PHOSPHORUS

TOTAL

PHOSPHORUS

ORTHOPHOSPHORUS

TOTAL ORGANIC

PHOSPHORUS

CONCENTRATION, IN 

MILLIGRAMS PER LITER 

AS NITROGEN

South Platte River at Denver

TOTAL

PHOSPHORUS

ORTHOPHOSPHORUS

TOTAL ORGANIC 

PHOSPHORUS

Outlier value

Includes 90 percent

 of data

Includes 75 percent

 of data

Median

Includes 25 percent

 of data

Includes 10 percent 

 of data
8

concentrations in effluent also may have been a 

factor. In contrast, concentrations of suspended sediment, which reaches the river largely through erosion 

from nonpoint sources such as construction sites and 

bare-ground areas, increased during this period in 

response to increases in construction and storm runoff. 

Nutrients, particularly phosphorus, can bind to sediment and also be transported to the river during 

erosion, but the lack of a similar increase in nutrient 

concentrations suggests that nonpoint sources of nutrients were small compared to point sources.

At the Kersey site, concentrations of orthophosphorus decreased and concentrations of suspended 

sediment increased from 1994 through 2000 (table 2). 

Streamflow also increased during this period. The 

decrease in concentrations of orthophosphorus probably resulted from the instream dilution of wastewatertreatment-plant discharge, likely still one of the largest 

sources of orthophosphorus in the river at this site. 

Concentrations of suspended sediment may have 

increased during this period in response to the increase 

in streamflow and storm runoff from agricultural and 

urban areas. 

Nutrients in the South Platte River at Denver 

and near Kersey are derived from different sources, 

which contributes to different trends in nutrient 

concentrations at the two sites. Because discharge 

from wastewater-treatment plants is the largest source 

of nutrients to the river at the Denver site, instream 

dilution and treatment-plant upgrades during the study 

period would have had more of an effect on nutrient 

concentrations in the South Platte River at Denver 

than near Kersey. Farther downstream toward Kersey, 

agricultural inputs become a larger source of nutrients, 

which may have begun to offset the decreases in 

nutrient concentrations observed upstream during the 

study period.

How have pesticide concentrations 

changed in the South Platte River at 

Denver and near Kersey from 1994 

through 2000?

Water samples for pesticide analyses were 

collected during 1994 and the 4-year period from 1997 

through 2000. Because of the gap in the pesticide data 

set in 1995 and 1996, statistical trend analysis was not 

appropriate. Instead, broad changes in the pesticide 

mixture at each site were examined and compared.

The concentration of pesticides in rivers is influenced by several environmental and anthropogenic 

factors, such as the amount of precipitation, which is 

reflected in streamflow, and the pesticide application 

rates in areas that drain to the stream. Pesticide 

samples generally were collected for this study from 

May through September, the time of year when application rates and streamflow typically are elevated. 

Therefore, these pesticide concentrations are not 

representative of an entire year.

Samples from the South Platte River were 

collected bimonthly at Denver and monthly near 

Kersey. Because of the difference in sampling 

frequency, median concentrations were used to 

compare changes in the distribution of pesticides

Table 2. Trends in streamflow and concentrations of 

selected constituents in the South Platte River at Denver 

and near Kersey, Colorado, from 1994 through 2000

[NS, nonsignificant; trends were considered significant when the 

p-value was less than 0.1]

Constituent or 

property Denver Kersey

Mean monthly streamflow UP UP

Dissolved nitrate DOWN NS

Dissolved nitrite DOWN NS

Dissolved ammonia DOWN NS

Total organic nitrogen NS NS

Dissolved orthophosphorus DOWN DOWN

Total organic phosphorus NS NS

Suspended sediment UP UP

Concentrations of nitrate, nitrite, ammonia, and 

orthophosphorus decreased at the Denver site 

during the study period, and concentrations of 

orthophosphorus decreased at the Kersey site. 

These trends were related to changes in streamflow, nutrient sources, and management practices implemented in different parts of the 

basin.
9

between the two sites. To examine changes in the 

distribution of pesticides over time, the data from each 

of the years for which data were available were 

grouped. Total median pesticide concentrations were 

calculated for each site and year by summing the 

median concentration of all pesticides for which analyses were made during this study. This total should not 

be viewed as all-inclusive because not every pesticide 

used in the South Platte River Basin during the study 

period was included in the analyses for this study. The 

analysis ultimately focused on the four pesticides that 

composed the largest portion of the total median pesticide concentration at each site during each year. 

Accounting for overlap, this focus resulted in a list of 

eight pesticides (table 1). 

The largest difference between the two sites was 

the predominance of insecticides in the South Platte 

River at Denver and the predominance of herbicides in 

the South Platte River near Kersey (fig. 4). Atrazine, 

its degradation product deethylatrazine, and metolachlor composed the largest portion of the total 

median pesticide concentration near Kersey. Both atrazine and metolachlor were among the most heavily 

applied pesticides in agricultural areas of the South 

Platte River Basin (Mike Majewski, U.S. Geological 

Survey, written commun., 1997). In contrast, carbaryl 

and diazinon composed a much larger portion of the 

total median pesticide concentration at the Denver site 

compared to the Kersey site. Pesticide-use data 

currently are not available for the Denver area, so it is 

not possible to evaluate the link between urban pesticide use and the concentrations detected in the river at 

Denver. In other major river basins throughout the 

United States, insecticides such as diazinon commonly 

occur at detectable concentrations in rivers influenced 

by urban drainage (Larson and others, 1997). 

Carbaryl, prometon, and diazinon also occurred at 

measurable concentrations in the river near Kersey, 

which is indicative of the mixed urban and agricultural 

sources at this site.

In addition to broadly comparing pesticide 

mixtures at the two sites, changes in pesticide concentrations from year to year were compared. Pesticide 

concentrations in the river vary temporally because of 

changes in environmental conditions such as precipitation and streamflow and because of changes in the 

amount and type of pesticides applied. Streamflow at 

Denver is derived in part from snowmelt, which can 

dilute instream pesticide concentrations; therefore,

concentrations at Denver were dependent on the 

amount of snowmelt each year. In contrast, streamflow at Kersey primarily is derived both from snowmelt and, to a larger degree than at Denver, from 

localized storms, which either can dilute or elevate 

instream pesticide concentrations depending on the 

amount of water in storm runoff and the amount of 

pesticide carried in that water. Consequently, during 

the extreme low-flow conditions in 1994 

(fig. 5), when there was relatively little snowmelt, total 

median pesticide concentrations at both sites were 

high (fig. 4). During the less extreme conditions in 

1997 through 2000, greater amounts of snowmelt led 

to lower total median pesticide concentrations at both 

sites. Near Kersey, where more of the water came 

from storm runoff, concentrations fluctuated more 

than at Denver in response to differences in the 

number of large storms. Because pesticide-use data are 

not available, the contribution of changes in the 

amount and type of pesticides applied on the land to 

changes in the concentration of pesticides in the river 

is not known but likely was substantial. 

The mixture of pesticides also varied during the 

period of study (fig. 4). At the Denver site, the proportion of diazinon increased during the study period. 

The largest proportion of carbaryl was observed in 

1994 and the smallest proportion in 2000; however, 

the proportion varied during the intervening years. At 

the Kersey site, the proportion of DCPA decreased 

sharply from 1994 through the following years, while 

the proportion of prometon and deethylatrazine generally increased. During 1998, a large increase in the 

proportion of carbaryl also was observed. Changes at 

both sites may be related to the rate at which the pesticides are applied, their ability to bind to soil, the speed 

at which they break down in the environment, and the 

timing of a rainstorm and surface runoff after application.

The relative total median pesticide concentrations at the Denver site compared to the Kersey site

Insecticides like carbaryl and diazinon predominated at the Denver site, whereas herbicides like 

atrazine and metolachlor predominated at the 

Kersey site.
10

Figure 4. A comparison between the total median pesticide concentration and the mixture of pesticides in 

the South Platte River at Denver and near Kersey indicates that extremely low streamflow during 1994 

contributed to the highest concentrations during the study period. Insecticides like carbaryl and diazinon 

predominated at the urban Denver site, whereas herbicides like atrazine and metolachlor predominated 

at the mixed urban/agricultural Kersey site. May – September 1998 May – September 1999 May – September 1997 May – September 2000

Atrazine

Carbaryl 

EXPLANATION

0.12 μg/L

0.11 μg/L

0.089 μg/L

DCPA 

Deethylatrazine

Diazinon

Metolachlor 

Prometon

Tebuthiuron

Sum of all other constituents

Total median concentration

in micrograms per liter (μg/L)

0.37 μg/L

0.25 μg/L

0.39 μg/L

KERSEY

DENVER

1.4 μg/L

0.13 μg/L

1.2 μg/L

0.25 μg/L

May – September

1994
11

also varied between the low-flow year and subsequent 

years. During 1994, the total median pesticide concentration was slightly higher at the Denver site than at the 

Kersey site (fig. 4), possibly because the impervious 

surfaces in urban areas – parking lots and roads, for 

example – allowed less water to infiltrate into the 

ground, leading to more runoff of pesticides to the river 

even during this extremely low-flow year. In later 

years, however, the total median pesticide concentrations were higher at the Kersey site. These results 

suggest that during years with average or high streamflow, higher concentrations of pesticides reach the river 

from mixed urban/agricultural areas, whereas during 

years with low streamflow, higher concentrations of 

pesticides reach the river from urban areas. The 

reasons for this have not been well established but 

likely include differences in the amount of pesticides 

applied and changes in the amount of storm runoff and 

instream dilution in each area.

What can be done to further reduce the 

concentrations of nutrients and pesticides in the South Platte River?

Nutrients and pesticides in the South Platte 

River are a concern to humans and aquatic life. From 

1994 through 2000, some progress was made in 

reducing nutrient and pesticide concentrations, but 

increases in streamflow at times counteracted and at 

times enhanced decreases in nutrient and pesticide 

concentrations. Because the variation in streamflow is 

in part natural, additional efforts are necessary to 

ensure further reductions. The combination of urban 

and agricultural sources in the South Platte River 

Basin means that efforts to manage nutrients and pesticides in the river will need to focus both on point and 

nonpoint sources. Potential management strategies 

include further reducing point-source concentrations, 

improving and implementing BMPs aimed at controlling runoff from nonpoint sources in urban and agricultural areas, improving emissions from automobiles 

and powerplants that discharge nitrogen into the atmosphere, and reducing home and commercial use of 

fertilizers and pesticides. Implementing these strategies could lead to reductions in nutrients and pesticides reaching the South Platte River in future years.

Figure 5. Pesticide data were collected for this study from May through September in 1994, 1997, 1998, 1999, 

and 2000. Streamflow fluctuated in the South Platte River during these years – streamflow during 1994 was 

extremely low, whereas streamflow from 1997 through 2000 covered a range more typical of the region. These 

fluctuations affected instream pesticide concentrations.

EXPLANATION

0

CUBIC FEET PER SECOND

25,000

20,000

15,000

10,000

5,000

Kersey Denver Data-collection period

MAY AUG NOV FEB MAY AUG NOV FEB MAY AUG NOV FEB MAY AUG NOV FEB MAY AUG MAY AUG NOV NOV FEB MAY AU FEB G

1994 1995 1996 1997 1998 1999 2000

Median pesticide concentrations were slightly 

higher at the Denver site than at the Kersey site in 

1994. In later years, however, median pesticide 

concentrations were higher at the Kersey site. The 

highest median pesticide concentrations at both 

sites occurred in 1994, when streamflow was 

lowest. 
12

REFERENCES

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

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

D.A., 1999, The quality of our Nation’s waters—

Nutrients and pesticides: U.S. Geological Survey 

Circular 1225, 82 p.

Hirsch, R.M., and Slack, J.R., 1984, A nonparametric trend 

test for seasonal data with serial dependence: Water 

Resources Research, v. 20, no. 6, p. 727–732.

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

Pesticides in surface waters—Distribution, trends, and 

governing factors: Chelsea, Mich., Ann Arbor Press, 

373 p.

Litke, D.W., 1996, Sources and loads of nutrients in the 

South Platte River, Colorado and Nebraska, 1994–95: 

U.S. Geological Survey Water-Resources Investigations Report 96–4029, 31 p.

Litke, D.W., 1999, Review of phosphorus control measures 

in the United States and their effects on water quality: 

U.S. Geological Survey Water-Resources Investigations Report 99–4007, 38 p.

Litke, D.W., and Kimbrough, R.A., 1998, Water-quality 

assessment of the South Platte River Basin, Colorado, 

Nebraska, and Wyoming—Environmental setting 

and water quality of fixed sites, 1993–95: U.S. Geological Survey Water-Resources Investigations Report 

97–4220, 61 p.

Stevens, M.R., and Sprague, L.A., 2003, Hydrology and 

water-quality assessment of Muddy Creek and Wolford 

Mountain Reservoir near Kremmling, Colorado: U.S. 

Geological Survey Water-Resources Investigations 

Report 03–4073.

U.S. Census Bureau, 2000, Census 2000 redistricting data 

summary file: U.S. Census Bureau Technical Documentation Public Law 94–171, 223 p.

U.S. Environmental Protection Agency, 1986, Quality 

criteria for water—1986: U.S. Environmental 

Protection Agency report EPA 440/5–86–001 

[variously paged].

Wetzel, R.G., 1983, Limnology: Philadelphia, 

W.B. Saunders, 767 p.