Water Quality in the Lower Susquehanna River Basin, Pennsylvania and Maryland, 1992-95

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Major issues and findings --
Pesticides in ground water and streams

Wells and streams in the Lower Susquehanna River Basin Study Unit were sampled and the waters analyzed for many of the most commonly used pesticides in the United States. The pesticides analyzed for are soluble in water. The samples were collected to provide information on the spatial distribution of pesticides in ground water and streams, as well as the seasonal patterns of pesticides in streams (Breen and others, 1995; Hainly and Kahn, 1996; Hainly and Bickford, 1997).

Concentrations of pesticides in water from the wells and streams sampled rarely exceeded levels established as drinking-water standards. Only 10 of the measured concentrations in untreated waters from streams exceeded a level established as a drinking-water standards. Seasonal factors, such as storm runoff of pesticides during the major application period in the spring, contribute to high concentrations of pesticides in streams. Very few storm samples were collected for this study; however, 8 of the 10 exceedances were measured in storm-affected samples. More work would be needed to understand fully the high-concentration pulses of pesticides in stormflow. None of the samples collected from household-supply wells had concentrations that exceeded drinking-water standards.

Photos: agricultural and residential pesticide application (48,872 bytes)

The timing and rate of agricultural pesticide applications were important factors in describing the seasonal and spatial concentration patterns detected in ground water and streams.

Indicators of residential, commercial, and industrial pesticide use helped to explain concentration pattern of pesticides not used extensively for agricultural purposes.

Guidelines for protection of aquatic life were exceeded in samples from nine streams. Concentrations of malathion, chlorpyrifos, and methyl-azinphos exceeded guidelines for protection of aquatic life (U.S. Environmental Protection Agency, 1991). Although no guidelines have been established in the United States for atrazine, cyanazine, and metolachlor, these pesticides exceeded the Canadian guidelines for protection of aquatic life (Canadian Council of Resource and Environment Ministers, 1996).

Bar chart: Detection rates (top six: atrazine, (desethyl atrazine), metolachlor, simazine, prometon, alachlor, cyanazine) (18,463 bytes)

The six most frequently detected pesticides were the same for water from wells and water from streams.

Although drinking-water standards, human-health advisory levels, and aquatic-life criteria were rarely exceeded, these criteria have not been established for many of the pesticides that were sampled for. In addition, mixtures and degradation products were not considered in developing the human-health criteria. Only a limited range of the potential effects of pesticides in drinking water has been assessed. Therefore, it is important to evaluate pesticide occurrence and trends even though current standards were rarely exceeded.

Spatial Distribution of Pesticide Concentrations

Pesticides were detected frequently in ground water and streams; usually, more than one pesticide was detected at a time. Spatial and seasonal patterns of pesticide concentrations were documented using 577 samples that were tested for 47 herbicides or insecticides. A subset of the stream-water samples and all ground-water samples were tested for an additional 38 pesticides. In total, nearly 40,000 analyses of concentrations for individual pesticides were made on waters from 155 stream sites and 169 shallow wells from 1993 to 1995. For this study, shallow wells were defined as those less than 200 ft deep.

Bar chart: Detection rate in ground water, and leaching potential (26,904 bytes)

In water from wells in areas of limestone bedrock, where water easily infiltrates into the aquifer, pesticides with high leaching potential, such as atrazine, were almost always detected. Less widely used pesticides with low leaching potential, such as cyanazine, were rarely detected in these same areas. In areas of sandstone and shale, where the infiltration is poor, even pesticides with high leaching potential and high use, such as atrazine, were rarely detected in water from wells.

In more than 90 percent of the samples collected, at least one pesticide was detected, and two or more pesticides per sample were frequently detected. More than 60 percent of well-water samples in which pesticides were present contained more than one detectable pesticide. In general, measured concentrations of individual pesticides were very low; only 22 samples of stream water and 2 samples of ground water had concentrations that exceeded 2µg/L (micrograms per liter, or parts per billion).

The most commonly detected pesticides were the herbicides used primarily on corn: atrazine, metolachlor, simazine, prometon, alachlor, and cyanazine (see graph on page 12). Metolachlor and atrazine are the two most used agricultural pesticides in the Study Unit. Atrazine was detected in 98 percent of the stream samples and in 74 percent of the ground-water samples. Desethylatrazine (a breakdown product of atrazine) was usually detected with atrazine. Metolachlor was detected in 95 percent of the stream samples and in 53 percent of the ground-water samples. Nearly half of all the pesticides analyzed for were not detected in any sample. Of the 45 pesticides that were detected at least once, only 5 were detected in more than half of the samples collected.

Average annual pesticide use for agricultural purposes and nonagricultural (residential, commercial, and industrial) pesticide-use indicators were related to patterns of pesticide concentrations in waters from wells and streams. The timing and rate of agricultural pesticide applications for the high-use pesticides described a major part of the seasonal concentration patterns observed in water from streams and the spatial patterns observed in water from streams and wells. Indicators of nonagricultural use helped to explain concentration patterns of pesticides not used extensively in agriculture.

Detections of pesticides were related to pesticide use, pesticide-leaching potential, and bedrock type. Pesticides were most likely to be detected in samples from agricultural and urban areas. Limestone areas were far more likely to have pesticides in well water than areas underlain by sandstone and shale. Bedrock type influences the movement and discharge of ground water and affects spatial patterns of pesticide concentrations, as shown in the graph to the left and in the figure below. Some commonly used pesticides with low leaching potential (low potential to infiltrate into the ground with the water), such as alachlor and cyanazine, were detected in streams more often than in wells because they are more likely to be transported in surface runoff.

Cross-sectional schematic (12,842 bytes)

The amount of water that runs off the land surface or infiltrates into the ground depends on factors such as slope and how easily water can flow through the soil and bedrock material. In areas of limestone bedrock, where water can readily infiltrate into the ground, pesticides were commonly detected in ground water. The same pesticides detected in ground water were usually detected in streams during times when the flow in the streams is sustained by flow from ground water. In areas of sandstone and shale, where water does not easily flow through the soil and bedrock material, the easiest pathway for the water is to run off over the land. Pesticides were rarely detected in ground water in these areas.

To help understand how differences in bedrock type control concentrations of highly soluble pesticides in stream base flow, Study Unit personnel compared atrazine concentrations in streams during the dry times of the year, when the flow is low and dominated by ground water (base flow), to concentrations in ground water from shallow wells. Results differed considerably between limestone systems and non-limestone systems. In subunits with limestone bedrock, atrazine concentrations in waters from streams and shallow wells were similar, indicating a system of ground-water flow through large fractures and springs to the streams. In the sandstone and shale subunit, atrazine concentrations in well waters were lower than concentrations in the streams, indicating that water reaching the stream may be flowing from the aquifer to the stream through a system of fractures in a shallower layer of the aquifer than the layer penetrated by the wells. The graphs to the right illustrate the differences between pesticide concentrations in streams in limestone settings and sandstone and shale settings.

Temporal Variation in Pesticide Concentrations in Streams

Seasonal variations in pesticide concentrations in water from streams are affected by the timing of pesticide application and the type of bedrock. The highest concentrations of pesticides were seasonal pulses lasting up to several months (see graph at right). Peak concentrations were smaller in a limestone stream compared with a stream in a sandstone and shale area. Elevated concentrations in streams were related to the seasonality of agricultural-use applications. The seasonal variations in climate also were an important factor in explaining seasonal patterns.

Mill Creek, a limestone stream, shows a slight rise in atrazine concentration after the major application period because some atrazine is in runoff. Some atrazine infiltrates into the ground water and provides constant levels of atrazine to the stream for the rest of the year. This is a limestone stream pattern. Levels of atrazine remain between 0.1 and 0.2 µg/L because ground water provides most of the water to the stream.

The pattern at East Mahantango Creek, a stream in an area of sandstone and shale, shows a pulse or increase in atrazine concentration after application, followed by lower concentrations throughout the rest of the year. Topography and soils in this basin favor runoff of atrazine over leaching to the ground water. The levels of atrazine after the application period when the streamflow is supplied from ground water are lower in East Mahantango Creek than in Mill Creek

Plots: Streamflow, atrazine concentrations, and major application period (22,075 bytes)

In limestone streams, such as Mill Creek, atrazine concentrations were nearly constant year round. In streams in sandstone and shale areas, such as East Mahantango Creek, atrazine concentrations increased significantly after the major pesticide application period.

Pesticide Concentrations in the Susquehanna River

Concentrations of pesticides in the Susquehanna River were generally less than 1 µg/L. Analyses of 11 water samples from the Susquehanna River at Harrisburg from June 1994 to August 1995 indicated that mixtures of pesticides and their degradation products were frequently present in the river but at concentrations generally less than 1 µg/L (see table below). The pesticides detected at this site were similar to those detected in water from streams in agricultural areas throughout the Lower Susquehanna River Basin.

Pesticides were not detected in samples collected during synoptic studies at the main-stem Susquehanna River at Danville and were detected in low concentrations in the West Branch Susquehanna River at Lewisburg (upstream from the Lower Susquehanna River Basin). This pattern indicates that the pesticides present in the Susquehanna River at Harrisburg are most likely introduced by the tributaries in the Lower Susquehanna River Basin.


Pesticide concentrations in the Susquehanna River at Harrisburg, 1994-95, were generally less than (<) 1 microgram per liter.


Minimum concentration

Maximum concentration

Median concentration

micrograms per liter (parts per billion)









Atrazine, desethyl





















U.S. Geological Survey Circular 1168

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Suggested citation:
Lindsey, B.D., Breen, K.J., Bilger, M.D., and Brightbill, R.A., 1998, Water Quality in the Lower Susquehanna River Basin, Pennsylvania and Maryland, 1992-95: U.S. Geological Survey Circular 1168, on line at <URL:>, updated June 22, 1998 .

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Last modified: Thu Sep 3 16:54:36 1998