Water Quality in the Central Nebraska Basins, Nebraska, 1992-95

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Major Issues and Findings --
Agricultural Activities Potentially Affect the Management of Public Water Supplies

Contamination of surface and ground water from nonpoint sources is a national issue. Examples of nonpoint-source contaminants from agricultural activities are pesticides (fungicides, herbicides, and insecticides), sediment, nutrients (nitrogen and phosphorus), and fecal bacteria [8]. Of the many possible contaminants, pesticides receive the most attention because of the potential toxicity to aquatic life and humans. Pesticides often are used to increase crop yields and values. Herbicides prevent or inhibit the growth of weeds that compete for nutrients and moisture needed by the crops. Herbicides are applied before, during, or following planting. Herbicides also are used for weed control in urban areas, often with large rates of application.

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Row-crop agriculture dominates much of the Central Nebraska Basins Study Unit.

Alachlor, atrazine, cyanazine, and metolachlor, which are referred to as organonitrogen herbicides, were the four most commonly applied herbicides for corn, sorghum, and soybean production in the Central Nebraska Basins Study Unit. Of all pesticides used in the Study Unit, atrazine was the most extensively applied.

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Atrazine concentrations in the Platte River are a function of the timing and intensity of rainfall with respect to the application of the herbicide.

Insecticides are used to protect the crop seeds in storage prior to planting and also to protect the plants once the seeds germinate. Like herbicides, insecticides are used in urban areas to protect property, including buildings, lawns, trees, and ornamental shrubs.

The temporal distributions of alachlor, atrazine, cyanazine, and metolachlor and the concentrations of each were defined on the basis of analyses of water samples collected from the Platte River at Louisville from September 1, 1991, through August 31, 1992 [ 9 , 10 ] . These data were collected more often when the temporal variability of concentrations was expected to be large and less often when variability was expected to be small. The graph shows the temporal distribution of the concentrations of atrazine in relation to streamflow. Although the graph shows only how atrazine concentrations vary with time, concentrations of alachlor, cyanazine, and metolachlor varied in a similar manner. Concentrations of atrazine were larger in the spring and summer, when nearly all atrazine is applied, and smaller in the fall and winter. During this period of data collection, the largest herbicide concentrations were measured during the growing season following intense rainfall shortly after the herbicide had been applied [ 11 ] . Herbicide concentrations peaked in May and then generally declined in the months that followed [ 9 ] . For example, atrazine concentrations in May ranged from 0.17 to 30 micrograms per liter (µg/L), and mean daily streamflows ranged from 3,750 to 18,100 cubic feet per second (ft³/s). In contrast, atrazine concentrations in February ranged from 0.08 to 0.10 µg/L, and mean daily streamflows ranged from 5,110 to 8,150 ft³/s. The decrease in quantity of atrazine available for entrainment in surface runoff is a result of factors such as volatilization into the atmosphere as a result of high land-surface temperatures, photo decomposition and bacterial degradation, adsorption onto soil particles, and uptake by plants.

Water samples from the Platte River at Louisville site were analyzed for 46 pesticides.

Of the 46 pesticides, it appears that only atrazine, cyanazine, and alachlor may at times pose potential concern for public water supplies withdrawn downstream from the confluence of the Elkhorn River with the Platte River [12]. Presently, the USEPA has established only a Health Advisory Level (HAL) for cyanazine and not an MCL; a HAL is a guideline whereas an MCL is enforceable. Thus, based on current MCLs, atrazine is the herbicide most likely to exceed the MCL at the Platte River at Louisville. However, the compliance of a public water-supply system relative to an MCL or a HAL for a pesticide is based on an average annual concentration. Therefore, one or more exceedances of the specified value does not necessarily indicate noncompliance. Also the MCL or HAL for a pesticide applies only to the pesticide and not to any of its degradation products.

Alachlor, atrazine, and cyanazine pose potential concerns for drinking-water supplies.

An analysis of these four herbicides at the Platte River at Louisville site indicated that the source of the herbicides was almost entirely in the Study Unit and most was contributed by the Elkhorn River [12]. Most herbicide movement in streams occurred during the growing season (May through August).

Lincoln, Omaha (outside the Study Unit), and smaller cities along the Platte River withdraw water from the adjacent alluvial aquifer for public supplies. The aquifer adjacent has a direct hydraulic connection to the river and thus is affected appreciably by the quantity and quality of water in the river [13, 8].

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Atrazine movement from the Platte River through alluvial aquifers can be induced by ground-water withdrawals for drinking water.

Studies have shown that conventional water treatment is ineffective in removing organonitrogen herbicides such as alachlor, atrazine, cyanazine, and metolachlor from finished drinking water. These herbicides remain in solution, in contrast to many other contaminants that are more easily removed by conventional treatment processes, such as coagulation and sand filtration.

--John K. Stamer

U.S. Geological Survey Circular 1163

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