In the world of water resource management, the industry of agriculture is always noted for the potential problems such as fecal contamination, which can lead to microbial contamination of drinking waters. However, a growing concern is also the use of pesticides. Agriculture is a major user of pesticides. Most crops grown in the U.S. receive some pesticide application. For most U.S. crops, for example, 85% of all acreage is herbicide-treated, representing some 220 million acres treated with pesticides. Detailed data on agricultural pesticide use is available from a variety of sources, including the National Agricultural Statistics Service (NASS): Agricultural Chemical Use Database and the National Center for Food & Agricultural Policy's (NCFAP) National Pesticide Use Database.
Both of these sources provide information about pesticide use that can be searched by active ingredient, crop type, or state. The EPA estimates that 70 percent of all pesticides used in the U.S. are used in agriculture. On a national scale, pesticide applications are clearly associated with intensive agriculture. In 1992, the heaviest applications by pounds of pesticides applied to major agricultural crops occurred in the north-central Mississippi River Basin and in the southeast.
There is no doubt that major benefits have resulted from the use of pesticides in U.S. agriculture. Significant yield increases, economic savings for growers and consumers, and reduced soil erosion have been attributed to the widespread use of herbicides as explained in the National Center for Food and Agricultural Policy's summary The Value of Herbicides in U.S. Crop Production. Without herbicides, production costs for alternative weed control could exceed 14 billion annually, more than double what the nation's growers spend on herbicides applications. For most crops, yields without herbicide use would be reduced significantly. Substantial reductions in cropland erosion have been accomplished through the use of conservation tillage, which usually relies on herbicide applications. Without herbicide use, reduced tillage would become difficult or impossible, leading to more than 150 million tons of increased soil erosion annually.
These benefits have not come without cost. The U.S. Department of Agriculture has evaluated the potential for runoff and leaching losses of pesticides from agricultural land as a function of both pesticide application and land and soil factors. Clearly, dissolved pesticide runoff losses are a potential risk in the north-central Mississippi River Basin. Leaching losses from farm fields are potentially high throughout the central U.S. and in much of the southeast. Water quality data clearly show that pesticides have reached surface and groundwater. U.S. Geological Survey investigations have revealed widespread contamination of the nation's water resources by pesticides. Concentrations of herbicides and insecticides in agricultural streams, and in most rivers in agricultural regions, were highest in those areas of the nation with the greatest agricultural use. Herbicide concentrations were greatest in central U.S. streams, where use is most extensive. A relatively small number of heavily used compounds accounts for most detections.
The most frequently detected pesticide compounds in agricultural areas were the major herbicides atrazine metolachlor, cyanazine, and alachlor, ranked first, second, fourth, and fifth in national herbicide use for agriculture. In most agricultural areas, the highest levels of pesticides occur as seasonal pulses-usually during spring and summer-lasting from a few weeks to several months during and following high-use periods. Total pesticide concentrations in streams draining urban areas are generally lower than in agricultural areas, but seasonal pulses last longer and the concentrations are more dominated by insecticides. Insecticide concentrations were highest in urban streams. Erosion caused by agriculture is also a major concern for the management of water sources.
Why? Erosion by both water and wind can be severe when bare soil is exposed and unprotected by vegetation. This is particularly true on steep slopes where runoff water can concentrate and flow straight downhill. All land-disturbing activities can cause erosion problems, including forest management, construction, urban areas, highways, and surface mining. Agricultural erosion is a major sediment source because of the large area involved and the repeated land-disturbing effects of cultivation and grazing.
Researchers estimate that sediments carried to the oceans by the world's rivers increased from 10 billion tons per year before the introduction of intensive agriculture to 25-50 billion tons per year thereafter. Of the 75 billion tons of soil eroded worldwide each year, about two-thirds are believed to come from agricultural land. In 1995, 4 billion tons of soil was estimated to be lost each year from 395 million acres of U.S agricultural land. This translates into a total economic loss of more than $44 billion each year, of which $27 billion represents a reduction in soil productivity.