Water Quality

Water Pollution: Agriculture
Image of a cow standing in a field with some mountains in the background. Agriculture is a common contributor to non-point-source pollution. Pesticides and fertilizers from crop fields, as well as animal wastes from feed lots, are often carried in runoff to streams. Pesticides are generally toxic and may lead to immediate health problems — even death — within the stream. Fertilizers and animal wastes, however, tend to enrich streams with large amounts of nutrients such as nitrogen and phosphorus. The immediate result of increased nutrients in a stream is increased growth of aquatic plants. While this may seem beneficial at first glance, it is actually quite harmful to the ecosystem. The overabundance of plants leads to an overabundance of plant detritus on the streambed at the end of the growing season. Microbes on the streambed must then use larger amounts of oxygen in order to decompose the increased amount of dead plant material. This leads to a depletion of the amount of dissolved oxygen in the streamwater. Macroinvertebrates and larger aquatic animals such as fish can die from a lack of oxygen. With the death of herbivorous (plant-eating) animals, the aquatic plant population will continue to increase. The use of oxygen by microbial decomposers will also continue to increase. This positive feedback process is referred to as eutrophication. Photo © 1994 philg@mit.edu

Another indicator of agricultural pollution is the presence of the microbe fecal coliform. Fecal coliform is a non-pathogenic bacteria that naturally occurs in the digestive tracks of warm-blooded animals. "Non-pathogenic" means fecal coliform does not cause diseases. However, it is often found in association with pathogenic (disease-causing) organisms.

Loose dirt, or sediments, from plowed fields is also a form of agricultural pollution. Like pesticides, fertilizers, and animal wastes, dirt from plowed fields can be carried by runoff to streams. Sediments cause increased suspended solids in streamwater. The suspended solids decrease light penetration through the water column. When the sediments finally settle to the bottom of the stream, they suffocate life there. This movement of loose dirt into streams is called siltation. The murkiness of the water is referred to as turbidity.

The problem of pesticides, fertilizers, animal wastes, and sediments in runoff can be increased by livestock grazing along the stream. Vegetation that grows near streams acts as a buffer. This vegetation absorbs toxins and nutrients and traps sediments before they reach the stream. Livestock remove the vegetation. Without a vegetation buffer, pollutants can move freely from a field into a stream. In addition, livestock increase erosion of banks along streams, and thereby increase the occurrence of siltation.

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