Agricultural operations can be the source of non-point pollution in surface water. The major causes of surface water pollution associated with farming and ranching are sediment and nutrients.
Sediment
Soil erosion and resulting sedimentation is the leading cause of surface water pollution. Siltation is the leading cause of water quality problems in rivers and is the fourth leading cause in lakes and estuaries (1992 EPA 305[b] report). Although soil erosion is a natural process, it can be greatly accelerated by human activities such as farming.
Soil erosion is not only detrimental to agricultural production, but it also:
- fills road ditches and reservoirs
- blocks navigational channels
- negatively impacts aquatic plants and animals
- reduces recreational opportunities
- increases cost to municipal water supplies, and
- can endanger human health.
Major sources of sediment include runoff from agriculture (cropland), forestry and urban/suburban development.
Nutrients
Nutrients, especially nitrates and phosphates, can create significant water quality problems in both surface and ground water.
Surface Water
About 37% of the assessed river miles in the U.S. are impaired by nutrients (1992 EPA 305(b) report). Phosphates, and to a lesser extent nitrogen, can cause excessive growth of aquatic plants (eutrophication) in rivers and lakes. Eutrophication can be natural or human-induced. Agricultural sources of nutrients come from either fertilizer runoff or nutrients attached to eroded sediments. In most instances, the rate of human-induced eutrophication is many times faster than the natural process. For example, in a span of about 25 years (1950-1975), Lake Erie aged to about the same degree under human influences as would have occurred in 15,000 years naturally. Eutrophication can result in:
- reduced water clarity
- infestations of floating plants and/or algal mats
- low dissolved oxygen levels and resulting fish kills
- reduced aesthetic quality of water, clogged pipes and intakes, reduced property values and recreational opportunities
- toxicity due to release of poisonous gases by micro-organisms
- toxicity due to secretions from some blue-green or dinoflagellate algae blooms
- loss of sensitive species, such as sports fish, reducing species diversity
- changes in the color, taste and odor of drinking water supplies
- increased cost of water treatment
Major sources of nutrients in surface water include municipal wastewater, agriculture (crop and livestock production), forestry, urban/suburban development including landscapes, and recreation.
Ground Water
The most serious nutrient adversely impacting ground water supplies in the U.S is nitrates. Nitrates have been found above 2.0 parts per million (ppm) in 57% of rural domestic wells, although, only 2.4% of the wells had nitrates above the drinking water standard of 10 ppm NO3-N (1990 EPA National Pesticide Survey). Nitrates are naturally occurring in ground water, but concentrations can be greatly increased by human activities. High levels of nitrates can cause:
- health hazards and even death in human infants and older adults
- increased cost of water treatment,
- and health risk to domestic livestock at levels above 40 ppm NO3-N
Major sources of nutrients in ground water include agriculture (crop and livestock production), urban/suburban landscapes, rural septic systems and land applications of waste.