Non-point source pollution is cumulative in nature. While any single contributor of non-point source contamination may be insignificant, the cumulative effect of many such sources is measurable and leads to significant pollution of ground and/or surface waters. Surface and groundwaters are interrelated. Groundwater is surface water (lakes, rivers, streams, or overland flow from precipitation) that has percolated into and then through the ground to an aquifer. Groundwater may move back into surface water bodies through seepage, springs, or base flow into a river or lake depending on the geology of an area. Contaminated groundwater can move into uncontaminated aquifers or return to surface water, depending on the geology. Section 319 of the Federal Clean Water Act mandates development of programs for control and reduction of non-point source pollution of both surface and ground water.
Non-point source contamination comes from run off both agricultural and urban as well as other small sources such as septic and AOSS. Agriculture is reported to be one or the main non-point sources of water pollution and in studies done in the Chesapeake Bay Watershed and Sacramento River Delta and other locations the contamination from agriculture runoff has been the major source of contamination. Pesticide runoff is a large contributor of known pollutants to the watersheds and may be a significant contributor of endocrine disruptors to the freshwater supply. Both rain feed and irrigated agriculture are sources of contamination of fresh water. In April of 2009 the US EPA issued the Final List of Initial Pesticide Active Ingredients and Pesticide Inert Ingredients to be Screened Under the Federal Food, Drug, and Cosmetic Act as potential endocrine disruptors. These pesticides need to be further investigated and our use of pesticides reexamined and rethought.
In rain fed agricultural land, the precipitation washes agricultural chemicals (pesticides and herbicides) along with soil sediment to surface water. In addition, irrigation of the fields can increase the run off. Other sources of non-point souce contamination are confined animal feed lots, grazing, plowing, pesticide spraying , fertilizing, planting and harvesting a crop which can all contribute to run off of contaminants and sediment. The National Water Quality Inventory Report to Congress was intended to identify widespread water quality problems of national significance. This has served as a proxy for the quality of the waters of the nation despite a non systematic approach to identifying water quality by the states and significant limitations to the substances tested for. Many states target their limited monitoring resources to waters they suspect are impaired and, therefore, assess only a small percentage of their waters. These may not reflect conditions in state waters as a whole and tend to reflect areas of concern in the “water community.” The US-EPA in its last report to Congress identified agriculture as the leading cause of water quality impairment of rivers and lakes in the United States. Agriculture is also cited as a leading cause of groundwater pollution in the United States.
There are other significant environmental impacts from agriculture. When agricultural land is irrigated, the water balance in nature is altered. Water is withdrawn from a river, spring, or groundwater and added to agricultural fields. The environmental impact of an irrigation system is dependent on the nature of the water source, the quality of water, the method of delivery and the local geology and climate. Withdrawing ground water beyond the recharge rate may cause the land to subside as happened in the Central Valley of California. Aquifers may become saline. All water contains dissolved salts that attached to the water molecules as it washed over the land or percolated in the ground. Rain also contains some salts. The salts are generally at very low concentrations in “fresh” water’ however, evaporation of water from dry earth leaves much of the salts behind. Over time the salts concentrate. The problem is acute in the Central Valley of California, in China’s North Plain, in Soviet Central Asia (the –istans), parts of the Middle East and the Colorado River Basin. These are all semi-arid areas where irrigation is the basis of agriculture.
Withdrawing both groundwater and surface water can dramatically change the natural hydrology of rivers and water streams, water temperature, and can impact the aquatic ecosystem associated with the surface water. The San Joaquin River in California has been dewatered as has been Owens Lake. The Colorado River runs dry before it reaches the Ocean most years as does the Yellow River in China. The riparian ecosystems and delta estuaries associated with these areas no longer receive fresh water recharge and have been destroyed.
However, irrigation has vastly improved crop yields in many semi-arid climates. As population grows, and the demand for food increases irrigation is unlikely to be discontinued. Methods and control of irrigation can determine the extent of the environmental impact from the irrigation. Improved field irrigation practices are critical to limiting the impact. It is reported that irrigated agricultural land is two and a half times more productive than rain fed agricultural land and the limits of irrigation really are the fresh water resources, the capital costs and the saline buildup over time in the farm land and aquifers. To feed the populations of the earth and protect the earth mankind needs to utilize intelligent and environmentally balanced farming practices employed .