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Sedimentation, turbidity, pesticides, agricultural and urban waste, and fertilizer are the primary causes of surface and ground water quality problems. The threat to water quality by runoff from feedlots and other confined animal operations has increased with feedlot population increases, especially in the eastern part of the region.

Surface Water Quality

Agricultural activity is responsible for most of the polluted surface water in the region, including rivers, streams, lakes, and reservoirs (USEPA, 1990).

Table 4 compares surface water quality in the Northern Plains with surface water quality nationally. Siltation of stream beds from accelerated soil erosion, nutrient loading (primarily nitrogen and phosphorus), and pathogens from urban and agricultural waste are the primary causes of the region's surface water quality impairment. The somewhat generic term "organic enrichment," which Table 4 uses, refers to the eutrophication effects of runoff, containing nutrients from agriculture and domestic waste and applied fertilizer, when that runoff enters local and regional surface waters.

Soil erosion data can provide indicators of surface water quality. Eroded soil that reaches stream courses is detrimental to water quality through the combined effects of siltation and nutrient loading. Erosion rates from croplands in the Northern Plains are about 25% higher than for areas outside the region (Figure 9). Notably, however, erosion in the Northern Plains is dominated by wind action, which probably has a greater impact on soil fertility than on off-site surface water quality.

Figure 9. Comparison of Northern Plains region and national erosion rates (USDA/NRCS, 1994).

Groundwater Quality

Groundwater quality in the Northern Plains also affects domestic health and agricultural uses of water. Generally, shallow alluvial aquifers in the eastern portions of the region and along major drainage systems are the most vulnerable to contamination.

The region's major groundwater quality concerns are salinity and contamination of the shallow alluvial aquifers from nitrates and pesticides. The contamination level in these aquifers directly relates to the soils' leaching potential, the amount of water available for deep percolation, pesticide solubility, and nitrate availability.

Fertilizer and pesticide leaching is a primary threat to the region's groundwater quality. Fertilizer and pesticide applications have increased significantly since 1965, and they are potential sources of pollution.

Madison and Burnett (1985) showed elevated nitrate-N (less than 3 parts per million [ppm]; assumed background level) concentrations in groundwater across most of Kansas and the western part of the corn belt (Figure 10). Later, Spalding and Exner (1991) illustrated nitrate-nitrogen concentrations greater than 10 ppm along the Central Platte River. Hatfield et al. (1993) reported that at least 480,000 acres (200,000 hectares) in the Platte River Valley between Kearney and Columbus are underlain by groundwater having NO₃-N concentrations greater than 10 ppm.

Figure 10. Distribution of Nitrate-N concentrations in groundwater in the contiguous United States (Madison and Burnett, 1985).

The contaminated area is expanding by more than 9,600 acres/year (4,000 ha/year) (Hatfield et al., 1993). The contaminated areas were characterized by irrigated corn monoculture on well to excessively well drained soils with a vadose zone less than 49 feet (15m) thick.