Reuse of surface and ground water for irrigation has resulted in increased salinity in the lower South Platte River and surrounding alluvial aquifer. High salinity can be detrimental to irrigation and drinking-water supplies.
Stream water having high concentrations of nitrate can be stored in agricultural reservoirs, where nitrate concentrations decrease during the summer; thus, reservoirs could be part of a nitrate management strategy.
- Lowest concentrations and fewest number of these compounds were measured in forested mountains or rangeland sites; highest concentrations and greatest number of compounds were measured in urban and mixed (urban/agriculture) areas.
A habitat degradation index (HDI) and an index of biotic integrity (IBI), indicates that most sites were moderately degraded and that human activities and local site characteristics had a greater effect on habitat and fish communities than basin-scale characteristics such as land use.
The relative abundance of families of fish were altered and the number of invertebrate taxa were lower in mining-affected sites and urban, agricultural and mixed land-use settings compared to minimally affected areas such as forest and rangeland.
Median uranium concentrations were highest in shallow ground water from the alluvial aquifer in the plains, whereas, median radon concentrations were highest in ground water from the crystalline aquifer in the forested mountain areas.
There currently are no Federal drinking-water standards for uranium or radon, although standards for both are under review by USEPA. Naturally occurring concentrations of both these elements were high compared to proposed and/or historical standards in most areas of the basin.
Ground-water quality in the forested mountain areas generally was better than in other land-use areas. However, the potential for degraded ground-water quality is likely to increase as mountain communities grow.
Surface-water quality generally was better in forested mountain areas than in other land uses. Development in mountain drainages correlated with elevated concentrations of dissolved solids, suspended sediment, and nutrients in surface water.
Pesticides were frequently detected in surface water and shallow ground water in the urban setting. However, pesticide concentrations were generally low, with only seven pesticides having median values above the method detection limit.
Nutrient concentrations in urban streams have many potential sources and generally were highest immediately downstream from wastewater treatment plants. Nutrient concentrations in streams did not exceed any existing USEPA drinking-water standards. However, three ground-water samples from the alluvial aquifer had nitrate concentrations that exceeded the drinking-water standard.
Banned compounds, such as PCBs and chlordane, widely used historically in the urban setting, were detected in fish tissue. Stream-channel modifications for flood control and bank stabilization in the urban setting have altered the habitat available to biological communities.
Pesticides were detected in ground water, surface water, fish tissue, and bed sediment. Several pesticides were detected throughout the agricultural areas of the basin during the growing season. Although concentrations of individual pesticides generally were low compared to established criteria, the ecological and human-health effects of long-term exposure to pesticide mixtures are unknown.
Nitrate concentrations in alluvial ground water were high in areas where agricultural fertilizers and manure were applied. These high nitrate concentrations have degraded the use of ground water as a rural drinking-water supply. Nitrate concentrations in surface water were smaller than in ground water because microbial activity in streambed sediments removes a substantial portion of the nitrate from the incoming ground-water return flows.
The average number of pesticides detected in surface-water samples from the South Platte River were greater in mixed land-use areas than in land-use settings that were exclusively urban or agriculture.
Federally banned compounds, DDT and dieldrin, were detected more often and at higher concentrations in bed sediment and fish tissue collected from mixed land-use sites than in urban or agricultural settings.
Because of the cumulative effects of contaminants from point and nonpoint sources and habitat alteration, the number of aquatic invertebrate taxa generally was lower in mixed land-use areas than in either urban or agricultural land-use areas alone, and the IBI for fish indicated moderately to significantly degraded conditions.
Water quality in the ground water beneath urban areas and surface water within mixed land-use areas are degraded, as indicated by the highest relative ranking for VOCs and organochlorines and PCBs in fish within the basin, respectively.