PHYSICAL SETTING
Location
Climate
Land Use

WATER QUALITY
(Results from 1992-1995)
A summary of major issues and findings from the first decade of NAWQA is provided below. A more detailed discussion of water quality in the South Platte River Basin is presented in Dennehy and others (1998).

How has water development affected water quality in the South Platte River Basin?

Does land use contribute to contaminant inputs and affect habitat characteristics and biological communities in streams?

What factors control the occurrence of uranium and radon, and are the reported concentrations a concern?

Have mining and residential development in the mountains affected water quality?

Does urban land use affect water quality?

Have agricultural chemicals affected water quality?

How do discharges from permitted municipal wastewater treatment plants affect nutrient levels in streams?

What are the cumulative effects of mixed (urban/agriculture) land use on water quality?

What is the relative status of water quality by land use in the South Platte River Basin?

WATER QUALITY
How has water development affected water quality in the South Platte River Basin?

• Withdrawals of large volumes of water from streams in the basin for agricultural and urban use resulted in less water to dilute contaminants in streams.

• Alteration of the natural flow regime has degraded native aquatic habitat along streams.

• Ground water (subsurface irrigation return flow) was a major nonpoint source of nitrate, dissolved solids, and pesticides (atrazine and prometon) in the lower reaches of the South Platte River.

• 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.

• Storage of stream water with high concentration of nitrate in agricultural reservoirs decreased nitrate concentrations during the summer.

Does land use contribute to contaminant inputs and affect habitat characteristics and biological communities in streams?

• Concentrations of contaminants, such as organochlorine pesticides and polychlorinated biphenyls (PCBs) in bed sediment and whole fish tissue were related to land use.

• Lowest concentrations and fewest number of these compounds have been measured in forested mountains or rangeland sites; highest concentrations and greatest number of compounds have been measured in urban and mixed (urban/agriculture) areas.

• A habitat degradation index (HDI) and an index of biotic integrity (IBI) indicated that most NAWQA monitoring 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 insects, snails, crayfish were lower in mining-affected sites and urban, agricultural and mixed land-use settings compared to minimally affected areas such as forest and rangeland.

What factors control the occurrence of uranium and radon, and are the reported concentrations a concern?

• High concentrations of uranium and radon in the South Platte River Basin were directly related to local geology.

• 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.

• Currently there are no Federal drinking-water standards for uranium or radon, although standards for both are under review by U.S. Environmental Protection Agency. Naturally occurring concentrations of both these elements are high compared to proposed and/or historical standards in most areas monitored in the basin.

Have mining and residential development in the mountains affected water quality?

• 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.

• Volatile organic compounds (VOCs) were detected in ground water in forested mountain areas. Ground water in crystalline bedrock is susceptible to contamination from surface sources.

• Bed sediments in forested mountain streams affected by mining or development had the highest concentrations of trace elements in mountainous areas.

• Biological communities were less diverse and had fewer fish species in tributaries affected by mining or development compared to undeveloped mountain streams.

Does urban land use affect water quality?

• Pesticides frequently have been detected in surface water and shallow ground water in the urban setting. However, pesticide concentrations were generally low, with only seven pesticides having median concentrations above the method detection limit.

• Volatile organic compounds (VOCs), derived from gasoline and cleaning solvents, were detected in 86 percent of shallow urban ground-water samples.

• Nutrient concentrations in urban streams have many potential sources and generally were highest immediately downstream from wastewater treatment plants. Nutrient concentrations in streams have not been found to exceed any existing U.S. Environmental Protection Agency drinking-water standards. However; three ground-water samples from the alluvial aquifer nitrate concentrations exceeded the drinking-water standard (10 mg/L).

• Banned compounds, such as polychlorinated biphenyls (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.

Have agricultural chemicals affected water quality?

• Pesticides have been detected in ground water, surface water, fish tissue, and bed sediment. Several pesticides have been 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 shallow ground water adjacent to the stream were high in areas where agricultural fertilizers and manure have been 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.

How do discharges from permitted municipal wastewater treatment plants affect nutrient levels in streams?

• Treated wastewater effluent can account for as much as 100 percent of streamflow downstream from Denver and was the primary source of nitrate, ammonia, and phosphorus to Front Range streams.

• Total nitrogen concentrations in streams along the Front Range urban corridor increased substantially downstream from wastewater treatment plants.

• Phosphorus concentrations in the South Platte River from Denver to Balzac, Colo., (150 mile length of river) have been found to be higher than generally accepted limits for control of eutrophication (USEPA, 1986).

What are the cumulative effects of mixed (urban/agriculture) land use on water quality?

• The average number of pesticides detected in surface-water samples from the South Platte River was greater in mixed land-use areas than in land-use settings that are 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 in mixed land-use settings 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 Index of Biotic Integrity (IBI) for fish indicates moderately to significantly degraded conditions.

What is the relative status of water quality by land use in the South Platte River Basin?

• Water quality in surface and ground water in the forested mountain region of the basin generally was of good quality and was relatively unaffected by humans.

• In contrast, water quality in the agricultural areas of the basin was the most degraded, primarily from nitrate and salinity in ground water and salinity and suspended sediment in surface water.

• Water quality in the ground water beneath urban areas was degraded as indicated by the highest concentration of volatile organic compounds (VOCs) and surface water within mixed land-use areas was degraded, as indicated by organochlorines and polychlorinated biphenyls (PCBs) in fish within the basin.