National Water-Quality Assessment (NAWQA) Program - South Platte River Basin
South Platte Basin Information
(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
- 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 groundwater 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.
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
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
groundwater from the alluvial aquifer in the plains, whereas median
radon concentrations were highest in groundwater 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?
- Groundwater quality in the forested mountain areas
generally was better than in other land-use areas. However, the potential
for degraded groundwater quality is likely to increase as mountain
- 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 groundwater 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 groundwater 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 groundwater 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 groundwater 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 groundwater,
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
- Nitrate concentrations in shallow groundwater
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 groundwater as a rural drinking-water supply. Nitrate
concentrations in surface water were smaller than in groundwater because
microbial activity in streambed sediments removes a substantial portion
of the nitrate from the incoming groundwater 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 (U.S. Environmental Protection Agency, 1986,
Quality criteria for use, 1986: U.S. Environmental Protection Agency Report
440 14-86-001, [variously paged]).
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
- 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 groundwater 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 groundwater and salinity and suspended sediment in surface water.
- Water quality in the groundwater 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.