National Water-Quality Assessment (NAWQA) Program - South Platte River Basin
Groundwater Quality in the Plains
PAST GROUNDWATER ACTIVITIES
Map and Data from Effects
of Agricultural Land Use on Groundwater Quality Study
Map and Data from Fate of Nitrate in the
Alluvial Aquifer Study
Two groundwater studies were conducted in South Platte alluvial
aquifer (groundwater adjacent to the stream) in the plains area: 1) a
large-scale study of the effects of agricultural land use on groundwater
quality, and 2) a small-scale local study nested within the land use study
on the fate of nitrate in the alluvial aquifer in the agricultural setting
of the South Platte River.
Effects of Agricultural Land Use on Groundwater
Provide a broad overview of the quality of groundwater in the irrigated agricultural areas of the plains by sampling
recently recharged groundwater in the South Platte alluvial aquifer.
Relate observed groundwater quality to overlying land use.
Sampled 30 wells located in the alluvial aquifer
that were randomly distributed throughout agricultural lands along
the South Platte River from Brighton, Colorado, to North Platte Nebraska.
Twenty-five wells were installed by USGS and five were existing wells.
The predominant crops were corn, wheat, and grains (greater than 20
percent of the surrounding land area).
- Targeted Physical and Chemical Data:
Major ions, nutrients, pesticides, volatile organic
compounds (VOCs), radon, trace elements, tritium, dissolved organic
compounds (DOC), and isotopes of nitrogen and uranium.
- Selected Results:
Nitrate concentrations in the alluvial groundwater
were higher in areas where agricultural fertilizers and manure have
been applied. The high nitrate concentrations (greater than the U.S.
Environmental Protection Agency 10 mg/L drinking water standard) have
degraded the use of groundwater as a rural drinking-water supply.
Pesticides were detected in 29 of the 30 wells sampled. Fifteen pesticides
were detected at low concentrations. Atrazine and prometon were the
most frequently detected pesticides in the shallow groundwater.
Median uranium concentrations in shallow groundwater were higher in the plains than in the mountains. High concentrations
of uranium in the basin were directly related to local geology.
Fate of Nitrate in the Alluvial Aquifer (Stream-aquifer
To describe stream-aquifer interactions and the
processes controlling the fate of nitrogen-contaminated groundwater
near areas of discharge to the South Platte River.
Monitoring wells were installed and sampled in the
alluvial aquifer along flow paths from areas of aquifer recharge to
areas of aquifer discharge to the South Platte River. Wells were nested
and sampled at the top, middle, and bottom of the aquifer.
Conducted a series of seepage runs to quantify contribution of groundwater to stream.
Sampled both groundwater and surface water.
Sampled one round in 1992, two rounds in 1993, and
two rounds in 1994
- Targeted Physical and Chemical Data
Major ions, nutrients, dissolved gasses, stable
isotopes of carbon, nitrogen, and sulfur, chlorofluorocarbons (compounds
used for age dating), tritium, and dissolved organic carbon (DOC).
- Selected Results
Nitrate-contaminated groundwater generally could
not be avoided by drilling deeper wells because nitrate concentrations
typically were similar throughout the depth of the alluvial aquifer,
most likely due to mixing of water in the aquifer caused by irrigation
pumpage. Mixing was documented by age-dating of the groundwater,
which showed small differences between the top and bottom of the aquifer.
Nitrate concentrations in surface water were smaller than in groundwater in part because microbial activity in streambed sediments removed
a substantial portion of the nitrate from incoming groundwater return