Ground-Water Quality in the Plains

Map and Data from Effects of Agricultural Land Use on Ground-water Quality Study
Map and Data from Fate of Nitrate in the Alluvial Aquifer Study

Two ground water studies were conducted in South Platte alluvial aquifer (ground water adjacent to the stream) in the plains area: 1) a large-scale study of the effects of agricultural land use on ground-water 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 Ground-Water Quality

• Objectives:
  Provide a broad overview of the quality of ground water in the irrigated agricultural areas of the plains by sampling recently recharged ground water in the South Platte alluvial aquifer.
  
  Relate observed ground-water quality to overlying land use.
• Approach:
  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).
• Timing:
  June through August 1994
• 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 ground water 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 ground water 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 ground water.

  Median uranium concentrations in shallow ground water 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 Interactions)

• Objectives
  
  To describe stream-aquifer interactions and the processes controlling the fate of nitrogen-contaminated ground water near areas of discharge to the South Platte River.
• Approach
  
  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 ground water to stream.
  
  Sampled both ground water and surface water.
• Timing
  
  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 ground water 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 ground water, which showed small differences between the top and bottom of the aquifer.
  
  Nitrate concentrations in surface water were smaller than in ground water in part because microbial activity in streambed sediments removed a substantial portion of the nitrate from incoming ground-water return flows.