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South Platte River basin in Colorado and Nebraska (SPLT)

Two well networks in the alluvial aquifer of the South Platte River basin in Colorado and Nebraska (SPLT) were sampled in 1993-1994 (Dennehy and others, 1998) and resampled in 2002-2004 (Paschke and others, 2008). Overall, there was a significant increase in nitrate concentrations between the two sampling events. Water from approximately 40 percent of the wells had dissolved-oxygen concentrations less than 2 mg/L, indicating the water was under reducing conditions. There were no significant changes in nitrate concentrations in water samples with reduced conditions, but there was a significant increase in nitrate concentrations in groundwater with oxidized conditions. The increase in groundwater nitrate concentrations in oxidized parts of the South Platte alluvial aquifer is attributed to the continued increase of corn and cattle production in the area since the 1950s. Synthetic fertilizer and manure application rates increased in parallel with agricultural production until 2000 when a regional drought caused a decline in agricultural output from the basin. Nitrogen isotope ratios for samples under oxidizing conditions indicate that synthetic fertilizers are the predominant source of nitrate to groundwater.

Map showing the location of the South Platte River basin in Colorado and Nebraska (SPLT) study area (from Paschke and others, 2008).

Location of the South Platte River basin in Colorado and Nebraska (SPLT) study area (from Paschke and others, 2008).

Concentrations of the herbicides atrazine, DEA, and prometon decreased in the SPLT alluvial aquifer from 1994 to 2002 (Paschke and others, 2008). Available pesticide-use data are sparse and are insufficient for observing relations between pesticide use and changes of groundwater quality, but the observed decreases in atrazine and DEA groundwater concentrations in the South Platte alluvial aquifer suggest possible declines in local atrazine application rates and/or pesticide degradation. The persistence of DEA in groundwater and the increase in the DEA to atrazine mole ratio between 1994 and 2002 indicates atrazine degradation is likely contributing to the observed atrazine concentration decrease. The DEA to atrazine mole ratio was greatest for oxidized samples, further indicating that atrazine degradation to DEA is promoted by oxidizing conditions.

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