Nitrate was the predominant form of nitrogen in ground and surface water within the basin. Nitrate concentrations in alluvial ground water were high in some areas where agricultural fertilizers and manure were applied to the land. Nitrate-contaminated ground water generally cannot be avoided by drilling deeper wells because nitrate concentrations typically are similar throughout the depth of the alluvial aquifer, most likely due to mixing of water in the aquifer caused by irrigation pumpage. This natural removal process does much to improve nitrate concentrations in the South Platte River, but concentrations are still large enough that, when excessive phosphorus concentrations also are present, increased growth of nuisance algae and other aquatic plants can occur in streams and reservoirs. Nitrate concentrations in surface water are substantially lower than concentrations in wells adjacent to the South Platte River.
Microbial action removes some nitrate from ground water as it moves from the aquifer to the river.
The use of agricultural fertilizers and manure has affected the use of ground water as a drinking-water supply in some areas. Drinking water for sale near Greeley, Colo., in an area where the ground-water supply is contaminated by high nitrate concentrations.