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Colorado Water Science Center

Nutrients in Agricultural and Recreational Reservoirs

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Lori Sprague 1

ABSTRACT

In 1995, the U.S. Geological Survey conducted a study to characterize nutrient concentrations in five off-stream reservoirs Riverside, Jackson, Prewitt, North Sterling, and Jumbo during the irrigation season from March through September. These reservoirs are critical sources of irrigation water for agricultural areas in the lower South Platte River Basin, and several also are used for fishing, boating, swimming, hunting, and camping. One threat to the recreational value of the reservoirs is an overabundance of nutrients such as nitrogen and phosphorus. Nutrients are necessary for plant and animal life, but in excess quantities they become a health concern to humans and aquatic animals and can cause algal blooms that deprive deeper waters of the sunlight and oxygen needed by aquatic plants and animals.

Nitrogen concentrations in the reservoirs were highest in March and decreased through September as a result of dilution from river inflows and biological activity. From March through June, decreases in nitrogen concentrations in the river contributed to corresponding decreases in reservoir concentrations. From July through September, inflows from the river were cut off, and biological activity in the reservoirs led to further decreases in nitrate concentrations, which fell to near or below detectable levels. The timing of minimum nitrate levels in each reservoir was related to the timing of river inflows and the depth of the reservoir. With the reservoirs acting as a sink for nitrogen, concentrations of total nitrogen in the outflow irrigation-supply water decreased over the study period.

Phosphorus concentrations in the reservoirs did not show the same consistent decrease from March through September. With the exception of Riverside, which was in closest proximity to wastewater dischargers, concentrations of biologically available orthophosphate were decreased to near or below detectable levels by July. Phosphorus was recycled back to algae, however, through processes such as excretion from fish, decay of aquatic plants and animals, and release of orthophosphate from bottom sediments during periods of low oxygen. Due to these fluctuations, concentrations of total phosphorus in the outflow irrigation-supply water did not consistently decrease over the study period. The results of this study indicate that the practice of storing South Platte River water in off-stream reservoirs decreases nitrogen concentrations. However, storage may also contribute to the growth of nuisance algae that could affect the recreational use of these reservoirs.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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