Historical contributions of phosphorus from natural and agricultural sources and implications for stream water quality, Cheney Reservoir watershed, south-central Kansas

An examination of soil cores collected from 43 nonagricultural coring sites in the Cheney Reservoir watershed of south-central Kansas was conducted by the U.S. Geological Survey in September 1999. The cores were collected as part of an ongoing cooperative study with the city of Wichita, Kansas. The 43 sites (mostly cemeteries) were thought to have total phosphorus concentrations in the soil that are representative of natural conditions (unaffected by human activity). The purpose of this report is to present the analysis and evaluation of these soil cores, to quantify the phosphorus contributions to Cheney Reservoir from natural and agricultural sources, and to provide estimates of stream-water-quality response to natural concentrations of total phosphorus in the soil.

Analysis of soil cores from the 43 sites produced natural concentrations of total phosphorus that ranged from 74 to 539 milligrams per kilogram with a median concentration of 245 milligrams per kilogram in 2-inch soil cores and from 50 to 409 milligrams per kilogram with a median concentration of 166 milligrams per kilogram in 8-inch soil cores. Natural concentrations of total phosphorus in soil were statistically larger in samples from coring sites in the eastern half of the watershed than in samples from coring sites in the western half of the watershed. This result partly explains a previously determined west-to-east increase in total phosphorus yields in streams of the Cheney Reservoir watershed. A comparison of total phosphorus concentrations in soil under natural conditions to the historical mean total phosphorus concentration in agriculturally enriched bottom sediment in Cheney Reservoir indicated that agricultural activities within the watershed have increased total phosphorus concentrations in watershed soil that is transported in streams to about 2.9 times natural concentrations.

Retention efficiencies for phosphorus and sediment historically transported to Cheney Reservoir were calculated at 92 and 99 percent, respectively. Most of the phosphorus was retained in bottom sediment. Sediment accumulation in Cheney Reservoir was less than reservoir design-life specifications on the basis of the age of the reservoir.

Estimates of mean total phosphorus concentrations for selected streams in the Cheney Reservoir watershed under natural concentrations of total phosphorus in soil and a historic set of watershed conditions indicate that water from two of the five streamflow sampling sites would not meet the total phosphorus water-quality goal of 0.10 milligram per liter established by the Cheney Reservoir Watershed Task Force Committee. These results imply that the water-quality goal for total phosphorus in some streams of the watershed may not be met simply by reducing the amount of phosphorus applied. Instead, meeting the goal could involve a combination of approaches-for example, reducing the agricultural distribution of phosphorus and implementing changes in watershed activities to mitigate phosphorus movement to surface water.