Phosphorous Sources in Gold Creek, a Tributary of the
Clark Fork River in Western Montana

Jennifer H. Carey
Environmental Studies
University of Missoula

   Gold Creek, a tributary of the Clark Fork River in western Montana, has regularly exceeded the Environmental Protection Agency's total phosphorous (TP) water quality criterion of 0.050 mg/ 1 since monitoring began in 1988 and often exceeds 0.100 mg/l during late summer and fall. Gold Creek drains forested uplands tU1d lower elevations of irrigated fields and pastures. Cattle manure, fertilized hay fields, sediment erosion, and septic tanks are possible anthropogenic sources of P. Geologic P sources include the Permian Phosphoria Formation and volcanics that are mixed with Cretaceous and Tertiary sediments.
The intent of thus study was to determine if the high P levels in Gold Creek have an anthropogenic source. Samples were collected twice a monthly from April to October 1990, from Gold Creek's mainstem, tributaries, and springs, and analyzed for soluble reactive phosphorus (SRP), TP, and N. Discharge, pH, and conductivity were measured and fall base flow samples were analyzed for Si and F, two elements expected to correlate positively were SRP of geologic origin. Groundwater samples were collected from ten domestic wells. Bank sediments suspected of releasing SRP when eroded by cattle 3Jld irrigation diversions were collected for SRP extraction.

    The results indicate that much of the SRP is from all unidentified geologic source. Gold Creek appears to be receiving groundwater (rich in SRP and other dissolved solids) that is surfacing where the mountain mass meets ti1e valley fill material. Groundwater samples and springs above the influence of human activity were high in SRP (0.034-0.524 mg/l). SRP correlated positively Si, F, and conductivity, but did not correlate with N. SRP levels in the stream increased as discharge decreased and the SRP/TP ratio in the mouth of Gold Creek during summer low flows was 0.8 to 1.0. The sediment in two tributaries fed by P-rich springs had extractable SRP, and irrigation practices and erosion by cattle appear to influence SRP loads in these creeks. SRP in the Clark Fork River significantly increased below Gold Creek although the increase averaged only 0.00 Img/ 1. Hence, geologic P sources contribute to) the nuisance algae problem in the upper Clark Fork River.