Soluble Reactive Phosphorous Concentrations in the Upper Clark Fork River:

A Study of the Contribution of 2 Nonpoint Sources of Sediment

Scott A. Luchessa
Environmental Studies
University of Montana

    Nuisance algal accumulations occur in the upper Clark Fork River because of near optimal growing conditions, including high nutrient levels. Bioavailable nitrogen and phosphorus are often high enough to saturate growth and standing crop. Anthropogenic activities are thought to contribute significantly to these high nutrient levels. Point and nonpoint nutrient sources are beginning to be identified quantified and evaluated.

    This study sought to estimate the contributions of eroding streambank and fluvially deposited mine waste (slicken) sediments to the upper rivers bioavailable phosphorus (HAP) levels. Soil samples were collected from eroding streambank and slicken surfaces, homogenized and sieved. Samples of the <2mm size fraction were suspended for 8 hours in filtered river water at a concentration approaching the upper river's maximum total suspended sediment concentration (500 mg/l). The resulting extract was filtered and analyzed for soluble reactive P (SRP), a good indicator of HAP. The contribution of these sediment sources to the rivers BAP levels was estimated by multiplying the average amount of SRP extracted per gram of sediment by the total suspended sediment (TSS) levels in the river. Some of the factors that influence phosphorus extractability were examined, including concentration of TSS extracted, pH of extractant and particle size and soil textural class of the extracted sediment. Over the ranges tested here, none of these factors had any apparent effect on the amount of SRP extracted from these sediments.

    Both sediment types were composed primarily of silts and find sands with little clay content (10-12%) and an average of 5- 7% organic matter content. Slightly alkaline (pH= 8) eroding streambank soils released amounts of SRP similar to the acidic slicken sediments (pH= 4). The SRP contribution from these sources (2-4 ug SRP per gram of sediment) to the total SRP in the river was an insignificant part of the rivers high SRP levels. Total soluble phosphorus extracted was analyzed for some extracts and found to be similar to the amount of SRP extracted.