Searching for Phosphorus Sources in a Mixed-Use Mountain Watershed.

Where agriculture dominates, nutrient and soil management practices have been widely implemented in reducing phosphorus (P) losses at the edge-of-field. However, in the mixed-use mountain watershed in Wallsburg, Utah, the surface waters still reflect undesirable P concentrations although the presence of livestock has decreased significantly over the years, and cover crops abound on the small fraction of agricultural land. This scenario suggests possible evidence of legacy P stores in the local soils and stream sediments. The objectives of this research are to identify P sources in the Wallsburg Watershed and to learn whether those loads could be artifacts of legacy phosphorus stores. Available historic water quality data is supported by our own recent synoptic water samplings during 2015. In a single sampling event, water samples from 19 locations are collected for analysis in laboratory for dissolved and total P fractions, along with other major chemical constituents. We find that during both peak flow and base flow conditions, both dissolved and total P concentrations at some sites exceed the regulatory limits. We observe one particular spring-fed stream as a significant source of P loading, and that P loads along the main tributary generally increase as it flows through the agricultural landscape. Through geospatial analyses we plan to model the correlation of the water chemistry and the soil and land properties. Through this approach we hope to discover legacy P sources and develop a P Risk Index that could be applied to other mixed-use mountain watersheds.