Analysis of Long-Term Nutrient Transport from the Goodwater Creek Experimental Watershed.

Taste and odor problems in 2006 and 2007 at the drinking water treatment plant in Mark Twain Lake have implicated nutrient loadings in the lake and its tributaries. The Goodwater Creek Experimental Watershed (GCEW), a 72 km² watershed within the lake drainage area, has been monitored for flow since 1971 and for dissolved nutrients since 1991 for 3 nested watersheds (12.1, 31.5 and 72.6 km² drainage area). This watershed includes row crop land (76%), pasture and other grassland (14%), woodland (6%) and a small town in the upper end of the watershed (4%). The objectives of this paper are to analyze nutrient loadings at the three gauging stations in GCEW over 14 years, from 1991 to 2004. Daily loadings for ammonium-nitrogen, nitrate nitrogen, dissolved phosphorus and atrazine were calculated from base flow and runoff samples’ concentrations and 5-min flow values. Atrazine was included in the analysis as one compound not implicated in the discharge of human waste. Monthly and annual loadings and flow-weighted concentrations of these constituents were calculated and analyzed using linear regression and the non parametric Mann-Whitney and Wald-Wolfowitz tests for homogeneity, sudden changes and trends. Concentrations of dissolved phosphorus and ammonium nitrogen show distinct differences for each weir while atrazine and nitrate concentrations are similar at all weirs. The relative importance of urban land in each of the three nested watersheds was considered to explain large differences of concentration in dissolved phosphorus and ammonium nitrogen and partition nutrient loadings between those due to human and agricultural sources. These findings provide insight in what needs to be included in a holistic analysis of the nutrient sources in the watershed and how stream processes affect nutrient loadings. They will also play a key role in deciding what to include and how to parameterize a model of the watershed.