FIELD INVESTIGATION OF WATER QUALITY AND THE ROLE OF COLLOIDS IN AN URBAN WATERSHED, MILL CREEK, TENNESSEE

Mill Creek, a watershed in the karst terrain of the Nashville Basin, subject to variable land use practices, is listed with the USEPA as an impaired watercourse [303(d)]. It has exceeded USEPA regulatory levels (>90^th percentile of allowable limits for the given ecoregion) for siltation, nutrient load, and pathogens, and failed to reach adequate dissolved oxygen concentrations (TDEC, 2004). Colloidal constituents (particulate matter <1µm) that are permanently dispersed within the water column can contribute to the downstream distribution of pollutants due to their high surface areas and surface charge characteristics. To assess this contribution, grab samples were collected from Mill Creek in its upper, middle, and lower reaches under variable flow conditions between August of 2004 and Feburary of 2005. Tangential flow filtration was used to filter and fractionate particulates into size ranges of >0.65µm, 0.65µm - 0.45µm, 0.45µm - 0.11µm and <0.11µm. The resulting permeates were analyzed for total and reactive phosphate and trace metals by spectrophotometer and ICP-MS, respectively. A GIS was generated to illustrate spatial relationships of pollutant loads within the watershed. Phosphate loads increased with increasing discharge. Phosphate loads also increased downstream suggesting additional contributions from runoff especially during high flow events. The majority of the phosphate load is in dissolved form under all flow conditions. Phosphate associated with particulate matter demonstrated different distributions under high and low flow conditions. At low flow, phosphate is associated with colloids in all size fractions, whereas at high flow it is associated predominantly with the smallest colloids.