ANTHROPOGENIC IMPACT ASSESSMENT OF THREEMILE CREEK, MOBILE, ALABAMA

Human impact upon rivers has become an increasingly important field of study as fresh water supplies diminish in both volume and quality. Studies concerning anthropogenic impact upon river systems have shown that human activity contributes unwanted pollutants to rivers in various forms, including nutrient overloading and heavy metal discharge. Sources of pollutants are commonly classified as either point sources or non-point sources, and may include sewage treatment effluent, industrial discharge, or fertilizer application. This research project has been initiated to assess anthropogenic impact upon the water and sediment chemistry of Threemile Creek, Alabama.

Threemile Creek is a small, increasingly urbanized watershed that drains a portion of Mobile, Alabama. The creek is approximately 14 miles long and drains 29.1 km². Threemile Creek empties into the Mobile River and is tidally influenced. The mouth of the basin is characterized by aging sanitary sewer lines and both industrial and wastewater treatment plant dischargers. The headwaters of the basin are subject to increasing residential development. In 1996, the Alabama Department of Environmental Management assessed Threemile Creek as not supporting its designated agricultural and industrial classification.

The objectives of this study have been: 1) to acquire chemical data for both stream water and sediments in Threemile Creek, and 2) to assess any relationship between elevated heavy metal/nutrient concentrations and specific land uses or point-source dischargers in the basin. Results show that elevated nutrient concentrations in the stream water occur very near the location of a sewage treatment facility. Aqueous heavy metal concentrations are low, but increase in proximity to both sewage (Ag, Mo, As, Zn, Cu, Pb, Ni, Cr) and lumber treatment (Cu, Zn, Sb) facilities. Partial microwave digestion of surface sediment samples reveals enrichment in heavy metal content near both the sewage (As, Zn, Cu, Pb, Ni, Cr, V, Co, Cd) and lumber (Cu) treatment facilities. These sediments also contain the highest organic matter contents and the highest percentages of silt/clay size fraction, both effective sinks for metals. Further research will examine the bioavailability of the sediment-bound metals utilizing a five-step sequential extraction technique.