Comparative major ion geochemistry of Piedmont streams in the Atlanta, Georgia region: possible effects of urbanization

The major ion stream chemistry for an urbanized basin (Peachtree Creek) in the Atlanta (Georgia, USA) metropolitan region was analyzed and compared upon a time-series basis with the stream chemistry of a nearby, but far less developed basin (Sweetwater Creek). The major ion chemistry from both streams indicated that all parameters fell below safe drinking water standards as would be expected in watersheds underlain by low-solubility aluminosilicate bedrock. Base flow TDS concentrations for the urbanized basin (Peachtree Creek) were similar to30% greater than Sweetwater Creek and were characterized by elevated concentrations of base cations (Ca, Mg, Na), alkalinity, sulfate, and chloride. The less-urbanized Sweetwater Creek basin is underlain by a higher percentage of more soluble amphibolite and, therefore, the higher concentration of solutes within the urban base flow can not readily be explained by differential mineral weathering. The increased inorganic solute loads might have resulted from pollution input (leaky underground sewer lines?) and/or evaporative concentration; however, these explanations are presently speculative. Major ion concentrations within the urban Peachtree Creek basin were diluted by an average of similar to50% compared with only similar to25% in the Sweetwater Creek basin as a result of the increased contribution of direct surface runoff. The regression correlation coefficients for the ions that evolve through mineral weathering (Ca, Mg, Na, and HCO3) are much higher in urban stream flow than in Sweetwater Creek waters. This might indicate that storm waters in the urban basin are more closely derived from a two end member mixture (groundwater and street runoff) than waters from Sweetwater Creek, which represent a more complex mixture.