COMPARATIVE HYDROLOGY, HYDROCHEMISTRY, AND METAL CHEMISTRY OF STREAMS IN THE ATLANTA METROPOLITAN REGION

The effects of urbanization upon stream runoff, water chemistry, and sediment chemistry were investigated within the Atlanta metropolitan region (AMR). Rainfall, discharge, stream water chemistry and sediment-metal chemistry were analyzed in the highly urbanized Peachtree Creek [PTC] basin (60% urban) and the nearby less-urbanized Sweetwater Creek [SWC] basin (13% urban). PTC annual hydrographs were extremely "spiky" and the urban stream generates 12% more runoff than SWC although rainfall in the two basins is nearly identical. Two-day storm recession coefficients were greater for PTC than SWC and all other basins that were analyzed in the Georgia Piedmont study area. Typical storm recession perods for the urban stream are reduced to <12 hours as compared to 3-4 days for SWC. PTC hydrographs were also characterized by lower seasonal base flow recession coefficients. Clockwise rotational patterns associated with solute-discharge hysteresis plots for PTC strongly suggest that storm flow approximates a two end-member mixture with the storm-water runoff component dominating both limbs of the hydrograph.

Base flow TDS concentrations for the PTC basin were approximately 30% greater than SWC and elevated concentrations of base cations, alkalinity, sulfate, and chloride were present in base flow within the urban basin. Although the major ion chemistry of PTC base flow meets drinking water standards the elevated concentrations suggest that shallow ground water has been impacted by leaky sewage pipes. Zinc and trace metal concentrations are significantly greater in PTC storm runoff than SWC and other less urbanized streams; however, they are a factor of 0.01 lower than within street runoff. Trace metals were selectively concentrated in fine-grained stream sediment. Greater masses of zinc and lead are absorbed upon PTC suspended sediment than upon sediment from SWC and other less-urbanized streams. The most likely source of zinc (which is the most concentrated of the metals)is tire-erosion; however, zinc and other concentrations are still lower than MCL values within all AMR stream flow.