Geochemical and solute-discharge hysteresis comparison of two Atlanta metropolitan region watersheds
The major ion geochemistry (Ca, Na, Mg, K, SiO2 aq, SO4, HCO3, and specific conductance) of Peachtree Creek and Sweetwater Creek was analyzed dur-ing the period between 2000–2001. Peachtree Ck. drains one of the most urbanized basins within the Atlanta met-ropolitan region while Sweetwater Ck. drains a far less developed basin, ~35km west of Atlanta. Although all major ion parameters met safe drinking water standards, total dissolved solute concentrations in Peachtree Ck. were ~30% greater than within Sweetwater Ck. Sweetwater Ck. is underlain by a higher percentage of relatively soluble amphibolite and therefore the higher solute concentrations cannot be attributed to lithological differences between the two watersheds. It is not clear what mechanism is respon-sible; however, it is possible that leaky sewer pipes may be at least partially responsible for the higher solute loads within the Peachtree Ck. watershed, particularly that por-tion of the basin underlying the City of Atlanta. Most of the concentration-discharge (C/Q) loops associated with Peachtree Ck. were characterized by clockwise rotation and concave curvature. Such hysteresis dynamics can be most readily explained by a two-end member mixing model where “pre-event water” mixes with “event” water during storm periods. In contrast, the C/Q loops for Sweetwater Ck. were for the most part characterized by “anticlockwise hysteresis” indicative of three-component mixing.