Geochemical and solute-discharge hysteresis comparison of two Atlanta metropolitan region watersheds

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Rose, Seth
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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.
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