Surface and ground water mixing, flow paths, and temporal variations in chemical compositions of karst springs

Karst aquifers provide important sources of water, but may be susceptible to contamination from mixing with polluted surface water. Characteristics of mixing, in particular the extent of conduit versus diffuse flow, have been studied through time-series observations of the chemical composition of water discharging from springs (chemographs). Annual and storm chemographs of Ca, Mg, DIC, and alkalinity are constant within error for six closely spaced springs that discharge from the Floridan aquifer in north-central Florida, reflecting equilibration with carbonate rocks of the aquifer. Similar chemographs from other karst aquifers have been interpreted to reflect diffuse flow, but dye trace and cave diving evidence indicate conduit flow is significant in the study area. Chemographs from Floridan aquifer springs may differ from other karst springs because of a shallow water table, which places most conduits in the phreatic zone and increases the volume of ground water available to mix with recharged water. Three of the springs, with previously identified connections to swallets, show temporal variations in Cl and SO4 chemographs, but little variations in temperature or oxygen chemographs. In contrast, the three other springs show little change in Cl and SO4 chemographs, but significant and systematic decreases in temperature and oxygen chemographs from summer (rainy season) to winter (dry season). The Cl and SO4 chemographs reflect mixing with dilute surface water, while the oxygen and temperature chemographs reflect the depth of the source of water. Seasonal NO3 chemographs vary widely because of contamination. The storm chemographs show minor dilution of Cl and SO4 concentrations, at the same time as an increase from no detectable NO3 to ~1.5 mg/kg NO3, presumably from contamination. These results show that information unique from individual chemographs is provided by simultaneous chemographs of several solutes from groups of karst springs.