MAPPING GROUNDWATER SPRINGS WITHIN SCOUR POOLS IN COASTAL RIVERS IN GEORGIA

Concentrated spring discharge from the Floridan aquifer to rivers on the Florida peninsula is common, but largely unknown in coastal Georgia because upward flow is inhibited by thick confining units. However, shallow artesian aquifers above the Floridan may provide small, but locally important spring discharge to coastal rivers in Georgia. The purpose of this study is to test the hypothesis that artesian springs are present in coastal Georgia and play a subtle, but important role in the ecology of coastal rivers. This hypothesis is currently being tested at two deep pools known to provide critical summer habitat for Atlantic and shortnose sturgeon. At these locations general chemistry (Na, NH4, K, Ca, Mg, Cl, NO3, and SO4) and other parameters (temperature, pH, dissolved oxygen etc.) are measured from discrete samples being collected along the streambed and in vertical profiles.

Preliminary data show only subtle differences in most chemical species between upstream control sites and sites within the deep pools - differences in physical parameters are also small. However, calcium and alkalinity along the riverbed show strong concentration gradients that increase within the deep pools - pH also increases slightly. Based on these data, groundwater from aquifers containing calcite appears to be entering the deep pools and mixing with surface water in the rivers. Moreover, spatial distribution of the geochemical data indicates that groundwater discharge is taking place along a linear trend as opposed to a single spring head.

Both deep pools are steep-sided and elongate scour depressions whose bottoms are 3-5m below the average depth of the riverbed. The combined geochemical and bathymetry data suggests that the scouring process may have breached confining beds of shallow artesian aquifers, creating linear zones of groundwater discharge. Another possible model is that scouring exposed a fracture zone, thereby allowing artesian groundwater to discharge into the stream. More detailed sampling within the river will produce a refined conceptual model. Eventually, nested groundwater wells will be installed which should establish the chemistry of the groundwater responsible for the mixing patterns being observed in the rivers. Stratigraphic information will be obtained during well installation.