DETERMINATION OF GROUNDWATER FLUX FOR SOUTH CHICKAMAUGA CREEK IN RINGGOLD GAP, CATOOSA COUNTY, GEORGIA

Strategically located discharge measurements along a stream that is underlain by contrasting geologic units provide a means of evaluating relationships between streambed seepage and bedrock geology. The relatively straight course of South Chickamauga Creek in Ringgold Gap provides an opportunity for such a study. This 450-m stream reach flows across strike through the gap 10 miles SE of Chattanooga, TN (UTM Z16, 0673605mE, 3864145mN), at the division of Taylor Ridge and Whiteoak Mountain.

A roadcut along I-75S in Ringgold Gap provides excellent exposures of Upper Ordovician to Lower Silurian strata. This outcrop, and others in the area, reveal potential hydrogeologic controls and, by simple extrapolation, provide an understanding of substream geology. These units dip shallowly (14°) to the southeast (dip azimuth=117°). Slight variations in strike and dip suggest a fold axis (3°, 040°) that is consistent with regional structures. These units also contain three distinct sets of joints: (1) near vertical cross-strike (cross-fold) joints, (2) northwest-dipping strike-parallel joints, and (3) near vertical oblique joints that strike approximately 080°. Joints and bedding-parallel partings are potential avenues for enhanced water flow.

A seepage run was conducted under baseflow conditions on October 20-21, 2001. Locations for gauging the northwest-flowing creek, L1 to L4 (SE to NW), were chosen such that major differences in lithology are bracketed by discharge measurements. Upstream from L1, the creek flows over alternating beds of sandstone and shale of the Red Mountain Formation. Between L1 and L2, the lithologic section consists mainly of shale with minor sandstone near the top and calcarenite near the bottom. L2 and L3 bound a heterolithic section of various siltstones, sandstones, and shales. Downstream from L3, the creek is underlain by alternating thin-bedded calcarenite and calcareous shale of the Cathey's Formation.

A simple water balance between each station indicates a significant gain in stream flow between L1 and L2, a loss between L2 and L3, and little change in flow between L3 and L4. An error analysis following Carter and Anderson (1963, J. of Hydrauls. Div. ASCE HY4 105-115) indicates that these determinations of groundwater flux exceed the total error associated with the streamflow measurements.