FLUID LOGGING AND DISCRETE SAMPLING OF THE FRESHWATER/ SALINE-WATER INTERFACE OF THE EDWARDS AQUIFER, SOUTH CENTRAL TEXAS: IMPLICATIONS FOR GROUNDWATER FLOW AND ORIGIN OF SALINITY (PART 1- FLUID LOGGING)
The Edwards Aquifer is the primary source of water in south central Texas for agriculture, municipal, industrial, and ecological needs. The aquifer is made up of limestones with interbedded dolostones of the Lower Cretaceous Edwards Group that dip to the southeast. Flow in the aquifer is influenced by the Balcones fault zone; a band of late Cretaceous-Miocene normal faults that run parallel to the strike of the aquifer. The up-dip, freshwater zone (FWZ) of the aquifer is recharged with surface water infiltrating along the northern area of the outcropping Edwards Group. The saline-water zone (SWZ) is adjacent to the FWZ and forms an interface at the down-dip limit of the fresh water. Though the region where the freshwater/saline-water interface (FSI) occurs is spatially defined, little is known about the nature of groundwater flow between, and along, its surface. Down-hole logging of fluid temperature and specific conductance was conducted in monitor well transects to define the vertical profiles of the FSI at different segments in the aquifer. Temperature gradients range from 0.7 to 1.0 oC/100m in the FWZ wells to 2.0 to 2.3 oC/100m in the SWZ wells. This difference between the thermal gradients mainly is caused by the presence of zones with higher rates of fluid flow as compared to lower flow zones in the aquifer. Specific conductance profiles also differ between wells, varying from wells with rather uniform values along the entire length of the open interval to wells that have a very sharp contrast between the FSI in the borehole. Values from the SWZ range from 500 to 3,100 mS/m while values from the FWZ range from 40-80 mS/m. In some wells, increased thermal gradients are associated with increased specific conductance indicating a low flow regime. In others wells, both the saline and fresh groundwaters are stratified and in continuous thermal contact (uniform gradient), indicating that both zones have circulation. Overall, the profiles indicate that groundwater flow in the eastern transects have some degree of vertical stratification within the aquifer as well as a strong structural control of flow between wells in a transect. In the westernmost transect there is a progressive, more gradational increase in salinity and temperature down dip of the FSI with less structural influence.