UTILIZING RESISTIVITY LOGS AND THE RWA METHOD TO MAP SALINITY ZONES IN THE EOCENE QUEEN CITY AQUIFER, CENTRAL TEXAS

The Texas Water Development Board is mapping brackish aquifers to better understand water resources in the state. However, public water quality measurements from existing wells occur in the outcrop and less saline portions of the aquifers. Calculations on select geophysical well logs with deep resistivity tools using the “R_wa Method” provide estimated total dissolved solids (TDS) values where directly measured samples were not available.

Data mining for aquifer properties and water quality and stratigraphic and lithologic mapping for the aquifer and adjacent aquifers had to be completed before salinity mapping could start. We located 61 wells with 146 TDS measurements for the Eocene Queen City Aquifer in central Texas. Of these, 108 TDS measurements could be correlated with specific conductance calculated from a resistivity log to create “ct” values. Another 348 wells with 538 TDS values were calculated solely from deep resistivity logs.

The R_wa Method (Resistivity Water Apparent) is based on Archie’s equation, which provides parameters for discerning the resistivity of water from the overall resistivity measured for the formation. Accurately defining those parameters is challenging since no lab measurements for the Queen City Aquifer were discovered. There is an improvement in performance as the water chemistry becomes more sodium and chloride dominated. Additionally, finding or collecting deeper and higher salinity water samples that can be paired with geophysical well logs would improve the calculated water quality values for these areas of the aquifer.

For the Queen City Aquifer in central Texas, the extent of salinity zones based on measured and calculated TDS showed a gradual but non-uniform increase in salinity with depth. The downdip extents and mixing of these zones seems to vary based on depositional environments and structure mapped by previous authors. Estimating TDS from resistivity logs allowed us to map the vertical and downdip variability in salinity where there were no measured water quality samples. Upon completion, all data related to the “Brackish Groundwater in Aquifers of the Upper Coastal Plains, Central Texas” study will be available at http://www.twdb.texas.gov/innovativewater/bracs/index.asp.