Revising the Fresh-Saline Water Interface in Eastern Kentucky

Abstract

Shallow drilling depths in the Devonian Berea Sandstone oil and gas play and potential high-volume hydraulic fracturing in the budding Cambrian Rogersville Shale play have generated a renewed interest in protecting groundwater quality in eastern Kentucky. The depth to the base of potable water was mapped by H.T. Hopkins in 1966 in his “Fresh-Saline Water Interface Map of Kentucky.” The map remains an important guidance document for well operators and the Kentucky Division of Oil and Gas when evaluating surface casing depth. To create the map, Hopkins assumed that the total depth of domestic water wells equaled the base of fresh groundwater (total dissolved solids equal less than 1,000 ppm) or the fresh-saline water interface. However, it is likely that most water wells did not penetrate the base of the deepest fresh water. As a result, Hopkins’ map likely underestimates the depth of the fresh-saline water interface.

To increase the accuracy of the map, post-1966 domestic water-well data were added to the Hopkins data in a 14-county area covering the Berea and Rogersville plays in eastern Kentucky. The number of wells increased from 50 used by Hopkins to 4,824 in this study. The elevation range for the interface increased from 300 ft to 1,020 ft in the Hopkins map to 75 ft to 2,198 ft in our analysis.

Despite the increased robustness, the data from shallow wells continued to underestimate the fresh-saline water interface in regions of the map. Groundwater depth is influenced by topography and surface water, and to ameliorate underestimation, the added wells were examined in relation to their respective watershed elevations defined by hydrologic unit codes (HUC). Specifically, wells with total depths deeper than the minimum stream elevation in a HUC (pour point) were used to map the fresh-saline water interface. Excluding wells with total depth elevations above their HUC pour points resulted in reducing the maximum fresh-saline water interface elevation by 1,173 ft. Despite the improvement, the true depth of the interface in any given area remains uncertain, and we suggest the alternative term, “deepest observed fresh water.”

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016