ASSESSING THE LIKELIHOOD OF GROUNDWATER CONTAMINATION FROM HYDRAULIC FRACTURING IN THE EAGLE FORD SHALE: A MAP-BASED ANALYSIS OF THE EAGLE FORD AND OVERLYING AQUIFERS

This study investigates the controversial hydraulic fracturing process (fracking), focusing on whether fracking itself is likely to initiate new fractures or widen existing fractures that could permit groundwater contamination in the region where oil and gas are currently produced from the Eagle Ford shale, in south Texas. Three different maps were used to compare the depth differences between the top of the Eagle Ford shale and the base of aquifers that overlie the regions of active production, assuming that the likelihood of drinking water being directly contaminated by the hydraulic fracturing process can be related to the thickness of rock separating the fracking operation from regional aquifers. These maps include: 1) a map displaying depths to the top of the Eagle Ford shale as well as the regions where oil and gas are currently produced; 2) a map showing the distribution of aquifers that would be affected by a contamination event, including the Carrizo-Wilcox and the Gulf Coast aquifers; and 3) a map showing depths to the base of the aquifers that overlie the Eagle Ford shale. When we compared the depth difference between the top of the Eagle Ford (which increases in depth towards the south) and the base of the aquifers, the thickness of intervening rock is significant in all areas. Where oil is produced, the maximum depth difference was 6000 feet (1830 m) and the minimum depth difference was 2000 feet (610 m); and where gas was produced, the maximum depth difference was 14500 feet (4420 m) and the minimum depth difference was 4000 feet (1220 m). Our reconnaissance analysis suggests that fractures initiated by the fracking process have little chance of permitting the migration of oil or gas upward into the aquifers overlying the Eagle Ford. However, new exploration is taking place in the units above the Eagle Ford, which will decrease the depth difference between the Eagle Ford and overlying aquifers, thus increasing the potential of groundwater contamination. We did not investigate other environmental risk factors related to surface operations, such as water use, fracking fluid disposal, or well casing blowouts within the aquifer zone, but these factors should also be assessed to evaluate the overall impact of hydraulic fracturing operations.