Changes in baseflow patterns in water-limited shale oil and gas regions: the Eagle Ford play

Quantifying and analyzing the contribution of groundwater from shallow aquifers to rivers as baseflow is very important for water supply and riverine ecosystem health, especially in water-limited catchments. Baseflow depends on the water available (precipitation), vegetation (land use, water use), aquifer properties and water-table depth. In this context, human activities such as groundwater abstraction for multiple purposes can alter the relationship between aquifer storage and baseflow. In this study, we analyzed observed changes in baseflow patterns of 40 catchments located across the Eagle Ford shale gas/oil play (Texas) during the period 1986-2015. The Eagle Ford sedimentary formation is actually the largest shale oil producing region in the US with large production in shale gas. Intensive unconventional resources extraction in the Eagle Ford play started in 2009 and gas/oil production increased faster than in other plays, accompanied by a rise in groundwater consumption for HF purposes. Spatial and temporal impacts on baseflow at the Eagle Ford play derived from HF were assessed by means of different patterns such as baseflow hydrograph separation, flow-duration curves, empirical storage-discharge relationships and streamflow recession curve analysis. A comparison during different periods of water use for HF activities was performed: pre-development period (1986-2000); moderate period (2001-2008); and intensive period (2009-2015). The pre-development period was considered as a baseline and catchments located inside and outside the play area were separately analyzed. The results show negative changes on baseflow patterns during the intensive HF period that were not observed during the moderate period, especially in catchments located inside the play. These changes were also characterized by a decline on mean annual baseflow volume and shorter hydrograph recession times, that led to a shift in the streamflow regime in some catchments from perennial to intermittent and from intermittent to ephemeral regimes. However, the intensive HF period occurred during the exceptional drought in Texas in 2011-2013, associated to an increase in groundwater demand for irrigation and municipal consumption, which may explain most of the observed prolonged decline in annual baseflow volume.