ASR Drought Management Operations 2014 South Bexar County

Macias, Roberto

The Edwards aquifer is the primary source of drinking water for the City of San Antonio and much of the surrounding region. Withdrawals are regulated by the Edwards Aquifer Authority (EAA). However, Edwards permits can be curtailed during drought to ensure adequate supplies and maintain flows in San Marcos and Comal Springs. To ensure the continued economic prosperity of the region especially during drought, the San Antonio Water System (SAWS) completed the 64 million gallons per day (mgd) South Bexar Aquifer Storage and Recovery system. In wet years, surplus Edwards aquifer water is stored in the Carrizo sand formation of the Carrizo-Wilcox aquifer in southern Bexar County. The stored water is recovered to augment supplies during extended drought periods. It is estimated that offsetting 60 mgd of SAWS withdrawals for six months, over two consecutive years, would increase the flow in Comal Springs by 28 cubic feet per second, an amount equal to 30 percent over the Stage IV minimum flow.

With the San Antonio region into a fourth year of drought, the Edwards Aquifer Authority imposed Stage IV pumping restrictions August 12, 2014, in response to declining Edwards levels. This resulted in a 40 percent reduction in the permitted withdrawal from the Edwards aquifer. SAWS was already operating in their Stage 2 water use restrictions but was able to avoid additional water use restrictions, and still comply with the forecasted 2014 Edwards pumping cap, by continuing recovery from ASR and use of their other non-Edwards water supplies.

SAWS has gone into recovery mode several times to reduce pumping pressure on the Edwards aquifer. Significant recovery events occurred in 2006, 2011, 2012, and late 2012 through the summer of 2013. For the most recent recovery event, recovery began on January 2, 2014, with a residual storage volume of 87,914 acre-feet (ac-ft) and was ongoing as of September 20, the last day of operational data. A total of 19,086 ac-ft were recovered year to date, with ASR meeting at least 10 percent of the daily system demands in the months of February through July, with the peaking being 16.1 percent in June. The largest previous recovery event ending in mid-August 2013 with a total withdrawal of 16,103 ac-ft from the South Bexar ASR.

This presentation summarizes the South Bexar ASR operational history with a focus on the 2014 recovery event. Strategies employed by SAWS to address changes in recovered water quality, resulting from increasing concentrations of the native Carrizo groundwater at several wells, will be discussed. Plans for modifying recharge and recovery operations in the future will also be presented.

Recovery from the SAWS’s ASR project allowed the utility to avoid severe drought restrictions resulting from a 40 percent reduction in Edwards supply.