Wastewater Disposal, Nitrate in Streams, and Contamination of Karst Groundwater

Groundwater in karst systems, such as the Edwards aquifer in Central Texas, is uniquely vulnerable to contamination.  The water quality of the Barton Springs segment of the Edwards aquifer is controlled largely by that of streams, which supply about 85% of aquifer recharge.  During 2000–2010, there was a rapid increase in the number of septic systems and volume of land-application as a means of wastewater disposal in the spring catchment area.  Nitrate concentrations and isotopic composition were measured for 17 months during 2008–2010 at Barton Springs and five streams recharging the aquifer.  During this period, hydrologic conditions transitioned from severe drought (dry period; spring discharge 13 ft³/s, low to no stream flow) to high flow (wet period; spring discharge 94 ft³/s, continuous stream flow).  Nitrate concentrations in the streams and Barton Springs were higher during the wet period than during the dry period.  During the wet period, nitrate concentrations in the streams were higher by a factor of 2 to 26 compared with historical concentrations (1990–2008) for similar flow conditions, and nitrate concentrations at Barton Springs were about 0.3 mg/L higher (25 percent) than historical concentrations.  Nitrogen isotope signatures indicate that, during the dry period, undiluted wastewater return flow and the atmosphere (rain or vehicle emissions) were the principal sources of nitrate to streams, and that during the wet period, soil water and wastewater were the principal sources of nitrate to streams.  An increase in δ¹⁵N(nitrate) at Barton Springs from the early 1990s to 2008–2010 indicated that the contribution of wastewater to nitrate in spring discharge increased during that period.  Our results indicate the projected doubling of population during 2010–2035 in the spring catchment area and associated increase in wastewater disposal likely will adversely affect water quality at Barton Springs.