Skip to main content
U.S. flag

An official website of the United States government

Source, variability, and transformation of nitrate in a regional karst aquifer: Edwards aquifer, central Texas.

June 21, 2016

Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO3) loading to surface and groundwater. We investigate variability and sources of NO3 in a regional karst aquifer system, the Edwards aquifer of central Texas. Samples from streams recharging the aquifer, groundwater wells, and springs were collected during 2008–12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO3 stable isotopes (δ15N and δ18O). These data were augmented by historical data collected from 1937 to 2007. NO3 concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO3 concentrations in the Barton Springs segment is controlled by occurrence of individual storms and multi-annual wet-dry cycles, whereas the lack of short-term variability in groundwater in the San Antonio segment indicates the dominance of transport along regional flow paths. In both segments, longer-term increases (years to decades) in NO3 concentrations cannot be attributed to hydrologic conditions; rather, isotopic ratios and land-use change indicate that septic systems and land application of treated wastewater might be the source of increased loading of NO3. These results highlight the vulnerability of karst aquifers to NO3 contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008–10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO3than does discharge, consistent with nitrification of organic N within the aquifer and consumption of dissolved oxygen. This study demonstrates that subaqueous nitrification of organic N in the aquifer, as opposed to in soils, might be a previously unrecognized source of NO3 to karst groundwater or other oxic groundwater systems.

Publication Year 2016
Title Source, variability, and transformation of nitrate in a regional karst aquifer: Edwards aquifer, central Texas.
DOI 10.1016/j.scitotenv.2016.05.201
Authors MaryLynn Musgrove, Stephen P. Opsahl, Barbara Mahler, Chris Herrington, Thomas Sample, John Banta
Publication Type Article
Publication Subtype Journal Article
Series Title Science of the Total Environment
Index ID 70173854
Record Source USGS Publications Warehouse
USGS Organization Texas Water Science Center