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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>Stephen P. Opsahl</dc:contributor>
  <dc:contributor>Barbara Mahler</dc:contributor>
  <dc:contributor>Chris Herrington</dc:contributor>
  <dc:contributor>Thomas Sample</dc:contributor>
  <dc:contributor>John Banta</dc:contributor>
  <dc:creator>MaryLynn Musgrove</dc:creator>
  <dc:date>2016</dc:date>
  <dc:description>&lt;p&gt;&lt;span&gt;Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;) loading to surface and groundwater. We investigate variability and sources of NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;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&amp;ndash;12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;stable isotopes (&amp;delta;&lt;/span&gt;&lt;sup&gt;15&lt;/sup&gt;&lt;span&gt;N and &amp;delta;&lt;/span&gt;&lt;sup&gt;18&lt;/sup&gt;&lt;span&gt;O). These data were augmented by historical data collected from 1937 to 2007. NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;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 NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;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 NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;. These results highlight the vulnerability of karst aquifers to NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008&amp;ndash;10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;than 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 NO&lt;/span&gt;&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;minus;&lt;/sup&gt;&lt;span&gt;&amp;nbsp;to karst groundwater or other oxic groundwater systems.&lt;/span&gt;&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1016/j.scitotenv.2016.05.201</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Elsevier</dc:publisher>
  <dc:title>Source, variability, and transformation of nitrate in a regional karst aquifer: Edwards aquifer, central Texas.</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>