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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>J.K. Böhlke</dc:contributor>
  <dc:contributor>M. W. Doughten</dc:contributor>
  <dc:creator>Niel Plummer</dc:creator>
  <dc:date>2006</dc:date>
  <dc:description>&lt;div class="hlFld-Abstract"&gt;&lt;div id="abstractBox"&gt;&lt;p class="articleBody_abstractText"&gt;Groundwater from remote parts of the Middle Rio Grande Basin in north-central New Mexico has perchlorate (ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) concentrations of 0.12−1.8 μg/L. Because the water samples are mostly preanthropogenic in age (0−28 000 years) and there are no industrial sources in the study area, a natural source of the ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;is likely. Most of the samples have Br&lt;sup&gt;-&lt;/sup&gt;, Cl&lt;sup&gt;-&lt;/sup&gt;, and SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;concentrations that are similar to those of modern bulk atmospheric deposition with evapotranspiration (ET) factors of about 7−40. Most of the ET values for Pleistocene recharge were nearly twice that for Holocene recharge. The NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;ratios are more variable than those of Br&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;or SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;. Samples thought to have recharged under the most arid conditions in the Holocene have relatively high NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;ratios and low δ&lt;sup&gt;15&lt;/sup&gt;N values (+1 per mil (‰)) similar to those of modern bulk atmospheric N deposition. The δ&lt;sup&gt;18&lt;/sup&gt;O values of the NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;(−4 to 0 ‰) indicate that atmospheric NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;was not transmitted directly to the groundwater but may have been cycled in the soils before infiltrating. Samples with nearly atmospheric NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;ratios have relatively high ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;concentrations (1.0−1.8 μg/L) with a nearly constant ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;mole ratio of (1.4 ± 0.1) × 10&lt;sup&gt;-&lt;/sup&gt;&lt;sup&gt;4&lt;/sup&gt;, which would be consistent with an average ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;concentration of 0.093 ± 0.005 μg/L in bulk atmospheric deposition during the late Holocene in north-central NM. Samples thought to have recharged under wetter conditions have higher δ&lt;sup&gt;15&lt;/sup&gt;N values (+3 to +8 ‰), lower NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;ratios, and lower ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;ratios than the ones most likely to preserve an atmospheric signal. Processes in the soils that may have depleted atmospherically derived NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;also may have depleted ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;to varying degrees prior to recharge. If these interpretations are correct, then ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;concentrations of atmospheric origin as high as 4 μg/L are possible in preanthropogenic groundwater in parts of the Southwest where ET approaches a factor of 40. Higher ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;concentrations in uncontaminated groundwater could occur in recharge beneath arid areas where ET is greater than 40, where long-term accumulations of atmospheric salts are leached suddenly from dry soils, or where other (nonatmospheric) natural sources of ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;exist.&lt;/p&gt;&lt;/div&gt;&lt;/div&gt;&lt;div class="hlFld-Fulltext"&gt;&lt;br data-mce-bogus="1"&gt;&lt;/div&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1021/es051739h</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>ACS</dc:publisher>
  <dc:title>Perchlorate in pleistocene and holocene groundwater in North-Central New Mexico</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>