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To establish the origin of this pattern, an additional 900 wells &lt;60\u2010m deep were tested for As and nine sites were drilled along a 35\u2010km transect perpendicular to the river. The field data show no relation between groundwater As concentrations ranging from &lt;1 to 660\u00a0\u03bcg\/L along the transect and (a) As concentrations of &lt;1\u20135\u00a0mg\/kg in cuttings of aquifer sand recovered while drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates, however, a marked increase in the thickness of a clay layer capping the aquifer starting from &lt;1\u20135\u00a0m near the river to over 60\u00a0m at the most distant site toward the base of the Naga foothills. Organic radiocarbon ages of 18\u201346\u00a0kyr obtained from all but one of 13 clay samples indicate pre\u2010Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of dissolved inorganic carbon of 0.2, 4.7, and 17.8\u00a0kyr were measured in groundwater from three monitoring wells installed to 30\u201360\u2010m depth at distances of 10, 20, and 40\u00a0km from the river, respectively. A conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.<\/jats:p>","DOI":"10.1029\/2017wr022485","type":"journal-article","created":{"date-parts":[[2018,8,7]],"date-time":"2018-08-07T05:31:01Z","timestamp":1533619861000},"page":"8160-8173","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":25,"title":["The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35\u2010km Transect Perpendicular to the Upper Brahmaputra River in Assam, India"],"prefix":"10.1029","volume":"54","author":[{"given":"R.","family":"Choudhury","sequence":"first","affiliation":[{"name":"Department of Civil Engineering Indian Institute of Technology  Guwahati India"},{"name":"Now at Department of Geological Sciences Gauhati University  Gauhati India"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"B.","family":"Nath","sequence":"additional","affiliation":[{"name":"Lamont\u2010Doherty Earth Observatory Columbia University  Palisades NY USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2960-9648","authenticated-orcid":false,"given":"M. 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