A stable isotope investigation of nitrate contamination at Many Devils Wash, Shiprock UMTRCA Site, Shiprock, NM
AdvisorWalker, Mark J.
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The Navajo Mill was a uranium and vanadium ore processing site located in Shiprock, NM, on a terrace adjacent to the San Juan River. The leaching of ore with ammonium and sulfuric acid introduced large amounts of contaminants such as uranium, nitrate, sulfate, manganese and selenium into groundwater of the terrace and the adjacent floodplain. Many Devils Wash (MDW) is a northward trending, predominately dry bedded wash steeply incised through terrain dominated by Mancos Shale, located approximately 0.2 miles east of the terrace area, terminating in the San Juan River. Elevated levels of the same constituents present in the terrace groundwater system are present in surface and ground waters of MDW. However, mill tailings were never placed in the vicinity of MDW, and transport mechanisms of contaminants from the former millsite to MDW are not clear. Two widely differing hypotheses are currently offered regarding the source(s) of contamination at MDW. One proposes that contamination originates at the former mill site 0.6 mile west of MDW and has reached MDW by flowing along a vaguely defined route eastward to the wash. The hypothesis presented in this research proposes that the contamination, especially nitrate, is from naturally occurring sources released by geochemical processes similar to those in other areas that are underlain by Mancos. Other studies have found high concentrations of nitrate, selenium, sulfate, and uranium from decomposition and weathering of Mancos Shale. The goal of this research was to examine the likelihood that Mancos Shale is a source of nitrate in groundwater of MDW. The research approach employed stable isotopes of hydrogen, oxygen and nitrogen to better define the contamination and hydrological processes occurring in MDW. The study relied on multiple lines of evidence including direction of groundwater flow implied by the piezometric surface, and relationships between stable isotopes of N, O, and H. While not conclusive, the results of this investigation coupled with previous work indicate that the contamination in MDW is likely the result of natural geochemical processes in the Mancos Shale environment of MDW, and may not be related to past milling activities on the terrace. This hypothesis could be evaluated by a combination of sampling approaches. These include spatially and temporally extensive sampling and inclusion of isotopes on uranium.