CONNECTING VADOSE ZONE NITRATE TO GROUNDWATER CONTAMINATION AT THE HANFORD SITE WITH STABLE ISOTOPES

    The δ¹⁵N and δ¹⁸O of nitrate in de-ionized water leaches of vadose zone core samples are used to identify likely sources of groundwater contamination at the USDOE Hanford Site in south-central Washington.  The isotopic compositions of nitrate from undisturbed vadose zone sediments (δ¹⁵N of 4 to 8‰, δ¹⁸O of -9 to 4‰) are consistent with natural soil nitrate from microbial nitrification.  The semi-arid climate of Hanford leads to seasonal shifts of 3 to 10 ‰ in the δ¹⁸O values of near surface pore waters, which may have caused the observed range in nitrate δ¹⁸O.  Buried caliche layers in the vadose zone have significantly elevated nitrate concentrations (>10x background) with similar δ¹⁵N (0 to 8‰) and high δ¹⁸O (18 to 42‰) relative to the background soil nitrate.  Vadose zone sediments from chemical processing areas at the site also have elevated nitrate levels where concentrated chemical and radioactive wastes leaked from buried tanks or were allowed to infiltrate into the vadose zone through disposal trenches.  The isotopic composition of the contaminant nitrate (δ¹⁵N of 10 to 35‰, δ¹⁸O of -5 to 5‰) was likely imparted by the chemistry used during plutonium recovery processing and is distinct from the background sources.  Significant amounts of dilute wastewater were also discharged to the vadose zone in these areas through infiltration ponds.

    The isotopic compositions of nitrate from 77 groundwater wells have been analyzed.  Groundwater samples from undisturbed areas of the site have nitrate isotope compositions similar to background vadose zone nitrate.  Samples from groundwater nitrate plumes in the vicinity of the chemical processing areas (concentrations up to 763 mg/L as NO₃-) have isotopic compositions consistent with a mixture of background soil nitrate and nitrate that has high δ¹⁸O values, such as the nitrate associated with the caliche layers.  These data indicate that most of the elevated nitrate concentrations in Hanford groundwater resulted from flushing nitrate out of the vadose zone during low-level wastewater disposal, rather than migration of the high δ¹⁵N vadose zone contamination.