Dating Groundwater Using Dissolved Organic Carbon and Estimating Flow-path Travel Times in Southern Nevada Aquifers
AdvisorHershey, Ronald L
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ABSTRACTThe study's objective was to evaluate the use of dissolved organic carbon (DOC) carbon-14 (14C) to estimate groundwater ages in southern Nevada aquifers. Hydrogeologists have traditionally used dissolved inorganic carbon (DIC) 14C to estimate groundwater ages. However, because multiple processes change DIC 14C activities in groundwater, complex corrections are often required, and the ages generated can be frustratingly inexact. The hypothesis was that DOC 14C dating is more accurate because DOC is less reactive with aquifer rocks. Water samples were analyzed for 14C by accelerator mass spectrometry. Laboratory experiments indicated that there is no significant dilution of groundwater DOC 14C by leaching of `dead' (no 14C) organic carbon from aquifer rocks. Similar DOC compounds were identified in both up- and down-gradient waters meaning that alterations from kerogen or bacteria likely have minimal effects on DOC 14C activities. Loss of DOC 14C through matrix diffusion is minimal because calculated diffusion coefficients were relatively small for DOC 14C compared to a conservative bromide tracer. Dissolved organic carbon 14C activity , expressed in percent modern carbon (pmc), for 14 recharge area locations ranged from 96.0 to 120.1 pmc, with an average of 105.8 pmc, yielding a recent (post-atmospheric nuclear weapons testing) age. Dissolved organic carbon 14C activity for eight down-gradient locations ranged from 45.8 to 75.2 pmc with an average of 62.3 pmc, or about 3,900 years. Flow-path travel times are thousands of years less for DOC than for both corrected and uncorrected DIC. This discrepancy likely is the result of water-rock reactions and other physical processes such as matrix diffusion that lower DIC 14C activities in groundwater. Results from this study demonstrate that the DOC 14C dating technique can be used to estimate groundwater ages in aquifers with low DOC and provides an alternative to the traditional DIC 14C dating.