Isotope hydrology of southern Nevada groundwater: Stable isotopes and radiocarbon

A new !O map of southern Nevada groundwater shows a systematic decrease in !O of !5‰ from 36" to 39"N latitude. The variation is consistent with higher-latitude recharge following continuous flow paths along north-south trending graben valleys and systematically increasing in !O due to mixing with lower-latitude, higher-!O recharge. The data do not suggest that large masses of groundwater with unusually low !O values were recharged during the last pluvial period as suggested by previous workers. Many !O-!D pairs in groundwater indicate variable amounts of evaporation relative to the global meteoric water line. The precipitation rate and type (rain versus snow) for a given geographic area controls the extent of evaporation. A “model” !O value was calculated from evaporated groundwaters by subtracting postcloud evaporation. The distribution of these model !O values suggest new regional groundwater flow paths previously undocumented. Dissolved inorganic carbon in groundwaters collected from deep regional flow systems typically has low C concentrations ("12% modern carbon). In contrast, groundwaters collected from the carbonate rock of the Spring Mountains and alluvium of Forty Mile Canyon have higher C contents, indicating more recent recharge. Because of nonlinear mixing of C, groundwater in the regional flow system likely acquired most of its observable C from mixtures of young, locally recharged groundwater.