Groundwater recharge timing based on 14C and 2H within Indian Wells Valley, California, USA

Abstract

A large carbon-14 dataset for groundwater in Indian Wells Valley, California, is used to identify groundwater recharged during the pluvial climate of the previous glacial period, more than 12,000 years before present. Additional data collected in 2019, including measurements of 13C and 14C in the recharge areas and dissolved organic carbon isotopes, substantiated the unusually heavy-isotope-enriched 13C and relatively low 14C compositions in the Sierra Nevada canyons on the northwestern margin of the basin. The carbon isotopic characteristics in this area may be due to aeolian deposits of sodium carbonate minerals from dried lake beds along the ancient Owens River, or dissolution of metasediments within the Sierra Nevada batholith, or geothermally sourced CO2 gas, or a combination of these. Recognizing these isotope characteristics is essential for correcting apparent 14C groundwater ages. Additional complications arise from sulfate reduction and related conversion of 14C-dead sedimentary organic material to dissolved inorganic carbon with a light 13C composition. The lacustrine deposits of pluvial China Lake and organic-rich spring deposits along the Sierra Nevada range front offer reducing environments across much of the valley. A similarly large 2H dataset provides an independent marker for recharge in a previous climate, facilitated by exhaustive regional studies of 2H content of precipitation, springs, and groundwater (Smith et al., 2002). The 14C data support an association of δ2H values less than −100‰ with pluvial-climate groundwater and δ2H values greater than −96‰ with groundwater recharged under current climatic conditions. Groundwaters with δ2H between these values cannot be assigned with certainty to either group. Evaporation is also evident for a small set of groundwater samples, rendering 2H unreliable as a climate indicator for them. Using 14C and 2H in combination, much of Indian Wells Valley is found to contain groundwater consistent with recharge during current climatic conditions, with ages generally less than 6000 years. Groundwater with pluvial characteristics occurs primarily in the far end of the flow system, in the eastern part of the valley near the evapotranspiration discharge area. The footprint of this older water is more extensive in the deeper parts of the flow system and the vertical distribution of ages indicates it moves upward from the deeper parts of the flow system to the shallow zone to discharge. Estimates of groundwater travel time from a numerical flow model of the basin are generally consistent with the isotopic interpretations.