Use of Groundwater Tracers to Assess Climate Change in North-Central New Mexico during the Holocene Epoch
Thursday, 18 December 2014
The groundwater system at Los Alamos, New Mexico encompasses a complex shallow mountain-block and mountain-front zone; intermediate-depth perched zones located east of the mountain front; and an extensive regional aquifer occupying the Rio Grande rift. In the study area, groundwater-flow paths in the regional aquifer are generally from the northwest to southeast that laterally extend 17 km before discharging to the Rio Grande. This system is unconfined and variable mixing occurs at greater distances east of the primary recharge zones. Since 1943, the regional aquifer has been mined for industrial processes and residential use and consumption at Los Alamos. A large data set for groundwater-age and inorganic solutes has been assembled for the intermediate-depth perched zones and regional aquifer, which provides a potential archive record of paleoclimate-change occurring during the past 10,000 years. Groundwater ages range from modern groundwater to approximately 9,700 years before present. Unadjusted radiocarbon-age results for groundwater samples collected from 58 background-monitoring wells and 10 springs correlate well with natural chloride and perchlorate concentrations and δ18O values. Background concentrations of dissolved perchlorate and chloride increase with increasing groundwater age and residence time. Values of δ18O are slightly heavier in groundwater samples having increasing age, suggesting that warmer climatic conditions occurred 10,000 years before present. Perchlorate and δ18O datasets correlate the strongest with average groundwater age, showing Pearson correlations of 0.81 and 0.71, respectively. The Pearson correlations for chloride to age and perchlorate to chloride are 0.62 and 0.81, respectively. Overall, this dataset suggests that climatic cooling has gradually occurred in north-central New Mexico during the Holocene, and does corroborate previous Holocene climate-change studies conducted in the southwestern United States.