Radiocarbon Dating and Paleohydrology of Regional Aquifer Groundwater Beneath the Pajarito Plateau, New Mexico

The regional aquifer beneath the Pajarito Plateau offers a unique opportunity to investigate groundwater age and its relationship to climate change during the Holocene. The aquifer material at the regional water table is mostly free of calcium carbonate (calcite), making radiocarbon in dissolved inorganic carbon an ideal age tracer. From 2005 through 2009, 129 groundwater samples from 69  regional aquifer wells and springs were collected and analyzed for radiocarbon and other dissolved constituents including tritium, major ions, trace elements, and stable isotopes of hydrogen and oxygen. Wells and springs exhibiting background conditions do not contain tritium above 0.3 tritium units, suggesting that no post-1943 recharge has occurred at these sampling locations. Results indicate that groundwater age increases west to east from about 500 years along the mountain-front recharge zone to 10,000 years at a discharge zone at White Rock Canyon springs discharging at the Rio Grande. Distributions of average age (radiocarbon) and modern age (tritium) confirm active recharge and mixing of pre-1943 groundwater occurs beneath perennially wet canyon bottoms dissecting the Pajarito Plateau. Groundwater flow velocities calculated from points along the regional flow path and outside the areas of active recharge range from 1 to 3 m/year. Through the 10,000 year radiocarbon record, the more climate sensitive ions such as chloride and sulfate tend to increase with age with R² correlations of 0.687 and 0.719, respectively. Stable isotope ratios of both oxygen and deuterium correlate with age and are more positive in older waters, suggesting that the climate in northern New Mexico during the Holocene was warmer than in recent times.