Presenter: Bruce K. Darling, Ph.D., P.G., C.P.G.,
AIPG Texas Section District II Representative
Abstract
Carbon-14 (half-life 5,730 yrs) has been widely used in hydrogeologic investigations to age-date groundwater … or the time elapsed since water entered an aquifer system through infiltration within a recharge area. The use of the radioisotope, however, is fraught with many problems related to subsurface processes that alter the original carbon-14 signature, and which lead to widely varying and erroneous estimates of age. Such processes include mixing of groundwaters and rock-water interactions that dilute active carbon with “dead” carbon derived from sources such as carbonate rocks or ancient coal deposits. Many correction factors have been proposed to adjust measured carbon-14 in samples of groundwater, but these factors often require assumptions based on processes that are not well-documented and data that are either incomplete or not often available to hydrogeologists.
This presentation will offer a different perspective on the use of carbon-14 based on the results of a multi-year investigation of the aquifer systems of the Eagle Flat and Red Light basins of Trans-Pecos Texas. The basins are located in Hudspeth County. The objective was to ascertain whether reasonable estimates of groundwater age could be drawn from unadjusted carbon-14 and oxygen-18, a stable environmental isotope with fractionation properties related to temperature, rainout effect, and to climate change over long spans of time. Data collected for the investigation show that the apparently oldest groundwaters (based on unadjusted carbon-14) are associated with oxygen-18 exhibiting the greatest amount of depletion. Depletion and apparent age both increase along flow paths from recharge areas along basin boundaries to the deepest groundwaters of the basins. Depletion with respect to oxygen-18 increases markedly between 15 and 20 percent modern carbon (pmc). This pattern is interpreted to indicate the transition from precipitation in the cooler and wetter Late Pleistocene climate of the Southwest to the semi-arid/arid modern climate over a span of 10,000 or more years. Researchers with New Mexico Institute of Mining and Technology documented similar association between C-14 and O-18 in groundwater of the San Juan Basin of Northwestern New Mexico. (This presentation is based in part on the 2023 AIPG-TX Field trip led by Bruce Darling (Here).
Attached: Presentation Slides (Here)
Bio
Bruce Darling has nearly five decades of experience in the energy, mineral, and groundwater industries. Over the course of his career, the Trans-Pecos region of Texas, especially Hudspeth County, has been a focal area of much of his geological consulting. Bruce earned a PhD in Geology and a MA in Energy & Mineral Resources (Mineral Economics) at UT-Austin. Before embarking on his PhD program, he was a doctoral student in Mineral Economics at Colorado School of Mines. He earned his MS in Geology at the University of Louisiana at Lafayette. As a mining economics student at Mines and UT-Austin, he studied international mineral economics.
