EVOLUTION OF DISSOLVED INORGANIC CARBON IN SAPROLITE AND BEDROCK GROUNDWATER AT THREE PIEDMONT RESEARCH SITES IN NORTH CAROLINA

This study focuses on using stable carbon isotopes to trace the production of dissolved inorganic carbon (DIC) as groundwater moves through the saprolite and bedrock to stream channels in a Piedmont crystalline rock terrane. The objectives of this study are to understand the controls on groundwater alkalinity and DIC production in order to (1) identify what causes pH to increase to near-neutral values along shallow groundwater flowpaths and (2) to better understand the geochemical signature of groundwater inputs into small streams in the Piedmont region. Dissolved inorganic carbon (DIC) can be modified by silicate weathering, which consumes CO₂, and microbial CO₂. DIC is linked to the development of groundwater pH and alkalinity, which in turn can impact stream water quality at baseflow.

Groundwater was sampled from 3 research sites on the Charlotte Terrane (North Carolina): Langtree Peninsula Research Station (LPRS), Redlair Observatory (RO), and Reedy Creek Restoration Project (RCRP). Filtered samples were analyzed for ion concentrations, DIC, and alkalinity. At LPRS, alkalinity increased from saprolite to bedrock wells (range 0.13-1.30 meq/L). δ¹³C-DIC at LPRS fell within a narrow range (-22.8‰ to -17.4‰, median -19.3‰). These trends agree with increasing pH values and SiO₂ concentrations at LPRS (5.5-8.0 and 9.2-56.1 mg/L, respectively). Preliminary data from 9 new saprolite wells at RO indicate a comparable median but broader range of δ¹³C-DIC (range -22.5‰ to -8.9‰, median -19.3‰). We also observed a considerable range of alkalinity and DIC concentrations at RO (0.50-3.63 meq/L and 0.7-2.5 mM, respectively). In shallow saprolite wells at RCRP, we did not observe a predictable relationship between hillslope, floodplain, and riparian shallow groundwater in terms of alkalinity, DIC, or δ¹³C-DIC. Overall, shallow groundwater at RCRP exhibits a large range of alkalinity and DIC concentrations. Groundwater at RCRP exhibits a similar median δ¹³C-DIC as the other sites but a larger range (range -24.1‰ to -11.0‰, median -19.2‰). At RCRP, we also observed temporal variation in alkalinity, DIC, and δ¹³C-DIC between seasons. In the aggregate, δ¹³C-DIC is consistent with a single microbial CO₂ source derived from soil gas during recharge. δ¹³C-DIC is likely affected by differing system openness depending on proximity to the water table. Work in progress includes analysis of additional groundwater samples, ion and nutrient concentrations, and examination of related stream water samples.