GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 33-10
Presentation Time: 9:00 AM-5:30 PM


BATIANIS, Elizabeth, HORGAN, Julianna S. and VINSON, David S., Department of Geography & Earth Sciences, University of North Carolina at Charlotte, 9201 University City Blvd., McEniry 324, Charlotte, NC 28223

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.