Relating estuarine geology to groundwater discharge at an oyster reef in Copano Bay, TX

Abstract

Coastal bays with fresh submarine groundwater discharge (SGD) have been found to provide a favorable environment to shellfish (e.g., oysters). Significant variability in the spatial distribution and magnitude of SGD may occur because of subsurface heterogeneity. We investigated the relationship between stratigraphic controls (i.e. paleovalley systems) and the spatial distribution of SGD in relation to oyster reefs. In Copano Bay, South Texas, major oyster reefs have been shown to be oriented perpendicular to shore, along interfluves, and near paleovalley margins. A novel combination approach of radioactive geochemical tracers, stable isotopes, geophysical and petrophysical techniques were used to identify estuarine depositional environments for individual hydrogeologic characterization and evaluation of SGD distribution with implications to ecological features such as oyster reefs. A 1-km2 area within a paleovalley system, in Copano Bay, yielded discernible differences in sediment type, apparent resistivity, and SGD rates among four different depositional environments: paleovalley, paleovalley margin, oyster reef and interfluve. Electrical resistivity tomography revealed a direct connection between the oyster reef and the underlying, more permeable interfluve, and the presence of a fresher SGD component in the subsurface. Measurements of SGD from stationary and mobile continuous radon-222 surveys across each depositional environment showed that, by and large, areas in proximity to reefs have larger SGD rates (62.6 ± 11.3 cm/d in January and 28.7 ± 10.8 cm/d in July) when compared to paleovalleys (31.9 ± 8.9 cm/d in January and 19.5 ± 7.2 cm/d in July). The role of depositional environments as hydrologic controls on SGD to oyster reefs revealed in this study is further confirmed by significant differences in stable and radioactive isotopes observed over small spatial scales offshore when compared to the entire estuary. Although constant mixing and radon-222 degassing of bay waters are likely altering the relationships between the different depositional environments and magnitude of SGD, this study provides an improved understanding of possible pathways of freshwater and solute inputs to oyster reefs for more informed decisions on reef restoration efforts.