Geochemical Characteristics of Submarine Groundwater Seeps on the South Carolina Continental Shelf

Groundwater discharge into the marine environment is a potentially significant pathway for exchange of nutrients, pathogens, and other dissolved chemical constituents between land and sea.  A combination of geophysical and geochemical techniques is geared towards quantifying the spatial distribution of groundwater seeps, chemical properties of the groundwater, and the total flux of groundwater across the sediment/water interface.  Funding through South Carolina Sea Grant Consortium has supported the preliminary work.

Geophysical Investigations

The continuous resistivity profiling (crp) method allows for measurement of the electrical structure of the shallow sediment layers in the marine environment. Instrumentation used during the preliminary investigation includes a Supersting R8 IP 8 channel resistivity meter and 100 meter streamer array, leased from Advanced Geosciences, Inc. The 8 channel array allows for simultaneous measurements of potential differences through a variety of depths, up to approximately 30% of the streamer length. For example, in 15 meters of water, the electrical structure of the upper 15 meters of sediment may be imaged with this system. A 2D inversion model, supplied by Advance Geosciences, Inc converts measured potential drops into resistivity profiles (figure 1). Positioning information for the array is achieved through constant logging of dGPS signals during the trackline surveys. On the processed profiles, warmer colors (reds) indicate areas of elevated resistivity signal. For conditions of constant porosity and fairly uniform sediment composition, elevated resistivity signals suggest the presence of relatively fresher pore water than the ambient ocean condition.

In order to reconcile the patchy nature of the resistivity signal, Chirp subbottom profiles are used to assess structure and variability in sediment layers. Profiles were collected with the BCCMWS Edgetech SB512i Chirp. The geologic framework of Long Bay is characterized by Cretaceous/Tertiary strata outcropping on the seafloor in some areas and unconformably overlain by a marine transgressive surface in others (Baldwin et al., 2004). The inner shelf and shoreface are covered with a discontinuous, thin layer of modern sediments. Strata are frequently truncated and incised by extensive paleochannel networks which are back-filled with sediments of varying age. Gently tilted and folded strata contain layers of variable erosional resistance resulting in complex outcrop patterns. Axes of low-amplitude antiformal features are frequently breached as a result of erosional patterns related to sea level fluctuation. Abrupt changes in geologic conditions imaged with the subbottom profiler often correlate with changes in resistivity (figure1). This relationship suggests that there are significant geologic controls on the distribution of groundwater seeps.

Geochemical Investigations

Preliminary geochemical investigations include measurement of radon activities along shore. Excess Rn in the water column is an indicator of a groundwater contribution. Geochemical tracers (e.g. Ra, Rn) are often used to determine groundwater fluxes, but are irrespective of the area over which any potential discharge is occurring. Flux estimates will be refined through careful assessment of the area through which discharge is occurring via the geophysical methods described above. Preliminary results from the ongoing Rn study suggest a significant groundwater contribution to nearshore waters (figure 2).

Additional ongoing work will assess groundwater chemistry (nutrient concentrations, Rn activity, redox potential) across the shoreface.

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