Seawater‐groundwater mixing in and fluxes from coastal sediment overlying discrete fresh seepage zones: A modeling study

• Published in: Journal of geophysical research. Oceans 2017-08, Vol.122 (8), p.6565-6582
• Main Authors: Zamora, Peter B., Cardenas, M. Bayani, Lloren, Ronald, Siringan, Fernando P.
• Format: Article
• Language: English
• Subjects: Biogeochemistry; Brackish water; Cells; Chemical analysis; coastal aquifer; Coastal ecology; Coastal environments; Coastal sediments; Computer simulation; Environmental impact; Fluxes; Fresh water; Freshwater; Geophysics; Gradients; Groundwater; Groundwater discharge; Groundwater flow; Hosts; Inland water environment; Intertidal environment; Intertidal zone; Measuring instruments; mixing; Modelling; Mud-water interfaces; Niches; Oscillations; Outlets; Philippines; Plumes; Pore water; Salinity; Salinity effects; Salinity measurements; Saturated flow; Seawater; Sediment; Sediment-water interface; Sediments; Seepage; seepage zones; Sensitivity analysis; springs; submarine groundwater discharge; Thickness; Tidal oscillations; Tides; Water analysis; Water discharge; Water springs; Width
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2017JC012769

Submarine fresh groundwater discharging from discrete zones such as conduits overlain by thick sediment remains poorly documented and understood despite common anecdotal accounts of it. We analyzed this phenomenon by a suite of variable‐density, variably saturated groundwater flow simulations guided by field studies. The field observations included point flux and salinity measurements using seepage meters spread across the intertidal and the subtidal zones. The discretely measured seepage fluxes in the subtidal region were as high and the salinity was as fresh as those from the intertidal zone. Both fluxes and salinities varied with tides. Two‐dimensional simulations of the idealized beach‐ocean vertical section with discrete freshwater springs at the base showed the development of seawater recirculation cells along the edge of freshwater discharge plumes emanating from the outlets, causing mixing. Brackish water discharges at the sediment‐water interface above where the plume mixes with the recirculating seawater. Sensitivity analyses showed that stronger terrestrial fresh groundwater flow increases the freshwater plume size but does not affect the mixing zone width. Tidal oscillations and a dynamic fresh groundwater flow widened the mixing zones and increased pore water flux across the sediment‐water interface. Sediment thickness had a minor effect on the width of both the freshwater plume and its surrounding mixing zone. The subtidal freshwater plume and mixing zone likely represent a unique but potentially ubiquitous setting which hosts dramatic ecological and chemical gradients. Their local biogeochemical niche and coastal scale ecosystem impacts will need to be considered in investigations of coastlines. Key Points Fresh groundwater discharge from discrete outlets to overlying sediment is modeled and observed A dynamic and broad freshwater plume forms around the discharge point The mixing zone around the plume is narrow but dynamic. Seawater recirculation occurs beside the plume