Using direct measurements of submarine groundwater discharge to investigate the coupling between surface and pore waters.

摘要

Submarine groundwater discharge (SGD) and its associated impact on coastal ecosystems was investigated at the sediment-water interface using diverse methods. This intercomparison of methods was the objective of a major project carried out in 5 diverse hydrogeological settings (Cockburn Sound, Australia; Donnalucata, Sicily; Shelter Island, USA; Ubatuba Bay, Brazil; and Flic-en-Flac Bay, Mauritius). Small-scale sedimentary processes were deemed very important in the control of local hydrogeological characteristics. Seepage meters were used to directly measure the flow of water across the sediment-sea interface. Coincident measurements of bulk ground conductivity (BGC) were made alongside seepage meters at four of these locations. An inverse relationship between BGC and SGD allowed for the extrapolation of point measurements of SGD to larger areas using BGC data. SGD estimates made using this method compared favorably with those obtained using other techniques.;Using seepage meters to measure flow rates, along with a manual drive point piezometer to measure pore water profiles, the coupling between pore water composition and advection due to SGD was investigated. The process of dispersion was found to determine both the shape and depth of salinity, nutrient, and radium profiles in the sediment. Dispersion may be controlled by biological or physical processes including the rate of advection itself, all of which change over time. Dispersion coefficients ranging from 0.02 m2d -1 to 2.8 m2d-1 were estimated from direct measurements.;This data also allowed for the investigation of anthropogenic impacts on the signature of SGD in coastal lagoons. At Shelter Island, the pilings of a pier altered the flow of groundwater into the sea by piercing a confining layer and allowing for a large influx of fresh groundwater from below. In the Venice Lagoon, the difference in water elevation between the lagoon and the sea has been investigated as a possible driver of SGD beneath the barrier lands, which separate the two bodies of water. A strong correlation was found between water level difference and SGD. This suggested that the hydraulic gradient caused by this difference drives a flow beneath the barrier island. The flow is enhanced by the presence of artificial conduits created when former inlets were in-filled. If the inlets are closed by storm surge barriers, as proposed, a groundwater exchange beneath the barriers could potentially be as large as 1.0 x 106 m3 d-1.