CHARACTERISTICS AND IMPLICATIONS OF SUBMARINE GROUNDWATER DISCHARGE OVER FRACTURED ROCK AQUIFERS

In fractured rock aquifer, clustering of fractures and the topography of the rock-sediment interface might focus or disperse submarine groundwater discharge (SGD) which also responds in chaotic ways to complicated connections between local recharge and withdrawals. Pore water chemical profiles must show similar variability in the face of upward advection, further complicating the calculation on geochemical fluxes. Benthic chambers vented to collection bags were used to measure SGD across the sea floor over fractured rock aquifers in Brazil and the island of Mauritius. Although a thin, unconsolidated sediment cover tended to diffuse the flow, SGD was unpredictably variable both in time and space. SGD at some locations exceeded 400 cubic centimeters of pore water per square centimeter of sea floor per day (400 cm/day). Large variations in SGD were seen over distances of a few meters, but high flow rates were easily found at between a quarter and half the spots tested indicating that high SGD is common. SGD rates at particular locations were observed to double (or halve) over hour's time for no apparent reason. A good correlation was found between pore water salinity and SGD. A SGD of 100 cm/day is capable of flushing the upper two meters of the sediment every day; the relatively high salinity observed in the pore water of the sediment blanket requires an efficient mixing process in the surficial sediments themselves. Interpretations of pore-water profiles in surficial sediment and calculations of chemical fluxes in the coastal zone both need to accommodate (a) rapid vertical mixing, (b) high, but transient, upward vertical advection and (c) meter-scale spatial variations. Although not as pronounced, similar characteristic have been seen in direct observations of SGD in unconsolidated aquifers which seem fairly heterogeneous (e.g. on the coastal plain of Long Island, NY and the deltaic sediments of the Venice lagoon); this may be due to the complicated shallow stratigraphy of sedimentary, coastal settings or to anthropogenic development which imposes pseudo-karst characteristics on the groundwater system.