Measurement and significance of the direct discharge of groundwater into the coastal zone

While the major rivers of the world are reasonably well gauged and analysed, thus allowing comparatively precise estimates of riverine inputs to the ocean, it remains very difficult to evaluate the influence of direct groundwater discharge into the ocean. In spite of the recognition that many land-sea interfaces of the world are characterised by 'leaky' continental margins, it is unclear how important groundwater-derived springs and seeps are in terms of overall marine geochemical budgets. The principal reason that groundwater estimates have not caught up to the precision base typical of other oceanic inputs is that the direct discharge of groundwater into the coastal zone is inherently very difficult to measure. Concerted efforts are now being made to improve this situation by a variety of hydrological and oceanographic techniques. Standard hydrological approaches include measurement of the hydraulic gradient on land with flow being estimated either analytically or numerically. Groundwater flow is also assessed using standard water balance considerations. Unfortunately, the uncertainties in the data used for water balance calculations are often on the same scale as the flow being evaluated. Oceanographers have been using seepage meters, mini-piezometers and geochemical tracers to estimate submarine groundwater discharge (SGD) in the coastal zone. Hydrogeologists and oceanographers thus tend to approach the same problem fro different ends. Rarely are methodologies combined or more than one approach applied in the same study. We suggest that intercalibration experiments be designed in order to provide a more complete recognition of the strengths and weaknesses of various approaches. (C) 2001 Elsevier Science B.V. All rights reserved.