Groundwater flow near salt domes is complex because groundwater is subject to a variety of driving forces including the release of geopressured fluids, large lateral density gradients, and regional hydraulic head gradients. The complexity of this environment is born out by recent geochemical and geophysical observations that indicate the occurrence of upward groundwater flow near some salt domes. In order to evaluate the relative importance of different mechanisms driving groundwater flow near salt domes, we have developed a numerical model that couples groundwater flow, heat transport, and transport of dissolved salt, and accounts for salt diapirism. Our calculations indicate that upward groundwater flow can occur as the result of thermal convection when the regional background salinity is greater than 15 weight percent, a value typical of many areas of the south Louisiana salt dome province. For lower background salinities, dissolution causes salt‐laden groundwater near the dome to sink, leading to depressed isotherms. While the release of geopressured fluids is difficult to quantify, it remains a likely mechanism for driving upward groundwater flow near some salt domes.
Complex groundwater convection patterns are possible near salt domes because groundwater is subject to both lateral heat and salinity gradients. In order to assess the mechanisms responsible for driving convection near salt domes we use dimensional analysis and numerical simulations to investigate coupled heat and salt transport in homogeneous sediments surrounding a cylindrical salt column. The dimensional analysis does not require the Boussinesq assumption. The coupled heat, solute, and groundwater transport equations are controlled by three dimensionless parameters: the Rayleigh number, the Lewis number, and the buoyancy ratio. The buoyancy ratio is the ratio of salinity to temperature effects on groundwater density, and it directly affects the groundwater flow equation. A finite difference numerical multigridding algorithm is used to iteratively solve the coupled transport equations. The multigridding technique was about 3 times faster than a point‐wise successive overrelaxation solution. Boundary conditions for the numerical simulations were adjusted to represent different contrasts in the thermal gradient between the salt and the overlying sediments. The contrast in thermal gradient is parameterized by the thermal conductivity ratio and is responsible for isotherms being elevated near the salt. The analysis suggests that a wide range of convective flow patterns are possible, with flow occurring either up or down along the salt flank. The sense of convection is dependent mainly on the value of the buoyancy ratio and how sharply isotherms are pulled up near the salt. These factors in turn depend on the regional salinity variation, the time since diapirism, and the thermal conductivity of water saturated sediments. While this analysis provides useful insight into the mechanisms driving free convection near salt domes, the assumptions about medium and fluid properties may limit the applicability of dimensional analysis in simulating flow in specific geologic settings.
Groundwater is an important resource, not least in south-eastern areas of England, where chalk is the dominant aquifer. Such chalk-fed stream ecosystems are rich and highly productive, they have particularly important springhead wetlands, and are characterised by fast-growing trout populations. The legislative framework, founded on the Minimum Acceptable Flow concept, is in place to protect these stream ecosystems. In surface water dominated catchments, normal operational rules use prescribed (usually hands-o)¯ows to protect in-river needs. Such rules are inappropriate for most groundwater-dominated catchments because of the slow response between change in groundwater abstraction and river¯ow. This paper illustrates the use of an Ecologically Acceptable Flow Regime (i) to determine a MAF based on the annual volume of¯ow, described as a¯ow duration curve, to protect the riverine ecosystem at the catchment scale, and (ii) to set a prescribed¯ow that must be maintained locally by river support.
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