T. M. Whitworth

Although geologic membrane effects are postulated to be important in the subsurface, their significance is uncertain. This paper investigates the potential significance ofgeologic membrane effects in perched aquifers. The steady-state analytical model presented herein uses parameters which can be measured, calculated, and/or estimated and is capable of addressing hyper filtration in multi-component groundwaters. Modeling was for differential heads ofbetween 1 and 2.5 meters across 10 to 100 cm-thick shales. Perched aquifer scenarios were chosen because the differential head across the lower bounding layer equals the water depth and horizontal flow is minimal. The results show the width ofthe concentration polarization layer (CPL) to be proportional to shale thickness and inversely proportional to the head differential across the membrane. Calculated solute concentration distributions indicate that it may be difficult to perform accurate groundwater sampling when a significant CPL is present. The modeling results suggest that membrane effects should be significant at gradients which naturally exist in perched aquifers. CPLs produced at these relatively low gradients contain relatively large masses of solute and have the potential to affect a number ofsaturated zone geochemical and biological processes. Ironically, one of the arguments against subsurface membrane effects has been that the high pressure gradients used in the laboratory do not exist in nature. study suggests that such high pressures are not necessary.

1998 New Mexico Geological Society Annual Spring Meeting April 9, 1998, Macey Center Online ISSN: 2834-5800

Pages: 13