FLUID-FLOW THROUGH CLAYEY SOILS - STABLE-ISOTOPE AND MINERALOGICAL EVIDENCE

The evaluation of clays as barriers to fluid movement can be improved by geochemical methods that provide ways to examine the reactivity and weathering of minerals in soils and sediments. X-ray radiography, X-r ay powder diffraction, and stable isotope geochemistry provide new dat a from field locations in the Mississippi River Delta of Louisiana ind icating that the clays are not effective barriers to the vertical migr ation of fluids in the shallow subsurface. Systematic changes in the m ineral assemblages, the soil structure and the delta D and delta(18)O values of fine clay fractions can best be explained by an alteration s equence produced as the originally smectite-rich clay mineral assembla ge was kaolinized by percolating groundwater.