RAPID EXCHANGE EFFECTS ON ISOTOPE RATIOS IN GROUNDWATER SYSTEMS .2. FLOW INVESTIGATION USING SR ISOTOPE RATIOS

Sr isotope ratios were measured in groundwater, whole rock digestions, and cation exchange extracts from a clay-rich groundwater system at E rnest O. Lawrence Berkeley National Laboratory and were used to constr ain flow velocities and search for preferential flow paths. In the Ori nda formation siltstone, Sr-87/Sr-86 increases strongly over tens of m eters along presumed flow paths, indicating slow groundwater flow. Dis solved Sr is close to isotopic equilibrium with the exchangeable Sr in the clays, and the observed Sr-87/Sr-86 increase is interpreted as a cation exchange front moving slowly through the unit combined with dis solution of minerals with relatively high Sr-87/Sr-86 ratios. The data are inverted using a one-dimensional transport-dissolution-exchange m odel; the results indicate long-term average flow velocities of less t han 0.2 m/yr which are consistent with C-14 measurements. The data sug gest a lack of strong preferential flow paths through this unit.