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 Ernest O. Lawrence Berkeley National Laboratory and were used to constrain flow velocities and search for preferential flow paths. In the Orinda formation siltstone, ⁸⁷Sr/⁸⁶Sr increases strongly over tens of meters along presumed flow paths, indicating slow groundwater flow. Dissolved Sr is close to isotopic equilibrium with the exchangeable Sr in the clays, and the observed ⁸⁷Sr/⁸⁶Sr increase is interpreted as a cation exchange front moving slowly through the unit combined with dissolution of minerals with relatively high ⁸⁷Sr/⁸⁶Sr ratios. The data are inverted using a one- dimensional transport-dissolution-exchange model; the results indicate long- term average flow velocities of less than 0.2 m/yr which are consistent with ¹⁴C measurements. The data suggest a lack of strong preferential flow paths through this unit.