The isotope patterns observed and estimated SGD rates can be driven by a combination of at least 3 major processes: (1) fresh groundwater discharge, (2) free convection of surface water in crab burrows, and (3) tidally-driven recirculation of creek water into crab burrows. The very similar patterns revealed by 222Rn, 223Ra and 224Ra in both summer and winter imply that
BSI201 SGD was entirely recirculated seawater. Table 4 demonstrates minor differences in estimated groundwater fluxes using the different isotopes. Larger fluxes calculated from radon compared to those calculated from radium would indicate a fresh groundwater input (Mulligan and Charette, 2006). However, the slightly lower 222Rn-derived fluxes imply negligible fresh groundwater inputs, and/or that we may have underestimated a radon
sink (potentially current-driven evasion to the atmosphere, see above). The suggestion that fresh groundwater discharge is negligible is consistent with the lack of upstream freshwater inputs to this creek (i.e., no freshening of creek water at low tide).
