The use of 36Cl and chloride/bromide ratios in discerning salinity sources and fluid mixing patterns: A case study at Saratoga Springs

The well-known Saratoga Springs of New York State are low-temperature (10–19 °C), mildly acidic, highly carbonated mineral springs which discharge along the many normal faults in the Saratoga–McGregor Fault System of Eastern North America. Fifteen saline and moderately saline fluids collected from the Saratoga Springs system and its surroundings were measured for 36Cl/Cl ratios, chloride concentrations, and Cl−/Br− ratios to determine the sources of salinity, fluid mixing patterns and history of these waters. The springs have total chloride concentrations between 300 and 6500 mg/L, far in excess of local freshwater, and Cl−/Br− ratios typically in the range of 100–140. 36Cl/Cl ratios for these waters range from (3–22) × 10− 15, markedly below both the modern rainwater 36Cl/Cl ratio of above 1500 × 10− 15, and the expected pre-nuclear rainwater 36Cl/Cl ratio for the region of 400 × 10− 15, indicating that Cl− and 36Cl in these springs are primarily derived from subsurface, non-meteoric sources. 36Cl concentrations and Cl−/Br− ratios can be effectively used to eliminate the possibility of magmatic, ancient marine, porefluid, and evaporite contributions to the springs. 36Cl production rates and secular equilibrium 36Cl/Cl ratios were derived for different subsurface lithologies in the region based on estimated subsurface neutron fluxes for different rock types. A comparison of 36Cl/Cl ratios and Cl−/Br− ratios in the Saratoga Springs with secular equilibrium 36Cl/Cl ratios and with Cl−/Br− ratios in potential host formations indicates water/rock interactions in the subsurface to be the main source of 36Cl to the springs, especially from the Ordovician carbonates which make up the aquifer, and the underlying Cambrian clastic rocks. The more dilute springs appear to be a mix of saline springs and a meteoric component, probably due to the addition of rainwater as the springs rose to the surface along the fault zone. The use of chloride/bromide ratios was found to be an effective way to support and clarify the information yielded by 36Cl/Cl ratios and chloride concentrations in the study of brine history.