Geochemical techniques to discover open cave passage in karst spring systems

Dissolved O2 (DO) and pH data provide a novel, inexpensive field method to detect open cave passages in karst spring systems that can be complemented by laboratory measurements of major elements and nutrients, total suspended solids (TSS), Escherichia coli (E. coli) levels, and stable isotopes. Karst springs in east-central Missouri that have no known air-filled passages (“phreatic” springs) typically have low DO and pH values (<80% saturation and <7.7, respectively), which are characteristic of groundwaters that do not communicate with the atmosphere. In contrast, springs draining vadose cave passages have higher DO and pH values (>60% saturation and >7.7, respectively) that resemble surface waters due to the equilibration of DO with the overlying cave atmosphere and the simultaneous degassing of dissolved CO2. Traverses down phreatic spring branches show exchange with the atmosphere causes an increase in DO only a short distance downstream of the spring orifice, while the pH concurrently increases due to the degassing of CO2. Further downstream both parameters tend to level off reflecting a general approach to equilibrium under surface conditions, though this process is more rapid for DO than for pH. In contrast, the DO and pH along cave spring branches change little from values at the cave entrance. Additionally, (1) degassing processes affect the saturation state of calcite, with cave springs being the most saturated with respect to calcite, (2) phreatic springs typically have lower TSS and E. coli levels than open cave springs due to slower and less variable flow delivery, longer residence times, and less turbulent flow, and (3) phreatic springs tend to plot on the global meteoric water line (MWL), while waters from open cave systems tend to be enriched in 18O and D and can plot below the line due to evaporation and exchange of the water with the overlying cave atmosphere.