A NOVEL TECHNIQUE TO DISCOVER OPEN CAVE PASSAGE IN KARST SPRING SYSTEMS

Most open cave passages remain undiscovered, either because they do not breach the surface or their entrances are concealed by obstructions. Ground penetrating radar, electrical resistivity tomography, diffraction tomography, and gravity methods can detect cave passage, but more commonly these methods are time consuming, ineffective, and expensive. We present a novel, inexpensive means to detect open cave passage in karst spring systems using dissolved oxygen (DO) and pH meters. Karst springs in east-central Missouri that have no known air-filled passages typically have low DO and pH values (< 60% saturation and < 7.7, respectively), which is characteristic of groundwaters that do not communicate with the atmosphere. In contrast, springs draining vadose cave passages have higher DO and pH values (60 – 90% saturation and > 7.7, respectively), which resemble surface waters due to the equilibration of DO with the overlying cave atmosphere and the simultaneous degassing of dissolved carbon dioxide. Traverses down the spring branch of Rockwoods Spring, a small perennial spring in the Rockwoods Reservation, MO, clarify the difference in chemistry between the two types of springs. In particular, 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 carbon dioxide. 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. These chemical responses are corroborated by total suspended solids (TSS), bacterial, and oxygen and hydrogen stable isotope data. Karst springs with no known air-filled passage 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. Springs with submerged passages also tend to plot on the meteoric water line (MWL), while waters from open cave systems can plot below the MWL, indicating isotopic enrichment by evaporation into the overlying cave atmosphere.