GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 171-4
Presentation Time: 9:00 AM-6:30 PM

IDENTIFYING HYSTERESIS PATTERNS BY DETECTING TRENDS IN CALCULATED AND DIRECTLY MEASURED CO2 CONCENTRATIONS


FINK, Madison S.1, FISHER, Emily1, BARNA, Joshua2, EDENBORN, Harry M.3, TORAN, Laura4 and HERMAN, Ellen K.1, (1)Department of Geology and Environmental Geosciences, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, (2)Department of Geology and Environmental Geosciences, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837; Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506-0053, (3)Geosciences Division, National Energy Technology Lab; U.S. Department of Energy, Pittsburgh, PA 15236, (4)Earth and Environmental Science, Temple University, Philadelphia, PA 19122, msf013@bucknell.edu

Geochemical fluctuations during storm events at two karst springs in Centre County, Pennsylvania, USA, can be used to assess flow paths and recharge processes. Changes in CO2 concentrations [CO2] show hysteresis through the rising and falling limbs of storms, and the direct measurement of [CO2] enables entire storms to be studied with more precision than calculated values using alkalinity, pH, temperature, and major ions. Nondispersive infrared (NDIR) sensors measured PCO2 directly at the two sites hourly. At Weaver Spring there were two different NDIR CO2 sensors, a Vaisala and a C-senseTM from Turner Designs. The C-sense is corrected to account for changes in atmospheric and hydrostatic pressure, though to date these variables have seemed relatively unimportant in [CO2] monitoring at shallow water springs. Logged values were checked with field grab samples analyzed by a CarboQC CO2 meter that employs multiple volume expansion.

The direct, logged measurement of [CO2] eliminated error such as pH electrode error, temperature changes, and degassing in storm sample concentrations calculated with PHREEQC. Overall, the directly measured values matched well with calculated values, although the logging of [CO2] showed higher resolution, particularly for capturing the rising limb of storms. Measuring [CO2] directly was particularly helpful in comparing arrival times of distinct waters during different storms at the same site as there was clearly variability in flow paths. During a storm at Springhouse Spring, clockwise hysteresis indicated possible flushing of soil water during the storm. During a storm at Weaver Spring, an ISCO automatic sampler captured only part of the falling limb of a large storm. The calculated [CO2] for this storm indicated a rise in concentration during this time. For the same storm, the logged [CO2] showed a double loop hysteresis that indicated the arrival of several distinct waters rather than the single pulse based on the calculated concentrations.