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

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 P_CO2 directly at the two sites hourly. At Weaver Spring there were two different NDIR CO₂ sensors, a Vaisala and a C-sense^TM 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.