Monitoring dissolved gases and ions in groundwater using an in situ technique
ABSTRACT We tested an in situ method for monitoring the concentrations of aqueous chemical species and dissolved gases in groundwater. We deployed in situ samplers in 8 wells completed in the glacial Glasford formation and underlying Banner formation of the Mahomet aquifer, a regional water supply in east-central Illinois. We distributed and retrieved samplers 25 times over 35 weeks. At one point in this interval, we pumped each well to retrieve a sample for standard analysis, for comparison to results obtained by the in situ technique. The in situ method relies on the diffusion of analytes through a permeable membrane integral to our samplers. The samplers can be deployed and retrieved from a well without the necessity of pumping groundwater. Since no pumping is required, the method allowed the samplers to be monitored from all of the wells within a few hours. SO42- concentrations ranged among the wells from below detection (0.01 mM) to 0.84 mM, and Cl- ranged from 0.01 to 3.08 mM. These results correspond closely to concentrations measured from pumped samples. Over the first 21 weeks, H2, CO2, and CH4 averaged 42.5 nM, 0.012 atm, and 0.0090 atm, respectively. At 21 weeks, about the time we pumped the wells, we observed that H2 increased by a factor of 5, and CH4 by a factor of 3, in two wells screened in the Glasford formation and three wells screened in the Banner formation. Measurements conducted in the remaining three wells changed little. The concentration of CO2 in each well remained constant throughout the sampling period. The in situ method may provide a useful technique for frequently testing groundwater chemistry over extended periods of time.
Passive sampling, Mahomet aquifer, Hydrogen gas (H2)
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign