VARIATION IN DISSOLVED GAS CONTENT AND MICROBIAL ACTIVITY IN GROUNDWATER DURING WELL PUMPING

To better understand subsurface microbiological processes, and to better define groundwater sampling strategies, we monitored the chemical and microbiological evolution of water pumped from an unconsolidated confined aquifer. The initial pumping rate of less than 0.5 L/min was increased and maintained at 6 L/min for 5 days. The concentrations of ferrous iron, sulfide and sulfate attained steady state within 8 hours. During this period, the dissolved H₂ and CH₄ concentrations rapidly increased from 48 to 95 nM and from 62 to 72 μM, respectively, then stabilized around 100 nM and 68 μM.

After about 30 hours, H₂ decreased slowly over the next 90 hours to 35 nM, while CH₄ increased to 83 μM. The total microbial numbers in the water increased during this interval from 5×10⁶ to 9×10⁶ cells/L. The results of T-RFLP microbial community analysis showed an initial bacterial population that disappeared within 4 hours. The community re-established itself much later, near the end of the five-day experiment. A population of archaea (methanogens) was maintained throughout the experiment.

We interpret the initial increase in H₂ to reflect the role of H₂-consuming bacteria resident within the wellbore. With increased pumping, H₂ was supplied to the wellbore more rapidly than bacteria there could consume it, causing the H₂ concentration to rise to a level likely reflecting approximate ambient conditions in the aquifer. Continued pumping increased the flux of substrates toward the well, promoting microbial growth. Biomass was calculated to increase more than twelve-fold, on the basis of the increase in the rate of hydrogen consumption. After about 30 hours, increasing rates of methanogenesis and potentially other microbial processes drove down the H₂ concentration and caused CH₄ to accumulate.

These results suggest the best time for obtaining a representative groundwater sample is after the initial stabilization phase and before the growth of microorganisms in response to continued pumping. In this study, the optimum interval extended from about 8 hours to about 30 hours of rapid pumping.