Fine‐scale measurement of gasoline vapors, major gases (O 2 , CO 2 , N 2 , and CH 4 ), residual nonaqueous phase liquid (NAPL) gasoline, and soil physical properties has allowed detailed assessment of the role of soil layering and seasonal variability on hydrocarbon vapor fate and biodegradation. In this study we conducted coring and static depth profile monitoring at the end of summer and end of winter for a layered sandy vadose zone in Perth, Western Australia. Transient on‐line monitoring of vapors and O 2 was also performed with in situ multilevel volatile organic compound (VOC) and O 2 probes. For high soil moisture contents at the end of winter, vapors were shown to accumulate beneath a compacted, cemented layer approximately 0.3 m below the ground surface, and O 2 penetrated only to depths of 0.4 m below ground. At the end of summer, when soil moisture was lower, O 2 penetrated to depths of up to 1.5 m, and hydrocarbon vapors remained at or below this depth. Regardless of seasonal changes, sharp separations were seen between the depth of O 2 penetration from the ground surface and the depth of penetration of the vapors upward from the hydrocarbon‐contaminated zone. Modeling of steady‐state O 2 profiles indicated that a number of simple O 2 consumption models might apply, including point‐sink, distributed zero‐order, or distributed first‐order models, each leading to different biodegradation rates. Combining independent data with modeling helped determine the most appropriate model, and hence estimates of O 2 consumption and hydrocarbon biodegradation. Also, reliable estimates of the biodegradation rate could only be calculated after consideration of the layered features.
Oxygen probes used for long-term in situ monitoring in the vadose zone generally cannot be retrieved for calibration purposes. Therefore, a method is needed for confirming the calibration of permanently installed O2 probes. We developed a novel in situ calibration checking technique for use with permanently installed probes. The technique was tested at a pyritic tailings disposal site where O2 probes were installed below a bentonite–polypropylene composite cover. The technique involved (i) opening a gas sampling tube to allow atmospheric O2 ingress to the probe's location via barometric pumping and (ii) analyzing the resultant probe data collected during periods of stable, maximum probe response. Probe calibration checking was done six times during a 12-yr period, and results showed that response data were within 5% of the original calibration. This confirmed the long-term reliability of the probes and identified that the probes' useable life span extended to at least 12 yr.
A bioaugmentation (addition of microorganisms) remediation strategy has been evaluated for atrazine in laboratory and field-scale experiments. Bacteria capable of degrading atrazine were isolated from the field site and characterized. Testing of the probable field strategy using large-scale columns under aerobic conditions showed bacteria in the pesticide-contaminated groundwater were capable of degrading atrazine at zero order rates of 240–400 µg L−1 day−1 or t 1/2 = 0.34 days. The field trial, at the leading edge of the plume, using a semi-passive oxygen delivery system combined with bioaugmentation of atrazine-degrading bacteria isolated from near the source of contamination is on-going and results are continuing to be interpreted.
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