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Corrective Action Plan Page 20 ' water table also introduce oxygen into the vadose zone because air permeates the soil when groundwater elevations are low and fresh, oxygenated water infuses this zone when the water nses The fresh supply of oxygen stimulates microbial activity and aids the process of biodegradation of hydrocarbons For the above-stated reasons, following an initial period when groundwater will be continuously pumped at a high rate to remove the most severely contaminated water from the subsurface, a schedule will be established whereby the pumping rate will be periodically changed so as to enhance soil flushing and oxygenation The schedule of periodic changes in pumping rates that will produce the best overall remediation rate depends on the relative balance between the rate at which groundwater affected by dissolved contaminants is recovered, the total percentage of contaminant load ' that is adsorbed on the soil and the rate at which natural bioremediation progresses This schedule can not be determined a priori An optimal program must be developed from observations of the rate of change in contaminant concentrations as the remediation program progresses However, for planning purposes, but subject to revision as empirical data from the field become available, the scheme described below is proposed The groundwater recovery and treatment system will be operated continuously at, or close to, its design capacity for a period of six months, or until the mean concentration of total petroleum hydrocarbons in samples recovered from the pumped and monitoring wells falls to around 25% of its initial levels Pumping will then be stopped for a period not in excess of one month to allow the water table to rise and flush the soil in the vadose zone After that delay, a high rate of pumping will be resumed and will be continued for three month If, as is expected, contaminant concentration in groundwater in the near field of the recovery wells are observed to rise after pumping is temporally stopped and then fall again after it is resumed, the cyclical method of operating the groundwater recovery pumps will be adopted as a standard procedure As experience is gained and data from regularly scheduled rounds of groundwater sampling becomes available, the duration of the pump-operating cycles and the pumping rates will be adjusted to obtain the best possible rate of remediation However, pumping will not be stopped for such a lengthy period, or the pumping rate so reduced, that there would be a risk that the original plume of contamination might expand further down gradient Contaminant Load in Recovered Groundwater Stream When groundwater is pumped from recovery wells located in the center of a contaminated area, the concentrations of contaminants in the stream of recovered water often fluctuate ' widely as water from different parts of the affected zone of the subsurface arrives at the well