Ground Water Recovery and Treatment

Abstract

Until the environmental revolution, the only ground water that was routinely treated to remove contamination was the impacted ground water that was extracted for beneficial use. With the recognition that contamination could cumulatively impact drinking water wellfields or entire areas and basins, there was an incentive to remediate ground water not yet extracted for use. Although it is now recognized that impacted ground water can be treated in situ or ex situ, at the time it was logical to extract impacted water, using wells similar to potable water wells, and treat and/or dispose of that water into surface waters. The underlying strategy was based on the concept that by removing the impacted water, you would gradually remove the underground contamination. Consequently, the first ground water remediation systems were generally extraction and ex situ treatment or simple discharge to surface water. This process is called pump-and-treat (Liu, 1997). As experience with these pump-and-treat systems accumulated, it became apparent that for many applications these systems were going to operate for a very long time, possibly decades or more, to approach the ground water standards for drinking water in the Safe Drinking Water Act regulations. In addition, it soon became clear that the rules of solubility still applied, and contaminant mass removed per mass of water diminished over time due, ironically, to the success of the prior removal. This was due in large part to the physics and chemistry of contamination by pure products of low solubility but high toxicity at low concentrations. Extractions became prolonged, not necessarily because the total mass of contaminant to be removed was very high, but because the dissolved mass of contaminants at any time was low. Then as the contaminant mass is lowered by treatment, the dissolved concentrations diminish correspondingly per constant time unit due to the lower equilibrium gradient. In addition, many free-phase or high concentration contaminants had penetrated lower permeability materials driven by gravity and/or high concentrations. Their extraction without a similar extraction driving force, i.e., overcoming the hysteresis effect, meant that after the cleanup goals were apparently reached and the system was shut down, sequestered contaminant concentrations would gradually equilibrate with the recoverable ground water, again increasing the contaminant concentrations (USEPA, 1996).