The frequency of MTBE occurrence in ground water was significantly higher in areas of high recharge, whereas the frequency of PCE occurrence was not significantly higher in areas of high recharge.  In addition, the frequencies of MTBE occurrence by aquifer had a significant positive correlation to recharge, whereas the frequencies of PCE occurrence by aquifer had no significant correlation to recharge.  Concentrations of both compounds showed less correlation to recharge than occurrence frequencies.

The difference in occurrence between these two VOCs relative to recharge probably is a result of their different transport properties.  When released from point sources, gasoline compounds like MTBE float at the top of an aquifer because they are less dense than water.  Only through downward movement of recharge water would dissolved concentrations of gasoline compounds be carried through an aquifer.  In contrast, when released from a point source, solvents like PCE tend to sink through an aquifer and move along deeper flowpaths that have little connection to younger recharge water.

Ground water resource managers should consider potential recharge in strategies to minimize the occurrence of gasoline compounds in regional-scale aquifers.  However, consideration of aquifer stratigraphy and structure may be more important than potential recharge in strategies to minimize the occurrence of solvents in regional-scale aquifers.