The TCE plume represents dissolved liquid solvent derived from a source or sources of liquid TCE that have entered the ground somewhere upstream of the LDS ballfield; liquid TCE is a denser-than-water solvent with low solubility that contaminates ground water by flowing downward by gravity through surface sediments and through the aquifer, producing a plume of dissolved solvent in water flowing past or in contact with the liquid solvent.
The TCE plume at its widest point occupies approximately half of the width of the aquifer (ca. 1500 ft in the LDS ballfield area), narrowing to approximately 600 ft in the Riverside Golf Course area.
Twice as many production wells have concentrations greater than 2 ppb in 1998 than in 1993.
Over time, TCE concentrations at individual wells have waxed and waned in a pattern that is consistent between wells (see Figures 2a and 2b), but the trend is for significant increases in concentration over the past six years of record (Table 1 and Appendix 2).
From the amount of ground water flowing past the LDS ballfield per year (3.7 - 5.3 billion gallons), whose average TCE concentration is estimated conservatively at 5 ppb, at least 13-18 gallons of liquid TCE per year or 90-130 gallons of TCE have flowed past the ballfield since 1992 (see Appendix 1 calculations for details).
The plume cannot be traced back towards the landfill in the "Data Gap" area, suggesting either that it is very narrow (less than ca. 100 ft wide) and sinuous, or that it does not exist in that area.
There is no evidence of a long-term decline in plume concentrations, and a significant overall increase in dissolved solvent mass since 1993, with average TCE concentrations of affected wells in 1998 showing a 92% increase compared to the same wells in 1993, and a 13% increase in the 1996-1998 period compared with 1993-1995 (see Appendix 2).
The amount of liquid TCE in the source(s) responsible for the plume is large (more than 100 gallons already accounted for), implying that much liquid TCE has entered the ground and will maintain high plume concentrations for a long time to come.
With (1) the continued expansion of the contaminated area and limited options for exploiting new areas, (2) no evidence that concentrations in the southernmost part of the plume are declining, and (3) the very large amount of TCE that has entered the system, it appears that some form of plume control should be implemented to safeguard existing wells and provide additional options and flexibility for future water supply management.