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. 4) Groundwater contamination at the site fluctuates in response to changes in the water depth When GT-1 and GT-2 were first drilled in late 1994, the water depth was at 75 feet, below the zone of contaminated soil, yet diesel was detected in the water samples This was probably due to contamination during drilling, causing impacted soil higher in the boring to slough into the saturated zone and contaminate the water temporarily Within a few months, the water depth began to rise and groundwater concentrations dropped below the detection limit When the water depth later rose above 63 feet and came in contact with the zone of residual soil contamination, concentrations spiked 'sharply, even though the water level was above the screened interval This situation had previously been predicted during our initial investigation in 1994, when we recognized that soil contamination was significant between 40 and 60 feet Since 1999, the water depth has increased again by more than 10 feet, and diesel concentrations in the groundwater have declined by more than 90% in all wells except GT-6 Even in GT-10, concentrations declined from 190,000 ppb in early 1999 to between 20,000 and 30,000 ppb in early 2000, a decrease of at least 85% Concentrations have since spiked and declined again, but in the most recent sample the concentration of 37,000 ppb still represents a decrease of over 80% from the 1999 peak This decline has occurred even though the water depth is presently at 58 feet,which is only 6 inches above a soil sample collected in 1999 that had a TPH-d concentration of 19,000 ppm Although it is conceivable that the groundwater concentration will rise again'in the near fixture, it is a virtual certainty that the concentration would decline greatly should the water level fall to its pre-1998 level, when drought conditions and agricultural use of groundwater resulted in regional lowering of the water table Hence, by and large, groundwater contamination at this site is a problem primarily when the water depth is above 60 feet 5) Active remediation of groundwater at this site would be difficult to implement, because contaminant concentrations are high only within the Modesto channel, which has a very limited volume of groundwater In recent years, this channel has been completely devoid of groundwater, and treatment would be impossible because the impacted water has been contained within the low-permeability aquitard that underlies the channel Wells that are completed within the Riverbank Formation yield water to wells much more readily, but contaminant concentrations are too low in this formation to warrant the difficulty and expense of treatment 6) The hydrogeology at this site was well understood as early as October 1996 Subsequent drilling in 1999 added some additional information regarding the vertical extent of contamination beneath the former UST facility, but for the most part this phase of drilling merely confirmed the hydrogeological interpretation that had been developed in 1996 Groundwater monitoring since that time has further confirmed the relationship between the water depth and changes in contaminant concentration, but has otherwise added little new information Continued monitoring is therefore unlikely to produce significant new insights in the future Further, it is difficult to imagine what public health benefits could be derived from additional attempts to locate the precise limit of soil or groundwater contamination {"non-detect line") in selected locations (e g north of GT-8), especially in view of the fact that this margin probably undergoes slight changes continuously as the water depth changes 8