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Former Manna Pro Site Page 6 Project No 207 3 ' Remedial Feasibility Study • 3.0 NATURAL ATTENUATION ' Groundwater monitoring has been conducted at this site for almost a decade Examination of the groundwater monitoring data suggests that natural attenuation is degrading the ' contaminant plume albeit at a very slow rate Data from the peripheral wells (i e MW-4, MW-5, MW-7, MW-8, MW-9, & MW-10) seem to suggest that the total "footprint" or areal extent of the contaminant plume is decreasing In the core of the contaminated area as ' represented by samples from MW-1, MW-2, MW-3, and MW-6, the rate of natural attenuation seems to be much slower than that observed in the peripheral wells Over all, ' contaminant concentrations in these wells have decreased about 35 percent since the wells were constructed The well with the highest absolute concentration (MW-1) has, however, exhibited the lowest percentage contaminant decrease Taken as a whole, the data appears to suggest that area with higher contaminant concentrations experience the slowest rate of natural attenuation This is believed to be due to oxygen-depleted conditions in the more heavily contaminated areas The length of time needed for natural attenuation to adequately mitigate the release was estimated using TPH-G concentrators from groundwater monitoring events for the four wells i cited in the preceding paragraph A log-normal cross-plot of TPH-G concentration against the time (in years) since the wells were constructed was generated using Excel software A copy of the cross-plot is included as Figure 5 A best fit linear projection was made for each i of the four plotted wells as shown in the figure Based on the rate of attenuation exhibited in the groundwater to date, and projecting forward, it appear that it may take at least an additional 25 to possibly as much as 100 years for natural attenuation to progress sufficiently i for a site closure to be considered possible The time range is due variations in the highest to lowest interpreted attenuation rate iShould the natural attenuation method of site mitigation be employed, no additional work would be completed except for continued groundwater monitoring It is likely that over this i period of time it would be necessary to repair or replace some, or all, of these wells Assuming that groundwater monitoring costs $12,000 per year in constant year 2000 dollars, the total cost of monitoring would be from $300,000 to $1,200,000, depending on the length i of the remedial project Based on a lifetime midway between the two (i e 62 years) the total estimated cost would be $744,000 i i i • i2937 Veneman Ave,#13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto,CA 95356 Fax(209)569-0295 i