ARCHIVED REPORTS_XR0003258 - Laserfiche WebLink

Home Browse Search

10 October 2003 . AGE-NC Project No. 98-0468 Page 3 of 4 observed from well.MW-4. Therefore, the distance the plume would move, prior to attenuation, ' should be less than the 100 feet beyond well MW-10; as demonstrated,by the'travel time from the former UST to well MW-lb. However, these .estimates are based solely on the changes in concentrations in four wells over three years or less. Figure 1 depicts the local land use in a large scale and Figure 2 depicts the well network and distance icts the well screen network and stratigraphic cross,section with of travel in a map scale.Figure 3 dep the MTBE has traveled. TRANSPORT MODEL ANon-Steady State Spreadsheet Analytical Model(available on the LA-RWQCB web site)was used to model travel time to the property boundary,demonstrating a facetious drinking water well.Certain assumptions have been made in the use of this model including a 1..7 horizontal dispersivity ratio and a 0.561,vertical dispersivity ratio, a 1,100 µg/L source concentration(MW-4) and T1 of 510 days. The 1.7 dispersivity ratio was adopted from the spreadsheet default value and seems to match . estimates ofthe site plume's lateral dimension. ntil several years MTBE analysis was not performed u data is available as to MTBE source concentration following the UST removal and, as a result, no at the time of removal. An assumed value of 1,100 µg/L was used for.modeling purposes. T, 510 days is based on the date of the first usable MTBE sample data(1999) from the well selected of , y for modeling purposes.Modeling was performed on one down gradient well MW-9 with well MW-4 constitutes the source. These well was selected due to their down-gradient location and their distance from the source area. When entering the analytical data for MW-4, only those results starting with the June 1999 were used.By using the June 1999 start date, all data entered into the spreaLsheet was obtained using the same EPA method (8260B) with minimum detection limits of 5 µg/ Matching of peaks between the model and analytical data was done by adjusting the ground water velocity. The model velocity result,.0.07 ft/day,-is consistent with thosedient directanes shown for ng was selected sandy silt to silty sand.The property boundary,at the assumed down gr modeled ground water velocity and for travel time modeling as a demonstration(1;700 ft).Using the distance to the property line, the estimated time for the plume to reach 4.96 µg/L is 24,000 day or . 4.3 years. This would suggest that the MTBE plume will be degraded or disperse to less 5 µg/L yorid.the property of the site. Pertinent portions of the Excel based modeling prior to migrating be are included in Appendix B.