ARCHIVED REPORTS XR0011751 - Laserfiche WebLink

Home Browse Search

Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 14 5.2.1 Thickness of Floating Product The thicknesses of floating product that have been measured in Monitoring Well MW-7 from April 11, 2002 through November 8, 2003 are recorded in Table 3. Over that period, the thickness of LNPL in the well has fluctuated between 0.18 ft. and 0.58 ft., but the fluctuations have been around mean values that have an increasing trend. The measurement on November 8, 2003 was made prior to the purging of floating product from MW-7 that was performed on that date. Since then, two additional purges of LNAPL from that well were performed, on November 15 and November 22, 2003. By the latter date, no floating product �. was measurable on the groundwater prior to its being purged and no more than a sheen was observed on the extracted groundwater. }.. It is important to recognize that the thickness of floating product measured in a monitoring well does not equate with the actual thickness of floating product in the adjacent formation. Due to hydraulic effects, particularly as they affect LNAPL in the capillary fringe in .r proximity to the wall of the well boring, the measured thickness of floating product in a well casing differs from the actual thickness of floating product on the water table a short distance from the well. The"apparent thickness," as measured in the well, is generally greater than the i- actual thickness of floating product in the formation. The relationship between the apparent thickness of floating product to the actual thickness of floating product in the formation can also be affected by the direction and speed at which the water table is rising or falling due to local or regional precipitation or anthropogenic effects such as seasonal changes in agricultural irrigation. Of the numerous methods available for estimating the actual thickness of floating product in the formation compared to the thickness measured in a monitoring well, one of the simplest that does not call for knowledge of field parameters that, in practice, cannot be measured in the field, is that developed by Hall, et al (1984). Those authors simply subtract a "formation factor" from the measured thickness of floating product in a monitoring well to estimate the thickness of floating product in a formation. The formation factor values range from 5.0 cm in a coarse sand to 12.5 cm for a fine sand. Because the method is apparently restricted to the soil types for which formation factors have been derived, the method is often criticized as having limited practical value in application to the general case. However, given the serious limitations that affect all of the other available methods, on a relative basis, the method published by Hall, et al is, in practice, as useful as any of the others. `~ If we assume that the thickness of 0.58 ft. (18 cm) of floating product in Monitoring Well MW-7 that was measured on November 8, 2003, reflects the current conditions with reasonable accuracy, then using Hall, et al's "formation factor" of 8 cm, which is in the mid- range of their values for that factor, an estimated thickness of LNAPL in the formation would be 10 cm(0.33 ft.). 5.2.2 Recoverable Floating Product When making estimates for the purpose of remedial engineering design, it is not sufficient to consider differences between the apparent and actual thickness of floating product. It is also sic