SITE INFORMATION AND CORRESPONDENCE_FILE 2

Home Browse Search

GEOMATRIX Ms. Mary Meays San Joaquin County Public Health Services 29 September 1993 Page 6 confined conditions, further evidence that the aquifer pumped during the test is confined. Second, the transmissivity value calculated by Alton is not what would be expected considering the description of the pump test. The test was run for 2840 minutes at a flow rate of 2 gallons per minute (gpm). If back calculating the drawdown that would have been observed during a test of this aquifer using the Alton transmissivity value, one gets a total drawdown of over 25 feet, which is much greater than the available drawdown in the wells. This indicates that the actual transmissivity value of the aquifer is greater than estimated by Alton. Third, the aquifer thickness used by Alton to estimate the hydraulic conductivity of the aquifer was 16 feet, which is incorrect. The aquifer at this site is the lower sand unit, and the correct thickness is 1 to 3 feet, or an average of 2 feet. The effect of this inconsistency is that the hydraulic conductivity estimated from the pump tests using Alton's transmissivity value should be higher, approximately 6.5 x 10' feet per minute (ft/min) instead of 7.5 x 10' ft/min. Combined with the error in estimation of the transmissivity by Alton, the correct value for hydraulic conductivity of the aquifer is much greater than estimated by Alton. Groundwater velocities corresponding to a corrected hydraulic conductivity would also be greater than estimated by Alton. Section 1.2.7.1, Page 1-10 - Alton states that the areal extent of the excavation was based on the results of-pre- excavation sampling. This is not accurate. The areal extent to the north, east, and west was clearly limited to the property boundaries as evidenced by the pre-excavation soil data, which show high petroleum concentrations near the north, east, and west property boundaries and no evidence that these concentrations drop off toward the property boundaries (Alton, 14 December 1990). Because no samples were collected during or after the excavation, there is no documentation of the areal extent of contamination, and no documentation that all of the high concentrations of hydrocarbons were removed. Section 1.3.1, Pages 1-11 through 1-13 - In this section Alton incorrectly portrays that the amount of hydrocarbons left in soil at the site is not significant and that cleanup of these hydrocarbons may be infeasible at this site. Considering that significant contamination is still present at the property, that the property is currently vacant, and that contamination that remains could also be