SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

10/22/2003 14:24 209-579---05 MODESTO ATC = PAGE 04 �+..� U A ■ 90C1ATF $ INC . Floating petroleum product has previously been observed in well MW4. On April 4,'2003, only a slight, unmeasurable sheen was observed in well MW4. No free product was recovered from the well because the quantity was too small to be collected into a bailer. During subsequent monitoring events, the sheen was not evident, Table 2 summarizes the product thickness 1 measurements and volume of product removed from previous monitoring events. Groundwater samples were collected on April 11, 2003 from downgradient wells MW2 and MW6. ATC attempted to collect a sample from well MW7 but following purging, the well did not sufficiently recover due to the nearby dewatering. On May 19, June 30, and August 12, 2003 groundwater samples were collected from all wells on site, MWI through MW7. Prior to sample collection, each sampled well was purged of a minimum of three well casing volumes of water. Purged water was contained in labeled 55-gallon drums (DOT H), and stored on site. Groundwater samples were collected after physical parameters (pH, temperature, and conductivity) were monitored and had stabilized. Dedicated disposable bailers with bottom. emptying devices were used to collect samples from the wells. Field purge logs are presented in Attachment 1, S LABORATORY ANALYSES Collected groundwater samples were analyzed by Argon Laboratories, a State- certified laboratory, for total petroleum hydrocarbons as diesel (TPHd) utilizing EPA method 8015. Analytical results are summarized in Table 3. Laboratory data sheets and chain-of-custody documentation are included in Attachment 2. Laboratory results indicate that TPHd was detected in groundwater samples collected from well MW4 on May 19, June 30, and August 12, 2003 at concentrations of 1,300, 6,900, and 85 parts per billion (ppb), respectively. Analysis of samples collected from wells MW2 and MW6 on April 1 11, 2003 and from wells Mwl, MW2, MW3, MW5, MW6 and MW7 on May 19, June 30, and August 12, 2003, did not indicate the presence of TPHd above method detection limits. CONCLUSIONS The detected concentrations of TPHd have been decreasing in groundwater samples collected 1 over the last five months of monitoring events, indicating that the dewatering has not had an adverse affect on the plume and the plume has remained stable during the off-site dewatering period. When off-site dewatering was terminated concentrations of TPHd in groundwater samples from MW4 decreased. Based on the results of these monitoring events ATC recommends proceeding with site closure and recommends destruction of wells MWl through MW7- w.%2S74.ozbu*rt Ty,,,r1002.&c 4