WORK PLANS - Laserfiche WebLink

Home Browse Search

These trends in the groundwater concentrations in the source wells maybe sufficient evidence to suggest that the dissolved hydrocarbon plume is not migrating(low risk site closure criterion#3). However, pending the discovery of an error with the initial monitoring well survey in 1991,anew downgradient well may need to be installed to define the horizontal extent ofthe plume. This well will be constructed in a similarmanner as was implemented in previous monitoring wells. Ifgroundwater flow is confirmed to be towards the southeast,WESTON recommends installing the new downgradient well on the sidewalk on the southeast comer of Sutter Street and Washington Street. 4.3.3 Vertical Extent of Groundwater Contamination According to EHD,the vertical extent of groundwater contamination in the source area at the site has not been adequately defined(EHD,2005). The monitoring well with the highest historical and current concentrations ofanalytes ofconcem is MW-2,located in the area of excavated USTs. Monitoring well MW-2A is located approximately 50 feet southwest ofMW-2 and is screened at a lower depth. This well also shows elevated levels ofTPH-g and BTEX;however,the concentrations detected are much lower than in MW-2. This suggests that contaminant levels decrease with depth. r In order to obtain a more defined picture ofthe vertical extent ofcontamination,a groundwater investigation adjacent to monitoring well MW-2 will need to be performed. The investigation would involve collecting depth discrete groundwater grab samples at 10 foot depth intervals from the top ofthe water table to 90 feetbgs and submitting the samples for TPH-g and BTEX analysis. It is anticipated that contamination will no longer be detected at this depth. r The samples will be collected using a hollow stem auger drill rig and hydropunch groundwater sampler. Soil samples will be collected from the boring at 5-foot intervals using a California-modified split spoon sampler and logged in accordance with the Unified Soil Classification System,American Society for Testing and Materials(ASTM)Number D2487-92. Soil samples will be screened with a Photoionization Detector (PID)for organic vapors,and selected soil samples will be submitted to the laboratory for TPH-g and BTEX analysis. Groundwater samples will be collected with a disposable polyvinyl chloride(PVC) hydropunch screen and stainless steel bailer. A 2-inch diameter groundwater monitoring well will be installed in the boring after collecting the soil and ` groundwater samples. Well construction will be determined by the on-site geologist,depending on the lithology, PID measurements, and any staining or odors encountered. The monitoring well will be developed at least 48 hours after installation and groundwater samples will be collected from the completed well on the quarterly schedule for monitoring at the site. In addition,ithasbeen discovered that the bottom ofmonitoring well MW-2 has silted in by approximately 8 feet,as the bottom of the well was measured down to 45.5 feet bgs initially and is now measured at 37.60feetbgs. While the well development rig is present on site,this well mayrequire redevelopment, ` which essentially consists ofcleaning the borehole and casing ofdrilling fluid to properly settle the gravel 15 r