ARCHIVED REPORTS_XR0012626 - Laserfiche WebLink

Home Browse Search

E `n monitoring wells (MW-5, MW-6, MW-7, and MW-8) are located based on the presently known compound distribution and apparent shallow groundwater gradient direction. MW-5 and MW-6 are placed to define the lateral plume boundaries �- to the northwest and the southeast, respectively. Monitoring well MW-7 is located northeast of MW-2 and will monitor for compound presence directly downgradient from the source area. MW-8 is also located directly downgradient and should provide an indication of the rate at which the compounds decrease downgradient due to dispersion or natural biological degradation. If the new downgradient wells MW-7 and MW-8 contain more compound concentrations than M11-2, additional wells may be required to define the extent of the plume. The decision to construct monitoring wells in addition to the five proposed above will be made only after an analysis of the data acquired by the Phase II Program is completed. 2.4 MONITORING WELL CONSTRUCTION The limited geologic data currently available for the site �• indicates that the aquifer is composed of fine to coarse-grained sand interlayered with discontinuous clay lenses. The aquifer is unconfined and contains no significant continuous aquicludes, at least within the upper 100 feet below the ground surface (person- al communication with the CRWQCB, August, 1989) . The monitoring wells will be drilled using hollow stem auger (HSA) techniques and split-spoon sampling at 5-foot intervals for lithologic control. The wells will be drilled to a total depth -' equal to approximately 20 feet below the top of the water table. Actual depths will vary depending on the elevation of the ground surface at each well. Wells drilled from street level will be -� approximately 60 feet deep. Based on presently available data, a total well depth of approximately 60 feet will provide a 20 foot water column in the wells. The wells will be constructed of 4-inch diameter schedule 40 PVC pipe with 20 feet of 4-inch diameter schedule 40, 0.010 inch slotted screen. The pipe will be flush-threaded; no glues or solvents will be used in constructing the wells. The well construction method will be as follows: o Screen and casing will be suspended within the hollow stem augers to the total depth so that the screen extends from 5 feet above the water table to 15 feet below the water table. This will leave a 5 foot sand plug beneath the screened section. o An appropriate sized sand/gravel pack will be placed around the well screen while removing the augers. The sand pack will extend 3 to 5 feet above the top of the screen. 2 - 3