ARCHIVED REPORTS XR0012580 - Laserfiche WebLink

Home Browse Search

E-11m P -- < ' lipM Erffl� - - EM :.. "vim v "FII Du Font Biiosi;st.nis Page 14 August 10, 1989 Job No.211-71-6 The boreholes were drilled using 8-inch O.D. hollow-stem augers. All bits, drill rods, and augers were steam-cleaned prior to use. All nine exploratory borings were subsequently backfilled with cement grout containing 5 percent bentonite. The grouting of the borings was witnessed by a representative of the County of San Joaquin Local Health District. Soil samples were collected from the borings using a 2 1/2-inch-I.D. split barrel sampler(containing three 2 1/2-inch x 6-inch brass tube liners), a 5-foot continuous core sampler,and a standard penetration, sampler. The 2 - 1/2-inch I.D.split-ban•eI and standard penetration were driven into the undisturbed soil beyond the tip of the auger using a 140-pound safety hammer with a 30-inch drop. The 5-foot continuous sampler was advanced along with the augers and the core was subsampled for analytical testing using 2 1/2-inch x 6-inch brass tubes. The sampling interval for all boreholes ranged from continuous to 5 feet apart. The boreholes were logged and all soil samples were visually classified ?. (ASTM D-2487-85). The boring logs and well logs are attached in Appendix A, Boring Logs and Well Logs. eT Prior to each sampling,the samplers and brass tube liners were thoroughly washed with a trisodium. ' v phosphate solution and then carefully rinsed to avoid cross-contaminating the samples. Soil samples were retained in the brass liners,Iabeled,covered with aluminum foil,capped with polyethylene lids, sealed with adhesive tape, and placed on ice. Selected samples were delivered to the State of -j California licensed Trace Analysis Laboratory (Kayward) for chemical analyses. Routine chain- of-custody procedures were employed. } Monitoring Well Installation { The eleven monitoring wells installed at the Douglas Gas Station site by Du Pont Biusystems are y- all similar in detail of construction(see Well Lags presented in Appendix A). The monitoring wells (MW-15 through MW-25)were constructed using 2-inch diameter PVC well screen and casing. The + boreholes for each well were sampled and advanced as discussed in the previous section. 1 Information regarding all of the monitoring wells installed at the Douglas Gas Station site(i.e.by Du Pont Biosystems and by previous investigators)is presented in Table A,Summary of Monitoring Well Construction. ,l - The proper lengths of screen and blank PVC well casing were steam-cleaned and then threaded _ together above ground and lowered inside the hollow-stem augers. The slotted portion(screen)was made long enough to intersect the water table and account for seasonal variations;no glue was used in this assembly. The well screen consists of Schedule 40 PVC with four 0.0I or 0.02-inch-wide horizontal slots per inch. The annular space between the PVC and the borehole was then backfilled - --- to about two feet above the well screen wtlh Lone Star#2/12 Monterey filter sand. The sand was slowly poured down the hollow-stem augers while the augers were slowly withdrawn. This method minimized the possibility of bridging and helped assure that the filter sand would surround the well casing before the native material could collapse into the borehole.