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Mr. John Welty <br /> April 10, 1990 <br /> Page 13 <br /> groundwater elevation in PW-1 to be higher than in Monitoring Well 11 which may <br /> alter the true-gradient direction. <br /> Two quarterly update reports have followed the County letter (November 1989 and February <br /> 1990). Future work proposed in the reports did not include new pumping well expansion. One <br /> additional monitoring wells cluster was proposed for the on-site underground storage tank <br /> location. <br /> GTI's April 1989 work plan was revised and resubmitted in June of 1989. On August 3, 1989, <br /> a letter was sent by the RWQCB to the County regarding the June 1989 revised work plan for <br /> additional site assessment_ The work plan proposed two additional groundwater monitoring wells <br /> up gradient and two monitoring well clusters to investigate the possibility of an up gradient <br /> source and to better define the vertical extent of contamination. <br /> The following comments were included: <br /> 1. The practice of installing 2-inch-diameter monitoring wells at this site should be <br /> discontinued. All future monitoring wells installed at this site should be constructed <br /> of nominal 4-inch diameter (or larger) casing and well screen to permit adequate well <br /> development and provide representative groundwater samples. <br /> 2. The proposed well cluster to be located in the vicinity of MW-1 and MW-4 is in an <br /> area of known contamination. Therefore, the proposal to drill to a depth of 70 feet <br /> with a hollow-stem auger should not be approved. Such a well installation would <br /> compromise the integrity of samples from the other depth-discrete wells in the cluster. <br /> The proposed shallow monitoring well would be screened from a depth of <br /> approximately 45 feet to 60 feet below ground surface. This would leave only a <br /> 10-foot stratigraphic interval between the bottom of the shallow monitoring well screen <br /> and the 70 foot deep aquitard for the screened intervals of the two depth-discrete <br /> wells. <br /> 3. Because of the difficulty of isolating specific hydrologic units, it does not seem that <br /> the proposed well cluster would be capable of providing depth-discrete samples. <br /> Samples obtained from these wells would be of limited value. <br /> 4. Based on the information provided, it would be more appropriate to construct the <br /> shallow monitoring well with a short screened interval to isolate specific hydrologic <br /> units. During drilling of the exploration borehole, a conductor casing should also be <br /> used to limit the possibility of lower zones becoming contaminated as the auger is <br /> employed deeper into the borehole. <br /> The County most likely did not relay the RWQCB's letter to Southland or GTI since they went <br /> ahead with the proposed cluster well design. <br /> Brown and Caldwell <br />