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Antonia K. J. Vorster 4 December 18, 1989 <br /> evidenced by the recent results from LVMD #2) , and that either the <br /> suspected pathway (fractured casing) for the contamination detected in LVMD <br /> #1, in 1984, was incorrect (or not the only pathway) , or that the sealing- <br /> off process for this well was ineffective. <br /> A potential pathway for the downward migration of contaminants is through <br /> the intervening substrata between the uppermost water bearing zone and the <br /> deeper substrata intercepted by the production wells. Drilling logs for <br /> the county production wells indicate that the depth to the start of the <br /> perforations is 160 feet for LVMD #1, 208 feet for LVMD #2, and 210 feet <br /> for LVMD #6. The screened intervals appear to be no greater than forty <br /> feet, though they may extend deeper. The existing monitoring wells are <br /> screened within the uppermost water bearing formation. Monitoring Wells <br /> #1, #2 and #3 are screened at 43 to 58 feet. Monitoring Well #4 is <br /> screened at 55 to 70 feet. <br /> The consultant in his Phase I report stated that these two water bearing <br /> zones are hydraulically separate, based upon a static water level <br /> differential of between 12 and 14 feet between Monitoring Well #4 and LVMD <br /> Well #1 located approximately 50 feet away. That is, he concluded that <br /> the deeper zone was a semi-confined aquifer separate from the uppermost <br /> water bearing zone. However, a semi-confined aquifer is not totally <br /> isolated from overlying aquifers, but rather the degree of hydraulic <br /> connection is dependent upon the nature (the hydraulic conductivity) of <br /> the intervening substrata (the aquitard) , and the magnitude and direction <br /> of the vertical hydraulic gradient between the aquifers. <br /> A cursory review of the drilling logs for the county wells indicate that <br /> the substrata is heterogeneous consisting of alternating layers of sands <br /> and clays. The cross section developed, by the consultant, in the Phase <br /> I report is incorrect. The sand layer shown in the cross section between <br /> 180 and 250 feet at LVMD #1 is reported in the driller's log to consist <br /> of a soft clay between 190 and 255 feet. Likewise the sand interval at <br /> LVMD #2 between 200 and 250 feet is reported in the dri l l er's log to <br /> consist of a brittle shale between 207 and 240 feet, and a sandy clay <br /> between 242 and 254 feet. The purported aquitard also has similar <br /> discrepancies in lithological interpretation between the drilling logs and <br /> the cross section. Though low permeable layers (i .e. , clays or sandy <br /> clays) are logged throughout the total depth of the borings, it has not <br /> been demonstrated that these layers exist between the uppermost water <br /> bearing zone and the semi-confined aquifer with sufficient competency (low <br /> hydraulic conductivity, thickness, and continuity) to isolate the deeper <br /> zones from the upper zone. In addition, the vertical hydraulic gradient <br /> between the two aquifers is approximately 0. 11 and in the downward <br /> direction. This vertical gradient is about 50 times greater than the <br /> horizontal gradient measured in the upper zone. Therefore, the driving <br /> force (the downward vertical gradient) appears sufficient at this site to <br /> drive contaminants downward, if the nature of the intervening substrata <br /> provides a pathway. <br />