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Mr. Kevin Graves April 28,2006 Page 4 After stating that the resumed monitoring of the other four existing wells "may continue to define the extent of the shallow groundwater contamination", emphasis added, "The Work Plan is not approved" letter went on to state that "At this time, EHD does not consider a geoprobe investigation of shallow groundwater to be warranted." However, in the Request for Work Plan letter EHD stated that the "work proposed should complete the investigation of the lateral . . . extent of the. . . groundwater contamination." With soil sampling boreholes open to first encountered groundwater, the additional effort to determine the lateral extent of any remaining shallow groundwater contaminations, as requested, through grab samples of that groundwater would seem to warranted! In the fourth paragraph of "The Work Plan is not approved" letter, EHD seems to concur with the proposed locations of the deeper screened monitoring wells as paired with existing monitoring wells MW-1, MW-2, and MW-4. The paragraph summarizes the log of Thornton Municipal Well. No. 2 (Attachment D), indicating that "there is sand from 25 to 60 feet below grade, so the deeper portions of this sand interval are of concern." The EHD does not mention that well is sealed to a depth of 70 feet, ten feet into that which on the log is shown as "blue clay & shale." Nor does EHD mention that the well is cased from 0 to 135 feet, and that the well screen is set from 135 to 150 feet; 160 to 185 feet; and from 225 to 235 feet. EHD seems to reject the proposed 10 foot screen interval (30 to 40 feet), in that it "may be too shallow and too long." Too long, perhaps. The undersigned has no objection to the EHD's recommendation of a five foot long monitoring well screen interval, nor determination through continuous coring, either through rotary or drive sampling to make said determinations. Though not detailed in the work plan, isn't this how such a determination would be made?. As to the length, for planning purposes, the log does, after all, show that the "sand and wood" sequence is perhaps 35 feet thick. However, as to "too shallow", one should be very weary this close to an active municipal well when punching vertical holes, as, regardless of the borehole and well sealing guidelines, a vertical borehole is still a disruption of the stratigraphy and poses the risk of introducing a vertical conduit of higher permeability than previously existed. For this reason, the predetermination of well construction details using a cone penetrometer is rejectedposing too high a risk since the small diameter of either the cone penetrometer or the Geoprobe' presents the most challenging borehole to seal. In addition, it should be expected that the logged wood would prevent achieving the desired depth anyway, or at best resulting in borehole advancing and sealing problems. The vertical distribution in the 35 foot "sand and wood" sequence should be ascertained incrementally, beginning at the depth of the deepest available nearby data. This is to say the workplan implied and initial screen interval of 30 to 40 feet may be followed by a screen interval of 50 to 50 feet, etc., now reduced to perhaps 35 to forty followed by 45 to 50, etc. H 2 OGEOL A GROUND WATER CONSULTANCY