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CDM CAMP DRESSER& McKEE ~toNr 3, 1990 Dr. William F. Pilkington Health Director Cabarr~s county Health Department 715 Cabarrus Avenue West Concord, North Carolina 28025 Geotechnical Work Needed for Methane Gas study and Proposed Demolition Area Dear Fred: The purpose of this letter is to request an amendment to our contract for the methane gas migration study to allow completion of additional geotechnical work for the proposed methane gas venting trench as well as the proposed demolition disposal area. As we have discussed, because of the presence of methane gas in som~ of the gas monitoring wells surrounding the old fill area, we have reco~ended construction of a cut-off trench around the old fill area and the existing buildings at the landfill. This trench, which will be excavated down to the ground water table, will intercept the migrating gas and vent it to the atmosphere before it can migrate off the landfill property or into the on-site landfill buildings. The ground water table acts as a lower barrier to the migration of methane gas, and, therefore, our vertical trench must reach the water table to insure that all pathways for gas movement have been intercepted. In evaluating the trench option, we are concerned that the ground water table in some areas where the trench will go may be deeper than a large backhoe can reach. This concern is based upon our review of ground water level readings around the landfill site. We believe %,~ can route the trench in areas where the ground water table is within reach, however, this must be verified in the field to reduce the possibility of problems when we' go to construction, other options, such as passive vent wells may have to be used in areas where the water t-able is not within reach of a backhoe_. With this amendment to our contract, wa are proposing to retain a soil drilling firm, who will work with a CDM staff geologist in the field, to determine the depth to the ground water table along the proposed trench route. Based upon the results, we will be able to better design the venting trench and other options if necessary, as well as provide a better bid package for perspective contractors. Recently, you also asked us to evaluate the use of the permitted stockpile area at the landfill as a demolition disposal area. The permitting of this area will reduce the ~mnunt of waste going into the sanitary portion of the landfill as well as assist you in c~,~lying with the Senate Bill 111 requirement that all yard waste be diverted fro~ sanitary landfills beginning Januaq~ 1, 1993. Mike Babuin of our staff recently met with Jim Coffey of the Division of Solid Waste Management to discuss the requirements for permitting this area. Mike sua~arized the results of his m~etlng in a letter which you were copied on. One of the requirements of the Division is that four to five soil borings be cu,~leted, and qonverted CAB7.1/30