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Advance eoEnv ronmental Inc. 12 April 2005 AGE-NC Project No. 95-0103 Mr.Nuel Henderson San Joaquin County-Environmental Health Department(EHD) 304 E. Webber Avenue, 3`d Floor Stockton, CA 95201 Subject: Ground Water Sampling and Monitoring Natural Attenuation Work Plan Addendum CHAPIN BROTHERS INVESTMENTS, INC. 1766 Monte Diablo Avenue, Stockton, California Dear Mr. Henderson: At your request Advanced GeoEnvironmental,Inc.(AGE)has provided the following sampling and monitoring plan for monitored natural attenuation(MNA)analysis of the ground water at and near the site. AGE has received the request through telephone conversations on 11 and 12 April 2005. This sampling plan was prepared with consideration to MNA monitoring techniques described and prescribed within the Wisconsin Department of Natural Resources publication PUB-RR-614 and other Environmental Protection Agency documents. The objective of the monitoring is to collect ground water samples through techniques which generally minimize undesired exposure to atmospheric conditions when and where allowable; and to monitor for parameters which signal the natural attenuation of dissolved hydrocarbons.Generally the geochemical parameters selected for monitoring are based on the use of terminal electron acceptors as microorganisms consume hydrocarbons.The list of acceptors for analysis are proposed in the previous sampling schedule. The sampling for MNA analysis will be conducted during the following four monitoring events, to established a trend of the geochemical parameters and demonstrate the presents of terminal electron acceptors metabolized by-products. The recommended methods for analyzing MNA from monitoring wells is low-flow purging as defined as less than 1 L/minute and low-flow sampling as defined at less than 300 mL/minute.The wells will be purged with a low-flow peristaltic pump with disposable tubing,or a low-flow down hole pump with the capability to reduce flow rates.Purged ground water will be monitored through a flow-through cell, not exposed to the atmosphere, for pH, conductivity and temperature. The volume of purged ground water will be limited,not reducing the water level within the wells,during purging. The purge of ground water will continue until ground water parameters have stabilized. Dissolved oxygen(DO)and oxidation reduction potential(OPR)parameters will be monitored from down-hole probes with the upper two feet of the water table; with measurements collected prior to purging,Pcollected 11 during each purge volume and after the final purged volume of 1water. �� G�� jhq �J I�� M f4jJ _ l In iFl �iVN lwry Zohf /K�c�/r.� W� � gq4UVLtP1A�6T�.r tKe4"J't tiles