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2.0 SITE BACKGROUND 2.1 Site History Fumigant pesticide residues have been detected in the unconfined aquifer system beneath and downgradient of the Site. In 1982, OCC installed a groundwater remedial system to control migration of these residues by extracting and treating the impacted groundwater. As part of the remedial system, 63 groundwater monitoring wells were originally installed to monitor groundwater conditions and the progress of the remediation. The wells were referred to as permanent wells (PW wells) to differentiate them from temporary wells initially installed during the investigation stage of the project. These wells are currently sampled two times per year and static water level measurements are collected from the wells once per month. The groundwater samples collected from these wells are analyzed for a variety of organic and inorganic constituents while the monthly water level measurements are used to construct potentiometric surface maps for the three defined aquifer zones. The combined water quality and gradient data is then used to monitor the effectiveness of the gradient control and the progress of aquifer water quality improvements. The remedial system and the associated monitoring well network has been in almost continual operation since startup of the remedial program. After the installation of the original 63 permanent monitoring wells, six additional monitoring wells, arranged into two well clusters identified as PW23 and PW24, were installed to monitor the southern and southwestern portions of the remedial area. In October, 1991, the four wells comprising the permanent monitoring well cluster PW14 were abandoned due to commercial development in a portion of the remedial program area. These wells were not replaced because it was determined that they were no longer needed for adequate monitoring of the remedial program. 2