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City of Stockton Draft Amended Report of Waste Discharge(AROWD) Austin Road Landfill Work Plan ■ VOC compounds reaching the shallow aquifer beneath the disposal site are a combination of dense vapors and liquid leachate. Other potential contaminant sources may also exist. ■ Contaminants will migrate along with the prevailing groundwater to the north-northeast,but will be attenuated by processes of dispersion,adsorption onto aquifer soil,and volatilization. ■ Due to the interbedded sand-silt-clay nature of the subsurface deposits,contaminants will migrate preferentially along a wandering pathway that follows interconnected zones of the more permeable deposits. ■ Migration will be predominantly in the horizontal direction and within the shallow aquifer unit. ■ The recent VOC detections in the state agricultural well are likely due to compound migration from the shallow aquifer along the well bore. ■ Deep irrigation and domestic wells locally create a downward pull on the plume. This preliminary conceptual framework will be refined by: 1. Assessingborehole geologic data to provide insight on the continuity of the shallow and deep g g aquifer layers. 2. Analyzing groundwater sampling results to provide information on longer-term plume migration rates. 3. Reviewing information on nearby irrigation and domestic wells such as depths,screened intervals,pumping rates,and pumping schedules to evaluate potential influences on the plume. This includes investigating the two residential wells at 7898 and 8106A Austin Road. 2.2 Task 2 - Field Investigation This section presents the methods and procedures that will be used to collect,manage,and analyze soil and groundwater samples and conduct measurements in the field. Included in this section are methods for: ■ Soil borings ■ Soil sample collection ■ Groundwater sample collection ■ Groundwater monitoring well installation ■ Water level measurements ■ Sample identification ■ Sample container labeling,sample preservation,shipping and chain-of-custody ■ Documentation ■ Contaminated materials disposal ■ Equipment decontamination The field investigation will address the data requirements needed to refine the conceptual framework. These requirements include the following: ■ location of the downg g radient edge of the plume, ■ determining whether the plume has migrated into the deeper aquifer, ■ effectiveness of the on-site extraction wells in capturing the plume and what effect they have on the deep aquifer, ■ the influence of attenuation mechanisms on the offsite plume and, ■ influence of irrigation and domestic wells in pulling the plume into,deeper zones. W:\REPORTS\STOCKTON\AUSnN\WORKPLAN.DOC Page 3