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to be installed in December,2001.Results of groundwater samples collected from these *� wells will be reported in the Fourth Quarter 2001 monitoring report. �.r Frequency of sampling The Disco site has accumulated a large data set of groundwater sample results since monitoring began in 1991_Monitoring wells containing petroleum hydrocarbons have either stable or reducing concentrations.Monitoring wells outside of the dissolved hydrocarbon plume have been consistently non-detect. A Groundwater Monitoring Frequency Decision Tree,Figure 5,is used to evaluate the frequency of sampling for each monitoring well at the site,based upon the location of the well with respect to the plume and gradient direction.Upon close review of the groundwater flow direction,it is evident that monitoring wells MW-DIS-3 and-10 are +� upgradient to the groundwater plume. They were previously defined as cross-gradient wells,however,it is appropriate to designate them as upgradient wells,and to sample them accordingly. The decision tree organizes the monitoring wells as upgradient,downgradient, ' crossgradient,or within the plume as follows: + Upgradient wells:MW-DIS-3,-4,-5,-6,-7S,-7D,-8S,-8D,and-10. • Downgradient wells: MW-136-11,-11D,:-12,-13,44,4413. • Crossgradient:MW-DIS-9 • Within Plume:MW-DIS-1,-2,NP-1-70',-80',-90',SVE-South,and SVE-North. Table 7 presents the updated recommended sampling frequency for all wells at the Disco +..-'` site. Monitoring wells MW-DIS-1,MW-DIS-2,NP-1-70,-80,-90,SVE-South,and SVE-North will continue to be sampled semi-annually for TPH-gas and BTEX compounds.TBA will continue to be analyzed in SVE-North on a semi-annual basis until there are 4 consecutive .r quarters of non-detects.All newly installed downgradient wells will be sampled on a quarterly basis for at least four quarters. Monitoring well MW-DIS-9 will be sampled during the fourth quarter monitoring event;this will be the annual sampling event for this i6e well. Note:In order to continue the accurate assessment of the groundwater flow direction, water level measurements in all wells and evaluation of the groundwater gradient will continue to be performed on a quarterly basis. 3