Groundwater Sampling Guidelines. Sampling of MWs for the appropriate laboratory analyses is required, as applicable, to determine the nature, degree and extent of groundwater contamination. The lateral placement of shallow MWs (water table xxxxx) for the delineation of the uppermost portion of the aquifer shall be initiated in the source area(s). MWs shall extend outward in either a grid or staggered pattern at intervals that are appropriate for the site (i.e., 15-foot intervals for small sites to greater than 15-foot intervals for large sites). The following should be considered, as applicable, when selecting locations for shallow MWs: the degree and extent of soil contamination, source area location(s), location(s) of nearby receptor(s), site-specific characteristics of the impacted aquifer (e.g., lithology, groundwater flow direction, gradient, conditions caused by drainage structures, preferential pathways, etc.), physical/chemical properties of the COCs (e.g., mobility, solubility, rate of degradation, degradation/transformation products, etc.), and direct push/hydropunch results. Intermediate/deep MWs for delineation of the intermediate/deep portions of the aquifer should be installed near to and slightly down gradient from the water table well(s) showing the highest degree of contamination. In evaluating the need to install intermediate/deep MWs, the following factors should be considered: site history, groundwater plume history, COC concentrations, physical/chemical properties of the COCs (e.g., density, mobility, solubility, rate of degradation, degradation/transformation products, etc.) and aquifer characteristics (e.g., vertical conductivity, lithology, etc.). Groundwater delineation shall be deemed complete upon achieving at least one of the following for each COC: • The groundwater and, if applicable, surface water CTLs set forth in Sections 24-11.1(2)(E)(1) and 24-11.1(2)(J)(1)(b) of the Code (i.e., no further action without conditions), • Natural background concentrations (see Natural Background Guidance No. 7C), or • The best achievable PQLs. In the event that delineation cannot be achieved because the placement of MWs is not practical due to physical constraints (e.g., buildings, power lines, utilities, roads, etc.), delineation to the CTLs may be estimated from available data by demonstrating a concentration gradient using appropriate contouring techniques (e.g., linear interpolation, Kriging technique, etc.). However, if the available data indicate that there is a poten...
