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<br />COMMENTS BY JAMES C. STEVENS <br />`J <br />I. An effective porosity value of 25% was estimated for the Santa Fe Fonnntion, but <br />die source of dais was not provided. Neun•on logs were rnrt,for the wells drilled <br />during this project, but tl:e we!! logs are not scaled irr porosity units. No core <br />data was referenced. <br />Since a lower effective porosity value would mean a higher seepage velocity and <br />affect the shape of any contaminant plume, the value used needs to be justified, <br />i. e. well log charts provided, core data noted whatever. <br />RESPONSE: Although neutron logs were run in these monitoring wells at the request of the <br />CDMG, due to the drilling methodology employed (to maintain Poole integrity), <br />the logs had to be run in cased holes. When neutron logs are run in cased Boles, <br />absolute porosity values cannot be obtained (only relative porosity values). <br />The porosity value that was used in the report was based otr the geology <br />encountered during the drilling and on our field geologist's experience, and is a <br />generally accepted value for porous material (see, e.g. "Basic Soils Engineering" <br />by B.K. Hough, Table 2.3 on pp 34-35, 1969). In the absence of Any ottrer data, <br />BMR does not have any reason to believe that this is not a representative value. <br />While it is acknowledged that a lower porosity value would result in a Higher <br />seepage velocity, varying the porosity in the range of typical values for this type <br />of material (20 to 35 percent} produces very small incremental changes in seepage <br />velocity that do not affect the assessment that the existing monitoring well system <br />is adequate (see sensitivity analysis results). <br />2. Lateral dispersivity and transverse dispersivity vahres of 100 and 30 were <br />employed in the determination of the width of the contaminant phone. Lower <br />values could reasonably be employed which would also affect the shape of the <br />contaminant plume, i. e. narrow it. <br />77:e lateral and transverse dispersivity values employed need to be justified. <br />RESPONSE: Since dispersivity values for an aquifer are difficult to obtain, and typically <br />require extensive field studies using many points of reference over a long period <br />of time, dispersivity values are typically estimated based on values available in <br />published literature. <br />It should be noted that dispersivity values can be large in Ireterog8neous aquifers <br />and BMR's experience in drilling the Santa Fe Formation in this area is that the <br />-4- <br />