Select the latitude adjusting value for each month from table 3-3. For latitudes lower than 50° North or 20° South, determine the monthly latitude adjusting value by interpolation. • Calculate monthly net precipitation by subtracting monthly evapotranspiration (or monthly potential evapotranspiration) from monthly precipitation. If evapotranspiration (or potential evapotranspiration) exceeds precipitation for a month, assign that month a net precipitation value of 0. • Calculate the annual net precipitation by summing the monthly net precipitation val ues. • Based on the annual net precipitation, assign a net precipitation factor value from table 3-4. Enter the value assigned from Figure 3-2 or from table 3-4, as appropriate, in table 31. TABLE 3-3-MONTHLY LATITUDE ADJUSTING VALUESa a Do not round to nearest integer. For unlisted latitudes lower than 50° North or 20° South, determine the latitude adjusting value by interpolation. 3.1.2.3 Depth to aquifer. Evaluate depth to aquifer by determining the depth from the lowest known point of hazardous substances at a site to the top of the aquifer being evaluated, considering all layers in that interval. Measure the depth to an aquifer as the distance from the surface to the top of the aquifer minus the distance from the surface to the lowest known point of hazardous substances eligible to be evaluated for that aquifer. In evaluating depth to aquifer in karst terrain, assign a thickness of 0 feet to a karst aquifer that underlies any portion of the sources at the site. Based on the calculated depth, assign a value from table 3-5 to the depth to aquifer factor. Determine the depth to aquifer only at locations within 2 miles of the sources at the site, except: if observed ground water contamination attributable to sources at the site extends more than 2 miles beyond these sources, use any location within the limits of this observed ground water contamination when evaluating the depth to aquifer factor a Use depth of all layers between the hazardous substances and aquifer. Assign a thickness of 0 feet to any karst aquifer that underlies any portion of the sources at the site. 3.1.2.4 Travel time. Evaluate the travel time factor based on the geologic materials in the interval between the lowest known point of hazardous substances at the site and the top of the aquifer being evaluated. Assign a value to the travel time factor as follows: • If the depth to aquifer (see section 3.1.2.3) is 10 feet or less, assign a value of 35. • If, for the interval being evaluated, all layers that underlie a portion of the sources at the site are karst, assign a value of 35. • Otherwise: -Select the lowest hydraulic conductivity layer(s) from within the above interval. Consider only layers at least 3 feet thick. However, do not consider layers or portions of layers within the first 10 feet of the depth to the aquifer. -Determine hydraulic conductivities for individual layers from table 3-6 or from insitu or laboratory tests. Use representative, measured, hydraulic conductivity values whenever available. -If more than one layer has the same lowest hydraulic conductivity, include all such layers and sum their thicknesses. Assign a thickness of 0 feet to a karst layer that underlies any portion of the sources at the site. -Assign a value from table 3-7 to the travel time factor, based on the thickness and hydraulic conductivity of the lowest hydraulic conductivity layer(s). TABLE 3-6-HYDRAULIC CONDUCTIVITY OF GEOLOGIC MATERIALS Type of material Clay; low permeability till (compact unfractured till); shale; unfractured metamorphic and igneous rocks Assigned hydraulic conductivity a (cm/sec) 10-8 10-6 10-4 10-2 alf depth to aquifer is 10 feet or less or if, for the interval being evaluated, all layers that underlie a portion of the sources at the site are karst, assign a value of 35. b Consider only layers at least 3 feet thick. Do not consider layers or portions of layers within the first 10 feet of the depth to the aquifer. 15 33555 35 15 15 1 1551 655 25 Determine travel time only at locations within 2 miles of the sources at the site, except: if observed ground water contamination attributable to sources at the site extends more than 2 miles beyond these sources, use any location within the limits of this observed ground water contamination when evaluating the travel time factor for any aquifer that does not have an observed release. If the necessary subsurface geologic information is available at multiple locations, evaluate the travel time factor at each location. Use the location having the highest travel time factor value to assign the factor value for the aquifer. Enter this value in table 3-1. 3.1.2.5 Calculation of potential to release factor value. Sum the factor values for net precipitation, depth to aquifer, and travel time, and multiply this sum by the factor value for containment. Assign this product as the potential to release factor value for the aquifer. Enter this value in table 3-1. 3.1.3 Calculation of likelihood of release factor category value. If an observed release is established for an aquifer, assign the observed release factor value of 550 as the likelihood of release factor category value for that aquifer. Otherwise, assign the potential to release factor value for that aquifer as the likelihood of release value. Enter the value assigned in table 3-1. 3.2 Waste characteristics. Evaluate the waste characteristics factor category for an aquifer based on two factors: toxicity/mobility and hazardous waste quantity. Evaluate only those hazardous substances available to migrate from the sources at the site to ground water. Such hazardous substances include: • Hazardous substances that meet the criteria for an observed release to ground water. • All hazardous substances associated with a source that has a ground water containment factor value greater than 0 (see sections 2.2.2, 2.2.3, and 3.1.2.1). 3.2.1 Toxicity/mobility. For each hazardous substance, assign a toxicity factor value, a mobility factor value, and a combined toxicity/mobility factor value as specified in the following sections. Select the toxicity/mobility factor value for the aquifer being evaluated as specified in section 3.2.1.3. 3.2.1.1 Toxicity. Assign a toxicity factor value to each hazardous substance as specified in Section 2.4.1.1. 3.2.1.2 Mobility. Assign a mobility factor value to each hazardous substance for the aquifer being evaluated as follows: lying the sources at the site, regardless of the aquifer being evaluated, assign a mobility factor value of 1. For any hazardous substance that does not meet the criteria for an observed release by chemical analysis to at least one of the aquifers, assign that hazardous substance a mobility factor value from table 3-8 for the aquifer being evaluated, based on its water solubility and distribution coefficient (Ka). • If the hazardous substance cannot be assigned a mobility factor value because data on its water solubility or distribution coefficient are not available, use other hazardous substances for which information is available in evaluating the pathway. • For any hazardous substance that meets the criteria for an observed release by chemical analysis to one or more aquifers underTABLE 3-8-GROUND WATER MOBILITY FACTOR VALUES a • If none of the hazardous substances eligible to be evaluated can be assigned a mobility factor value, use a default value of 0.002 as the mobility factor value for all these hazardous substances. Determine the water solubility to be used in table 3-8 for the hazardous substance as follows (use this same water solubility for all aquifers): • For any hazardous substance that does not meet the criteria for an observed release by chemical analysis, if the hazardous substance is present or deposited as a liquid, use the water solubility category "Present as Liquid" in table 3-8 to assign the mobility factor value to that hazardous substance. • Otherwise: -For any hazardous substance that is a metal (or metalloid) and that does not meet the criteria for an observed release by chemical analysis, establish a water solubility for the hazardous substance as follows: -Determine the overall range of water solubilities for compounds of this hazardous substance (consider all compounds for which adequate water solubility information is available, not just compounds identified as present at the site). -Calculate the geometric mean of the highest and the lowest water solubility in this range. -Use this geometric mean as the water solubility in assigning the hazardous substance a mobility factor value from table 3-8. -For any other hazardous substance (either organic or inorganic) that does not meet the criteria for an observed release by chemical analysis, use the water solubility of that hazardous substance to assign a mobility factor value from table 3-8 to the hazardous substance. For the aquifer being evaluated, determine the distribution coefficient to be used in table 3-8 for the hazardous substance as follows: For any hazardous substance that does -For any hazardous substance that is a |