Four main hydrogeologic and geologic factors were identified that influence vertical travel time of water: hydrogeologic setting, surficial sediment, unsaturated thickness, and target zone lithology. Two factors are based on spatial data from Plate 3 and from Plate 1 of Part A (hydrogeologic setting and surficial sediment, respectively), and two are based on point data from field measurements and well logs (unsaturated thickness and target zone lithology). Each factor was subdivided into categories. The categories (in parentheses) and their associated factors are hydrogeologic setting (outwash plain, collapsed outwash, hummocky moraine, ground moraine), surficial sediment (sand and gravel, sandy loam, loam, loam to clay loam), unsaturated thickness (0 to 20 feet, 21 to 50 feet, greater than 50 feet), and target zone lithology (no clay, some clay, all clay). A vertical time of travel for water to reach a depth of 20 to 50 feet (the target zone) was assigned to each of the categories.
Beginning in the upper left corner of the study area, groups of polygons (from the coverage SGNT from the MGS) within a selected geographic area were combined into map polygons based on identical hydrogeologic setting and sediment type as well as similar unsaturated zone thickness and target lithology (point data similarity was verified or changed based on StatsFromMike.xls). I selected groups of polygons that met this criteria by clicking on them and assigning them the same map polygon number in the attribute table: Sens12_21). Conversely, if a polygon from SGNT contained unsaturated zone thickness and/or target lithology that was highly variable from one portion of the polygon to another, this polygon was split into one or more map polygons (using ArcView tools). I moved methodically through the study area, selecting workable (whatever fit on my computer screen) geographic areas to assess.
The information from this exercise was entered into an Excel table, the table was used in Access to calculate values and this table was exported: SensitivityTable.xls.
A table that summarizes the values and categories is included in MapPolyFactors.xls, ?final summary of methodology? sheet. This workbook also contains the abbreviated version in the ?Methodology compact size? sheet.
Vertical ground-water travel time to the target zone was approximated for each category within a spatial factor based on hydraulic conductivity values from literature (Stephenson and others, 1988 and Soil Survey staff, 1998) see HydraulicConductivityfromStephenson.xls. These hydraulic conductivity values represent vertical flow rates through saturated porous sediment. Some or all of the sediment above the target zone is unsaturated throughout most of this study area. Except for flow through fractures and other pathways, ground-water flow through unsaturated sediments is slower than flow through saturated sediments. Applying the faster saturated flow rates to unsaturated sediment may result in a shorter than realistic travel time. However, this effect is diminished by the unsaturated thickness category, which assigns longer travel times to thicker unsaturated sediment.
Each category within the setting and sediment factors was given a value on a scale of 1 to 5 corresponding to the range of travel time (Figure 1 of Plate 4) for water to enter the target zone: 1 = hours to months, 2 = weeks to years, 3 = years to decades, 4 = decades to a few centuries, and 5 = decades to several centuries. The categories within the settings factor and their associated travel time value in parentheses are outwash plain (1), collapsed outwash (2), hummocky moraine (3), and ground moraine (4). The categories within the sediment factor and their associated travel time values are sand and gravel (1.5), sandy loam (2.5), loam (4), and loam to clay loam (4.5).
The point data were assigned a numerical value for each category. The unsaturated thickness factor was divided into the following categories: 0 to 20 feet (some to all of the sediment above the target zone is unsaturated) = 1, 21 feet to 50 feet (the unsaturated sediment continues into the target zone) = 2, and greater than 50 feet (the sediment is unsaturated to below the target zone) = 3. A representative value for each map polygon was calculated based on the percentage of data points within each of these categories. This representative value was adjusted to a scale of 1 to 5 to correspond to the spatial data scale. The target zone lithology factor accounts for the possibility that sediments within the target zone are different from those mapped at the surface, a difference that would affect vertical time of travel. This factor was divided into three categories: no clay = 1, some clay = 2, and all clay = 3. Again, a representative value was calculated and adjusted to a scale of 1 to 5.
The numerical values for the four factors were averaged for each map polygon, and an initial sensitivity rating was assigned based on that average as follows: 1 to 1.7 = very high, 1.8 to 2.5 = high, 2.6 to 3.4 = moderate, 3.5 to 4.2 = low, and 4.3 to 5 = very low. The matrix in Figure 3 on Plate 4 shows the results of this process. Sensitivity ratings of map polygons that lacked point data were compared with similar neighboring map polygons to determine their geologic sensitivity rating. If the average value for a map polygon fell close to a boundary between two ratings, the value for that map polygon was re-examined using qualifying information.
Qualifying information was used to validate or adjust the initial sensitivity rating for each map polygon based on the physical and chemical environments. The following are examples of features of the physical environment. Many isolated lakes or wetlands in low-lying areas of a map polygon suggest sluggish ground-water movement. Well-developed stream networks (dendritic streams) suggest a preference for precipitation to run off rather than infiltrate. The chemical environment was also examined to validate or adjust sensitivity ratings. The presence of tritium in ground-water samples or evidence of enhanced recharge, for example, ruled out the possibility of a low sensitivity rating.