Todd County Geologic Atlas Part B

This file was created by combining the estimated infiltration time for soil units with the estimated infiltration rate for the underlying surficial geology units, the soil?s parent material. Field [TRVL_TIME] is the best estimate of travel time of water (in hours) to move from the surface to a depth of 10 feet.

The sensitivity to pollution assessment for near-surface materials estimates the time of travel for water to travel from the land surface to a depth of ten feet, and is shown in Figure 13, Plate 10. Soil properties are used to estimate the travel time from land surface to a depth of three feet and surficial geology properties are used to estimate the travel time from a depth of three feet to ten feet. The near-surface materials sensitivity assessment was created by estimating infiltration rates through soils and surficial geology units based on the Natural Resources Conservation Service (NRCS) hydrologic rating for soils and the geologic unit texture of deeper parent materials, from Plate 3, Part A, Surficial Geology. Estimates of infiltration rates are shown in Table 1, Plate 10. The NRCS defines hydrologic groups based on a somewhat complicated system, but it is primarily based on the soil unit?s texture and the presence or absence of dense, low permeability layers.

The near-surface materials sensitivity rating is determined by using the minimum transmission rates for the soil and surficial geologic units to calculate the estimated travel time. For this calculation, the water table is assumed to be 10 feet below the land surface throughout the county. The GIS polygons from both the soil survey and the surficial geologic map are converted to grids with a 30 meter cell size. Each cell in the respective grids is assigned a minimum transmission rate as explained above. The travel time for the upper 3 feet is calculated using the minimum transmission rate of the soil unit. The travel time for 3 feet to 10 feet below land surface is calculated using the minimum transmission rate of the surficial geologic unit. The total travel time to 10 feet is then used to estimate the near-surface materials geologic sensitivity (Figure 2, Plate 10). Some soil units have not been assigned a hydrologic group (for example gravel pits) and therefore have no assigned minimum transmission rate. If a minimum transmission rate was not available for a soil unit, then the surficial geology unit minimum transmission rate was used to calculate the travel time for the entire 10-foot distance.

Soils with a dense material layer are primarily from a dense basal till parent material. These soils have a hydrologic group rating of C or C/D. This implies that these soil layers are less sensitive than most other layers. These soils may transmit water through vertical fractures much more quickly than through the matrix material. Therefore, an overlay polygon representing the soils with dense material layers is shown in feature class soils_dense. The sensitivity rating calculated for these areas of dense soils may be too low, if vertical fractures exist.

The [infiltration rate grid] was calculated using the combine command from spatial analyst. It combined grids [soils_rate_int] and [sgpg_rate_int], where [soils_rate_int] is the infiltration of water through soil (from 0 to 3 feet below land surface) and [sgps_rate_int] is the infiltration rate of water through the underlying surficial geologic material (soil parent material) from 3 to 10 feet. To facilitate the use of integer grids, both infiltration rates are stored as inches/hour * 100.

The travel time to a 10-foot depth, field [TRVL_TIME], was calculated using the following field calculation:

Dim TTime As Double
If [SOILS_RATE_INT] = 0 Then
If [SGPG_RATE_INT] <> 0 Then TTime = ROUND((1000 *12) / [SGPG_RATE_INT], 0)
elseif [SGPG_RATE_INT] = 0 Then TTime = -9999
endif
elseif [SGPG_RATE_INT] = 0 Then TTime = -9999
else TTime = ROUND((300 *12) / [SOILS_RATE_INT] + (700 *12) / [SGPG_RATE_INT], 0)
endif

Field [SOILS_RATE_INT] is from [min_t_rate] in soils.shp. It is the infiltration rate in inches/hour * 100. (To use an integer grid, it was necessary to multiply the infiltration rate by 100.)

Field [SGPG_RATE_INT] is from [min_t_rate] in sgpg_cat_py. It is the infiltration rate in inches/hour * 100. (To use an integer grid, it was necessary to multiply the infiltration rate by 100.)

Field [TRVL_TIME] is the estimated travel time from the surface to 10 feet below the surface in hours. If the value is this field is -9999, then the cell should be considered null. -9999 was used when both [mintratcor] in soils.shp and [min_t_rate] in sgpg_cat_py were equal to 0. It was used when water is at the surface, so infiltration to 10 feet is meaningless. If the soil data are null, but [min_t_rate] is not null, then [min_t_rate] from sgpg_cat_py was used to calculate the travel time for the 10 feet of travel.

Sensitivity ratings are based on hours of travel to a depth of ten feet and are applied as follows:

The final raster was converted to a polygon shapefile.

Sensitivity interpretation was based on estimated time of travel ranges as follows: