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Hydrologic Soil Group —Snohomish County Area, Washington <br />USDA Soil Map <br />Hydrologic Soil Group <br />Map unit symbol <br />Map unit name <br />Rating <br />Acres in AOI <br />Percent of AOI <br />3 <br />Alderwood gravelly <br />sandy loam, 15 to 30 <br />percent slopes <br />B <br />6.2 <br />100.0% <br />Totals for Area of Interest <br />6.2 <br />100.0% <br />Description <br />Hydrologic soil groups are based on estimates of runoff potential. Soils are <br />assigned to one of four groups according to the rate of water infiltration when the <br />soils are not protected by vegetation, are thoroughly wet, and receive <br />precipitation from long -duration storms. <br />The soils in the United States are assigned to four groups (A, B, C, and D) and <br />three dual classes (A/D, B/D, and C/D). The groups are defined as follows: <br />Group A. Soils having a high infiltration rate (low runoff potential) when <br />thoroughly wet. These consist mainly of deep, well drained to excessively <br />drained sands or gravelly sands. These soils have a high rate of water <br />transmission. <br />Group B. Soils having a moderate infiltration rate when thoroughly wet. These <br />consist chiefly of moderately deep or deep, moderately well drained or well <br />drained soils that have moderately fine texture to moderately coarse texture. <br />These soils have a moderate rate of water transmission. <br />Group C. Soils having a slow infiltration rate when thoroughly wet. These consist <br />chiefly of soils having a layer that impedes the downward movement of water or <br />soils of moderately fine texture or fine texture. These soils have a slow rate of <br />water transmission. <br />Group D. Soils having a very slow infiltration rate (high runoff potential) when <br />thoroughly wet. These consist chiefly of clays that have a high shrink -swell <br />potential, soils that have a high water table, soils that have a claypan or clay <br />layer at or near the surface, and soils that are shallow over nearly impervious <br />material. These soils have a very slow rate of water transmission. <br />If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is <br />for drained areas and the second is for undrained areas. Only the soils that in <br />their natural condition are in group D are assigned to dual classes. <br />Rating Options <br />Aggregation Method. Dominant Condition <br />USDA Natural Resources Web Soil Survey 12/10/2021 <br />Conservation Service National Cooperative Soil Survey Page 3 of 4 <br />