Group A . Group D. Soils having a very slow infiltration rate (high runoff potential) when This group's infiltration rate ranges from about 12 millimeters to 25 millimeters per hour. These soils vary in composition of clay, silt, and sand. A common rule of thumb for infiltration feasibility is that hydrologic soil groups A and B are suitable, C soils are questionable, and D soils are not good candidates. The Natural Resource Conservation Service has grouped soils that have similar runoff characteristics into four classes. Hydrologic Soil Group . According to Rawls 1982, the lower end of soils assigned to Hydrologic Soil Group C have an average infiltration rate of 0.17 inches per hour. Group C. Soils having a slow infiltration rate when thoroughly wet. They have NRCS-reported infiltration rates that … These soils have an infiltration rate ranging from 4 to 7 millimeters per hour. For each soils hydrologic group… See Table 2.3.3. Hydrologic Soil Group Hydrologic soil groups are based on estimates of runoff potential. The NRCS soil hydrologic group B includes sandy loam soils with moderate textures. Hydrologic Soil Groups A and B are more conducive to stormwater recharge than “C” soils, so care must be exercised when designing stormwater recharge system in “C” soils. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. According to the NRCS (formally the Soil Conservation Service), 64% of Mecklenburg County’s land area is composed of soils in hydrologic group B. Group C soils are typically silty loam, with moderately fine to fine texture. These soils have a high rate of water transmission. Native soils in proposed infiltration areas must have a minimum infiltration rate (permeability or hydraulic conductivity per -A) of 1/2 inch per hour (typically Appendix 8 Hydrologic Soil Group A and B soils meet criterionthis ). Hydrologic Soil Group C Soils having slow infiltration rates when thoroughly wetted, consisting chiefly of (1) soils with a layer that impedes the downward movement of water, or (2) soils with moderately fine or fine textures and slow infiltration rate. Required Infiltration Rate: Native soils should have a minimum field infiltration rate of 0.5 inches per hour (Hydrologic Soil Group A and B soils) at the actual location and soil layer where infiltration is proposed. Selection of a hydrologic soil group should be done based on measured infiltration rates, soil survey (such as the NRCS Web Soil Survey), or judgement from a qualified soil science or geotechnical professional. These soils have a slow rate of water transmission. These soils have a very slow rate of water transmission (final infiltration rate less than 0.05 in (1.3 mm) per hour). Initially, soil infiltration rates can be and recharge rates. Similarly, a soil’s permeability is a critical parameter in the design of stormwater BMPs that utilize infiltration. Hydrologic Soil Group Assumed Infiltration Rate Description Group A 4”/hour Soils having a high infiltration rate (low runoff potential) when thoroughly wet. The four Hydrologic Soils Groups are A, B, C and D. Where A's generally have the smallest runoff potential and Ds the greatest. This method has been adopted in several other northeastern states with similar climates and average annual precipitation. These consist chiefly of deep, well drained to excessively drained sands or gravels. NRCS Soil Survey maps and estimates of average annual infiltration rates for each hydrologic soil group. Presented below are three sets of guidelines: Section 1 demonstrates how to identify an appropriate Hydrologic Soil Group (HSG) for a soil with an unknown or questionable HSG including a method to Soils. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. These soils have a slow rate of water transmission. Soils are classified by the Natural Resource Conservation Service into four Hydrologic Soil Groups based on the soil's runoff potential.