Site soil variability site with spatially heterogeneous soils (vertically or horizontally) – as determined from site investigations are more difficult to estimate average properties for resulting in a higher level of uncertainty associated with initial estimates. Depth to seasonal high groundwater/impervious layer groundwater mounding may – become an issue during excessively wet conditions where shallow aquifers or shallow clay lenses are present. Table VII.3: Suitability Assessment Related Considerations for Infiltration Facility Safety Factors Consideration High Concern Medium Concern Low Concern Direct measurement of ≥ 50 percent of infiltration Direct measurement area with localized Use of soil survey of 20 percent of ≥ infiltration measurement maps or simple infiltration area with Assessment methods methods texture analysis to localized infiltration (see explanation below) estimate short-term measurement or infiltration rates methods (e.g., Use of extensive test pit infiltrometer) infiltration measurement methods Silty and clayey Granular to slightly Texture Class soils with significant Loamy soils soils fines Highly variable indicated from site Soil borings/test pits Multiple soil borings/test assessment or Site soil variability indicate moderately pits indicate relatively limited soil borings homogeneous soils homogeneous collected during site assessment Depth to groundwater/ <5 ft below facility 5-10 ft below facility >10 facility bottom impervious layer bottom bottom Localized infiltration testing refers to methods such as the double ring infiltrometer test (ASTM which measure infiltration rates over an area less than 10 sq-ft, may include lateral flow, and do not attempt to account for heterogeneity of soil. The amount of area each test represents should be estimated depending on the observed heterogeneity of the soil. Extensive infiltration testing refers to methods that include excavating a significant portion of the proposed infiltration area, filling the excavation with water, and monitoring drawdown. The excavation should be to the depth of the proposed infiltration surface and ideally be at least 50 to 100 square feet.
Site soil variability – site with spatially heterogeneous soils (vertically or horizontally) as determined from site investigations are more difficult to estimate average properties for resulting in a higher level of uncertainty associated with initial estimates. Depth to seasonal high groundwater/impervious layer – groundwater mounding may become an issue during excessively wet conditions where shallow aquifers or shallow clay lenses are present. Table VII.3: Suitability Assessment Related Considerations for Infiltration Facility Safety Factors Consideration High Concern Medium Concern Low Concern Assessment methods (see explanation below) Use of soil survey maps or simple texture analysis to estimate short-term infiltration rates Direct measurement of ≥ 20 percent of infiltration area with localized infiltration measurement methods (e.g., infiltrometer) Direct measurement of ≥ 50 percent of infiltration area with localized infiltration measurement methods or Use of extensive test pit infiltration measurement methods Texture Class Silty and clayey soils with significant fines Loamy soils Granular to slightly soils Site soil variability Highly variable indicated from site assessment or limited soil borings collected during site assessment Soil borings/test pits indicate moderately homogeneous soils Multiple soil borings/test pits indicate relatively homogeneous Depth to groundwater/ impervious layer <5 ft below facility bottom 5-10 ft below facility bottom >10 facility bottom Localized infiltration testing refers to methods such as the double ring infiltrometer test (ASTM which measure infiltration rates over an area less than 10 sq-ft, may include lateral flow, and do not attempt to account for heterogeneity of soil. The amount of area each test represents should be estimated depending on the observed heterogeneity of the soil. Extensive infiltration testing refers to methods that include excavating a significant portion of the proposed infiltration area, filling the excavation with water, and monitoring drawdown. The excavation should be to the depth of the proposed infiltration surface and ideally be at least 50 to 100 square feet.
OC_TGD_Appendices_5-19-11.docx