2.3.3 Soil and Subsurface Hydrology Characterization

In-depth characterization of soil and subsurface hydrology is vital to LID planning and design. Typically, the goals of this task are to evaluate the site’s feasibility for infiltration and, where appropriate, determine long-term native soil design infiltration rates. Soil characterization is also important to help specify materials to be used in design. For example, geotextile layers for separation may not be needed on the sides or bottom of excavations for bioretention or permeable pavement if the native site soils are not expected to migrate into the various BMP layers based on grain size distributions (see Figure 2.2: Soil Texture Triangle).

This section documents well-accepted practices that may be used to characterize the soils and subsurface hydrologic conditions and evaluate long-term native soil infiltration rates to be used for design of infiltration-based BMPs. Designers should consult local jurisdiction requirements to determine the soil and subsurface hydrology characterization method that will be required to support the design.

Soil and subsurface characterization relies to a large extent on infiltration test pits, soil test pits, or soil borings. The type and number of these tests for initial site assessment is variable and site specific; however, some general guidelines are appropriate. A few strategically placed tests are generally adequate for initial soil and infiltration assessment. Test locations are determined by topography, estimated soil type, hydrologic characteristics, and other site features. Consult a certified soil scientist, professional engineer, geologist, hydrogeologist or engineering geologist registered in the State of Washington for the infiltration test pit, soil test pit, and soil boring recommendations for initial assessment. A more detailed soil and infiltration capacity assessment may be necessary after the preliminary site layout with location of LID stormwater controls is established.

The methods described in this section are used to determine the measured saturated hydraulic conductivity rate for existing subgrade soils for overall site assessment and beneath bioretention areas and permeable pavement. The measured saturated hydraulic conductivity with no correction factor may be used as the design infiltration rate if the professional engineer deems the infiltration testing described below (and perhaps additional tests) are conducted in locations and at appropriate distribution capable of producing a soil profile characterization that fully represents the infiltration capability where the bioretention or permeable pavement areas are located (e.g., if the small-scale PITs are performed for all bioretention areas and the site soils are adequately homogeneous).

If deemed necessary by a professional engineer, a correction factor may be applied to the measured saturated hydraulic conductivity to determine the longterm design native soil infiltration rate. Whether or not a correction factor is applied and the specific number used will depend on heterogeneity of the site soils and number of infiltration tests in relation to the number and type of infiltration areas. For bioretention, the overlying bioretention soil media provides excellent protection for the underlying native soil from sedimentation; accordingly, the measured native soil infiltration rate for bioretention does not require a correction factor for influent control and potential clogging over time.

For recommendations on test frequency and correction factors specific to bioretention, see Section 4.4: Bioretention. For recommendations on test frequency and correction factors specific to permeable pavement, see Section 4.6: Permeable Paving.

The depth and number of test holes or test pits and samples should be increased if, in the judgment of the licensed certified soils professional, conditions are highly variable and such increases are necessary to accurately estimate the performance of the infiltration system. Qualified soils professionals include: certified soil scientists, professional engineers, geologists, hydrogeologists or engineering geologists registered in the State of Washington. The exploration program may also be decreased if, in the opinion of the licensed certified soils professional, the conditions are relatively uniform and omitting the test pits or borings will not influence the design or successful operation of the facility. In high water table sites, the subsurface exploration sampling need not be conducted lower than two feet below the groundwater table.

Prepare detailed logs for each test pit or test hole and a map showing the location of the test pits or test holes. Logs should include, at a minimum: depth of pit or hole, soil descriptions, depth to water, and presence of stratification. Logs should substantiate whether stratification does or does not exist. The certified soils professional may consider additional methods of analysis to substantiate the presence of stratification that may influence the design or successful operation of the LID practice.

Soil stratigraphy should also be assessed for low permeability layers, highly permeable sand/gravel layers, depth to groundwater, and other soil structure variability necessary to assess subsurface flow patterns. Soil characterization for each soil unit (soil strata with the same texture, color, density, compaction, consolidation and permeability) should include:
  • Grain size distribution.
  • Textural class.
  • Percent clay content.
  • Cation exchange capacity.
  • Color/mottling.
  • Variations and nature of stratification.

If the groundwater in the area is known to be less than five feet below the proposed LID facility, the ground water regime should be assessed. At a minimum, groundwater monitoring wells should be installed to determine groundwater depth and seasonal variations, considering both confined and unconfined aquifers. Monitoring through at least one high groundwater period is necessary, unless site historical data regarding groundwater levels is available.

If on-site infiltration may result in shallow lateral flow (interflow), the conveyance and possible locations where that interflow may re-emerge should be assessed by a certified soil scientist, professional engineer, geologist, hydrogeologist or engineering geologist registered in the State of Washington (or suitably trained persons working under the supervision of the above professionals or by a locally licensed on-site sewage designer). In general, a minimum of three wells associated with three hydraulically connected surface or ground water features are necessary to determine the direction of flow and gradient. Alternative means of establishing the groundwater levels may be considered. If the groundwater in the area is known to be greater than five feet below the proposed LID facility, detailed investigation of the groundwater regime is not necessary.

Special considerations are necessary for highly permeable gravel areas. Signs of high groundwater will likely not be present in gravel lacking finer grain material such as sand and silt. Test pit and monitoring wells may not show high groundwater levels during low precipitation years. Accordingly, sound professional judgment, considering these factors and water quality treatment needs, is required to design multiple and dispersed infiltration facilities on sites with gravel deposits.

A groundwater mounding analysis should be considered if the minimum depth to bedrock, water table, or other impermeable layer is less than five feet. Groundwater mounding analysis may also warrant consideration if the contributing drainage area to an LID BMP is large relative to the BMP footprint area.

See Appendix B: Evaluating Soil Infiltration Rates for detailed discussion of the following methods for evaluating native soil infiltration rates:
  • In-situ small-scale Pilot Infiltration Test (PIT) method.
  • Soil grain size analysis method.
  • In-situ large-scale PIT.

Generally, the small-scale and large-scale PITs are similar; however, the small-scale PIT reduces cost and test time and is appropriate for LID facilities that have relatively low hydraulic loadings. The large-scale PIT is preferred for large-scale permeable pavement facilities where stormwater from adjacent impervious surfaces is directed to the pavement surface, resulting in higher hydraulic loads. The soil grain size analysis method can be used if the site has soils unconsolidated by glacial advance. Consult local jurisdiction for soil testing and reporting requirements that pertain to the project site

Figure 2.2 - Soil Texture Triangle
Illustrates the range of soil types that will be encountered in planning for LID practices. It also illustrates the general range of hydrological soil groups characterized by low to high infiltration capability. Source: Courtesy of the University of Arkansas Community Design Center