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Soils affect stormwater and stormwater management in two ways.
- Soil is a source of sediment and associated pollutants (e.g. metals, organic compounds) for stormwater runoff.
- Soil affects the types of stormwater management practices that can be implemented at a site (e.g. potential for infiltration, vegetation)
Importance of healthy soils
Soil serves several important functions, including but not limited to the following.
- Provide a media for vegetation
- Retain pollutants
- Promote biologic activity, including degradation of some pollutants
- Control air and water storage and transport
- Capture and retain carbon
- Well managed soils minimize erosion and prevent compaction
Soil erosion and sediment runoff to waterways are significant problems in Minnesota. According to the Minnesota Pollution Control Agency (MPCA 2016), approximately 30 percent of the state’s rivers and streams are impaired by sediment. Poorly managed construction sites can be substantial sediment sources to these surface waters. Up to 100 tons of sediment per acre can be lost annually from unmanaged construction sites (EPA 1999).
The Universal Soil Loss Equation (USLE) and it's update, the Revised Universal Soil Loss Equation (RUSLE) are used to predict sheet and rill erosion. Soil loss, typically expressed on an annual basis in tons per acre, is affected by rainfall characteristics, soil erodibility, slope length and gradient, soil cover, and erosion control practices. Soil erodibility is the intrinsic susceptibility of a soil to erosion by runoff and raindrop impact. In general, the following affect soil erodibility.
- Increasing amounts of soil organic matter result in decreasing values of K
- Soil type effect on K: silt > silt loam = fine sand > loam > clay loam > clay > coarse sand. Note that wet clay soils that have expanded have increased risk.
- Coarse sand particles are too large to transport
- Clays are cohesive with good soil structure and it is difficult to dislodge soil particles
- Silts and fine sands are not cohesive and are easily transported
- Texture is the principal factor affecting Kfact, but structure, organic matter, and permeability also contribute
- The soil erodibility factor ranges in value from 0.02 to 0.69.
For more information on soil erodibility, link here.
Managing soil loss involves erosion protection and sediment control, with erosion protection being preferred.
Erosion protection practices
Implementation of stormwater practices
The types of stormwater practices that can be implemented at a site are largely determined by soil conditions. Of particular importance are soil suitability for infiltration, suitability for implementing vegetated practices, and suitability for retaining pollutants.
- Infiltration is suitable for most soils in hydrologic soil groups A and B. Some infiltration can occur on C soils, but generally this is not sufficient to meet performance goals. Slope also affects infiltration, but development sites can generally be graded. Some coarse-textured soils are not suitable for infiltration because of contamination or proximity to restricting features, such as drinking water wells, underground utilities and structures, and shallow bedrock.
- Soils are generally suitable for supporting vegetation unless contaminated or compacted. However, the type of vegetation that can be utilized at a site varies with soil fertility and soil hydraulic properties. Coarse-textured soils often must be amended with organic material due to low fertility, and they may not support plants requiring greater amounts of water. Clay soils may restrict root development for some vegetation. Wet soils, often having high organic matter content, may be restrictive for vegetation requiring good drainage.
- Retention of pollutants is a function of the soils ability to adsorb/absorb pollutants and the rate at which water infiltrates through the soil. Organic matter is the primary factor affecting pollutant retention, but retention may be poor in soils with very high infiltration rates.
Fore more information, see Assessing soil suitability for stormwater applications and Assessing soil health and function