Chapter 4 of Siting Criteria for Vegetative Treatment Areas provides a risk assessment tool for VTA characteristics including preferred soil properties for minimizing both surface and groundwater risks. This document recommends the following for surface water protection:
“Soils with moderate permeability are best for VIBs [vegetative infiltration basins] and VTAs. Soils with high permeability will reduce potential for discharge from a VTA, but increase the risk to groundwater. Soils with a low permeability improve protection of ground water, but increase the potential for a discharge from the VTA. For VIBs, soils with 0.6 to 2 inches per hour to a 5-foot depth are recommended. For VTAs, soils with 0.2 to 2 inches per hour to a 5-foot depth are suggested.”
This reference further suggests the following for groundwater protection:
“Many biological, physical, and chemical processes break down, lessen the potency, or otherwise reduce the volume of contaminants moving through the root zone of surface soils. These processes, collectively called attenuation, retard the movement of contaminants into deeper subsurface zones. The soil’s attenuation potential increases as clay content increases, the soil deepens, and distance increases between the contaminant source and the well or spring. The cation exchange capacity of clay soils limits movement of positively charged contaminants such as ammonium (NH4+). Clay also has a very low permeability, thus slowing contaminant movement and increasing the contact time that allows more opportunity for attenuation. Deeper soil increases the contact time a contaminant will have with mineral and organic matter of the soil. Longer contact time provides greater opportunity for attenuation.”
Author: Rick Koelsch, University of Nebraska
