Physical and Hydraulic Properties of Engineered Soil Media for Bioretention Basins

The composition of engineered soil media largely determines the stormwater treatment efficiency of urban bioretention basins. Laboratory flow-through experiments were conducted to quantify infiltration, bulk density, and moisture holding capacity as a function of different composite mixtures of sand, soil, and compost, and to assess the effect of compaction on bulk density, moisture holding capacity, and saturated hydraulic conductivity. Eleven mixtures were evaluated that varied in volumetric proportions of sand (30% to 70%), sandy or silt loam soil (0% or 20%), and organic compost (20% to 70%). Steady-state infiltration rates were high for all mixtures, ranging from 87 to 178 cm h-1, and followed the order of mixtures containing sand and compost only > mixtures containing sand, compost, and sandy soil > mixtures containing sand, compost, and silt loam soil. Infiltration rates for mixtures containing sand and compost only and mixtures containing sand, compost, and sandy soil exhibited a significant linear relationship with the ratio of sand to compost. Bulk density of the mixtures was inversely related to the proportion of compost and followed the order of mixtures containing sand, compost, and silt loam soil > mixtures containing sand, compost, and sandy soil > mixtures containing sand and compost only. Conversely, moisture holding capacity increased with the proportion of compost and followed the order of mixtures containing sand and compost only > mixtures containing sand, compost, and sandy soil > mixtures containing sand, compost, and silt loam soil. Compaction as a result of an initial wetting process and the infiltration tests led to increases in bulk density and decreases in moisture holding capacity, with mixtures containing a silt loam component showing the greatest resistance to these effects. Bulk density, moisture holding capacity, and compaction were all linearly related to the ratio of sand/compost in the mixture. Air permeability measurements were used to estimate saturated hydraulic conductivity of four of the mixtures. Reductions of compost and additions of soil decreased saturated hydraulic conductivity. For the same proportions of sand, soil, and compost, the mixture containing silt loam soil was less compactable and incurred greater changes in saturated hydraulic conductivity compared to the mixture containing sandy soil. Although, at least initially, compost controlled the physical density of these mixtures, the textural class of the mineral component appears to help stabilize infiltration and dampen the effect of changing the ratio of sand to compost on the physical and functional characteristics of these mixtures.