Effect of soil disturbance in native and engineered soils used in stormwater infiltration systems

Hydrologic cycle restoration is the primary objective of stormwater management. Infiltration and bioretention systems composed of engineered and/or native soils are preferred tools for achieving this objective while also providing pollutant removal. However the disturbance of native soils can cause releases of nutrients and suspended solids in the early life of these systems. To limit the potential of replacing one problem with another, a better understanding of the behavior of soil components as they contribute to water transport and pollutant treatment is needed. This project investigated the ability of the various soil horizons in a Wharton silt loam (Pennsylvania) to treat runoff from simulated storm events. The soil was collected intact, but had to be air-dried and the columns repacked when soil shrinkage caused bypassing of runoff along the walls of the laboratory columns. This process is similar to the disturbance of soil by bioretention construction and provided a unique opportunity to evaluate resulting nutrient releases. The effluent water from this reconstructed silt loam had elevated concentrations of total nitrogen (leaching > 100 mg/L of N initially from all soil horizons) and of total phosphorus from the organic horizon (∼1.5 mg/L) during application of the first 0.6 m of stormwater runoff. A release of calcium (∼500 mg/L) from all soil horizons also occurred, likely due to the destruction of cement bonds between soil aggregates. Potassium was also released from the O-horizon of the disturbed silt loam (∼30 mg/L) but leveled off after 0.2 m of applied runoff.