Remediating Compacted Urban Soils with Tillage and Compost

Urban areas typically have a high degree of impervious surfaces— that is, roads, parking lots, rooftops, and other surfaces that impede the movement of water into the soil. In predevelopment areas with natural ground cover, over half of precipitation infiltrates the soil to replenish groundwater tables, and only a small amount (10%) remains as runoff into stormwater systems, rivers, and lakes. In areas with 75 to 100% impervious surfaces—which is typical of urban development—far less precipitation is able to infiltrate the soil, and more than 50% of rainfall typically becomes surface runoff (Figure 1). Stormwater runoff from urban areas creates many problems. The increased rate and volume of this runoff increases the risk of flash flooding and can alter the temperature and aquatic habitat in streams. In addition, roads and parking lots are full of contaminants, including debris, wear from tires and brake pads, oils and other chemicals, toxic metals, animal pathogens, soil and sediment, road salt, and excess nutrients, that are picked up and transported into lakes and rivers by runoff. Municipalities and developers use several stormwater management practices to mitigate the effects of runoff caused by urbanization. Current approaches focus on implementing strategies that allow more rainwater to infiltrate into the ground. In addition to reducing peak flow rates during a storm, infiltration can remove pollutants through filtration of particles and the molecular binding of pollutants to the soil. Infiltration also reduces the need for stormwater sewers and storage by reducing the volume of stormwater runoff that must be handled. Unfortunately, the soils in most residential developments have lower stormwaterinfiltration capacity than the native soils they have replaced, because topsoil depth is reduced and subsoil compaction is increased as land is reshaped and worked with heavy equipment during development. A number of techniques are available to increase stormwater infiltration, Remediating Compacted Urban Soils with Tillage and Compost