Compacted Urban Soils and their Remediation

The effects of urbanization on soil structure can be extensive. Infiltration of rain water through soils can be greatly reduced, plus the benefits of infiltration and biofiltration devices can be severely jeopardized. Our research group (along with others), have been studying disturbed urban soils for several decades, both in the laboratory and in the field. The effects of compaction on infiltration capacities and plant growth for different types of soils have been the greatest interest, along with methods to restore soils to their natural capacities. Stormwater pollutant movement through urban soils, along with the benefits and problems associated with different soil amendments (and groundwater contamination potential) has also been examined during our research. Long-term infiltration performance degradation associated with clogging (and therefore the need for pre-treatment and other suitable design issues), pollutant retention capacity, and effects of snowmelt on clayey soils are other areas of current research interest. This short review paper will focus on soil compaction and some basic restoration recommendations. Infiltration in Disturbed Urban Soils Soil disturbance/compaction in urban areas occurs during construction cutting and filling operations, general grading operations, and other processes of running heavy equipment over the soil. After construction, continued compaction can occur with site activities such as walking, sports, and even parking heavy vehicles on grassed areas. Slow improvements in soil compaction may occur with time in relatively undisturbed areas by deep rooted plants or by soil insects or other boring animals. Basically, soil infiltration performance is usually significantly degraded compared to natural soil conditions and is commonly overlooked during hydrologic analyses and design. Knowing the likely effects of this soil compaction on urban hydrological conditions is critical for designing safe drainage systems. Restoring the infiltration capacity of a soil is also possible and can provide significant benefits in stormwater management. The following discussion presents observations from a number of field and laboratory measurements and describes likely degraded infiltration rates for a variety of conditions. A later discussion presents comments pertaining to restoring infiltration rates. Field Tests of Compacted Soil Infiltration Rates A series of 153 double ring infiltrometer tests were conducted in disturbed urban soils in the Birmingham, and Mobile, Alabama, US, areas as part of an EPA project that investigated disturbed urban soils and soil amendments (Pitt, R., J. Lantrip, R. Harrison, C. Henry, and D. Hue. Infiltration through Disturbed Urban Soils and CompostAmended Soil Effects on Runoff Quality and Quantity. U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory. EPA 600/R-00/016. Cincinnati, Ohio. 231 pgs. December 1999, available at: http://www.unix.eng.ua.edu/~rpitt/Publications/BooksandReports/Compacted%20and%20compost%20amended%2 0soil%20EPA%20report.pdf). The tests were organized in a complete 2 factorial design to examine the effects of soil-water, soil texture, and soil density (compaction) on water infiltration through historically disturbed urban soils. Ten sites were selected representing a variety of desired conditions (compaction and texture) and numerous tests were conducted at each test site area. Soil-water content and soil texture conditions were determined by standard laboratory soil analyses. Compaction was measured in the field using a cone penetrometer and confirmed by the site history. During more recent tests, compaction is directly measured by obtaining samples from the field from a known volume (digging a small hole and retrieving all of the soil into sealed bags that are brought to the lab for moisture and weight analyses. The hole that is carefully cleaned of all loose soil is then filled with free-flowing sand from a graduated cylinder to determine the volume. The laboratory dry weight of the excavated soil is dived by the hole volume to obtain the density). From 12 to 27 replicate tests were conducted in each of the eight experimental