Infiltration Through Compacted Urban Soils and Effects on Biofiltration Design

Abstract

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 jeopardized. This paper is a compilation of results from several recent and on-going research projects that have examined some of these problems, plus 2 possible solutions. Basic infiltration measurements in disturbed urban soils were conducted during the EPA-sponsored project by Pitt, et al (1999b), along with examining hydraulic and water quality benefits of amending these soils with organic composts. Prior EPA-funded research examined the potential of groundwater contamination by infiltrating stormwater (Pitt, et al, 1994, 1996, and 1999a). In addition to the information obtained during these research projects, numerous student projects have also been conduced to examine other aspects of urban soils, especially more detailed tests examining soil density and infiltration during lab-scale tests, and methods and techniques to recover infiltration capacity of urban soils. This paper is a summary of this information and it is hoped that it will prove useful to both stormwater practice designers and to modelers. Prior research by Pitt (1987) examined runoff losses from paved and roofed surfaces in urban areas and showed significant losses at these surfaces during the small and moderate sized events of most interest for water quality evaluations. However, Pitt and Durrans (1995) also examined runoff and pavement seepage on highway pavements and found that very little surface runoff entered typical highway pavement. During earlier research, it was also found that disturbed urban soils do not behave as indicated by most stormwater models. In an attempt to explain the variations observed in early infiltration tests in disturbed urban soils, tests were conducted in the Birmingham, AL, area by the authors, assisted by UAB hydrology students. About 150 individual double-ring infiltration tests were conducted, separated into eight categories of soil conditions (comprising a full factorial experiment). Factors typically considered to be responsible for infiltration rate variations are texture and soil-water content. These Alabama tests examined texture and soil-water content, plus soil compaction (as measured by a cone penetrometer). It was also hoped that age since disturbance and cover condition could also be used to explain some of the variation, but poor distributions of these conditions over the complete range of the main experimental test conditions did not allow complete statistical examinations of these additional factors. The initial exploratory analyses of the data showed that sand was mostly affected by compaction, with little change due to soil-water content levels. However, the clay sites were affected by a strong interaction of compaction and soil-water content. The variations of the observed infiltration rates in each category were relatively large, but four distinct soil conditions were found to be significant, as shown in Table 1. The data from each individual test were fitted to the Horton equation, but the resulting equation coefficients were relatively imprecise, with the noncompacted sandy soil tests being the only soil category that had obvious infiltration rate variations that were well described by time since the start of the tests. When modeling runoff from most urban soils, it may be best to assume relatively constant infiltration rates throughout an event, and to utilize Monte Carlo procedures to describe the observed random variations about the predicted mean value. Table 1. Infiltration Rates for Significant Groupings of Soil Texture, Soil-Water Content, and Compaction Conditions Group Number of tests Average infiltration rate (in/hr) COV noncompacted sandy soils 36 13 0.4 compact sandy soils 39 1.4 1.3 noncompacted and dry clayey soils 18 9.8 1.5 all other clayey soils (compacted and dry, plus all wetter conditions) 60 0.2 2.4 Amendments to the soil were also found to significantly improve both the infiltration capacity of the soils and to better capture pollutants from the infiltrating water, significantly reducing the potential of groundwater contamination. Some organic amendments may leach nutrients for several years, but all were found to significantly reduce the transport of toxicants.