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• Technical Notes. sands or clays are on the lower end of the range, whereas silts and silt loams are on the high end of the range. Glacial tills, which were compressed by thousands of feet of ice in the last ice age, can have a bulk density ranging as high as 1.6 to 2.0 gms /cc, depending on how much they have weathered. Highly organic soils, like peat, can be as low as 0.3 gms/cc. In general, bulk density increases with soil depth, reflecting the compression by the overlying soil, and the decline in the abundance of soil fauna and organic matter. Figure 107.1 shows a typical profile of how bulk density changes with depth for soils of different land use (Smith, 1999). In contrast, many urban soils and surfaces have much higher bulk densities (Table 107.1). The highly disturbed soils of urban lawns range from 1.5 to 1.9 gms/ cc, while athletic fields and fill soil typically range from 1.8 to 2.0 gms /cc. These bulk density values approach the density of concrete (2.2 gms /cc). Soils adjacent to building pads and along the road rights of way are intentionally compacted to meet engineering specifica -' tions, and can range from 1.5 to 2.1 gms /cc, depending 0 08 10 20 0 ac so 0 40 50 60 Bulk Density (gm /cc) 1.2 1.6 2 2.4 2.8 Figure 107.2: Relationship Between Soil Bulk Density and Soil Porosity Table 107.1: Comparison of Bulk Density for Undisturbed Soils and Common Urban Conditions (Compiled from various sources) Urban Lawns Crushed Rock Parking Lot 1.5 to 1.9 Urban Fill Soils Athletic Fields 1.5 to 2.0 1.8 to 2.0 Rights of Way and Building Pads (85% Compaction) Rights of Way and Building Pads (95% Compaction) 1.8 to 2.0 1.5 to 1.8 Concrete Pavement Quartzite (Rock) 1.6 to 2.1 2.2 2.65 on local compaction standards and the compressibility of the underlying soil. The Consequences of Compaction The extensive compaction of urban soils has many adverse hydrologic impacts on a watershed. The pri- mary impact relates to the change of porosity within a soil. Figure 107.2 illustrates how soil porosity dimin- ishes as bulk density increases. Porosity is important because it governs the soil's capacity to hold water, infiltrate runoff and allow roots to penetrate. As poros- ity declines, compacted urban soils can produce much more surface runoff than is normally expected for grass or meadow cover. While pervious areas are not gener- ally thought to contribute much stormwater runoff, when urban soils become highly compacted, their runoff response more closely resembles that of an impervious surface, particularly during large storm events. For example, Wignosta et al. (1994) found that compacted soils produced from 40 to 60% of the annual runoff for a small developed catchment, and that the soils had an effective runoff coefficient as high as 0.5. Other researchers have also noted that com- pacted urban soils can have effective runoff coeffi- cients in the 0.2 to 0.45 range (Pitt, 1992, and Legg et al., 1996). While these runoff coefficients are still lower than those commonly reported for completely paved areas (0.50 to 0.99), they are very significant since lawns can comprise as much as 50 to 70% of residential cover. Thus, from a practical standpoint, soil compaction increases watershed runoff and creates Watershed-Protection Techniques •!Vol.. 3, No. 2 • January 2000?