THE SECONDARY PERMEABILITY OF IMPERVIOUS COVER

Impervious cover is a term used to signify buildings, roads, and parking lots with essentially zero permeability and is an important consideration when estimating recharge to aquifers underlying urbanized areas. It is commonly assumed that increasing impervious cover causes a decrease in recharge from precipitation, but observations of most roads, sidewalks, or parking lots reveal that there are abundant fractures that may provide sites for infiltration. We estimate the secondary permeability of pavements using a double-ring infiltrometer extrapolating the infiltration rate as the water depth approaches zero and using it as a hydraulic conductivity proxy. Data were collected on concrete and asphalt pavements at fractures or expansion joints intersected along 16 30-meter scanlines. By dividing the sum of the discharges for each fracture by the area represented by the scanline, we determine an equivalent-porous-media hydraulic conductivity. For discrete fractures, these range over three orders of magnitude from >10-2 to <10-5 cm/sec; scanline hydraulic conductivities range two orders of magnitude from >10-4 to 10-6 cm/sec; permeability along the scanlines tends to be dominated by one or two highly conductive fractures; and that the hydraulic conductivity of the entire paved surface is approximately 5.9·10-5 cm/s. By multiplying the paved surface hydraulic conductivity by the time the surface can be expected to be saturated, we find that 170 mm or 21-percent of mean annual rainfall is available as potential recharge. When coupled with an enhanced subsurface permeability structure resulting from the installation of utilities and the reduction of evapotranspiration from the reduction of vegetation, the net effect of roads and parking lots on recharge is uncertain but, in most cases, there is likely either no effect or an increase in recharge from precipitation.