THE SECONDARY PERMEABILITY OF “IMPERVIOUS” COVER

The term "impervious" is comcommonly used in urban settings to describe the permeability of buildings, roads, and parking lots. When estimating recharge to an aquifer underlying an urbanized area, impervious cover becomes a prime consideration. It is generally assumed that an increase in impervious cover leads to a decrease in direct recharge. However, even a cursory glance at most roads, sidewalks, or parking lots reveals that, far from being impervious, there are abundant fractures that may provide avenues for infiltration. We have developed a method to determine the secondary permeability of pavements using a double-ring infiltrometer to measure the infiltration rate of water into fractured pavements. Linear extrapolation is employed to determine the infiltration rate as the water depth approaches zero, which is used as a proxy for hydraulic conductivity by assuming that the gradient is unity. Data were collected on concrete and asphalt pavements located in Austin, Texas, at each point a fracture or expansion joint intersected along 30-meter scan lines. By dividing the sum of the discharges for each fracture by the area represented by the scan line we are able to determine an equivalent-porous-media hydraulic conductivity. For discrete fractures, we find that equivalent hydraulic conductivities range four orders of magnitude from >10^-2 to 10^-5 cm/sec; scan line hydraulic conductivities range two orders of magnitude from 10^-4 to 10^-5 cm/sec; and that permeability along the scan lines tend to be dominated by one or two of highly conductive fractures. 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. In some cases, even direct recharge could be increased with urbanization.