EFFECTS OF URBANIZATION ON GROUNDWATER RECHARGE

Urbanization is yet frequently assumed to increase stream flows because of the inevitable increase in “impervious cover” and, therefore, to decrease groundwater recharge because less precipitation may infiltrate through paved surfaces. Our studies indicate, however, that several processes generally cause recharge to increase. Indirect recharge may increase in area with losing streams if stream flow durations increase. Artificial recharge commonly increases because of leakage from water, sewage, and storm sewer systems; over-irrigation of lawns; the construction of storm water retention and detention ponds; and designed recharge structures. Direct recharge from precipitation may either decrease or increase. Using direct infiltration experiments, we find that fractures in pavements are permeable with hydraulic conductivities ranging from >1.0E-2 to ~1.0E-5 cm/sec. Average scanline permeabilities are on the order of 1.0E-04 to 1.0E-5 cm/sec (which is consistent with storm-water runoff tests) and are dominated by the few most conductive fractures. This can increase localized recharge. When coupled with reduction of evapotranspiration, the net effect of roads and parking lots on recharge is uncertain and in some cases, combined direct and localized recharge could increase with increasing impervious cover. World wide data show utility line leakage rates varying from 5 to ~80%. When the lines are in the vadose zone, the resulting recharge can be quite significant. Recharge increases with urbanization because of leaky water, storm-water, and sewer systems; urban over irrigation and other artificial recharge; and possibly increased direct, localized, and indirect recharge. This effect is most dominant in urban areas in arid climates. Finally, the rate of porosity and permeability increase caused by urbanization is far greater than natural rates and urban areas function as pseudo-karstic systems regardless of the geological setting of the city.