THE PHYSICAL EFFECTS OF URBANIZATION ON SHALLOW SURFACE WATERS AND GROUNDWATER SYSTEMS

Urbanization is one of the major geomorphic, hydrologic, and geological processes shaping the Earth. It drastically affects shallow groundwater systems and surface waters. When land surface is leveled and low areas are filled for construction, small streams and springs are commonly buried. Dewatering of basements and subsurface infrastructure can lower water tables and decrease springflows. For instance, in Washington DC, none of the 40 historic identified springs currently flow. Much of any urban area is covered with commonly mis-termed “impervious cover” and stream flows become more flashy and flooding gets worse. However, based on infiltration studies, pavements can have secondary permeabilities on the order of 10^-5 cm/sec. Groundwater recharge patterns are highly altered spatially, but, in most cases and particularly in arid zones, recharge increases on a city-wide scale because of increased artificial, localized, and indirect recharge and decreased evapotranspiration, which together can serve to maintain low stream flows. A network or reticulation of subsurface conduits, tunnels, and utility lines are installed that create: 1) zones of enhanced but highly anisotropic and heterogeneous permeability and 2) enhanced shallow secondary porosity (essentially an epikarst). These networks can provide significant recharge sources and/or can serve as drains that limit water table rise. In subsequent construction, abandoned utility lines are commonly left in place which further increases heterogeneity. These factors make flow system and transport modeling in urban areas difficult. Nonetheless, with increasing urban population and urban sprawl, consideration of the alterations to shallow hydrogeologic systems is necessary for water supply, public health, and environmental reasons.