EFFECTS OF URBANIZATION ON SHALLOW HYDROGEOLOGIC SYSTEMS

Shallow hydrogeologic systems are drastically altered by urbanization and at rates are very rapid geologically. Urbanization is documented to increase secondary permeability and secondary porosity and these permeability and porosity fields are very inhomogeneous and anisotropic. Urbanization has levelled land surfaces and filled in low areas, rerouted (and buried) small streams, created ephemeral lakes in the form of flood control structures, and increased “impervious”, but not impermeable cover. In fact, groundwater recharge in cities generally increases. Direct recharge and, perhaps, evapotranspiration decrease, but artificial, localized, and indirect recharge commonly increase, which can maintain urban streams during low-flow conditions. The need for large water supplies to serve growing cities is often accompanied by decreased use of shallow local aquifers. Small streams and springs are commonly buried, water table elevations are altered, and flooding is exacerbated. Current and abandoned utility lines can provide recharge or serve as drains to make flow and transport modeling difficult. Finally, recent events show numerous sinkholes, contamination events, and water main breaks causing flooding in urban settings. These effects are documented with examples primarily from Austin, Texas, and Washington, DC, USA. The effects of urbanization on shallow hydrogeologic systems in urban design need to be considered in the context of water supply, public health, and environmental protection.