EFFECTS OF URBANIZATION OF GROUNDWATER RESOURCES, RECHARGE RATES, AND FLOW PATTERNS

Over half of the world’s population lives in cities and this percentage is growing. Urban sprawl is also progressing at high rates. Urban areas have profound effects on groundwater systems because they require water resources; produce sources of pollution; may alter local climate systems; change the geomorphology; alter the permeability field; and, generally, increase recharge. Where groundwater supplies the cities, problems of subsidence, salt-water intrusion, aquifer overexploitation, and loss of wetlands can occur. Where surface water is used, problems of rising water tables can occur along with the other problems involved in water transfer and reservoirs. In addition, the protection of environmentally sensitive aquatic ecosystems has become a sensitive issue. Future urban water resources must utilize both surface and groundwater. The latter is underutilized in some settings because of management issues, economy of scale, scientific uncertainties, and public policy. Recharge is inhibited in urban areas where impervious cover impedes infiltration and promotes runoff, but urbanization also creates additional recharge: leakage from water and wastewater systems, leaks from storm sewers, and irrigation return flow from lawns, parks, and golf courses. The more efficient cities report a 10% water loss from the water distribution system. Typical values range between 20 to 30%, in developed countries and between 30 to 60% in the less developed countries. Loss from sewer systems may approach similar amounts. World-wide data demonstrate that increased recharge to the groundwater is the rule. This creates an important local resource. Some streams (e.g., in Austin, Texas) are now fed primarily by leakage from water and sewer mains. In addition, urbanization hides existing fluvial systems and the utility systems form networks of hyper-permeability pathways. Where situated beneath the water table, utility systems collect flow similar to a karstic system. When above the water table, they act as line and points sources of recharge and contamination. Urban development should be designed protect future water resources and environmentally sensitive areas. This requires detailed hydrogeological maps, data, and analyses and continuous monitoring of local hydrogeological systems is necessary.