CHANGING RECHARGE AND HYDROGEOLOGY IN AN URBANIZING AREA – EXAMPLE OF AUSTIN, TEXAS, USA

Global urban populations and areas are expanding rapidly. Urbanization is a major process shaping the landscape that typically increases "impervious" cover, alters shallow permeability fields, buries existing stream channels, increases local flooding, creates new sources of contamination, alters local climate, disrupts ecosystems, and increases groundwater recharge. The shallow urban underground is a complex network of tunnels, conduits, utilities, and other buried structures comparable to a natural karstic system, but which evolves at a much faster rate. These changes are examined for Austin, Texas, which has grown steadily since 1885 with a five-fold population increased since 1960. In particular, we find that urbanization introduces new sources of water and increases recharge by leakage from water mains and sewers, irrigation return flow, increased infiltration of surface runoff, and artificial recharge structures. The difference between the amount of treated water supplied by the City of Austin and the amount of sewage that arrives in the wastewater treatment plants represents the amount of water potentially available for recharge. This is significantly higher than the 12% unaccounted for water claimed by the City. About 7% of the treated water-loss has been attributed to leaks from mains; the rest is irrigation-return flows, septic tank flows, sewer leakage, and evapotranspiration. Our data show that urban (impervious) pavements can have significant secondary permeability. Analyses of dissolved 87Sr/86Sr of show that Waller Creek in downtown Austin is derived from water mains during periods of low flow. Water balances for Barton Springs and the Edwards aquifer in Austin indicate that, relative to precipitation, spring flow has increased with urbanization. This is attributed to both increased recharge from urbanization and increased flow to losing streams from an urbanizing catchment area. These flows are input into a decision support system being tested for aquifer management.