Urban effects on groundwater recharge in Austin, Texas
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Cities and urban populations are growing at a high pace, but groundwater remains an underutilized resource in most urban areas. The general impacts of urban development on groundwater include overexploitation; subsidence; decreasing quality; salt-water intrusion; disruption of ecosystems; variations in the local climate; properties of the soil; natural drainage network; and the quantity, quality, and location of both recharge and discharge. The shallow urban underground is an intricate network of tunnels, conduits, utilities, and other buried structures comparable to a natural karstic system, except that "urban karst" is generated much faster. Urbanization also introduces new sources of water, resulting in an increase of groundwater recharge. These sources include irrigation of parks and lawns, leakage from water mains and sewers, and infiltration structures. The areal extent of Austin, Texas, has grown steadily since 1885 but has increased five-fold since the 1960's. The difference between the amount of tap water treated in the City of Austin and the amount of sewage that arrives in the wastewater treatment plants (or excess urban water), represents the amount of urban water potentially available for recharge. A water balance shows that about 7% of the treated drinking water is estimated to be lost to leaks from the distribution network and 5% to leaks from sewers. The rest of the excess urban water is used in irrigation of parks and lawns, some of which will be evapotranspired and some will turn into recharge. Smaller fractions are recharged in septic tanks and other designed infiltration devices. Direct recharge from rainfall has decreased as a result of the introduction and expansion of impervious pavements, from 53 mm/a under preurban conditions to 31 mm/a in the year 2000. However urban sources of recharge contribute an average of 85 mm/a of excess urban water, resulting on an urban recharge of 63 mm/a, and a total recharge rate that could equal 94 mm/a. Several hydrogeochemical parameters were tested as tracers of urban recharge in Austin. Chlorination by-products (trihalomethanes) were found in high concentrations in tap water and in low concentrations in wastewater. However, they were not detected in either surface water courses or groundwater. δ¹⁵N is a commonly used indicator of leakage from sewers, but unusually low values were obtained. Finally, ⁸⁷Sr/⁸⁶Sr of dissolved strontium shows a strong trend that can be related to the degree of urbanization over the Barton Springs segment of the Edwards aquifer. Values of this ratio from the lesser urbanized wells indicate groundwaters close to equilibrium with the limestone, while samples from the more urbanized wells show higher values, which are closer to those of tap water.