RECENT TRAVERTINE DEPOSITS AS RECORDS OF GROUNDWATER PROCESSES IN URBANIZING ENVIRONMENTS

Urban development has multiple impacts on hydrogeology, including increased recharge through utility leakage and irrigation, alteration of groundwater flow paths, and increased water contamination. Many of these impacts have only recently begun to be appreciated, and in many instances temporal records of water coincident with urbanization are nonexistent, making planning of future development difficult to predict. Recently deposited spring water calcite (travertine) may provide a proxy record of changing groundwater composition. The city of Austin, Texas, has undergone a large increase in urbanization over the past 40 years. Because the chemistry of municipal water in Austin is distinctive from groundwater, it is possible to examine the amount of urban water input to recharge using the isotopic composition of strontium (⁸⁷Sr/⁸⁶Sr). We have examined a 30-year-old spring-fed travertine deposit on the wall of a roadcut through Lower Cretaceous marine limestones. The spring is actively precipitating calcite, and appears to have grown continually since the construction of the road in 1976. The contributing area to this spring has undergone an increase in urban land use of approximately 40% since 1983. Preliminary ⁸⁷Sr/⁸⁶Sr data for the travertine and spring waters show a small but measurable increase over time, from 0.70790 to 0.70797. These values are higher than those for Lower Cretaceous marine limestones, and lower than values for municipal water as well as soil exchangeable Sr. This increase in ⁸⁷Sr/⁸⁶Sr may be accounted for by increased amounts of municipal water input to the groundwater in the spring's contributing area. Modification to the area's recharge patterns and soils may also play a role in the temporal isotopic changes. Longer temporal records obtained from multiple sites in the area will help evaluate the role of urbanization in groundwater composition.