If it weren’t for leaky city pipes and irrigation, it’s possible water flowing through popular Austin swimming holes would be much lower.
A recent study led by The University of Texas at Austin has found that in urbanized areas, much of the water that flows through Bull Creek – which feeds Bull Creek District Park and St. Edward’s Park swimming holes – can be traced back to municipal sources, such as sprinkler runoff, and leakage from municipal water and wastewater pipes.
“That means if you turned off all the leakage and the irrigation, then some urbanized regions of Bull Creek and its tributary channels would lose a large amount of stream flow, more than 50% during non-rainstorm conditions,” said lead author Lakin Beal, who conducted the research while earning her master’s degree at the UT Jackson School of Geosciences. She is now a hydrogeologist at INTERA Inc.
The study was published in the journal Water Resources Research and is part of the university’s Planet Texas 2050 initiative, which looks to Texas research as a model for solving environmental challenges facing the globe.
The findings raise concern about water quality in the watershed over time. Elevated sodium and chloride concentrations in urban areas suggest that wastewater is making it into the stream and spring water that flows naturally through the watershed.
The results also showcase a promising new geochemical technique that could help other cities learn more about their watersheds, said co-author and Jackson School Professor Jay Banner.
“To recognize that the municipal water is getting out there, that’s not a surprise because every city is leaking. But what is novel is the way we are able to chemically identify it and track how it changes,” said Banner, who is also director of UT’s Environmental Science Institute and an organizer of Planet Texas 2050.
The study examined water samples from throughout the Bull Creek Watershed, which spans about 50 square miles. About half of the watershed is in a rural setting, and half in an urban setting, which allowed the researchers to study how stream and spring water changed as the area became more urbanized.
The scientists were able to distinguish municipal water from natural water by looking for differences in the isotope signatures of strontium in the urban stream and spring water. Strontium is a trace element naturally found in Austin’s underlying limestone bedrock. Bedrock from different regions can have unique isotope signatures. Austin’s municipal water supply comes from the Highland Lakes, which has a different isotope signature than the local groundwater.
By comparing the chemical composition of the water samples against values from a model that estimates how fluids would need to mix in order to have a particular composition, the researchers inferred that 50-90% of the water in the urbanized portion of the Bull Creek watershed may be traced to municipal sources during non-rainstorm conditions.
They also found evidence that municipal water takes multiple flow paths through the watershed, with some water entering more quickly into the watershed, in comparison to other samples that took longer paths to percolate through the limestone and reach the water table.
Beal said she wasn’t surprised by the results, given that leaking municipal water infrastructure is common to most major urban centers. And the city’s roads and sidewalks combined with its porous limestone landscape offer an easy path for municipal water to infiltrate the environment.
Still, the technique used to study the watershed is an important tool to look at how water changes in a rapidly urbanizing watershed. The study points out other areas where water interacts with bedrock geology that has a distinctive chemical signature – such as St. Louis, Missouri, and Spokane, Washington – as good candidates for applying the technique.
“That underscores what the goal of the Planet Texas 2050 project is about,” Banner said. “We want our results to be transferable.”
The research was funded by the National Science Foundation, the UT Bridging Barriers initiative, the Jackson School and the City of Austin.