172-7 Geochemical Characterization of Quaternary Alluvial Terrace Springs in Austin, Texas for an Urban Groundwater Source Identification Program

Session: Urban Geochemistry

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Abstract:

Austin, Texas straddles the Balcones Fault Zone, which separates the karst Edwards Plateau to the west and Blackland Prairie to the east. Within city boundaries over 1500 springs have been mapped. While the region’s karst springs, such as Barton Springs, are well studied, the smaller, perched-aquifer springs emerging from Quaternary Colorado River Alluvial Terrace Deposits east of the fault have received comparatively little attention.

Austin’s Watershed Protection Department has a groundwater forensics protocol called “Urban Discharge Assessment” (UDA) to investigate the source of urban seeps. This program uses geochemical profiles of local municipal tap water, wastewater, reclaimed water, and natural springs based on confidence intervals for 17 aqueous parameters. Although the UDA framework is well calibrated for Austin’s natural karst springs using over 30 years of data, the lack of equivalent data for alluvial springs has resulted in a skewed reference baseline for groundwater.   Field investigations in historically underserved east Austin revealed numerous springs where standard parameters were measured outside UDA-defined natural spring confidence intervals. These anomalies are compounded by more industrial activity and less protected land in the area, complicating source attribution.

To address this knowledge gap, we conducted quarterly sampling over two years to establish a robust geochemical dataset specific to the Alluvial Terrace springs of east Austin. Results indicate geochemical distinctions from karst springs. Notable differences between the two types of springs include higher calcium, sodium, and Nitrate-N concentrations, lower magnesium concentrations, and higher pH   in Alluvial Terrace springs. Our data also suggest that Austin’s Alluvial Terrace springs are more sensitive to anthropogenic influences than previously recognized, which challenges assumptions that land east of the Balcones Fault Zone requires less environmental protection than its karst counterpart. In one instance, we documented a spring impacted by a nearby irrigation system’s transition from municipal to reclaimed water.

Using these results, we calculated new UDA confidence intervals specific to Austin’s Alluvial Terrace springs to support future investigations. Future work will continue to monitor select Alluvial Terrace springs and may expand to include understudied springs in other geologic units, such as the Late Cretaceous Austin Chalk or Buda formations.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6609

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