Environmental forensic investigation of mystery sediment plumes at Barton Springs, Texas

The City of Austin Watershed Protection Department (WPD) in Austin, Texas maintains a spill response plan to protect Barton Springs, one of the largest springs in the state that also functions as a municipal swimming pool, endangered species habitat, and well-studied karst research site. From December 18-20, 2018, three discrete sediment plumes of unknown origin emerged from Main Barton Spring and Eliza Spring, presenting an atypical spill scenario. Unlike the majority of spills, where staff respond to a known location, pollutant, and spill volume, WPD responders and staff at the Barton Springs Edwards Aquifer Conservation District (BSEACD) had to work backward from evidence at the springs to identify the plumes’ source. Of the four springs that make up the Barton Springs Complex, the highest turbidity was observed at Main Barton and Eliza Springs, which narrowed the source to the Manchaca groundwater basin based on previous regional groundwater traces. The concentrated spikes in turbidity associated with each plume indicated minimal dispersion and thus a source relatively near the springs. The white color of the plume on the 19th was typical of either construction runoff or bentonite grout used in well installation. No significant rainfall had occurred in the days prior to the event, and the plumes were not typical of storm responses. Following these lines of evidence, the source was identified as nearby drilling of a residential geothermal heat pump well system located approximately 1,250 meters from Main Barton Spring. The timing of the plumes correlated to voids encountered and well grouting as noted by the driller during initial discussions on site, yielding approximately a 315 - 415 meter/hour (1,030 – 1,360 feet/hour) groundwater velocity: within the range of velocities documented by groundwater tracing. Following this incident, WPD and BSEACD officials updated drilling protocols and communication procedures regarding future drilling projects in the area to improve WPD’s ability to respond quickly to similar events in the future. Because the sediment plumes only increased turbidity for short periods (no other parameters were affected) and posed no other significant threat to the health and safety of humans or the resident endangered salamanders, the incident presented unique opportunities to (1) test our Barton Springs Spill Response Plan readiness, (2) gather data similar to a groundwater tracing study, and (3) improve the Barton Springs Spill Response Plan and update drilling and notification protocols between overlapping regulatory agencies