Dye tests show connection between Onion Creek and Middle Trinity

Initial results from an ongoing study have found Onion Creek is “hydrologically connected” to the Middle Trinity Aquifer, according to a memo to the Barton Springs Edwards Aquifer Conservation District (BSEACD).

Doug Wierman, an independent geoscientist working with the Meadows Center in San Marcos, said the results show an important connection that could have an impact on the area.

“It confirms that whatever is in the creek can get into the aquifer,” Wierman said.

The study, which was conducted on Dec. 4, 2017, was a project involving the BSEACD, the Hays Trinity Groundwater Conservation District, the Meadows Center and the city of Austin.

Hydrologists used a dye-trace test method to test whether surface water in Onion Creek was contributing to recharge in the Middle Trinity Aquifer.

The test involved injecting non-toxic dye into karst features, or openings, along Onion Creek in the vicinity of the city of Dripping Springs, according to the memo.

The test is a “long-established, safe and scientifically sound approach” to identify potential surface and groundwater interactions, according to the memo.

Dye was injected into the Bigote Swallet, a recharge feature near Ranch Road 12, a karst in the Howard Ranch subdivision and into a karst south of Onion Creek downstream of the Gatlin Creek Road bridge. Wierman said entities received cooperation from local landowners to conduct the tests.

Wierman said hydrologists monitor the dye trace tests three different ways. One involves color change in the water, which is the “least likely thing you see.”

Other ways include running water samples through a spectrometer, along with testing for carbon in the water.

Hydrologists didn’t have to wait long for results to come back.

John Dupnik, general manager of the BSEACD, said the “most compelling evidence” of direct connection was dye showing up in a landowner’s well that was 1.25 miles from the injection point. Dupnik said it’s believed the dye took less than 24 hours to show up in wells from two of the injection points. The wells are connected to the Middle Trinity Aquifer.

“That’s indicating very rapid transit in the sub-surface to the aquifer from Onion Creek,” Dupnik said.

Dupnik added that he wasn’t surprised by the rapid results based on the fractures in the limestone of karst aquifers, such as the Middle Trinity. He said rapid water movement has been seen in previous studies.

“We have a limestone karst environment, where water travels through fractures and water movement is very fast,” Wierman said. He added that a mile per day of travel for water to an aquifer is not uncommon.

Dupnik said hydrologists and scientists are continuing to study water samples and conduct tests.

How such results could impact Dripping Springs’ permit to expand its wastewater treatment system is unknown at this time.

Dripping Springs’ permit, which was submitted to the Texas Commission on Environmental Quality, aims to expand capacity to its wastewater treatment plant to 995,000 gallons per day. The permit could allow the city to discharge treated effluent into Walnut Springs, a tributary of Onion Creek.

Over the past year, city officials have said they intend to avoid discharge, with the city enacting agreements with developers for beneficial reuse of the effluent.

Ginger Faught, Dripping Springs deputy city administrator, said city officials have not had an opportunity to review the study at this time.

Dupnik said the study provides “hard science” about the connection between Onion Creek and the Middle Trinity. BSEACD provided the theory to TCEQ to consider with Dripping Springs initial permit application.

Currently, TCEQ is determining which parties could be part of a contested hearing for Dripping Springs’ permit. Dupnik said he hopes the BSEACD is a part of the hearings.

Blayne Stansberry, BSEACD president, said the initial results confirm what the organization’s scientists had suspected.

“There’s a lot of connection and Dripping Springs’ proposed wastewater discharge permit is upstream of that in Onion Creek,” Stansberry said. “One could infer that the wastewater discharge, although treated, is going to go into the creek, into the aquifer and private wells as soon as the next day.”

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