• science

    Contaminants of Emerging Concern (CECs)

    This was presented to the HTGCD Board on June 15th, 2016 by Douglas Wierman, P.G. from Meadows Center for Water and the Environment at Texas State University. View .pdf presentation here.

    These CECs will enter Onion Creek if there is a wastewater discharge.

    City of Austin Onion Creek Study (WASP Modeling)

    Link To The April 2016 City of Austin Study
    Link To The August 2016 Supplemental Comments on Application for Proposed Permit No. WQ0014488003

    Because no data analysis or water quality modeling accompanied the Preliminary Engineering Planning Report (PEPR), the City of Austin constructed a Water Quality Analysis Simulation Program (WASP) model to asses the creek's response, which will demonstrate the potential impacts of nutrient enrichment on the surface water of Onion Creek. Due to the connectivity between Onion Creek and local groundwater resources, impacts to the surface water of Onion Creek can impact local groundwater sources.

    The study concluded that Onion Creek would be degraded from a low productivity oligotrophic system (pristine, low nutrient, low plant growth) to a higher productivity mesotrophic system (higher nutrient, higher plant life) for approximately 9-12 miles depending on rainfall in a given year.

    The City of Austin concluded the current proposal should not be accepted as it results in significant degradation of the water quality on Onion Creek based on the modeling results in this report.

    BSEACD Surface Groundwater Interactions Memo 

    Link to the 6/24/2015 BSEACD Surface Groundwater Interactions Memo


    In summary, the studies referenced in this memo provide strong evidence that water flowing in Onion Creek is recharging the Trinity and Edwards Aquifers along certain reaches of the streams. The greatest uncertainty of these studies is how much, how quickly, and under what conditions does recharge occur into the Trinity Aquifer.

    Aquifer Impact

    (Update: Mayor's "Statement of Clarification" added below evidence)

    At the City of Dripping Springs City Council Meeting on the evening of June 14th, 2016 upon responding to 5 individuals speaking in opposition to Dripping Springs’ pending permit filed with TCEQ to discharge treated sewage effluent into Walnut Springs and subsequently into Onion Creek, Mayor Todd Purcell made these verbal comments in his response to us at exactly 7:20PM CST:

    “I will push to pull the permit if that water is going into our aquifers” and “show me evidence and I will pull the permit”

    We present the following 8 documents and web link as evidence that Onion Creek waters do in fact go into our aquifers. There are at least 12 names associated with this material – all very well respected professionals in their fields of study including hydrology and geology.

    We respectfully ask that the Honorable Mayor of Dripping Springs act now upon his commitment given to us on the evening of June 14th, 2016 at 7:20PM CST and we trust that Mayor Purcell will stay true to his word and will in fact immediately “pull the permit.”


    Evidence 1:
    BSEACD Resolution #063016-01


    Evidence 2:
    HTGCD Resolution #20160707

    Evidence 3:
    USGS Report Report

    Evidence 4:
    Surface-Water and Groundwater Interactions Along Onion Creek

    Evidence 5:
    BSEACD Summary of Recent Studies Regarding Surface and Groundwater Interactions in the Blanco River and Onion Creek

    Evidence 6:
    Contemporaneous Notes Taken 7/7/2016, 7:30 PM from HTGCD Special Meeting

    Evidence 7:
    TCEQ Interoffice Memorandum

    Evidence 8:
    Abstracts of Most Pertinent Studies to Barton and Onion Creek Watershed Rule Stakeholders Meeting

    Evidence 9, link:



    Notarized signatures swear and attest to Mayor’s comments 7 individuals

    Mayor Todd Purcell's "Statement of Clarification"  

    (In response to his statements above)

    Many conversations are ongoing around the City of Dripping Springs’ pursuit of a necessary discharge permit from the Texas Commission on Environmental Quality (TCEQ) in order to expand our wastewater treatment facility. It’s a complex topic of great interest to many in our community and for good reason. It is also a topic that has initiated a difficult, but necessary dialogue throughout the area and, in particular, at City Council meetings.


    I’d like to take this opportunity to clarify a comment I made at the Council meeting on June 14th in regard to the proposed wastewater expansion and discharge permit. At that meeting, I said, “If we are going to put water into the aquifer, we need to pull the permit.” Citizens have brought up that comment on multiple occasions since the meeting. And, I did, in fact, say exactly that. However, I should have been more careful with the words I chose to use.


    What I should have said is, “If the discharge permit is going to negatively impact the quality of water in the aquifer, then we should re-evaluate disposal options.”


    I call this out for two reasons: one, as I mentioned, several community members have brought up my statement, so, I wanted simply to clarify my position; and two: I stand firmly behind the plan the City is moving forward with to address our growing wastewater challenges. I am confident in the science and the many years of research and analysis that has been performed.


    I know this a complicated and emotional discussion, and it should be grounded in facts. To that end, my offer still stands: City staff and I are willing to meet with any interested stakeholder in the community to further discuss this issue. Thank you for the opportunity to clarify my statement from the City Council meeting on June 14.





    Mayor Todd Purcell

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