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Colorado River below Lady Bird Lake: Alluvium Case Study of Pathogens

The Texas Water Resources Institute investigated pathogen movement between the Colorado River below Lady Bird Lake and its adjacent alluvial aquifer.

Background

Lady Bird Lake is a reservoir on the Colorado River in downtown Austin. The segment below the lake (Segment 1428) has been identified as being at high risk for bacteria contamination due to the density of septic systems in the area, and groundwater chemistry and hydrology. Prior to 2012, Segment 1428 did not meet all water quality standards because bacteria concentrations exceed the criteria used to evaluate attainment of the contact recreation use. These bacteria are naturally found in the intestines of humans, livestock, wildlife, and pets. Although they are not generally disease-causing, their presence in water indicate the potential presence of disease-causing microorganisms from fecal contamination. Therefore, higher levels of fecal bacteria in water mean a higher risk to humans of contracting diseases by ingesting contaminated water while swimming, wading, or kayaking—activities, called “contact recreation,” in the state’s standards for water quality.

A 2009 University of Texas study conducted on this segment indicated that the surges in flow below Lady Bird Lake following upstream dam releases strongly affected the hydrology of downstream riparian aquifers. However, that study did not document whether this connectivity affected the movement of pollutants between surface water and alluvial aquifers. This is important to understand because private well owners pumping water from such aquifers may be at higher risk of contracting pathogens if there is high movement of fecal bacteria into groundwater.

In 2010—in conjunction with this project—the Texas Water Resources Institute (TWRI) conducted a technical literature review on this topic and found numerous instances where groundwater quality has been degraded by surface water infiltration. However, microbial methods of transport through the soil zone, transport rates, and survivability were less understood.

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Project Description

To answer some of these questions, TWRI investigated E. coli movement between the Colorado River and its alluvial aquifer downstream of Lady Bird Lake. They installed four wells at different distances from and perpendicular to the river, and collected groundwater from each well over three 24-hour periods. TWRI recorded field parameters and analyzed samples for E. coli.

During the sample periods, dam releases resulted in a half foot change in river height and little fluctuation in field parameters and fecal bacteria concentrations. Fecal bacteria concentrations were higher in river samples than in well samples. These results suggest that there was little-to-no mixing of river and groundwater and the risk of contracting diseases from contaminated groundwater is low in this area. However, dam releases during this study were much smaller than those occurring during the 2009 study. Therefore, mixing and pollutant movement may be greater with larger dam releases.

As part of this project TWRI also developed educational materials for private well owners to assess the vulnerability of their wells to pathogen contamination and best practices to mitigate them. The project was completed in May 2015.

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For More Information

To find out more about the NPS Program, call 512-239-6682 or e-mail us at nps@tceq.texas.gov.

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