E. Coli. Fate and Transport below Subsurface Septic Tanks in the Lake Granbury Area.

Pathogens have been identified as a major polluter of water resources in the state of Texas. In the North Central Texas, Lake Granbury which is a part of three lake system of the Brazos River is a critical water supply reservoir, providing water for over 250,000 people in more than 15 cities. Recently, it has been observed that some of the coves of the Lake Granbury have been infested with Escherichia coli bacteria (E. coli). Lake coves closer to residential area are suspected to have E. coli contamination due to large number of underground septic systems. A study is conducted to track, describe, and model pathogen fate and transport, so water quality can be protected for Lake Granbury. In this study, a vadose zone model ‘HYDRUS-3D’ is used to simulate the impact of onsite septic systems and E. coli affecting the water quality of lake in Granbury area. Water flow and E. coli fate/transport are simulated for a 2-D cross sectional domain encroached by lake Granbury on one side and an open drainage canal on the other side with underground septic tanks as the input source in the center. Hydrus simulations were run for nearly two year duration. Initial results indicate that modeled E. coli concentrations decrease with depth and its transport through subsurface is correlated with matric potential. The presentation will also include a sensitivity analysis of the model to various parameters and the assessment of E. coli contamination in the Lake Granbury.