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Modeling the Effects of Low Flow Augmentation by Discharge from a Wastewater Treatment Plant on Dissolved Oxygen Concentration in Leon Creek, San Antonio, Texas

Tejal A. Gholkar

A GIS-based hydrological/water quality model called Non Point Source Model (NPSM) was used to simulate various physical, chemical and biological processes taking place in the Leon Creek Watershed, near San Antonio, Texas. The model was then used to evaluate base flow augmentation scenarios to remedy dissolved oxygen problems during dry, low-flow periods. The effects were demonstrated by increasing base flow in a stream by discharging recycled water from Leon Creek Wastewater Treatment Plant during a three month low-flow period in 1993, 1994 and 1995 respectively. Five scenarios were evaluated in addition to the control scenario (no flow augmentation). Each of the five scenarios represented an increase in base flow by a factor of 0.25, 0.5, 1, 2 and 4 respectively.

The study indicated that increasing base flow in the stream increased the mean daily DO concentration in the stream. The most significant effect was observed when the base flow was increased by a factor of 1 onwards, with no data point falling below the DO criterion of 5 mg/l. From the results of DO modeling developed for this project iv and from the scenario analysis, it can be concluded that a minimum flow augmentation of one times base flow (i.e. doubling the base flow) is required in order to see a significant increase in mean daily DO concentration in Leon Creek and associated tributaries and remedy DO problems during low-flow periods. Since there is uncertainty involved in the modeling process, it is recommended that a higher flow augmentation of two times base flow or four times base flow be implemented in order to reduce uncertainty and significantly improve water quality of Leon Creek.

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