Extension of an LA-QUAL Model for Evaluating a Proposed Wastewater Discharge to Realistic Receiving Water Temperature and Flow Conditions

Multiple steady-state LA-QUAL(version 8.0) models for varying temperature and flow conditions were evaluated and used to generate a time-series prediction of water quality impacts from a proposed wastewater discharge to Bear Creek (Hays County, Texas) at the upstream boundary of the Barton Springs portion of the Edwards Aquifer Recharge Zone. LA-QUAL predicted output is sensitive to dilution (amount of natural creek flow present) and temperature (removal rates are temperature-dependent) (Herrington 2008). Although LA-QUAL (or QUAL-TX) models are commonly used by Texas Commission on Environmental Quality staff to evaluate potential dissolved oxygen degradation for new discharges (TCEQ 2003), typically only a single critical condition (low flow, high temperature) is considered. The use of multiple models is a more accurate representation of the temporal variability in flow and temperature. Using this approach, the proposed discharge is predicted to degrade the water quality of Bear Creek and the Edwards Aquifer. However, this analysis is still only a conservative approximation of the actual degradation potential and does not simulate more dynamic ecological responses such as nutrient spiraling or storm event driven sediment re-suspension.