WHAT IS NATURAL DO IN SOUTHEASTERN GEORGIA?

The State of Georgia identified impaired stream segments in the Georgia East 2 basins, Ogeechee and Savannah, for dissolved oxygen (DO). Typically, these segments do not have long-term flow or water quality measurements. Among the challenges present were determining natural and critical conditions for the TMDL process. An approach was developed to address these needs in data limited segments. A segment-by-segment approach using locally and regionally observed data and published values was used to address these needs.The development of an approach required an analysis of existing data from regional stations and a series of educated assumptions based on the local characteristics of the system. The calibration process was challenging with some parameters being measured while others were reasonably estimated or provided using best professional judgement.A key obstacle in this study was the definition of natural DO conditions. Many of the segments addressed were low-gradient streams and naturally exhibited DO values typically lower than the criteria, conditions not unfamiliar in Southeastern GA. It was important to develop a TMDL approach for these segments involving the setup of steady-state models, calibrating to low-flow conditions, and determining the natural and critical conditions relative to existing conditions. Allocations were then derived from the results of these conditions.