A Simplified Approach for Determining Bacterial TMDLS in Tidal Systems

The application of complicated numerical models to the determination of bacterial total maximum daily loads (TMDLs) in small tidal systems is often compromised by several factors, including: a lack of available data needed to adequately describe tidal dynamics in the impaired systems; difficulty in accurately quantifying bacterial loadings from non-point sources; and budgetary constraints that can limit the scope of a numerical modeling effort. This paper describes a simplified, cost-effective modeling approach to the bacterial TMDL determination process that was applied to shellfish waters in two tidal estuaries in the State of South Carolina. The application of this approach to the TMDL determination for impaired waterbodies within the Murrell's Inlet Estuary system is discussed as an illustrative example. As part of a weight-of-evidence approach, multi-segment box models were developed to assess the importance of the various point and nonpoint sources of fecal coliform bacteria to in-stream levels measured within the Murrell's Inlet Estuary, the most economically important shellfish producing area along the northern coast of South Carolina. Several water quality stations within this saltwater, tidally-influenced system are classified as Restricted, indicating that shellfish harvesting for direct marketing is not allowed due fecal coliform levels at locations that exceed one or both water quality criteria established for shellfish harvesting waters. The box models were constructed on the principles of conservation of mass, included computations of freshwater and tidally-influenced flows, and bacterial fate and transport, and facilitated the quantification of fecal coliform loadings in the freshwater runoff from nonpoint source areas within the watershed, which were the primary contributors to fecal coliform contamination in the impaired waterbodies. The models were calibrated to the available data and used to determine bacterial TMDLs on a watershed-specific basis. TMDLs for each impaired watershed were developed by estimating the reductions in fecal coliform loadings predicted by the model needed to achieve the geometric mean standard at each impaired station. Due to the tidal nature of the estuary, which results in the upstream movement of fecal coliform bacteria through the systems with the tidal flows, these evaluations were performed on a watershed basis. That is, a single percent reduction was assigned to a watershed having multiple impaired stations such that the prescribed reduction results in the attainment of water quality standards at all stations within the system. Similarly, cumulative probability distributions of the measured fecal coliform levels for each of the impaired stations were used to determine the average percent reductions in instantaneous source loadings to the system needed to satisfy the not to exceed 10% standard. In both instances, an explicit margin of safety (MOS) of 5% was assumed. The more stringent (i. e., higher) of the two percent reductions for each watershed was then selected in the final TMDL determination to ensure that both State standards will be achieved. Results of the TMDL determination indicated that 71 to 82% reductions in non-point source fecal coliform loadings to the Murrell's Inlet Estuary were necessary to satisfy both water quality criteria for fecal coliform in shellfish harvesting waters. The box models served as an efficient, technically defensible management tool and as the basis for the development of a management plan aimed at mitigating fecal coliform sources to the impaired estuaries. The modeling approach also provided a cost-effective means to evaluate fecal coliform contamination within other impaired estuaries within South Carolina.