Applicability and Lessons Learned from Using the Load Duration Curve Method to Develop TMDLS for Hardness-dependent Metals

A load duration curve (LDC) is a graphic representation of the assimilative capacity of a water body at a full range of flow regimes. The LDC is frequently used to represent the Total Maximum Daily Load (TMDL) of a given pollutant or constituent for the watershed draining into the impaired water body. This article draws TMDL examples from approximately two dozen TMDLs developed for impaired water bodies within the Neosho River and the Lower Arkansas River basins, Kansas USA, and focuses on how different challenges posed by various watersheds can be met using the LDC method. Specifically, this article describes how the LDC method is applied to multiple watersheds with hardness-dependent metals impairment. Even when key challenges arise such as disparities in data availability, stream or river flow characteristics, or hardness-flow relationships, these environmental factors may be effectively incorporated into the LDC developed for each watershed to yield an implementable TMDL for the hardness-dependent metal(s) in question. This article demonstrate how the LDC approach may be used to calculate waste load allocations for point source discharges, load allocations for non-point discharges, and explicit or implicit margins of safety required to ensure adequate protectiveness of the watershed. The selection process for hardness values (single values and/or regression-derived equations) to be integrated into the TMDL water quality target at specific flow percentiles is also discussed in this article. Modifications that are occasionally required when a stream is intermittent and/or dominated by point source discharge flow are presented and explained as well. Discharge monitoring data from point sources are sometimes limited and example modifications to waste load allocation and current point source load calculations for watersheds with or without discharge monitoring data for specific point source discharges are discussed. TMDLs that apply to sub-watersheds within a watershed are complicated by differences between hardness on the tributaries and main stem. These differences are addressed through LDC displays segregating relative contributions from each tributary. Point source and non-point load reductions are critically important in implementing the TMDL, and this article provides examples of how point and non-point load reductions will be calculated depending on data availability. For example, in cases where site-specific water quality data is limited, the LDC may be integrated with a Generalized Watershed Loading Function or other hydrodynamic model to estimate non-point loading to the watershed. The discussion also addresses the various working assumptions, uncertainties, and examples of implicit and explicit margins of safety required to develop and implement an achievable TMDL for hardnessdependent metals.