SEDIMENT AND NUTRIENT MODELING FOR TMDL DEVELOPMENT AND IMPLEMENTATION

Abstract

At present, there are over 34,000 impaired waters and over 58,000 associated impairments officially listed in theU.S. Nutrients and sediment are two of the most common pollutants included in the list. States are required to identify andlist those waters within their boundaries that are not meeting standards, to prioritize them, and to develop Total MaximumDaily Loads (TMDLs) for the pollutants of concern. Models are used to support development of TMDLs, typically to estimatesource loading inputs, evaluate receiving water quality, and determine source load allocations so that receiving water qualitystandards are met. Numerous models are available today, and selection of the most suitable model for a specific TMDL projectcan be daunting. This article presents a critical review of models simulating sediment and nutrients in watersheds andreceiving waters that have potential for use with TMDL development and implementation. The water quality modelsdiscussed, especially those with sediment and/or nutrient components, include loading models (GWLF and PLOAD),receiving water models (AQUATOX, BATHTUB, CE-QUAL-W2, QUAL2E, and QUAL2K), and watershed models havingboth loading and receiving components (AGNPS, AnnAGNPS, CASC2D/GSSHA, DWSM, HSPF, KINEROS2, LSPC, MIKESHE, and SWAT). Additional models mentioned include another receiving water quality model (WASP), watershed models(ANSWERS storm event, ANSWERS continuous, PRMS storm event, SWMM, and WEPP), and BMP models (APEX, REMM,and VFSMOD). Model sources, structures, and procedures for simulating hydrology, sediment, and nutrients are brieflydescribed for the reviewed models along with an assessment of their strengths, limitations, robustness, and potentials for usingin sediment and/or nutrient TMDLs. Applications of AGNPS, APEX, BATHTUB, CE-QUAL-W2, GWLF, and SWAT in TMDLdevelopments are presented. Applications of some of the other models (DWSM, GSSHA, and KINEROS2) relevant to TMDLstudies are also presented. The models proved to be useful; however, they require a learning process. Simple models are easyto use but have limitations; comprehensive models are labor and data intensive but offer extensive analysis tools. Finally,recommendations are offered for advancing the sediment and nutrient modeling technologies as applied to TMDLdevelopment and implementation. Advances could be made towards: making the best use of existing models, enhancing theexisting models, combining strengths of existing models, developing new models or supplemental components with physicallybased robust routines, numerous field applications, sensitivity analyses, full documentation, and rigorous education and training.