Total Maximum Daily Loads

In contrast to waste load allocation investigations performed to meet Clean Water Act requirements to determine the allowable point source loading of pollutants from municipal and industrial wastewater facilities, modern water quality management TMDL studies must explicitly account for all point and nonpoint sources of pollutants discharged within a watershed that may contribute to the water quality impairment of designated uses of specific river segments within the watershed. In addition to pollutant loads contributed by point source municipal and industrial wastewater facilities, pollutant loads contributed by nonpoint sources of urban stormwater and combined sewer overflow systems, atmospheric deposition and rural, agricultural and forested land uses must also be quantified to determine TMDLs for specific pollutants that have been identified as the cause of impairment of a waterbody. In a watershed–based TMDL study, an inventory of point and nonpoint source loads of the pollutants of concern is used as the starting point for the assignment of reasonable reductions of point and nonpoint source loads within a watershed. Since it is difficult, if not impossible, to correlate an inventory of point and nonpoint source pollutant loads with water quality problems in a watershed solely by the evaluation of water quality monitoring data, hydrodynamic and surface water quality models must be developed to provide the quantitative cause-effect linkage between pollutant loads, source locations and water quality impairments under different hydrologic and seasonal conditions.

TMDL development thus requires experienced professionals that can define watershed problems, define project objectives, collect and prepare data for use in water quality models and select and calibrate the most appropriate water quality model for the site-specific waterbody and impairment.
Since Dynamic Solutions was part of a team contracted by USEPA’s Ecosystem Research Division (ERD) to provide an evaluation of currently available watershed, hydrodynamic, water quality/eutrophication, sediment transport, chemical fate and bioaccumulation models, we are highly experienced in selecting the most appropriate model to achieve the objectives of a site-specific TMDL project. While the availability of data, and the quality of that data, generally guides the model selection process, experienced professional judgment and communication are the keys to successful TMDL development. A critical component of our TMDL development process is ensuring that all stakeholders are fully represented in a well-documented process. We typically start the TMDL development process with simplified approaches and gradually increase complexity, if necessary, based upon results of the preliminary more simplified work.

Selected examples of our TMDL-related projects include:

Evaluation of Contaminated Sediment Transport and Fate Models and Evaluation of Chemical Bioaccumulation Models. In a study for EPA’s National Exposure Research Laboratory, Office of Research and Development in Athens, Georgia, Dynamic Solutions was a team member for the review and evaluation of currently available surface water models for hydrodynamics, sediment transport, toxic chemical transport and fate, conventional pollutants, eutrophication and chemical bioaccumulation. As part of these projects, Dynamic Solutions recommended upgrades to currently available toxic chemical and eutrophication models to link organic carbon for partitioning of toxicants with organic carbon.

Norwalk Harbor Hydrodynamic and Water Quality Model
Dr. Andrew Stoddard, Dynamic Solution’s senior water quality modeling expert, served as the principal engineer for a team that developed a 3-dimensional hydrodynamic and water quality model of the Norwalk River, Norwalk Harbor and nearshore Long Island Sound in Connecticut. Water quality issues for Norwalk Harbor included bottom water dissolved oxygen depletion and nutrient enrichment which were related to eutrophication of the estuary. Water quality issues also included urban runoff stormwater loading of pathogenic organisms and the periodic closures of recreational beaches and shellfish beds in Norwalk Harbor and the beaches of Long Island Sound. The objective of the modeling study was to determine the total maximum daily load (TMDL) of total nitrogen that can be discharged to Norwalk Harbor without violating Class SB water quality standards for dissolved oxygen. EFDC was used to provide two-layer tidally driven transport for input to EPA’s WASP5 model for eutrophication and oxygen depletion. A field sampling program was also undertaken by the project team that included measuring water quality parameters at a number of stations in the water body; collecting sediment-water interface flux rate measurements for dissolved oxygen and nutrients; and obtaining benthic microflora data for incorporation into a macrophyte sub-model that was incorporated in WASP5. The allowable nitrogen loads were compared to existing and projected nonpoint and point source loads to determine the degree of effluent controls needed for the planned expansion of the Norwalk WWTP as well as other best management practices (BMPs) that might be needed to control nonpoint source loading of total nitrogen from the Norwalk River watershed. The findings of this TMDL modeling study resulting in a savings of $25 million for the City of Norwalk since NPDES permits for total nitrogen were less stringent than originally required by CTDEP.