Under the U.S EPA’s Chesapeake Bay Total Maximum Daily Load (TMDL) established in 2010 to restore and protect the bay, many state and local entities are required to implement changes in their processes to lower both Point Source (PS) and Non-Point Source (NPS) loading contributions. In this study, effect of a PS loading modification from a mining facility in Catawba, VA on the water quality and subsequent pollutant transport characteristic in the receiving body of water were investigated. The study site, a mining facility in Catawba, VA, situated in the north of Roanoke, releases a point source discharge into the Catawba Creek which is within Chesapeake Bay watershed boundary. The main objective of this study was to model the water quality response in nearby stream sections to a modified ammonia point source discharge. In 2015, the mining facility implemented a retention pond to store its wastewater before discharging to comply with its new VPDES permit limit of ammonia concentration from 20mg/L to 10mg/L. During pre-2015 TMDL regulation, the facility had discharged its effluent directly into the Catawba Creek.
Water quality and subsequent pollutant transport characteristic in the Catawba Creek was characterized by using a Mixed Flow Reactor (MFR) model framework with advective and diffusive/dispersive transport components. Flow characterization based on channel morphology was carried out first to characterize advective transport, then the critical dispersion coefficient for Catawba Creek was determined from by in situ tracer study with saline solution as well as with empirical equations for comparison and verification. Coefficient and the ammonia decay rate in advection-dominant Catawba Creek was then calculated from a temperature-adjusted first order kinetic equation. Model results show primary transport characteristic is advective and concentration of ammonia stays constant within the Catawba Creek.
For the retention pond, a Continuous Stirred Tank Reactor (CSTR) model framework was applied. The model determined that with an average influent flowrate of 0.85 MGD into a 900 MG retention pond, and an effluent flowrate of 2.8 MGD, the mining facility will be in VPDES compliance with the new discharge limit of 10 mg/L into Catawba Creek if the influent ammonia concentrations into the retention pond do not exceed 200mg/L or 1,400 lb/day for winter.
Detailed background, and modelling approach, data, and results will be presented and discussed.