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Water Quality Management: Wasteload Analysis for Developing Permit Limits
Introduction | Modeling | Download Model | Additional Information
Introduction
A wasteload allocation (WLA) is the portion of a receiving water's assimilative capacity that is allocated to one of its existing or future point sources of pollution. DWQ conducts a wasteload analysis to calculate the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. Computer models are used extensively for water quality management and determining water quality based effluent limits (WQBEL) for discharge permits. The models predict the water quality in a water body in response to changes in seasons, pollutant loading, and various wasteload allocation strategies.
DWQ has adopted the QUAL2Kw model for the determination of UPDES permit limits for nutrient related discharges to dissolved oxygen sensitive rivers and streams. Other models and tools are used to evaluate the discharge of conventional and toxic pollutants to rivers and streams, as well as discharges to lakes and reservoirs
Mathematical Modeling—QUAL2kw
Model Overview/Abstract
QUAL2Kw is a water quality model that simulates nutrient and oxygen dynamics in rivers and streams. It is applicable to rivers that are vertically well mixed and for periods with steady flow. QUAL2Kw is widely used for Total Maximum Daily Load studies of rivers for evaluation of temperature and eutrophication, including nutrients, dissolved oxygen, and pH. QUAL2Kw is based on the QUAL2K model and is maintained and distributed by the Washington State Department of Ecology. Following are the characteristics and capabilities of QUAL2Kw:
- One dimensional: the channel is well-mixed vertically and laterally.
- Steady state hydraulics: non-uniform, steady flow is simulated.
- Diel heat budget: the heat budget and temperature are dynamically simulated as a function of meteorology on a diel time scale.
- Diel water-quality kinetics: all water quality variables are dynamically simulated on a diel time scale.
- Heat and mass inputs: point and non-point loads and abstractions are simulated.
- Software environment and interface: Q2Kw is implemented within the Microsoft Excel/VBA environment. It is programmed in the Windows macro language: Visual Basic for Applications (VBA). Excel is used as the graphical user interface.
- Model segmentation: QUAL2Kw can use unequally-spaced reaches. In addition, multiple loadings and abstractions can be input to any reach.
- Carbon speciation: QUAL2Kw uses two forms of carbon, rather than BOD, to represent organic carbon. These forms are a slowly oxidizing form (slow dissolved organic carbon) and a rapidly oxidizing form (fast dissolved organic carbon). In addition, non-living particulate organic matter (detritus) is simulated. This detrital material includes particulate organic carbon, nitrogen, and phosphorus.
- Anoxia: QUAL2Kw accommodates anoxia by reducing oxidation reactions to zero at low oxygen levels. In addition, denitrification is modeled.
- Bottom algae: QUAL2Kw explicitly simulates attached bottom algae using either zero-order or first-order growth kinetics.
- Luxury uptake: Variable stoichiometry of nitrogen and phosphorus in bottom algae is simulated.
- Light extinction: Light extinction is calculated as a function of algae, detritus and inorganic solids.
- pH: Both alkalinity and total inorganic carbon are simulated. These are used to determine pH.
- Pathogen indicator: A generic pathogen indicator is simulated (e.g. fecal coliform or E coli). Pathogen indicator removal is determined as a function of temperature, light, and settling.
- Sediment-water interactions: Sediment-water fluxes of dissolved oxygen and nutrients are simulated internally rather than being prescribed. Sediment oxygen demand (SOD) and nutrient fluxes are simulated as a function of settling particulate organic matter, diagenesis reactions within the sediments, and the concentrations of soluble forms in the overlying waters.
- Sediment heat flux: Sediment-water heat flux and sediment temperature is simulated using a Fick's law formulation to account for conduction between the water and sediment and hyporheic flow and heat exchange.
- Hyporheic respiration: Exchange of water between the surface water column and the hyporheic zone, and simulation of sediment pore water quality, including optional simulation of growth and respiration of heterotrophic bacteria biofilm in the hyporheic zone.
- Automatic calibration: A genetic algorithm is included to determine the optimum values for the kinetic rate parameters to optimize the goodness of fit of the model compared with observed data.
- Monte Carlo simulation: Ready to run Monte Carlo simulations with either the YASAIw add-in, also available from the Department of Ecology, or Crystal Ball, including an example using YASAIw.
QUAL2Kw Model and Users Manual Download from Department of Ecology
Model Calibration
DWQ staff, in collaboration with researchers at Utah State University, are currently developing standard operating procedures for data collection, and model build and calibration. These procedures are anticipated to be completed by spring 2012.
Site Specific Data Collection
Facilities can submit site-specific information on both the receiving waterbody and the discharge characteristics for consideration. Wastewater treatment facilities are encouraged to plan ahead when considering any data gathering effort. Many of these efforts require seasonal data particularly collected during low stream flow conditions.
Model Application to Wasteload Analysis
Once the models are built and calibrated, they are used by DWQ staff for determining discharge permit limits. The results of the wasteload analysis are included as an addendum to the Statement of Basis for the UPDES permit. All supporting data and models used in the determination of permit limits are available for review by the permittee and other interested parties.
Additional Information
If you need additional information and/or assistance in the application of QUAL2Kw to determining UPDES permit limits in the state of Utah, please contact:
Nicholas von Stackelberg
Utah Division of Water Quality
801-536-4374
