MODELING THE BIG BLACK RIVER: A COMPARISON OF WATER QUALITY MODELS1

ABSTRACT:

The Mississippi Department of Environmental Quality uses the Steady Riverine Environmental Assessment archetype (STREAM) to establish effluent limitations. While the U Environmental Protection Agency has approved of its use, questions arise regarding the model's simplicity. The objective of this research was to compare STREAM with the more commonly utilized Enhanced Stream Water Quality prototype (QUAL2E). The comparison involved a statistical evaluation process based on sensitivity analyses, input probability distribution functions, and Monte Carlo simulation with site-specific data from a 46-mile (74-km) reach of the Big Black River in central Mississippi. Site specific probability distribution functions were derived from measured rates of reaeration, sediment oxygen demand, photosynthesis, and respiration. one as well as the other STREAM and QUAL2E reasonably predicted daily average dissolved oxygen (DO) based upon a comparison of output probability distributions with observ DO. Observ DO was consistently within 90 percent confidence intervals of mould predictions. The STREAM approach generally overpredicted while QUAL2E generally matched observ DO. Using the more commonly assumed lognormal distribution as oppos to a Weibull distribution for sum of two units of the sensitive input parameters followed in minimal differences in the statistical evaluations. The QUAL2E approach had distinct advantages above STREAM in simulating the growing cycle of algae.

(KEY TERMS: dissolved oxygen; QUAL2E; STREAM; wastewater discharge; water quality modeling; Big Black River.)

INTRODUCTION AND BACKGROUND

Water quality management attempts to patronize the uses of a water material part while using the water as an economical means of waste disposal. The amount of waste a water material part can assimilate depends on numerous factors (McBride, 2002; Mohamed et al., 2002; de Azevedo et al., 2000; Somlyody et al, 1998) Environmental engineers must quantify the assimilative capacity for each water material part by predicting the water quality as a function of the waste loading (Chapra, 1997; Davis and Cornwell, 1998) The water quality parameter of regard for waste load allocations (WLAs) and water quality based effluent limitations (WQBEL) is repeatedly the instream concentration of dissolved oxygen (DO) (Lung 2001; Edinger, 2002) The Mississippi state standard for DO, as defined in the State of Mississippi Water Quality Criteria for Intrastate, Interstate, and Coastal Waters (MDEQ 2003) is a daily average concentration of at least 50 mg/L with an instantaneous minimum of no les than 40 mg/L

The greatest in quantity commonly utilized model for predicting the impact of discharges upon DO is the Enhanced Stream Water Quality protoplast (QUAL2E) (Walton and Webb, 1994; Melching and Yoon 1996; Chaudhury et al, 1998; Lung 1998; Bowen and Hieronymus, 2003) The QUAL2E mould is a comprehensive and versatile water quality protoplast widely used for WLAs, discharge permit determinations, and other conventional pollutant evaluations (Brown and Barnwell, 1987; Walton and Webb, 1994; Melching and Yoon 1996; Chaudhury et al., 1998; Lung 1998; Bowen and Hieronymus, 2003) It can simulate up to 15 water quality constituents in any combination desired by dint of the user. Simulated constituents include DO, biochemical oxygen demand (BOD) temperature, algae as chlorophyll a, organic nitrogen as nitrogen (N) ammonia as N nitrite as N nitrate as N organic phosphorus, dissolved phosphorus, coliforms, an arbitrary nonconservative constituent, and three conservative constituents. The QUAL2E archetype applies well to dendritic streams that are well mixed and assumes that the major transport mechanisms are significant alone in the main direction of roll on The model can operate in either steady state or dynamic manner (Brown and Barnwell, 1987).

The major differences between QUAL2E and STREAM are that QUAL2E simulates the out and out nitrogen cycle, the complete phosphorus round of years and growth cycle of algae. The extension cycle of algae is directly influenced by the agency of the concentrations of nitrogen and phosphorus. STREAM does not simulate phosphorus, and the single form of nitrogen simulated is ammonia nitrogen. Within STREAM, the user has the option of entering a value for P R and turf However, these values are constant within the protoplast and do not respond to changes in the ammonia nitrogen concentrations.

Questions arise regarding the simplicity of STREAM in comparison to more commonly used archetypes such as QUAL2E. Minimum analysis has been performed upon STREAM to assess the horizontal of confidence associated with original predictions, especially with respect to more commonly utilized water quality archetypes The objective of this research was to compare STREAM and QUAL2E using a statistical management that converts input parameter uncertainty into output prediction uncertainty. Evaluation of STREAM and other hydrologie and water quality originals has been historically based upon comparison of a model prediction with a single observation for a specific location (Sabbagh and Fox 1999) However, modeler rarely know input parameters with exact certainty (Haan et al., 1995; Parker et al., 1995) An alternative evaluation strategy being increasingly used in hydrologie and water quality modeling involves uncertainty and probability analysis. This process involves uncertainty analysis to identify the greatest in quantity sensitive input parameters, input probability distribution functions (pdfs) to quantify site specific input parameter variability, output probability distributions derived using Monte Carlo simulation (Cheney and Kincaid, 1994) and type assessment based on a comparison of the output distributions with field observations (Sabbagh and Fox 1999) The conduct is valid when input parameters are showed by singular values. Researchers have applied this statistical evaluation to a number of different patterns (Haan et al., 1995; Haan and Zhang, 1996; Prabhu, 1995; Zhang et al., 1995; Sabbagh and Fox 1999; and Haan and Skaggs, 2003) on the contrary not instream water quality prototypes