Development of a water quality model using finite segment method

Abstract

Mathematical models have gained wide acceptance as invaluable tools to support the effective management of impacted rivers and lakes. Simulation of water and pollutants kinetics and movement are the key processes in the water quality modelling. The process has proved to be a reliable and economic method of assessing water quality of surface water. The basic concept underlying the development of a water quality model is the formulation of appropriate mass transport equations. In this study, the finite segment method was used to develop the one dimensional water quality model. The advective dispersive transport phenomenon of the mass transport equation was addressed by the well-established Water Quality Analysis Simulation Program (WASP), developed by the USEPA. The kinetic phenomenon involving phytoplankton, nitrogen, phosphorus and dissolved oxygen in the water column was developed in a kinetic module using FORTRAN 77 computer language. The basis of the kinetic transformations was adopted from literature. The model was applied to the lower reach of the river Sitalakhya to test the model performance. The model was calibrated and verified for the low flow period. Field data were collected to substantiate the calibration and verification process. However, the lack of recorded data during that period prompted the use of a hydrodynamic model to simulate the ".. low flow hydrodynamics. The model reproduced the system behavior reasonably, thereby substantiating the validity of the model. Sensitivity of the model was analyzed to determine the effects of different key kinetic parameters on the spatial distribution of the water quality constituents. Model results indicate that the river contains an abundance of nutrients. Lower light penetration resulting from high level of turbidity limits the algal growth under low flow conditions. Most concentration profiles do not show much spatial variations in the longitudinal direction. River flows dominate the kinetics in the water column and uniform mixing caused the water quality parameters uniformly distributed along the river. The dissolved oxygen concentration remains above the critical level, indicating that the water is still suitable for the survival of aquatic life including fish. Some water quality parameters. such as ammonia nitrogen, suspended solids and carbonaceous biochemical oxygen demand (BODs) are at a critical state. Although their steady state concentrations remained above the recreational and fishing standards, set by the Department of Environment, immediate pollution control and waste management measures are needed to prevent further deterioration. . The model indicates that the effluent loadings from the major point sources have minor effect on the overall spatial distribution of pollutants. The enormity in the width of the river usually represents a well mixed condition at the mid stream of the river for one-.sided discharge, prompting the need for a well defined three dimensional water quality model for the system. The model can be used for waste load allocation and for addressing policy issues regarding existing and future industrial and urban development. The model is a generalized program addressing nutrient transport and kinetics interaction of phytoplankton, nitrogen, phosphorus and dissolved oxygen in river system. It can be applied to any river system in Bangladesh with appropriate calibration and verification.