DEVELOPMENT AND APPLICATION OF A NUMERICAL-MODEL OF KINETIC AND EQUILIBRIUM MICROBIOLOGICAL AND GEOCHEMICAL REACTIONS (BIOKEMOD)

This paper presents the conceptual and mathematical development of the numerical model titled BIOKEMOD, and verification simulations perform ed using the model. BIOKEMOD is a general computer model for simulatio n of geochemical and microbiological reactions in batch aqueous soluti ons. BIOKEMOD may be coupled with hydrologic transport codes for simul ation of chemically and biologically reactive transport. The chemical systems simulated may include any mixture of kinetic and equilibrium r eactions. The pH, pe, and ionic strength may be specified or simulated . Chemical processes included are aqueous complexation, adsorption, io n-exchange and precipitation/dissolution. Microbiological reactions ad dress growth of biomass and degradation of chemicals by microbial meta bolism of substrates, nutrients, and electron accepters. Inhibition or facilitation of growth due to the presence of specific chemicals and a lag period for microbial acclimation to new substrates may be simula ted if significant in the system of interest. Chemical reactions contr olled by equilibrium are solved using the law of mass action relating the thermodynamic equilibrium constant to the activities of the produc ts and reactants. Kinetic chemical reactions are solved using reaction rate equations based on collision theory. Microbiologically mediated reactions for substrate removal and biomass growth are assumed to foll ow Monod kinetics modified for the potentially limiting effects of sub strate, nutrient, and electron acceptor availability. BIOKEMOD solves the ordinary differential and algebraic equations of mixed geochemical and biogeochemical reactions using the Newton-Raphson method with ful l matrix pivoting. Simulations may be either steady state or transient . Input to the program includes the stoichiometry and parameters descr ibing the relevant chemical and microbiological reactions, initial con ditions, and sources/sinks for each chemical species. Output includes the chemical and biomass concentrations at desired times. BIOKEMOD has been coupled with a hydrologic transport code, HYDROGEOCHEM, to allow the simulation of coupled advective-dispersive transport and biogeoch emical transformation of pollutants in groundwater. Three verification exercises are compared with analytical solutions to demonstrate the c orrectness of the code. Two validation simulations of batch laboratory systems are compared with the laboratory data to demonstrate the code 's ability to replicate behavior observed in real systems, and two val idation exercises simulating reactive transport are presented to demon strate the code's performance in simulating mixed equilibrium and kine tic biogeochemical reactions coupled with hydrologic transport. (C) 19 98 Elsevier Science B.V. All rights reserved.