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16 2) Natural Recharge of Oxygen The natural recharge of 02 refers to the dissolved oxygen that would be transported into the aquifer due to the flow of ground water The user can assign an 02 ' concentration to the constant head nodes This value is input as the variable FCTR4(Data Set 7, Appendix A) 3)Vertical Exchange of Oxygen with the Unsaturated Zone Vertical exchange of 02 or reaeration is simulated as a first order decay in HC concentration A constant decay coefficient(DEC2)is required as input to the model(Card 4,Appendix A) The first order decay is applied at the nodes and not to the particles (This technique prevents any numerical instabilities that could occur if the ' coefficient of decay was small relative to the particle movement time step) 4) Injection of Oxygenated Water The fourth source of 02 into the aquifer Is through injection of 02 rich water Injection wells specified in Data Set 2(Appendix A)can be simulated as contaminant ' sources, 02 sources, or both The concentration of 02 Is Input as the variable CNRECO (Data Set 2,Appendix A) i ' The first three sources of 02 are used to simulate natural biodegradation of a contaminant plume Test Problem#1 (Section 41)illustrates the simulation techniques for a contaminant plume undergoing natural biodegradation using the first two sources of oxygen (initial oxygen and natural recharge) Test Problem#2 (Section 4 2)Illustrates the use of reaeration as an additional source of oxygen Into an aquifer 2 2 2 Simulation of In-Situ Blorestoration In order to simulate In-situ biorestoration schemes,the user must specify the following 1)The concentration distribution of HC in the aquifer The HC plume to be cleaned up is Input in the array CONC(Data Set 9,Appendix A) This plume could be obtained from field measurements or from a previous BIOPLUME it simulation effort(with or without natural biodegradation depending on the particular field conditions) 2)The concentration distribution of OZIn the aquifer The 02 distribution in the aquifer is Input in the ' array CONC1 (Data Set 10,Appendix A) This plume can be obtained from a previous BIOPLUME 11 simulation if the HC plume was being naturally biodegraded For other cases,the 02 distribution would be obtained from field measurements ' 3) Injection well data The location, rate, and concentration of infected water are input in Data Set 2 (Appendix A) The concentration of 02 in the injected water is input as the variable CNRECO (Data Set 2, Appendix A) ' Test Problem#3 illustrates modelingof an in-situ biorestoration scheme A doublet injection/production scheme is used to predict the clean-up time required to restore the contaminated ' aquifer modeled in Test Problem#1 2.3 Sensitivity Analysis In order to define which parameters have the most effect on biodegradation in BIOPLUME II,the following detailed sensitivity analysis was performed The parameters that were Investigated Included hydraulic conductivity, dispersivity (longitudinal and transverse), porosity, reaeration,and retardation A hypothetical contaminant plume was generated using a single continuous hydrocarbon source The above mentioned parameters were then vaned individually to determine their effect on biodegradation The results from the sensitivity analyses indicate that biodegradation in the model is most sensitive to hydraulic conductivity,the coefficient of reaeration, and the coefficient of anaerobic decay The following input data was utilized in the base run ' 2-3