Mathematical modeling of the formation of reduced environments in a coastal aquifer under the bacterial mediation

A numerical study of the bio-geochemical aspects of the coastal aquifers is an interesting subject to address. So far, numerical simulations which discuss about the bacteria mediated reduction processes are not common in the scientific literature. Snyder et al., (2004) made detailed surveys on the bio-geochemical oxidation-reduction approach for the mixing zone formed in a shallow aquifer at the estuary of Sapelo Island, Georgia, USA. Based on the bio-geochemical approaches, they have shown that the distribution of manganese, iron and sulphate ions are affected by the availability of natural organic carbon and bacteria mediated reducing processes, which take place at the both sides of fresh and seawater transition zone. Hiroshiro et al., (2006) analyzed the redox (oxidation-reduction) potential distribution in the monitoring wells and found out that the redox potential drastically decreased below the mixing zone from fresh water to reduced seawater. In this paper, a numerical investigation (redox model) of the formation of reduced environments under the aerobic and anaerobic bacterial mediation at the availability of organic carbon is discussed. Developed redox model is applied to the selected cross section of Kujyukurihama beach of Chiba prefecture, Japan. Field investigations conducted in Kujyukurihama beach have found out that the measured oxidation-reduction potentials (ORP) at the seawater region of the aquifer show -380 mV, -99 mV and -66 mV at 8.0 m, 7.0 m and 5.0 m respectively. Moreover, the measurements conducted at different depths of this aquifer show high concentrations of reduced species in the anaerobic seawater region. The availability of oxygen and other electron acceptors such as NO3-, MnO2, Fe(OH)(3) and SO42- with the organic carbon as the electron donor encourage different bacteria to activate and create reduced environments in the subsoil. The present numerical simulation considers four bacterial groups named as X-1, X-2, X-3 and X-4 to simulate the reduction processes. Bacteria X-1 consumes oxygen in the aerobic condition and reduces NO3- in the anaerobic condition while X-2, X-3 and X-4 bacterial groups reduce MnO2, Fe(OH)(3) and SO42- respectively at the anaerobic condition utilizing the available organic carbon. At the initial stage of modeling, a density dependent two dimensional model is used to obtain the steady state seawater and freshwater distribution of the selected cross section of the Kujyukurihama beach. The initial chemical species concentrations are assigned according to the distribution of seawater and freshwater in the aquifer. A 4.0 m thick organic carbon layer is assumed at the bottom of the aquifer in the numerical simulation. Then bacteria mediated redox model is applied to simulate the formation of reduced environment in the aquifer. In the redox model, three different phases are considered named as mobile phase, matrix phase and bio phase. All the bacterial activities are taken place in the bio phase. The species allocated in three phase can transfer between phases according to the concentration gradients created by bacterial metabolism take place in the bio phase. The numerical results are presented for the different chemical species and four bacterial groups which are considered in model. Numerical results show that the availability of organic carbon is an important factor which determines the formation of different reduced environments in the aquifer. Moreover, the results of this study show that, generally it is possible to simulate the formation of reduced environments at the subsurface of coastal aquifer using the bacteria mediated redox model. So far, this kind of numerical studies are very limited in the field of numerical modeling of coastal aquifer related phenomena. Therefore, in future more detailed numerical modeling studies are essential to explain the biogeochemical processes of coastal aquifers.