Assessment of vulnerability of groundwater resources by industrial pollutants of Sari oil reservoir

Introduction: The study area is Sari-Neka plain where majority of population are involved with agricultural activities. Sari petroleum products distribution company is situated in the south of the plain with many storage tanks. Storage tanks are rather old and leakage from them is likely to occur. In this research the possibility of leakage and transport of dissolved hydrocarbons in Sari aquifer is investigated.
Material and methods: Based on geological, geographical and hydrological data the conceptual model of the plain is first constructed then the numerical groundwater flow model of the aquifer is developedusing MODFLOW. The isopotential map of August 2004 is used for the steady state calculation and distribution map of hydraulic conductivity is constructed. At the unsteady state, model calibration is conducted for the next 12 month following August 2004. The velocity field result from the flow model, is used as input to transport model, MT3DMS. Then the model was appliedfor the TPH concentration was measured in a 14-year period. Considering advection process, hydrodynamic dispersion and the process of adsorption, MT3DMS model is calibrated by simulating the plume of TPH and the absorption rate is determined. Based on the value of absorption rate, the fate of pollutant and the natural attenuation potential is evaluated. To predict the aquifer potential for natural attenuation in the next years, the model was run with the obtained absorption rate. Finally, the attenuation potential is presented in form of simple exponential equation. This equation could be a simple tool in the hand of practitioners for predicting the natural changes of concentration of TPH with time.
Results and discussion: According to the attenuation equation, the required time for clearing pollution with initial concentration of 10 mg / L in the Sari aquifer was estimated 62 years and if there is no injection of pollutant into the aquifer in the next years, in 2057, the pollution plume will be naturally cleared by the aquifer. Based on the calibrated parameters of the pollution model, the retardation factor in Sari aquifer was calculated 1.57. Also, with model designed can simulate each scenario of the pollutant injection and extraction. In order to predict TPH in the next years with the continuous injection of petroleum pollutants into the aquifer, the TPH concentration measured in March 2009 was considered as the initial concentration of the pollution model. Then continuous injection of petroleum pollutants into the aquifer at constant concentration of 10 mg / L was applied to the model at 10, 20, 30, 40 and 60 years intervals. Finally, the pollution model was implemented and the distribution of pollution plume in the aquifer was predicted. The pollution model was also implemented by applying three scenarios of (1). increasing the injection concentration to 20,000 mg/m3, (2). 30% reducing the pumping rate of wells around the site and (3). 30% increasing the Tajanriver water level. These conditions caused changes in the concentration of pollutants and the spread of the pollution plume.
Conclusion: If there is a leakage and continuous injection of pollutants into the Sari aquifer in the next years, the oil pollution plume develops but can prevent of its expansion by pump and treat.