Optimization of aquifers' hydrodynamic coefficients by numerical modeling (Case study: Damaneh-Daran Plain Aquifer)

Introduction
Water is a renewable resource that naturally follows a hydrologic cycle. Huge groundwater tables are essential resources that are utilized as underground water. Irregular population increase in the last three decades, limited surface waters, and too much utilization of groundwater tables damaged the Iranian aquifers irreparably, both quantitatively and qualitatively. Today, with the introduction of extensive industrial activities and chemical fertilizers in agriculture, the most important source of human life is at risk. Groundwater pollution is often caused by toxic wastewater from industries, agriculture, or sewage storage sources. The effects of different pollutants in the environment are different. One of these pollutants is nitrate. The contamination of underground water with nitrate ions is an important global problem, the most important source of which is arbitrary agricultural activities and the excessive use of nitrogenous chemical fertilizers. Numerical simulation of groundwater flow and contamination transport is a vital aquifer resource management tool that estimates the hydraulic and hydrologic parameters of the aquifer. In previous studies conducted by researchers, only flow modeling or only pollution transfer has been discussed, which we have studied both cases in the present study. Therefore, in this research, the flow and transfer of pollution has been investigated separately and completely.
 
Materials and Methods
In this research, the flow and transfer of pollution were investigated separately and completely. In this paper, the general aquifer software simulation algorithm is first discussed. The Damaneh aquifer is then introduced. Damaneh is located in Isfahan Province in the catchment basin of the Falat Markazi. The region’s specifications are given, and its hydrologic and hydrogeological features are stated. The process of creating the conceptual and numeral model is discussed, and the aquifer modeling process is examined in detail. The last section discusses the modeling of contaminant transport. According to the climatic conditions of the region, the Damaneh plain faces alternating wet and dry periods. For this reason, for the model to better match the nature of the aquifer, a wet and dry period has been applied to the model. According to the monthly rainfall statistics, from 2005 to 2008 was a dry season and from 2009 to 2013 there was a large relative drop in rainfall, which was applied to the model as a drought period. This study presents the results of mathematical modeling of the groundwaters’ flow and contaminant transport and their simulation in the Damaneh aquifer. Groundwater modeling system (GMS) software was used. The necessary information, such as geological, hydrological, hydrogeological information, and topographic maps are gathered. The aquifer flow model in stable and unstable states was run next. The model is optimally calibrated for hydraulic conductivity and specific yield up to a particular standard error. It was verified using an unsteady period to ensure the simulation results’ accuracy. To predict the water level fluctuations and the response of the two optimized coefficients, the Damaneh aquifer was simulated for a specific time interval using two distinct scenarios.
 
Results and Discussion
Statistical analysis of flow model errors in a stable state gived maximum observed and calculated water levels of 0.87 m and RMSE and correlation coefficients of 0.99 m and 99.9 % and 1.113 m and 99.7 % for calibration and verification periods, respectively. The correlation coefficient between the observed and calculated data for transport modeling was 89.1 %. The error values indicate that the designed flow and contamination transport mathematical models match the aquifer’s natural conditions well. They give a good simulation of the hydrological system. In the aquifer, the presented results of these discoveries have high uncertainty, because, in some wells of the adjacent logs, drastic differences between the type and profile of these logs can be seen. To calculate the path length and movement time of pollution particles on the surface of the aquifer, points were determined and the MODPATH model was run in Forward mode. The results showed that the transit time was between 2 and 7 years, and the shortest and longest paths traveled by the particles in this period of time were 312 m and 6200 m, respectively. As the modeling results showed, the concentration of nitrate in most areas of the Damaneh aquifer exceeded 80 mg/l during the past years 2007 to 2008 and according to drinking water consumption standards, it is necessary to reduce the nitrate concentration or clean the Damaneh aquifer. A sensitivity analysis shows that the flow model is more sensitive to hydraulic conductivity and specific yield. The transport model is most susceptible to the surface recharge, porosity, and longitudinal coefficient of variation parameters, respectively. It is not sensitive to recharge from the aquifer’s borders. The results of the transfer model showed that only one halo of pollution is forming in the Damaneh aquifer and it is spreading in the entire aquifer at an approximate speed of 15 m d^-1.
 
Conclusion
According to the error values, it can be concluded that the mathematical model designed for the flow and transfer of pollution has a good match with the natural conditions of the aquifer and has well simulated the behavior of the hydrological system. It is suggested to prepare accurate and new statistics of the exploitation wells in the region, although it is very expensive at the level of plains such as the Doman plain, but the special conditions of this plain require more attention. Providing smart meters is the most effective harvesting control programs in the region.