Assessment of The Long-Term Quantitative Effects of Management Conditions and Current Abstraction on Varamin Plain Aquifer

Abstract Today, use of models to illustrate the facts and better understanding them, as well as make accurate decisions about phenomena is very common. One of these models is groundwater flow simulation model used by experts in many countries, also, the accuracy and validity of their results has been proved. The object of this study is to evaluate the effect of current management plans of aquifer and Jajrood River basin on changes of the water table of unconfined aquifer and piezometric level of confined aquifer by numerical simulation. Using numerical models, it can be predicted that with the governing process or different management scenarios, how the aquifer reacts to abstraction and recharge. In this study, hydraulic behavior of Varamin Plain aquifer was simulated using the Modflow code in GMS software in stable and unstable states. After calibrating the model, hydrodynamic coefficients were corrected, then using the resulted model, quantitative behavior of the aquifer was predicted for two management scenarios in the future years. The first scenario expresses the existing quantitative conditions of the Varamin Aquifer. That is, water do not flow to the plain from southeast wastewater treatment plant of Tehran, but water flows to the Varamin aquifer from Jajrood River. The second scenario indicates situation of the aquifer if the planning are carried out by the relevant organizations. Under this scenario, assigned values from discharging of the southeast wastewater treatment plant of Tehran enter the study area, though due to the implementation of Mamlou storage dam, a significant portion of the surface water resources of Jajrood River will not enter the plain. According to the maps resulted from the first scenario, decline in groundwater level continues as in previous years, but in the second scenario, considering the utilization of wastewater treatment plant discharge, despite the cutting off the water entering from the Jajrood River to the plain, the groundwater decline rate will decrease, although this decline continues. Comparison of the model implementation under first and second scenarios shows that with the implementation of planned projects, part of the aquifer’s quantitative problems, particularly in the northern areas of the plain, will be decreased, so that in some northern areas the expected decline in groundwater (results of the first scenario) has decreased about 30 to 40 meters by 2041. In the second scenario, a balance of the groundwater is still decreasing, so that storage reduction rate of the aquifer is about 100000000 m3 annually. Results of the simulation show that under both scenarios (current trend and after implementation of ongoing plans), balance of the groundwater is decreased and decline in water table will be continued, however in comparison to the first scenario, decline rate of water table under the second scenario will be decreased. It can be stated that in order to stop the decline in water level in Varamin aquifer, in addition to water entering plan from Tehran wastewater treatment plant to Varamin plain, recharge of the aquifer from Jajrood River (as the main source of aquifer natural recharge) should be continued.