ارزیابی آثار آتشفشان بر تغییرات کیفی منابع آب زیرزمینی با استفاده از مدلسازی کمی و کیفی آبخوان خاش

The quality of groundwater resources has particular importance due to the necessity of utilization and type of consumption. The aquifers adjacent to the volcanoes are affected by these sources exhibiting different behaviors. These aquifers are highly vulnerable and their water quality is affected by sources of volcanic pollutants. In this study, the qualitative status of the Khash aquifer in the south of Taftan volcano has been investigated. Qualitative problems are caused by natural factors and human activities. There are some situations that human activities intensify the natural factors in qualitative problems, such as: the chemical exchange of arsenic and chromium in the aquifer, the invasion of saline water fronts by the sea and saline areas, the qualitative effects of natural events such as volcanoes, changes in the Earth's motion due to earthquakes, etc.
Khash Aquifer is located at the down part of Taftan Mountain in Sistan and Baluchestan Province, Iran. The purpose of this study was to identify the contaminant sources of this aquifer. Therefore, in addition to sampling and qualitative analysis, MT3D qualitative simulation model was used. Simulation of qualitative model and regional analysis of groundwater flow in the area revealed that the southern part of the aquifer isis charging by underground nutrient flows from the northern part. Also direct underground feeding from the higher altitudes, with geological formations affected by volcanoes, increases TDS and sulfate concentrations in the southern part of the aquifer.` Groundwater resources are the main source of water supply for the consumptions in this aquifer. The two processes of convection and diffusion in the aquifer were introduced as the main components of qualitative simulation in the model. For the purpose of qualitative simulation of aquifer, the time step was performed as 6 months, from October 2011 to September 2016, including 10 time simulation steps. The qualitative model of aquifer is based on a quantitative model of aquifer and the initial concentration of sulfate and TDS in the aquifer was considered as the basic conditions for qualitative simulation. In order to define the initial conditions using the measured results of sulfate concentration and TDS in October 2011, Craig's internalization method was used.
The results of simulations showed that in this section more than 12% of TDS and 30% of sulfate increased. This increase, in addition to the lack of operational management, also indicates the existence of underground currents that have exacerbated the situation of human interference. Investigations showed that the amount of gas in groundwater resources in the northern part of the aquifer was high and this increase also acidified groundwater resources in the region. Also, an examination of the exploitation wells in the northern part showed that the water in these sources has a large amount of gas, and this amount of gas in some places is so high that it has even changed the taste of the water. Evaluation of various studies showed that a geological activity plays an important role in the formation of gas in groundwater resources. Examination of other quality parameters in the groundwater of the region indicated that the more groundwater routes, the higher amount of solutes dissolved in it. Therefore, increasing the TDS concentration is a clear feature of these changes. The results of numerical simulation using MT3D model also confirmed this trend of changes in aquifer. Also, the results of qualitative analysis of pH in the groundwater resources of the region showed that, most of the wells in the aquifer were acidic; only the well of Dasht-e Robat village had a high alkalinity. Another source of acidification could be the surface oxidation of H2S gas, which exacerbates rock dissolution. These types of water have low chloride but high sulfate. The average concentration of sulfate in the aquifer highly fluctuated.  Nikabad village (at the beginning of the aquifer and down of the Mehran power supply dam) has the most concentration of sulfate due to its proximity to the Taftan Mountain and the dissolution of sulfur in surface waters entering the plain. This fact is the opposite of the assumption  that the southern wells have the more the amount of sulfate. So that the village of Kalki at the end of the aquifer has the highest amount of sulfate 1660 mg/lit. Based on the results obtained from the aquifer quality modeling, the simulation of water quality status for the TDS and sulfate parameters for the forward 3-year period was analyzed It indicated that there would be 12% and 30% increase respectively in TDS and sulfate concentration, in southern part of aquifer. Hence, appropriate management should be applied to address the water acidity.