Environmental impacts of Gachsaran formation in Maydavood plain, East of Khuzestan province

Introduction:
At present, the increase in the Earth's population has led to an increase in consumption, and an abnormal harvest of water and soil resources.MaydavoodPlain is a very fertile plain with rich water resources in the east of Khuzestan Province.In recent years, water contamination and sinkholehazards have happened in this plain.To date, very few environmental studies have been carried out on this plain. 
Material and methods:
In this study, field and laboratory studies have been used to study the environmental hazards of the Maydavood Plain. During field studies, many sinkholes around the Maydavood Plain were identified and their dimensions were measured. Samples were collectedfrom the rocks and water resources of the region. The solubility test was carried outon gypsum samplesof Gachsaran Formationusing the circulation method. By using the Geographic Information System (GIS), isopotential map of sulfate and calcium ions was prepared for the plain. The ICP-OES analysis was used to determine the amount of heavy metals in water.GISwas also used toprepare theisopotential map ofheavy metals in the Maydavood Plain.
 Results and discussion:
According to the office studies, the MaydavoodPlain is surrounded by Asmariand Gachsaran Formation and the bedrock of this plain is Gachsaran Formation that consists of gypsum and marlstones.All of thesinkholes are formed near the Gachsaran Formation. Results of the solubility test showed that gypsums had high solubility. Base on the isopotential map of sulfate and calcium ions, the amount of these ions were more concentrated around Sarelah and Dogacheh villages than other parts of the plain due to the dissolution of Gachsaran Formation gypsum. Hydrological studies showed that during the last 15 years, the groundwater level of the Maydavood Plain has decreasedabout 5 meters. In recent years, a few sudden sinkholes were formed, especially around Sarelah and Dogacheh villages due to the high solubility of Gachsaran Formation gypsum and a decrease in groundwater level. The type of these sinkholes a is cover collapse. Hydrochemical analysis of groundwater showed that 81, 31, 44, and 100 % of deep wellshad concentrations greater than permissible valuesof arsenic, iron, manganese,and nickel, respectively.Also, concentrations of vanadium in all of the deep wells were lower than the permissible value. Base on heavy metals isopotential map, the amount of heavy metals around of South MaydavoodPlain (Sufla) and MaydavoodVasati villages were higher than other parts of the plain. The source of heavy metals in the plain was the petroleum material pollution from oil spring existing. 
Conclusion:
By controlling the excessive pumping of groundwater and using artificial nourishment, it is possible to prevent the decrease of groundwater quality and the formation of new sinkholes in the plain. Otherwise, the new sinkhole may be formed and cause damage. High concentrations of heavy metals in groundwater can be hazardous for the health of people living in this region. To avoid this hazard, it is suggestedto providethe drinking water for the MaydavoodPlain’s populationfrom another area with a permissible value ofheavy metals.