JWSS - Journal of Water and Soil Science

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Gypsiferous soils, as the most characteristic soil in arid and semi-arid regions, are widespread in Isfahan Province. The north-west region of Isfahan is a closed basin and is surrounded by geologic formations. This area is a part of Zayandehrood watershed. The origin of gypsum in this area was investigated in this study. Considering the geologic distribution of gypsum, twenty-two locations, suspected to have been affected by gypsum, were selected for sampling. Natural and artificial excavations were used to discover their relationships. To distinguish the differences between fresh and weathered shales, some fresh samples were taken from the depth of Shemshak formations (a lead mine).

Field observations showed that in Shemshak and Nayband formations, gypsic crystals were present within the layers of weathered shales, and pyrite was observed only in the mine shales. In weathered red conglomerates (Lower Cretaceous), considerable amounts of gypsum had accumulated. In the hydrothermal veins of Shemshak and Nayband formations, gypsic crystals were found in matrix or between rock layers. In some parts, the Qom formation (Evaporate sediments) contained plenty of gypsum.

From field observations and analytical data, such as mineralogy of untreated and floated rock powders, determination of residual sulfides in shales, and the examination of thin and polished sections of rocks, we conclude that the origin and main resources of gypsum in our study area are as follows:

1. Gypsum being released from Cretaceous limes particularly from red conglomerates

2. Oxidation of pyrite and any sulfidic minerals existing in shales

3. Gypsum being released from evaporates (Qom formation and Oligomiocenic Marls)

4. Gypsum formed through hydrothermal reactions during late Tertiary.

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In this study, the simulation of the BORKHAR plain aquifer located in north-east of Isfahan was done for the estimation of the hydrodynamic coefficients and for the preparation of the prediction and management model with the purpose of the study of the water table surface situation in the next years. The study of the geological situation of the plain and the report of the drilling of its exiting wells indicated that the BORKHAR plain has two kinds of aquifer: confined and unconfined. According to the field data related to the unconfined aquifer, a part of this aquifer was selected for the mathematical simulation. The calibration model for the estimation of the K and Sy. coefficients was done by dividing the plain into four geohydrologic units and by using the PEST, a module of the MODFLOW model. The situation of the water table level during 1380-1390 was studied according to the different management options by the calibration model. If the trend of the exploitation doesn’t change during the next ten years, we will confront with a maximum drop of 48 meters. As a practical way for preventing from this drop, it was suggested that the exploitation management reduce the 30 percent of the demand. One of the most important result of this will be the 26 percent reduction of the drop. By developing an optimization model and by imposing the necessary constraints on the critical regions, and transferring water from other parts, it seems that the trend of the drop will be controllable at a admissible level (less than 10 meters until 1390). Using the optimization model will make a change in the increasing trend of the drop and an improvement in the situation of the aquifer.