The present study was conducted to investigate the effect of increasing uncontrolled drilling of water wells and subsequent extraction of renewable capacity of Gorgan plain aquifer on the decrease in groundwater level and some water quality parameters in a specific period of 30 years. For this purpose, the trend of rainfall changes, drilling wells, groundwater level fluctuations, as well as some physicochemical parameters were investigated in the studied period. Data analysis of variance was used to investigate the statistical differences between quantitative and qualitative parameters. The annual changes in drilling of authorized and illegal wells in the province shows an increasing trend in the number of wells drilled in the Gorgan plain, followed by an increase in the volume of water extracted since the early 1980s. The results of statistical studies, histograms of annual changes in groundwater level of deep and semi-deep wells as well as hydrographs of deep and semi-deep aquifers studied show a reduction in groundwater level in both semi-deep and deep aquifers in the study period. Since the average annual rainfall in the study period is almost constant, the drilling of shallow and deep wells followed by uncontrolled abstraction of deep and semi-deep aquifers is the most likely major factor in the decline of groundwater levels in the study plain. Based on the statistical results, a significant difference was observed between most of the physicochemical parameters of semi-deep and deep wells in the old and new periods. However, these changes are greater in semi-deep wells than the deep ones. Also, the two parameters Cl and Na, which are the main factors of water salinity, show the greatest changes. Hydrogeochemical diagrams of plain aquifers show an increase in soluble solids as well as changes in the type and hydrochemical facies of groundwater in the new period. However, the trend of changes in the shallow aquifer is more intense.
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