عنوان مقاله [English]
Identifying the permeable zones in Dare-h-Zar copper mining area in Sirjan using time series analysis of the precipitation and groundwater level
The Dareh-zar copper mining area is located approximately 50 km northwest of Sirjan and 10 km south of Sarcheshmeh copper mine in Kerman province. The general direction of the groundwater flow is approximately from north to the south. Without groundwater pumping in the mining area, temporal variations of the groundwater levels were dependent only on the rainfall and the river’s discharge fluctuations.
Matherials and Methods
In this research, the time series analysis of the precipitation and the water table has been performed to determine their relationship. In order to prepare the water table and precipitation data, the components of the trends were identified and then deleted for the calculation of time series in the frequency domain. Also, for preliminary study of the location of permeable zones in the hard rock layers, the three-dimensional graphical model of permeable zone has been drawn based on the geo-statistical method using ROCKWORK software.
Discussion of Results
In the time series of the groundwater levels, a dominant decreasing linear trends were recognized during the period of 2014-2017. After de-trending the time series of rainfall and water table, using autocorrelation functions, a seasonal trend with a period of 12 months was recognized in the data, which was due to the annual changes in climatic conditions. According to the time series analysis, the time lags between the precipitation and the groundwater levels vary between 1 and 2 months. The time lags have been increased to one year in the southeastern of the mining pit. It is probbaly resulted from a zone with very low permeability. The lag time of impact response functions (IRF) for water table in the observation wells (in related to the spatial variations of hydraulic head) were plotted versus the direct distance between the wells. In general, the lag time is expected to be increased with increasing spatial spacing between wells, while in the study area there was no strong relationship between the lag time and the direct distance. In fact, the IRF of the water table to variations of the hydraulic head in the hard rock aquifer of study area (unlike to alluvial aquifers) was not dependent on the distance between the observation wells. So hydraulic connectivity was probably through high-permeability zones in the study area. The north and south area of the pit show the lowest lag time with the maximum number of lag times less than 1 month. Therefore, high-permeability zones were probably located in the north and south of the pit. It was confirmed by three-dimensional model (fence diagram) of lithology showing the locations of the seepage faces and permeable zones in the area.
The northern and southern parts of the pit are suitable places for drilling wells for dewatering plan of the mining area. Finally, three-dimensional model and fence diagram of the lithology showed that a permeable zone has been extended in elevation 2500 to 2600 m above sea level. The locations exactly correspond to the seepage faces level in the mine pit.
Keywords: Groundwater, Time series analysis, lag time, water table