عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The Sharifabad alluvial aquifer is located in the northeast of Qom City. As a result of surface water scarcity this unconfined aquifer supplies water requirement for agricultural purposes. Since the past decades, the Sharifabad aquifer has been subjected to contamination due to pollutant of the adjacent metropolitan Qom City and agricultural activities. The most important contaminants in the study area include effluent of the Qom wastewater treatment plants, agricultural and horticultural drainage water, leakage water from sanitary landfills and so on. Though, relative impact of these contaminants on groundwater quality is noticed but the further effects would be observed in future. At present, water quality suffers from geogenic contamination which results in increasing salinity.
Individual polluted groundwater sites generally are not large, but once polluted, groundwater may remain in an unusable or even hazardous condition for decades or even centuries. Low velocity of groundwater prevents a great deal of mixing and dilution; consequently, a contaminant plume may maintain with high concentration as it slowly moves from points of recharge to zones of discharge. One of the features of groundwater contamination that makes it so serious is its long-lasting. Wastes buried long time ago may cause groundwater contamination that takes decades to be discovered.
In order to assess the contamination of the Sharifabad aquifer, an extensive sampling with an appropriate distribution of station was carried out in May 2014 and water samples were collected from 50 deep wells in the area. The samples were analyzed to find out concentration of the following parameters including; TSS, pH, EC, TDS, DO, BOD5, COD, total alkalinity, CO3, HCO3, total hardness, Ca, Mg, SO4, Cl, Na, K, NH4, NO2, NO3, PO4, Br, I, Ba, and Sr. Apart to aforementioned parameters, the total coliform, fecal coliform, fecal streptococci, Fe, Hg, Pb, Co, Cd, Ni, As, Zn, B, and Sn were measured for 14 wells. It is to noted that color, temperature and turbidity were also determined.
The quality is sometimes relative and cannot be expressed numerically; however, qualitative indexes and standards can relatively solve the problem. In this study, Iranian National Standard, Potable water Standard- Physical and Chemical Characteristics, No. 1053, 5th edition and water quality manual of World Health Organization (WHO, 2011) were used to evaluate the contamination of the Sharifabad aquifer.
Due to dominantly saline nature of groundwater in the Sharifabad plain, the main ions concentration, total dissolved solids and the indices exceed the maximum allowable limit of the defined instructions. All the wells depicted pH values in favorable range and turbidity was observed only in one well. The concentration of BOD and COD were low and DO value of groundwater samples were in normal range. Sometimes, the nitrate concentration of groundwater was considerable but usually it ranged between 0.1 to 1 mg/l. The nitrate concentration in deep wells of the study area varied between 5 to 38 mg/l, while based on Iran potable water standard, maximum permissible amount is 50 mg/l. In all wells, ammonium, nitrite and nitrate concentration were less than the maximum suggested amount. Concentrations of the non-toxic metals of iron and zinc and toxic element of arsenic, mercury, barium and nickel in all wells were less than the maximum permissible limit. The concentration of the two toxic metals of cadmium and lead were below detection limit of the laboratory and therefore, it is needed to be measured by advanced equipments. In some samples concentration of boron was more than maximum level. Results for the maximum probable number (MPN) of total coliforms and fecal coliforms indicated that four and two wells have been contaminated, respectively. Contamination by fecal streptococci was only observed in one sample and based on FC/FS ratio the contamination source of the well is likely to be manure.
The Sharifabad aquifer is mostly suffers from salinity which is a geogenic contaminant, although, heavy exploitation and saline water aggression can be considered as anthropogenic pollutant. Though, since recent decades, the Sharifabad aquifer has been subjected to pollutants, but the anthropogenic contamination is commonly still lower than the maximum permissible limit, defined by standards, despite, nitrate concentration is relatively high in a few wells. The thick soil above water table depth (more or less 42 m) is probably a reason to absorb the pollutants.
In areas such as Sharifabad where transmisivity coefficient of the aquifer is not high and groundwater movement is low results in decrease of mixing and dilution processes of pollutants. Therefore, to prevent further aquifer contamination, measures such as reduction volume of contaminants injection into the aquifer has to be considered. The present effluent input into aquifer is about 24 MCM/ year and estimated to rise up to 44 MCM/ year by 2025. It is evident that when effluent input increase the aquifer has to tolerate this load so its condition would be critical. Thus, the only remain alternative to save the aquifer is to treat the effluent before usage.