Document Type : Research Paper
university of tabriz
In the cases that two or more factors affect inceasing of dissolve solid, the source identification in groundwater could be usable in planning, managing and choosing the right strategies in exploitation and protection of water resources. In this study, Hydrogeochemical, geological and geophysical investigations were carried out to assess the geochemical water composition and also governing processes groundwater hydrochemistry in Malekan Plain Coastal Aquifer. The study area is located between latitudes 56˚36' to 37˚44' N and longitudes 45˚45' to 46˚40' E, with an area of 450 sq. km. The Malkan plain is bordered from the northwest by the Urmia Lake salty flat plains in the North western Iran. The main amis and objectives of the present study are evaluation of groundwater salinization and delineation of ion offspring in Malekan Plain Coastal Aquifer.
2- Matherials and Methods
In order to assess the groundwater quality, including the source of salinity, and the hydrogeochemical processes which affect the quality of water, 41 groundwater samples were collected and analyzed for various physical parameters such as pH, electrical conductivity (EC), and also hydrochemical parameters such as major, minor and trace elements, in July 2016, using standard procedures (APHA, 2008).
Statistical and geochemical modeling were performed to understand the processes controlling groundwater geochemistry. The chemical composition of groundwater was evaluated by several natural factors including aquifer lithology, interaction with geological formations. In addition, anthropogenic activities such as agriculture and industry have been shown to impact on the groundwater chemistry using hydrogeochemical techniques. In addition, the Piper diagram The piper diagram was used to evaluate the geochemical evaluation and groundwater types.
The results of field and laboratory measurements of physicochemical parameters, major minor and trace elements showed that the high electrical conductivity (EC) (691-14710 μS/cm) in groundwater is due to high concentrations of gradual increasing in major elements, Cl-, Ca2+, Na+ and SO42-, through groundwater flow.
On the basis of relative proportions of major-ions, the chemical composition of groundwater in the coastal aquifer is classified. The sequence of abundance of the major cations and anions is as follows: Ca2+ > Na+ + K+ > Mg2+ = HCO3 -> SO42- > Cl- and Na+ + K+ > Ca2+ > Mg2+ = Cl- > HCO3- > SO42-. Piper diagram reveals higher bicarbonate, calcium and magnesium along the recharge zones and tend to decrease along the flow path and vice versa for ions like sodium, potassium and chloride.
Surface and groundwater evaporation have had a significant effects on groundwater salinity and solute increasing at the end of the plain. To understand the origin of groundwater salinity in Malekan plain, the values of saturation index of calcite, dolomite, gypsum, anhydrite and halite in all 41 groundwater samples has been calculated using the Phreeqc Interactive 3.0.0. computer code. Studying the ion ratios revealed that sodium has been entered to the aquifer in consequence of salt water intrusion form the evaporate brines, contemporary playas and connate groundwater from northwest of aquifer adherent to SouthEast of Lake Urmia, and also weathering and leaching of salty-clayey layers. In other hand, calcium and magnesium ions originate from limestone and dolomite deposits. The hydrogeochemical ratios such as Cl− / sum Anion, HCO3− / sum Anion, Cl−/ (HCO3− + CO32-), (Na+ / Cl-), Na+/ (Na+ + Cl−) and Ca2+ / (HCO3− + SO42-) were estimated to determine the origin of ions and salinity in the groundwaters.
Evidences like certain ratios, hydrogeochemical graphs and saturation index which arose from geochemical modeling indicates that the dissolution of evaporation deposits and cation exchange are the most important processes affecting the chemical composition of groundwater. The results of applying statistical and Hydrogeochemical techniques confirmed that both the evaporation-crystallization reactions and water/rock interaction in the aquifer and exposed the effect of human activities on groundwater quality. The chloride ion, as an indicator of groundwater salinization, is highly correlated with EC, which play an key role in groundwater quality. The ratios of conservative elements (Cl- / Br-) for groundwater samples in the plain ranged from 93 to 2184 and showed the effect of evaporation deposits, agricultural returned groundwater and domestic sewage.
Li+ / Cl- ratio is one of the most important used ratio to separate different salinity origins. Samples related to seawater can be distinguished by the value of Br- / Cl-. The Br- / Cl- relationship helps to identify the possibility of seawater intrusion since it is relatively constant in the seawater considering the extremely long residence time of Sea masses.
Compound Diagram of ionic ratios (Li+ / Cl-) vs. (Br- / Cl-) were identified as an adequate criterion of separating different salinity sources. The groundwater samples located in evaporation playas and geothermal or groundwater part of diagram and have far composition than Samples related to seawater or lake can be distinguished by the value of Br- / Cl-.
All evidence like certain ratios, hydrogeochemical graphs, saturation index and Compound Diagram of ionic ratios (Li+ / Cl-) vs. (Br- / Cl-) indicate the groundwater samples which origin from different sources as different clusters. Based on results of the present study, likely the saltwater intrusion from the Urmia lake highly likely would be rejected and other factors like cation exchange, salt water intrusion form the evaporate brines, contemporary playas, and also weathering, leaching of salty-clayey layers from northwest of aquifer adherent to South East of Lake Urmia, and groundwater evaporation at the end of the plain could be the main causes regarding to used techniques.