Document Type : Research Paper
Authors
1
member of Young researchers and elite club, Science and Research branch, Islamic Azad University, Tehran, Iran
2
Head of Earth Sciences college in Kharazmi University
3
PhD candidate of hydrogeology, Department of Geology, Earth Sciences College, Kharazmi University
Abstract
Introduction
The special regional and geographical characteristics of Iran, the flowof streams with Mediterranean regime, topographic status, and diversity of relieves have provided an extensive areafor the penetration of water into the ground. All of these factors together with the shortage of ground water reservoirs have enhanced the significance of ground water in Iran. Since the volume of ground water is so limited, the management of maintenance and protection of this resource is highly significant. One of the factors which must be noted in the management and maintenance of ground water is the maintenance of the quality of this valuable resource. To accomplish this objective, first, the status of ground water must be examined so as to be able to take the proper managerial strategies based on examination of the parameters affecting the quality of ground water. The chemical composition of ground water is determined by means of factors such as the composition of rain water, the geological and mineralogical structure of catchment basins, and geological processes throughout water path. Accordingly, the reasons underlying the changes in the quality of water can be found out by studying the chemical composition of water reservoirs. To examine the chemical composition of thereservoirs, various methods have been used in different studies. Among these methods, the followings can be pointed: drawing Piper diagram to determine the type and profile of ground water, using ionic ratios to set the origin of ions, studying the environmental factors controlling the chemical composition of the ground water, and etc. Regarding the studies on the quality of ground water with an emphasis on studying the chemical composition of water reservoirs, numerous studies have been conductedin Iran and in the world. Each of the studies has examined various qualitative parameters with respect to the kind of water demand.
This study aims to examine the changes of ions dissolved in the ground water of Golpayegan Plain using ionic ratios. It also assesses environmental factors controlling the chemical composition of the ground water in the area under study.
Materials and methods
To study the quality of groundwater in the plain under study, the qualitative statistics of 32 wells related to 2010 were applied. These statistics included K, Na, Mg, Ca, EC, So4, C1, TDS, PH, TH, and HCo3 parameters.
The percentage of ionic balance error was used toensurethe correctness of the results fromthe analysis of the samples. Accordingly, estimating the percentage of ionic balance error, it was determined that the difference between cations and inions based on mEq/l in all samples was lower than the standard amount (%5). Hence, the values of the samples were reliable.
AqQA1.1 was applied to analyze and compare these parameters and draw comparative diagrams and qualitatively rank water. Due to the dispersion of measurement stations, Kriging interpolation was used todraw ionic equivalent maps suitable for heterogeneous distributions. This process was created by means of ArcGis10.1. To study the correlation between the qualitative parameters of underground water, Kendallcorrelation coefficient was applied in SPSS19. Ionic ratios and the diagrams of pairwisecomparison between ions were used for examination ofthe origin of soluble substances in underground water and the existing reactions in the plain.
Results and discussion
Hydrogeochemical studies of the plain under study were carried out using two approaches. In the first, the origin of ions in ground water was examined based on the environmental factors controlling the chemical compositions of underground waters. In the second, the origin of the ions was examined based on ionic ratios. With respect to the significance and main role of determining the type and profile of underground water and also correlation between ions in the trend of hydrogeochemical studies and finding the origin of ions existing in the underground waters, the two following stages were executed in the first step:
Type and Profile of Underground Waters: Based on Piper diagram and the distribution of anions and cations, the dominant type and profile of the ground water of the area were clorursodic. However, in someareas, other profiles including sodic bicarbonate (well No. 5), calcic bicarbonate (wells No. 1&3), and calcic sulfate (well No. 2) were also observed.
Correlation of Chemical Ions: To study correlation and determine the relationships between chemical variables, correlation coefficient was computed and correlation matrix was drawn. Based on the computation of correlation coefficients, maximum correlation value is between EC and T.D.S variables with a positive relation(direct correlation) and minimum correlation between HCO3- variables and Mg+2 with a positive relation.
Origin of Ions based on Environmental Factors Controlling the Chemical Composition of Underground Waters: Various factors like the depth of underground water, lithology, the size of particles and sediments on the path through which ground water pass and the distance travelled by this ground water have changed the amount of ions. In some areas, they have resulted in the pollution of the ground water of the area under study. In the forthcoming, the effect of each factor on the ionic changes of the ground water is explained in detail:
Distance Traveled by Ground water and the Size of Particles and Sediments on Flow Path: The effect of the distance travelled by underground water on the quality of ground water can be observed well in the northern areas of the plain (the area in Gharghab Valley where ground water exit). This is with respect to the increase inall the ions existing in water and the decrease inthe quality of water. Regarding particle size factor, again, it is seen in the equivalent maps of cations and anions that the amount of all ions toward the southern area of the plain with coarser alluvial sediments is increasing toward the northern area of the plain (muddy flat area). In this area, the percentage of sand particles is low based on the surveyed geotechnical logs and field sedimentology visits. They are mostly consisted of sand and clay. The thickness of the mud layer on the ground also reaches 6m in this area. Generally, the finer the particles are, the lower the penetration will be. Principally, low penetration increases the time of the closeness of underground water to surrounding substances. As a result, underground water will be saltier due to a further chance for dissolution.
Depth of Underground Water and its Evaporation level: The shallower the underground water is located, the further it will be affected by external factors like evaporation and/or agricultural and industrial activities. The effect of this factor is highly apparent in the northern areas of the plain and in muddy flat areas. In these areas, the depth of underground water is lower as compared withthe other areas of the plain. The amount of evaporation is so high. As a result, the amount of all ions in this area is so higher than other areas.
Lithology: One of the very important factors affecting the quality of the ground water isthe rocks and sediments located inthe path of underground water flow. In the area under study, it is approved based on Gibbs model. According to the model, based on the location of the samples of the underground water on the diagram, the main process controlling water chemistry in over %90 of the samples (except wells No. 30, 31, and 32 which are the evaporation of the main process) is the mutual reaction of water and rock (the weathering of geological formations). Hence, lithology factor has maximum effect on the quality of ground water in the area under study.
Based on the abovementioned issues and with respect to lithology factor, the origin of ions can be described as follow:
The concentration of chlorine in the plain is changingfrom 21.3ml/l (minimum) to 1603.18ml/l (maximum). The high anomaly of Cl (especially, in the northern areas of the plain) can be attributed to the considerable extension of metamorphosis structures; in specific, schist, phylite, and the sediments containing chlorine.
Regarding the significant role of TH and its conformity with limy outcrops in a small part from south to east of the plain, it is again possible to ascribe the origin of Ca and Mg increase in the mentioned area to the lithology factor.
Sulfate enters into ground water mainly due to the dissolution of evaporativeminerals including gyps and anhydrite. In dry areas, sulfate leaping from the top layers of soil is considerable. It leads sulfate to be the main anion of ground water in those areas. The conformity of the areas containing a high anomaly of So4 can be attributed to this issue.
Regarding the increase of Na and K ions in the northern areas of the plain (before muddy flat area), since the type of geological structures in a part of this area is igneous and pyroclastic, weathering of the igneous rocks containing sodium and potassium can be considered as the trigger forincrease inthese ions in this area.
Origin of Ions based on Ionic Ratios
Na/Cl, Ca/So4, and Na/ So4 Ionic Ratios: Based on a correlation analysis between the samples, sodium representeda very good correlation with chlorine. It can be concluded that these two elements are resulted from the same source. In this study, Na/Cl ratio in ground water was 1. Accordingly, in most areas of the plain, the origin of sodium wasevaporation process, reversed water from agriculture and human activities.
The presence of a high correlation and linear relationship between two Na and Cl ions show the dissolution process of halite. The presence of a high correlation and linear relationship between two Ca and So4 ions show the dissolution process of gyps in the area under study. Regarding the considerably stronger correlation between Na and Cl ions (as compared to Ca and So4) and also the more linear relationship between these two ions (as compared to Ca and So4), the process of halite dissolution has affected the quality ofground water in the area more than gyps dissolution process.
Cl/HCO3 Ionic Ratio: If Cl/HCO3>2.8, it will show serious water pollution due to the penetration of salty water and also intensive evaporation. Again, the reduction of the ratio of chlorine to bicarbonate below unity indicates the feeding of tables from limy sources. Its reversal shows anincrease insalinity and the penetration of salty water front from sources like salty lakes, salt dome, chemical composts, salty geological formations, and etc. Regarding the lack of salty dome in the area under study, the increase inthe above ratio can be attributed to chemical composts and/or salty geological formations (e.g. muddy flat) and also metamorphic structures.
Mg/Mg+Ca Ionic Ratio: If Mg/Mg+Ca ratio increases, it can show the exit of Ca due to calcite sedimentation. Again, if the areas with maximum index of calcite saturation do not conform to the areas where Mg/Mg+Ca ratio also increases, it will be inferred that the extra Mg of the underground water has entered into this water from another source like weathered schist. Hence, if the areas with maximum calcite saturation index do not conform to the areas where Mg/Mg+Ca ratio also increases, it will be inferred that the extra Mg of the underground water has entered into this water from another source like weathered schist. On the other hand, for the areas where Mg/Mg+Ca>0.5, the weathering of ferromagnesium minerals will be considered as the origin of the high concentrations of magnesium in underground water.
Ca/Ca+So4 Ionic Ratio: If Ca/Ca+So4ratio is high yet, Ca saturation index is low, it indicates non-carbonate resources for dissolved calcium. The conformity of calcite supersaturated areas to the increase of Ca/Ca+So4ratio indicates the effect of dissolved gyps on the increase of calcite saturation index in these areas.
Conclusion
Piper diagram of the samples in the area under study shows that Cl anion and Na+Kcation are dominant in the plain. Accordingly, the type and profile of the ground water in the area are clorursodic. Based on Gibbs diagram, lithology factor is the main and most effective environmental process. Based on ionic ratio and correlation coefficients, halite dissolution process is the main chemical process affecting the ground water of the area under study. Among other factors affecting the quality of the ground water in Golpayegan Plain, the followings can be pointed: depth of underground water, size of particles and sediments on the path through which ground water move, and the distance travelled by ground water (as environmental factors), and the dissolution process of gyps and sulfates containing sodium (as chemical factors).
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