Feasibility Study for Hydraulic Connection in Azarshahr Plain Coastal Aquifer and Urmia Lake, by Geochemical Ratios and Inverse Modeling

Document Type : Research Paper


1 Tabriz,29 Bahman Blvd,Tabriz University,science Faculty,geology department

2 Hydrogeology professor,Tabriz University,science Faculty,geology department


Azarshahr plain is one of the plains adjacent tothe Lake Urmia that has been faced, during recent years,withover extraction of groundwater and alsoexposed to salinity enrichment. On the other hand, Urmia Lake with about 5000 square kilometers is the relatively vastlake with hyper-saline water as a rare biosphere resource in Iran and the world. This lake has a great economic, social and bio-spherical importance in Iran. It has faced withenormous water depletion during the recent years. Many studies has been conductedabout climate change, drought, rivers discharge declining and changing in land use so far that increasing in water utilization in agricultural activities have taken into account as the most important factor in water table reduction of Urmia Lake.However,there is no study about the hydraulic connection between Lake and its adjacent aquifers.
Perception of spatial changes in groundwater components that is resulted from various inputs is a difficult work, especially if chemical composition or boundary conditions change over time, as it is popular in coastal aquifers. Utilization of geochemistry in different water resources connection has done in many studies. Geochemists generally seek to findchemical ratios such as /  , / , / +  and / +  andand  isotopes as saltwater intrusion indicators. The present study for the first time investigates the hydraulic connection between Urmia Lake and Azarshahr plain aquifer using geochemical ratios and geochemical modeling.
Materials and methods
Urmia Lake is a kind of the widest and shallowest lakes in the world and has hyper saline water.A lot of permanent and seasonal river and streams feed the lake. Broad and unwanted changes in water and ecology of this lake have been started about 2 decade before and during this time.Thewater volume of the lake decreased toless than 14 billion in 2011from about 32 billionin 1995.
Azashahr study area is considered as one of the Lake twelve adjacent aquifers that is located between 45°,46’ to 45°,50’ longitudinal and 37°,43’ to 37°,52’ latitudinal. Its total area is about 457 km  that its plain has an area about 124 km. The highest and lowest height in the study area is 3700m and 1282m, respectively. Its average annual precipitation is about 221.2 mm whereas the average annual evaporation is about 1579 mm. and themost important stream in Azarshahr plain is Azarshahrchai.Ithas a southeast- northwest trend and is eliminated to the lake because of wide agricultural usage. On the other hand, the total annual discharge ofthe aquifer is about 90.64 million  that is one of the groundwater depletion and saltwater intrusion factors in the study area.
Groundwater Salinity is increasedin Azarshahr plain aquifer because of wide discharge in one side and decrease inUrmia Lake water level on the other hand. This probably shows that saltwater intrusion to Azarshahrplain aquifer is in relation with water level decreasesin Urmia Lake and consequently reveals the hydraulic connection between them. To investigatethis connection, groundwater sampling and analysis were conductedand hydraulic connection evaluation was done via three steps as below.
Graphical methods
Three popular Piper, Stiff and Pie chart graphical method was selectedand the samples were plotted after the analysis and were compared to Urmia Lake water samples. Star diagram also was plotted for the samples and their map was drawn. On the other hand, pie chart that shows the percentage of water major components was calculated and their distribution in study area was plotted and compared to Urmia water component percentage.
Geochemical ratios
General status of saline water is  and  dominance with 0.86 molar ratio and of course  exceed over the alkali metals ( and ). Also,  is more than  absolutely (i.e. / =4.5-5). On the other hand, continental fresh water is known with its variable composition andits dominant anions are, , and . Geochemists try to find alsothe below geochemical ratios generally for saltwater intrusion resource determination: / , / , / +  and / +  and also some isotopes such as Boron, Oxygen and Hydrogen. In the present study,geochemical ratios were calculated and plotted in study area for saltwater intrusion resource finding and also evaluation of hydraulic connection between Urmia Lake and Azarshahr plain aquifer.
Geochemical Inverse Modeling
Hydro-geochemical modeling is an important tool for recognition of geochemical reactions in groundwater. In the modeling process, geochemical reactions during the flow path such as solution, precipitation, mixing and cation exchange are simulated and compared withfinal water composition.
Solute aggregation in groundwater can be supposed as a reaction of water with surrounding minerals in porous media. Hydro-geologists utilize the water chemical data for classification of various mineral balance situations. In this study, Ion Activity Product (IAP) and Balance constant (K) and also Saturation Index (SI) were used for probable minerals, composition creation and comparison with probable phases in Urmia Lake water sample.
Therefore,because of enormous volume of calculations in balance reactions and also finding the balance constant for reactions, PHREEQC computer code has been used and probable phases in groundwater mixed samples and Urmia Lake water sample were created and compared to each other. These were carried out to find if there is any chemical connection between lake and Azarshahr aquifer. The mixed samples were created in three paths near the lake. Mixing reaction was done between samples number 18 and 19, 20 and 23 and also between 25 and 22 in porous media presentation. 
Results and discussion
Geochemicalanalysis results of groundwater samples and Urmia Lake indicatedthat dominant cation in Azarshar aquifer and Urmia Lake are and , respectively. Also groundwater dominant anions are ,  and during the flow direction, whereas dominant anion in lake water is that is distinct characteristic of marine composition.
Piper diagram revealed that groundwater type in study area are - -  and - -  whereas Urmia Lake water is in -  type. The nearest sample to Urmia Lake water composition is sample number7 that is located out of aquifer domain and is affected by anthropogenic activities probably. Also stiff diagram of groundwater samples and Urmia Lake aren’t similar to each other and show the different sources.
/  Ratio in saltwater intrusion is lower than marine values (i.e. Molar Ratio<0.86). On the other hand, ratio more than 1 is the indicator of anthropogenic activities such as residential area wastewater. /  Ratio distribution in the aquifer domain shows that in the northwest and west of aquifer, there is / ratio less than marine ratio.Therefore,there is saltwater intrusion from northwest of the study area.
Also evaluation of / ( + ) and /  ratios revealed that, these ratios in aquifer domain are more than those in Urmia Lake water spectacularly. As it can be clearly seen, minimum value for /  ratio in aquifer domain starts from about 29 and for / ( + ) ratio starts from about 0.2, whereas these ratios are 0.01 and 0.03 for Urmia Lake, respectively. This shows no relation between aquifer groundwater and Urmia Lake.
Other popular geochemical ratio for saltwater intrusion is Revelle ratio explained as [ / ( + )]. Ratio more than 1.5 depicts the saltwater intrusion. Revelle Distribution plot representedthat there is saltwater intrusion from domain northwest direction but it is not from the lake because of its direction and also South-North trend.
Hydro-geochemical modeling of different groundwater sample mixing in flow direction and the nearest distanceto the lake has been indicatedas a comparsion between probable phases saturation index. The results show thatmixing of samples had been done but probable phases did not, same as the probable phases in Urmia Lake water.
Saltwater intrusion from Urmia Lake to adjacent aquifers because of their heavy pumping rate is one of probable factors in water level depletion of Urmia Lake. On the other hand, finding the relationship between different types of water resources to each other by geochemistry is a popular way that have not been done so far in Urmia Lake. Therefore,useof geochemistry studies and its different branches werethe base of this study. At last, after the useof geochemical ratios and geochemistry modeling, their results indicatedthat there is saltwater intrusion in Azarshahr plain aquifer, but it is not from Urmia Lake water. Probably it comes from adjacent salty pans to the aquifer. Geochemistry did not show the logical relationship between groundwater samples of aquifer and water samples of Urmia Lake.Itdidnot findhydraulic connection between them subsequently.Thegeochemistry revealed that it is a applicable tool in finding the relationship between various resources of waters.


Main Subjects

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