Hydrogeochemical Characteristics Assessment of Springs and Wells in the East of Gorgan plain

Groundwater is one of the most important water resources that is always used by humans in different sectors. The composition and chemical quality of groundwater sources are always threatened by natural factors such as climate, soil type, geology, topography, precipitation and evaporation, as well as human factors such as population growth, extensive agricultural activities, and industrial development. Hence, hydrogeochemical researches can provide very detailed information about the effect of the aquifer materials of the study area, the route of recharge and discharge, the areas of evaporation from the groundwater, the effect of surface water on the groundwater of the region, as well as its quality in various uses for decision makers. This research  was conducted to 1) compare and determine the significant difference (using the paired samples t-test) between physicochemical parameters in the statistical period under review (between wells and springs), 2) use geochemical ion ratios in order to discover the origin of ions and influencing processes that are responsible to alter the concentration of ions in the studied water resources, 3) investigate and identify the ion exchange process (normal and reverse) using chloro-alkaline indices and some hydrogeochemical graphs, and 4) identify saline water intrusion from subsurface layers or human pollution into the groundwater sources.

 In this study, 14 piezometric wells in the eastern part of Gorgan plain and 8 springs located in the heights between the cities of Ramyan, Minoodasht, Kalaleh and Gonbad were selected for hydrogeochemical study for which hydrogeochemical processes and qualitative changes of physicochemical parameters results (including Ca, Mg, Na, K, HCO3, Cl, SO4, TDS, pH and EC) were used over a period of 10 years. To compare and determine significant differences between physicochemical parameters in the statistical period under study between wells and springs, paired samples t-test and analysis of variance were performed in Minitab software. Before performing the paired sample t-test, normality of the data was checked using the Anderson-Darling test. In  paired samples t-test, if the p-value estimated by the software is greater than or equal to 0.05, with a probability of 95%, the hypothesis H0, which means that there is no significant difference between the average of the investigated treatments, is accepted and the hypothesis H1 is rejected. However, if the calculated p-value is less than 0.05, the hypothesis H0 is rejected with a probability of 95% and the hypothesis H1, which means that there is a significant difference between the averages of the investigated treatments, is accepted. Next, the maps related to the spatial changes of the groundwater parameters were plotted in the GIS environment and by using the IDW method, and then the trend of their changes in the studied area was interpreted. Aq.QA software was also used to better analyze the hydrogeochemical data and to draw the stiff and piper diagrams. Finally, to identify the factors governing changes in groundwater quality and also to investigate the relationships between variables, Gibbs diagram, ion ratios, and compositional diagrams have been used. All the mentioned graphs were prepared using Excel software.

   The spatial distribution of hydrochemical parameters showed that the concentration of most physicochemical parameters due to groundwater flow (from west to northwest) is increasing. The results of the investigation of the water type in the springs showed that magnesium bicarbonate (Mg-HCO3) is the dominant type of water in the studied springs. Unlike springs, a wide range of water type is observed in piezometric wells (Mg-HCO3, Mg-Cl, Na-Cl and Na-HCO3). Also, Ca-Mg-CO3-HCO3 bicarbonate facies and then mixed Ca-Mg-Cl-SO4 facies are the most abundant in water resources. Based on the distribution of water source samples in the Gibbs diagram, the chemical composition of all the samples, except for one sample related to the well, is controlled by the reservoir rock of springs or piezometric wells in the plain. Since the main ions show a linear relationship with TDI, it can be concluded that there is a possibility of intrusion of saline water containing sodium and chlorine ions or dissolution of halite minerals in the groundwater of the plain. The results of ion ratios showed that groundwater is recharged by dolomite, dolomite-limestone, and lime-dolomite formations associated to different geological periods is the main factor controlling hydrogeochemical parameters in the springs. But in the observation wells of the plain, in addition to the above cases, the weathering of silicates and the small intrusion of saline water into the ground water table have caused changes in hydrogeochemical parameters. The results of ion exchanges showed that the normal ion exchange process has a more effective role than the reverse ion exchange process in final concentration of main elements in the groundwater of studied water sources.

The changes in the concentration of ions in the springs are much less than the observed wells in the plain. Bicarbonate is the dominant anion in both well and spring water sources. Nevertheless, in the two wells no. 3 and 10 chloride ions prevail, which can be caused by the intrusion of saline water from the lower layers into the groundwater. Based on the spatial distribution of the physicochemical parameters, the concentration of most of the parameters in the direction of the groundwater flow is increasing due to the presence of layers with poor quality, the infiltration of urban sewage and agricultural effluents, as well as the intrusion of saline water from the subsurface layers. The investigation of the water type of the springs shows that magnesium bicarbonate is the predominant water type. Therefore, it can be concluded that the reservoir rock that recharges the springs is more than dolomite limestone. Similar to the springs, all the wells drilled on the edge of the highlands have the dominant type of magnesium bicarbonate. But by moving away from the mountain front and changing the concentration of some ions due to environmental and human factors, the type of groundwater becomes more diverse. Based on the composition graphs, it can be concluded that there is a possibility of intrusion of saline water containing sodium, magnesium, sulfate and chloride ions or the dissolution of some minerals such as halite and gypsum sulfate, especially in the groundwater of the plain. The results the ionic ratios related to the intrusion of salt water in water resources show the good quality of the groundwater and the recharge of the aquifer by limestone formations. Considering that the springs are located in the groundwater recharge area (fresh water), they are less affected by human pollution and, as a result, they have better chemical quality. On the other hand, the wells situated in the plain, due to human activities such as the excessive use of nitrogen fertilizers for agricultural products and the infiltration of urban or domestic sewage into the aquifer along with the intrusion of saline water into the groundwater in some areas of the plain show a greater tendency to become salty.