Hydrochemical Study of surface and groundwater resources of Lanjanat Plain

Extended abstract
Background and Objectives
Groundwater quality assessment and related hydrochemical characters are critical for water resources management in the arid and semi-arid regions. Water quality is a function of physico-chemical parameters and strongly influenced by geological formations and human activities. The hydrochemical evolution of groundwater mostly depends on the chemistry of the aquifer materials, water and rock interaction and water residence time. Water quality assessment requires knowledge of mineralogy of the surrounding rocks and identification of relevant chemical reactions. Ions in groundwater may be of natural source, human origin or a combination of both factors. The Lanjanat is the largest Plain in Isfahan Province and is located in the Gavkhouni Basin where the Zayandehroud River is passing. According to the geological classification of the country, Lanjanat Region is located in the Sanandaj-Sirjan structural zone. The hard rocks exposed in the heights surrounding area include Permian rocks, Jurassic sediments (including dark gray shales with interlayers of sandstone, quartzite, limestone, and volcanic), shale, marl, and Cretaceous limestone, conglomerates, and Paleogene sandstones. The Quaternary sediments exposed in the plain have formed an alluvial aquifer over the impermeable Jurassic shales. The main recharge source of the Lanjanat Aquifer is meteoric water, which decreases from west to east and from south to north of the basin. Lanjanat plain is one of the most fertile agricultural centers in Isfahan Province, which in recent years has faced a quantitative and qualitative crisis of water resources due to the increasing population, development of industries and agriculture and also excessive extraction of groundwater. In this research, an attempt has been made to update the information related to the quality of surface and groundwater and to determine the origin and hydrochemical evolution of the groundwater in the plain.

Methodology
In this research, 13 locations of surface and underground water sources, including rivers, springs, Ghanats, agricultural wells, and fields were selected for sampling. The pH, temperature and electrical conductivity of water were measured in fields. Also, total dissolved solids and major ions concentration were determined in the laboratory. After determining the physicochemical characteristics and concentrations of the main ions of the samples, to understand the relationship between the main hydrochemical variables, a Pearson correlation matrix was provided. Besides, the ion ratio diagrams (including sodium vs. chlorine ratio, bicarbonate vs. sodium, calcium vs. sulfate, calcium and magnesium vs. bicarbonate and sulfate, calcium vs. bicarbonate, and bicarbonate vs. magnesium) together with the combination charts, and Piper's diagram were used to determine the origin of ions in water sources.

Finding conclusion
The results showed that the minimum and maximum concentrations were respectively observed in the river samples and the deep wells located in the central sector of the plain. The range of water electrical conductivity was between 264 5755 µ/cm in the rice field and 5755 µ/cm in the agricultural well. The total dissolved solids was between 271 mg/liter in the river sample and 3889 mg/liter in the agricultural well. The amount of dissolved solids was increased by moving from the sides towards the central sector of the plain, as well as by increasing the depth of the wells. The water type of the samples varies from calcium bicarbonate near the carbonate formations to calcium sulfate or sodium chloride in the central sector of the plain. The Pearson correlation matrix showed a very strong positive correlation between the total dissolved solids and electrical conductivity of water and a strong inverse correlation between pH and total dissolved solids and electrical conductivity. The concentration of the main anions and cations in the surface and underground waters of the region was mostly affected by natural factors, especially the dissolution of carbonate and evaporative minerals. According to the results of the ion ratio diagrams, the origin of the main ions in the water resources of the region was mostly from the dissolution of geological formations, although human activities were effective in some places. Agricultural activity has caused an increase in concentration of some ions by using fertilizers (such as sodium and sulfate). The present research is only based on the results of a sampling period of 13 surface and underground water sources. In order to better understand the hydrochemistry of the plain, it is suggested to carry out monthly sampling during a water year, and at more points. Also, in addition to the main anions and cations, the concentration of nitrate, phosphate and heavy metals should also be measured and analyzed.