Geochemical processes affecting groundwater chemistry in Khosh_yailagh carbonate formation, north of Iran

Geochemical processes affecting groundwater chemistry in Khosh_yailagh carbonate formation, north of Iran
Mohsen Rezaei1, Rashid Zivari2, Javad Ashjari3, Abdolreza Kaboli4
1- Associate Prof., Applied Geology Department, Faculty of Earth Sciences, Kharazmi University (m_rezaei@khu.ac.ir)
2- Msc Student, Applied Geology Department, Faculty of Earth Sciences, Kharazmi University
3- Assistant Prof., Geology Department, Faculty of Geosciences, Tehran University
4- Golestan Regional Water Authority

Introduction
Carbonate formations (including karst springs and wells), have important role in supplying domestic and irrigation water in our country. Along with the exploitation of these resources, it is required to be considered geochemical characteristics and factors affecting water quality changes. Khosh-Yailagh carbonate formation is a formation of Devonian age in the north of Iran. The outcrops are located in mountainous areas and in the margins of Gorganroud plain. The local base level of erosion is high due to particular tectonic behavior. Because of low thickness and coarse texture of alluvium, well yields are not respond to different uses.
Groundwater hydrochemical assessment is usually based on a set of comprehensive information about the chemistry of groundwater. Chemistry of groundwater is affected by different factors such as geology, climate, type of rock weathering reactions, and recharge water quality (Guler et al, 2004; Subramani et al, 2005; Coetsiers, 2006).
The purpose of this study is to evaluate the evolution of geochemical processes and water quality of Khosh-Yailagh carbonate formation. Water quality graphic methods, multivariate statistical methods, hierarchical cluster analysis (HCA), principal components factor analysis (PCA) and chemical mass balance were used in determination of hydrochemical parameters and assessing geochemical process of water of carbonate formation.

Materials and Methods
The study area located in the east Alborz structural-sedimentary zone in Golestan province, north of Iran between the N36º48' 00" to S37º00'00" and W54º47'00" to W55º08'00". Minimum height in the region of 118 meters and a maximum altitude is 1834 meters. Various formations outcrop in the study area. Khosh-Yailagh is the most important of these formations, which is carbonate with karst development potential.
20 water samples were collected from springs. Field analysis of Temperature, pH and conductivity was done when the sample is collected. The major elements (Na+, Ca2+,Mg2+, K+, Cl-, SO42-, HCO3-, NO3-) were analyzed in the lab using standard methods. Composite diagrams were used to indicate relation between different ions. The statistical technique of multivariate analysis was used to characterize hydrochemical processes through data reduction and classification. The factor analysis derived principal components from a correlation matrix and rotated axes with a quartimin rotation.



Discussion of Result
All of the groundwater samples are low salinity with electrical conductance ranges from 220 to 706μs. All groundwater samples have carbonate-Calcium type. Clustering analysis was used for combining cases (water samples) into clusters. This clustering routine resulted in three groups of water samples on the basis of variables (pH, EC and major elements).
Group 1: low salinity waters (309 ≤ EC ≤ 589μs) (samples 1, 2, 5, 10, 11, 12, 13, 14, 18, and 20). Average TDS of this group is 41.27 mg/l.
Group 2: low salinity waters (451 ≤ EC ≤ 538μs) (samples 4, 7, 8, 15 and 16). Average TDS of this group is 37.22 mg/l.
Group 3: electrical conductance of this group ranges between 495 to 706μs. This group contains samples 3, 6, and 19. These samples indicate more water-rock interaction. Groups 1 and 2 are more similar and could be consider as one group.
Most measured parameters showed weak correlation, which is the evidence on the effects of different processes in the water chemistry. Factor analysis was applied to identify the dominant processes controlling major chemical components of groundwater. The variables for factor analysis were Na+, K+, Mg2+, Ca2+, HCO3-, Cl-, SO42-, EC and pH. The virmax orthogonal rotation method was applied. Four factors are extracted to represent the contributions influencing chemical composition of groundwater (Table 1). The variables of TDS and Ca, have high positive loading of factor 1. The variables K and HCO3 have high positive loading of factor 2. The factor loading of pH show high positive value on factor 3. The variables Na and Cl have positive loadings on factors 4.

Table 1. Results of principle component analysis
VARIABLES Component
P1 P2 P3 P4
TDS .916 .507 -.770 -.040
pH -.082 .007 .969 .017
Ca .895 .263 -.004 .009
Mg -.907 .316 .211 .056
Na -.306 .195 -.325 .785
K .190 .844 -.199 .131
HCO3 -.211 .774 -.015 .287
Cl .193 .201 .335 .796
% of variance 24.072 23.207 20.463 17.252

Ion-exchange reactions occurs, both positive and negative direction that depends on the flow distribution and mixing of groundwater. In order to understand the dominant lithology of environment in which water is flowing, the molar ratio of calcium and magnesium was used. Molar ratio of 1.44 represents interaction of water with dolomitic limestone and dolomite formations. Saturation indices of calcite, dolomite and gypsum indicate that the groundwater is not at the chemical evolution.
The processes that govern changes in the groundwater composition, as interpreted from the factor analysis are mainly determined. In plotted Gibbs diagram, the water samples lie in field of water rock interaction. Dissolution and precipitation of carbonate minerals is the main factor controlling chemistry of groundwater in the study area. Cation exchange processes influence the concentration of cation such as Ca, Mg and Na. Rainwater that is charged with biogenic and atmospheric CO2 is another component determines chemistry of groundwater in the recharge area.