Hydrogeochemical investigation of an epithermal mineralization bearing basin using multivariate statistical techniques and isotopic evidence of groundwater: Kestanelik Sub-Basin, Lapseki, Turkey

Abstract

This study investigates hydrogeochemical processes controlling the groundwater quality using 64 groundwater samples in the epithermal mineralization bearing Kestanelik Sub Basin, NW Turkey. The groundwater dominantly exhibits a mixed-ion hydrochemical facies characterized by Ca-Mg-HCO3, Mg-Ca-HCO3_, Ca-Mg--HCO₃-SO₄ and Ca-Mg-Na-HCO₃ reflecting weakly mineralized waters circulating within calcareous sandstone and colluvium. The molar ratios of (Ca²⁺+Mg²⁺)/(SO₄^2-+HCO₃^-) and Na⁺/Cl^-ratios and the linear plot between (Na⁺+K⁺)-Cl^- and (Ca²⁺+Mg²⁺)-(HCO₃^-+SO₄^2-) indicate that of dissolution of carbonates and silicate weathering, ion and reverse ion exchange processes influence the hydrogeochemistry of groundwater sources. Three and four factors account with 84.48 and 75.27 % of the total variance for spring and well waters, respectively. FC1 in the groundwater samples is described by strong loadings of TH, Ca²⁺, Mg²⁺, HCO₃^-, Ba, TDS, Cl^-, EC and SO₄^2-, and moderate loading of Na⁺, Mn, K⁺, TDS, pH. FC2 in the samples has strong positive loadings on Al, Fe, Zn, Na⁺, SO₄^2-, Cl^- and moderate positive loadings on B, TDS and K⁺. FC3 shows strong positive loading on Cu, Al, DO and moderate factor loadings for Fe in the well water samples. FC4 exhibits high positive loadings for As, Zn, and moderate positive loading for EC. Geochemical variables of FC 1, 2, 3 and 4 for the groundwater indicates mixed geogenic source of groundwater contamination through water-rock interaction processes dominantly from (i) the calcareous sandstone, serpentinite and colluvium lithologies and (ii) quartz, illite, pyrite and iron oxide minerals sourced from epithermal mineralization in the area. pH and As values exceed the desirable limits of WHO and TSE 266 guidelines for drinking purpose. The tritium levels show that some well waters have experienced longer water-rock interaction and residence time and are thus recharged from older groundwater sources compared to other well and spring waters.

Introduction

Sustainable freshwater resources, one of the most important natural resources, have an important role on both development of industry, agriculture, urbanization, and improvements in quality of life and eco-environment maintenance. These resources in general, are supplied from different resources, depending on the availability of surface water bodies (rivers, lakes, ponds, etc.) and groundwater (aquifers and springs) on earth (Baig et al., 2009; Ravikumar and Somashekar, 2017; Dişli, 2017). Recently, water demand for agricultural, industrial and domestic water uses has increased dramatically owing to constantly rising socio-economic development and population in today’s developing world, and therefore many areas are expected to experience supply and demand imbalance for water resources in both arid and semi-arid regions (Li et al., 2016; Caraballo et al., 2016). It is estimated that groundwater consumption worldwide is considered to be 50, 40 and 20 % of the amount of domestic, industrial and irrigation water, respectively (Su et al., 2017). In recent years, due to the contamination of water resources which is usually caused by human activities such as open and/or closed pit mining operations, expansion of the urban scale with population growth, agricultural activities, domestic and industrial wastewater discharge and problems in managing sustainable landfills, the quality of freshwater has equal importance with its quantity because of its suitability to be used for various purposes (Tai et al., 2012; Su et al., 2017; Dişli, 2018a). Consequently, meeting the demand for clean or fresh drinking water, and achieving the sustainability of waters in terms of quality and quantity for almost all living organisms in the present and future are some of the most significant problems in the world. In addition, water quality could get changed with the direction of movement in the hydrological cycle and the other processes containing evapotranspiration, oxidation/reduction, ion exchange, water-rock interaction, mineral dissolution, deposition of secondary minerals, mixing of waters and domestic and industrial wastewater (Su et al., 2017; Wagh et al., 2017).

The most known contaminants causing pollution of freshwater resources are mainly arsenic, fluorine, nitrogen and organic ones (Su et al., 2017). Recently, the existence of elevated arsenic (As) in water resources, especially in groundwater, used as drinking water in many countries have risked the health of more than 100 million people worldwide (Mukherjee et al., 2009). The literature show that the long-term low-dose arsenic exposure in water (i) could induce varying chronic health effects such as cardiovascular diseases and diabetes, black foot disease, keratosis, gastrointestinal and renal disorders (Buschmann et al., 2008; Tsuji et al., 2014) and (ii) has also been associated with various cancers such as bladder, kidney, skin and live (Tchounwou et al., 2003; Lamm and Kruse, 2005). The arsenic (As) is a systemic toxic metalloid, which has a natural element with features intermediate between metals and nonmetals (Baig et al., 2009; Singh et al., 2015). Arsenic is generally present in nature in four oxidation states, mainly arsenate (As⁵), arsenide (As³), arsenic (As°) and arsin (As^-3) and its solubility varies depending on pH and ionic environment. Arsenic is chiefly present in groundwater sources containing minimum amounts of methyl and dimethyl arsenic compounds in inorganic forms such as As³ and As⁵, and the most stable arsenic form in nature is As⁵ (Zhao et al., 2010; Gupta et al., 2011; Singh et al., 2015). The World Health Organization (WHO) and Turkish Drinking Water Standards (TSE 266) have set the maximum value of arsenic in drinking waters at 10 μg L^-1 (WHO, 2011; TSE 266, 2005). On the other hand, contamination of water resources due to mine drainage and discharge of groundwater from the mining area is another major environmental issue affecting the quantity and quality of water sources surrounding the project area. Additionally, in the mine's deposits, the generation of acidity from weathering of sulfide waste generally causes acid mine drainage (AMD), a frequently documented consequences of mining that influences freshwaters quality worldwide (Hogsden and Harding, 2012). AMD generally occurs when pyrite group of minerals such as pyrite, chalcopyrite, arsenopyrite are subjected to hydrological and/or biological weathering (oxygen, water and chemoautotrophic bacteria), atmospheric effects, becoming oxidized and producing sulphuric acid (low pH), dissolved metal ions, elevated sulfate concentrations (Skousen et al., 1994). Common heavy metals such as Fe, Pb, Mn, As, Zn and Cu and sulphates are generally present in elevated concentrations, and they become more soluble depending on the amount of acidity in waters (Harding and Boothroyd, 2004; Luis et al., 2009).

Geogenic factors are also important as antropogenic factors on influencing the quality of the water resources. The geological formations commonly determine the physicochemical properties of both ground and surface waters of an area. Additionally, an existing mineralization in the area may negatively affect the quality of the water. Dissolution of various minerals from the geological formations as well as the water-rock interactions in mineralized and associated alteration zones causes both spatial and temporal changes in the quality of the groundwater and hence may pose risk in both environmental and human health (Jarup, 2003).

In Turkey, drinking water is generally provided from groundwater (well and spring) and surface water (rivers, lakes, and dams) sources. The exploitation of groundwater has rised significantly in Turkey especially since 1990 to supply safe water for households (drinking and potable water) owing to agricultural, industrial and domestic wastewater discharge into freshwater resources and sustaining wetland agriculture depending on climate change seen in the Mediterranean Basin. Numerous cases of natural arsenic pollution have also been reported in Turkey including Simav Plain (Gündüz et al., 2010), Heypeli Spa-Afyon (Gemici and Tarcan, 2004), Şarkışla Plain-Sivas (Kobya et al., 2015), Hisarçık (Çöl and Çöl, 2004), İzmir, Afyon, and Van (Baba and Tayfur, 2011).

In this paper, we focus on the Kestanelik Sub Basin which is located in the Biga Peninsula in northwestern Turkey approximately 35 km east-northeast of the city of Çanakkale. In the study area, geological formations as well as epithermal mineralization and associated wall rock alteration create suitable conditions for influencing the quality of subsurface water resources. The main aims of the study area to (1) investigate the groundwater chemistry and main hydrogeochemical processes in an epithermal mineralization bearing Kestanelik Sub-Basin and (2) evaluate groundwater quality and assess its suitability for drinking by comparing the calculated parameters in relation to the drinking water standards and guideline. We used statistical analyses such as Pearson correlation analysis and factor anaysis to investigate the sources controlling the groundwater chemistry. We also performedstable isotopes (D and ¹⁸O) and radioactive isotopes (³H) analyses to find out the origin of the groundwater. This study sheds some light on the water-rock interaction in areas of epithermal mineralization and associated wall rock alteration. In addition, it has significant implications for the effects of epithermal mineralization and associated wall rock alteration on the quality of the groundwater resources before the commencement of any mining activities in the area.