6 th International Young Researchers Workshop on River Basin Environment and Management, 19-21 October 2018, Jimbaran Bay Beach Resort & Spa, Bali, Indonesia HYDRO-GEOCHEMICAL APPRAISAL TO PROMOTE GROUNDWATER SUSTAINABILITY AT KALPITIYA AREA, NORTH-WESTERN COASTAL AQUIFER, SRI LANKA P. RATHA (1,2) , K. D. W. NANDALAL (2) , H. M. T. G. A. PITAWALA (3) , H. A. DHARMAGUNAWARDHANE (3) , S. B. WEERAKOON (2) (1) Department of Water Resources and Rural Infrastructures, Institute of Technology of Cambodia, Phnom Penh, Cambodia, (E-mail: rathaphok0621@yahoo.com) (2) Department of Environmental Engineering, Faculty of Civil Engineering, University of Peradeniya, Peradeniya, Sri Lanka, (E-mail: kdwn@pdn.ac.lk, sbweera@pdn.ac.lk) (3) Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka, (E-mail: apitawala@pdn.ac.lk, dharmag@pdn.ac.lk) ABSTRACT Groundwater (GW) though vastly important resource for man, however, it may not meet the standards because it contains dissolved constituents coming from natural sources. Hydro-geochemistry is very vital from perspective of environmental impact assessment. The present study covered Kalpitiya area, one of the most potential GW zones in the north-western coasts of Sri Lanka, as dominated by unconsolidated sand underlain by clay and limestone. Continuous increased population, intensive agricultural activity, and over-exploitation of GW associated with geogenic factors make it imperative to assess the quality of the GW system to ensure long-term sustainability of the resources. This study aimed to appraise hydro-geochemical characteristics of GW using statistical and graphical analysis and irrigation water quality evaluation. The result of the current study reveals that the major cations and anions in GW in Kalpitiya area are Na + and HCO3 - , respectively followed by Ca 2+ and SO4 2- contributing to the salinity of GW, and the mean values and coefficient of variations of GW elements are relatively large (except pH and Mg 2+ ), which refers to high content and large spatial variation. High evaporation rate and nutrient enrichment are the main features for EC and TDS enrichment. The evolution of GW form the Piper diagram was identified the dominance of water facies as mixing Na-Ca-HCO3 type and Na-Ca-HCO3-Cl-SO4 type and Na-Ca-Cl-SO4 type. Comparison of GW quality and irrigation suitability standards proves that majority of water samples are suitable for irrigation purpose. Based on the Wilcox diagram classification, majority of GW samples fall within good (38%) and permissible (18%) category, while 15% of the samples represents excellent category. The rest is recorded as doubtful and unsuitable. Generally, chemistry of water is guided by complex weathering process, ion exchange and influence of agricultural acts. It is implied that contamination of nutrients of evaluated GW could result from agricultural practice as that of major amount of other ions also rely on salt water sprays to the soil and anthropogenic actions. It is proposed that the current drive towards the attendant urbanization and socio-economic development means a greater requirement for the quality and quantity of portable GW in the area. Keywords: Hydro-geochemistry; statistical and graphical analysis; Piper diagram; Kalpitiya; Sri Lanka. 1. INTRODUCTION Groundwater (GW) is globally pivotal for resolving fresh water desire for human life. Coastal aquifer has become one of the most potential GW regions for socio-economic development. As there is an increasing interest in environmental issues, hydro-geochemistry is very necessary from viewpoint of environmental impact assessment (Šráček & Zeman, 2004). GW quality variation is a function of physical and chemical constituents in an area influenced by geological and anthropogenic activities (Srinivasamoorthy et al., 2014). Hydro-geochemistry that constitutes the other issues deals with processes such as dissolution, precipitation, absorption and desorption, ion exchange reactions and the residence time along the flow path which controls the chemical composition of GW, which alter GW quality (Kazemi & Mohammadi, 2012). Understanding the principal processes that control GW chemistry is either crucial, in terms of sustainable management of GW resources, or scientifically challenging. Research and knowledge of hydro-geochemistry and evolution of GW can disclose the present situation of GW and the interacting mechanism between GW and environment and is essential to determine the origin of chemical composition of GW. Moreover, it is as vital as its quality since it is the key factor to determine its suitability for drinking, domestic, engineering, agricultural, and industrial aspects (Subramani, Elango & Damodarasamy, 2005). These days, as recognized that the quality of GW is just as vital as its quantity because man’s perspective of using water is firstly focused on the quality by ignoring the quantity. During last recent decade, rapid urbanization, intensive utilization of GW, agricultural development, multiple socio-economic expansion, and consequent over- exploitation lead to extremely vulnerable to pollution in north-western coastal aquifer (Jayasingha, Pitawala & Dharmagunawardhane, 2013; Thilakerathne, Chandrajith & Schüth, 2014). This study is dedicated to evaluate the hydro-geochemical characteristics of sandy GW to understand the root causes of GW quality deterioration, types, and sources by using statistical and graphical analysis and irrigation water quality assessment in Kalpitiya area, a typical agricultural area in north-western coastal region, where GW is the main source of drinking water, agricultures, and other domestic purposes. 2. DESCRIPTIONS OF STUDY AREA Kalpitiya area is located in the Puttalam district of the north-western province. The coordinates of the area are