Characterization of groundwater in Malala Oya river basin, Sri Lanka using geochemical and isotope signatures

Groundwater for Sustainable Development 9 (2019) 100225

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Research paper

Characterization of groundwater in Malala Oya river basin, Sri Lanka using geochemical and isotope signatures

T

S.L. Senarathnea, J.M.C.K. Jayawardanaa, E.A.N.V. Edirisingheb, Rohana Chandrajithc,∗ a

Department of Natural Resources, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Sri Lanka Isotope Hydrology Section, Atomic Energy Authority, Orugodawatta, Sri Lanka c Department of Geology, Faculty of Science, University of Peradeniya, Sri Lanka b

ARTICLE INFO

ABSTRACT

Keywords: Environmental isotopes Irrigation suitability Rock-water interactions Silicate weathering Tritium in groundwater

A comprehensive study was conducted to investigate characteristics of groundwater in the Malala Oya River basin using hydrogeochemistry and environmental isotopes of 2H, 18O and 3H as this is an important source of water for the southern dry zone of Sri Lanka. Twenty-two groundwater and 8 surface water samples were collected and measured for major ions, metals, isotopes of 2H, 18O and tritium (3H). The prominent water type found in the basin was Ca–Mg–Cl while silicate weathering was found to be the main process that controls the groundwater quality. Irrigation suitability calculations indicated that the groundwater in the basin is highly susceptible to salinity hazard. The isotope data suggested that the shallow groundwater in the basin is recharged from the northeast monsoon rains while the deep groundwater (> 20m) is recharged from the groundwater flow from adjacent highland regions. Tritium analysis indicated that the deep groundwater in the basin were stagnant for a long period in aquifers. The findings of the present study suggest the need for a sound groundwater management plan for the basin with an emphasis on salinity control measures.

1. Introduction In many regions of the world, groundwater plays a vital role in catering to the needs of the water for domestic, agricultural and industrial sectors. Globally, groundwater is increasingly put under pressure by anthropogenic activities (Ouyang et al., 2014). Excessive abstraction of groundwater for irrigation and industrial purposes, discharge of urban and domestic waste, agricultural chemicals and subsequent contamination of aquifers have led to the deterioration of quality of groundwater in many parts of the world (Kumar et al., 2005). According to the recent predictions of global climate change scenarios, surface water sources are going to be limited. As such, the world's largest storage of fresh water; the groundwater plays a crucial role in sustaining ecosystems and enabling human adaptation to climate variability (UNFCCC-COP, 2011). Having recognized the importance of groundwater as a reliable source of fresh water, sustainable management and protection of groundwater have been given a top priority in the Sustainable Development Goals (SDG's) of the UNDP (Bhattacharya and Bundschuh, 2015; Megdal et al., 2017). A significant advancements in exploration of groundwater resources and remarkable progress in many areas, including the global-level characterization of groundwater systems, their properties and conditions have been achieved (Döll and

∗

Fiedler, 2007; Siebert et al., 2010). However, in many regions of the world, particularly in developing countries, groundwater has not yet been given appropriate attention by investigating potential threats and sustainable management (Morsy et al., 2018). However, stress on groundwater resources are being observed mainly due to excessive abstraction and pollution of aquifers (Bhanja et al., 2017). Groundwater is the major source of water for domestic, agriculture and industrial uses in Sri Lanka (Panabokke et al., 2007). However, in recent years, an increasing pressure on groundwater resources in the country such as excessive abstraction and pollution of aquifers due to industrial, agricultural and urban waste disposal have been encountered (Villholth and Rajasooriyar, 2010). Further, a direct link between drinking water quality and human health has also been observed in many parts of Sri Lanka (Chandrajith et al., 2012; Dissanayake and Chandrajith, 1999; Rubasinghe et al., 2015). Therefore, much attention is required to investigate the quality of groundwater particularly for domestic and agriculture purposes. Despite emerging global awareness, groundwater management in Sri Lanka has still not been converted into a sustainable approach. A key limitation to sustainable management of groundwater in Sri Lanka includes lack of baseline data on groundwater dynamics and their interrelations with different environmental compartments (Panabokke and Perera, 2005).

Corresponding author. E-mail address: [email protected] (R. Chandrajith).

https://doi.org/10.1016/j.gsd.2019.100225 Received 14 December 2018; Received in revised form 19 April 2019; Accepted 7 May 2019 Available online 09 May 2019 2352-801X/ © 2019 Published by Elsevier B.V.

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Fig. 1. Map of the Malala Oya River basin in southern Sri Lanka and sampling locations.

Environmental isotopes such as 2H, 18O and 3H provide useful information on processes which influence groundwater such as evolution, recharge processes, rock-water interaction and the contaminant processes in comparison to general groundwater quality analysis (Aggarwal et al., 2000; Chandrajith et al., 2014, 2016; Edirisinghe et al., 2016; Jayasena et al., 2008; Song et al., 2006). However, very limited studies have been carried out in Sri Lanka on geochemical evolution, recharge processes, rock–water interaction of groundwater using stable isotopes. Sri Lanka is characterized by two climatic regims with marked differences in precipitation patterns (Fig. 1). The region denoted as ‘wet zone’ that receives over 2500 mm average annual precipitation while ‘dry zone’ region that covers two third of the island receives only about 1000 mm annual precipitation. The dry region of the island is characterized by 9 months long dry season, followed by extremely heavy precipitation during the year. Higher evapotranspiration and low precipitation received to the dry zone region has led to a water scarcity, especially during the dry periods. Limited surface water availability of the region has led to the utilization of groundwater supply as the main source for domestic purposes and irrigation. To cater to the expanding water demand in the southern dryer area of Sri Lanka, the government of Sri Lanka has initiated a trans-basin river diversion project to provide water from the wet zone. The water from Uma Oya, a tributary of Mahaweli River, which flows in the wet region, will be diverted through a tunnel system to the southern dry zone of the country. The diverted water preliminary flows to Malala Oya River before it distributes to the small irrigation reservoirs in the basin. Trans-basin river diversions have been found to create numerous social, economic and environmental impacts. The changes of river flow, volume of water in supply and receptor basins, water salinization, changes of local climate are the possible impacts of trans-basin river diversions (Chen et al., 1999). Drastic changes in the quality and quantity of surface and groundwater in the Malala Oya river basin is expected due to the Uma Oya diversion project. Considering the benefits and risks associated with such trans-basin diversions, it is advisable to monitor the environmental conditions before implementation of the project. However, no systematic evaluation of groundwater status of the Malala Oya basin has been carried out in the recent past. Therefore, this study aims at characterizing the groundwater and surface water in the Malala Oya basin using geochemical traces and stable isotopes signatures. The finding of this study is expected to generate baseline data for future groundwater management in the region and

help to evaluate the environmental changes that will be expected due to trans-basin diversion. 1.1. Geological and hydrological background Malala Oya River that flows through the southern dry area of Sri Lanka is one of the important river basins in the region. The basin extends approximately 405 km2 with 17 sub-watersheds (Fig. 1). The study area is mostly flat and undulating at an altitude of about 50 m asl to the sea level. The area receives heavy precipitations from November to February from the northeast monsoon. The ambient temperature of the region varies from 27 to 32 °C. The mean annual precipitation of the basin is 1096 mm with average annual surface runoff of 192 MCM. Annual groundwater recharge of the basin is estimated as 41 MCM from the total precipitation. Annual extraction of groundwater is 0.13 MCM which is 3.42% of the total demand (Imbulana et al., 2002). The entire river flows in a high-grade metamorphic basement complex that mainly comprises granitic gneisses, biotite-hornblende gneisses and scattered bands of charnockitic gneisses. Therefore, the groundwater in the region occurs mainly in fractures and other weak zones in crystalline basement rocks while shallow groundwater is generally extracted from the weathered overburden. Due to the scarcity of water, lowering of the groundwater table during the dry spells and subsequent changes of water quality have been made profound impact on the consumers in the region (Imbulana et al., 2002; Jayatillake, 2002). In addition, increased salinity of the water is also found to affect the suitability of water for drinking and irrigation in some parts of the basin (IGES, 2006). 2. Materials and methods 2.1. Groundwater sampling A total of thirty (30) samples of water from shallow (< 10 m) dug wells constructed in the saprolite (n = 7) and deep tube wells (> 20 m) that drilled to fractures and other weak zones in crystalline rocks (n = 15) and from water reservoirs (n = 8) were collected covering the entire Malala Oya basin (Fig. 1). When collecting groundwater samples, pumping was carried out for 10 min by a submersible pump while shallow well samples were collected from the deepest layer using a depth sampler. 2

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Table 1 Geochemical composition of water resources from the Malala Oya basin (EC - Electrical Conductivity in μS/cm, Alk- Alkalinity, Hard – Hardness; GW- Groundwaters; SW- Surface water). units are in mg/L except for pH and EC. Sample No.

Well depth(m)

pH

EC

TDS

Alk

Hard

Cl−

SO42-

NO3-

PO43-

F−

NO2

Na

Mg

K

Ca

MO01 GW MO02 GW MO03 GW MO04 GW MO05 SW MO06 GW MP07 SW MO08 GW MO09 GW MO10 SW MO11 GW MO12 GW MO13 SW MO14 GW MO15 SW MO16 SW MO17 GW MO18 SW MO19 GW MO20 GW MO21 SW MO22 GW MO23 GW MO24 GW MO25 GW MO26 GW MO27 GW MO28 GW MO29 GW MO30 GW

38.4 29.0 3.6 7.6 12.0 54.0 38.5 30.0 10.5 8.3 9.4 15.7 36 32.0 29.5 33.0 30.0 35.0 30.0 9.0 9.0 12.0

7.46 7.41 6.53 7.15 6.42 7.32 8.02 6.58 7.03 7.42 7.08 7.20 7.48 7.97 8.15 7.72 8.17 7.20 7.42 6.61 7.89 7.01 7.46 7.23 7.82 7.32 6.82 7.20 7.42 6.97

849 1980 159 850 854 1665 902 1826 1395 517 2082 6.7 467 1099 739 1271 1369 272.7 3595 11110 271 1131 2717 5224 1890 1664 1972 2160 2106 2269

416 971 78.1 417 419 867 442 895 684 254 1028 3.31 229 540 362 623 671 134 1760 5442 133 555 1332 2562 926 816 966 1059 1032 1113

500 610 97.6 561 61 513 415 85.4 805 256 757 744 110 647 269 842 305 85.4 464 635 146 683 879 659 683 635 588 537 561 635

136 268 48 276 44 336 204 344 356 128 376 676 104 212 160 248 324 64 1980 328 148 248 1300 620 452 568 764 692 664 756

54 280 18 67 22 332 106 322 104 64 244 307 114 68 118 176 174 36 281 1286 38 60.5 432 940 272 215 336 410 370 414

42 75 126 25 54 56 43 63 65 30 235 690 31 57 55 130 170 32 124 1080 14 69 198 560 90 165 130 150 155 165

0.7 0.8 13 0.6 3.8 0.8 1.1 0.6 0.5 2.4 0.4 0.6 3.6 0.4 1.8 1.4 0.6 5.8 0.6 1.3 1.9 1.1 0.5 1.9 0.5 0.5 2.3 1.0 1.7 1.5

0.26 0.69 0.74 0.22 0.25 0.18 0.36 0.50 0.53 0.23 0.84 0.49 0.15 0.40 0.08 0.36 0.35 0.20 0.45 0.33 0.13 0.35 0.47 1.97 0.78 0.36 0.26 0.55 0.45 0.6

1.92 1.84 0.12 0.58 0.10 0.42 0.55 1.10 2.48 0.56 2.78 3.42 0.25 2.08 1.15 1.12 1.22 0.32 1.08 1.88 0.30 1.73 2.92 0.56 2.98 1.76 0.64 0.62 0.67 0.47

0.003 0.004 0.009 0.005 0.013 0.003 0.014 0.007 0.008 0.043 0.003 0.006 0.035 0.004 0.008 0.014 0.023 0.032 0.006 0.008 0.023 0.020 0.006 0.031 0.007 0.006 0.027 0.009 0.024 0.016

99.1 152 43.8 29.7 12.4 61.5 117 137 184 67.2 144 233 59.7 168 106 206 200 55.4 233 233 13.9 233 233 233 233 133 213 213 232 222

10.8 58.7 5.8 22.8 2.2 54.5 18.0 52.8 57.2 13.3 34.4 184 11.1 20.3 17.5 29.1 41.4 13.5 81.9 156 7.0 71.3 104 86.3 41.2 35.7 64.7 62.6 89.4 72.1

1.89 7.28 5.52 1.01 5.44 2.19 15.7 6.75 3.40 7.45 44.0 1.72 10.2 4.03 12.3 12.5 3.64 78.8 3.63 19.1 4.38 12.2 3.30 2.35 8.75 6.58 3.77 1.83 3.55 2.10

32.6 113 12.7 139 8.1 133 56.2 133 84.4 27.2 78.4 128 22.6 32.5 34.2 62.8 69.1 27.6 155 272 20.8 119 122 73.2 57.8 96.4 157 118 115 160

2.2. Field investigations

Sri Lanka. The International Atomic Energy Agency (IAEA) standard operating procedures were adapted for the preparation and quality controlling of the measurement. The tritium (3H) activity of groundwater samples was measured on a liquid scintillation counter (Packard 3170 TR/SL, minimum detection limit – 0.3 TU) after the electrolysis enrichment process. The isotope compositions were expressed in delta per mill (0/00) (δ) notation with respect to the Vienna Standard Mean Ocean Water (V-SMOW) where δ18O or δ2H = (Rsample−Rstandard)/ Rstandard) × 1000 where R denotes the ratio of 2H/1H or 18O/16O.

On-site measurements of pH, Total Dissolved Solids (TDS), Electrical Conductivity (EC), and salinity were performed using Orion STAR 329 multi-parameter kit. Water samples were collected into pre-cleaned high-density polyethylene bottles. Samples were filtered using 0.45 μm disposable nylon syringe filters and potions of samples were acidified with conc. HNO3 for cation analyses. Water samples were collected into 15 ml polypropylene centrifuge tubes and sealed with parafilm for isotope analyses. Unfiltered water samples were collected for tritium (3H) analysis. All samples were kept at 4 °C till the analyses were performed.

2.4. Data analyses Piper trilinear diagram, US salinity hazard diagram and Wilcox plot were used to assess the hydrogeochemical characters of groundwater. The molar ratios of major anions and cations were used to evaluate the contribution of cation exchange processes during the weathering of rocks. The percentage pollution of groundwater was calculated to determine the anthropogenic impacts to the groundwater. The irrigation suitability of groundwater was examined based on Sodium Absorption Ratio (SAR) and Percentage Sodium (%Na) as calculated by the formulas 1 and 2;

2.3. Laboratory analyses Major anions and cations, and metal elements in water samples were determined following the American Public Health Association procedures (APHA, 2012). Alkalinity, total hardness and chloride contents were measured using titrimetric methods while sulphates, phosphates, nitrite, nitrate and fluoride were measured using Hach DR 2700 spectrophotometer. Major cations (Na, K, Ca and Mg) were measured by the Atomic Absorption Spectrophotometer (AAS) (Varian 240FS) while other elements (Li, Be, Al, Co, Ni, Cu, Cr, Mn, Fe, Zn, As, Hg, Sr, Cd, Ba, Pb) were measured using Thermo ICapQ Inductivity Coupled Plasma Mass Spectrometry (ICP-MS). The internal drift of the instrument was corrected using 103Rh and 186Re as internal standards. The accuracy of the analysis was performed using the certified standards TM 25.4 (Environment, Canada) that measured to assure the quality of the analytical procedures. The recoveries were less than 1.0% for Cr, Mn, Co, Ni, Cu, As and Pb; within 1.0–3.0% for Zn, Sr, Cd and Hg; within 3.0–5.0% for Be, Ba and Al and within 5.0–8.0% for Li and Fe. Stable isotopes (18O and 2H) in samples were determined by Laserbased liquid-Water Isotope Analyser (LWIA; model LGR-DLT 100) available at the Isotope Hydrology Laboratory of Atomic Energy Board,

SAR =

Na+ Ca2 + + Mg 2 +

(1)

2

%Na =

(Na+

K+)

+ 100 Ca2+ + Mg 2+ + Na+ + K+

(2)

3. Results and discussion The maximum and minimum values of analyzed water quality parameters for 22 groundwater samples and 8 surface water samples are shown in Table 1. The pH of groundwater samples varied from 6.5 to 8.1 with an average of 7.2 while, it varied from 6.4 to 8.1 with an 3

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Table 2 Metal element content in water sources from Malala Oya basin (in μg/L). Elements

Li Be Al Cr Mn Fe Co Ni Cu Zn As Sr Cd Ba Hg Pb

Groundwater

Surface Water

Min.

Max.

Mean

Min.

Max.

Mean

0.210 0.002 7.680 0.018 2.477 12.93 0.074 0.829 0.592 8.107 0.074 92.80 0.055 18.48 0.001 0.120

46.6 0.21 625 3.80 2241 2981 2.85 9.06 432 241 0.65 2315 0.92 673 0.06 12.1

8.66 0.02 114 0.97 268 321 0.53 3.50 29.6 41.2 0.25 811 0.19 139 0.01 1.39

0.120 0.005 25.14 0.191 3.480 40.95 0.115 0.960 1.886 13.92 0.141 47.50 0.079 31.79 0.003 0.750

0.41 0.03 1253 2.10 74.0 368 0.80 11.5 11.0 210 1.03 607 1.08 281 0.09 3.06

0.30 0.01 291 1.27 30.0 150 0.40 4.50 4.64 60.6 0.44 287 0.35 139 0.02 1.72

Fig. 2. Variation of total hardness in groundwaters in the Malala Oya basin.

rich with fluoride bearing minerals such as micas and amphiboles (Dissanayake and Chandrajith, 1999). Dental fluorosis is one of the common health problems among communities in the dry zone of Sri Lanka due to drinking of water with high fluoride content (Chandrajith et al., 2012; Ranasinghe et al., 2019). Metals in groundwater are received greater attention in recent years due to their higher toxicity and tendency of bioaccumulation. Weathering of rocks, sewage discharge and leaching from other waste materials are the major sources of trace metals in water (Jain et al., 2010). Among studied metals, the iron (Fe) content in all groundwater samples of the basin was within the standard permissible level of 300 μg/L (WHO, 2008). The Mn level in the basin varied from 2.48 μg/L to 2242 μg/L with a mean of 268 μg/L. Twenty three percent (23%) of groundwater samples in the basin exceeded the permissible level of Mn that is recommended for drinking water. Mn usually exists at low concentration in groundwater due to its geochemical control (Jain et al., 2010). Adverse effects on water supply structures and offensive taste in drinking water are possible impacts of high Mn levels in groundwater. The speciation of Fe and Mn is influenced by the redox characteristics of groundwater (Biswas et al., 2012; Hasan et al., 2007). Iron content in groundwater may induced by reduction of Mn in the aqueous system leading to precipitation of Fe-oxyhydroxides (Appelo and Postma, 2005). Dissolved organic matter (DOC) in groundwater would also further facilitate the reductive dissolution processes that affect the content of dissolved Fe in groundwater system (Bhattacharya et al., 2002). The Al content in groundwater samples in the basin varied from 7.68 μg/L to 625 μg/L with a mean value of 114 μg/L. The presence of high Al levels in drinking water can induce a neurotoxic effect (Rajasooriyar et al., 2013). The Sr content of groundwater samples varied from 93 μg/L to 2315 μg/L with a median of 811 μg/L while Ba contents varied from 18.5 μg/L to 673 μg/L with a median value of 139 μg/L. Lead (Pb) content in the basin varied from 0.12 μg/L to 12.1 μg/L with a mean value of 1.39 μg/L. Only one deep well showed a slightly higher Pb content (12.1 μg/L) than the WHO recommended value of 10 μg/L. The arsenic (As) and cadmium (Cd) contents in groundwater of the basin varied from 0.074 μg/L to 0.65 μg/L and from 0.055 μg/L to 0.92 μg/L, respectively. Both As and Cd contents in groundwater of the study area were several fold lower than the permissible levels. The contents of other elements such as Ni, Zn, Cu, Cr, and Hg in both groundwater and surface water from the Malala Oya basin were lower than the WHO recommended limits. Mercury is another important toxic element that has mainly of anthropogenic origin (Chandrajith et al., 1995; Matsunaga et al., 1999) however, in the Malala Oya basin Hg levels recorded is very low and that varied from 0.001 to 0.056 μg/L.

average of 7.5 in surface water samples. The alkalinity of groundwater ranged from 85.4 to 879 mg/L with an average value of 572 mg/L. Alkalinity of surface water samples ranged from 61 to 842 mg/L. Almost all groundwater samples, except two exceeded the recommended alkalinity level for drinking purposes. This may possibly due to dissolution of carbonates and phosphates minerals in groundwater. Anthropogenic activities such as cleaning and washing could also contribute to high alkalinity in surface water. The Electrical Conductivity (EC) in groundwater samples varied from 6.7 to 11110 μS/cm. An exceptionally high EC value was recorded in a deep well sample (MO-20). Over 60% of studied wells exceeded the permissible levels of EC for drinking purposes recommended by the Sri Lanka Standard Institute (SLSI). The total hardness of groundwater varied from 48 to 1980 mg/L while significantly higher values (1300 mg/L) were recorded in a deep well and also in a shallow well (1980 mg/L). In the study area, over 90% of groundwater samples exceeded the recommended maximum permissible level for total hardness for drinking purposes. Collected water samples were classified into different categories based on the total hardness (Fig. 2). Accordingly, most groundwater samples collected from the study basin could be categorized under very hard category (> 300 mg/L). The total hardness of surface water samples varied from 44 to 248 mg/L. Drinking hard water with high fluoride contents is suspected be a causative factor for high incidences of chronic kidney disease with uncertain etiology in the dry zone of Sri Lanka (Wickramarathna et al., 2017). The Cl− and SO42- content in groundwater samples varied between 18 to 432 mg/L and 25–690 mg/L, respectively. Nearly 50% of groundwater samples exceeded the permissible level of Cl− for drinking purposes. However, only three groundwater samples exceeded the maximum permissible level for SO42-. Leachates from fertilizers from paddy fields would add SO42- into both surface and groundwater. The nitrate-N of groundwater in the study area varied from 0.4 to 13 mg/L from which one shallow well showed significantly high nitrate-N level (13 mg/L). The nitrite concentrations of groundwater samples varied from 0.003 to 0.030 mg/L. The sources of nitrite and nitrate inputs to groundwater are N-fertilizers, improper sewage discharges or leachates from pit latrines that are usually located in the overburden. The phosphate content of groundwater varied between 0.18 - 0.84 mg/L. Only one deep well (MO24) showed a significantly higher phosphate level (1.97 mg/L). The fluoride content of groundwater in the region varied from 0.12 to 3.42 mg/L. About 80% of groundwater samples in the Malala Oya basin exceeded 0.5 mg/L level that considered as the optimal level of fluoride in drinking water for Sri Lanka (Chandrajith et al., 2012). The source of fluoride in groundwater is mainly due to leaching from metamorphic aquifer materials that are 4

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Fig. 3. Piper triliner plot for the Malala Oya basin.

3.1. Geochemical evolution of groundwater The Piper trilinear diagram is one of the widely used tools for identifying hydrogeochemical evolution of groundwater based on major cations and anions (Ray and Mukherjee, 2008). Accordingly, water are classified into 6 major types as Na–Cl type, Ca–HCO3 type, Ca–Na–HCO3 type, Ca–Mg–Cl type, Na–HCO3 type and Ca–Cl type (Kumar, 2013). Based on the results obtained in this study, the prominent water facies in the basin was Ca–Mg–Cl type (Fig. 3). Majority (53%) of deep well samples showed Ca–Mg–Cl type while some wells were associated with Ca–HCO3- and Ca–Na–HCO3- types. 86% shallow wells belonged to either Na–Cl or Ca–Na–HCO3- facies. The results suggest a significant contribution of weathering of mica, amphibole and pyroxenes in the high-grade metamorphic rocks in the evolution of groundwater types in the basin as shown below; Dissolution of pyroxenes;

Fig. 4. Major ion relationships of groundwater in the study area.

silicate minerals are the major contributors of Ca2++ Mg2+ and HCO3of groundwater in the basin. Since there is no evidences for saline water intrusion and also absence of geothermal activities in the area, silicate weathering is primarily responsible for major ions in the study groundwater. 3.2. Suitability of groundwater for irrigation The suitability of groundwater for irrigation is an important consideration in dry climatic regions of Sri Lanka since it is the main source of irrigation water. Higher levels of dissolved ions in irrigated water leads to physical and chemical damages of crops and soil (Ravikumar et al., 2011). A plot of sodium hazard vs. salinity hazard was used widely to evaluate the irrigation suitability of groundwater (Fig. 5). In the study region, most groundwater samples were scattered in the lower sodium and high salinity hazard zone. Groundwater belonging to these groups can be used for irrigation activities with proper salinity control (Anudu et al., 2011). Similar results were also obtained from the Wilcox diagram that plot %Na against EC (Fig. 6). Accordingly, groundwater from the study area plotted in good to permissible and also in doubtful to unsuitable regions. Nearly 36% of groundwater samples in the study area fall in good to permissible regions while 23% of samples were in doubt for unsuitable regions.

CaMg(Si2O6) + 4CO2 + 6H2O → Ca2+ + Mg2+ +4HCO3- + 2Si(OH)4 Dissolution of amphiboles; Ca2Mg5Si8O22(OH)2 + + 14HCO3- + 8Si(OH)4

14CO2

+22H2O→

2Ca2+

+

5Mg2+

The relationships between major ions are generally used to identify the evolution of groundwater and their mixing mechanisms (Thilakerathne et al., 2015). The molar ratio of Na+/Cl− is widely used to evaluate silicate weathering, seawater intrusion, halite dissolution and ion exchange processes (Rubasinghe et al., 2015). The Na+/Cl− ratio of samples collected from the Malala Oya basin indicated that samples were scattered alone the 1:1 line (Fig. 4a). The Na+/Cl− molar ratio, which is greater than ≥1, represents Na+ is released to the groundwater by silicate weathering (Rubasinghe et al., 2015). The Ca/ Mg molar ratio of all groundwater samples in the basin scattered very close to or below the 1:1 line (Fig. 4b). Usually the Ca/Mg ratio between 1 and 2 indicates calcite dissolution process while the molar ratio > 2 indicates the silicate mineral dissolution processes (Thilakerathne et al., 2015). The plot of (Ca2++Mg2+) vs (HCO3+ Cl− + SO42-) showed a linear relationship (Fig. 4c). Samples from the study area were scattered between 1:1 and 2:1 lines in the plot of HCO3- vs. Ca2+ + Mg2+ (Fig. 4d). The groundwater samples that plotted above the 1:1 line indicate the weathering of silicate minerals (Abeywickarama et al., 2016). These results suggest that weathering of

3.3. Isotope composition of groundwater Environmental isotopes of δ18O and δ2H in ground- and surface water are important traces that can be used to evaluate hydrological processes such as infiltration, evaporation, surface water-groundwater interactions and rock weathering (Clark and Fritz, 2013). The δ18O and δ2H of groundwater in the Malala Oya river basin varied from -8.37‰ to -1.64‰ and from -54.70‰ to -9.93‰, respectively. The surface water samples showed δ18O and δ2H values of -5.35‰ to -3.01‰ and -34.28‰ to -17.08‰, respectively (Table 3) (Fig. 7). Groundwater isotopes in the region were plotted on the regression line (GWL) denoted by δ2H (‰) = 6.97δ18O (‰) + 5.53 (r2 = 0.915). The Local 5

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Table 3 Isotope compositions of waters in the Malala Oya basin.

Fig. 5. US Salinity Laboratory's diagram for the classification of Salinity Hazard in groundwater from the Malala Oya Basin.

Sample no.

δ2H (‰)

δ18O (‰)

3

MO01 MO02 MO03 MO04 MO06 MO07 MO08 MO09 MO10 MO11 MO12 MO14 MO16 MO17 MO19 MO20 MO21 MO22 MO23 MO24 MO25 MO26 MO27 MO28 MO29 MO30

-31.2 -29.2 -54.7 -16.1 -29.5 -22.8 -33.0 -34.8 -34.3 -32.5 -31.3 -12.6 -17.1 -33.9 -23.7 -36.7 -25.4 -17.8 -31.7 -51.3 -9.93 -26.5 -23.0 -22.0 -23.3 -22.5

-4.73 -4.66 -8.37 -2.13 -4.64 -3.18 -5.80 -5.75 -5.35 -5.42 -4.95 -1.64 -3.01 -5.37 -3.38 -5.36 -3.87 -3.92 -5.93 -7.87 -2.06 -5.29 -4.05 -4.03 -4.11 -3.98

_ 0.25 _ _ _ _ _ _ _ 0.21 _ _ _ _ _ _ _ _ _ _ _ 0.17 _ _ _ _

H (TU)

Fig. 6. A plot of percentage sodium (%Na) and electrical conductivity (EC) for classification of the groundwaters of Malala Oya basin.

Meteoric Water Line (LMWL) for the southern dry zone was estimated using the data available at the International Atomic Energy Agency (http://isohis.iaea.org) for Hambantota meteorological station that located close to the Malala Oya basin (Fig. 1). The LMWL for the study basin is represented by δ2H (‰) = 8.01δ18O (‰) + 12 (r2 = 0.848). The isotope composition of surface water from the basin represent the evaporation line (EL) of δ2H (‰) = 6.48δ18O (‰) + 0.09. Deep wells sample in the study region were plotted along the LMWL indicating a direct recharge from the monsoon rain. However, the deviation of shallow wells from the LMWL indicates the intensive evaporation under prevailing dry conditions in the basin. The confluence point of GWL and LMWL represents the rainfall index of the northeast monsoon. The average isotopic composition of the northeast monsoon rain is -6.01 for δ18O and -36.7 for δ2H (Edirisinghe et al., 2017). These evidences suggest that the main source of groundwater in the basin is the monsoon rain. However, it is evident that isotope composition is modified due to high rate of evaporation particularly in the shallow regolith aquifers. The enriched isotope composition of shallow groundwater could have been attributed to mixing with surface water. Two groundwater samples (MO03 and MO24) showed the most depleted isotope value with high EC (5224 μS/cm) value. The most depleted isotope compositions can be expected from higher elevation recharge (Poage and Chamberlain, 2001). Thus, deep groundwater would have recharged from rains in the highland regions located adjacent to

Fig. 7. Comparison of isotopic compositions (δ18O vs. δ2H) of groundwaters and surface water of the Malala Oya basin (GMWL: Global meteoric water line; LMWL: Local meteoric water line; EL- Evaporation line).

the study area. The high EC values in these deep groundwater samples would be due to the dissolution of minerals in its flow path. To evaluate approximate resident time of groundwater, tritium (3H) with 12.32 years of half-life was adapted in this study. Three deep wells were selected to measure the tritium contents. The observed 3H values were below the detection limit of the counter (< 0.3 TU). The mean 3H value in rainwater in Colombo is approximately 1 TU during 2009–2011 period (Chandrajith et al., 2014). The observed tritium values of deep groundwater samples indicate a prolonged retention time with very low recharge in comparison to tritium values of the rain water. 4. Conclusions In this study, geochemical and environmental isotope evidences 6

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were used to characterize water resources in the Malala Oya river basin. Higher EC and hardness were the main characteristics of the Malala Oya river basin. Over 90% of groundwater samples exceeded the recommended levels of alkalinity (200 mg/L) while nearly 50% groundwater samples exceeding recommended Cl− level of drinking water. Nitrate-N and phosphate concentrations were within the acceptable limits in majority of samples. However, the fluoride concentrations of groundwater were significantly higher and probably causing adverse health effects. The dominant water types in the basin were in the order of Ca–Mg–Cl > Ca–HCO3 > Ca–Na–HCO3 > Na–Cl. The results of this study indicate that silicate weathering is the leading process that controls the groundwater geochemistry while significant contributions are also made from ion exchange processes and carbonate dissolution. Irrigation suitability calculations of groundwater indicated a low sodium hazard condition in the Malala basin. Isotope of oxygen and hydrogen of surface water indicated effects of intensive evaporation while most shallow and deep water is directly recharged from local rainfall, however, some of the deep groundwater is being recharged by meteoric water that seep through hard rock fracture systems, possibly from nearby highland region.

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