Hydrogeochemical and isotopic signatures of groundwater in the Andasa watershed, Upper Blue Nile basin, Northwestern Ethiopia

Journal of African Earth Sciences 160 (2019) 103617

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Hydrogeochemical and isotopic signatures of groundwater in the Andasa watershed, Upper Blue Nile basin, Northwestern Ethiopia

T

Getnet Taye Bawoke∗, Zelalem Leyew Anteneh, Alebachew Tareke Kehali, Mohammed Seid Mohammedyasin, Gashaw Wudie Department of Geology, School of Earth Sciences, Bahir Dar University, Bahir Dar, Ethiopia

A R T I C LE I N FO

A B S T R A C T

Keywords: Groundwater evolution Rock-water interaction Hydrogeochemical facies Anthropogenic pollution Andasa watershed

This study aims to evaluate the hydrogeochemical and isotopic signatures of water (surface and groundwater) of the Andasa watershed in the upper Blue Nile Basin, northwest Ethiopia using integrated hydrogeochemical and isotopic methods with the help of Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) techniques. To achieve this, major cation and anion (n = 61) and stable isotope (δ2H and δ18O; n = 36) analyses have been done. The result revealed that geogenic (rock-water interaction) and anthropogenic (waste disposal landfill site, agricultural practices, diverted river via open canal) effects are the major water quality controlling factors. Rock-water interaction is more dominant factor that produced alkaline earths bicarbonate hydrogeochemical facies. Moreover, the Cl and δ18O relationship shows a shift towards the right from vertical axis (δ18O) which signifies anthropogenic effects. Using HCA and PCA techniques, EC, TDS, Ca, Na, Mg, HCO3, SO4, Cl and NO3 were identified as the leading hydrogeochemical parameters in determining the area. Accordingly, Mg-Ca-HCO3, Ca-Mg-HCO3 and Ca-Na-HCO3 found to be the main water types. Silicate minerals are responsible for controlling the hydrogeochemical property of the waters in the area through hydrolysis and cation exchange processes. Most of the samples signify evaporation prior to recharge in the wet seasons and irrigation return flow in the dry seasons. Most river samples show depleted δ18O and δ2H reflecting that their source is highland area of the watershed. From hydrogeochemical and isotopic signatures, the north and northeast parts of the area are the discharging zones and southern and central parts are the recharging zones.

1. Introduction Hydrogeochemical and isotopic studies are imperative in resolving different groundwater related problems (e.g., Fontes and Edmunds, 1989; Gat, 1996; Mazor, 2004; Appelo and Postma, 2005; Edmunds et al., 2006; Ma et al., 2007; Prasanna et al., 2010a, 2010b; Clark and Fritz, 2013). Since groundwater is concealed from physical observations in most of its natural existence, it is difficult to understand the different processes and changes in the hydrologic system (Mazor, 2004). The secret of groundwater in relation to its chemical reactions, origin, recharge mechanisms, interaction with surface water, resident time and extent of pollution can be portrayed with the aid of geochemical and stable isotope techniques (Mazor, 2004; Appelo and Postma, 2005). The isotopic fractionation of oxygen and hydrogen help to trace the recharge and discharge zones of groundwater (Gat, 1996; Clark and Fritz, 2013). Even though diversified use of surface and groundwater resources are highly increased in Ethiopia, hydrogeochemical and isotopic studies ∗

are at their infant stages. There is also less awareness about effects of human induced pollution and well fields are not protected from pollution in the country (Demlie and Wohnlich, 2006; Demlie et al., 2007; Kebede, 2013). Groundwater is easily fragile resource threatened by pollution and over exploitation (Freez and Cherry, 1979; Ayenew, 2005; Kalaivanan et al., 2017). Unregulated growth of urban areas without infrastructural services for proper collection, transportation, treatment and disposal of domestic waste water led to increased pollution and health hazards (Jeong, 2001; Sharp et al., 2003; Ayenew, 2005; Demlie and Wohnlich, 2006; Naik et al., 2008; Misra, 2011; Kebede, 2013). Upper Blue Nile (Abay) river is diverted via open canal for irrigation in north and northeastern part of the research area typically surrounding one of the satellite towns of Bahir Dar, Tis Abay. However, its effect is not considered to the existing groundwater in relation to water logging, irrigation return flow and recharging the highly porous quaternary aquifer type in the area. Bahir Dar city open waste disposal landfill is also found in the area which may bring considerable pollution

Corresponding author. E-mail address: [email protected] (G.T. Bawoke).

https://doi.org/10.1016/j.jafrearsci.2019.103617 Received 26 August 2018; Received in revised form 28 August 2019; Accepted 28 August 2019 Available online 29 August 2019 1464-343X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Study area relative location from Eastern Nile Sub basins including elevation in meter. The inset map at the top right corner shows part of Nile: Eastern Nile sub basins (Blue Nile, Main Nile, Tekeze-Setit-Atbara and Barro-akobo-Sobat) and Ethiopia. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

and groundwater resources from anthropogenic pollution. Besides the open waste disposal landfill site situated within the study area, majority of the watershed is urban area hence an integrated hydrogeochemical and isotopic study have significant roles on scientific understanding of the hydrogeochemical evolution and help for water resources planning and management options. In the study area, there are no comprehensive hydrogeochemical and isotopic studies except those conducted in nearby basins (e.g. Kebede, 2005; Mamo, 2015; Abiy et al., 2016; Nigate et al., 2016; Enku et al., 2017). Hence, this work tries to assess the groundwater evolution, rock-water interaction and anthropogenic effects, and determines the recharge-discharge zones using an integrated hydrogeochemical

challenge to existing ecosystem. The Andasa watershed is one of the important sheds in the upper Blue Nile Basin in which urban expansion and farming practices are taking place and known for its huge groundwater (Asrat, 2017) as well as surface water potential. The area has variable topographic setting ranging from 1585 to 3209 m and lithological complexes of Oligocene-Miocene flood basaltic units, quaternary basalts and recent quaternary deposits (Figs. 1 and 2). Water resources usage in the area is significant, but natural as well as human induced activities on groundwater composition are not deciphered. Bahir Dar city (capital of Amhara National Regional State) is expanding and 66% of the area is included within the city's administration. Irrespective of the city's expansion little has been done to protect surface 2

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Fig. 2. Geological map of the study area and locations of collected water sample sites. The geological units are modified after Asrat (2017) and BECOM (1998). The geological structures are extracted from satellite images by the authors.

2017). The rainfall of the area shows bimodal pattern with minimal rainfall during spring and maximum rainfall in summer periods. The landscape lies between elevation ranges of relatively gentle and low lands (1585 m a.s.l) to hilly and rugged relief types (3209 m a.s.l). The elevated and rugged terrains are found in southern parts of the study area where as low lands are found in the northern parts with some pockets of river incision and structurally collapsed grabbens occupied by quaternary deposits. The area is characterized by geological structures such as faults, fractures and joints which are manifestations of tectonic activities (Asrat, 2017).

and isotopic approaches in the Andasa watershed, northwest Ethiopia (Fig. 1).

2. Study area description The study area is situated at the head of upper Blue Nile Basin, between 309785 and 348444 m east and 1233162–1282834 m north UTM, Northwestern Ethiopia, about 560 km far from Addis Ababa (Fig. 1). The watershed has 828.26 km2 total areal coverage which consists of three administrative districts: Bahir Dar Zuria, Yilmana Densa and Mecha. Of which Bahir Dar Zuria shares the largest areal proportion (66%), followed by Mecha (23%) and Yilmana Densa (11%). Bahir Dar city and other small satellite towns such as Meshenti, Andasa, Tis Abay and Debre Mawi also found within the study area which are highly dependent on groundwater. Additionally, there are several rural communities who are dependent on groundwater for their domestic water supply, livestock and sanitation uses. Unaccounted small household irrigations are held in the watershed following the main river system. As stated by Nigate et al. (2016) the diverted surface water through open canal supports the livelihood of surrounding local small satellite towns (Tis Abay and Andasa) for vegetables and planting Khat (Catha edulis). This is also evidenced and supported by intensive field visits, continuous close observations and communication with local people via focal group discussions. Inter Tropical Convergence Zone (ITCZ) position is responsible for variable rainfall in Ethiopia as a whole. Specifically, when ITCZ lies on the north part of the country, extended rainfall happened in the study area. This season is called summer (Kiremt) in which both south Atlantic Ocean equatorial westerlies takes the largest contribution for more than half of the country. Southerly and easterly winds from Indian Ocean contribute almost to all parts of the country to be under rainfall (Chernet, 1993; Alemayehu, 2006; Berhanu et al., 2014; Gashaw et al.,

3. Geological and hydrogeological settings 3.1. Geological and stratigraphic setting The Ethiopian flood basalts were previously classified into Ashengie, Aiba, Alaj and Tarmaber Formations (Mohr, 1983; Chernet, 1985). Currently, based on geochronological studies the former representations are equivalently used by lower basalt (Tv1), middle basalt (Tv2), upper basalt (Tv3) and uppermost basalt (Tv4) units (Haro et al., 2011; Hailemariam et al., 2012; Asrat, 2017). The lower basalt is composed of porphyritic texture with pyroxene, plagioclase and olivine as the most common phenocrysts. The middle basalt, forming gentle slope in the Andasa watershed is characterized by phyric basalts of pyroxene-plagioclase, plagioclase, and occasional olivine-pyroxeneplagioclase, and olivine. The upper basalt which reaches up to 650 m thickness in the study area is showing pyroclastic nature towards its top part. It is mainly characterized by columnar basaltic units (Asrat, 2017). In general, the mineralogical composition of Ethiopian flood basalts is almost uniform dominated with plagioclase, pyroxene and with or without olivine (Kieffer et al., 2004). More than 80% of upper Blue Nile (Abay) Basin in Ethiopia is 3

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covered with Oligocene-Miocene flood basalts and associated recent quaternary deposits and about 40% of the whole land mass of the country is covered with tertiary volcanics (Alemayehu, 2006). Cenozoic volcanic rocks in the basin forming prominent plateau with altitudinal variations between 2000 and 2500 m a.s.l and associated many shield volcanoes reaching above 4000 m a.s.l. The Cenozoic volcanics consisted Oligocene Miocene flood basalts of trap series, quaternary flood basalts, scoria cones and craters, alluvial quaternary deposits (eluvial, fluvial and alluvial). The Blue Nile river deeply cut off these thick geological successions and gives a chance for exposures of high-to-low grade metamorphic rocks, intrusive igneous units (mafic and felsic) and Paleozoic-Mesozoic outcrops (sandstones, evaporites, limestones). On top, the youngest volcanic plugs (trachyte and rhyolitic), quaternary flood basalts, scoria cones and ongoing recent quaternary deposits (filling river valleys, flat and gentle topographic plateau surfaces, and basaltic foothills) are prominent features of the Blue Nile basin (Alemayehu, 2006; Asrat, 2017). Quaternary flow basalts are scoraceous dark-gray to greenish-grey which are strongly vesicular composed of olivine phyric, pyroxeneplagioclase phyric, thin basalt flows; zeolite-rich strongly vesiculated basalts; basalt lava flows separated by basaltic agglomerate and basaltic breccia. Scoria cones are gray, brown to pink, vesicular to scoriaceous olivine, pyroxene-plagioclase phyric basaltic cones; commonly horizontally stratified scoria falls. The last unit, quaternary deposit, is found at depressed-low-lying areas, incised-bottom valleys, hills foot, marshy and flood plains of the area. The thickness reaches 1.5–2 m represented by black cotton and clayey soils. Fluvial sediments constitute clayey silt, sandy silt and silty clay materials, at places mixed with pebbles, cobbles and boulders of basalt (TCE, 2009; Beshawered et al., 2010; Zewdie and Yoseph, 2012; Asrat, 2017). Thick black, brown and reddish-brown soils are commonly believed to be evolved from sand, silt and clay bearing clasts of basalt, rhyolite and scoraceous materials. The Andasa watershed comprises the above units, i.e., lower basalt, middle basalt, upper basalt, quaternary flow basalt, scoria cones and recent quaternary deposits (Fig. 2).

found forming variable landscapes. The lower basalt (basal sequence) is exposed extensively in the eastern, northeastern and central parts of the area making undulating and gently slopped topography (Fig. 1) covering 334 km2 areas; characterized by high degree of weathering. NWSE aligned dyke swarms cut the unit and control the groundwater and surface water flow directions. There are several structurally controlled high yield springs towards northern and northeastern directions (Asrat, 2017). These springs are found in the lower flood basalts (exposed up to 500 m thickness) and characterized by low permeability in the watershed (Asrat, 2017). The study area is described by two most groups of aquifer systems which are extensive aquifers of intergranular permeability and extensive aquifers with fracture permeability. Bicarbonate is dominant natural inorganic anion for groundwaters found in the northwestern and southeastern plateau whereas in sedimentary terrains sulfate occurred in addition to bicarbonate (Alemayehu, 2006). Evidence showed volcanics of the plateau has CaMg-HCO3 and Na-Ca-HCO3 later evolve to Na-HCO3 water type in the rift valley of the country with substantial increasing of salinity from plateau to rift valley. Good to excellent water quality is also common to high plateau of the country and the rift valley is confined with high levels of Na, HCO3 and F ions with some localized exceptions due to geological processes (Appelgren et al., 2000; Kebede, 2005; Alemayehu, 2006; Ayenew et al., 2008; Mamo, 2015; Nigate et al., 2016). 4. Methods A total of 61 water samples from different sources: surface water (n = 10), groundwater (n = 35) and spring (n = 16) were collected for hydrogeochemical analysis. Standard polyethylene plastic bottles were washed with deionized water and dried up for storing water samples and emphasis was given to avoid contamination in all conditions. In situ measurement of pH, EC, TDS and temperature parameters were done with portable water analyzer kits, calibrated properly before use and checked each day with a standard solution. Duplicated samples were collected and checked in laboratory for data QA/QC purpose. The ions (Ca2+, Mg2+, Na+, K+, HCO3−, NO32−, SO42−, F−, Cl−, Fe, B, PO42), total Alkalinity, total hardness and turbidity were analyzed in water, soil and geotechnics laboratory of Amhara Design and Supervision Works Enterprise (ADSWE) following APHA (2005) guidelines. Ca2+, Mg2+ Fe, B− and Mn were measured using Atomic absorption spectrophotometry (AAS) whereas Na+ and K+ concentrations were evaluated using flame photometer emission and absorption methods. Titration procedures were applied to measure HCO3−, CO32−, SO42− ions even though CO32− is almost nil in almost all samples. Using colorimetric methods, Total Alkalinity, Total Hardness, PO42−, F−, NO32− and Cl− ionic concentrations were measured. Turbidity was measured using turbidity meter. Electrical methods were applied in some selected samples for further checkup of pH, EC and TDS in addition to field measurements. The accuracy of laboratory results were checked using cation-anion equilibrium relationship empirically in meq/liter using Eq. (1):

3.2. Hydrogeological setting Volcanics found in highland and in the rift are the main groundwater sources which are essential for rural and urban societies (Demlie et al., 2008). The volcanic rocks in central Ethiopia are also the main sources and accessible aquifers because of their stratigraphic superimposing (Kebede, 2013). Groundwater occurrence in the Cenozoic volcanic rocks in Ethiopia is along fractured, flow contacts and weathering profiles. Though the Miocene to Quaternary sediments areal coverage is less extensive, they form loose sediments of higher primary porosity leading to the largest groundwater storage in Ethiopia (Alemayehu, 2006; Kebede, 2013). Based on aquifers property of groundwater occurrence, flow, recharge and geomorphic features, four hydrostratigraphic units are recognized for the whole volcanic province of the Ethiopian plateau (Chernet, 1993); Basal sequence, Upper sequence, Shields and Quaternary basalts. Basal sequence which is equivalent to the traditional name “Ashengie basalts” is forming the base of all the flood basalts, highly weathered and perceived as low permeability. The next major classification, upper sequence is equivalent to the former classification names of “Aiba, “Alaji” and “Tarmaber” formations. This unit is slightly weathered and fall under permeable and relatively higher productive aquifer. The third formations are shields encompassing composite stratigraphy of ashes, rhyolites, trachytes and basalts. High yield springs emergence is distinctive property of this unit (Kebede, 2013). The last hydrostratigraphic formation, quaternary basalts, are scoraceous basaltic units having thinly bedded and central volcano related features. They are considered as high productive aquifers (Kebede, 2013). In Andasa watershed, all the above listed lithological formations are

Electron balance (%) =

∑ Ycations − Xanions ∑ Ycations + Xanions

(1)

where Y and X are charges on cation and an anion. The charge balance error should be less than 5% and samples up to 10% charge balance error have been considered for analysis and interpretation in different studies (Singhal and Gupta, 2010; Piña et al., 2018). Except three samples, all the water samples are showing electron balance error less than 10%. The remaining 61 samples were used for hydrogeochemical analysis. For some secondary water samples which did not have TDS values (mg/l) but do have EC (uS/cm) measurements, the following relationship is adopted from Hiscock and Bense (2014) with ke (correlation factor) varied between 0.5 and 0.8. This factor is found to be 0.6 based on the inputs from field collected samples (Eq. (2)): 4

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(2)

TDS = k e*EC

where, δ ( 0 00 ) is the deviation for deuterium (D) and O; Rsample refers 18 O/16O or 2H/1H water samples and Rstandard is the corresponding values in VSMOW. Global Network of Isotopes in Precipitation (GNIP) database for Addis Ababa is used as a proxy for precipitation data. A 24 year (1990–2014) stable isotopic data of δ18O and δ2H were used for Local Meteoric Water Line (LMWL) with equation (δ2H = 7.08 δ18O + 12, R2 = 0.92). Despite the considerable distance between Addis and the study area, similarity in topography (1600–3200 m for Andasa and Addis) and sharing mono-modal (between June to September) rainfall distribution helps to use GNIP data.

for spring (SP8) with a mean value of 6.56. Minimum, maximum, and mean values of EC are 137, 5610, 657.61 in μS/cm and TDS 65, 3647, 412 in mg/l recorded, respectively. The highest TDS and EC values are observed in northern part of the area (SP1) and fall under G1, brackish water class, based on TDS value. Most of the samples are drop within a range of fresh water class in their TDS and EC contents. Though average values are within WHO (2011) permissible limits, there are 12 samples associated with TDS and 10 samples related to EC in which their values are out of the allowable limit. The samples that disclosed higher EC values (> 1000 mg/l) are two springs (SP1and SPs1) which are located in the same place but sampled in different times. 8 borehole samples (SB6, BHs1, BHs3, BHs4, BHs5, BHs12, BHs14, BHs20) found in different parts of the watershed also shown elevated EC. However, all samples which are unfit for drinking in terms of EC values are also above the permissible limit of WHO (2011) in TDS standards too (> 500 mg/l) in addition to BH3 and BHs8. There are no surface water samples which display higher values from WHO (2011) standards for EC and TDS parameters. This study is similar with Ayenew (2005) stated as highland area surface waters (rivers, lakes) found to be fresh together with the surrounding shallow circulating meteoric groundwater recharge systems. Ca values are deviated from WHO (2011) allowable limit in SP1 (114.8 mg/l) and BHs12 (198.4 mg/l). SP1 and SPs1 samples are above permissible values for Mg. Na in SPs, and K in SP1, BHs8, BHs9, BHs10, BHs12 and BHs20 are deviated from the standards of WHO (2011). In most of the samples (n = 44), HCO3 lied above the permissible limits of WHO (2011). From 10 river samples analyzed, 8 samples are failing to fulfill WHO (2011) standard in HCO3. The minimum, maximum and mean values for HCO3 are found to be 71.5, 3550 and 268.69 mg/l in their order. The rest anions (chloride, sulfate, Nitrate and Fluoride) are within the standard. Isotopic results (δ2H) show minimum, maximum and mean values of −10.3, 4.7 and 0.5 respectively, whereas, −3.94, 0.52 and 0.76 are recorded as minimum, maximum and mean values for values for (δ18O) respectively (Table 1). Using the static water level, the groundwater contour (water table map) has been produced and thereby groundwater flow direction is generated (Fig. 3). This map is discussed with the spatial evolution of hydrogeochemical parameters.

5. Results

5.2. Hierarchical Clustering Analysis (HCA)

5.1. Statistical analysis of physicochemical and isotopic signatures

Based on similarities that have been obtained from physicochemical variables, samples are grouped in to four major categories and each of the main groups are further clustered in to sub groups (Fig. 4). Dendrogram plot which is useful in tracing HCA based on the Ward's cluster and square Euclidian distance method is employed (Belkhiri and Mouni, 2012). Accordingly, four major hierarchical taxonomies and sub assemblages have been produced for the total of 61 samples based on a reference line (phenon line) with a rescaled distance of 6.5. A complete record of data having pH, EC, TDS parameters, major cations of Ca2+, Mg2+, Na+, K+ and major anions of HCO3−, Cl−. SO4−-, NO3−, and F− were considered (Fig. 4; Table 2). Table 2 shows the 4 major clustered groups and their corresponding sub categories. The first major group (G1) is possibly clustered based on elevated values for most parameters (EC, TDS, Ca, Mg, Na, K, HCO3) out of WHO (2011) guideline and dominant Mg-Na-HCO3 water type (Tables 2 and 5). Increasing orders of Cations are Na > Mg > Ca > K and that of anions are HCO3 > Cl > SO4 > NO3. The second major group (G2) contains 8 total samples. Most of the mean values of the parameters in G2 are found within the limit of WHO (2011) except HCO3 and some maximum values of EC, TDS and K. Cations and anions dominating this group are: Ca > Na > Mg > K and HCO3 > NO3 > Cl > SO4 respectively. These samples are characterized by existence of deep boreholes and springs found around discharge zones and dominancy of Na ion over Ca ion in the boreholes. NO3 is also

Using different scatter plots and ratio diagrams, overall groundwater evolution has been traced from recharge to discharge areas. Standard diagrams like Piper, Gibbs and Chadha employed for the interpretation of major cations and anions for their source deduction (rock-water interaction, cation exchange, evaporation, anthropogenic pollution). Moreover, Principal Component Analysis (PCA; dimension reduction to uncorrelated axes by different exploring loading factors) and Hierarchical Clustering Analysis (HCA; data clustering to groups) were employed for determination of dominant hydrogeochemical parameters in the samples and groundwater evolution analysis. The water samples for stable isotopes of oxygen and deuterium (δ18O and δ2H) for water samples were collected with 500 ml polyethylene bottles which were rinsed ahead of sampling. A total of 36 samples from surface water (n = 8), groundwater (n = 18) and spring (n = 10) were used for deuterium and Oxygen-18 isotopic composition analysis. The laboratory analysis was carried out in Addis Ababa University, School of Earth Sciences using liquid water isotope analyzer (LGR). The isotopic composition of water is commonly expressed in per mill (‰) as deviation from the standard mean ocean water (Craig, 1961); the standard solution is later modified as Viennan Standard Mean Ocean Water (VSMOW). The laboratory results are expressed in concentrations per mil (‰) relative to VSMOW as expressed in Eq. (3):

δ (‰) = (

R sample R standard

− 1 )* 1000

(3) 18

The chemical and isotopic compositions of water samples from wells (hand dug, shallow boreholes and deep wells), springs and surface waters (rivers) are presented in Table 1. Based on the references set by WHO (2011) guidelines, comparison has been made for some selected major cations and anions including other variabilities (pH, EC, TDS) and stable isotopes (δ2H‰ and δ18O ‰). Table 1 showed 12 parameters encompassing major cations and anions, two stable isotopes (δ2H, δ18O), EC, TDS and pH analyzed for their respective minimum, maximum, mean and standard deviation of 61 samples. Hence, pH values varied from 5.77 to 8.54 with a mean value of 7.21 which is under WHO (2011) acceptable range. The minimum recorded values are found in central and south west parts of the area (HD6 = 5.77, SB7 = 6.19, SP8 = 6.49) in which agricultural practices are common and confirmed by relatively elevated sulfate (9.57 mg/l) and lowered Mg (2.01 mg/l) and Ca (1.363 mg/l) values. This phenomenon is related with anthropogenic effects because the above two wells are confined by agricultural activities and they are left unprotected. In the same way, spring sample, located south west of the area is also open spring in which anthropogenic effects (wastes from a small local town, Merawi) may intensify human induced effect for water quality deterioration. This is in line with the result of Nigate et al. (2016) which has lower threshold boundary of WHO (2011) guideline 5

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Table 1 Major cations, anions and isotopic compositions of the samples. Sample ID

pH

Unit

EC

TDS

Ca

Mg

Na

K

HCO3

Cl

SO4

NO3

F

δ2H

18O

μS/cm

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

(‰)

(‰)

2.60 3.40 0.90 4.20

0.04 −0.13 0.06 −0.04

1.70 2.10 −5.40 −0.30 1.90 1.10 −1.50 −10.3 −6.20 −0.50 1.20 −1.70 −1.50 0.10

−0.32 −0.10 −1.17 −0.82 −0.71 −0.59 −1.87 −3.9 −3.53 −1.86 −0.76 −2.39 −1.42 −1.22

1.90 −1.40 3.20 3.30 2.80 4.70 1.80 3.60 −4.10 1.70 2.90 2.00 −7.30 0.50 0.60 −0.70 2.80 2.40

−0.40 0.10 0.15 −0.10 −0.21 0.52 0.05 0.38 4.34 −0.27 −0.79 −0.35 −3.34 −0.69 −0.80 −1.01 0.30 −0.28

0.502 −10.3 4.70 3.30

0.76 −3.94 0.52 1.1

BH1 BH2 BH3 BH4 BHs1 BHs10 BHs11 BHs12 BHs13 BHs15 BHs16 BHs17 BHs18 BHs19 BHs2 BHs20 BHs4 BHs6 BHs7 BHs8 BHs9 HD1 HD2 HD3 HD4 HD5 HD6 RV1 RV10 RV2 RV3 RV4 RV5 RV6 RV7 RV8 RV9 SB1 SB2 SB3 SB4 SB5 SB6 SB7 SB8 SP1 SP10 SP2 SP3 SP4 SP5 SP6 SP7 SP8 SP9 SPs1 SPs2 SPs3 SPs4 SPs5 SPs6

8.14 7.72 7.00 7.39 6.94 7.08 7.31 6.60 7.63 7.73 6.61 7.44 7.88 7.56 7.11 7.08 6.58 8.25 7.52 8.26 6.75 7.10 6.52 6.90 6.65 6.62 5.77 7.96 6.92 7.43 7.24 7.48 8.03 7.49 7.65 7.78 7.82 7.02 7.50 8.05 7.05 7.54 8.45 6.19 7.15 7.08 6.71 7.12 7.32 7.09 7.06 7.25 6.64 6.49 6.61 7.19 7.13 6.53 6.59 6.80 7.60

470.00 470.00 810.00 491.00 1090.00 302.00 300.00 2840.00 511.00 360.00 301.00 381.00 431.00 417.00 287.00 1247.00 1650.00 303.00 316.00 910.00 205.00 230.00 280.00 320.00 680.00 330.00 137.00 380.00 251.00 420.00 420.00 350.00 340.00 360.00 450.00 220.00 210.00 490.00 460.00 480.00 580.00 390.00 1140.00 190.00 405.00 5610.00 283.00 280.00 300.00 510.00 290.00 650.00 310.00 207.00 239.00 4410.00 327.00 192.00 220.00 187.00 507.00

306.00 306.00 527.00 235.00 697.60 196.30 195.00 1817.6 332.15 234.00 195.65 228.60 258.60 250.20 160.00 748.20 1072.5 196.90 205.40 515.56 133.30 150.00 182.00 208.00 442.00 215.00 65.00 247.00 120.00 273.00 273.00 228.00 221.00 234.00 293.00 143.00 137.00 319.00 300.00 312.00 377.00 254.00 741.00 90.00 195.00 3647.0 134.00 182.00 195.00 332.00 189.00 423.00 202.00 99.00 113.00 2646.0 196.20 115.20 132.00 112.20 304.20

16.90 30.00 17.32 18.65 16.78 20.30 13.10 198.4 28.10 25.40 17.80 47.00 16.00 28.00 28.00 41.00 56.55 18.50 35.60 56.16 24.50 18.00 14.00 16.00 26.80 10.20 9.36 21.50 28.65 23.00 28.00 23.40 30.40 18.60 24.30 11.30 12.00 39.20 20.00 37.50 37.50 16.00 36.40 11.94 18.98 126.8 12.75 12.40 17.30 22.40 10.00 25.40 16.00 11.88 8.74 56.00 23.00 14.00 17.00 17.00 39.00

17.70 15.00 24.00 7.26 2.15 7.60 3.39 44.50 3.40 4.02 2.60 13.00 8.00 16.00 16.80 47.00 6.26 2.50 4.84 9.29 6.80 9.20 14.00 17.00 30.90 25.00 6.13 11.40 9.12 13.50 9.70 13.00 10.40 17.30 16.00 9.00 8.00 13.00 19.30 17.00 23.00 13.20 24.40 5.83 5.88 137.2 8.98 10.50 8.90 21.30 13.40 21.00 12.50 6.97 9.82 364.0 18.00 9.00 12.00 9.00 25.00

15.92 11.27 21.28 20.16 20.16 26.40 14.50 105.2 43.50 16.80 33.40 17.00 83.00 38.00 12.00 189.5 57.09 16.21 9.82 61.20 20.30 0.67 1.46 3.15 7.20 2.13 13.96 5.81 8.67 4.13 4.69 3.85 7.70 3.04 8.75 5.11 3.18 4.27 7.63 7.49 4.76 19.04 12.70 6.68 18.99 174.1 16.88 5.64 4.30 5.32 2.60 9.94 5.11 11.76 14.86 543.0 12.00 8.00 5.00 8.00 26.00

2.59 1.86 6.58 2.64 0.47 28.5 0.53 25.2 4.35 1.47 1.34 5.20 3.40 1.80 3.30 14.7 6.64 1.50 1.42 30.3 14.5 1.37 0.36 1.26 0.88 1.23 3.54 3.22 3.90 3.43 3.85 3.08 1.89 3.85 3.71 2.87 3.85 2.27 3.22 2.52 0.55 4.69 8.90 0.92 3.50 21.2 6.26 2.10 2.12 5.03 2.02 1.68 2.45 3.50 5.70 10.7 2.20 1.20 1.60 1.00 1.10

206.50 196.00 252.50 172.20 100.65 176.30 95.40 1209.5 185.00 153.50 135.40 238.00 262.00 281.00 175.68 830.00 317.20 91.20 136.30 330.00 170.50 71.50 94.50 108.50 244.00 94.50 113.20 122.50 152.30 156.50 149.50 133.00 134.50 125.40 164.50 76.50 73.40 196.00 150.50 203.00 210.00 157.50 295.00 97.60 170.30 1825.0 167.00 75.50 105.40 115.50 84.00 231.00 94.50 126.00 143.40 3550.0 185.00 99.00 128.00 105.00 307.00

3.00 1.90 18.50 3.50 2.50 2.20 2.20 30.40 2.20 0.60 1.20 2.00 14.00 2.00 3.64 13.04 15.50 1.80 5.60 21.50 4.50 5.80 5.60 8.10 12.50 12.50 0.01 3.60 18.20 2.60 9.10 1.60 5.70 4.40 4.70 3.10 2.40 5.40 5.90 5.80 15.50 1.50 15.00 0.32 0.41 25.00 1.42 4.90 2.20 44.00 10.00 10.50 13.50 0.09 0.00 30.00 4.00 1.00 1.00 1.00 7.80

0.50 0.50 2.00 3.40 1.20 5.30 7.10 20.50 3.20 4.30 0.80 2.60 3.00 3.60 0.11 0.41 3.40 11.00 0.67 4.00 3.60 13.00 10.00 6.00 13.00 32.00 1.52 0.50 5.22 0.60 0.70 0.50 0.50 0.50 8.00 2.00 0.50 9.00 4.00 7.00 7.00 5.00 6.00 2.00 1.10 5.50 0.15 5.00 0.80 3.00 0.80 0.40 0.70 1.75 1.75 36.20 5.00 3.20 1.00 4.00 17.80

0.20 0.20 0.10 1.44 8.80 11.0 0.90 0.46 2.40 2.30 0.25 0.40 1.80 0.90 8.10 1.77 1.44 4.65 2.12 2.16 0.20 0.10 0.20 0.10 0.50 0.20 0.84 0.70 3.01 0.00 3.50 0.60 0.40 0.60 2.40 0.40 0.60 0.20 0.70 0.10 0.20 0.10 0.80 0.70 0.30 0.90 0.11 0.40 0.20 0.00 0.60 0.30 0.20 0.74 0.74 0.40 14.6 14.4 6.20 12.0 4.40

0.84 0.84 0.71 0.05 0.23 0.37 0.21 0.15 0.24 0.22 0.32 0.44 0.64 0.42 0.60 0.41 0.22 0.70 0.84 0.60 0.68 0.51 0.60 0.93 0.73 0.44 0.13 0.62 0.12 1.24 0.52 0.52 0.49 0.61 0.78 0.47 0.77 0.46 0.40 0.32 0.60 0.92 0.92 0.09 0.07 0.00 0.15 0.34 0.91 0.50 0.47 0.82 0.45 0.04 0.12 0.25 0.16 0.12 0.16 0.10 0.17

Mean Min Max St.Dev WHO

7.21 5.77 8.45 0.55 6.5–8.5

625.03 137.00 5610.00 919.09 1000.00

391.02 65.00 3647.0 583.67 500.00

28.14 8.74 198.4 28.27 75.00

21.16 2.15 364.0 48.16 50.00

30.33 0.67 543.0 75.49 200.0

4.87 0.36 30.3 6.46 12.0

271.32 71.50 3550.0 505.04 120.00

7.51 0.00 44.00 8.68 250.0

4.98 0.11 36.20 6.87 250.0

2.05 0.00 14.6 3.46 50.0

0.45 0.00 1.24 0.29 1.50

relatively higher in this major group. Major group three (G3), contains 22 samples and classified into 4 subgroups (Table 2). Subgroup 1 is characterized by low TDS and

relatively low HCO3. Na in this subgroup is in equal or a little higher than Ca which may derive from trapped sediments in hand dug wells and springs. Subgroup 2 shows fast circulation of water in hand dug 6

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Fig. 3. Groundwater flow direction and water table map (reduced water level) of the study area derived from depth to water table and elevations.

path. One exceptional sample (SB5) showed elevated SO4 because it is situated besides landfill site. Subgroup 2 and 3 are characterized by low TDS and representing fresh water types (Table 2). The order of cations and anions is the similar to cluster 3. In summary, Mg-Na-HCO3, Ca-Mg-Na-HCO3, Mg-Ca-Na-HCO3 and Ca-Mg-Na-HCO3 are water types for Cluster 1, 2, 3 and 4 correspondingly derived from average values of major cations and anions (Tables 3 and 5).

wells and rivers with absolute Ca dominance from cations and relatively elevated Cl indicator of anthropogenic effect. Subgroup 3 accompanied with high Cl, NO3 and SO4 likely from anthropogenic impact. The last subgroup (4), portray distinctly elevated anthropogenic effect for SO4 and Cl. Mg is also in proportion with Ca and is likely sourced from weathering of olivine bearing quaternary flow basalt. The last main group (G4) contains 28 samples. This group is again classified into sub clusters based on their similar hydrogeochemical properties (Table 2). Subgroup 1 represents all intermediate values except BHs7 which is Ca dominated (Ca-HCO3 water type) and may attribute to fast meteoric water circulation via highly porous and permeable quaternary flow basalt. BHs18 which is found in the discharge zone of the area dominated with Na cation (Na-HCO3 type) is likely from underlying lower basalt and facilitated by long trajectory

5.3. Correlation matrix analysis of hydrogeochemical variables Helsel and Hirsch (1991) described correlation as measure of association between two continuous variables whether the variables are co-vary together or not, and it will be measured by correlation 7

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Fig. 4. Dendrogram used for clustering of water samples using Ward's linkage and squared Euclidian distance measurement.

Table 2 List of major groups and sub hierarchical clusters of the hydrogeochemical parameters and number of samples included in each class. S/N

Major Groups

1

G1

2 3

G2 G3

4

G4

Sub Groups

G3SG1 G3SG2 G3SG3 G3SG4 G4SG1 G4SG2 G4SG3

Sample ID's

No. of Samples

Focused variables for discussion in the cluster

SP1, SPs1, BHs12

3

BHs1, BHs10, BHs2, BHs8, SPs2, SPs3, SPs4, SPs5 SP9, SP10, SP16, BHs16, SB7, HD6 HD1, SB1, HD2, HD4, BH3, SB4, SP5, Sp7, RV3 BHs4, RV10, HD3, BHs20, BHs9 SP4, HD5 BH2, RV9, BH1, BHs18, SB6, SB3, SB5, BHs7, SP6, RV7, RV2 BHs6, SPs6 BH4, BHs11, BHs13, BHs15, BHs17, BHs19, RV1, RV4, RV5, RV6, RV8, SB2, SB8, SP2, SP3

8 6 9 5 2 11

EC, TDS, Ca, Na, Mg, K, HCO3; Elevated with WHO (2011) Standards comparison Mean of HCO3 > WHO (2011) Standards comparison Low TDS and HCO3; Na ≥ Ca Ca - dominated Relative elevated values in Cl, NO3 & SO4 Elevated SO4 and Cl Intermediate values

2 15

Low TDS – Fresh water

Total number of samples

61

8

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Table 3 Summarized statistical figures of each major cluster groups (WHO, 2011). Cluster 1

pH EC TDS Ca Mg Na K HCO3 Cl SO4 NO3 F 2 H 18 O

Cluster 2

Cluster 3

Cluster 4

WHO (2011)

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

6.60 2840 1817.6 56.00 44.50 105.20 10.70 1209.5 25.00 5.50 0.40 0.00 −16.13 −4.95

7.19 5610 3647 198.4 364 543 25.2 3550 30.4 36.2 0.90 0.25 −4.10 4.34

6.96 4286.7 2703.5 127.07 181.90 274.10 19.03 2194.8 28.47 20.73 0.59 0.13 −10.12 −0.31

6.53 187 112 14.0 2.15 5.00 0.47 99.0 1.00 0.11 2.16 0.10 −10.6 −3.23

8.26 1090 698 56.2 18.0 61.2 30.3 330 21.5 5.3 14.6 0.6 3.54 −1.06

7.06 439.4 265.6 24.03 10.48 19.10 8.57 162.5 4.61 2.98 9.66 0.29 −0.86 −1.88

5.77 137 65 8.74 2.6 0.67 0.36 71.5 0.00 0.15 0.00 0.04 −15.5 −3.34

7.24 1650 1073 56.6 47 190 14.7 830 44.0 32.0 3.50 0.93 3.30 0.30

6.75 452.7 281.2 21.75 14.66 19.81 4.04 180.4 9.76 5.35 0.71 0.42 −0.42 −0.79

7.12 210 137 11.3 2.50 3.04 0.53 73.4 0.41 0.40 0.00 0.05 −7.75 −3.53

8.45 1140 741 47.0 25.0 83.0 8.90 307 15 17.8 4.65 1.24 4.70 0.52

7.65 421.1 265.4 23.74 12.10 15.70 2.95 167.8 4.32 3.57 1.08 0.56 0.31 −0.92

6.5–8.5 1000 500.0 75.0 50.0 200.0 12.0 120.0 250.0 250.0 50.0 1.5

wells, hand dug and shallow boreholes, spring and surface waters. Correlation matrix among major ions and some selected determining parameters are observed and better correlation has been depicted in spring and groundwater samples (Fig. 5D). While assessing the overall summary statistics in groups, clustered groups have mixed water type dominancy (Table 5; Fig. 6).

coefficients. Pearson's correlation is commonly used as measure of linear correlation (Helsel and Hirsch, 1991). In the study area, Pearson's correlation is done through scatter plots (Fig. 5) and presented in Table 4 for major determinant parameters. In effect, moderate to highly strong linear correlations have been observed between TDS and EC, Mg and HCO3, Mg and Na, HCO3 and Na, HCO3 and EC, TDS and HCO3, Ca and EC, TDS and Ca, Na and TDS, EC and Na, Mg and EC, TDS and Mg, Cl and TDS, EC and Cl etc. Simple and nested scattered matrix plots have been generated to show the strength of the above listed relationships (Fig. 5). Effort was made to assess effect of altitude/elevation on water chemistry of the area. However, impact of elevation in water samples is not significant as a general trend and level of significance is not achieved either 0.001 or 0.005 levels. Box plots for TDS, pH and EC (Fig. 5A) indicate elevated values for deep groundwater samples, springs, shallow boreholes, hand dug wells and surface water samples respectively. These results agreed with the above statistical values and clustered groups. Fig. 5B shows distribution of major cations with dominated parameters. Deep groundwater is dominated by Na and Mg, hand dug wells (Ca and Mg), shallow boreholes (Ca), springs (Ca) and surface water samples (Ca) ions respectively. The Mg and Na in deep wells are related with deep circulation with rock-water interaction and mixing of fresh meteoric water through porous media especially quaternary basaltic lithology. Sediment ion trapping and cation exchanging might be responsible for Ca and Mg enrichment in hand dug wells. Lastly, the dominancy of Ca (shallow boreholes, springs and surface water samples) may be related with ferromagnesian olivine weathering, fast flushing circulation and recharging are common to these scheme types. Fig. 5C revealed that in all water source types, HCO3 is dominated followed by Cl and SO4 for deep

5.4. Principal Component Analysis Principal Component Analysis (PCA) is a method to reduce dimensions by forming a new set of variables or axes and viewing in to two axes which are linear combinations of original variables. All axes are uncorrelated to each other and the first principal component explains more the variance of the data followed by the second and the remaining can be treated as residuals (Helsel and Hirsch, 1991) and the main factors that affect most groundwater chemical properties can be detected easily. In the study area; EC, TDS, Ca, Mg, Na, HCO3, Cl and SO4 are the first 8 hydrogeochemical parameters which are linked strongly with PC1 that explains 49.6% of the variability deciphering major ions rockwater interaction mainly and anthropogenic effects somehow (Table 5). Factor 2 explains 13.57% of the data variability and associated with pH and F may be by anthropogenic influence. PCA3 and PCA4 are represented by K (rock-water interaction) and NO3 (anthropogenic origin) which accounts 10.64% and 7.84% variability respectively. It can be observed that the first four principal components (eigenvalues) explain hydrogeochemical property of the area about 81.6% (Table 6; Fig. 7). As Fig. 8A indicates SP1, SPs1, BHs20, BHs12, and BHs4 are highly evolved water samples via rock-water interaction representing deep

Table 4 Correlation matrix for hydrogeochemical parameters and selected major ions.

pH EC TDS Ca2+ Mg2+ Na+ K+ HCO3− ClSO42− NO3− F− Elv

pH

EC

TDS

Ca2+

Mg2+

Na+

K+

HCO3−

Cl−

SO42-

NO3−

F−

El

1 -.022 -.018 -.003 -.020 -.008 .049 -.024 -.018 -.089 -.117 .460** -.224

1 .999** .733** .779** .769** .512** .880** .599** .430** -.106 -.213 -.212

1 .741** .755** .742** .509** .859** .597** .416** -.108 -.197 -.217

1 .352** .402** .614** .555** .553** .333** -.089 -.178 -.109

1 .938** .262* .964** .489** .612** -.097 -.123 -.209

1 .383** .961** .474** .540** -.069 -.190 -.197

1 .421** .439** .139 .047 -.093 -.197

1 .537** .580** -.096 -.194 -.204

1 .408** -.208 .004 -.283*

1 -.062 -.122 -.062

1 -.309* -.14

1 -.298*

1

Note: ** indicates “correlation is significant at the 0.01 level”, * indicates “Correlation is significant at the 0.05 level”. 9

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Table 5 Summarized statistical analysis and water type classification. Except EC (uS/cm), the other parameters are in mg/l. Water Type (Group)

pH

EC

TDS

Na

K

Ca

Mg

F

Cl

HCO3

NO3

SO4

Mg-Na-HCO3 (G1) Ca-Mg-Na-HCO3 (G2) Mg-Ca-Na-HCO3 (G3) Ca-Mg-Na-HCO3 (G4)

7.0 7.1 6.8 7.6

4286.7 439.38 456 421.07

2702 265.6 282 265.9

274 19.1 19.8 15.7

19.0 8.57 4.04 2.95

127 24. 21.8 23.7

182 10.5 14.7 12.1

0.13 0.29 0.41 0.56

28.5 4.61 9.76 4.32

2195 162.5 180.4 167.8

0.59 9.66 0.78 1.02

20.7 2.98 5.35 3.57

hydrogeochemical property of the area (Fig. 8D).

boreholes and spring samples found at the discharge zone of the area. In Fig. 8B along PCA2 axes tells the presence of elevated pH and F on HD5, SP8, RV10, SB6, RV1, SPs3, SPs5, BH1, HD6 etc. Interrelated with anthropogenic impacts characterized by incredible low values of PCA1. BHs8, BHs10, HD1, BHs9 are related with PCA3 by water-rock interaction and releasing K to water from K-feldspars and cation exchanging process (Table 6; Fig. 8C). F5 has low involvement in controlling

6. Discussion 6.1. Hydrogeochemical facies and rock-water interaction Piper diagram explain and classifies different types of water groups

Fig. 5. (A) Box plots showing pH, EC and TDS distributions with outliers categorized as deep wells, shallow boreholes, springs and surface water samples. (B) Box plots showing major cations distribution with outliers categorized as deep wells (GWDP ≥ 60 m), shallow boreholes (GWSB 30–60 m), springs and surface water samples. (C) Box plots showing major anions distribution with outliers categorized as deep wells, shallow boreholes, springs and surface water samples. (D) Matrix scatter plots for major ions, pH, EC and TDS with trending line for deep, shallow, hand dug (GWHD < 30 m) and groundwater samples; spring and surface water samples. 10

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Fig. 6. Stiff diagrams to illustrate mean concentrations of major ions among clusters: G1, G2, G3 and G4 in diagrams A, B, C and D correspondingly. Table 6 Factor loading vectors on major ions, pH, EC and TDS. F1

F2

F3

F4

F5

pH EC TDS Ca Mg Na K HCO3 Cl SO4 NO3 F

−0.043 0.945 0.933 0.717 0.876 0.882 0.559 0.956 0.686 0.606 −0.118 −0.218

0.725 0.005 0.017 0.036 0.007 −0.028 0.045 −0.015 0.201 −0.059 −0.605 0.829

0.058 0.112 0.134 0.534 −0.425 −0.311 0.637 −0.218 0.189 −0.401 0.156 −0.083

0.555 0.016 0.008 −0.127 0.128 0.175 0.129 0.110 −0.290 −0.099 0.665 0.068

−0.006 −0.185 −0.185 0.069 −0.068 −0.119 0.096 −0.109 0.331 0.586 0.284 0.165

Eigenvalue Variability (%) Cumulative (%)

5.949 49.575 49.575

1.628 13.570 63.145

1.277 10.644 73.789

0.941 7.841 81.630

0.673 5.612 87.242

based on their hydrogeochemical facies. This can be verified along flow path of water evolution (recharge-discharge area) across lithological variations in contact with water and evaporation (Hem, 1970; Freez and Cherry, 1979; Fetter, 2001; Domenico and Schwartz, 1998; Singhal and Gupta, 2010). Hence, in line with various clustered sample groups, Piper plot also reveals Mg-Ca-HCO3 as main water type followed by CaMg-HCO3 and Ca-Na-HCO3 which can give easy understanding of whole water evolution in Andasa watershed (Fig. 9). Gibbs (1970) plot used to decipher how hydrogeochemistry of water is affected and altered by different natural processes such as rock weathering, evaporation, precipitation or in combination of these all natural processes. It is executed based on ratio plots of TDS versus Na+/ (Na+ + Ca2+) and TDS versus Cl/(Cl− + HCO3−) for cations and anions correspondingly (Fig. 10). According to the plots shown in Fig. 10A and B, rock-water interaction dominates over precipitation and evaporation. Some of the samples (G1) fall near to the evaporation line. This can be attributed to a deep groundwater circulation slowly and this

Fig. 7. Principal components with eigenvalues and their variability contribution.

may have a contact with Mesozoic sedimentary formations overlain by the thick tertiary volcanics (Hautot et al., 2006). There might have a connection with upper tertiary formations through tectonic faults and make local pockets of saline waters (Kebede, 2005; Ayenew et al., 2008). The high salinity signature is observed in boreholes found in the nearby area. In addition to Gibbs plot, Chadha (1999) hydrogeochemical facies classification plot also supports the above assertion in which most samples fall on the alkaline earth exceeded alkali metals and weak acid anions over ride strong acid anions zones. However, there are some deviations in which alkali metals and weak acid anions exceed alkaline earths and strong acid anions regions, respectively (Fig. 11).

11

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Fig. 8. Biplot factor scores with variables of the area: (A) F1 versus F2, (B) F1 versus F3, (C) F1 versus F4, and (D) F1 versus F5.

6.2. Rock water interaction and major ions distribution As stated by Fetter (2001) Ca, Mg, Na, K, from cation groups and HCO3, SO4, Cl, CO3 from anion classes are the most constituents (> 90%) of groundwater. Furthermore, EC, TDS, pH and NO3 are parameters in which focus is given for discussion and different types of plots are presented for water chemistry analysis. Crystalline rocks (igneous and metamorphic) are composed of mainly quartz and aluminosilicate minerals (micas and feldspars) which are far from ambient temperature and pressure during their formation. Hence they become unstable both in soil and groundwater horizons (Freez and Cherry, 1979; Domenico and Schwartz, 1990; Appelo and Postma, 2005). Of all the silicate minerals, Olivine and Ca-plagioclase are easily weatherable whereas quartz is most weathering resistant mineral. The dissolution processes make an alteration of hydrogeochemical water composition which is facilitated by CO2 involvement and produces clay mineral residuals like kaolinite, elite and montmorillonite. This incongruent dissolution increases pH and HCO3 in water. Calcium and bicarbonate can be released to water congruently (Eq. (4)) calcite dissolution with the presence of carbonic acid in water and incongruent way (Eq. (5)) albite dissolution to kaolinite as follows (Appelo and Postma, 2005):

Fig. 9. Major and subgroup clustered cations and anions of Piper (1953) diagram.

12

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Fig. 12. (A) Ca + Mg versus HCO3 and (B) Ca + Mg versus TC to show alkaline earths dominancy for total compositional load of water chemistry.

Fig. 10. Gibbs diagrams to reveal water chemistry controlling mechanisms; cations: (A) TDS vs Na/(Na + Ca) and anions: (B) TDS vs Cl/(Cl + HCO3).

HCO3 + CaCO3 = Ca2 + + 2HCO−3

olivine and residuals of clay minerals which can provide a significant role in determining the chemical composition of water. This can be accompanied by different processes: weathering, ion exchange, dissolution and hydrolysis so that production of major ions (Ca, Mg, Na, K, HCO3, Cl, SO4, and NO3) becomes prevalent. This idea is stated by earlier studies (Ayenew, 2005; Alemayehu, 2006). These authors also described as most Ethiopian highland waters are enriched with Ca-Mg and Na dominated water types derived from basic igneous rocks. Ca is produced from silicates of basic volcanics derived minerals (pyroxene,

(4)

2NaAlSi3O8 + 2H2 CO3 + 9H2 O= Al2 Si2 O5(OH)4 + 2Na+2HCO−3 + 4H 4 SiO4

(5)

The six lithological units (lower basalt, middle basal, upper basalt, quaternary flow basalts, quaternary deposits and spots of scoria cones and fall outs) are found in the study area. They comprise mainly with mineralogical compositions of plagioclases, feldspars, pyroxenes,

Fig. 11. Chadha (1999) plot for hydrogeochemical facies display in milli-equivalent percentage. 13

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Fig. 13. Ca versus Na to understand the calcium dominancy in the recharge zone and evolved to sodium along the flow path to discharge and depth horizons.

6.3. Water type classification

amphiboles and feldspars) and Mg is from basic rocks of ferromagnesian minerals (olivine, pyroxene and amphiboles) (Ayenew, 2005; Alemayehu, 2006). HCO3 is the principal anion group in the study area that can be attributed from atmospheric and soil horizons through CO2 dissolution. Na and K can be produced from acidic igneous rocks. Water which is retained in sediments with the presence of clay mineral (high cation exchanging ability) may adsorb Na and raised Na cation load in waters around sediments (Hem, 1970). In relation to main ions, different types of diagrams have been produced to identify compositional loads of water chemistry dominantly. As HCO3 versus Ca + Mg plot revealed, all data are laid below the equal line 1:1. This signifies excess alkalinity is adjusted by alkali metals release (Na + K) (Fig. 12A). Additionally, Ca + Mg against Total Cation (TC) plot confirmed that all the samples data fall below the equiline suggesting continuous supply of alkali metals in the water chemistry from silicate minerals (Fig. 12B). The Ca versus Na plot depicts the dominancy of Ca in recharge areas and Na in discharge areas where groundwater evolution is clearly observed with almost no connection between these two major cations (R = 0.18; Fig. 13). As a result, hand dug wells (shallow and near to surface as well as fresh), HD1 and HD2 as an example, have highest values for their Ca and found in the southern part of the area (recharge zone). Similarly, SPs1 (spring), BHs20 (deepest borehole = 500 m) and BHs18 (deep borehole) are found in the northern part of the area (discharge zone) and the samples have shown dominat evolved groundwater as they have highest Na values due to rock-water interaction and cation exchanges. Using scatter plot of total alkalinity against total cation, it is possible to show weathering consequence on water chemistry. Weathering and dissolution of silicate minerals (one of the main derivers in determining geochemistry of waters in the area) were evident (Fig. 14). The samples are very near to a 1:1 equiline (R2 = 0.91) displaying high relationship between total cations and corresponding total alkalinity (Gibrilla et al., 2010). Due to ionic exchange, shortage of Ca + Mg is observed with respect to SO4 + HCO3 along flow path and this alkaline earth metals depletion and relative SO4 + HCO3 excess is balanced with Na increment in the area (Ahialey et al., 2010, Fig. 15).

Using Piper and Chadha diagrams, different types of hydrogeochemical facies have been produced which depend on rock water interaction (residence time and trajectory flow length). Moreover, anthropogenic impacts existed mainly around urban centers and agricultural areas have taken their own part in changing hydrogeochemical compositional load of waters. A study in Bahir Dar (located at the study area border) have also been identified by previous studies for the presence of anthropogenic impacts (Ayenew, 2005; Kebede, 2013). In the study area, the major water types are Mg-Ca-HCO3 followed by Ca-Mg-HCO3 and Ca-Na-HCO3. Around Bahir Dar area (Zewdie and Yoseph, 2012) and Tana sub basin (Mamo, 2015) grouped as basic and transitional water types. The study revealed that hydrogeochemistry of the area is mainly controlled by geogenic and anthropogenic factors. The geogenic controlling processes includes rock weathering (silicates hydrolysis and dissolution, ionic exchange and adsorption). Anthropogenic impact is brought by urbanization (open waste disposal landfill) and agriculture (irrigation mainly with the diverted water via open canal).

6.4. Stable isotopic signatures (δ2H and δ18O) Global and local meteoric water lines (GMWL and LMWL) used as reference lines for tracing water in hydrologic cycle (e.g. Gat, 1996; Clark, 2015). Any deviation from these lines is controlled by climatic (temperature, humidity, rainfall amount) and geographic (altitude, latitude, continental, etc) variables ((Dansgaard, 1964)). Similarly, Clark (2015) described the various parameters or processes that alter isotopic composition of water as evaporation, condensation, re-evaporation, mixing during recharge and groundwater flow as well as isotopic exchange during mineral-water and gas-water interactions. In the plot of δ18O Vs δ2H (Fig. 16), most samples (from boreholes, springs and hand dug wells) show a deviation from the LMWL. Such deviation from meteoric water line and their alignment along distinctive evaporation line indicates isotopic fractionation by temperature effect (Geyh, 2000; Clark, 2015). During rainy season (June to September), the study area is characterized by generation and movement of flood from adjacent hilly topography to lowland areas where it stays long time in poor drainage water stagnating areas. The 14

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Fig. 14. Total alkalinity versus total cation.

may be related to the spatial distribution of samples which is concentrated in more or less similar elevations. The effect of altitudinal variation has also been clearly depicted in which all existing hydrogeochemical variables have weak correlation with altitude (Table 4). Most river samples show depleted δ18O and δ2H isotopic signature reflecting that their source is from the highland area of the watershed. This depletion could be related to rapid flow of the river which provides little time for evaporation as proved by Kebede and Travi (2012). In normal natural groundwater Cl vs δ18O scatter plot is very close to the vertical axis (δ18O). However, Fig. 17A shows a shift towards the right which signifies anthropogenic effect. Demlie et al. (2007) came up with similar result on human induced pollution of Akaki basin near to Addis Ababa. The spring near to the landfill site (SP4) shows similar isotopic signature (depleted) to Abay River (RV10, diverted water via open river canal) and Andasa river (RV2). This is suggesting interaction between surface water (the rivers) and groundwater (the spring) in the

preferential orientation of groundwater samples (Fig. 16) along the evaporation line suggests possibly evaporation of the water prior to recharge. Most of the samples falling below LMWL and GMWL signify evaporation of the water prior to recharge in the wet season and irrigation return flow in the dry season (Fig. 16). From the field visit especially north and northeast parts, they have high water logging capacity in the wet season owning to thin sheet of clay layer. Intensive irrigation with diverted river canals reveals irrigation return flow recharges in the above mentioned portions of the area. This is also supported by existence of anthropogenic pollution which is found in different water sources like spring, hand dug wells and boreholes (e.g., SP4, BHs18, RV10). Identifying groundwater recharge site using altitude effect on isotopic composition of rain helps to demarcate groundwater recharge area (Gat, 1996). However, little variation is observed in the area. This

Fig. 15. Scatter diagrams to disclose carbonate and silicate weathering mechanisms for water chemistry: SO4 + HCO3 vs Ca + Mg in meq/l unit. 15

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Fig. 16. δ18O versus δ2H plot showing relative locations of the collected water samples with respect to GMWL and LMWL.

area, especially at the highly porous quaternary flow basaltic lithological facies.

6.5.2. Hydrolysis Hydrolysis and dissolution of silicate minerals change composition of water (Domenico and Schwartz, 1990). Specifically, continuous hydrolysis of biotite and plagioclase will promote up concentrations of cations. For example, Ca2+ can be produced from dissolution of small quantities of carbonate minerals and montmorillonite occurs as a weathering product of plagioclase in addition to kaolinite in deeper part of the system (Domenico and Schwartz, 1990). If Ca/Mg ratio is > 1, then sources of Ca and Mg cations are from silicate minerals hydrolysis. Moreover, with the presence of CO2 silicate mineral hydrolysis can also produce HCO3. Andasa watershed is exhibited with silicate minerals hydrolysis and responsible for the formation of Ca–Na–HCO3 water type resulted from intermediate rock water interaction in most highland areas (Ayenew et al., 2008; Kebede, 2013; Kawo and Karuppannan, 2018). Most samples fall in Ca/Mg ratio > 1 domain apart from few saline samples (deep boreholes, shallow boreholes and springs) in northern part of the area. The other evidence for hydrolysis occurrence is taking Na/Cl ratio and interpreted for tracing Na cation from silicate minerals hydrolysis whenever the ratio of Na/Cl > 1. In the area, a few samples are out of this ratio consequently most samples are within ratio range (Kawo and Karuppannan, 2018).

6.5. Source of major ions deduction based on ionic ratios 6.5.1. Ionic ratio plots Based on ionic ratios of (Na+K-Cl)/(Na+K-Cl+Ca), (Na/(Na+Cl)), Ca/(Ca+SO4), Cl/Sum anions, HCO3/Sum anions and TDS values for each sample, possible origin of water composition analysis can be determined (Calmbach and Waterloo Hydrogeologic, 2003; Bonotto, 2016). Samples with major ions composition that have complete data were considered. Therefore, possible rock source deductions have been made. The above five listed ratio values were used for the examination of plagioclase weathering possibility/unlikely; possible sodium source other than halite - albite, ion exchange; possible calcium sources other than gypsum–like carbonate or silicates; rock weathering or others like evaporites, rainwater; Silicate or carbonate weathering, respectively (Calmbach and Waterloo Hydrogeologic, 2003; Bonotto, 2016). Similarly, TDS values can be used for silicate weathering or carbonate weathering or others like brine, seawater intrusion happening reflections (Calmbach and Waterloo Hydrogeologic, 2003; Bonotto, 2016). In summary, most samples meet ratio result found between, > 0.2 (Na+K-Cl)/(Na+K-Cl+Ca) < 0.8 which is interpreted as plagioclase weathering possibility. Based on this ratio results, most of hand dug wells, shallow boreholes and spring samples were found out of likely source of plagioclase weathering. The evaluation made for (Na/(Na +Cl)) > 0.5, Ca/(Ca+SO4) < 0.5, Cl/Sum anions < 0.8 and HCO3/ Sum anions > 0.8 were likely happened in the area and understood as sodium ion sources other than halite, calcium source other than gypsum, rock weathering and finally silicate-carbonate weathering have been identified in their order. It has been realized that TDS values can possibly model the study area effectively. Data from deep and shallow boreholes (irrespective of their location) and most spring samples (located in the discharge zones of the watershed) deciphered similar to carbonate weathering or brine conditions signifying long residence time and rock water interactions. The situation is also supported with dendrogram clustering (G1), piper diagrams, silicate-carbonate interface weathering scatter plots, groundwater table map and Gibbs plots.

6.5.3. Cation exchange Slight increment of Na without significant increase along recharge to discharge zone and deep groundwater boreholes can be sign for cation exchange processes. Commonly used cation exchanging order is mentioned as follows (see Karamouz et al., 2011): Ba2+ > Sr2+ > Ca2+ > Mg2+ > Cs+ > K+ > Na+ > Li+. From the above sequence of replacing order, ions in the left will replace ions in the right. For instance, Mg will exchange Na. For ion exchange of Na+ for K+ and exchange of Na+ for Ca2+, the following reactions are derived, respectively (Eqs. (6) and (7)):

Na+ + K− X= Na − X+ K+

(6)

Na+ + 0.5Ca − X2 = Na − X+ 0.5Ca2 +

(7)

Gibbs diagram (Fig. 10), rock water interaction which is the main controlling factor defining hydrogeochemistry of the area portrays various hydrogeochemical facies. Two most common types of indices used for indexing cation exchange chloro-alkaline indices (CAI) used the following formula (Schoeller, 1965) to reveal water chemistry evolution (Eqs. (8) and (9)): 16

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Fig. 17. (A) Cl versus δ18O showing anthropogenic pollution of water resources. (B)

CAI − I= (Cl− − (Na+ + K+ )) /Cl−

(8)

CAI − II = (Cl− − (Na+ + K+)/SO24− + HCO−3 + CO−3 + NO−3 )

(9)

18

O versus TDS to show recharge-discharge area.

6.6. Hydrogeochemical evolution and groundwater recharge/discharge zonation Groundwater chemistry can give a clue to recharge and discharge areas identification and hence evolution. The concept will be stronger whenever supported with isotopic data analysis and interpretation. Therefore, in the area under investigation, hydrogeochemical data ratio analysis has been depicted to infer recharge-discharge relationships. Obviously, recharge areas are confined with higher elevations although tectonic activities alter the situation and local pocket areas formed which are not considered as recharge areas regardless of their location. The area is confined with alkaline earths dominated water types (MgCa-HCO3 – 27.87% and Ca-Mg-HCO3 – 24.59%) followed by mixed water types (Ca-Na-HCO3 and Mg-Na-Ca-HCO3) and lastly basic (Na-CaHCO3) and transitional (Mg-Ca-HCO3-Cl, Mg-HCO3-SO4) water types. This phenomenon is an indication for less evolved water (fresh) as most of the area is dominated with recharge regions. Most of the samples including river waters satisfy being recharge area based on TDS (< 1000 mg/l) and discharge areas are found to be

Based on situations held whether Na and K cations (from water) exchanging with aquifer materials (found in rock) with Ca and Mg, results might be negative or positive. Negative results are indications for Ca and Mg (in groundwater) replacing Na and K in aquifer material. Whereas positive results are indications for Na and K (in groundwater) exchanging Ca and Mg in aquifer materials thereby increasing water compositions by Na and K cations (Schoeller, 1977). This shows that groundwater and spring samples were considered for chrolo-alkaline calculation and resulted in 85.25% negative indices depicting exchange of Na and K from water with Ca and Mg in aquifer material for CAI-I and II and only 14.75% positive indices meaning Na and K in water exchanging Ca and Mg in aquifer material.

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Fig. 18. Recharge-discharge area spatial mapping based on (A) (HCO3 + CO3)/total anions ratio map, left and (B) TDS spatial distribution map, right.

controlled with rock weathering (silicate hydrolysis and dissolution), depth, faulting, recharge-discharge zones, and anthropogenic effects such as urbanization, agriculture and waste disposal landfills. In addition, waste disposal landfill site, agricultural practices, diverted river via open canal (anthropogenic) were identified as water quality determining factors related with SO4, NO3 and Cl. This has been evidenced from the Cl and δ18O relationship which signifies anthropogenic effects around the open landfill site.

all clustered G1 sampling areas with higher TDS and structural architectures derive their discharge volumes. Moreover, SB6, BH3, BHs1 and BHs4 are found in the same locality and they fulfilled discharge area criteria based on higher TDS. Water types also confirmed the situations of recharge (Alkaline Earth bicarbonate = AE-HCO3 dominated – 52%) and recharge-discharge transition zone (Mg-, Na-, with HCO3 dominated – 41%) circumstances. If the ratio of (HCO3 + CO3)/total anions < 1 accompanied with Ca-HCO3, Ca-Na-HCO3, Na-HCO3 facies it is considered as a recharge area. If the ratio of (HCO3 + CO3)/total anions > 1 with Na-Cl and CaCl facies, it is perceived as discharge area (Fig. 18) (Kumar and James, 2017). Therefore, one can conclude the northeastern portion of the area is discharge region while northwestern, southern and southwestern parts are proved to be recharge zones (Kumar and James, 2017). However, some local pocket areas are found sandwiched between recharge to discharge zones specifically following faulted, depressed areas and incised valleys. This condition is also verified by hydrogeochemical as well as isotopic analyses in the study area.

Acknowledgements We would like thank Bahir Dar University for providing this research fund under School of Earth Sciences, Bahir Dar University research grant ID (BDU/RCS/SOE/06/10). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jafrearsci.2019.103617. 1.

7. Conclusions

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The rock-water interaction (geogenic) effects including hydrolysis and cation exchange processes control hydrogeochemical property of the area. Considering chrolo-alkaline indices, 85.25% have negative results showing exchange of Na and K from water with Ca and Mg (in aquifer) and 14.75% of positive results showing Na and K in water exchanging Ca and Mg in aquifer material. Using HCA and PCA techniques, EC, TDS, Ca, Na, Mg, HCO3, SO4, Cl and NO3 are the vital hydrogeochemical controlling parameters and alkaline earths bicarbonate is dominating the hydrogeochemical facies. The dominant cations found are Ca, Mg, Na and that of anions HCO3, Cl and SO4. Based on hydrogeochemical and isotopic results, the southern and central parts of the area are recharging zones with Ca and HCO3 dominating ions whereas the northern and northeastern parts are discharging zones with Na and HCO3 dominancy. Mg-Ca-HCO3, Ca-Mg-HCO3 and Ca-Na-HCO3 are the main water types. Hydrogeochemistry of the area is mainly

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