Introduction

Groundwater is subsurface water that occurs in soils and rocks beneath the water table (Aminiyan et al. 2016). Groundwater plays a significant role in the development of a nation. All over the world, groundwater is widely used for different purposes such as domestic, irrigation, industry, and recreational. The use of groundwater has increased rapidly in the recent few years due to rapid population growth, drought, and rapid growth of the world economy (Aminiyan et al. 2020; Ostad-Ali-Askari et al 2021).

The chemistry of groundwater is derived from the interaction between groundwater and subsurface earth materials, the chemistry of rainwater, and various anthropogenic sources, such as mining, land clearance, saline intrusion, and industrial and domestic wastes (Babiker et al. 2007; Llamas and Martínez-Santos 2005; Talebmorad et al. 2022). The most important hydrogeochemical processes that control the composition of groundwater are the dissolution of minerals from the aquifer materials, precipitation of minerals from groundwater, and ion exchange processes between the aquifer materials and groundwater. Analysis of groundwater chemistry is vital to understanding the influence of geology, climate, and anthropogenic activities on groundwater composition, and it is important to evaluate the suitability of water for different uses, such as drinking, agriculture, and industries (Bucci et al. 2017; Shayannejad et al. 2022). The principal factors that have significant effects on the evolution of the chemical composition of groundwater can be identified from the relationships of major ions (Ahmed & Clark 2016; Fisher and Mullican 1997; Ostad-Ali-Askari et al. 2019).

So far, various hydrogeochemical works have been conducted to understand the geochemical processes that are affecting groundwater chemistry and quality (Tirumalesh et al 2010; Rajesh et al 2012; Moussaoui et al 2023; Argamasilla et al 2017; Choi et al 2014; Dehbandi et al 2018; Liu et al 2017; Rao et al. 2012). Sufficient knowledge of the geochemical processes that govern the evolution of groundwater chemistry important to groundwater-related issues, and the hydrogeochemical processes are significantly variable based on time and space (SubRamani et al. 2010).

Currently, the main source of water supply for domestic purposes is groundwater in most parts of Ethiopia both in urban and rural areas. Besides this, groundwater is also extensively used for irrigation purposes in the Tigray region due to the scarcity of surface water and insufficient rainfall in most parts of the region. The Ellala and Aynalem wellfields are the main sources of groundwater for water supply in Mekelle City and surrounding areas. The regional hydrogeological setting of the whole or large part of the Mekelle basin was studied by various researchers (Abreha 2014; Grmay et al. 2015).

Understanding the roles of hydro-geochemical processes in the evolution of groundwater chemistry in developing countries like Ethiopia is limited, rather most of the studies were focused on the assessment and evaluation of groundwater quality for different uses. As the catchment is close to Mekelle City which is fast-growing than ever, the water supply for different purposes such as domestic, industrial, and irrigation is mined from groundwater. So far the groundwater assessment and some unintegrated works on groundwater chemistry have been routine previous works as mentioned above. However, for a better understanding of the hydrogeochemical processes which have been undergone in the groundwater, comprehensive and integrated analysis with multiple binary cations and anion analysis is vital due to the heterogeneity of the aquifer system (Grmay et al 2015), and active industrial practice in the catchment. Moreover, the knowledge of the scientific justification of the hydrogeochemical dynamism as a factor affecting the gradual evolution of groundwater chemistry is limited. Consequently, people and scholars who have a routine idea about groundwater quality, have doubts about the sources of groundwater chemistry and geochemical processes that control the evolution of groundwater chemistry in different areas. Therefore, this study focuses on the hydrogeochemical factors with comprehensive and integrated multi-binary cation and/or anion analysis. Furthermore, particular attention has been given how their impact on the evolution of groundwater chemistry, thereby adding input to the knowledge and understanding of the area of hydrogeochemical process between groundwater and aquifer interaction in various aquifer systems.

Description of the study area

Location

The Ellala catchment is located in the central part of the Mekelle Basin and west of the Afar Rift Valley. It is found in northern Ethiopia in Tigray regional, state, and bounded between 1475000 m–1515000 m N and 520000 m–580000 m E (Fig. 1A). The study area covers about 296.5 km2.

Fig. 1
figure 1

A Location and drainage map B 3D physiographic view C Geological Map D Hydrogeological map Basin (Kassa et al., 2015)

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Physiography and climate

The catchment is characterized by cliffs at the northern and southern peripheries of the catchment, ridges in the eastern, northeastern, and southeastern parts, and a flat area in the western part of the catchment (Fig. 1B). The maximum elevation is 2620 m asl in the eastern part of the catchment, whereas the minimum elevation is 1960 m asl in the western part of the catchment.

This catchment is part of the Tekeze drainage system. In this catchment, there is one main river which is known as the Ellala River and many tributaries. This main river follows the structurally weak plane of the Mekelle fault which intersects different lithological units, whereas the tributaries follow the general slope inclination (Abreha 2014; Grmay et al 2015). The low density of streams may indicate that the bedrock is either highly resistant or highly permeable. The highly resistant bedrock may be related to dolerite dykes, whereas the permeable nature is associated with highly fractured and jointed bedrock. It is characterized by a dendritic drainage pattern (Fig. 1A).

The climate of Ethiopia is mainly controlled by the seasonal migration of the Intertropical Convergence Zone (ITCZ) and the topographical features of the area (Fazzini et al. 2015). According to Ethiopian temperature zoning, the catchment is located under the semi-arid climatic zone. There are two rainy seasons, June to September and March to April. However, most of the annual rainfall occurs during July and August with a mean annual rainfall of 650 mm. The mean annual temperature ranges from 15 to 16 ℃.

Geological setting

Regional geology and geological structures

The Ellala basin lies almost at the center of the Mekelle Basin. Scholars (Beyth, 1972b; Kazmin, 1972), have studied the regional geology of the Mekelle Basin. Accordingly, the Mekelle Basin is one of the five well-known sedimentary basins in Ethiopia, which is composed of very thick Palaeozoic—Mesozoic sedimentary rocks. The Palaeozoic sediments rest on the folded and eroded surface of the Precambrian basement and are unconformably overlain by the Mesozoic sediments (Beyth, 1972b; Kazmin, 1972). The Mesozoic sediments unconformably are overlain by flood basalts of the Tertiary age (). The Mesozoic sediments from the oldest to the oldest are the Adigrat sandstone, Antalo limestone, the Agula shale, and Amba Aradom sandstone. These sediments are the results of major transgression–regression cycles and are related to major regional tectonic events that have affected the entire East African region (Ayele and Gangadharan 2016). In the basin, there are regional brittle and ductile structures. The Negash Geosynclinal Fold is the Neoproterozoic structure in Mekelle Outlier and is an overturned fold plunging southward by about 15 to 200 (Gebreyohannes 2009). The four regional normal fault belts in the Mekelle Basin are the Wukro, Mekele, Chelekwot, and Fuicea Mariam fault belts (Beyth 1972a). These faults were probably active after the deposition of the Agula Shale but before the deposition of the Amba Aradam Formation.

Local geology and geological structures

The major geological units in the Ellala catchment are limestone, limestone–shale–marl intercalation, dolerite intrusion, and Quaternary sediments (Fig. 1C; (Grmay et al 2015).

Generally, the limestone and limestone–shale–marl intercalation beds are horizontal but tilted near the dolerite intrusion and in fault zones and the dip angle of the beds varies from 10° to 25°. These rock units are affected by minor and major faults and joints. These joints are both systematic and non-systemic and regular joint spacing and an aperture range from 0.4 to 2 m and from 0.6 to 10 cm, respectively. Besides, karstic features are common in limestone. Dolerite dykes and sills have intruded on these units in different places. The dolerite unit covers a large area and is affected by joints. However, the joints are widely spaced and have very narrow apertures as compared with other rock units. It is hard and massive, but in some places, it is highly weathered and an exfoliation type of weathering is common in this rock unit. The dolerite intrusions follow the fault zone of the main Mekelle fault belt. The quaternary sediments are composed of boulders, gravel, sand, silt, and clay-sized particles. These sediments are mainly found in flat topography and along the main Ellala River and their thickness varies from 0.8 to 5 m.

The major geological structure in the catchment is a fault, and the Mekelle fault belt is found in this catchment with an orientation of NW–SE and dipping southward. The orientations of regional and local faults are grouped into two NW–SE and NE–SW. Faults with an NW–SE orientation are mainly found in the vicinity of Mekelle City, whereas faults trending in the NW–SE are found widely distributed throughout the catchment (Fig. 1C; Beyth 1972a).

Hydrogeology of Ellala catchment

The hydrogeology setting of the catchment is very complex because the sedimentary rocks are intruded by complex networks of dolerite dykes and sills. The rock units in the catchment are highly affected by regional and local structures (faults and joints). Moreover, solution cavities and caverns are common in the limestone unit. Hence, these features have a primary role in the occurrence and movements of groundwater in the catchment. The main aquifer units of the Ellala catchment are highly weathered and fractured limestone and shale–limestone intercalated, dolerite, and shale units (Fig. 2; (Grmay et al 2015)). These units show a wide range of hydraulic properties and the highest hydraulic conductivity values were recorded at the contacts between the sedimentary rocks and dolerite intrusion due to a high degree of fracturing and weathering at the contact zones (Yehdego 2003). The types of aquifers in the catchment are mainly unconfined and confined to semi-confined (Fig. 2) and the aquifer productivity in the catchment ranges-from poor to moderate potential (Fig. 1D; (Grmay et al 2015)).

Fig. 2
figure 2

Lithological and hydrogeological Log a Limestone Aquifer b Limestone-Shale-Marl Intercalation Aquifer c Dolerite Aquifer. (Modified after Grmay et al 2015)

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A groundwater level contour map was prepared using the groundwater level of 49 wells, and the groundwater, flows from east to west, is the same as the surface water flow direction (Grmay et al 2015). The groundwater flow path can be inferred from the ratio of bicarbonate to chloride (Signal, 1999). Decreasing the ratio of bicarbonate to chloride concentration from the east toward the western part of the catchment supports the idea that groundwater flows from east to west (Fig. 1D). The Ellala River is parallel to the fault plane of the Mekelle fault. Therefore, the groundwater movement or flow path in the catchment is mainly controlled by the major and minor structures.

Materials and methods

Water sample collection

The depth-integrated groundwater sampling method has been applied because the groundwater sampling was conducted after the wells' drilling processes were completed. Collection of groundwater samples from all wells and springs in the catchment is time-consuming and it is not economically viable to conduct chemical analysis for all samples. Hence, before sample collection, in situ measurements of electrical conductivity, pH, temperature, and TDS were conducted on 105 groundwater points with portable EC and pH meter devices during the field investigation. Once the groundwater samples have been stratified, the selection of representative samples for analysis from each stratification or class is the precondition for laboratory analysis of the groundwater samples. Both groundwater sample stratification and selection have been done based on the Quality classification of water for irrigation (after Wilcox 1955). Accordingly, 38 representative groundwater samples were selected by applying a random sampling technique from each class by considering the distribution of wells in the catchment and the different lithological units. As a result, twenty-two samples from shallow wells, nine samples of hand-dug wells, and seven samples from springs were selected. After the groundwater sample selection for analysis had been done, two samples per each selected groundwater point were collected using one litter capacity of plastic bottles. From these water points, two deep wells, two springs, eight shallow wells, three hand-dug wells, and one open well are in quaternary deposits, two springs, three shallow wells, and one hand-dug well and two open wells are in dolerite, one spring is in limestone and one deep well, nine shallow wells, two springs, and one open well are in limestone–shale–marl interaction.

In situ measurements physical parameters

The most valuable physical parameters of groundwater, such as temperature, pH, TDS, and electrical conductivity, were made in the field.

Laboratory analysis

The groundwater samples were analyzed for their chemical concentration in the hydrogeochemistry laboratory of Earth Sciences at Mekelle University.

Laboratory analysis for all samples was done for their major cations and anions (Na+, Ca2+, Mg2+, K+, Cl−, SO42−, HCO3−, NO31−, and PO43−. To analyze the cations (Na+, Ca2+, Mg2+, and K+) of the samples, an atomic absorption spectrometer was used and a titrimetric method in the lab was used to analyze HCO3− and Cl−. Moreover, anions, such as SO42−, NO32−, and PO32−, of the samples were measured using an Ultra Spectrophotometer. (Tables 1, 2, 3, 4).

Table 1 Physical- chemical parameters of groundwater samples of the Ellala catchment (concentration of ions in mg/l, temperature (T) in 0C and EC in µS/cm)
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Table 2 Groundwater type of the Ellala catchment
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Table 3 Chloro-alkaline Indices I and II
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Table 4 Saturation indices of major minerals and Base Exchange Index (BEX) of the Samples
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Personal and instrumental errors

Since the chemical analysis was done based on the predefined objective of the work, it was clear that there some chemical concentrations that had been remained from the analysis. As a result, it was planned to use duplication analysis rather than ionic error balance. Hence, the sample analysis was conducted two times where the analysis was run twice, and the chemical analysis of these samples showed a good repetition with very small variations (Table 5 & 6).

Table 5 Double Chemical Analysis Results of Cations
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Table 6 Double Chemical Analysis Results of Anions
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Hydrochemical data analysis

Schoeller and piper diagram analysis

To determine the dominant ions in the groundwater and hence the groundwater facies, both the cations and anions of the groundwater samples were plotted on the Schoeller diagram (Schoeller 1955) and Piper diagram (Piper 1944), using AquaChem 3.7 software. From these analyses, it is possible to identify the various groundwater facies and water types. In addition to this, the Schoeller diagram (Schoeller 1955) was done to identify the dominant cations and anions which can provide clues on the respective sources of these components.

Multi-variant binary analysis

Different binary plot analyses were done to identify the predominant hydrogeochemical processes that govern the groundwater chemistry in the catchment (Abid et al. 2011; Kloppmann et al. 2011). Hence, analysis of the binary plots and ratios between commonly bonded cations and anions was performed to identify the geochemical and anthropogenic factors that control the evolution of the groundwater chemistry in the catchment.

  1. a)

    The degree of rock-water interaction and its impact on the evolution of groundwater chemistry were examined by the binary plots of TDS versus Na/ (Na + Ca) and TDS versus Cl/ (Cl + HCO3) (Gibbs 1970).

  2. b)

    The binary plot analysis of Ca/Na versus HCO3/Na and Ca/Na versus Mg/Na was performed to determine the rock-water interaction domain which includes dissolution such as carbonate dissolution, evaporate dissolution, and silicate weathering (Gning et al. 2017; Ibrahim et al. 2019; Kumar 2014; Mukherjee et al. 2008).

  3. c)

    A binary plot and analysis of SO42+  + HCO3− versus Mg2+  + Ca2+ was done to evaluate the effect of carbonate and silicate weathering on the evolution of groundwater chemistry (Datta and Tyagi 1996; Behera et al. 2019; Pradhan et al 2022). As per the objective of the work, the following binary plot analysis was done:

  4. d)

    The binary plots of Na+ versus HCO3− and HCO3− versus Na+ + Cl− were done to evaluate the degree of silicate weathering in the groundwater rock interaction.

  5. e)

    Binary of Ca versus SO4, Ca versus HCO3−, and Ca2+  + Mg2+ versus HCO3− were also done to see the main sources of calcium.

  6. f)

    Further binary plot analysis of Ca + Mg versus Na was done to identify the type of cation exchange groundwater–aquifer interaction.

  7. g)

    The binary plot of nitrate versus chloride was performed to examine the anthropogenic effect on groundwater chemistry.

Chloro-alkaline indices (I and II)

Cation exchange is one of the most common hydrogeochemical processes of water–rock interactions in groundwater, which controls the ionic components of groundwater (Xiao et al. 2012). Hence, the chloro-alkaline indices (I and II) were performed to identify the cation exchange which is occurring during the interaction between the groundwater and its host environment while residence or travel. The chloro-alkaline indices were calculated using the following equations (Schoeller 1977):

$$CAI-I=\frac{{Cl}^{-}-\left({Na}^{+}+{K}^{+}\right)}{{Cl}^{-}}$$
(1)
$$CAI-II=\frac{{Cl}^{-}-\left({Na}^{+}+{K}^{+}\right)}{{HCO}_{3}^{-}+{SO}_{4}^{2-}+{CO}_{3}^{2-}+{NO}_{3}^{-}}$$
(2)

where the concentration of ions is in meq/l.

Base exchange index (BEX

The Base Exchange index (BEX) analysis was done to assess the exchange of water between groundwater and seawater which in turn was used to infer the origin of groundwater in the basin as the terrain and the groundwater-bearing formation are mainly sedimentary. This analysis is performed as the sum cations, such as Na+, K+, and Mg2+ (in meq/L), corrected for a contribution of sea salt, which assumes the absence of other sources (Pieter 1999), and is given as:

$$BEX=\left[Na+K+Mg\right)-1.0716Cl$$
(3)

From this analysis, the positive values of BEX revealed the base flow from the groundwater to the sea and are freshening process. The negative value of BEX indicates the salinization of groundwater where the seawater flows to the groundwater zone, but the zero value of BEX is an indication of an equilibrium circumstance where there is balancing (Stuyfzand 1993).

Saturation index calculation

The saturation index calculation was done using the PHREEQC interactive software program, which was used to interpret and evaluate the further tendency of interaction between the aquifer material and the water (Parkhurst 1995). This analysis is popularly used to understand the degree of equilibrium between mineral species and water. The changes in the state of saturation of the dominant minerals in water are helpful in providing significant information on the hydrogeochemical processes which govern the groundwater chemistry (Belkhiri et al. 2012; Behera et al. 2019; Pradhan et al 2022). The saturation index of a mineral is calculated by the following equation (Deutsch 1998).

$$SI=log\frac{IAP}{Kmineral}$$
(4)

where, SI = saturation index, IAP = ion activity product of the dissociated chemical species in solution, and Kmineral = the solubility constant of the mineral. When SI > 0, the specific mineral is oversaturated and precipitation is possible; when SI = 0, the specific mineral is in equilibrium with the solution, and when SI < 0, the specific mineral is undersaturated and dissolution is possible.

Results

Physico-chemical parameters

The in situ and laboratory analysis results for the cations, anions, and other chemical parameters of the groundwater samples have been compiled (Table 1). The pH in the groundwater ranges between 5.71 and 8.1, recording in SHW7 and HDW10 respectively (Table 1) and about 68.42% of the groundwater samples revealed a pH value ranging from 5.71 to 6.9, which indicates that the groundwater in the catchment is non- alkaline nature. The pH values show an incremental from eastern, southern, and northern toward the western part of the catchment. The temperature of the groundwater samples ranges from 17.5 to 25.8 ℃. The maximum value was recorded in SP6 from the lower catchment whereas the minimum value was recorded in SHW40 from the upper catchment.

The groundwater samples from the catchment indicate a wide range of electrical conductivity (Table 1), ranging from 50 µs/cm (SP20) to 3121 µs/cm (SHW40) and this indicates the water quality varies from fresh to saline groundwater. From the spatial distribution diagram of EC (Fig. 3A), it was observed that the EC value is increased from the recharge at the east toward the discharge in the western part of the catchment. It is observed that the EC distribution is parallel to the groundwater flow path where significant anomalies are recorded surrounding the city and close to the industries (Fig. 3A). The degree of salinity of the groundwater increases along the general groundwater flow direction, from the eastern to western. The liquid and solid wastes released from the city and the industries could be responsible for the high value of the EC in the catchment but, the aquifer material contribution is also significant.

Fig. 3
figure 3

A Spatial Distribution of EC B Spatial Distribution of TDS C Groundwater samples on a Schoeller Diagram

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Concentrations of Mg2+, Ca2+, Na+ and K+ are ranging from 8 to 160 mg/l, 45 to 216 mg/l, 3 to 106 mg/l and 0 to 120 mg/l respectively. Concentrations of HCO3−, PO43−, Cl−, SO42− and NO3−vary from 244 to 701.5 mg/l, 2.5 to 94.6 mg/l, 1.6 to 30.6 mg/l, 2.3 to 85.5 mg/l and 0.2 to 10.7 mg/l respectively. The minimum and maximum values for total dissolved solids in the catchment are 228.205 mg/l (SP12) and 2175.08 mg/l (DPW19) respectively (Table 1). The TDS value revealed a generally increasing trend from the eastern to the western part of the catchment parallel to the groundwater flow path (Fig. 3B), with a high concentration around the city and adjacent to the industrial zones, which has a similar fashion to the EC spatial distribution. This confirms that EC is controlled by TDS, where highly mineralized water is more conductor than water with low concentration, and hence, the salinity of the water is increased as one goes from the eastern to the western of the catchment under investigation. The waste materials and chemicals that are discharged from the city and the small-scale industries could be the ground sources in addition to the effect of the aquifer materials.

The nitrate concentration in the groundwater samples ranges from 0.188 mg/l (SHW31) to 9.5 mg/l (DPW19) (Table 1) and more than 76% of groundwater samples revealed that much more than the natural concentration of this ion, which is generally less than 1 mg/l (Agrawal et al. 1999). In contrast to this, the nitrate concentration in the groundwater samples is more exaggerated than the natural background of the ion and the significant concentration of this shows an anthropogenic source of nitrate. The phosphate concentration in the groundwater samples ranges from 2.4 mg/l (SHW39) to 94.6 mg/l (SHW37) with an average of 30.3 mg/l (Table 1). The main sources of phosphate in groundwater are overlying soil, dissolution of phosphate-bearing minerals in the aquifer, agricultural fertilizer, animal waste, and leaking septic systems or infiltration of wastewater (Fuhrer et al. 1999). In this catchment, phosphate-bearing sediments have not been reported yet, so that, the sources for the high concentration of phosphate in groundwater are mainly related to anthropogenic activities such, as fertilizer and wastes, from the city and industries.

Discussion

Ionic dominance and groundwater facies

To identify the dominant ions in the groundwater and hence the groundwater facies, both the cations and anions of the groundwater samples were plotted on the Schoeller diagram (Schoeller 1955) and Piper diagram (Piper 1944). The dominant major ions in the groundwater in descending order are Ca2+, Mg2+, Na+, and K+ and HCO3−, PO43−, SO42−, Cl−, and NO3− for cations and anions, respectively (Table 1; Fig. 3C & 4a). Bicarbonate is the dominant anion for all groundwater samples (Fig. 4a), which is possibly due to the carbonate material weathering and dissolution of carbonate minerals. Even though almost all the groundwater samples are plotted in the bicarbonate dominant type in the case of cations, the concentration of calcium and magnesium is much higher than sodium and potassium (Fig. 4a). As shown in Fig. 4a, the groundwater facies in the catchment is predominantly Ca-Mg-HCO3 and Mg-Ca-HCO3 (Fig. 4a) types and this is consistence with the dominance of calcium, magnesium, and bicarbonate components (Table 2. Generally, the trends in the Schoeller diagram are the same for all samples and this suggests a common source, and needs further analysis which will be addressed in the following sections.

Fig. 4
figure 4

Groundwater Facies and BEX a Piper Plot b Durov c Base Exchange index (BEX)

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Rock–water interaction

As per the binary plots of TDS versus Na/ (Na + Ca) and TDS versus Cl/ (Cl + HCO3) (Fig. 5a & b), the groundwater samples are plotted dominantly toward the rock dominance zone, which implies the rock–water interaction is the main governing factor for the evolution of groundwater chemistry in the catchment. Dissolution, such as carbonate dissolution, evaporate dissolution and silicate weathering are components in the rock–water interaction domain (Gning et al. 2017; Ibrahim et al. 2019; Kumar 2014; Mukherjee et al. 2008). Further, the binary plot analysis of Ca/Na versus HCO3/Na and Ca/Na versus Mg/Na (Fig. 5c & d), was performed to identify the component of rock interaction which is contributed as a main natural process involved in changing groundwater chemistry. Accordingly, most of the groundwater samples are plotted from the end zone of silicate weathering toward the carbonate dissolution, which reveals carbonate dissolution is the major geochemical process that controls the evolution of groundwater chemistry in the catchment (Fig. 5c, d). The sources of major ions in the groundwater and their evolution mechanisms can be identified from the compositional relations of the dissolved species (Fisher and Mullican 1997). Hence, the analysis of binary plots of the major ions has been discussed in the following sections.

Fig. 5
figure 5

Ionic Binary plots a Na/(Na + Ca)versus TDS b Cl/(Cl + HCO3) versus TDS c Ca/Na versus HCO3/Na) Ca/Na versus Mg/Na

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Further from the scatter plot of SO42+  + HCO3− versus Mg2+  + Ca2+, most of the samples are plotted below the 1:1 line toward the Ca2+ and Mg 2+ side, and four samples fall in the silicate weathering zone (Fig. 6c). This indicates that the major hydrogeochemical process occurring in the catchment is dominantly by carbonate dissolution (calcite and dolomite) with some extent of silicate weathering. From the cation–anion binary plots, most of the groundwater samples are plotted close to the 1:1 line in the Ca2+ versus HCO3− plot (Fig. 6a), hence, the dominance of carbonate dissolution for the evolution of groundwater chemistry is also confirmed from this binary plot. In the Mg2+ versus Ca2+ plot (Fig. 6d), almost all of the groundwater samples fall very close to the 1:1 line, reflecting the concentration of the calcium and magnesium is more or less equal in almost all groundwater samples. However, the groundwater samples are somewhat far from the 1:1 line and pointing toward the Ca2+  + Mg 2+ side in the case of Ca2+ + Mg2+ versus HCO3− plotted (Fig. 6b), which indicates there are also other sources of magnesium ion in addition to the dissolution of dolomite.

Fig. 6
figure 6

Ionic binary plots a Ca versus HCO3 b Ca + Mg versus HCO3 c Ca + Mg versus HCO3 + SO4 d Ca versus Mg e Ca versus SO4 and) Mg versus SO4

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The finding of calcium and magnesium dominance in the groundwater is also confirmed by the plots of Ca2+ versus SO42− (Fig. 6e) and Mg2+ versus SO42− (Fig. 6f), where the concentration of calcium and magnesium increase over the sulfate. This idea is also supported by the analysis of saturation indices minerals where calcite and dolomite reflect a similar range of saturation indices (Table 4 and Fig. 8a, c). Furthermore, the groundwater samples are dominantly laid from the 1:1 line pointed toward the bicarbonate in the binary plots of Na+ versus HCO3− and HCO3− versus Na+ + Cl− (Fig. 7a, c) indicating that carbonate weathering is the dominant geochemical process over silicate weathering, involved in the evolution of groundwater chemistry of the catchment. A significant number of the samples are plotted below the equiline toward the sodium dominance in the binary plot of sodium versus chloride (Fig. 7d), which revealed a concentration of sodium ion is much higher than chloride ion. This suggests that silicate weathering or ion exchange also controls the evolution of groundwater chemistry in the catchment. However, there are few samples laid along the equiline, suggesting a common source of halite dissolution for both ions, and few samples plotted above the equiline which indicates a signal of other anthropogenic sources of chloride (Fig. 7d). Furthermore, in the binary diagram of Ca versus SO4 (Fig. 6e), and Ca versus HCO3− (Fig. 6a) analyses were performed on the groundwater samples in the catchment (SU et al. 2007), and most of them are laid toward the calcium zone from the 1:1 equiline, indicating the dissolution of carbonate minerals (possibly calcite). Since a significant number of samples are plotted above the equilibrium line toward bicarbonate (Fig. 6a), it should be noticed that there are also some limited and common sources for calcium and bicarbonate. Besides, from the Ca2+  + Mg2+ versus HCO3− binary plot (Fig. 6b), it was observed that the samples dominantly indicate the high contribution of carbonate dissolution. This shows an excess of calcium over sodium due to reverse ion exchange, but few samples revealed the dominancy of sulfate and bicarbonate over the calcium and magnesium, which are plotted above the equiline, explaining the silicate weathering due to the dolerite intrusion, which also shares the above interpretation (Harrington and Herczeg 2003).

Fig. 7
figure 7

Binary plots a Na + Cl versus HCO3, b Mg + Ca versus Na, c Na versus HCO3, d Na versus Cl and e NO3 versus Cl

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Ion exchange process

The chloro-alkaline indices I & II were calculated using Eqs. 1 and 2. From this calculation, both chloro-alkaline indices I and II are negative for all groundwater samples (Table 3). According to Schoeller (1977), negative values for both indices reflect the presence of a reverse ion exchange process which causes the enrichment of sodium and potassium ions in the groundwater from the aquifer materials and the removal of calcium and magnesium ions from the groundwater to the aquifer materials. Furthermore, binary plot analysis of Ca + Mg versus Na was performed to identify the type of cation exchange that is active in the groundwater (Fig. 7b). Relatively higher concentrations of Ca + Mg over Na in a Ca + Mg versus Na plot show a reverse cation exchange between the groundwater and aquifer materials (Rajmohan and Elango 2004). All of the groundwater samples are plotted below the 1:1 line toward the calcium and magnesium zone, which indicates the presence of reverse ion exchange in the catchment (Fig. 7b). The presence of a shale bed in the aquifer system, which is rich in clay could be the source of sodium and potassium ions in the catchment. The active reverse ion exchange in the groundwater in the catchment is also confirmed by the Durov plot analysis in which all the groundwater samples are laid along the reverse ion exchange zone (Fig. 4b).

The assessment of the Paleo groundwater origin, freshening, and salinization between groundwater and seawater is well examined with the Base Exchange index (BEX) analysis (Stuyfzand 1993). This analysis is important as the catchment is part of the sedimentary basin and groundwater-bearing formations are mainly sedimentary units. In this study, the Base Exchange index was calculated using Eq. 3. Accordingly, all the groundwater samples revealed a positive Base Exchange index value which is much more than two (Table 4 and Fig. 4c) and is suggested that the groundwater in the catchment is dominated by the meteoritic origin, although it needs further groundwater chemistry study with isotope dating analysis. Moreover, the base-exchange softened water process could be interpreted from the Base Exchange index analysis as far as the high base-exchange reaction of alkali earth elements exchanged for Na + ion under high concentrations of bicarbonate is over calcium and magnesium (Jeevanandam et al., 2012).

Saturation index of minerals

The saturation index analysis of minerals was done to predict the tendency of mineral dissolution and/or precipitation into the groundwater or to the aquifer materials so that the state of equilibrium between groundwater and the mineral in the aquifer will be identified (Appelo & Postma 2005). Moreover, the result of this analysis is important for the assessment of the nature of the interaction between the groundwater and the minerals of the water-bearing formation, and the current and further geochemical processes that govern the evolution of the groundwater chemistry can be identified. The saturation index of the major minerals of the aquifer materials was calculated using the PHREEQC interactive software program (Table 4 and Fig. 8), and more than 63% of the samples revealed both calcium and magnesium are undersaturated (Fig. 8c). As the groundwater reservoir is dominated by carbonate materials, it is surprising that the groundwater is significantly undersaturated in terms of calcite, dolomite, and aragonite minerals (Fig. 8a, c), which supports the activity of the normal ion exchange between sodium and/or potassium with calcium and/or magnesium, suggesting the parallel dissolution and precipitation reactions occurring in the groundwater. Hence, further dissolution of calcite, dolomite, and aragonite minerals will be possible in the groundwater where the minerals are undersaturated state. Also, the groundwater is entirely found under saturated with a mineral such as halite, sylvite, gypsum, and anhydrite (Table 4 and Fig. 8b), which indicates the further dissolution of these minerals.

Fig. 8
figure 8

Saturation Indices of Major Minerals a Super Saturated b Equilibrium and Under Saturated c Saturated Comparism of Calcite and Dolomite

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Groundwater chemistry along the flow path

The recharge and discharge zones in the catchment are the eastern and western parts of the catchment respectively, which can be identified not only from the surface topography but also from the hydrogeological map of the catchment (Fig. 1D). The spatial distribution of the bicarbonate ratio to chloride (Fig. 4C), confirmed that the groundwater flow in the catchment is from eastern to western, and this indicates dissolution and other hydrochemical processes are significantly strong toward the western part of the catchment, which is supported by the spatial distribution of EC (Fig. 3A) and TDS (Fig. 3B), where their concentration is increased along the groundwater flow path from eastern to western and is higher around Mekelle City and adjacent to the industry in the western part of the catchment. Moreover, the overexploitation of groundwater supply for the city and the nearby industries can increase the ion concentration in the groundwater in the western part.

Anthropogenic processes

The contaminants induced by human activity termed anthropogenic sources such, as agricultural (animal compost and fertilizer), infiltration of sewerage as well as discharges from septic tanks, and domestic and industrial wastes together with the depletion of groundwater, contributed to the evolution of groundwater chemistry and affecting its quality (Ben Brahim et al. 2012). To examine the impact of anthropogenic activities on groundwater chemistry, binary plots, and the concentration analysis of anthropogenic processes sensitive anions, such as chloride, sulfate, and nitrate, were performed. The binary plot analysis of nitrate versus chloride was performed to investigate the input for these ions (Fig. 7e). Accordingly, it was observed that the ions have derived from different sources, and the animal composts and other fertilizers for agricultural application, and infiltration wastes from sewerage, and septic tanks in addition to the domestic and industrial wastes could be the sources of these ions in the catchment. However, the limited halite input for chloride and the atmospheric sources of nitrate can be also interpreted from the binary analysis of these ions (Fig. 7e). The high concentration of chloride in the drinking water supply leads to toxicity which cannot be sensed in the lower stage of the human body, and is lead even to heart failure (Shahid et al. 2014). Besides, the high level of chloride in the water can cause an increase in electrical conductivity which in turn advances the corrosivity capacity of the water. Moreover, when the water flows in metal pipes and pumps the chloride can react with metal and lead to the addition of metals in the drinking water. Hence, the environmental unfriend application of fertilizers and the poor sewage and waste management system in the area should be addressed to improve the groundwater quality in the area in terms of chloride and nitrate. Since, most of the communities settled away from the mega-cities where the treatment of the groundwater quality gets little attention and the communities are using untreated water, thereby the health risk caused by the nitrate and chloride component in the drinking water is significant from time to which needs to maintain and removal of these anions.

Conclusion

Hydrogeochemical investigations were conducted in the Ellala catchment to understand the main governing factors in the evolution of the groundwater chemistry of the catchment. The groundwater in the catchment is non-alkaline. The pH, TDS, and EC increase from east to west and are significant around Mekelle City and the industrial zones due to the wastes of the city and the nearby industries, and the degree of salinity of the groundwater indicates a similar fashion with the EC which revealed the direct relationship with TDS. The major ions in descending order are Ca2+, Mg2+, Na+, and K+ (cations) and HCO3−, PO43−, Cl−, SO42−and NO3− (anions). Calcium, magnesium, and bicarbonate are the dominant ions in the catchment. Hence, the groundwater of the catchment is predominated by Ca-Mg-HCO3 and Mg-Ca-HCO3 water types.

The groundwater chemistry of the catchment is controlled by the rock–water interactions, mainly carbonate dissolution mainly calcite and dolomite, and this idea is supported by saturation indices of minerals where calcite and dolomite revealed a similar range of saturation indices. The reverse cation exchange between the groundwater and aquifer materials is also the dominant geochemical process due to the sodium and potassium ions sources from the presence of a shale bed in the aquifer system, which is rich in clay in the catchment. Although to a limited extent, the silicate weathering due to the dolerite intrusion and halite dissolution from the gypsum layer are also geochemical processes that undergo in the groundwater–aquifer interaction in the catchment. Besides the natural geochemical processes, the groundwater chemistry of the catchment is also modified by anthropogenic activities. The possible source for the high concentration of calcium, magnesium, and bicarbonate in the groundwater is carbonate dissolution. Furthermore, silicate weathering originating from dolerite and other clay minerals is also a source for the enrichment of magnesium ions in the groundwater. The excess concentration of sodium ion over chloride ion is due to reverse ion exchange and/or weathering of sodium-bearing materials, such as shale and dolerite. The sources for the high concentration of phosphate, nitrate, and chloride in the groundwater are mainly anthropogenic activities, sources that need treatment, and groundwater quality control and management in the catchment. The groundwater in the catchment is dominated by the meteoritic origin, although it needs further groundwater chemistry study with isotope dating analysis. The groundwater is undersaturated with calcite, dolomite, and aragonite minerals where further dissolution of these minerals is going in the groundwater.