The effect of hydrogeological and hydrochemical dynamics on landslide triggering in the central highlands of Ethiopia

The study area is marked by its complex lithological and tectonic settings. The Cenozoic era is characterized by extensive faulting accompanied by widespread volcanic activity and uplift. The area is represented by two major litho-stratigraphic formations, which are the Tertiary volcanic rocks associated with volcanic ash and the Quaternary superficial deposits. The major rock and soil types in the area include: aphanitic basalt, porphyritic basalt and agglomerate basalt (aphanitic basalt–porphyritic–agglomerate); ignimbrite–tuff–volcanic ash; intercalated porphyritic basalt and scoriaceous agglomerate (porphyritic basalt–scoriaceous agglomerate); Tarmaber basalt; upper ignimbrite; and unconsolidated deposits (colluvial and alluvial deposits; Fig. 3). The volcanic rocks have experienced intense weathering, which resulted in the occurrence of deep weathering profiles and weathered landforms. The aphanitic basalt–porphyritic–agglomerate units crop out in the gully areas and series of cliffs and benches in deeply dissected valleys. The unit exhibits notable textural and compositional variations vertically, which are constituted by the aphanitic basalt, porphyritic basalt, and scoriaceous agglomerate basalt.

The ignimbrite–tuff–volcanic ash unit mainly consists of pumiceous lapilli tuff and volcanic ash with subordinate ignimbrite, trachyte, and rhyolite. The ignimbrite–tuff–volcanic ash beds form small cliffs that are highly altered and intensely weathered, and are vertically jointed and highly shattered by faulting (Mebrahtu et al. 2020a). The porphyritic basalt–scoriaceous agglomerate unit consists of dominantly porphyritic basalt and scoriaceous agglomerate with subordinate aphanitic basalt and vesicular basalt. The porphyritic–scoriaceous agglomerate basalt shows a high rate of spheroidal weathering and breaks easily to very small-sized material, and the weathering and fracturing prevails more in the major joints and layering. This unit is highly weathered and fractured, favoring the circulation and storage of subsurface water. The Tarmaber basalt unit is mainly exposed in the western part of the study area in the high-rising mountain chains (Fig. 3). This rock formation forms vertical cliffs and ridges trending in the N–S direction as well as some E–W offsets and shows well-developed columnar joints (Mebrahtu et al. 2020a). The upper ignimbrite unit is exposed in the western part of the study area overlying the Tarmaber basalt. This unit is fine-grained, highly weathered, and crossed by subvertical to vertical fractures (Mebrahtu et al. 2020a).

The slopes with lower inclination are covered by Quaternary sediments. The colluvial deposits are associated with rock pediments originating mainly from the basalt, presumably transported downslope by the action of gravity and slope wash (Mebrahtu et al. 2020a). These colluvial deposits mainly contain rock fragments and soil derived from fragmented and weathered bedrock. The alluvial sediments are deposited along the major riverbeds and convey large volumes of sediment during the wet season, mostly in the form of debris slides and flows. They are derived from the weathering, transportation, and reworking of different rocks from the steep cliffs and the escarpment. The area is traversed by four major trends of faults (N–S, E–W, NE–SW, and NW–SE; Fig. 3) and they can be assumed to be a major conduit for groundwater flow. However, this behavior can be sometimes lost or sealed by precipitation of secondary material or clay (Guglielmi et al. 2000). The landslides coincide with the tectonically active geological structures.

A multi-techniques investigation strategy combining hydrogeochemical, isotopic and geophysical methods, was followed in this study. Groundwater chemistry and stable isotopes analyses were used to characterize the groundwater flow system and rock–water interactions. The water samples were collected in two field campaigns (April–June 2016 and October–November 2017) from 65 sites. The samples were taken directly from two different sources, which include cold springs and rivers. Spring water samples were collected directly at their discharge points under natural pressure by using a plastic syringe. The water sampling point locations were systematically selected in order to be representative of: different rock formations in the stratigraphic column; recharge and discharge areas; and landslide areas and surroundings. The locations of the sample sites are shown in Fig. 3, and the hydrochemical results, including isotope data, are presented in Table 1. All the water samples were filtered through a 0.45-μm membrane on site and filled 50-ml polyethylene bottles. The samples taken for major cations analysis were acidified to pH 2 with HNO₃ (nitric acid). All the major ions except HCO₃⁻ were analysed using ion chromatography (Dionex 1000 Ion Chromatography System) in the laboratory of Applied Geology at the Ruhr University of Bochum (RUB), Germany. HCO₃⁻ was analysed in the field by employing a burette titration method by using HCl, and the total Fe (Fe_tot) was determined by atomic absorption spectrometry 240F (AAS) in the RUB. Measurements of electrical conductivity (EC), pH and temperature were made in-situ by using WTW Multi340i handheld meters, and each electrode was calibrated. Conventional field hydrogeological observations and a landslide inventory were done to support the results from hydrochemical and isotope analyses.

The stable isotopes (¹⁸O and ²H) were measured in 39 water samples (33 cold springs and 6 rivers). The sampling bottles were repeatedly rinsed with the water to be sampled and then completely filled leaving no space for air. Water samples for stable isotopes (¹⁸O and ²H) were collected in high-density polyethylene bottles (50 ml) and analyzed following standard procedures at the laboratory of Isodetect (Environmental Monitoring) in Munich and the Department of Materials and Earth Sciences at the Technical University of Darmstadt, Germany. The determination of ¹⁸O and ²H in groundwater was carried out by using a laser absorption device (PICARRO L2130-i δD/δ¹⁸O Ultra High-Precision analyzer). The measured values ​​of the samples (mean of 10 individual measurements) were calibrated with international standards (Standard Light Antarctic Precipitation (SLAP), Standard Mean Ocean Water (SMOW), Greenland Ice Sheet Precipitation (GISP)) and any drift or memory effects were corrected. The general measurement error is ±0.1 or ± 0.5‰ (standard deviation) based on the Vienna Standard Mean Ocean Water (VSMOV). The measurement inaccuracy (simple standard deviation) of the analysis carried out reached a maximum of ±0.09‰ for ¹⁸O and ± 0.3‰ for ²H. Long-term isotopic data of rainfall (from 1961 to 2016) from the Addis Ababa (Fig. 1) Global Network of Isotopes in Precipitation (GNIP) station (190 km from the study area) is taken from the International Atomic Energy Agency database (IAEA 2020). The resulting stable isotope data are interpreted by plotting them with the Global Meteoric Water Line (GMWL; Craig 1961), and the local meteoric water line (LMWL) of Addis Ababa (Kebede et al. 2008). In this study, Statistica version 8.0 was used to conduct hierarchical cluster analysis (HCA). The softwares ArcGIS 10.5 (Esri), Geochem (US Geological Survey), computer program Diagrammes v 6.5 and CorelDRAW X7 were used for database creation, spatial data analysis and to prepare high-quality maps and illustrations. The final results and interpretations were then combined to formulate a conceptual hydrogeological model of the Debre Sina landslide area.

The groundwater flow in the study area is controlled by geological structures, topography and rock type. The groundwater flow direction in the whole basin coincides with the topography following the surface-water flow direction (Fig. 4) because small intermittent and particularly perennial rivers form local drainage basins and shallow aquifers. The flow is partly controlled by the structure and partly by the geomorphology of the area; local groundwater flow directions vary from place to place according to the local topography (Fig. 4). The groundwater divide between the Rift Valley and the Jemma basin do not generally conform to the surface-water divide; the divide is slightly shifted to the west into the Jemma basin particularly in the northwestern part of the study.

The lithostratigraphic, geomorphologic, isotopic and hydrochemical evidence indicates that two groundwater flow systems (shallow/local and intermediate-deep) exist in the study area. The shallow groundwater flow is mainly localized to the highland areas and adjacent escarpments and its water table is a subdued replica of the surface topography (Figs. 5 and 6), which is generally characterized by lower concentrations of dissolved ions, depletion in heavy isotopes and higher d-excess. A significant part of the groundwater is discharged to rivers (in the form of baseflow) and as contact springs within the highland plateau and its margins. The intermediate-deep groundwater flow is strongly influenced by the lithostratigraphy and the major faults in the area rather than the surface geomorphology. At the top of the slope, groundwater directly flows through the vertical to subvertical joints with different flow paths mainly guided by the highly permeable gravitational features that correspond to interconnected tension cracks. This intermediate deep groundwater is recharged through the deep-seated fractures adjacent to the major faults in the highland plateaus and discharges mainly in the form of high-discharge springs and baseflow in the eastern lowland sections of the Dem Aytemashy, Robi, Majete and Shenkorge rivers (Figs. 4 and 6). The shallow aquifer system is drained by the perennial springs located at the top of slopes, at the basal aquifer in the lower part of the slopes and at the landslide toe.

The Fig. 4 depicts that there is a swamp area (discharge of groundwater) around Argaga/Asfachew in the north-central part of the area, which corresponds well with field observations in the study area. This marshy area is developed where the rocks are impermeable and rock–water intact near the surface. Around Yizaba and Majete areas, the groundwater contours are closed indicating flow from all directions towards the center (Fig. 4). As the water table drops below the stream level, water infiltrates from streams and rivers into the aquifer. Aquifers along the rivers are recharged by the surface water of streams, and the flow of many streams is controlled by geological structure. The area is characterized by large faults that play important roles in the occurrence and movement of the groundwater. The plateau volcanic rocks retain rainwater for a long time and create favorable conditions for infiltration through a highly weathered, jointed and permeable upper layer. The shallow groundwater is partly drained by rivers and the remaining water recharges the underlying aquifers. The highly permeable fracture network facilitates subsurface flow to the lowlands as the primary recharge source of the deeper aquifers.

The highly fractured volcanic rock of the plateau, consisting of basalt, ignimbrite, rhyolite and/or trachyte, is one of the major water-bearing formations in the area. It also covers large gently-to-steeply undulating areas of the eastern part of the area. In accordance with to the distribution of springs (Fig. 3), the inter-bedded volcanic rocks of the ignimbrite–tuff–volcanic ash act as a semiconfined aquifer. The vertical to subvertical joints and tensional features in the Tarmaber basalt covering the plateau area create a favourable condition for rainwater percolation. Most springs are located at topographic breaks such as hillsides.

Generally, there is a clear zonation in the total ionic concentration of natural waters following the direction of groundwater flow from the highlands to the lower elevations. This zonation corresponds with the spatial variations of recharge and discharge conditions and the geological setting. The slight increase in the total ionic concentration towards the lowland implies that the residence time of the groundwater and the magnitude of rock–water interaction are likely to increase in the same direction. The faults in the area are not only weak zones, but also mostly characterized by deeper weathering and higher potential for concentrated groundwater flow, which can act as a lubricant and produce water pressure, causing landslides. The most favorable condition for landslides in the Debre Sina area and its surroundings is considered to be the fractured state of the bedrocks, especially near the tectonic lines. As a result, most mass movement occurs in the NNE–SSW and N–S directions, which coincides with set of lineaments. The lapilli tuff, tuff breccia and tuffaceous strata within the pyroclastic unit make the strata susceptible to slaking, which, in itself, can also be one of the triggering factors of landslides in the area. Triggering mechanisms can also be aggravated by the development of pore-water pressure, seepage forces, seepage erosion and mechanisms related to high plasticity. The hydrogeological conditions of the terrains are generally favourable for the development of seepage forces within the pyroclastic sediments (tuff and pumice horizons) and unconsolidated deposits during periods of rainfall.

All the groundwaters and the surface waters in the area are fresh, characterized by low total dissolved solids (TDS) ranging from 61 to 522 mg/L. The pH values show that the groundwater is slightly acidic to alkaline (6.3–9.8) in springs and rivers (Table 1). The chemical groundwater types of an area can be distinguished and grouped by their position in a Piper diagram (Piper 1944). Different hydrochemical facies were identified in the study area on the basis of the Piper diagram (Fig. 5), and four major water types, identified as Ca–Mg–HCO₃, Ca–HCO_3, Ca–Mg–Cl–SO₄ and Na–HCO_3, classified according to their dominant chemical composition (Fig. 5) were found. Groundwater and surface water from the higher elevations typically have a Ca²⁺(Mg²⁺)–HCO₃⁻ hydrochemical facies, whereas groundwater in the lower altitude displays a Na⁺–HCO₃⁻ type (SP46). There is a general compositional change from a Ca²⁺(Mg²⁺)–HCO₃⁻ type water to a Na⁺–HCO₃⁻ hydrochemical facies along the groundwater flow path from higher to lower altitude (Fig. 6). This result is consistent with other hydrochemical studies conducted along the central-western highlands and margins of the Afar depression (Darling et al. 1996; Chernet et al. 2001; Ayenew 2005). As topography controls the fluxes in the hydrological cycle, it also controls the hydrochemical signature of the groundwater. The TDS of the groundwater increases towards lower altitude as the hydrochemical facies changes along its flow paths. The low TDS and bicarbonate groundwater type in the highland part indicate the fast hydrogeological regime of the plateau receiving a relatively high volume of precipitation. The TDS content increases along the flow direction as water flows from the recharge to the discharge areas. In the study area, Ca–Mg–HCO₃ is the dominant water type in the basic volcanics and Na–HCO₃ in the acidic volcanic rocks. In general, the TDS increases from the infiltration area along the watershed on the plateau to the drainage area formed by the valleys of the Robi River, Shenkorge River, and their tributaries (Fig. 4).

From west to east, the Na⁺ concentration increases due to cation exchange. Similarly, there is a facies change, from being slightly mineralized in the west, to a significantly mineralized water type in the east. High Na⁺ and K⁺ concentrations in springs located in the lower part of the landslide show that the water was in contact with acidic volcanic rocks at the head scarp. In addition to cation exchange, weathering of silicate minerals controls the hydrogeochemical facies. The hydrochemical data provided useful insight into the main hydrogeochemical processes involved in the water mineralization. Water groups represented by Ca–Mg–HCO₃ are weakly mineralized waters within the basaltic and scoriaceous aquifers. Water groups represented by Ca–Na–HCO₃ and Ca–HCO₃ are draining the fractured rhyolites, ignimbrites, tuff, and trachytes, and, as could be expected, have a more dilute chemistry. The Na–Ca–Mg–HCO₃ and Ca–Na–Mg–HCO₃ water types are mixtures of the water types Ca–Na–HCO₃ and Ca–HCO₃. Ca–Mg–HCO₃ and Ca–HCO₃ groups represent shallow groundwater circulation and short residence time that contain early stages of geochemical evolution (recent recharge) or rapidly circulating groundwater that has not undergone significant rock–water interactions (Edmunds and Smedley 2000; Kebede et al. 2005, 2008). The location of the landslide occurs within a formation that is poorly welded and composed of tuffaceous material that readily weathers to clay minerals and is capped by pervious basalt and/or ignimbrite and colluvial deposits (Fig. 6).

Gibbs plots are employed to understand the processes affecting the geochemical parameters of groundwater (Gibbs 1970, 1971). In these diagrams, TDS is plotted against the concentrations of Na⁺/(Na⁺ + Ca²⁺) for cations, as well as TDS versus Cl⁻/(Cl⁻ + HCO₃⁻) concentrations for anions. From these diagrams the natural mechanism controlling groundwater chemistry, including the rock-weathering dominance, evaporation and precipitation dominance, can be derived. The Gibbs plot of samples from the study area (Fig. 7) shows that all of the groundwater samples fall into the rock-weathering dominance group. The results indicate that the surface water had active interaction with groundwater, since the samples are not located in the rainfall dominance cell. All data points in the domain of water–rock interaction (Fig. 7a,b) indicate that chemical weathering controls water chemistry. As stated in the preceding, the interaction between rocks and water results in leaching of ions into the groundwater system, which influences the water chemistry. The chemistry of the spring water is mainly controlled by the residence time and the intensity of recharge. The upslope springs show low-mineralized water types, whereas the springs at the toe of the landslide area show higher mineralized water.

Hierarchal cluster analysis (HCA) is a typical multivariate statistical algorithm that puts observed data into meaningful clusters in their hierarchal order (Davis 2002). Three groundwater groups have been identified from the preliminary HCA based on major-ion chemistry (Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃⁻, SO₄²⁻, F⁻, Cl⁻) of the water samples collected in this study (Fig. 8). The groundwater samples from the higher-altitude areas lie within the low-EC group I (Fig. 8). Most samples that lie in this group are characterized by low concentrations of all major ions. These samples are located in the highlands bounding the Rift Valley and are characterized by low salinity, with TDS below 216 mg/L. They were collected mainly from basaltic and scoriaceous aquifers, indicating fast groundwater flow. The vertical/subvertical joints and tensional fractures create a favourable condition for rainwater percolation. This area is generally acting as a recharge zone for the surface water as well as subsurface water that flows to the down-slope areas. This low EC (72–222 μS/cm) characteristic arises from the sample-point location within the recharge area (low residence time) and the presence of aquifer material with lower solubility, indicating that groundwater in the highland areas is getting recharge from rainwater. The samples in group II have an EC range of 115–465 μS/cm and low concentrations of all the major ions similar to group I, but they are found at middle altitudes. These samples were collected close to the escarpments, and recharge seems to have taken place by precipitation over the highlands and transported through large faults. They were collected from highly fractured and shattered ignimbrite, rhyolite, trachyte associated with basalt, indicating that the groundwater movement is shallow to intermediate. However, they have relatively higher concentrations of Na⁺, K⁺, Cl⁻ and SO₄²⁻ as compared to group I, which is mainly related to the solution or interaction between water and secondary minerals or clay that precipitate into faults. The local freshwaters in the middle altitude are controlled by the normal faults ultimately derived from fast circulating recharge from the high rainfall of the plateau. Groundwater mainly emerges as high-discharge cold springs on the slope and at the bottom hill of the escarpment formed by steep faults. The groundwater samples in group II therefore correspond to groundwater in the intermediate flow systems. In the middle part of the study area, transitional types Ca–Na–HCO₃ and Ca–HCO₃ occur.

The water samples of group III were collected from the lower-elevation areas (below 1,500 m asl) in the eastern and northeastern parts of the study area, which are covered with volcanic ash-dominated units and sporadic colluvial-alluvial deposits. In these litho-units, the groundwater movement is slow, which together with the presence of soluble minerals, enhances the effects of rock–water interaction giving rise to relatively higher concentrations of Na⁺, K⁺, Cl⁻ and SO₄²⁻. The EC values of the groundwater samples within this group are between 215 and 573 μS/cm and increase towards the Shewa Robit Valley, which indicates that there is intermediate to deep groundwater circulation and relatively higher residence time of the groundwater. The low hydraulic gradient of the groundwater in the lowland plain (Fig. 4) also indicates slow groundwater velocity. This leads to the longer residence time and enhancement of rock–water interaction. Sodium bicarbonate-rich groundwaters as well as higher sulphate concentrations were found in this discharge area; however, there are also localized freshwaters at the lower elevation, indicating that there is also fast circulating recharge from the high rainfall of the plateau along regional faults.

The average isotopic composition of the water samples collected from the study area is −5.70‰ for δD and − 3.31‰ for δ¹⁸O, which is not very far from the long-term weighted average isotope composition of the summer rainfall for Addis Ababa IAEA station (Kebede et al. 2008). This suggests that the groundwaters in the study area are mainly recharged from the summer rainfall on the highlands under cold air conditions. Therefore, they are generally of meteoric origin and they are not affected by some processes (like evaporation) during or before recharge. The residence time is short, the soil/rock–water interaction is low, and the water is little mineralized mainly in the highland and intermediate regions. Therefore, it is possible to conclude that the main cause of the landslide is not the active soil/rock–water interaction. It is rather because of the steep slope topography and the pressure formed during precipitation, which leads to an increase in the weight of the loose and weathered materials (increasing its shear stress). The material loses its shear resistance which finally results in land mass failure or landslide. The springs and water ponding in the study area are usually seen at the upper failure section or main scarp of landslides, and their discharge comes from the overlying unit with a high discharge (Fig. 9).

Intermittent springs emerge along the highly conductive layers of porphyritic-agglomeratic basalt, as well as where there is an intersection with the low-conductivity pyroclastic layers (groups I and II; Fig. 9). In areas where such highly permeable zones/layers are covered by colluvium, the groundwater can build up pore-water pressure from below and favour the triggering of shallow landslides. Below the spring horizons, humid zones are also formed which could additionally favour landslide triggering, especially during heavy or long-term rainfall. Such ponded water (Fig. 9e) can infiltrate into the slope and increase pore-water pressure, which decreases the shear strength, thereby causing instability to the slopes. In many of the landslide-affected sites, springs and seepage zones were observed to emerge along more fractured zones of the rocks or along the coarser soil horizons (Fig. 9a–d).

Geophysical studies were carried out at two selected sites, namely, Yizaba and Armaniya. Vertical electrical soundings (VES) were conducted along the deep-seated landslide in Yizaba and along the shallow to intermediate landslide in Armaniya in order to trace the orientation and location of the faults and geological contacts, which can have considerable effect on the groundwater circulation, as well as to map the various aquifer systems. These investigations were also aimed at determining the thickness of the overburden materials, to characterize the vertical distributions of subsurface layers, to estimate the depth to the water table, to identify probable aquifer beds and to characterize the nature of the bedrock. The VES interpretation of the three profiles is based on lithological outcrops, surface observations and the overall geological setup of the area.