Implications of groundwater quality to corrosion problem and urban planning in Mekelle area , Northern Ethiopia

Surface and groundwater chemistry being an important factor in urban planning and infrastructure development, present paper tries to present the problems of corrosiveness due to groundwater chemistry in Mekelle city. Iron corrosion in distribution systems and engineering structures are common problems in many urban areas. Corrosiveness of groundwater at different localities in Mekelle and its environs has been evaluated on the basis of AAS-UV spectrophotometer-generated hydro-geochemical data. Corrosiveness of water was estimated by using corrosion indices like Larson Index, LI and Aggressive Index, AI and total dissolved solids, total carbonate hardness, chloride and sulphate data were evaluated to estimate aggressiveness of the water samples on iron pipes. Analyses of the results have shown that most of the samples from boreholes and hand dug wells compared to spring samples, are potentially aggressive. The result shows that 66.7% spring, 81.3% shallow hand dug wells and 81% borehole water samples have a Larson index (LI) above 0.5, a threshold of corrosiveness of water. This study highlights the basic characteristics of surface and groundwater chemistry and its potential hazard for corrosion of pipes, and provides a baseline information and awareness to the city planners for urban management.


INTRODUCTION
With over one billion people worldwide lacking access to clean water and over 2 million deaths annually attributable to water-borne diseases, there has been massive increased reliance upon groundwater resources in many rapidly developing countries, including those of east, south-east Asia and Africa.More than 50% of the world's population lives in cities and the proportion is rapidly increasing (Afonso et al., 2006).The urban subsurface includes a network of pipes, conduits, metallic rods, reinforced concrete footings and other structures that modify the natural hydraulic conductivity of the geological materials and result in interaction with subsurface water.Being the major structural material used in the construction industry, steel corrosion problem has been attracting much attention only in developed countries (Hong et al., 2012).The problem of corrosion and scaling is quiet significant in developing countries and researches towards understanding and solving the problem are extremely low.Surface water and groundwater system is one of the most important influencing factors in foundation engineering and urban development and is required for design of structures.Hence monitoring and conserving this important resource is essential (Chatterjee et al., 2010).Understanding the hydrogeology of cities is imperative from various Gebremedhin, B., Tesfahunegn, A and Solomon, G (MEJS) Volume 5(1): [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70]2013 © CNCS, Mekelle University 52 ISSN:2220-184X perspectives such as; urban planning, water resource utilization and protection, foundation of structures, waste disposal and septic tank site selection etc.In general, the magnitude of damage that could occur on urban structures due to surface and groundwater depends on the amount of water table lowering and its fluctuation and chemical characteristics.
Describing the overall water quality and its impact on water pipes in the Mekelle area is complicated due to the spatial variability of the geology, hydrogeology and wide range of indicators that could be measured.Many organizations and scholars have studied the hydrogeology of Mekelle area (e.g.Abdelwassie Hussien, 2000;Hailmariam Hailu, 2010, Samuel Yehdego, 2003;Teklay Zeray, 2006;WWDSE, 2007).All of these studies focused on groundwater potential assessment, recharge and discharge estimation and few of these dealt with hydrogeochemistry but none on the impact of water quality on water pipes and other engineering structures.So, present paper tries to fill this gap and report the possible impact of water quality on water supply metal pipes.

GEOLOGICAL CONTEXT
Regionally, the geology of northern Ethiopia is well documented (e.g.Beyth and Shachnai, 1970;Beyth, 1971;Levitte, 1970;Bossellini et al., 1997;Russo et al., 1996) and locally few studies were conducted (e.g.Worash and Valera, 2001;Gebremedhin Berhane, 2010;Gebremedhin Berhane and Tenalem Ayenew, 2010;Tesfamichael Gebreyohannes, et al., 2010).The geology of northern Ethiopia is generally sub-divided into four major units: Basement complex, Paleozoic-Mesozoic Sedimentary Sequence, Cenozoic Trap volcanics and Sediments of the Ethiopian Rift (Levitte, 1970).The study arealies within the Mesozoic sedimentary sequences of the Mekelle Outlier (Fig. 1) which is a near circular area of about 8000 km 2 .According to Danielle (1943) the formation of sedimentary basins and deposition of sediments during Mesozoic in east Africa is supposed to be the result of Mesozoic transgression, which covered east Africa.This transgression is believed to be caused by general subsidence and/or the worldwide rise of the sea level (Bosellini et al., 1997).The history of the sedimentary basin in Mekele Outlier began in either Ordovician or Carboniferous and probably ended in lower Cretaceous before the eruption of the Trap volcanics (Beyth, 1971).Beyth (1971) studied the structure and tectonics of the sedimentary rocks of the Mekele Outlier and the escarpment.He also identified two main fault systems: the WNW running fault belts (Wukro, Mekelle, Chelekot and Felega Mariam) and Rift Valley fault system, which formed the escarpment and the Danakil depression (Fig. 1).
The study area, located at the center of the Mekelle Oultier, is mainly covered by limestone- , B., Tesfahunegn, A and Solomon, G (MEJS) Volume 5( 1   19 samples collected in the present study were collected in properly rinsed plastic 500 ml bottles and kept in refrigerator until delivered to the laboratory for analysis.The parameters like pH, electrical conductivity and temperature were measured in the field itself and the cations and anions were measured in the hydrogeochemical laboratory, Department of Earth Science, Mekelle University using atomic absorption spectrophotometer (VARIAN50B) and UV-spectrophotometer (Lamda EZ201) respectively.For reliability of the results the charge balance was computed and found it to be less than 5% for all data used in this study.Similar analytical technique was employed in the case of secondary data.Samples location (coordinates), geology, all measured parameters and simple statistics like maximum, minimum and average are given in tables 1, 2 & 3.

Gebremedhin
As the study is mainly focused on effects of water quality on metallic pipes, Larson Index (LI) proposed by Larson (1975) and Aggressive Index (AI) (American Water Works Association, 1977) are used in the present paper to check the corrosiveness of water to The Larson Index (LI) is an empirically derived ratio of specific ions which expresses the corrosive nature of a particular water sample with regard to the rate of metal corrosion.Water with LI values more than 0.5 values indicates high corrosive character (Singley et al., 1985).
The LI emerged from work with experimental solutions containing bicarbonate, chloride, and sulphate ions (Larson, 1975), and is not designed to be applied to waters with low hardness and small concentrations of dissolved solids (Singley et al., 1985).The LI may be applicable the water is moderately aggressive, and for values less than 10.0, the water is considered to be highly aggressive.

Groundwater dynamics
The study area forms upper part of AtbaraTekeze River basin.Surface drainage is generally in east to west direction and is controlled by geological structures.During the wet season, runoff from the upland area and hills is channeled by a number of gullies, rills and small streams, which in turn joins the major rivers.Floods generated from nearby cliffs passes through the city posing considerable challenges and threat to people by affecting and eroding footpaths, roads, culverts and houses.Depending on the degree of weathering and discontinuities, groundwater exists in all rock units in the form of a stratified or multi-aquifer system.Previous drilling data (e.g.Gebremedhin Berhane, 2002, WWDSE, 2007)      particularly the rising water levels affect the structural properties of soils and can reduce bearing capacity, cause swelling, and create hydrostatic uplift pressures (Brassington, 1990).
Groundwater level fluctuation is increasing from 3m in 2002 (Gebremedhin Berhane, 2002) up to 5m in 2012 (present study) indicating influence of expanding city on groundwater level fluctuation.It is expected to increase further in future if present trend continues, due to excessive abstraction and other anthropogenic impacts.Thus causing subsidence of structures and drying up of grazing and wet lands.

pH, Total dissolved solids, Electrical conductivity and Total hardness
In water samples pH varies from 6.9 to 8.6 with mean value of 7.6 and temperature from 25.6  to 18.7C with a mean value is about 22.4  C.Electrical conductivity (EC) varies from 542 to 5300 S/cm with a mean value of 1289.7S/cmandTotal dissolved solids (TDS) from 330-1,312 mg/l for spring, 43613007.13mg/l for hand dug wells and 3502195 mg/l for boreholes (Figs.4&5; Tables 1-3).The relationship of conductivity versus total dissolved solids (Fig. 4a) resulted in to the equation: TDS = 0.8089EC -117.59 with coefficient of correlation r = 0.9892.Most of the TDS values (Fig. 5) for the study area are beyond the permissible limits for corrosion (27 samples) and other domestic purposes (WHO, 2008).
Higher TDS shows longer residence times and increased water-rock interaction.TDS content is usually the main factor, which limits or determines the use of groundwater for any purpose and controls corrosion (Mahadevaswamy et al., 2011).Except in spring water, which represent rainwater composition, TDSin most of the hand dug wells and boreholes is high due to high solubility of carbonate minerals in the Mesozoic carbonate rocks which is dominant in  Calcium and magnesium, along with sulfates, chlorides, and bicarbonates, account for water hardness.The total hardness (TH) as CaCO 3ranges from 273 to 947 mg/l for spring, 228.42041 mg/l hand dug wells and 2101743 mg/l boreholes.The data indicate that only two samples from springs, two samples from hand dug wells and three samples from boreholes (only 5%) have TH values below 300 mg/l, which is the portable limit (WHO, 2008).The remaining samples exceed the limit, which accounts for the encrustation on watersupply distribution systems.Water with TH greater than 80 mg/l cannot be used for domestic purposes, because it coagulates soap lather.

Major Cation Chemistry
The relation between anions and cations (r0.95) and the average ionic distribution of major cations and anions are shown in Fig. 6a& b.Among cations, Ca 2+ is the most dominant ranging from 96.6324 mg/l for spring water, 84268.14mg/l for shallow hand dug well and 53.8631.8mg/l for borehole water followed by Na + , Mg 2+ and K + .Na + is next to calcium, varies 11-61 mg/l for spring, 18260 mg/l hand dug wells and 6.6202 mg/l for boreholes.
High concentrations of Ca 2+ and Na + in the groundwater are attributed to presence of calcite and gypsum in the rocks, mobile nature and cation exchange among minerals.Na + causes corrosion of metals and at high concentrations reduce the clay rich soil permeability and affects the soil structure (Seneviratne, 2007), but it is highly soluble and do not cause scaling.

Major anion chemistry
Bicarbonate is the dominant anion in shallow hand dug wells (340691 mg/l) and springs (178.6475.55mg/l) followed by SO 4  , Cl  and NO 3


. Samples from boreholes show SO 4  (10.51347 mg/l) as dominant anion followed by HCO 3  , Cl  and NO 3  (Fig. 6b).The main source of bicarbonate seems to be calcite and gypsum present in limestone and marl.In  chloride in shallow hand dug wells could be attributed to human and animal activities or contamination of surface and subsurface water and saline residues in soils.Chloride ions accelerate corrosion of stainless steel even at values as low as 50 mg/L (Seneviratne, 2007).
Nitrate is seldom contributed by decaying organic matter, sewage wastes, and fertilizers (Subrahmanyam and Yadaiah, 2001).Nitrate values range from 0.27 to 98.6; 0.21 to 336.1 and 0.29 to 272.8 mg/l for springs, hand dug wells and boreholes respectively.Higher values for nitrate in many shallow hand dug wells and few springs and borehole water samples is due to contamination directly from surface run-off and sewage.

Effect of groundwater chemistry on pipes, foundations and distribution system
Many studies suggest that urban and suburban development is closely associated with groundwater quality (Shanahan, 2009;Shanahan and Jacob, 2007).Chemical composition of groundwater is a measure of its suitability for domestic consumption; industrial purposes, and is considered as one of the important parameters in the selection of type of pipes that are used for water supply(lifting, transportation and distribution).Groundwater data in and around Mekelle (Tables 13)not only show lateral variability but also show higher values for SO 4 , TDS, Ca and HCO 3 which are mainly attributed to geology and to a limited extent to anthropogenic processes.In one of the studies, Mebrahtu and Zerabruk (2011) reported 1000mg/l mean value for TDS by for Mekelle City.
Generally, TDS above 1000 mg/l indicates its ability to conduct electric current, which is enough to cause serious electrolytic corrosion (USACE, 1994).As per the data about 27% samples were found to be above the minimum limit (Fig. 7).Thus suggests potential problem of corrosion for metal pipes in water supply systems and other engineering structures buried in the ground or in contact with water.Samples from limestone aquifer (e.g.Messebo area and Mekelle city) are found to be high in TDS compared to Quiha and Aynalem area.Total carbonate hardness in excess of 300 mg/l is considered as an indicator of incrusting water.
Water can cause corrosion involving many metals.Its deposition results in the development of scale depending on pH and alkalinity, hardness of water above 200 mg/l CaCO 3 , temperature (WHO, 2008).All samples from city show hardness above this threshold.
Alkalinity, pH, chloride, calcium and sulphate are primary water quality parameters that facilitate iron corrosion (Tang et al., 2006, WHO, 2008).Corrosion may uniformly attack a metal surface (uniform corrosion) or it may be focused at specific sites (Sarin et al., 2001).
The concentration of these major cations and anions were found to be high (Tables, 1-3).The corrosiveness of water can be estimated by the calculation of one or more corrosion indices.Higher concentration of chloride and sulphate also increases the rate of corrosion.The presence of chloride and sulphate in water promotes corrosion and pitting of iron and copper (Seneviratne, 2007).Because both ions form strong acids, they tend to increase the acidity of water.In addition to this they may increase corrosion rates by increasing the conductivity of the water.Sulphate may inhibit the formation of protective films by ion-pairing with calcium and magnesium (Schock and Neff, 1982).

CONCLUSION AND RECOMMENDATIONS
Water from shallow hand dug wells and boreholes not springs, shows high content of TDS and unsuitable water quality and its implications to corrosion and related problems.
The Larsen Index (LI)is found to be above 0.5 for 84% of the water samples, an indication for corrosive character of water and Aggressive Index (AI) is found below 10 for 82 % of Detailed and integrated study on corrosion, possible chemical reactions and impact of groundwater and soil/rock chemistry on engineering structures (e.g.concrete footings) is intercalation, dolerite, limestone, minor sandstone and alluvial deposits.Detail geological and structural descriptions of the study area are documented in Gebremedhin Berhane (2010); Gebremedhin Berhane and Tenalem Ayenew (2010) and Gebremedhin Berhane and Kristine Walraevns (2011) (Fig. 2).

Figure 1 .
Figure 1.Location of Mekelle City and schematic representation of the outlier and major structures near the city (modified after Bossellini et al., 1997).

Figure 2 .Figure 3 .
Figure 2. Geological map of Mekelle area and simplified cross-section along line AB.Grids are in UTM zone 37 (in meters East and North)(after Gebremedhin Berhane and Tenalem Ayenew, 2010).
for Mekelle City and its environs.Mekelle city is a good example for cities in the developing countries which rely on groundwater for their water supply.In general, the aquifers in Quiha and Aynalem area are dominantly related to fractured dolerite with minor limestone and at places fractured sandstone (e.g.BH-4 to BH-110, BH-89, BH-98), while the aquifer in the Mekelle city and Messebo area are dominantly found to be fractured limestone and marl/shale intercalated with gypsum layers (e.g.BH-52, BH-66, BH-70, BH-72, BH-73, BH-78, MCF-1, MCF-2, CF-3).Recent groundwater level measurements along the Illala River in five boreholes show variation in depth from 2 m to 6.3 m b. g. l.In the case of shallow hand dug wells, the depth is about 0.3m b. g. l. during rainy season in many parts of the city.An inventory of 70 boreholes and hand-dug wells within the city and its environs resulted in an average water level of 18 m b. g. l with a range of 0 to 70 m b. g. l.Fall and rise of groundwater, City and its environs.Anthropogenic activities could be another reason for high TDS in the case of hand dug wells.The electrical conductivity (salinity) of the groundwater in Mekelle area is mainly associated to the intercalation of gypsum layers in the shale and limestone, which dissolves to increase the sulphate value in groundwater.It is shown in Fig.5bwhere electrical conductivity shows high coefficient of correlation (0.8) with sulphate (EC=1.845SO 4 +692.39).

Figure 4 .
Figure 4. Average values of physico-chemical properties of spring, hand dug well and borehole water samples from Mekelle area.TH: total hardness; all concentrations are in mg/l, except EC (μS/cm).

Figure 5
Figure 5.a.Relationship of total dissolved solids (TDS) and electrical conductivity (EC) and b. electrical conductivity (EC) and sulphate concentration (SO 4 ) of water samples from Mekelle City and its environs (100 samples).
shallow hand dug wells CO 2 facilitates dissolution and solubility of calcite in comparison to springs and deep boreholes.Sulphate values range from 14.77 to 712; and 24.3 to 1607 mg/l for springs and shallow hand dug wells respectively.Higher values for sulphate are observed mainly in Messebo area and Mekelle city and gypsum beds which are reported in many boreholes logs(Samuel Yihdego, 2003;Hailmariam Hailu, 2010; WWSDE, 2007)  form the main source.Pyrite present in shale, also contributes but very limited.Sulfate concentrations more than 250 mg/l are objectionable for many domestic purposes(WHO, 2008).

Figure 6
Figure 6.a) Relation between concentration of anions and concentration of cations and b)Average ionic concentrations in water samples from springs, hand dug wells and boreholes (100 samples).

Figure 7 .
Figure 7. Variation of TDS in the water samples from Mekelle City.

Figure 8 .
Figure 8.Variation of total hardness in the water samples from Mekelle City and its environs.
Figure 10 presents variation of concentration of Ca + , Cl -and Sulphate of the water samples.In general, the methods used in this research have shown that the groundwater in Mekelle City is potentially aggressive or corrosive to metallic materials (pipes, iron used foundations, etc.) and has the tendency to affect behaviour of geologic materials used as foundations and as construction materials.

Figure 9 .
Figure 9.Selected indications of metallic pipe corrosion and scale from Mekelle City and Messebo Cement Factory water distribution systems: a) gate valve out of use, b) tread of pipe damaged, c) gate valve start to scale and rusting and d) metallic material, pipe, almost changed into soil due to corrosion.

Figure 10 .
Figure 10.Variation of calcium, chloride and sulphate in the water samples, Mekelle.
water samples, similarly an indication for corrosive character.Hence, both indices show similar result and in close agreement with field observations.The hydrogeological conditions and chemical composition of groundwater are important constraints and limiting factors in the future developments, type of materials used in water distribution systems and quality of constructions in the City.Unless such information is valued and updated during planning, design and construction, the damages and concerns presented in this paper are only warnings to other costly hazards.The paper highlights the importance of adequate groundwater studies in terms of dynamics and composition, for integration into design and city planning.As a summary, the following geo-hydrological issues in Mekelle City are important to planners and decision makers: Shallow groundwater systems susceptible to pollution and with high TDS and TH, which may interfere with construction activities and completed engineering structures.Potentially aggressive groundwater to pipes and other metallic materials used in water distribution systems and engineering structures.

Table 1 .
Chemical constituents and other properties of spring water from Mekelle area.

Table 3 .
Chemical constituents and other properties of water from borehole source from Mekelle area