Geochemical processes influencing groundwater quality for drinking and irrigation uses in Hombolo catchment

AHP

Analytic Hierarchy Process

ASS

atomic absorption spectroscopy

BOD

biological oxygen demand

COD

chemical oxygen demand

DRPC

Directorate of Research, Publication and Consultancy

EC

electrical conductivity

GIS

geographic information system

HPLC

high performance liquid chromatography

IRDP

Institute of Rural Development Planning

PI

permeability index

PSS

percentage soluble sodium

SAR

sodium adsorption ratio

TDS

total dissolved solids

UDOM

The University of Dodoma

WQI

water quality index

The sustainable development of any country in the world, particularly in semi-arid regions, is constrained by the availability of freshwater (Zaki et al. 2019). Water is required for both domestic and economic development of all sectors including industrial and agricultural sectors. The suitability of water for intended purpose is determined by its water quality parameters. The water quality parameter for both surface and groundwater resources is affected by many factors including climate, geology, soil type, land geomorphology, vegetation, flow condition, and human activities (Uddin et al. 2021). Since the quality of water is critical to life (Sheikhy Narany et al. 2014) understanding the major factors contributing to water quality is important.

Groundwater is freshwater stored and moving slowly in subsurface geological material (Jackson et al. 2001; Todd & Mays 2004). It is a fraction of precipitation that is recharged to the ground and reaches the water table. Groundwater accounts for approximately 30% of the world's freshwater (Mkongwa 2013; NGS 2021) and 97% of all freshwater liquids potentially available for human consumption (Machiwal et al. 2011). It is a crucial source of fresh water in a semi-arid region (Khan et al. 2011; Li et al. 2017; Mallick et al. 2018) for agricultural and domestic uses. A semi-arid region is the type of dry land characterized with a precipitation to evapotranspiration ratio of 0.2–0.5 (Lal 2004). This region has limited availability of surface water resources due to low rainfall and high evaporation rate (Heyns 2009).

Water resources in urban areas are under dynamic loading conditions as a result of rapid population growth, urbanization, and economic development (Gumbo 2004; Sato et al. 2013). The water demand is constantly increasing and exceeding supply in many cities around the world, both developed and developing (Gumbo 2004; Sarantuyaa 2010).

The high demand for water in developing cities especially those in semi-arid areas has resulted in frequency or over-pumping of groundwater resulting in a decline in the water table and allowing mixing of water from the different aquifers and deteriorating the quality of the groundwater (Kamra et al. 2002; Seyam et al. 2020). Water quality refers to the chemical, physical, and biological characteristics of water that meets the requirement for its intended use (Meybeck et al. 2006; Tyagi et al. 2013). Groundwater quality is influenced by human activities and natural processes (Nkotagu 1996a, 1996b; Tredoux et al. 2000; Rwebugisa 2008). As said earlier, the groundwater is the fraction of precipitation that recharges the aquifer. The chemical composition of the recharge water, interaction between water and soil or rock matrix, the residence time and the proceeding geochemical processes within the vadose zone, determines the quality of the groundwater (Babiker et al. 2007; Rwebugisa 2008; Tizro & Voudouris 2008; Vasanthavigar et al. 2010; Gad & El Osta 2020). The proceeding human activities on the surface of the Earth such as livestock keeping, agricultural irrigation, and urbanization influence the vadose zone's moisture content and geochemical reaction rate and in turn affect the hydrological characteristics and hydro-chemical reactions in the saturated zone (Li et al. 2017). Furthermore, physical alterations of the land, groundwater resource utilization, and waste emissions from anthropogenic activities cause periodic change in groundwater quality (Burri et al. 2019).

The quality of the groundwater for specific use is assessed based on the groundwater's chemical composition, physical parameters, and biological indicators (Babiker et al. 2007; Gray 2008; Rwebugisa 2008; Spellman 2017) as summarized in Table 1. The limiting level of each parameter for the definition of good quality of groundwater is defined by World Health Organization (WHO) standards or using a local standard developed by individual countries or authorities. The impact of each groundwater quality parameter is explained well by Omer (2019). For instance, chloride is found naturally in groundwater, but a high chloride concentration in freshwater could indicate wastewater pollution (Chatterjee 1996) and a higher concentration of biological indicators that the contaminant is due to human activities (Elisante & Muzuka 2017).

Groundwater quality deterioration is a well-known problem that has gotten a lot of attention since industrialization (Von der Heyden & New 2004; Arias-Estévez et al. 2008). Globally, groundwater quality deterioration has been accelerated by the increased water demand due to increased population, industrialization, expansion of agricultural activities, and urbanization (Heyns 2009; Kanagalakshmi & Nagan 2013; Zhang et al. 2019a, 2019b). When aquifers become contaminated with hazardous chemical products, they may become unusable for decades. Contaminants’ residence time in groundwater aquifer can range from weeks to decades, depending on the physicochemical properties of the compound and environmental conditions (Burri et al. 2019). Water quality assessment is critical for protecting and managing groundwater resources, which are increasingly vulnerable to human-induced physical and chemical pressures (Machiwal et al. 2018). Therefore, understanding the physical and geochemical processes that define the evolution of groundwater properties in arid and semi-arid regions can lead to a better understanding of hydrochemical systems and ultimately contribute to sustainable development and effective water resource management (Rezaei & Hassani 2018).

Tanzania has abundant water resources from rivers, lakes, and dams, but many piped urban water supplies in semi-arid areas (Shemsanga et al. 2017; Sangea et al. 2018) and other areas with insufficient and underdeveloped water supply infrastructure (Elisante & Muzuka 2016a, 2016b) rely on groundwater resources. The groundwater quality in Tanzania is variable in space and time. The quality of groundwater is influenced by the nature of lithology (Chacha et al. 2018; Makoba & Muzuka 2019), population density, land use, climate condition (Makoba & Muzuka 2019), groundwater overdraft (Mtoni et al. 2013), seawater intrusion on the coast (Mato 2015), and anthropogenic activities that affect shallow aquifers (Kassenga & Mbuligwe 2009; Elisante & Muzuka 2016a, 2016b). The major contaminants in the groundwater in Tanzania include potential toxic element (Tomašek et al. 2022), microbial contaminants (Elisante & Muzuka 2016b), nitrate, and others depending on the geology and anthropogenic activities in the region.

Dodoma city, the capital of the Tanzanian government, is located in the central part of the country. The city depends on a single source of groundwater located in the Hombolo water catchment. According to the 2012 population census, the Dodoma city population was 410,956 people (URT 2013). However, since the official and physical relocation of the national capital activities from Dar es Salaam to the Dodoma city in recent years, development of academic institutions like The University of Dodoma (UDOM), St. John University, Institute of Rural Development Planning (IRDP), among others, the population has steadily increased. In line with agricultural activities such as irrigation, livestock keeping, and so forth, urbanization, the increased population in Dodoma urban area has been accompanied by an increase in water demand and waste disposal (Kongola 1999; Shemsanga et al. 2018). Recent studies indicates that, groundwater over-extraction have resulted in potentially lowering the water tables and finally deteriorating the groundwater quality (Kongola 1999; Shemsanga et al. 2018). The study by (Nkotagu 1996a, 1996b; Rwebugisa 2008) quantified the recharge using different techniques such as chlorine mass balance and isotope. Another study was done by Onodera 1995 applying the isotope technique to confirm stable isotopic composition in deep groundwater caused by the partial infiltration process. (Nkotagu 1996a, 1996b; Rwebugisa 2008; Kisaka 2018) again analyzed the groundwater chemistry in the catchment. Furthermore, a study by (Elisante & Muzuka 2017; Nkotagu 1996a, 1996b; Kongola 1999) quantified the level of nitrate in the catchment. Massawe et al. (2017) analyzed the impact of human activities on groundwater vulnerability while Batakanwa et al. (2013); Shemsanga et al. (2017) posed a question on groundwater and soil salinity sources in the catchment. Recent studies have engaged in developing groundwater vulnerability map using geographic information system (GIS) and analytic hierarchy process (AHP) on identifying the most vulnerable areas (Kisaka & Lema 2016). There have been no studies that have attempted to explain the physicochemical evolution of the groundwater quality in the catchment. Therefore, at this stage it is necessary to gain a better understanding of the complex natural processes that generate the observed composition of the available groundwater chemistry, as well as all anthropogenic activities that could jeopardize its safety and security. The purpose of this study is to collect fragmented and existing groundwater quality information from various researchers in the Hombolo catchment to describe the geochemical processes governing groundwater quality and its suitability for irrigation and drinking uses. The overall goals of this study are to describe the groundwater's physicochemical and biological parameters, identify the major pollutant in groundwater, analyze its suitability for irrigation and domestic uses, and finally describe the mechanism and physiochemical processes that are responsible for the presence of major ions in groundwater.

Water that is deemed unsafe should be treated according to the standard of the intended user. Treatment of water is not feasible economically (Molinos-Senante et al. 2010). Understanding the threats to groundwater resource is critical for human health, food security, and ecosystem conservation. It draws the community's attention to groundwater quality issues and encourages it to avoid practices that may contribute to groundwater quality deterioration near groundwater resources. Furthermore, it assists policymakers and management personnel in better utilizing the information gathered for better groundwater management and monitoring for sustainable resource utilization.

The Hombolo catchment is located northern side from the Dodoma urban district along main road to Arusha region and lies between latitudes 05° 056′S to 06° 09′S and longitudes 35 °48′E to 35 °58′E (Figure 1). The Hombolo catchment is situated at an elevation between 1,016 m and 1,948 m above mean sea level. The catchment consists of two sub-catchments located north and south of the catchment. The Kinyasinguwe stream drains the northern part of the region, including the western sides all the way down to the northern sub-catchment, commonly named the Makutupora sub-catchment. The second major catchment area drains the southern hills of Dodoma to the Ilazo stream, which flows into Hombolo dam (Massawe et al. 2017).

The Hombolo catchment receives an average annual rainfall of 550 mm–614 mm (Rwebugisa 2008, Kisaka 2018) concentrated from November to April (Sandström 1995; Batakanwa et al. 2013). However, rainfall is unreliable and highly variable in time and space within the catchment resulting in insignificant or non-existent recharge (Kisaka 2018). The rainfall distribution is highly affected by topography, heavy rainfall is observed in the hilly regions, which are believed to be a recharge zone (Onodera et al. 1995; Rwebugisa 2008). The estimated recharge flux is between 1 and 2% of the annual rainfall (Rwebugisa 2008). The minimum and maximum temperatures are 14.1 °C and 31.4 °C observed in July and November respectively (Rwebugisa 2008, Kisaka 2018). The catchment is characterized with an annual average evapotranspiration rate of 2,000 mm (Nkotagu 1996a, 1996b; Rwebugisa 2008).

The Hombolo catchment is situated in the fractured crystalline basement rock such as granites and gneiss from the Precambrian Dodoma system (Nkotagu 1996a, 1996b; Kisaka & Lema 2016). It consists of granite, hornblende, and biotite gneiss with foliation development, pink pegmatite, and basic dike. The rocks are highly fractured and weathered, with three prominent faults (Kisaka 2018), which are Mlemu fault, Kitope fault, and Kirungule fault. Quartzites, ironstones, micaceous quartzites, quartzo-feldspathic schists, and ferruginous quartzites are some of the most common metamorphic rocks in the catchment (Massawe et al. 2017). The basin is mostly flat, punctuated by small hills, and covered in recent unconsolidated deposits, with crystalline rocks exposed primarily along the basin's margin (Nkotagu 1996a, 1996b). The regolith's thickness ranges from 50 m to 100 m, with an average of 60 m; it sits on top of a fractured aquifer with a hydraulic conductivity of m/s (Nkotagu 1996a, 1996b). The water table in the area is about 2 m–10 m measured from the surface of the earth (Nkotagu 1996a, 1996b; Rwebugisa 2008). Soil and other detrital deposits derived from granitic rocks, which are generally silty and sandy, cover a large portion of the area. Mbuga, clay, sand, and gravel, as well as concretion limestone, make up the surface deposits. White sandy soil, red loam soil, and black clay soil are the three main soil types (Rwebugisa 2008; Kisaka 2018). The mineralogical variation with depth is well described by Bowell et al. (1996) as presented in Figure 2.

Table 2 shows the range of the results of various studies on the physical and biological parameters of groundwater samples, as well as their comparisons to the WHO standard for drinking water.

The groundwater temperature is the only parameter whose range falls within the prescribed limit stated by WHO. The maximum value of other parameters their maximum value obtained fall outside the limit defined by WHO. The good feeling of the presented data can be judged when using an average value as done by Shemsanga et al. (2017); Kisaka (2018), where all the parameters were within the range except the fecal coliform and total coliform. Bacteria found in the soil, groundwater influenced by surface water percolation, and human or animal waste are all included in total coliforms. Fecal coliforms are a subset of total coliforms that are thought to be present only in the gut and feces of warm-blooded animals, for example birds and mammals.

The groundwater composition is determined by several factors such as composition and mixing ratio of end embers, hydrodynamic process (advection and hydrodynamic dispersion), a chemical reaction taking place within a phaneritic zone, unsaturated zone, and with the aquifer matrix.

Major cation and anion have been discussed by previous studies in the study area to deduce the water type or facies in the catchment. Shemsanga et al. (2017) analyzed about 27 samples from shallow dug well and deep groundwater samples and found the concentration of major cations and anion reported in meq/L as Na⁺>Mg²⁺>Ca²⁺>K⁺ and respectively leaving the major cation and anion as Na+ and Cl- respectively and major water type were Na-Cl followed by CaMgCl and CaHCO₃ in order of decreasing. Another study done by Rwebugisa (2008) analyzed the chemical composition of groundwater from shallow dug well and the deep ground boreholes and found the major cations in the order of Ca²⁺>Na⁺>Mg²⁺>K⁺ while the anions were in the order of HCO₃⁻>Cl⁻>NO₃⁻>SO₄²⁻ giving out the major type of water as CaHCO₃. A study by (Kisaka 2018) deduced CaHCO₃ as the water type with Ca²⁺ and HCO₃⁻ being major cation and anion respectively while other cations were ordered as Na⁺>Mg²⁺>K⁺ and anions as SO₄²⁻>Cl⁻>NO₃⁻. (Nkotagu 1996a, 1996b) sampled about 51 samples from 10 m deep and shallow groundwater for geochemistry analysis in the catchment using High-Performance Liquid Chromatography (HPLC) for cation analysis and Atomic Absorption Spectroscopy (AAS) for anion analysis. Major cation and anion were ordered as Na⁺>Ca²⁺>Mg²⁺>K⁺ and Cl⁻>HCO₃⁻>SO₄²⁻>NO₃⁻>SiO₄⁻ and giving NaCl the water type in the catchment. Other studies were done by Shindo 1989 for both confined and unconfined aquifer as presented by Rwebugisa (2008) and found that the dominant anion was HCO₃⁻, with a Cl⁻ percent higher than that of soil water, and the groundwater composition was between Ca+Mg and Na+K. The chloride concentrations in the groundwater ranged from 27 to 143.5 mgL⁻¹, with a standard deviation of 28.5 mg L⁻¹. Higher concentrations were found at greater depths, and they increased alongside the direction of groundwater flow. Another study that analyzed the geochemical content of water in the area was done by Bowell et al. (1996), which found the water was essentially Na-Ca-Cl-HCO₃ with minor K⁺, Mg²⁺, F⁻ and SO₄²⁻.

Groundwater nitrate contamination is a global issue that requires technical and policy solutions to mitigate its effects on humans and the environment (Huno et al. 2018). Waters containing more than 50 mg/L of nitrate can cause health problems in humans and animals (WHO 2003; Gatseva & Argirova 2008) such as methemoglobinemia (Reynolds-Vargas et al. 2006). Nitrate is a chemically stable (Reynolds-Vargas et al. 2006; Elisante & Muzuka 2017) and non-reactive nitrogen species occur naturally as part of the nitrogen cycle. Under various environmental conditions, nitrate can be reduced to nitrite, ammonia, and nitrous oxides, as well as nitrogen gas, by microbial and chemical processes (Elisante & Muzuka 2017; Huno et al. 2018). Groundwater occurs naturally with low concentrations of nitrate (Elisante & Muzuka 2017). The presence of nitrate in groundwater at high concentrations could be an indirect indicator of the presence of other contaminants resulting from human activities (Reynolds-Vargas et al. 2006). Leaching and oxidation of nitrogenous compounds incorporated in rocks are natural sources of nitrate in groundwater (Holloway & Dahlgren 2002) and others are fixation of leguminous plants and microorganisms (Edmunds & Smedley 1996).

Several studies indicated the nitrate concentration in the Hombolo catchment have been undertaken by previous researchers. Kongola (1999), Rwebugisa (2008), and Nkotagu (1996a, 1996b) reported that a maximum of about 100 mg/L were observed in the early 1990s and coinciding with the maximum pumpage rate of 30,000 m³ per day. The sought source of nitrate was the intensive use of fertilizers in grape farms (Rwebugisa 2008). Approximately three tons of nitrate were pumped per day, a value that is significantly higher than that of any known agricultural input (IAEA 2002; Verhagen 2003). Several villages were relocated outside the basin's catchment area, and an agricultural production program was suspended to mitigate the threat of pollution and in turn the concentration decreased.

An International Atomic Energy Agency (IAEA) regional project study used a tritium trace to learn the turnover time and found no tritium which implies a mean residence time of greater or equal to 200 years. The increase of nitrate levels in certain boreholes over 1–2 years could not have affected the entire groundwater aquifer, as determined by crude mass balance calculations (Verhagen 2003). Radiocarbon measurements done by (Verhagen 2003) confirmed this conclusion with a narrow range of pMC, putting the upper limit on mean residence time at around 3,000 years. The only explanation for the nitrate spread is that the pressure gradient near the wells is reversed, drawing in shallow groundwater rich in nitrate. Increasing the pump rate reduced the upward pressure gradient. The high nitrate concentrations in shallow groundwater may have accumulated over time from non-anthropogenic sources.

A recent study by Massawe et al. (2017) found that the nitrate concentration averaged 51.35 mg/L for shallow wells and 37.91 mg/L for deep boreholes. Rwebugisa (2008) reported it to range between 0.89 mg/L and 150 mg/L with an average value of 37.78 mg/L while Kisaka (2018) found it to range between 1.35 mg/L and 82.87 mg/L. The historical concentration of nitrate in the catchment is presented in Figure 3. This indicates that the nitrate concentration in recent years has been increasing again, coinciding with frequency pumpage to meet the water demand in the city of Dodoma. In the period between 2007 to present, the population has increased considerably.

The nitrate is highly found in the shallow depth (Kisaka 2018) and is mainly attributed to anthropogenic activities such as agricultural activities, natural weathering of minerals such as the dissolution of plagioclases and amphibole, along with the leaching of surficial and salt and nitrification processes (Nkotagu 1996a, 1996b). The spread of nitrate is promoted by the mixing of water from the shallow aquifer and deep aquifer due to over-pumping to meet the demand for water (Verhagen 2003).

Water quality parameters were weighted according to influence on the general nature of drinking water. The EC and TDS were given 5 as the highest weight, while Ca²⁺, Mg²⁺, pH, and HCO₃⁻ were given 2 as the lowest weight. Other parameters were assigned a value of 3 or 4 based on their influence on groundwater quality as determined from the literature. Table 3 shows WHO allowable limits (S), average concentrations of each parameter in the catchment (C) and the weight of each parameter.

Table 4 is used to classify the suitability of water for drinking. The WQI ranges between 51.07 and 178 with an average of 104 in the catchment. The WQI clusters from different researchers for year 1989, 1996, 2008, 2017, and 2018 was determined and indicated that water is poor for human consumption and with time its quality is deteriorating exponentially as shown in Figure 4. This may be due to population increase and rapid urbanization process taking place in the city of Dodoma.

The value of SAR, PSS, and PI were compared with the suitability classes shown on Table 5. Low value of SAR and PSS and high value of PI are suitable for water to be used for irrigation.

SAR, PSS, and PI were determined from 140 samples. SAR values ranged from 0.03 to 40.04, with an average of 4.44, and 75% of the sample had a SAR value less than 5.16. The PI ranged from 25.43 to 168.83, with an average of 74.49, and 75% of the sample had a PI between 57.49 and 168.83. The PSS, on the other hand, ranged between 6.54 and 99.06, with an average value of 50.89, and 75% of the sample had a PSS value less than 59.95. Figure 5 depicts the box plot of each index. Based on the statistical values obtained and the ranges shown on Table 5 for water quality, the groundwater in the catchment is suitable for irrigation because the SAR, PSS, and PI are within acceptable ranges.

The pH value of water in the catchment ranges between 5.98 and 9.4 from different kinds of literature with the most area having a pH greater than 7. This implies HCO₃ depletion. Since HCO₃ doesn't form carbonic acid, a high ratio of m (Ca²⁺+ Mg²⁺)/HCO₃. Figure 6(c) indicates other sources of Ca++ and Mg++ such as cation exchange or gypsum dissolution. A low ratio indicates an additional HCO₃ from weathering of albite.

The groundwater chemistry data were drawn on Gibb's diagram (Figure 7) and the majority of samples fall within the rock–water interaction zone and evaporation zone. The rock–water interaction zone indicates the chemistry of the groundwater is influenced by the rock-water reactions when water percolates. The evaporation also influences the groundwater chemistry as clearly shown by Gibb's diagram and this explains the reason for the high TDS value in the Hombolo catchment.

The rapid growth of urban area affects groundwater quality due to increased population density, building density, demand of water, and high waste discharge rate. Various contaminants from urban through surface runoff contaminant with the aquifer and deteriorate its quality. The groundwater quality of Hombolo catchment has been analyzed to evaluate the suitability for drinking and irrigation use. The study developed a geochemical process that influences the quality of water in the catchment. The Hombolo catchment has a variety of physical, biological, and groundwater facies. The presence of fecal coliforms in groundwater indicates surface runoff interaction with groundwater reservoir and possible pathogen presence in groundwater. This reduces the suitability of water for domestic use. The TDS is vital for life because it affects the temperature of groundwater. Understanding this parameter is critical because too low or too high levels can be harmful to water users. In the catchment, higher TDS values reflect both anthropogenic and natural influences on groundwater chemistry. Also, groundwater facies vary in space and time. The increase in population increases water demand, causing overpumping of wells and mixing of shallow and deep aquifers, contributing to the increase in nitrate concentration. To avoid nitrate mixing, pumping should be monitored from high-yielding boreholes. The groundwater in the catchment is suitable for irrigation, but its quality for drinking purposes is deteriorating over time. Cation exchange, evaporation, and dissolution of minerals such as amphibole, albite, and pyroxene are the primary processes that influencing groundwater chemistry. The dominant nitrate transport mechanism and origin are unknown in the catchment. More research is needed on the spatial distribution of water quality parameters. Geospatial distribution of groundwater quality is critical for environmental assessment and future groundwater management plans.

The author wishes to thank The University of Dodoma, particularly the Directorate of Research, Publication, and Consultancy (DRPC), through the Junior Academic Staff portal 2020 for providing fund.

The author declares no conflicts of interest with this article.

All relevant data are included in the paper or its Supplementary Information.