The sustainability of groundwater in semi-arid regions: the case of Makutupora Basin in Tanzania

Globally, groundwater plays a significant role in water supplies and sustainable development. Over two billion people rely on groundwater for daily water needs, and more than half of the world's population access their water from groundwater (Guppy et al. 2018). Groundwater systems offer 24% of direct industrial water supply, 36% of potable water, and 42% of agriculture worldwide (Lee et al. 2018). Due to its widespread occurrence, relatively good quality, high reliability during droughts, and generally affordable development costs, the use of groundwater has substantially increased in recent decades, much so within the developing world (Tuinhof et al. 2011; Baguma et al. 2017). Further, groundwater is widely regarded as a common pool resource which largely impacts its sustainable development. Yet, as a common pool resource that sustains humans and ecosystems, groundwater is often subject to unsustainable levels of exploitation and depletion. This is due to the increase in population and hence a growing demand for food (McLaughlin & Kinzelbach 2015). Due to intensive groundwater exploitation, saltwater intrusion and land subsidence may become severe concerns in some areas (Michael et al. 2017). Additionally, the poor are frequently disproportionately affected by these new groundwater risks (Baguma et al. 2017). Sustainability should be practiced through the principles of safe yield, sustainable yield, sustainable groundwater development, and sustainable groundwater management to combat these risks and increase the usability of this essential natural resource.

Groundwater sustainability depends on the environment, which ranges globally from arid to humid (Cuthbert et al. 2019), and changes in quantity and quality. Additionally, not all aquifers are renewable at a human time scale. Generally, groundwater sustainability is defined as maintaining long-term, dynamically stable storage and flow of high-quality groundwater using inclusive, equitable, and long-term governance and management (Gleeson et al. 2020). Groundwater sustainability is increasingly incorporated into groundwater policies, laws, and regulations in several places worldwide (Owen et al. 2019). As a policy instrument, the goals might be variable but often aim to prevent groundwater over-drafting, including measures to ensure water supply into the future or to protect groundwater-dependent systems (Ross 2016).

To ensure groundwater sustainability, it is important to understand the dynamics of groundwater systems, the impacts of human activities, and the needs of different stakeholders through an integrated approach. An integrated approach brings together experts from various fields, including hydrology, engineering, geology, ecology, sociology, economics, and law, to help identify and address the complex interactions between groundwater and surface water systems and develop more holistic and sustainable management strategies (Bren d'Amour et al. 2017). However, in Africa, the lack of an integrated management approach significantly impacts the interactions between groundwater sustainability policy and technology implementation (Gaye & Tindimugaya 2019). This is due to establishing environmental standards, appropriate solutions, and implementing technology and problem-solving requiring joint problem-solving between science, technology, policy, environment and society (Bren d'Amour et al. 2017). Integrated groundwater management helps improve the management strategy's effectiveness by identifying and addressing the root causes of problems rather than just the symptoms. This approach leads to more targeted and efficient management solutions that are more likely to be successful in the long term (Maheshwari et al. 2014).

Groundwater sustainability indexing is vital for groundwater management as it involves technical, environmental, social, and economic aspects of sustainability (Senent-Aparicio et al. 2015). Much research has been conducted in the Makutupora basin, a semi-arid region in Africa to ensure groundwater resource sustainability (Shindo 1991; Rwebugisa 2008; Seddon 2019; Zarate et al. 2021). However, these studies relied on technical hydrogeological parameters, including recharge and ignored environmental, social, economic, and policy aspects. Sustainability should ensure both technical and management aspects, thus the development and use of the resource without causing unacceptable environmental, economic, or social consequences (Senent-Aparicio et al. 2015). Inadequate to no consideration of all sustainability aspects may result in overexploitation, water quality degradation, stakeholder conflicts, economic losses, and legal challenges. It is crucial to consider these factors when developing and implementing management strategies to ensure the sustainable use and protection of groundwater resources and the environment. This paper helped to raise attention, particularly in developing countries, that sustainable groundwater management includes not only hydrogeology, i.e., the balance between recharge and abstraction, but also the environment, that the use and development of groundwater go hand in hand with the protection and preservation of the natural environment, social-economic factors that influence groundwater resource, and policy to ensure guidelines and regulations in place. As a result, a thorough assessment of groundwater sustainability in semi-arid regions is required.

Hydrogeologically, the water table is 40 m in the well-field area and remains at 2.5 m depth around Maya Maya, generally varying according to topography. The aquifer in the area is sensitive to rainfall, characterized by abrupt changes in groundwater levels during wet and dry seasons. The main formations are mbuga clay, clay, calcrete, sand, red silt, silt, sand gravel, weathered rock, and basement rock. The topmost stratum is mbuga clay, with an average thickness of 40 m, while the water-bearing formations are calcrete, sand, gravel, and weathered and fractured granite (Shindo 1991).

From a geology point of view, the Dodoma area lies in a groundwater basin known as the Hombolo basin. The basin is situated in the Dodoma Craton's fractured crystalline basement region. It is believed that the Chinene Mountains represent a fault block. The mountains resemble an uplifted plateau, and their surface was assumed to be level before uplifting with the flat-lying area to the south. The prominent topographic features of the area are the northwest-trending Chinene Mountains, which exceed about 2,000 m.a.s.l in elevation and the gently rolling plains dotted with inselbergs and ‘mbuga’ with an elevation of about 1,100 m. The well-field is formed by two grabens interconnected with each other. In the north, the Kitope graben extends in the northeast direction parallel to the northeast part of the Little Kinyasungwe River, and it is bounded by Zanka and Kitope faults. South of this graben, there is a Makutupora graben which follows the Mlemu fault extending in the northeast direction. They are downthrown for about 100 m (Rwebugisa 2008).

The northeast and southwest trending faults or shear zones structurally control drainage within the mountain range. There are no perennial rivers, while ephemeral rivers flow only after heavy rainstorms in the rainy season (Shindo 1991). Little Kinyasungwe is the main river in the area, originating from the northeast part of the catchment. Its first part flows in the southwest direction and forms a swamp at the Makutupora well-field, which dries up during the dry season. After developing a swamp at the well-field, the river discharges to the Hombolo dam in the Southeast part of the basin. In areas around Maya Maya, Mtungutu and Mkondai villages, streams disappear and reappear in the alluvial fans and buried stream channels. In the west part of the well-field, streams flow in the southwest direction and discharge in the swamp area.

This study incorporates both primary and secondary data sources. The primary data was obtained through laboratory analysis of electrical conductivity (EC) and nitrate concentration in groundwater samples collected from 14 boreholes during the study period. The analysis was conducted at the College of Earth Sciences and Engineering (CoESE) laboratory, University of Dodoma. Additionally, during the study period, the progress in protected natural areas in the basin was gathered through discussions with catchment experts/staff who regularly monitor the conservation progress of protected natural areas.

The secondary data utilized in this study include various sources. Groundwater abstraction and groundwater level data and the financial expenditures and estimated costs associated with sustainable water resource management were obtained from the Wami/Ruvu Basin-Dodoma Office. Information regarding the annual volume of water consumed and the population size benefiting from the aquifer was acquired from the Dodoma Water Supply and Sewerage Authority (DUWASA). The assessment of aquifer vulnerability was based on the work of Kisaka & Lema (2016), while land use and land cover data were sourced from Mseli et al. (2021). The Basin Organizational Capacity Index, which pertains to water management rules, was provided by the Ministry of Water, which is responsible for formulating and implementing water management rules and policies. Human development index (HDI) income data were obtained from the National Bureau of Statistics and the Tanzania Revenue Office in Dodoma. The Ministry of Education, Science, and Technology's Dodoma office provided information on the variation in the aquifer's HDI-Education indicator.

Each of these indicators was analyzed separately following a pressure-situation-response model. It consists of analyzing the relationship between human activities (pressure) and their impact on the state of the environment (situation), causing a series of actions to be taken to solve the problems that are created (response). This model incorporates cause and effect relations, informing users and decision-makers of the relationship between various parameters and therefore helps establish or reorientate policies (Parris 1999; Olsthoorn et al. 2001). Several parameters adequately represent the individual processes in each indicator (Table 1) as modified from Senent-Aparicio et al. (2015). According to Equation (1), each indicator was given the same weight to avoid bias in the results, and the ASI, like the other indicators, ranged between 0 and 1. The weight of each indicator was established by consensus among the various stakeholders in the basin.

From the final ASI value, sustainability could be considered unstable if ASI < 0.5, moderate if the range varies between 0.5 and 0.8, and stable if ASI > 0.8 (Nations 1990).

Hydrogeology indicators explain aspects of groundwater's quantitative and qualitative state and possible trends and impacts on planning and sustainable management of available water resources (Lambán et al. 2011). This study used the classification score method to integrate the indicators. The score was defined with quantitative values (ranging from 0 to 1) and qualitative categories depending on the parameter. For all indicators, a score of 1 indicates extreme sustainability, while a 0 indicates unstable status.

The four indicators calculated in the quantitative hydrogeological category are presented in Table 2. These indicators include renewable groundwater volume per capita (⁠⁠), groundwater abstraction and recharge index (I_Ab/R), index of change in groundwater storage (I_(ΔH)i), and the water level trend index in observation wells (I_SV).

Table 2

Quantitative hydrogeological indicators

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		⁠: Renewable groundwater volume per capita GWR: Total renewable groundwater resources (m3/yrs.) P: Population size in the study area	< 500	0
			500 ≤ ≤ 1,000	0.25
			1,000 ≤ ≤ 1,500	0.5
			1,500 ≤ ≤ 2,000	0.75
			> 2,000	1
		I(ΔH)i: Index of change in groundwater storage Hi: Groundwater level in a month i (December 2018) Hmax: Maximum groundwater level in the study period (2010–2018) Hmin: Minimum groundwater level in the study period (2010–2018)	I(ΔH)i < 20%	0
			20% ≤ I(ΔH)i ≤ 40%	0.25
			40% ≤ I(ΔH)i ≤ 60%	0.5
			60% ≤ I(ΔH)i ≤ 80%	0.75
			I(ΔH)i > 80%	1
Situation		IAb/R: Groundwater abstraction and recharge index ΣAb: Total groundwater abstraction ΣR: Total groundwater recharge	IAb/R > 80%	0
			60% < IAb/R ≤ 80%	0.25
			40% < IAb/R ≤ 60%	0.5
			20% < IAb/R ≤ 40%	0.75
			IAb/R ≤ 20%	1
Response	(Canadian Water Sustainability Index, CWSI)	ISV: Water level trend index in observation wells r: Number of wells with increasing water level n: Number of wells where the water level has not changed ∑W: Total number of observation wells	ISV < 20%	0
			20% ≤ ISV ≤ 40%	0.25
			40% < ISV ≤ 60%	0.5
			60% < ISV ≤ 80%	0.75
			ISV > 80%	1

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		⁠: Renewable groundwater volume per capita GWR: Total renewable groundwater resources (m3/yrs.) P: Population size in the study area	< 500	0
			500 ≤ ≤ 1,000	0.25
			1,000 ≤ ≤ 1,500	0.5
			1,500 ≤ ≤ 2,000	0.75
			> 2,000	1
		I(ΔH)i: Index of change in groundwater storage Hi: Groundwater level in a month i (December 2018) Hmax: Maximum groundwater level in the study period (2010–2018) Hmin: Minimum groundwater level in the study period (2010–2018)	I(ΔH)i < 20%	0
			20% ≤ I(ΔH)i ≤ 40%	0.25
			40% ≤ I(ΔH)i ≤ 60%	0.5
			60% ≤ I(ΔH)i ≤ 80%	0.75
			I(ΔH)i > 80%	1
Situation		IAb/R: Groundwater abstraction and recharge index ΣAb: Total groundwater abstraction ΣR: Total groundwater recharge	IAb/R > 80%	0
			60% < IAb/R ≤ 80%	0.25
			40% < IAb/R ≤ 60%	0.5
			20% < IAb/R ≤ 40%	0.75
			IAb/R ≤ 20%	1
Response	(Canadian Water Sustainability Index, CWSI)	ISV: Water level trend index in observation wells r: Number of wells with increasing water level n: Number of wells where the water level has not changed ∑W: Total number of observation wells	ISV < 20%	0
			20% ≤ ISV ≤ 40%	0.25
			40% < ISV ≤ 60%	0.5
			60% < ISV ≤ 80%	0.75
			ISV > 80%	1

View Large

Groundwater contamination by nitrates from farming and livestock is the most severe environmental and public health problem (Senent-Aparicio et al. 2015). In the study area, drinking water standards for the World Health Organization (WHO) and Tanzania Bureau of Standards (TBS) have been used to characterize the current status and trends in groundwater quality and identify challenges over time. The indicators used are presented in Table 3, which include the index of change in groundwater quality in a study period (I_Q), the index of change in groundwater quality compared to short-term and long-term periods (I_ΔQ), the nitrate index, and the electrical conductivity (EC) index.

Table 3

Qualitative hydrogeological indicators

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		IQ: Index of change in groundwater quality in a study period (2010–2018) pmax: Maximum measured EC during the study period pmin: Minimum measured EC concentration during the study period p: Current EC	IQ ≤ 20%	0
			20% < IQ ≤ 40%	0.25
			40% < IQ ≤ 60%	0.5
			60% < IQ ≤ 80%	0.75
			IQ > 80%	1
		IΔQ: Index of change in groundwater quality compared to short-term and long-term periods PS: Average EC in the short period (2015–2018) PL: Average EC over the long period (2010–2018)	IΔQ ≥ 20%	0
			10% ≤ IΔQ < 20%	0.25
			−10% ≤ IΔQ < 10%	0.5
			−20% ≤ IΔQ < −10%	0.75
			IΔQ < −20%	1
Situation		Nitrate index	IN ≥ 100	0
			50 ≤ IN < 100	0.5
			IN < 50	1
		IN: Sampling point index N50: Number of sampling points below 50 mg/L NT: Total sampling point	IQ ≤ 20%	0
			20% < IQ ≤ 40%	0.25
			40% < IQ ≤ 60%	0.5
			60% < IQ ≤ 80%	0.75
			IQ > 80%	1
Response		Improvement of nitrate concentration in the study area	Very poor	0
			Poor	0.25
			Medium	0.5
			Good	0.75
			Excellent	1

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		IQ: Index of change in groundwater quality in a study period (2010–2018) pmax: Maximum measured EC during the study period pmin: Minimum measured EC concentration during the study period p: Current EC	IQ ≤ 20%	0
			20% < IQ ≤ 40%	0.25
			40% < IQ ≤ 60%	0.5
			60% < IQ ≤ 80%	0.75
			IQ > 80%	1
		IΔQ: Index of change in groundwater quality compared to short-term and long-term periods PS: Average EC in the short period (2015–2018) PL: Average EC over the long period (2010–2018)	IΔQ ≥ 20%	0
			10% ≤ IΔQ < 20%	0.25
			−10% ≤ IΔQ < 10%	0.5
			−20% ≤ IΔQ < −10%	0.75
			IΔQ < −20%	1
Situation		Nitrate index	IN ≥ 100	0
			50 ≤ IN < 100	0.5
			IN < 50	1
		IN: Sampling point index N50: Number of sampling points below 50 mg/L NT: Total sampling point	IQ ≤ 20%	0
			20% < IQ ≤ 40%	0.25
			40% < IQ ≤ 60%	0.5
			60% < IQ ≤ 80%	0.75
			IQ > 80%	1
Response		Improvement of nitrate concentration in the study area	Very poor	0
			Poor	0.25
			Medium	0.5
			Good	0.75
			Excellent	1

View Large

Pressure on the environment has been evaluated based on the variation in the area of the basin used for agriculture and the population variation throughout the study period since the proportion of urban and agricultural areas correlates with the quality of the resources. Population growth is a critical feature affecting the long-term sustainability of water resources. The state of the environment has been defined from the percentage of basin area under natural vegetation (A_v). In contrast, the response to these pressures has been evaluated based on increased vegetation cover during the study period and legally declared as protected natural areas (Table 4). The municipal population data were projected from the National Census of 2012–2020. The land use land cover area was calculated from the remote sensing map (Mseli et al. 2021). These recent data have also been used to determine the percentage of each part of the basins with natural vegetation.

Table 4

Environment indicators

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		GRI: Groundwater drought index Dy,m: Water level in month m of year y μD,m: Average water level data in month m in year D δD,m: Standard deviation of water level data for month m in year D (2010–2018)	GRI ≥ 2	Very intense wet situation
			1.5 ≤ GRI < 2	Wet situation
			−1 ≤ GRI < 1.5	Normal situation
			−2 ≤ GRI < −1	Drought
			GRI < −1	Intense drought
Situation		Iv: Groundwater resources vulnerability index ΣAv: Area of the aquifer that has a high potential for vulnerability ΣA: Total area of the aquifer	Iv > 80%	0
			60 < Iv ≤ 80%	0.25
			40 < Iv ≤ 60	0.5
			20 < Iv < 40	0.75
			Iv ≤ 20	1
		IS: Stress index C: Volume of water consumed annually (m3) GWR: Renewable groundwater resources (m3)	IS ≤ 20%	0
			20% < IS ≤ 40%	0.25
			40% < IS ≤ 60%	0.5
			60% < IS ≤ 80%	0.75
			IS > 80%	1
	Percentage of basin area under natural vegetation (Av)		IAv ≤ 20%	0
			20% < IAv ≤ 40%	0.25
			40% < IAv ≤ 60%	0.5
			60% < IAv ≤ 80%	0.75
			IAv > 80%	1
Response		Progress in protected natural areas in the basin, in the period studied	Δ ≤ −10%	0
			−10% < Δ ≤ 0%	0.25
			0% < Δ ≤ 10%	0.5
			10% < Δ ≤ 20%	0.75
			Δ > 20%	1

Dimension .	Index .	Parameter .	Level .	Score .
Pressure		GRI: Groundwater drought index Dy,m: Water level in month m of year y μD,m: Average water level data in month m in year D δD,m: Standard deviation of water level data for month m in year D (2010–2018)	GRI ≥ 2	Very intense wet situation
			1.5 ≤ GRI < 2	Wet situation
			−1 ≤ GRI < 1.5	Normal situation
			−2 ≤ GRI < −1	Drought
			GRI < −1	Intense drought
Situation		Iv: Groundwater resources vulnerability index ΣAv: Area of the aquifer that has a high potential for vulnerability ΣA: Total area of the aquifer	Iv > 80%	0
			60 < Iv ≤ 80%	0.25
			40 < Iv ≤ 60	0.5
			20 < Iv < 40	0.75
			Iv ≤ 20	1
		IS: Stress index C: Volume of water consumed annually (m3) GWR: Renewable groundwater resources (m3)	IS ≤ 20%	0
			20% < IS ≤ 40%	0.25
			40% < IS ≤ 60%	0.5
			60% < IS ≤ 80%	0.75
			IS > 80%	1
	Percentage of basin area under natural vegetation (Av)		IAv ≤ 20%	0
			20% < IAv ≤ 40%	0.25
			40% < IAv ≤ 60%	0.5
			60% < IAv ≤ 80%	0.75
			IAv > 80%	1
Response		Progress in protected natural areas in the basin, in the period studied	Δ ≤ −10%	0
			−10% < Δ ≤ 0%	0.25
			0% < Δ ≤ 10%	0.5
			10% < Δ ≤ 20%	0.75
			Δ > 20%	1

View Large

Social and economic indicators are used to assess the impact of groundwater sustainability on society and the economy. Groundwater provides public water resources worldwide, so consideration of social dimensions and the impact of water on social factors are significant issues in sustainable groundwater management and play an important role in achieving sustainable development. Hence, quantifying sustainable water resource management should consider social dimensions (Bui et al. 2018). Social sustainability indicators depend on the status of the study area and may differ according to the nature and requirements of the local community. The indicators used in the study for the social and economic dimensions are presented in Table 5. These indicators help to identify the social and economic implications of groundwater sustainability and inform decision-making about groundwater management and policy.

Table 5

Social-economic indicators

Dimension .	Index .	Parameter .	Level .	Score .
Pressure	(Bui et al. 2018)	IDp: Index of population dependence on groundwater GWps: Population that relies on groundwater resources ΣP: Total population	IV > 80%	0
			60 < IV ≤ 80%	0.25
			40 < IV ≤ 60	0.5
			20 < IV < 40	0.75
			IV ≤ 20	1
Situation	(Senent-Aparicio et al. 2015)	IHDI Income: index of change in HDI-Income during the study period (2010–2018) HDI IncomeStart: HDI-Income at the beginning of the study period HDI IncomeEnd: HDI-Income at the end of the study period	IHDI-E ≤ −20%	0
			−20% < IHDI-E ≤ −10%	0.25
			−10% < IHDI-E ≤ 0%	0.5
			0% < IHDI-E ≤ 10%	0.75
			IHDI-E > 10%	1
Response	(Bui et al. 2018)	II: Management expenditure index S: Amount of money spent on sustainable water resource management in the study area E: Estimated cost of sustainable water resource management in the study area	II ≤ 20%	0
			20% < II ≤ 40%	0.25
			40% < II ≤ 60%	0.5
			60% < II ≤ 90%	0.75
			II > 90%	1

Dimension .	Index .	Parameter .	Level .	Score .
Pressure	(Bui et al. 2018)	IDp: Index of population dependence on groundwater GWps: Population that relies on groundwater resources ΣP: Total population	IV > 80%	0
			60 < IV ≤ 80%	0.25
			40 < IV ≤ 60	0.5
			20 < IV < 40	0.75
			IV ≤ 20	1
Situation	(Senent-Aparicio et al. 2015)	IHDI Income: index of change in HDI-Income during the study period (2010–2018) HDI IncomeStart: HDI-Income at the beginning of the study period HDI IncomeEnd: HDI-Income at the end of the study period	IHDI-E ≤ −20%	0
			−20% < IHDI-E ≤ −10%	0.25
			−10% < IHDI-E ≤ 0%	0.5
			0% < IHDI-E ≤ 10%	0.75
			IHDI-E > 10%	1
Response	(Bui et al. 2018)	II: Management expenditure index S: Amount of money spent on sustainable water resource management in the study area E: Estimated cost of sustainable water resource management in the study area	II ≤ 20%	0
			20% < II ≤ 40%	0.25
			40% < II ≤ 60%	0.5
			60% < II ≤ 90%	0.75
			II > 90%	1

View Large

Groundwater policy indicators are metrics used to assess the effectiveness of policies that manage and sustain groundwater resources. These indicators can measure various aspects of groundwater policy, including the legal and institutional framework level, the availability of data and information, the level of public participation, and the effectiveness of management and conservation efforts.

For the institutional capacity of the basin, we have used a quantitative classification ranging from poor (0) to excellent (1), accepting that if there are appropriate water resource management laws, but they have not yet been implemented or regulated, an intermediate score (0.5) was assigned. Thus, the starting point is a medium level. The score increases depending on legal capacity, the effectiveness of the institutional framework and public participation in the sustainable water resource management process. Legal and institutional capacity in managing water resources is subject to the administration's ability to perform its functions effectively, efficiently, and sustainably.

Policymakers, water managers, and other stakeholders can use groundwater policy indicators to evaluate the progress in implementing effective groundwater management practices and identify areas where additional action may be needed. Groundwater policy indicators are essential in the sustainable use and management of groundwater resources.

In policy response analysis, data were provided from the Wami/Ruvu Basin-Dodoma Office responsible for investments in the management of aquifer water resources. The indicator used in this category is presented in Table 6.

The hydrogeology indicator score was calculated as the aquifer's quantity and quality parameters average.

In the analysis of hydrogeological quantity, various indexes, such as a change in groundwater storage, groundwater abstraction and recharge, and water level trend in observation wells, were used (Table 7). The renewable groundwater volume per capita score of 0 indicates poor sustainability regarding the availability of groundwater resources per population served. This means that the amount of renewable groundwater available to each individual is insufficient to meet their needs and maintain long-term sustainability. A higher score would signify a better balance between groundwater supply and demand and a greater likelihood of ensuring reliable access to this critical resource for future generations.

Furthermore, the natural availability of water was calculated to be 144 m³ per person per year between 2010 and 2020. This amount is reasonable for a semi-arid environment with low rainfall and scarce surface water resources, as natural water availability can be less than 100 m³ per person per year (Guppy et al. 2018). Although this amount is reasonable, particularly for semi-arid environments, it is higher than that reported in Finland (126.2), which has a humid climate. This value, however, is lower than in South Africa (261.0) because South Africa has a diverse range of climates, ranging from arid and semi-arid in the west to subtropical in the east, resulting in a high value. Iran has a higher value of 1,103, corresponding to a humid area where natural water availability can range from several thousand m³ per person per year (Carvalho et al. 2009; Hosseini et al. 2019).

The groundwater storage change index has a value of 0.417 and a score of 0.5, indicating moderate sustainability. This suggests that there is room for improvement. Based on this score, stakeholders should focus on implementing strategies that can help preserve groundwater resources, such as reducing water usage by finding an alternative source to substitute the demand; and implementing conservation measures.

The groundwater abstraction and recharge index has a value of 1.59 and a score of 0.25, indicating poor sustainability. It implies that the groundwater abstraction rate exceeds the recharge rate, which is not long-term sustainable. This indicates that groundwater resources are being depleted faster than they can be replenished naturally, posing a significant threat to the area's water resource availability. As a result, excessive groundwater extraction occurs without due consideration for recharge, potentially leading to resource depletion or long-term consequences such as land subsidence. Research in the study area has confirmed decreased recharge and increased groundwater pumping (Shindo 1991; Rwebugisa 2008; Seddon 2019; Mseli et al. 2021). In semi-arid regions, however, the groundwater abstraction and recharge index value ranges between 1.5 and 2.5, indicating a balance between groundwater extraction and recharge. The area's value of 1.59 is within the acceptable range. This value is higher than in other semi-arid countries such as Iran and South Africa (0.88 and 0.058, respectively) (Carvalho et al. 2009; Hosseini et al. 2019). At the same time, it is lower than in Iraq (2.79), depending on climate, geology, land use, and groundwater management practices (Carvalho et al. 2009; Hosseini et al. 2019).

The stability of water levels in observation wells is an important indicator of the health of groundwater resources. Currently, the stability of water levels in the observation wells is fair, with a score of 0.5. While this is a promising indication, suggesting that there is room for improvement in managing groundwater resources. Stakeholders should consider implementing strategies that can help preserve groundwater resources, such as reducing water usage and promoting recharge methods to maintain water supplies and ecological well-being. Additionally, they should regularly monitor water levels in observation wells to identify potential issues early and take corrective actions.

The sustainability of the aquifer was assessed using the pressure, situation, and response parameters (Table 8). The study evaluated groundwater resources' sustainability by analyzing quality changes over short-term and long-term periods. The results showed that the pressure indicator scored 0.75 and 0.5 for the short-term and long-term periods, respectively, indicating a reasonably sustainable resource. This suggests an improvement in water quality over time. In a semi-arid environment, the values for the index of changes in groundwater quality over the study period and short-term and long-term periods depend on various factors such as the location, the type of pollutants, and the hydrogeological characteristics of the aquifer (Nemčić-Jurec et al. 2022). A similar study in a semi-arid region of Iran discovered that the minimum and maximum values for the index of changes in groundwater quality over 5 years were 0.32 and 0.8, respectively, indicating moderate to high degradation of groundwater quality due to human activities. Over 3 years, another study in a semi-arid region of India found index values ranging from 0.04 to 0.88, with lower values indicating greater degradation of groundwater quality. Over 10 years, the index values in a semi-arid area of South Africa ranged from 0.2 to 0.7, indicating moderate to high vulnerability of groundwater resources to contamination (Carvalho et al. 2009; Senent-Aparicio et al. 2015; Hosseini et al. 2019).

The situation parameter evaluated the number of sampling points with average nitrate levels that comply with regulations (<50 mg/L) and the nitrate concentration levels. The average number of sampling points with average nitrate levels for sustainable management of groundwater resources varies depending on several factors, including the specific regulatory limit, the hydrogeological characteristics of the aquifer, and the level of human activity in the area. Based on these factors, a reasonable estimate for the average number of sampling points for sustainable management of groundwater resources in a semi-arid environment with average nitrate levels would be around 10–15 sampling points for a small- to medium-sized resource (less than 1,000 acre-feet), and 20–30 sampling points for a larger resource (greater than 1,000 acre-feet) (Khadija et al. 2021). In the study area, the number of sampling points with average nitrate levels that comply with regulations (<50 mg/L) is 19, the reasonable number to ensure that the resource is not degraded.

The value of the sampling point index for a semi-arid environment depends on several factors, such as the size and complexity of the groundwater system, the level of precision required for monitoring, and the nature and sources of contaminants. Generally, a reasonable value of the sampling point index for a semi-arid environment ranges from 0.5 to 2.0 sampling points per square kilometer. A lower value of the sampling point index of 0.5 is appropriate for a groundwater resource with relatively uniform hydrogeological conditions and low variability in contaminant concentrations. In contrast, a higher value of the sampling point index of 2.0 is required for a groundwater resource with complex hydrogeological conditions and high variability in contaminant concentrations (Nemčić-Jurec et al. 2022). In the study area, a value of 0.95 was found, implying a reasonable value for monitoring groundwater quality.

The response analysis, measuring efforts to reduce nitrate levels, scored 0.75, indicating good sustainability, thanks to the 2008 Ministry of Water Program's efforts to relocate nearby communities and ban livestock within the basin. However, the situation is becoming increasingly uncertain due to population growth caused by the government's relocation to Dodoma and illegal activities such as livestock farming, agricultural practices, and unauthorized borehole drilling, which may negatively impact water quality in the future.

The environmental sustainability of the aquifer was evaluated using pressure, situation, and response indicators (Table 9). The pressure was measured using the groundwater drought index, while the situation was determined using the stress index, groundwater resource vulnerability index, and percentage of area under natural vegetation. A groundwater drought index value of 0.257 indicates mild to moderate stress. However, the exact range of index values for groundwater management in a semi-arid environment depends on local conditions and management objectives.

The situation showed moderate to excellent sustainability, with scores of 0, 0.5, and 1 for stress index, groundwater resource vulnerability index, and area under natural vegetation, respectively.

A low groundwater stress index suggests the groundwater resource is under low stress, a positive indicator for groundwater sustainability. A low-stress index means that the aquifer can recharge at a rate equal to or greater than the rate of withdrawal or use, which is important for maintaining groundwater levels and avoiding depletion. However, it is important to note that a low-stress index does not necessarily mean the groundwater resource is in optimal sustainability. Other factors, such as water quality and groundwater management practices, can also impact the overall sustainability of the groundwater resource.

Meanwhile, a high percentage of the area under natural vegetation positively affects groundwater sustainability. Natural vegetation is critical in supporting groundwater recharge by facilitating water infiltration into the soil and promoting water storage in the aquifer. This is particularly important in semi-arid and arid regions, where groundwater resources are often limited and under stress. When natural vegetation is removed or degraded, the amount of water that can infiltrate into the soil and recharge the aquifer is reduced, leading to a decline in groundwater levels. This can negatively impact the environment and human populations that rely on the groundwater resource. Therefore, a high percentage of the area under natural vegetation helps to maintain groundwater sustainability by promoting groundwater recharge and ensuring the long-term availability of the resource. This emphasizes the importance of protecting and restoring natural vegetation in semi-arid and arid regions and the need for sustainable land use practices that prioritize the conservation of natural ecosystems.

The values of the groundwater resource vulnerability index in a semi-arid environment can vary depending on a range of factors, such as the characteristics of the aquifer, land use practices, and potential contaminant sources. In general, semi-arid environments are considered more vulnerable to groundwater contamination due to the lower groundwater recharge rates and higher susceptibility to contamination from human activities. The groundwater resource vulnerability index of 53% in the study area suggests moderate vulnerability to groundwater contamination, which is higher than Iran's (21.85%), a similar semi-arid environment. However, land use practices in the area may increase the risk of contamination, such as intensive agricultural activity in Iran (Senent-Aparicio et al. 2015).

A progress score of 0.75 in protected natural areas suggests that there has been significant improvement in the protection and conservation of natural areas. This implies that there has been an increase in the quantity and quality of protected natural areas, which can have positive implications for groundwater sustainability and the environment as a whole. Improving the protection and management of natural areas can have direct and indirect implications for groundwater sustainability. For example, protecting forests and wetlands help to maintain or improve groundwater recharge rates, which is important for maintaining the quantity and quality of groundwater resources. In addition, protecting natural areas help to reduce the risk of contamination from human activities, such as agriculture or waste disposal. Progress in protected natural areas scored 0.75 in the study area results from conservation efforts by the Greening Dodoma Project championed by the Vice President's Office and supported by World Wildlife Fund (WWF) through environmentally friendly initiatives contributing to environmental cleanliness and tree planting.

The social sustainability of the aquifer was evaluated using population dependence on groundwater, change in human development index (HDI) income, and groundwater management expenditure (Table 10). The population's dependence on groundwater was high, with an index of 69.5% and a score of 0.25, indicating low sustainability. This implies that a significant portion of the population in the study area relies on the aquifer for water supply, putting pressure on the aquifer. Thus, more efforts are needed to promote sustainable groundwater use, such as implementing regulations to limit groundwater extraction, improving water management practices, and promoting alternative water sources.

On the other hand, the change in HDI income over the study period showed excellent sustainability, with an index of 69% and a score of 1. The improvement in HDI income was sustainable over the study period, and this suggests that the improvement in HDI income was sustainable, meaning that it could be maintained over time (Kubiszewski et al. 2013). This implies that the policies or interventions that led to increased HDI income were effective and had lasting impacts.

Finally, the groundwater management expenditure index was 67%, with a score of 0.75, indicating good sustainability. This suggests that the government or other responsible entities are investing significant money and resources into managing and maintaining groundwater resources (Mahoo et al. 2015). This includes monitoring groundwater levels and quality, implementing measures to reduce contamination, and promoting sustainable groundwater use. A score of 0.75 indicates that these management efforts are effective and sustainable, meaning that they will likely have positive long-term impacts on groundwater resources and the communities that rely on them (Jordan et al. 2021).

The policy indicator assessed the sustainability of the aquifer by evaluating the pressure, situation, and response. The pressure was measured by the variation in aquifer HDI-Education, while the situation was evaluated through the basin organizational capacity index (water management rules) (Table 11; Dumont 2021). The response was assessed by the aquifer institutional capacity in IWRM (legal and organizational). The scores for the variation in aquifer HDI-Education and the basin organizational capacity index were 0.625, indicating moderate to good sustainability. This suggests that access to water resources can boost local economic development and improve education through increased investments in the education sector (Dumont 2021).

However, the aquifer institutional capacity in IWRM (legal and organizational) had a score of 0.5, indicating moderate sustainability, reflecting the need for improvement in water management laws and regulations, including implementing a water resource management plan and groundwater management framework (Dumont 2021).

The situation improved in the study area with the reallocation of people by the Ministry of Water. In 2015, establishing the Water Basin Authority, which was given ownership of the basin, made management easier than the previous arrangement under the Ministry of Water. Further improvement was seen in 2020 with changes to the Water Policy, which enabled better enforcement.

The findings are useful for policy makers and development planners as the aquifers' sustainability dimensions showed satisfactory sustainability (Senent-Aparicio et al. 2015) with an overall ASI score of 0.59 (Table 12) according to Equation (1). This suggests that the aquifer is not being replenished as quickly as water is being withdrawn. This could lead to declining water availability and quality over time, making the aquifer less sustainable. However, the extraction and usage of groundwater is also governed by socioeconomic factors prevailing at the local level (Wyatt et al. 2015; Suhardiman et al. 2018; Kaini et al. 2020a). The pressure, situation, and response parameters showed no significant differences. Yet, lower score in the hydrogeological pressure is hindered by the quantity of groundwater use per capita, potentially requiring measures to reduce water usage or find alternative water sources to sustain the aquifer over the long term to ensure sustainable yield.

The value of ASI in semi-arid regions is relatively similar worldwide, compared to Spain's semi-arid region, with an ASI of 0.55. This is because water resources are typically limited in the semi-arid regions, and water management is critical to ensure a sustainable water supply. In the semi-arid region of Spain, the value is hindered by groundwater contamination with intensive local farming (Senent-Aparicio et al. 2015).

To maintain a sustainable water supply in semi-arid regions, implementing effective water management strategies is essential, including improving water use efficiency, promoting water conservation, implementing policies to limit water withdrawals, and promoting using alternative water sources such as recycled wastewater, especially for agricultural activities. Additionally, efforts to increase the recharge of aquifers through measures such as artificial recharge (Seddon 2019) can help to improve the sustainability of water resources in these regions.

Generally, the identification and optimal extraction of groundwater are necessary due to its vital role in adapting to the impacts of climate change on water resources. Groundwater's reliable supply, buffering capacity, support for ecosystems, agricultural resilience, drinking water security, and overall water management flexibility make it a crucial resource for ensuring water sustainability in the face of future climate uncertainties. As seasonal variations are expected in rainfall and surface water flow in future (Kaini et al. 2020b, 2021), identification and optimal extraction of groundwater is necessary.

The ASI is based on subjective judgments and requires consideration of multiple factors. An uncertainty analysis was performed to account for uncertainties related to the computation process and subjective choices. The resulting mean ASI and its 95% confidence interval were between 0.3856 and 0.7944 (Table 13), indicating that the sustainability of the Makutupora aquifer was low to slightly moderate during the studied period. It's important to note that the uncertainty of the results is reasonable.

Groundwater sustainability is a crucial aspect of ensuring the long-term availability of this vital resource for both human and natural systems. Sustainable groundwater use is a complex problem with technical, social, economic, policy, and political dimensions. This study used four indicators in six dimensions, i.e., quantitative and qualitative hydrogeology, environmental, social-economic, and political factors, to evaluate the sustainability of the Makutupora aquifer in Dodoma, Tanzania. The aquifers' sustainability dimensions showed satisfactory sustainability, with overall sustainability values of 0.59. This results from the sustainability index values of 0.6, 0.58, 0.67, and 0.52 for policy, environment, socioeconomic, and hydrogeological indicators, respectively. The hydrogeological analysis revealed that the groundwater sustainability situation is promising, although groundwater use per capita is alarming. However, the situation worsens due to the population increase caused by the government shifting to Dodoma, leading to illegal activities such as illicit livestock keeping and agricultural activities; and illegal borehole drilling in the basin that affects water quantity and quality in the future. This study contributes to developing and implementing effective and sustainable water management for decision-making. It also recommends effective water management strategies, including improving water use efficiency, promoting water conservation, implementing policies to limit water withdrawals, and promoting alternative water sources such as recycled wastewater, especially for agricultural activities. Overall, this study lies in its ability to guide water resource management, inform decision-making, prioritize interventions, promote sustainable practices, and foster stakeholder engagement to achieve long-term groundwater sustainability in the Makutupora basin and ensure a reliable water supply for current and future generations.

The authors gratefully acknowledge the financial support from the University of Dodoma provided for conducting this study. We would also like to sincerely thank the Wami/Ruvu Basin-Dodoma Office and Dodoma Water Supply and Sewerage Authority for their invaluable assistance during data collection.

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

The authors declare there is no conflict.