Physical factors limiting access to clean groundwater in Tanzania villages

Low yield, poor water quality, and nonfunctional infrastructure impede physical access to clean groundwater in rural Tanzania. We studied boreholes in 45 villages as part of a rehabilitation program led by the Global Water Institute at The Ohio State University. Villages were chosen because their groundwater supply systems were inoperative or unsustainable. The most common cause was pump failure, which occurred in more than half of the villages. Even if broken pumps were repaired or replaced, low pump capacities and potential yields would limit physical access in many villages. Low potential yield is often mistaken for a broken pump, but easily diagnosed with a pump test. Pump test records were available for only eight villages, highlighting the need for more testing and data accessibility. One-third of the villages had low water quality. In comparison to secondary water sources such as springs, impoundments, and dug wells, boreholes tended to have lower levels of nitrate and fecal coliform, greater total dissolved solids, and similar ﬂ uoride levels. In many villages, groundwater is the only viable water resource to support development, but drilling records and hydrogeologic data are sparse. We recommend better digital data archiving with governmental water supply authorities and the assessment of potential well yields and sustainable yields.


INTRODUCTION
Hundreds of millions of people across sub-Saharan Africa suffer due to inadequate access to clean water (WHO & UNICEF ). Three decades ago, the United Nations set a Millennium Development Goal to halve the proportion of the population without sustainable access to safe drinking water and sanitation, but the goal was not met in Tanzania

Survey approach
The 45 villages are distributed across seven regions and have populations ranging from 1,000 to 7,000 people.
Information on climate and hydrogeology is provided in the online Supplementary Materials. In each village, information on water accessibility was obtained through field observation and the assessment of infrastructure by technicians. The assessment form, which was adapted from the WHO guidance on sanitary surveys and the National Groundwater Association's recommendations on water point inspections, included observations of water body sources and characteristics, water usage, site-specific attributes, potential pollution sources (especially those related to human or animal wastes), and an intervention appraisal.
Driller's reports and other available water data were acquired, where possible, from district water engineers and village executive officers in person. Data were also sought from the Ministry of Water and Irrigation offices, including aquifer type, thickness, static water level, information on well yield or specific capacity, and water quality.

Hydrogeological analysis
Completion reports and pump test results were only available for boreholes in eight villages. For those villages, we used pump test data to estimate the specific capacity and potential well yield. Potential well yield is important because it reflects the maximum pumping rate a well can sustain without experiencing excessive drawdown. At greater pumping rates, the well quickly goes dry, and the water level must recover before pumping can resume.
Without an assessment of potential well yield, pumps cannot be effectively sized for wells. Specific capacity (SC) was calculated as follows: where Q is the pumping rate and s is the drawdown. In some cases, a step test was conducted, and we approximated Q as a weighted average of the pumping rates during each period.
Potential well yield (Y pot ) was calculated as follows: where s a is the available drawdown, which was assumed to be 3 m less than the difference between completion depth and static water level.
Where driller's reports were not available, the district water engineer supplied information such as completion depth and reported yield. The reported yield was typically inferred from the time required to fill a storage tank and is not equivalent to potential well yield based on aquifer pump tests (Equation (2)). The district water engineer also reported static water level for about half the villages.
Samples were collected for water quality from the borehole or the nearest access point. In addition to boreholes, many villages had secondary water sources, including springs, charco dams (hand-made earthen dams that store overland flow during the rainy season), and dug wells (shallow wells dug by hand to the water table). In three cases, a river was also accessed as a secondary source.
Samples were collected from these secondary sources (mostly consisting of surface water and shallow groundwater from dug wells) to compare against borehole water.
More information on water quality testing is available in the Supplementary Materials, including the type of secondary source for each village.

Infrastructure assessment
More than half of the villages (26 out of 45) had an inoperative pump, which prevented groundwater withdrawals altogether ( Figure 1). Other infrastructure problems also limited access or contributed to unsustainability ( Figure 1). In fact, 85% of villages had more than one infrastructure problem. with inadequate yield were randomly distributed across the regions of Kagera, Kilimanjaro, Mara, Singida, and Tabora.
The reported yield tends to reflect the capacity of the existing pump (Figure 2(b)), rather than the potential yield of the borehole, which is measured with a pump test.
In three of the 17 villages with low reported yields, well test records were available to calculate potential yields In the absence of pump test records, we compared the pump capacity with the reported yield, which was typically based on the time to fill a storage tank (Figure 2(b)). For most villages, the reported yield and pump capacity were nearly the same, suggesting that the well was capable of yielding water at the rate it was being pumped (Figure 2(b)).
In two villages, the pump capacity was significantly less than the reported yieldin other words, the pump is undersized.
Although both villages' pumps meet the domestic needs of the current populations, the wells could yield more water for other needs with a more powerful pump. In Mubaba   intercept more conductive fractures. The four wells with above-average specific capacity are all 150 m or deeper.
Three of these wells may have been drilled deeper because the static water levels were deep (greater than 60 m).

Water quality assessment
Of the 35 boreholes that were tested, 15 did not meet Organization does not have a guideline for total dissolved solids (TDS), water becomes increasingly unpalatable above 1,000 mg/L (Bruvold & Ongerth ). Three of the 35 tested boreholes had TDS concentrations above 1,000 mg/L, one of which also contained elevated fluoride (Figure 3).
TDS in 35 tested boreholes had a median value of 308 mg/L and ranged from 31 to 2,387 mg/L (Figure 3(a)).
The three boreholes that had TDS concentrations above 1,000 mg/L were located in different regions (Mara, Tabora, and Singida), but all had completion depths greater than 90 m (Supplementary Materials). One was the only well observed to have a substantial specific capacity (more than 50 m 2 /d). Secondary water sources had comparatively lower TDS concentrations than boreholes, with a median of 100 mg/L and a range of <1-1,500 mg/L (Figure 3(a)).
Fluoride in 35 tested boreholes had a median value of 0.8 mg/L and ranged from 0.1 to more than 3.0 mg/L (the maximum concentration measurable with a field photometer) (Figure 3

DISCUSSION
The physical factors that limit access to clean and safe groundwater in rural Tanzania are multifaceted and widespread. One-third of all villages had low yields that could not meet domestic water requirements for the population, one-third had poor water quality, and more than half had inoperable pumps (Figure 4). Although the villages in this study were selected because of known infrastructure problems, the prevalence of broken pumps has been observed previously in Tanzania and other rural developing countries (Nkongo ; van den Broek & Brown ).
Other infrastructure challenges that hindered access included leaky pipelines, a lack of distribution points, and high costs of powering the pump (Figure 1).
A third of all villages faced more than one physical challenge ( Figure 4). Four villages had sufficient water quality, but had low yield and an inoperative pump. These villages need reliable and affordable pumps and power but may also need additional boreholes to increase supply in lowyielding aquifers. Seven villages had poor water quality and an inoperative pump. This implies that even if the pumps are repaired or replaced, the produced water will require treatment to improve quality. Two villages experienced the triad of the inoperative pump, low reported yield, and poor water quality. These villages need a reliable pump and power source to bring the existing well online, but they may also require additional boreholes to increase yield and water treatment to improve quality.
Some of the low reported yields were due to pump constraints, while others were due to the aquifer and well constraints (Figure 2(b)). Aquifer productivity is known to be low across most of Tanzania  Groundwater from boreholes is also prone to fluoride, a geogenic contaminant, but perhaps no more so than water from secondary sources (Figure 3(b)). Fluoride has been reported to be a major groundwater quality problem in  We also recommend installing more boreholes in areas with low aquifer productivity but good groundwater quality, particularly in areas where nitrate and fluoride concentrations tend to be low but aquifer transmissivity is also low. Adding more wells with smaller pumps distributes pressure on the aquifer and reduces the drawdown, facilitating a steady, reliable flow at each well. In this study, Tabora had relatively good groundwater quality (low concentrations of nitrate and fluoride). In regions such as Singida and Kilimanjaro with higher fluoride concentrations, it is important to evaluate health benefits and risks of groundwater development. However, fluoride is often high in dug wells and surface water as well, so installing new boreholes is likely to improve access to water that is lower in nutrients and pathogens and perhaps similar in fluoride levels.

RECOMMENDATIONS
Despite the physical challenges of infrastructure maintenance, low yield, and marginal water quality, groundwater development will continue to be an important ingredient for water security in rural Tanzania. Groundwater is less susceptible to anthropogenic contaminants like pathogens and nutrients that plague surface water during the rainy season, as long as boreholes are properly constructed and protected with concrete aprons. Groundwater can have higher total dissolved solids, but this is fortunately one of the easiest and most affordable water quality parameters to test and monitor. Groundwater is also accessible year-round and is less influenced by climate than surface impoundments and rainwater harvesting systems. To support groundwater development, improved understanding of potential well yields and sustainable yields is needed. An evaluation of sustainable yield requires analysis of diverse factors such as natural recharge rates, connections to surface water bodies such as rivers and springs, and connections to other aquifers (Theis ; Zhou ). We advocate for further assessment of sustainable yield as part of the decision-making process for groundwater development in Tanzania.