4.1. Discussion of Results
Hot springs are well-documented sources of particularly increased groundwater fluoride concentrations, often displaying the highest concentrations globally (Figure 1
), so it was not unexpected that all hot springs in Malawi exceeded the WHO standard [9
] for fluoride in drinking water and possessed the highest range of groundwater fluoride concentrations (Figure 4
a). Hot springs produce groundwater from deeper reservoirs (i.e., different hydrogeological system) to those associated with shallow rock weathering, so they were classified separately as ‘Excessive Geogenic Fluoride’. This classification reflects: (i) deep hydrothermal source reservoir; (ii) significantly higher groundwater fluoride concentrations associated with hot springs; (iii) 100% likelihood of groundwater fluoride concentrations > 1.5 mg/L (Figure 4
a); and (iv) the only site-specific groundwater geogenic fluoride risks identified by this study. Fluoride concentrations > 4 mg/L are associated with skeletal and crippling fluorosis (Figure 4
) so hot springs represent almost all of the risk of those conditions in Malawi. They constitute the biggest human health risk from fluorosis and are the priority targets for replacement water supplies. The 63 hot springs represent highly localised ‘hot-spot’ areas of probable endemic fluorosis which currently affect an estimated 11,029 people in Malawi (Figure 7
a). It is likely that incidences of both conditions (skeletal, crippling), plus increased incidences of dental fluorosis relative to other locations will be present at and immediately surrounding those sites.
Rocks of alkaline igneous composition consistently produce the highest groundwater fluoride concentrations, relative to other rocks (Figure 1
) and our results support this conclusively (Figure 4
a). Our results contain groundwater fluoride concentrations from three different granitoids which all displayed > 60% likelihood of groundwater fluoride concentrations > 1.5 mg/L, reasonably justifying the extrapolation of the ‘Elevated Geogenic Fluoride’ classification to other granitoid rocks. Hydrochemical data from syenitic rocks were absent so reasonable justifications for inclusion into this classification along with other alkaline igneous rocks had to be established: lithologies of syenitic composition are well-documented sources of elevated groundwater fluoride, comparable to granitic types [10
], some syenites often possess significantly higher fluoride content than granites [53
]. The fluoride content of the Chipala-Kasungu nepheline syenites of northern Malawi range from 1550–2400 mg/kg [50
]. Quartz syenite of the Zomba and Mulanje plutons in southern Malawi have very similar composition to granite [47
]. All syenitic-type rocks in Malawi were thus classified as ‘Elevated Geogenic Fluoride’ and mapped as zones of increased generic risk.
Carbonatites are categorised as alkaline igneous rocks but have different composition to granites and syenites. They contain high relative sodium content which increases groundwater fluoride potential via fluorite (CaF2
) equilibration [5
]. Malawi carbonatites additionally have extensive occurrences of fluorite and fluoro-apatite veins [48
], a result of secondary precipitation from post emplacement hydrothermal processes and an additional source of groundwater fluoride. Carbonatites were therefore classified as ‘Elevated Geogenic Fluoride’ alongside other alkaline igneous rocks and mapped as zones of increased generic risk.
Lithologies within the ‘Moderate-low Geogenic Fluoride’ classification contained exclusively unconsolidated sediments and meta-sedimentary rocks (Figure 4
a). Unconsolidated sediments display 14% likelihood of encountering groundwater fluoride > 1.5 mg/L, however, this is a category which carries some uncertainty. Unconsolidated sediments in the Southern Region are dominated by rift basin colluvium, alluvium, lacustrine and fluvial sediments (Figure 3
). In the Central and Northern Regions, there is an additional category of colluvium which is characterised as ‘thin basement cover’ of unknown thickness and is extensive (Figure 3
). Those areas represent a two-tier aquifer system where weathered basement aquifers occur beneath unconsolidated sediment aquifers of variable and unknown thickness. An individual well may be tapping into either, or both of those aquifer systems, depending on its depth. This poses uncertainty when extrapolating geogenic fluoride risk categorisation for the unconsolidated sediments at those locations, as a well may be drilled into an elevated geogenic fluoride lithology (e.g., granite) which is hidden beneath the sediment cover, but showing as low risk on a map (unconsolidated sediments). High hydraulic conductivity and transmissivity values identified in wells within these areas [54
] suggest that from the perspective of drinking water boreholes, these areas abstract from unconsolidated sediment aquifers (for the most part) and are thus classified as ‘Moderate-low Geogenic Fluoride’. Further groundwater sampling to identify elevated fluoride, pumping test data to determine hydraulic conductivities (proxy for aquifer type: sedimentary aquifers classified by high hydraulic conductivity and transmissivity values; weathered basement aquifers characterised by low hydraulic conductivity and transmissivity values [6
]), and/or borehole drilling logs to identify sediment depth vs. well depth, would increase confidence in this classification. All remaining lithologies in Malawi within the ‘Meta-sedimentary Rocks’ lithological group (Figure 3
a) were reasonably classified as ‘Moderate-low Geogenic Fluoride’ based on existing statistical correlations for this group (Figure 4
‘Low Geogenic Fluoride’ lithologies were characterised by sandstones, marbles, basic igneous rocks and two meta-sedimentary rocks (Perthite gneiss and Hornblende-biotite gneiss) so all remaining sandstones, limestones, marbles and basic igneous rocks across Malawi were grouped within this classification. The two meta-sedimentary rocks present a potential anomaly within the Moderate-low classification; however, relatively high sample numbers (Figure 4
a) present an increased confidence so they were grouped within this classification. Low sample number (n
= 3) for basic igneous rocks present low confidence in prediction for those lithologies, however, all samples had groundwater fluoride concentrations < 1.5 mg/L (Figure 4
a), hence the classification as ‘Low Geogenic Fluoride’. Groundwater fluoride concentrations within basic igneous rocks elsewhere generally reflect this classification, although can sometimes exceed 1.5 mg/L (Figure 1
). Further testing of the map (Figure 7
a) with additional fluoride analysis will allow confidence to increase in predicting geogenic fluoride risk to groundwater from basic igneous lithologies. While all samples within sandstone (n
= 3) displayed groundwater fluoride concentrations well below 1.5 mg/L (Figure 4
a), a low sample number also presents decreased confidence in prediction, however, global data on groundwater fluoride concentrations from sandstones (Figure 1
) conclusively support the classification of these lithologies within ‘Low Geogenic Fluoride’. Results for limestones and marbles in Malawi fall within this classification (Figure 4
). Limestones elsewhere are associated with low concentrations of groundwater fluoride but can also be associated with more elevated concentrations [10
], sometimes up to 5 mg/L (Figure 1
). Malawi limestones (incl. meta-equivalents) were classified as ‘Low Geogenic Fluoride’ based on our data (n
= 8) where all samples were <1.5 mg/L, however, this prediction may change with increased sampling and testing of the map.
Anomalous elevated groundwater fluoride concentrations located within the rift basin may reveal the locations of fluoride-rich groundwater movement along faults hidden beneath unconsolidated basin sediments, and indeed was previously hypothesised in the Lower Shire Basin to explain the linear appearance of >6 mg/L fluoride concentrations found at that location [5
]. A separate geological study to investigate fault strikes currently hidden beneath basin sediments may assist in the delineation of anomalous fluoride concentrations from that lithology but the data do not yet exist. Investigating those fluoride anomalies may reveal concealed fault strikes in the rift valley where high-fluoride groundwater is mobilised along faults and in contact with shallow groundwater. Mapping of faults beneath basin sediments (inferring based on existing geological, topographical and hydrochemical data) would therefore be a useful a tool for identifying target locations for fluoride assessment and priority replacement water supply. Such an approach may result in an additional zone of increased risk (hidden faults), increasing prediction accuracy for our screening method. Removal of anomalous groundwater fluoride concentrations related to fault strikes (inferred or observed) to be classified and targeted separately, may additionally result in a new, much lower (and more accurate) generic fluoride risk classification for unconsolidated sediments. Dental fluorosis data collected and confirmed by dentists would be additionally useful in identifying areas where there may be fluoride-rich groundwater beneath basin sediments as they would appear as locations with a high number of fluorosis incidences, this would be particularly useful in areas with an absence of groundwater data.
In total, water points (mostly rural) covered by this study serve an estimated 13,075,237 people (Table 4
), accounting for 74% of Malawi’s total population [55
] and our analysis suggests up to 1.9 million people may be at some risk from geogenic fluoride. An estimated 248,963 people currently use water points within ‘Elevated Geogenic Fluoride’ zones (the highest statistical risk of encountering groundwater fluoride > 1.5 mg/L from shallow rock weathering: >60%) and may be at risk from dental fluorosis. Water points within the ‘Moderate-low Geogenic Fluoride’ zones carry significantly less statistical risk of encountering concentrations of groundwater fluoride high enough to cause dental fluorosis (10–17%), however they number 74% of water points covered by this study. This equates to 1–1.6 million people using water points within these zones. With such a high number of people potentially at risk, water points within this category cannot be ignored and will require assessment within the SDG framework. Similarly, ‘Low Geogenic Fluoride’ carries a low statistical risk (<10%) of elevated groundwater fluoride but still represents up to approximately 267,594 people (Figure 7
a) who currently may be at risk. Water points within zones with insufficient data for a fluoride classification currently affect 181,666 people (Figure 7
a). While the expectation is that these lithologies will be relatively low risk, these will still need to be assessed for completeness within the SDG period and likely will contain some level (albeit low) of fluoride risk.
Overall, the results for Malawi (Figure 4
a) reflect global trends in groundwater fluoride concentrations and aquifer type (Figure 1
), with hot springs and groundwater points within alkaline igneous rocks posing the biggest human health risks. The map produced (Figure 7
a) provides a high-resolution preliminary screening tool which can be used to target areas for groundwater fluoride assessment. The screening method was designed to be dynamic, providing increased prediction confidence with the continued addition of new groundwater and geological data. Areas can be prioritised for sampling and potential replacement water supply depending on the statistical likelihood of encountering groundwater with fluoride concentrations > 1.5 mg/L, thus, risk to human health from fluorosis, from highest–lowest geogenic risk.
The national geogenic groundwater fluoride vulnerability map (Figure 7
a) is provided and should be used as an investment planning (or review) tool for Donors, NGO’s and Water Sector Stakeholders for assessing geogenic fluoride risk. It should be reiterated that this map is presented as a preliminary screening tool to target and prioritise future sampling efforts for existing wells and new ones being drilled, with the addition of groundwater and geological data increasing prediction confidence. To facilitate management of groundwater abstraction, the fluoride risk map for each WRA was individually produced and 15 geogenic fluoride risk maps for each of Malawi’s 15 major WRAs can be found in the Supplementary Materials (Figures S11–S27)
. Water resources (groundwater and surface water) in Malawi are managed by the National Water Resources Authority (NWRA) at (surface water) catchment level under the Water Resources Act (Malawi) 2013 [56
]. The national fiscal resources in Malawi are managed through the 28 District Councils, and each District Water Development Officer (DWDO) would have responsibility for planning and monitoring of water resources. Therefore, it is also important to provide detail on fluoride risk for planning at the district scale as provided in Table 4
. It is likely that the ministry responsible for water will delegate to local government the responsibility of implementing new groundwater standards in the first instance, and there will be a need to determine the fiscal burden of monitoring and potentially replacing water supplies as the fluoride standard is lowered.
4.2. Policy Review and Implications
A review of groundwater policy, specifically the fluoride standard is needed for Malawi to achieve SDG 6 targets as our results show that the Malawian drinking water standard MS 733:2005 (groundwater and boreholes) for fluoride (6 mg/L) (Figure 8
) is not aligned with geogenic and acceptable health risks. The WHO guideline drinking water standard for fluoride is 1.5 mg/L [9
] and is aligned with geogenic risk (the link with the WHO guideline standard being that concentrations below 1.5 mg/L are not associated with causing dental fluorosis). Concentrations exceeding 1.5 mg/L cause varying degrees of fluorosis (Figure 4
), so retention of this standard runs the risk of (undocumented) dental and skeletal (and possibly crippling) fluorosis in populations who drink from wells with fluoride concentrations above risk standards. The Ministry responsible for water in Malawi and CJF Programme are together developing a plan to incrementally reduce the MS 733:2005 fluoride standard over time, aligning their drinking water fluoride standard with the WHO. Phase 1 of the plan is to reduce the standard from 6 mg/L to 4 mg/L. If implemented, this will manage the risk of skeletal and crippling fluorosis. Phase 2 will see a reduction to 2 mg/L and phase 3 will see a final reduction to 1.5 mg/L (Figure 4
a), which will manage the risk of all fluorosis and bring groundwater fluoride policy in line with the WHO. The geogenic fluoride groundwater risk map of Malawi produced in this study (Figure 7
a) is proposed primarily as the preliminary screening tool to be used in national efforts to manage groundwater assets for fluoride, but additionally as the basis for policy review. The map is based on the WHO standard of 1.5 mg/L which reflects geogenic risk, if it was based on the Malawian standard of 6 mg/L, only hot springs would show as risk areas as >6 mg/L groundwater fluoride concentrations are exclusive to them in Malawi (except one shallow well in the Southern Region). The gap between the WHO and Malawi standards for fluoride in drinking water reflect the “troublesome” fluoride concentrations (1.5–6 mg/L) identified previously as a key management issue due to diffuse occurrence and out of date fluoride standards [5
The Malawian standard MS214 for fluoride in general drinking water (piped networks) is in line with WHO guidelines and does not require assessment, with an upper limit and range of 0.7–1.0 mg/L [57
]. The standard MS 733:2005 for “raw water” that covers groundwater abstracted from boreholes and wells requires assessment (Figure 8
) and was the target of this study. These groundwater sources are the primary water points for rural areas (85% of Malawians). There is a lack of emphasis on fluoride in the Malawi standard documents which were written before the JMP classification as a priority chemical contaminant (WHO and UNICEF, 2017). Fluoride (along with arsenic) will require more focus in the redefinition of standards if SDG 6.1 targets are to be set and achieved in Malawi, specifically 6.1.1 which states the global indicator as: “safely managed drinking water” and describes it as: “Improved source located on premises” … ”free from priority chemical contamination” [58
]. Lowering the fluoride standard will render many water points unsuitable under this heading and alternative supplies will be required, a significant challenge for the Malawi Government.
MS 733 (Figure 8
) refers to siting boreholes at least ‘100 m away from sources of pollution’, Fluoride is a geogenic contaminant in groundwater; alkaline igneous rocks have been identified as high-risk sources of fluoride in groundwater and therefore, could be classed as a pollution source (particularly given the JMP classification), although large-scale geogenic sources (e.g., geology) could not be legislated in the same way as an anthropogenic point source (e.g., pit latrine). Hot springs on the other hand can and should be legislated separately in the standards as point sources of various pollutants, including fluoride. More detail on pollution sources (geogenic, anthropogenic) will be required when revising these documents. In the document, under “6.2 Handling”, the MS 733:2005 document states that use of a borehole or well should cease immediately if a pollution source is identified, until the cause is eliminated. This is unrealistic for water points within a generic geogenic fluoride source but there is no indication of protocols for such an occurrence. Elevated geogenic fluoride zones cover large areas and many water points (Figure 7
a). Site-specific chemical analysis of abstracted water is therefore necessary as it is unknown how many water points within elevated geogenic fluoride zones may be producing water with low fluoride concentrations (there is currently no systematic measurement of fluoride from groundwater supplies). Under MS 733:2005 only hot springs would require assessment.
Additional to reviewing and updating standards for fluoride in drinking water, the WASH sector has the responsibility to ensure that Malawi Government standards for installation are and were followed (including a requirement for chemical testing before installation of a pump or lifting device and use of the water as a drinking water supply). Our data show that many boreholes have been drilled which contain elevated groundwater fluoride which would have been within standards at the time of drilling but would fail water quality tests under proposed new standards. Updated, more stringent standards for fluoride will further the need for more focus on proper due diligence and accurate reporting. If these issues are not addressed, Malawi will continually gain new borehole infrastructure which later fails water quality tests and become stranded assets [2
]. Local-scale studies where elevated groundwater fluoride concentrations exist are recommended to better understand the geogenic processes causing them. Such localised investigations may produce site/area-specific management solutions, such as avoiding faults, hot springs, or high risk lithologies.
As Malawi’s population continues to rise, stress on water resources will increase and the need for safely managed rural water supply and stricter policy on groundwater quality will become increasingly difficult to achieve, highlighting the need for proactive management of water resources. This study was designed to inform policy makers with an evidence-based prediction method for both environmental management and policy review. Recommended best practices for advocating science-based policy review includes (but are not limited to): (i) accurately characterising the best available policy-relevant science; (ii) presenting a clear and concise argument; (iii) accurately characterising any uncertainty; (iv) transparent representation of scientific basis for policy recommendation, and; (v) avoidance of hyperbole [59
]. Thus, prediction maps have been produced here to form the basis for both assessment of existing groundwater supplies for geogenic fluoride (asset management), and as monitoring and investment planning tools as review of the Malawi fluoride standard is taken forward by the government. For Malawi to achieve SDG 6.1 scientists and policy makers must work closely and collaboratively to develop a consistent and accurate approach to reducing both current and future risks.