Nkhotakota is bounded to the north and south by latitudes 12˚30’ and 13˚30’S to the east of Lake Malawi, and to the west by longitude 34˚E.
This study was aimed at assessing the extent of the underlying geology and geological structures in relation to the fluoride concentration in groundwater. The area is underlain by the basement complex which is dominated by biotite gneiss and this is overburdened by buff pale pink sandy soils in some areas closer to the lake. The study was achieved through the collection of water samples from randomly selected boreholes and through the observation of the surrounding geology. Several chemical analysis techniques were employed, to quantify the elements in the water that may have a bearing on the concentration levels in the water samples. Maps showing the distribution of fluoride ion distribution were drawn using ArcMap.
The results were in support of the geology and geological structures influencing the fluoride levels in the groundwater, the highest concentration registering 14.41mg/l underlain by biotite gneiss, which undergoes isomorphic substitution with fluoride under high temperatures and these conditions are provided by the fault that traverses through the area, the tension is manifested as in situ temperatures thus favoring fluoride enrichment. The groundwater samples were analyzed for various ions which included; F-, SO4, NO4, Cl, Na, Ca, K, Mg and HCO3- ions, a strong positive correlation exist between fluoride ion concentration and sodium ion concentration, a negative trend between NO3, SO4 and Mg was established. The pH of most waters sampled ranged from 6.7-9.1 with a mean of 7.2, a positive correlation was established, high pH values; indicate alkalinity, which in turn favor high fluoride levels on the pH scale. The results of statistical analysis also indicate that NO3- has the highest mean for anions.
The groundwater fluoride concentration of four sample points in the study area exceeds the set standard limit of 1.5ppm fluoride, Lunda, Bondo and Ngoma with 2.104mg/l, 2.336mg/l and 2.677 respectively, the forth area, Madzimawira, a hot spring exceed both the WHO and MBS standards(6.0mg/l).
Groundwater is the main source for drinking water in most areas. Having a clean source is of great importance, but unfortunately this is not always the case because groundwater has a tendency to recharge with depth thus increasing the likelihood of rock water interaction than that of the surface water, this in turn increases the concentration of chemical elements in the groundwater (Leybourne & Cameron, 2010). All rocks dissolve in water thus adding hardness, salts and alkalinity into the water. The basic rock forming minerals that are hard and resistant to weathering such as quartz and feldspar dissolve the slowest whereas those that easily erode are the fastest to dissolve into the water causing the total dissolved substances in the water to increase (Hoch. T, 2008).
Fluoride in drinking water has both positive and negative impacts on a person’s health. When taken according to the recommended threshold by the World Health Organization (WHO) which is 1.5mg/L, it has a beneficial effect such as the prevention of any dental caries, but when the threshold is exceeded, the effects lead to fluorosis which affects the teeth and in extreme cases affects the bones, with the exposure time increasing the fluoride concentration leads to a more severe skeletal deformity (Sajidu et al., 2007).
For most cases, the occurrence of fluoride in water is due to the geological setting, (Sajidu et al). A number of FC cases have been reported from the areas along the great East African Rift valley which include Ethiopia, Kenya, Tanzania and Malawi just to mention a few. (Jovine et al). Baseline studies in Malawi have shown evidence of high fluoride levels in groundwater in areas such as Nkhotakota, Lilongwe, Karonga, Nsanje, Chikwawa, Mwanza and Mangochi (Sajidu et al., 2007)
Nkhotakota has a tropical type of climate and consists of two main seasons. These are the wet season from November to April and the dry season from May to October. On average, the district receives annual rainfall of about 1400mm but might fall as low as 860mm to as high as 1600mm. Steady rains falls within the months of December, January, February and March. Nkhotakota District experiences an average monthly maximum temperature of 28.7 degrees Celsius and minimum temperature of 20 degrees Celsius. The warmest month of the year is November while the coolest month is July. (Nkhotakota 2010)
Nkhotakota lies within the Malawi province of the Mozambican Orogenic Belt. The western area is referred to as the Basement complex whilst the eastern area is mostly overlain by unconsolidated and superficial deposits of the Lakeshore plain. (Nkhotakota Socio-economic Profile , 2010). Biotite and hornblende bearing gneiss variably modified by migmatization are the most usual and developments of the basement complex. Limited development of pyroxene hornblende gneiss occurs in the west, Quartzo-feldspathic gneiss have been mapped within the biotite and hornblende bearing gneiss particularly in the southern and central sections of the area. Calc-silicates are widely distributed but comprise only a small portion of the basement complex of the area. Thin stony sands developed throughout most of the Rift Valley escarpment zone to the west of the lakeshore plain. A major body of late tectonic grade has been mapped at Sani and minor granite and pegmatite intrusions and quartz reefs occur throughout the area, alkaline dykes of upper Jurassic-lower Cretaceous age are sparsely developed. The area underwent 3 phases of large scale metamorphism, regional metamorphism under amphibolite facie conditions are associated with the development of foliation and gneiss banding. Later episodes of deformation are considered to be of lower metamorphic grade. The development of the Rift valley later subjected the area to intense faulting (Harrison & Chapusa, 1986).
The research used several tools in order to achieve reliable results. Tools that were used include:
● Polyethylene bottles were used for water sample collection
● Sealed cooler boxes for collected sample storage
● GPS was used to collect location information for sample points
● EC meter was used for conductivity measurements
● pH meter was used for both pH and temperature(˚C) readings
● A field notebook and a pen were used to note down any necessary information
● Marker pen was used to label samples
Fifteen (15) samples were collected around Nkhotakota Boma using random sampling. Water samples were collected in duplicates, two for anion analysis and two for the cation analysis. A volume of 350ml was collected from each sampling point in each bottle; the samples collected for cation analysis were treated with nitric acid (HNO3) for preservation purposes right on the spot. Before sampling, the water from the borehole was left to run for 2 minutes so as to remove any stagnant water and the bottles were rinsed at least twice with the water from the borehole being sampled.
The Universal Transverse Mercator (UTM) coordinates of the sample points were collected using a Global Positioning System (GPS). These were then superimposed on a geological map obtained from the Geological Survey Department (GSD) that was digitized in ArcMap.
The physico-chemical parameters such as temperature, electric conductivity and pH were recorded in the field using probes. Chemical parameters such as Na, Ca, K, Mg, F, Cl, NO, SO, HCO3 were analyzed in the Chemistry labs, procedures such as titration, ion selective method, flame photometer, atomic absorption spectrum and turbidimetry where used for chemical analysis in the collected water samples.
The study area is dominated by biotite_gneiss and buff, pink and pale brown sandy soils with the alluvial cover being localized near the lakeshore. It is covered by soils with little to no outcrops exposure; the soils that are predominant are the buff, pale sandy soils.
Sandy lithosols with stoney colluvial also cover part of the lakeshore plain. They are usually quartzo_feldspathic in nature due to the underlying bedrock (Carter & Bennett, 1973).Red clay loam and sandy clay soils predominate throughout much of the district and clays (Nkhotakota Socio-economic Profile , 2010)
Biotite and biotite hornblende gneiss of semipelitic composition make up most of the basement complex lithology around these areas.
Table 1: Physico-chemical parameters of sampled boreholes
Four areas, Ngomba, Bondo mosque, Lunda and Madzimawira have fluoride levels above the WHO threshold being 1.5mg/l but below the MBS which is 6.0mg/l, 2.677mg/l, 2.336mg/l, 2.104mg/l and 14.410mg/l respectively. The rest 12 sampled areas have fluoride levels ranging from 0.3322mg/l-1.439mg/l.
Figure 2: Fluoride concentrations of the four localities that exceed the WHO threshold and one exceeding the MBS threshold
The pH is a measure of the alkalinity or acidic of water soluble substances, the scale ranges from 1-14. From the sampled locations, the maximum pH value recorded is from Madzimawira with 9.1, the minimum is 6.7, from Chande vlg, therefore the pH range of the sampled areas is 6.7-9.1 with a mean of 7.2. The r2 between the two parameters is 0.822 which signifies a positive correlation, high pH values, indicate alkalinity, favor high fluoride levels, areas that registered levels above the threshold set by WHO exhibit pH values higher than 7.0. The pH has a significant role in the solubility of F-, in acidic conditions; fluoride is absorbed in clay minerals (biotite/mica) whilst in alkaline conditions the fluoride is desorbed ( (Saxena & Ahmed, 2003).
EC is the electrical conductivity of the water solution. The sampled locations have an average EC of 354.2µS/cm, with a range of 122-784µS/cm, the highest EC value is found in Lunda vlg and the lowest is in Chande vlg 2. It experiences a weak positive correlation with fluoride concentration, the value being r2=0.153. The EC is determined by the magnitude of anions dissolved in the water being sampled.
The temperatures range from 27.4-53.8˚C, the average temperature was 30.2˚C. The majority of the sample points fell in the range of 27.4-29.4 and Madzimawira, one of several hotsprings located in Nkhotakota exhibits a temperature of 53.8˚C. The temperature is significant factor in case of the spring because of the geothermal activity and faults associated with the area. The faulting releases heat that increases the temperature thus increasing the solubility and mobility of F-.
Figure 3: Piper diagram showing the water types for sampled areas
Madzimawira has a water type of Na-Cl where the alkalies exceed the alkaline earths and also strong acids exceed weak acids whereas 93.3% of the sampled water fall under Ca-Cl water type where weak acids exceed the strong acids and alkaline earths exceed the alkalies.
According to (Edmunds & Smedley, 2013), crystalline basement rocks register high fluoride levels. Malawi is one of the countries in Africa that exhibit this trend although this is not the case for Nkhotakota boma. This may be due to several contributing factors such as depth of the
borehole, (information is not available thus cannot infer), and the amount of rainfall received –contributing to the leaching out of the fluoride from the water sources
According to Table 1, most of the sampled areas registered fluoride levels within the recommended threshold of both the WHO and MBS. The highest record being the sample collected from Madzimawira hotspring, which has an underlying geology of biotite gneiss, since biotite gneiss provides a site of fluoride exchange with the OH ion, we can conclude that this is not the controlling mechanism for fluoride enrichment in the study area due to the great variation between the areas that are too underlain by biotite gneiss. We can articulate that the concentrations were therefore due to other physio-chemical parameters or geological structures.
Hotsprings are typical in areas that are faulted or fractured, this seems to be case for the site, Nkhotakota is located along the transverse of the EARS thus the area has a network of faults (Carter & Bennett, 1973) a fault passes through the Madzimawira, and this may result in high temperatures that are optimum for solubility and mobility. Faults tend to go through tensional strain which manifest as geothermal activities and these may be responsible for the high insitu temperatures that facilitate the solubility and mobility of the fluoride ions. The depletion of Ca2+ in the Madzimawira sample is due to the precipitation of calcite in the high pH setting.
Figure 4: Map showing the distribution of fluoride ions in groundwater, Nkhotakota boma (interpolated map)
Areas such as Lunda, Bondo and Ngoma register values above the WHO threshold and exhibit underlying geology of pink and pale brown sand soils. These are derived from the lacustrine and alluvium which give rise to sandy and clayey soils through subaerial weathering. Fluoride occurs in primary minerals such as micas (biotite and muscovite), and the soils in the study area are locally micaceous (Carter & Bennett, 1973), thus providing an environment that favors the exchange of the OH- ion in the biotite structures;
Substitution of OH- with F in biotite: K2(Mg,Fe)4(Fe,Al)2[Si6Al2O20](OH)2(F,Cl)2,
Through the subaerial weathering, the resulting formation is that of the buff, pink and brown sandy soil, the F from the OH site in the biotite is released into the water.
The F- concentration and physico-chemical parameters such as the pH have a strong correlation, the higher the pH, the more the fluoride ions in the groundwater. All the other properties such as EC, Temp, NO3, HCO3, CO3, and SO4 showed weak correlation or none whatsoever, NO3 and SO4 and Na exhibiting a negative weak correlation.
Finally, the severity of fluoride contamination of borehole water in the area occurs as one cluster closer to the lake.
Most of the water samples fell under the Ca-Cl type whereas only the hot spring (Madzimawira) sample falls under Na-Cl water type according to figure 12.
Further research needs to be conducted to fully assess the controlling factors, a combination of soil data, geomorphology and geology would show areas with potential fluoride levels with much higher confidence levels in the results by producing a weighted fluoride index map. This can then be used by organisations to make informed decisions as to where to construct boreholes with accepted water standards, both at a national and international level.
Another research must be conducted to assess the extent of the influence of hydrogeological factors on the fluoride levels. This would also help in better understanding the other factors that influence concentrations of fluoride. Characteristics such as aquifer type, depth would shed more light on the influencing factors.
Lastly, there is a need for an updated database on extensive borehole information readily available in the district water departments.
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