Geochemical processes controlling the groundwater chemistry and

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Geochemical processes controlling the groundwater chemistry and fluoride contamination in the aquifer systems on the eastern,

western and northern flanks of Mount Meru, Tanzania

George BENNETT

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Arusha volcanic region in northern Tanzania

• Groundwater – source of drinking water

❖ High F⁻ conc.

• Dental and skeletal fluorosis

Introduction

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Two

field campaigns;

• July – September 2017

• March – September 2018

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Methods

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Methods

181

groundwater points

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Methods

158

water samples

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Results and Discussion

Dominant ions: Sodium (Na⁺) and bicarbonate (HCO₃⁻ )

NaHCO

₃ ^{type water}

Average pH value = 7.8 Alkaline groundwater

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Results and Discussion

High values of F⁻ were recorded Range: 0.15 – 301 mg/l

Average value = 23 mg/l Median value = 10 mg/l

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Results and Discussion

F⁻ concentration

• 91% of samples (143 samples)

– above 1.5 mg/l (WHO limit for drinking water)

• 9% of samples (15 samples) – below 1.5 mg/l

❖ Springs at higher elevations on Mount Meru

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Results and Discussion

F:Cl ratios

• 99% of samples (156 samples) – F:Cl > 0.10

• 1% of samples (2 samples) – F:Cl < 0.10

F⁻ is derived from chemical weathering of rocks

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Results and Discussion

Correlation analysis

y = 0.0467x - 2.6587 R² = 0.1427

R = 0.38

0 100 200 300 400

0 300 600 900 1200 1500 1800

F⁻(mg/l)

HCO₃^- (mg/l) (a)

y = 0.0914x - 0.1922 R² = 0.211

R = 0.46

0 100 200 300 400

0 300 600 900 1200 1500

F⁻(mg/l)

Na⁺ (mg/l) (b)

y = 0.6599x - 6.6264 R² = 0.2163

R = 0.47

100 200 300 400

F⁻(mg/l)

(c)

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Results and Discussion

Significant positive linear correlations of F⁻ with HCO₃⁻, Na⁺, K⁺ and pH

• Weathering of silicate minerals

❖ Na-K-rich volcanic rocks

r p-value

HCO₃⁻ 0.38 9.97E-07

Na⁺ 0.46 1.27E-09

K⁺ 0.47 7.45E-10

pH 0.33 2.00E-05

α = 0.05; r- correlation coefficients; n=158

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Results and Discussion

Significant negative linear correlation of F⁻ with Ca²⁺

• Calcite precipitation

• Dissolution of fluorite (CaF₂)

r p-value

Ca²⁺ - 0.21 6.84E-03

α = 0.05; r- correlation coefficients; n=158

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Results and Discussion

F⁻ (mg/l) increase with a decrease in elevation

STP4: F⁻ = 4.15

STP5: F⁻ = 4.34

S13: F⁻ = 6.16

S15: F⁻ = 7.19

S6: F⁻ = 35

S19: F⁻ = 41.12 STP6: F⁻ = 36.5

1300 1400 1500 1600 1700 1800

0 1 2 3 4 5 6 7 8 9 10

Elevation (m)

Distance (km)

A

Longer flow path --> longer residence time --> higher F⁻ A'

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Results and Discussion

F⁻ conc. vs Geology

F⁻ (mg/l) Nzd₁

Nzd₁

Nzd₂

Nvf Nvm

Nvm₁

Nvf

Geological formations

• Debris avalanche deposits - Nzd₁, Nzd₂

• Mantling ash – Nvf

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Results and Discussion

F⁻ conc. vs Geology

• Lower F⁻ values

❖ Pyroclastics with nephenelitic and phonolitic lavas

❖ Nephelinite lavas and breccias

• Higher F⁻ values

❖ Debris avalanche deposits

❖ Mantling ash deposits

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Conclusion

Chemical evolution of groundwater;

• Weathering and dissolution of silicate minerals

❖ Chemical weathering of Na-K-feldspars

• Calcite precipitation and dissolution of fluorite (CaF₂)

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Conclusion

Factors controlling F⁻ concentrations in groundwater;

• Nature of the geological formations

• Long residence time

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Recommendation

For safe drinking water

• Tapping water from the

springs with lower F⁻ values

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Ash cone, Mount Meru (Picture by George Bennett) ¹⁹

Thank You

Email: George.Bennett@UGent.be