1. Hydrogeochemistry and groundwater quality in Nyos area about three decade after the CO2 gas burst (North-western Cameroon)
4B-P5
*B.T. Kamtchueng 1 , W.Y. Fantong 2 , E.R. Tiodjio 1 , K. Anazawa 3 , M.J. Wirmvem[4], J.O. Mvondo 5 , M. Kusakabe 1 , G. Tanyileke 2 , T. Ohba 4 , J.V. Hell 2 , A. Ueda 1
1 University of Toyama, 2 IRGM Yaounde, Cameroon, 3 University of Tokyo, 4 Tokai University, 5 University of Yaounde-Cameroon.
Located in a volcanic crater in north-western Cameroon, Lake Nyos captured the world's attention in 1986, when an explosive release of CO2 from the lake's depths asphyxiated 1,700 people in the surrounding villages. 
people are moving back into the evacuated region with groundwater resources as the unique source of water supply
1. Background
10 m
Lake Nyos,
January 2013
45m
February 2001
Fig. 3 Hydrological map of study area
2. Methodology and study area
Geology
- Pluto-metamorphic basement (quartz monzonite)
- Basaltic materials, pyroclastic surge deposits
Sampling description
- Jan. 2013
- 20 water samples
- Major elements (cations and anions) were done using ion chromatography (Metrohm 761).
- PHREEQC program included in the DIAGRAMMES software version 5.9 was used to calculate saturation indices
3.3 Lake Nyos - Groundwater Relationship
Lake Nyos subsurface discharge contribute to recharge ground water in the basin
Deep CO2-rich water of lake do not affect groundwater quality in Nyos basin. 1 3 2 4
&
Fig. 9 Chemical distribution of groundwater flow patterns
Rocks + H2CO3 → cations + H4SiO4 + HCO3- + solids
Silicate hydrolysis of rocks-forming minerals
Volcanic terrains
Na-HCO3 (earlier stage)
Short residence time
CaMg-HCO3
Long residence time
Water rocks interaction a b
1:2 line
1:1 line c d e f
Fig. 5 Hill Piper trilinear plot
1 CaMg-HCO3
2 Na+K-Cl or Na+K-SO4
3 Na+K-HCO3
4 CaMg-Cl or CaMg-SO4
5 Ca-Cl
6 Na-HCO3
7 Ca-HCO3
8 Na-Cl 5 6 8 7
Evaporation dominance
Water-rock interaction dominance
Precipitation dominance
Fig. 6 Gibb’s plot
Groundwater hydrochemistry in Nyos watershed is dominated by Na-HCO3 type which evolve to Ca-Mg-HCO3 water type;
Dissolution of dolomite and calcite minerals following by weathering of silicate minerals are the major hydrochemical processes governing the groundwater chemistry;
Ion exchange process also occurs in the aquifer matrix within the basin;
Quality assessment shows that groundwater in Nyos basin are within the World Health Organization (WHO) guidelines for drinking and domestic uses;
Based on the sodium percentage (Na%) and sodium absorption ratio (SAR), Nyos groundwaters are considered suitable for agricultural purposes;
Toxic CO2-rich lake water may do not affect the quality of groundwater within the Nyos basin.
Wilcox diagram and USSL classification of Nyos GW
Fair
Poor
Good
Excel.
Very
C1-S1
C4-S3
C4-S2
C3-S2
C2-S2
C1-S2
C2-S1
C3-S1
C4-S1
C2-S3
C3-S3
C1-S3 C1 S3 S2 S1 C2 C3 C4
Low
Medium
High
Very high
Electrical Conductivity (㎲/cm)
Sodium Absorption ratio (SAR)
Located in a volcanic crater at the edge of Cameroon Volcanic Line (CVL), Lake Nyos captured the world's attention in 1986, when an explosive release of CO2  from the lake's depths asphyxiated 1,700 people in the surrounding villages. The similar disaster had been already observed two years before at Lake Monoun located of about 100 km toward south of Nyos with luckily much less casualties. After the gas disasters, many studies have been done on geochemistry of the lakes in order to clarify the cause(s) of the gaz burst. Holding the severity of the disaster and imminent risk, subsequent studies focused mainly on the interpretation and modeling of the mechanisms of gaz burst. Although significant finding already done on the Nyos and Monoun disaster, the hydrogeochemistry aspect of groundwater around those lakes remains little known.
Success of the degassing process
Population growth in Nyos area and surrounding villages with groundwater resources as the unique source of water supply.
Fig.2 Evolution of dissolved CO2 showing increase rate of CO2 before degassing operation and decrease rate of CO2 from Lake Nyos since 2001 when the degassing operation started. (Kusakabe et al., 2008)
… then the need of study groundwaters resources and related issues in Nyos area
Fig.1 Location of Lakes Nyos and Monoun on the CVL
Stoichiometric relations and Scatter plots
Range
Ionic ratio
Groundwaters and Soda Springs (n=20)
 Discusion
(Ca2+ + Mg2+) /T cations
> 0.5 cation contribution to GW by silicate weathering (Matini et al., 2012)
(Na+ + K+) /T cations
(Na+K) vs. TZ+ falling on or below the 1:2 line suggests silicate weathering (Datta and Tyagi, 1996) 
Ca2++Mg2+/HCO3-
> 0.5 and Ca+Mg vs. HCO3+SO4 below the 1:1 line confirms the effect of silicate weathering, (Datta and Tyagi, 1996)
Ca2+/Mg2+
=1dolomite dissolution; >1 calcite dissolution; >2 silicates dissolution
(Mayo and Loucks, 1995) (Katz et al., 1998)
Na+/Cl-
>1 suggests no halite dissolution and indicates ion exhange process (Fisher and Mullican, 1997)  
Na+/Ca2+
Na vs. Ca plotted below the 1:1 line suggests ion exchange process
(Lavitt et al., 1997)
CAI1
 Negative value indicates ion exchange process (Shoeller, 1967), (Jankowski et al., 1998)
(Kumar et al., 2007) 
CAI2
Fig. 8 Scatter plots
From both stoechiometric relations and Scatter plots, dissolution of carbonates (dolomite and calcite) following by silicates weathering and cation exchange are the major hydrogeochemical processes governing groundwater chemistry in Nyos wathershed.
Fig. 8 (e) and (f) which shows no correlation between NO3 vs. Cl and NO3 vs. K indicates no source of pollution from agricultural activities such as uses of fertilizers.
3. Results
3.1 Physical parameters
Deep lake water
The spatial variation of electrical conductivity (EC) measured on the Nyos groundwaters shows a wide range from the recharge area (upper Nyos) to the discharge area (Nyos village). This variation indicates inhomogeneity distribution of groundwaters within the catchement (local variation in point source) suggesting heterogeneity in the mineralization process.
Groundwaters samples along the dwelling houses at lower altitude expected to be the most mineralized seems to be diluted by discharge from another aquifer system not related with Lake Nyos (see Fig. 9).
This onbservation might be an indicative of multiple aquifer system in Nyos basin.
Groundwater near Lake outlet
Lake Nyos
subsurface
EC (μS/m)
Soda spring
Groundwater at
Nyos villages
Groundwater
at upper Nyos
150
200
250
300
100
3.4 Groundwater usability
Compliance of groundwater samples in Nyos basin to drinking standards
Groundwater parmeters
WHO standard (1996a, 2004)
BIS: 10500: satandard after ISO 10500: 1991
Maximum analytical results
Desired Limit (DL)
Max Permissible Limit (PL) DL PL
Groundwater around Lake Nyos (n=18)
Soda Spring
(n=2) pH
9,2
No relax
8.28
5.35
EC (μS/cm)
500
1400
1200
1000
226
536
TDS (mg/L)
2000
215
463
TH (mg/L)
100
300
600
113.66
248.83
Na+ (mg/L)
K (mg/L) - 12
200
13.25
6.30
25.74
8.11
Ca2+ (mg/L) 75
21.25
27.99
Mg2+ (mg/L) 50
150 30
17.86
42.93
Cl- (mg/L)
250
0.67
1.26
NO3- (mg/L) 10 45
2.88
0.24
SO42- (mg/L)
400
4.05
4.15
HCO3- (mg/L)
155.84
345.97
F- (mg/L) -　
1.5 1
0.04
0.09
Fig. 4 Spatial variation of electrical conductivity of the study area
3.2 Processes controling groundwater chemistry
Fig. 7: Relation δ2H/δ18O of groundwater in the Nyos watershed
Groundwaters in Nyos basin are within the WHO guidelines and in the range of excellent to good for irrigation.
4. conclusions
Plots of SI against TDS (Fig. 5) indicate that with respect to carbonate minerals and sulfate minerals, Nyos groundwaters are widely undersaturated.
The slight tendency toward saturation and oversaturation with respect to dolomite and calcite may be due due to kinetic effect such as dedolimitization which may inhibit their precipitation following this equation: CaMg(CO3)2 + Ca2+ → 2CaCO3 + Mg2+
Aknowlegment
The financial assistance from SATREPS Ny-Mo project is highly acknowledged. Our thanks are also due to Mr. Issa, Boris T. Chako and Asobo N. E. Asaah.
Fig. 7 Saturation indices (SI) of groundwaters