HYDRAULIC PROPERTIES OF UNCONSOLIDATED SEDIMENTARY AQUIFERS IN KRIBI, CAMEROON: ENVIRONMENTAL IMPLICATIONS

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1 HYDRAULIC PROPERTIES OF UNCONSOLIDATED SEDIMENTARY AQUIFERS IN KRIBI, CAMEROON: ENVIRONMENTAL IMPLICATIONS Vol. 13, n 1, Anatole Djieto Lordon 1*, Nenita Ntumba Bukalo 1 and Christopher M. Agyingi 1 *Department of Geology, University of Buea, P.O Box 63, Buea-Cameroon alordon@yahoo.com ABSTRACT In order to construct the Deep Sea Port of Kribi (Cameroon), it was important to evaluate the hydrogeological characteristics of the aquifers in the area; so as to determine if there could be any environmental impact on the population. To achieve this, both a step-drawdown and a constant rate pumping test were carried out in two boreholes (PS41 and PS21). The hydrogeological units underlying the test sites are sands, silts and weathered rock layer for PS 41, and sands, clays and weathered rock layer for PS21. The coarse sand and the top of the weathered rock are the main aquifers in the investigation site. At PS41, the step-drawdown test was done in two steps with an interval of one hour and increasing pumping rates ( m 3 /h) while the constant rate pumping test was done at a pumping rate of 0.15m 3 /h for 3 hours. The step-drawdown test was done in 3 steps in PS21 at increasing pumping rates (1, 2 and 3m 3 /h), with an interval of 1 hour; and the constant rate pumping test was done at 3m 3 /h and lasted 24 hours. The results of these tests were analyzed using the Theis-Jacob s method, and transmissivity, hydraulic conductivity, storage coefficient and radius of influence were computed. These results reveal that the area around PS 21 is a low permeability area. It can therefore be concluded that there is minimal risk of groundwater contamination by substances from the port. Keywords: Pumping test, Kribi, Environmental impact, Deep Sea Port RESUME Dans le but de construire le port en profonde de Kribi (Cameroun), une étude sur le comportement de la nappe aquifère a été menée. Cette étude permit de déterminer s il y aurait un impact environnemental sur la population, dû à la construction de ce port. Pour ce faire, des essais par paliers à débits croissants et des essais longue durée furent exécutes sur deux sites, PS41 et PS21. Les formations géologiques se trouvant sous ces sites sont : le sable, le silt et la roche altérée, sous PS41, et le sable, l argile et la roche altérée, sous PS21. Le sable grossier et la roche altérée sont les principaux aquifères dans la région d étude. Sur PS41, l essai par paliers a été fait en deux paliers enchainés d une heure chacun, avec des débits croissants de 0,1 et 0,2 m 3 /h. Alors que l essai longue durée a été exécuté à un débit moyen de 0,15 m 3 /h, jusqu à l arrêt, après 3 heures. Un essai par paliers enchainés (3 paliers) d une heure chacun avec des débits croissants de 1 ; 2 et 3 m 3 /h a été fait sur PS21. Sur ce même site, l essai longue durée a été exécuté à un débit moyen de 3m 3 /h pendant 24 heures. En utilisant la méthode de Theis-Jacob, la perméabilité, la transmissivité, le coefficient d emmagasinement et le rayon d influence ont été calculé. Ces résultats ont révélé que la région d étude a une faible perméabilité. Donc, il y a un minime risque de contamination de l eau souterraine par des substances provenant du port en eau profonde. Mots clés : Essai par palier, Kribi, Impact environnemental, Port en eau profonde Djieto Lordon et al. Vol. 13, (2012), n 1,

2 I. INTRODUCTION Hydraulic properties of an aquifer are parameters governing groundwater flow in that aquifer. Pumping tests are useful for providing information on aquifer properties that influence contaminant transport. These characteristics include (1) performance characteristics of a well (storativity and specific yield) and (2) hydraulic characteristics of an aquifer (hydraulic conductivity and transmissivity) (Poehls and Smith, 2009). Estimation of these parameters controlling groundwater flow and solute transport in a groundwater system is a major concern in hydrogeological research (Lee and Lee, 1999). This study comes in with a need of determining the aquifer s properties at the site of construction of the Deep Sea Port of Kribi. This paper, therefore, presents the performance and interpretation of pumping tests in sedimentary aquifers in Kribi (Cameroon). Geologic setting The study area is located in the Kribi-Campo subbasin (Fig.1). It lies on the south coast of Cameroon, both onshore and offshore. This subbasin is part of the Douala-Kribi-Campo basin, which is a passive margin basin (Burke and Whiteman, 1972). The Douala-Kribi-Campo basin has undergone a complex history that can be divided into the three classical stages of the evolution of Atlantic-type margins: pre-rift stage (Late Proterozoic to Late Jurassic), syn-rift stage (Late Jurassic to early Cretaceous), and post-rift stage (Late Cretaceous to Holocene) (Ntamack- Nida et al., 2008). The oldest rocks in this basin are Precambrian metamorphic rocks which make up the basement complex (Fig. 2). These are overlain by the Mundeck Formation (Lower Cretaceous). The sedimentary sequence in the basin is capped by the Wouri Formation (Quaternary). Based on lithologic logs of boreholes PS41 and PS21 (Fig. 3), the study area is made up of gneisses which underlie sands, clays and silts. Comparing these logs with the tectono-stratigraphy of the Kribi-Campo sub-basin, the gneisses in the study area are of Precambrian age while sands, silts and clays belong to the Wouri Formation of Quaternary age. The coarse sands (PS41) and the reddish sands (PS21) constitute the aquifers in the study area. Fig. 1. Location of the study area showing pumping boreholes PS41 and PS21 13 Djieto Lordon et al. Vol. 13, (2012), n 1,

3 Fig. 3. Lithologic log of borehole PS 41 Fig. 2. Tectono-stratigraphy of the Kribi-Campo subbasin (Ntamack-Nida et al., 2010) Fig. 3. Lithologic log of borehole PS 41 14

4 PS41 and PS21 (about 10.5 km apart). The stepdrawdown pumping test was used to evaluate the individual well performance with regards to yield, while the constant rate pumping test was used to further assess the nearby hydraulic properties of the aquifer. Step-drawdown tests The first step-drawdown test was carried out at Location 1, at borehole (PS41) which was used as a pumping well and a piezometer, 10m away from the borehole was used as the observation well. The step-drawdown pumping test was done in 2 steps with an interval of 1 hour between the 2 steps, and with an increase pumping rates ( m 3 /h). The second step-drawdown test was carried out at Location 2, at borehole (PS21) which was used as pumping well and a piezometer, 10m away from the borehole was used as the observation well. The step-drawdown pumping test was done in 3 steps with an interval of one hour between 2 steps, and increasing pumping rates (1, 2 and 3 m 3 /h). The pumping rate in the step-drawdown pumping test was determined manually using a bucket of 10 litres. Constant rate pumping test At Location1, the constant rate pumping test was done at a rate of 0.15m 3 /h for 3 hours, while at Location 2, the constant rate pumping test was done at 3m 3 /h for 24 hours. Drawdowns were taken manually using a water level indicator. The results of the constant rate pumping test were analysed using Theis-Jacob method. III. RESULTS AND DISCUSSION Step-drawdown pump test Fig 4. Lithologic log of borehole PS 21 II. MATERIALS AND METHODS Step-drawdown pumping tests and constant rate pumping rests were carried out in two boreholes The results of the step-drawdown pumping tests in both locations are summarised in tables 1 and 2. PS41 has a specific capacity less than 0.02 m 3 /h/m at pumping rates of 0.1 and 0.2 m 3 /h. This shows that the aquifer has very low permeability. The productivity of the aquifer is therefore very low; hence it is a clayey sand aquifer with low permeability. PS21 has a higher specific capacity, which varied from 0.41 to 0.52 m 3 /h/m. Step Table 1. Summary of the step-drawdown pumping test at PS41 Duration (hours) Pumping rate (m 3 /h/m) Drawdown (m) Specific capacity (m 3 /h/m)

5 Step Table 2. Summary of the step-drawdown pumping test at PS21 Duration (hours) Pumping rate (m 3 /h/m) Drawdown (m) Specific capacity (m 3 /h/m) The plots of the drawdowns versus discharges are shown in Figs.5 and 6. The results of the two steps fall on a parabola, and the characteristic curve of the borehole can be extrapolated to 0.07l/s to obtain a drawdown of 30m (Fig.5). In figure 6, it is shown that the results fall on a parabola. Therefore, the characteristic curve of the borehole can be extrapolated to determine the drawdown at Q = 1.5l/s Drawdown (m) Q (l/s) Fig. 5. Characteristic curve of PS41 (Plot of drawdown vs discharge) Fig. 6. Plot of drawdown vs discharge of PS21 16

6 Constant rate pumping test The recession and recovery curves of PS41 and PS21 are shown in Figures The recession curve of the constant rate pumping test at PS 41 shows that pumping was seriously affected by well leakage (Todd, 1980). That is the reason why the last measurements during pumping were not taken into account. While at PS21, pumping was slightly affected by well storage effects. Fig. 7. Recession curve s=f(t) at PS41 Fig. 8. Recovery curve s=f(t) at PS41 17

7 Fig. 9. Recession curve s=f(t) at PS21 Fig. 10. Recovery curve s=f(t) at PS21 Using Theis-Jacob s method for estimating transmissivity and storativity (equations 1 and 2), the average transmissivities and the storativity of each borehole were determined. eq. 1 Where, T= transmissivity Q = pumping rate Ds = slope of the straight line segment S = storage coefficient eq. 2 t 0 = time when the drawdown equals 0 X = distance between the pumping well and the observation well The radius of influence of pumping was determined using the equation 3. eq. 3 Where, R = radius of influence t = duration of pumping. The results of the constant pumping test are summarised in Table 3. Table 3. Summary of results of the constant-rate pumping test Well T (recession) in m 2 /s T (recovery) in m 2 /s T (average) in m 2 /s Storage coefficient Radius of influence (m) Hydraulic conductivity (K) in m/s PS x x x x x 10-5 PS x x x x

8 IV. DISCUSSION AND CONCLUSION These results confirm the results obtained from step-drawdown pumping. PS41 has lower transmissivity than PS21. The smaller drawdown in PS21 yielded the higher transmissivity, compared to PS41 which had a higher drawdown. This is due to the homogeneity assumption included in Theis- Jacob s method (Lee et al., 2003). The path taken by groundwater as it transports contaminants through an aquifer is controlled by the groundwater flow system. So, groundwater will move slowly in the aquifers of the study area because of the low hydraulic conductivity (Heath, 2004). The hydraulic conductivity of sands varies in orders of magnitudes between 10-3 and 10-1 m/s (Heath, 2004). PS41 has a hydraulic conductivity that is below this range; while PS 21 s hydraulic conductivity is a relatively low. The results of both pump testing methods corroborate and show that the study area has low transmissivity and conductivity. The low transmissivity of the formations and the fact that there are no settlements close to the deep sea port, show that there is minimal risk of contamination of groundwater by substances from the port. ACKNOWLEDGEMENT This study was financed by TOBE-HYDROTECH. We thank Mr Totcha Beka Etienne for permitting us to take part in the study and providing us with the data. REFERENCES Burke, K and Whiteman, AJ Uplift, rifting and break up of Africa: studies in Earth and spaces, Geological Society of America 132, Heath, RC Basic groundwater hydrology; 10 th Edition. U.S Geological Survey Water Supply Paper 2220, 91 p. Lee, JY and Lee, KK Analysis of the quality of parameter estimates from repeated pumping and slug tests in a fractured aquifer system in Wonju, Korea. Ground Water 37, Lee, J-Y, Kim, J-W, Cheon, J-Y, Yi, M-J and Lee, K-K Combined performance of pumping and tracer tests: A case study. Geosciences Journal 7, No. 3, p Ntamak-Nida, MJ, Baudin, F, Schnyder, J, Makong, J-C, Komguem, PB, Abolo, GM Depositional environments and characterization of the organic matter of the Lower Mundeck Formation (Barremian? Aptian) of the Kribi-Campo sub-basin (South Cameroon): implications for petroleum exploration. Journal of African Earth Sciences 51, Ntamak-Nida, M-J, Bourquin, S., Makong, J-C, Baudin, F, Mpesse, JE, Ngouema, CI, Komguem, PB and Abolo, GM Sedimentology and sequence stratigraphy from outcrops of the Kribi-Campo subbasin: Lower Mundeck Formation (Lower Cretaceous, southern Cameroon). Journal of African Earth Sciences 58, Poehls, DJ and Smith, GJ Encyclopedic Dictionary of Hydrogeology. Academic Press, 527p. Todd, DK Groundwater Hydrology. John Wiley and Sons, 535p. 19