Decreasing groundwater levels in hand-dug wells in Ouédo, southern Benin

The fate of shallow groundwater resources in a fast-growing suburban area

Based on MSc Thesis of Kambiri Cox (IGRAC Mentor: Arnaud Sterckx)

Decreasing Groundwater Levels

Hand-dug wells are the primary sources of potable water for many in West Africa, yet the security of shallow groundwater supplies is compromised by climate variability, land-use and land-cover change, population growth and groundwater abstraction. In Ouédo, a fast-growing suburban town in southern Benin, residents are encountering declining shallow groundwater levels and attributing this drawdown to the development of a new wellfield in Ouédo which supplies Cotonou, the largest city and economic capital of Benin.

Groundwater Well

It is known that the hand-dug wells in Ouédo tap into the unconfined Quaternary aquifer, while the boreholes of the wellfield access water from the underlying Mio-Pliocene sandstone aquifer known as the Continental Terminal. If the hand-dug wells are hydraulically connected to the deeper wellfield, then abstraction of significant volumes daily from the Ouédo production wellfield can deplete the shallow reserves with time and decrease water availability for communities. However, urban encroachment and population growth are also straining groundwater resources in the area. With the increase of impervious cover there will be less recharge to the shallow aquifer. This groundwater recharge can also decrease due to climate variability with decreased precipitation inputs and/or increased evapotranspiration. Moreover, with population growth in Ouédo there will be increased demand. In this regard, the water levels in hand-dug wells will decrease due to the multiplication of hand-dug wells throughout the area. Increased anthropogenic activities with population growth will also heighten the risk of bacterial contamination in the unprotected hand-dug wells. Should the aquifers be hydraulically connected, therefore, there is enhanced risk of these surface contaminants being transported to the Continental Terminal through downward fluxes.

Men in a canoe

By evaluating the driving forces of declining groundwater levels in the hand-dug wells (climate variability, land use/land cover change, population growth and abstraction), the impact of each can be compared within a well-researched context and assist in management practices in the study area. Results of this study confirm that recharge in the shallow Quaternary aquifer is strongly connected to local precipitation, but analysis of recent climate variability provided no clear evidence to support the hypothesis of decreased precipitation and/or increased evapotranspiration causing decreased recharge. On the other hand, increased urban encroachment in and around the study area has led to increased impervious cover and therefore decreased infiltration capacity of the soils. With urban encroachment, the study area experienced rapid population growth in recent years. The subsequent multiplication of handdug wells allowed for increased groundwater abstraction from the shallow Quaternary aquifer by local residents. Isotopic and hydrogeochemical characterization of the aquifers were largely inconclusive with respect to mixing between the two groundwater bodies as a result of wellfield abstraction but directed recommendations for further monitoring in the Ouédo study area.

MSc Thesis Kambiri Cox

Thesis Cover

The foundation of this story has been the MSc thesis of Ms. Kambiri Cox. Cox has followed the GroundwatCH programme, which is an Erasmus Mundus programme with IHE Delft, IST Lisbon and TU Dresden. The final stage of this programme was a thesis research, with supervision of Prof. Michael McClain (IHE Delft) and mentoring by Dr. Arnaud Sterckx (IGRAC), Dr. Johannes van der Kwast (IHE Delft), Prof. Abdoukarim Alassane (UAC-Benin), Mrs. Valerie Kotchoni (UAC/IRD-Benin). The full MSc Research thesis is now available for download and is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.