In the frame of a technical cooperation project between the Lake Chad Basin Commission and the German Federal Institute for Geosciences and Natural Resources, four groundwater sampling campaigns were conducted in the Chadian capital N’Djamena in 2013, 2016, 2017, 2018. 253 samples were taken from 67 private bore wells equipped with handpumps and 16 large-capacity production wells of the public water supplier. The first campaign was designed to get an overview of the chemical and bacteriological groundwater quality and included samples from 52 private wells from across the entire city area and 13 production wells within the city. The subsequent sampling campaigns were conducted for monitoring purposes. Many 2013 sampling points were broken or inaccessible in the following campaigns and were replaced by wells in the vicinity. Only 32 private and nine production wells were sampled in all four campaigns.
The 2013 campaign identified bacteriological contamination, elevated nitrogen, and fluoride concentrations as the main areas of concern. While 40 % of the private and 23 % of the production wells contained fecal bacteria, nitrogen species and fluoride were found (with one exception) to be below the WHO guideline values. Although the subsequent sampling campaigns confirmed this general picture, the overall benefit of the additional 188 samples was limited to the finding that nitrogen concentrations were increasing in some points. In 2016, 2017, 2018 two, five, and four samples, respectively exceeded the WHO guideline value of 50 mg NO3/L. However, the distribution of the nitrate hot spots were somewhat random and it was not possible to draw more than general recommendations (e.g. protection zones around wells) from these findings.
In 2019, a survey was started to measure the total depths of all sampled wells and to make an inventory of possible sources of contamination. Re-evaluating the collected data in the light of this additional information leads to some insights on contaminant pathways, but most importantly, clearly shows that the monitoring network needs to be optimized. (1) Before choosing a sampling location, the well depth must be known and the future accessibility ensured: Replacing a broken well by any random well in the proximity does not help, unless both wells are filtered in the same depth and additional sources of pollution can be out ruled. (2) The construction of new observation wells is inevitable, as our data proves that random wells close to production wells cannot be used for monitoring purposes. (3) An additional focus on mapping of the sewerage system (design and construction years) is necessary to make predictions about the future development of nitrogen concentrations. In technical cooperation projects, baseline samplings are an important tool to identify problems, but groundwater monitoring needs more careful planning to be cost effective