Study and assessment of trace metal contamination (As, Cd, Pb, and Hg) in water resources from the headwaters of the Ouémé and Pendjari watersheds in the Copargo Municipality (Northwestern Benin)

Abstract

Headwaters, considered strategic zones for the preservation of water resources and crucial for evaluating water quality and streamflow, have been increasingly subjected to anthropogenic pressure in recent years. This pressure, exerted by human activities, has notable negative repercussions on the quality of both water and soil resources. The aim of this study is to assess the level of contamination of water resources in the headwaters of the Copargo municipality by trace metals (arsenic, cadmium, mercury, and lead). To this end, water samples were collected during two sampling campaigns conducted during the high-water (august 2023) and low-water (november 2023) periods. In total, 42 sampling sites were selected, comprising 9 wells, 7 boreholes, 25 rivers, and 1 dam. These samples were properly preserved and transported to the laboratory for analysis of trace metals, arsenic, cadmium, mercury, and lead using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), according to the EPA 6010B method. The results indicate that the concentrations of arsenic, cadmium, mercury, and lead measured in water resources during the high-water period are significantly higher than those observed during the low-water period (p<0.05). Regarding water quality in relation to Beninese drinking water standards, the concentrations of trace metals in the water from wells, boreholes, and rivers are generally below the established thresholds, with the notable exception of mercury. Specifically, mercury concentrations of 1.56 µg/L, 1.67 µg/L, and 1.79 µg/L in well, borehole, and river water, respectively, exceed the Beninese standard of 1 µg/L. Furthermore, the results reveal that the average concentrations of arsenic (0.9 µg/L), cadmium (0.71 µg/L), and lead (0.32 µg/L) are slightly higher in groundwater than in surface waters, where they are 0.71 µg/L, 0.2 µg/L, and 0.31 µg/L, respectively. In contrast, the average mercury concentration is higher in surface water (0.9 µg/L) than in groundwater (0.8 µg/L). The contamination and pollution indices calculated indicate that 88.89% of the wells, 14.3% of the boreholes, and 46.15% of the rivers exhibit a moderate level of contamination (1 < DC < 3), while only one borehole (14.3%) shows a high contamination level (DC = 4.28). Regarding the pollution load index, all water resources studied show a low level of pollution. Despite the low observed contamination, the mercury concentrations suggest that most of the water resources in the municipality are unsuitable for human consumption. Although water resource contamination is relatively low, the toxicological risk remains concerning due to the cumulative nature of trace metals. This situation underscores the need for a modeling study of pollutant transfer to enable dynamic monitoring and better prediction of the quality of these water resources.