The Presence of Nitrate in Ground Water
Despite its natural occurrence in ground water (Hess and Jacobson, 1981; Arumi et al., 2005; and Stadler et al., 2008) the presence of nitrate in ground water is considered as a
contaminant, due to its potential for harm to the environment. It is deemed to be the most pervasive chemical contaminant of the aquifers, and the levels of contamination are projected to increase (Fried, 1991; Goodchild, 1998; Joosten et al., 1998; Birkinshaw and Ewen, 2000; Liu et al., 2005). Nitrate removal of the water bodies is thus crucial. Indeed, excessive levels of nitrate in drinking water cause methemoglobinemia in infants, which decreases the blood's oxygen transport ability ( Hegesh and Shiloah, 1982). Moreover, the excess input of nitrates in water body induces eutrophication which causes problems in the aquatic ecosystems such as toxic algal grow, lack of oxygen in the water and loss of aquatic biodiversity (Smith, Tilman, and Nekola, 1999).
The Loulo site had experienced in the last few years an increase in levels of nitrate and heavy metals (arsenic, copper, aluminum, and iron) in the underground water. This water is collected underground from leaking into the workings and is cascaded in the mine through a series of sumps and pumped to the settling ponds located underground, before pumping it to surface settling dams. On the surface, the residual solids are supposed to decant from the wastewater before being discharged to a stream, the Faleme river. It is an international watercourse shared by Senegal, Mali and Guinea which form the natural border between Senegal and Mali, and Senegal and Guinea.
The poor water quality has resulted in increasing levels of similar contaminants being measured at a monitoring point close to the Faleme River. Loulo has thus consulted an environmental consulting firm, Digby Wells, to address the water management issues and to enable the mine to reduce further impact to the water resources.
The firm proposed to build an artificial wetland. It is an alternative to traditional water treatment based on complex physical, biological and chemical process with the association of plants, soil, and microorganisms.
The wetland had been implemented in March 2017. The Environment and Community Service department is responsible for ensuring that the contaminant removal is effective by sampling and monitoring the water quality of wetland. The water analysis is conducted by a third party laboratory, the equipment not being available on-site.
The results of the first two months indicate that the levels of nitrates, contrary to those of heavy metals, increased after the water passed through the wetland (Table 3).