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).

The sources of the high nitrate concentration in the underground water have always been assumed to be the explosives used to extract the ore. Indeed, is a well-studied fact that explosives use is one of the main sources of nitrates in the mining industry (Morin and Hurt, 2009).

However, the employees of Maxam Mali, the company providing SOMILO with the explosives, deny any correlation between the water contamination and their explosives. They are assuring that this problem was never encountered in other mines.

The objective of this section to investigate the reasons for the increase of nitrate in the outlet of the wetland by evaluating the different sources of nitrate on Loulo site.

An interpretation of the monitoring data of the water bodies located on-site and in the neighboring area has been made.

Wetland characteristics

The wetland is installed downstream to the Gara sludge plant and the sewage treatment plant. It is a pool of 9300 m2, destined to treated the rich nitrate water before it is discharged in the Faleme river.

The actual design of the system had been guided by the United States Environmental Protection Agency (EPA) design manual for constructed wetlands.

The model has been built in six compartments of equal dimension, in addition to an entry pool where the particle in suspension can decant from the water.

Five meters of medium porosity rocks coming from the waste dumps had been laid over the compartments.

The evaporation rate of the area is evaluated at 12 mm/day and the retention time of the water between the inlet and the outlet is approximatively 33 hours.

After the particles had been decanted from the water, the latter is drained through the pores of the rocks into the different compartments. The nitrates are captured and mineralized by the microorganisms located in the plants' rhizomes (or roots).

The plant used is the cattail (typha latifolia) due to its effectiveness and the fact that it is indigenous to the area.

By photosynthesis, the cattail increases the concentration of oxygen in the water which stabilizes its rhizomes while limiting the speed of the water flow. It allows the process of denitrification. Denitrification is a microbially facilitated process where nitrate is reduced to nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products (1)

NO3−