The combined effect of abandoned mines and agriculture on groundwater chemistry.

Although it is well known that both mining and agriculture disturb groundwater quality, their mutual interactions are much less well documented, though agricultural activities may prevail once mining operations have ended. To study these potential interactions and their impacts on water chemistry, we monitored the chemical composition of groundwater at the outlet of a gold exploration gallery in an area of intensive agricultural activity along with an isotopic study of the groundwater, a reactive artificial tracer test that involved injecting H2O2 into the gallery, and geochemical modelling. The isotopic study revealed denitrification of the NO3-bearing groundwater that takes place through oxidation of the sulphide minerals associated with the gold deposit and leads to anomalous concentrations of some metals such as Zn, Co and Ni. It also contributes to liberating As into the groundwater, where the tracer test confirmed that As is sensitive to the redox conditions. The currently observed high arsenic concentrations in the groundwater are interpreted as resulting mainly from the former mining activities through a remobilization of As sorbed on or co-precipitated with the iron oxides that formed when the gallery was excavated. The geochemical modelling enabled us to calculate the respective role of each process involved in the As accumulation in the groundwater. It is also inferred that NO3 contamination from agricultural activities disturbs arsenic remobilization--by consuming available electron donors (e.g. organic matter), NO3 limits the reduction of iron oxides and consequently the release of arsenic.