Kinetic Controls on Iron and Arsenic Precipitation in Acid Mine Drainage (Carnoules, France)

Reigous Creek, located in the Rhone river basin (SW France), is affected by acid mine drainage. Mining has generated 1.5 million tons of waste (10% sulphides) stored at the springs where the creek originates. As a result of pyrite oxidation, the creek water displays low pH (down to 2.5) and high levels of sulphate (about 40 mmol/L), Fe (12-25 mmol/L), and As (1-3.5 mmol/L). The water chemistry, which has been monitored monthly for 4 years, varies seasonally. At the spring of the Reigous Creek, iron (mainly Fe(II)), exhibits concentrations decreasing from 25 mmol/L in summer to 12 mmol/L after rain events. Arsenic varies from 3.5 to 1 mmoUL and is mainly (50 to 95%) in the reduced form, As(III). Natural attenuation of As concentrations occurs as a result of microbiologically mediated As-Fe coprecipitation. From 10 to 47% of the Fe and 17 to 46% of the As are removed from the aqueous phase within the first 30 m of the stream. The efficiency of this attenuation varies seasonally. The solid that precipitates in the stream has a mean As/Fe ratio of 0.15 with seasonal variations. Suspended sediments consist of an As-rich amorphous phase of schwertmannite; most laminated concretions contain tooeleite. Laboratory experiments demonstrated that As-rich schwertmannite is formed during rapid Fe oxidation while a slow oxidation rate rather favours the formation of tooeleite. Short-term variation (24 h) of As and Fe concentrations were evidenced in Reigous Creek due to photoreduction of iron-bearing phases, which results in fluctuations of about 10% in Fe and As concentrations over a 24 h-period.