NATURAL REMEDIATION OF ALKALINE CONTAMINATION OF GROUNDWATER THROUGH BUFFERING: IMPLICATIONS FOR MANAGEMENT

At alumina refineries worldwide, there is the potential for contamination of groundwater by alkaline process fluids (caustic soda, Bayer liquor, bauxite residue leachate). Sources are leaks in containment areas for refinery operations and for bauxite residue, and spills on soils in unsealed areas with subsequent infiltration and leaching to groundwater. Alkaline contamination of the receiving groundwater not only increases pH and alkalinity of the groundwater but also results in associated increases in concentrations of certain trace elements. In particular groundwater concentrations of anions such as fluoride and (hydr)oxyanions such as arsenate and aluminate can rise rapidly with increasing alkaline pH.

Solution alkalinity and CO₂ pressure are the main determinants of pH. Increases in groundwater pH can be buffered naturally by high CO₂ pressures, by ion-exchange with clays, by precipitation as Ca and Mg carbonates or hydroxides in saline waters and also by H₂SO₄, generated from oxidation of pyrite on lowering of the water table. The relationship between pH, alkalinity and CO₂ pressure is discussed, both in the presence and absence of calcite, which is a key component of limestone formations.

Effects of ion exchange, precipitation and pyrite oxidation become apparent when molar increases in solution concentrations of Na and alkalinity from NaOH are not equal. Examples are presented of these buffering processes as observed for groundwater affected by alkaline process fluids at the three Alcoa World Alumina refineries in Western Australia: Kwinana, Pinjarra and Wagerup. Implications of these alkaline buffering processes for the management of alkaline groundwater contamination are discussed. It is concluded that long term management of impacts of alkaline contamination on groundwater quality can benefit from an understanding of hydrochemical and geochemical reactions in groundwater systems, such as occur through: elevated CO₂ pressures; ion-exchange with clays; precipitation of Ca and Mg carbonates or hydroxides in groundwater, especially saline waters; and H₂SO₄ generated from oxidation of pyrite.