Comparison of diversion well substrates for the treatment of acid mine drainage, Bellvue Mine, West Coast, New Zealand

Abstract

Bellvue Mine is an abandoned mine north of Greymouth, Rapahoe, West Coast. Although abandoned since the 1970’s, acid mine drainage is still being discharged from the mine and contaminating the nearby, Cannel Creek. A significant environmental problem globally, acid mine drainage is characterised by high dissolved metal concentrations and low pH conditions. As a result, Cannel Creek has become subject to ecosystem degradation and a loss of aquatic biodiversity.

Diversion wells are a method of passive treatment of acid mine drainage. A typical well consists of a cylinder-shaped container, filled with limestone aggregate, and a pipe centred down the well to allow water from an upstream dam to provide hydraulic head and entry to the system. Dissolution of the calcium carbonate raises the pH level, adds alkalinity and thus allows for precipitation of metal contaminants out of solution. Mussel shells are an alternative source of calcium carbonate and method of passive treatment. This research aimed to test the efficiency of a diversion well using mussel shells in treating acid mine drainage at Bellvue, in comparison to the more traditional diversion well using limestone.

An initial experiment was set up to test several combinations of variables which influence diversion well mechanics and function, to achieve optimal fluidization of substrate grains. These results could then be applied to diversion well installation at the site. The variables looked at were flow rates of water flowing into the well, the inlet pipe diameter, well height and diameter and substrate type and grain size. Several findings were made: • Optimal fluidization occurred with inlet pipe sizes 32-50 mm. • Grains were flushed over the well indicating the greater the well height, the greater the amount of substrate the well can contain, the more space for fluidization. • The larger the well diameter, the less fluidization occurred horizontally. • Optimal fluidization occurred with 2.36-10 mm limestone and 4.5-12 mm mussel shells.

Following this, a diversion well was installed at Bellvue. Limestone and mussel shells were tested individually in the well, a mixed substrate was also trialled. Chemical analysis and water quality data was collected to determine which substrate was more effective at treating acid mine drainage. Several findings were made: • Initial operation of a diversion well allowed for effective improvements in water chemistry and quality, with each substrate. The pH levels were raised to near neutral and metals precipitated out of solution. • The limestone proves to be the most effective treatment in this diversion well system compared to the mussel shells. Greater increases in pH levels and greater decreases in dissolved metal concentrations were achieved using the limestone substrate. • Long term operation of a diversion well could not be achieved with this setup. The size of the well was too small for the volume of substrate necessary to achieve long term effective treatment.

Future work, involving larger scale treatment using a diversion well at Bellvue, will need to involve increasing the height and therefore, volume of the well, allowing the use of more substrate and thus, longevity of treatment. Limestone is the more desirable substrate for the most effective treatment in a diversion well system.