environmental management | permitting Martha Mine | noise | dust | vibration | Water Treatment Plant | water management | groundwater monitoring | water sampling | The New Zealand dotterel | how have environmental considerations changed over time?

The Water Treatment Plant
The rainfall in Waihi is relatively high, averaging 2,147 mm/year, and unlike many mines around the world, the Martha Mine site operates with a net surplus of water. Because of this net water surplus, water management is an integral part of operations, and the Water Treatment Plant exists to treat surplus water prior to discharge to the Ohinemuri River.

The Water Treatment Plant is an integral part of the Water Management System at Martha Mine.

Prior to building the existing Water Treatment Plant, substantial laboratory trials were undertaken to develop the optimum treatment process design. This work looked at maximising the oxidation of cyanide in the form of free cyanide and weakly bound complexes of copper, nickel and zinc (WAD cyanide) which are recognised as the forms having most potential to affect aquatic biology.

The Water Treatment Plant was constructed and commissioned in 1989, and has performed well, with a very high level of compliance with the consent conditions. In 1999, as part of the Extended Project, and following the granting of further resource consents, the plant was upgraded to provide sufficient capacity to cater for the predicted increase in water requiring treatment.

As described in the Water Management section, excess water on site can be derived from a number of sources; some contain cyanide—added during the gold and silver extraction process—some do not. To manage this, the Water Treatment Plant incorporates two distinct treatment stages:

  • a cyanide destruction process, using hydrogen peroxide—a strong oxidising agent—to destroy the cyanide, and copper sulphate as a catalyst to speed up the reaction
  • a metal and trace ion removal process, using ferric chloride, lime and flocculants.

The upgraded plant has the capacity to treat up to 15,000 cubic metres per day of non-cyanided water and 5,000 cubic metres per day of cyanided water.

left; The Water Treatment Plant treats water to meet the conditions of the discharge consents.

above: Polishing ponds hold treated water prior to discharge into the Ohinemuri River.
Water decanted from the tailings pond (1), and collected in the underdrainage system (4) contains cyanide and passes through a pre-treatment, cyanide destruction stage. Hydrogen peroxide, copper sulphate and lime are added to the cyanided water (2) in a rapid mix tank. Cyanide oxidation begins.

The solution moves through a number of retention tanks (3) to ensure that there is sufficient time to allow cyanide oxidation reactions to take place during times of peak flow. Hydrogen peroxide in the presence of copper destroys all free cyanide through chemical oxidation. Weak acid dissociable (WAD) cyanide is also oxidised during the process. On oxidation, cyanide yields simple carbon and nitrogen compounds.

Mine water (5), collection pond water (6) and some underdrainage water (4) do not require treatment for cyanide removal. This water is treated to remove metals, trace ions and suspended solids.

Similarly, cyanided water goes through this same process once the cyanide has been destroyed.The water passes into tanks (7) to which lime and ferric chloride are added (8). The lime raises the pH of the water to 9.5-10. This is necessary to reduce the concentration of copper, and other metals, to the low levels required to allow the water to be discharged. In the presence of ferric chloride and lime, insoluble hydroxides and carbonates form. To separate the solids from the water, polyelectrolyte (flocculant) is added (9), along with more lime. Polyelectrolyte is used in potable (drinking) water treatment. It achieves a very low concentration of residual solids carry over. The result is a very high clarity water discharge.

All treated water then passes through clarifiers to allow the suspended solids and metals to be removed from the water. The suspended solids and metals fall to the bottom of the clarifiers (10), forming a slurry. The slurry is pumped to the tailings pond (11) via a thickener.

Carbon dioxide is added (12) to the clean water overflow from the clarifier to reduce the pH of the water to meet the compliance limits. The water then passes to polishing ponds (13). The polishing ponds hold the treated water for a sufficient time to allow laboratory testing, and the results to be received and interpreted prior to the water discharging to the Ohinemuri River. Water that meets the discharge criteria is discharged (14) to the Ohinemuri River. If the water does not meet the discharge criteria, it is recycled (15) back through the plant, used in processing, or piped to the tailings storage facility.

The conditions relating to the treated water discharge can essentially be divided up into the following:

Water Quality
The water quality limits are based on protecting both the aquatic biology of the river and downstream users, and the limits are based on the USEPA guidelines for the protection of aquatic life (instream) where such appropriate limits exist, and other guidelines where appropriate. Flow measurement is an important aspect. Compliance limits for the treated water quality parameters are set based on a dilution factor.

Water Flows
The consents allow up to 20,000 cubic metres per day to be discharged from the Water Treatment Plant, or 15% of the river flow, depending on which is the lesser. For this reason, flow meters exist to record the treated water discharge volumes. The river flows are monitored using a river level recorder. Regular river gaugings are carried out to ensure that the river level provides an accurate indication of the river flow.

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above: Measuring pH and conductivity in river water. This is the Ohinemuri River downstream from the Water Treatment Plant discharge point.
left: This structure houses the river level monitors. The Waste Rock Embankment can be seen in the background.
River Water Quality and Aquatic Biology
Monitoring of the water quality and aquatic biology of the Ohinemuri River and the Ruahorehore Stream is an important part of the treated water discharge consents. It is also important to compare the existing situation in the river with the pre-mining situation.

Baseline Monitoring
Baseline studies of the water quality and aquatic biology of the Ohinemuri River and its tributaries commenced in 1981, and these indicated that the quality of the river and stream habitats was largely determined by the land use within the respective catchments. This situation is commonly found in rivers and streams throughout New Zealand where the waters flow from bush covered catchments downstream through open farmland.

In the headwaters of streams located in native bush, the water quality was found to be good and the streams supported a diverse range of aquatic plants, benthic macroinvertebrates and fish. In the lower reaches of the tributaries and in the Ohinemuri River the tendency was for:

  • water quality to decrease slightly downstream and under higher flows
  • aquatic plant (macrophytes and algae) cover to increase
  • diversity of the benthic macroinvertebrates to decrease and abundance to increase
  • fish habitat quality to decrease.

These changes were considered to be due primarily to the removal of native bush and conversion to farmland. This resulted in increased nutrient and sediment input to the waterways. Riparian willows, which provided cover for fish, shaded the river and maintained lower water temperatures, were also removed by the council. Further changes were caused by the discharge of waste from cowsheds, which increased the organic and nutrient loading to the river and its tributaries. For information on how H.E.L.P. is redressing these issues see HELP.

In summary, a general lowering of the water quality of the Ohinemuri River and changes to the habitat quality of the river resulted from the development of approximately 65% of the catchment for farmland, and willow clearance (prior to 1989) along the tributaries and main stem of the Ohinemuri River. Catchment development resulted in increased inputs of dissolved nutrients and suspended solids (sediment). The clearance of willows reduced instream cover for aquatic organisms, raised water temperatures and increased the potential for algae and other aquatic plants to grow in the river.

Operational Monitoring
Monitoring for water quality, fish, periphyton, sediments, and benthic macroinvertebrates is carried out at a number of sites both upstream and downstream of the discharge points. The characteristics of the river are compared to those that existed prior to the discharge of treated water.

Monitoring to date indicates that the treated water discharge has had no significant adverse effect on the Ohinemuri River.

For information on how water at Martha Mine is tested see Water Sampling.