It's easy to say that gold mines pollute water. This was certainly true in the first part of last century when little thought was given to the environment by anybody, but things have changed today.

Gold mines and water quality

Gold mines pollute the environment. In particular they pollute our waterways. We read this in Letters to the Editor and on placards all the time. If you’ve got a mine then you must have pollution, right?

Well, no.

Let’s use the Martha and Favona gold and silver mines in Waihi as an example.

This is going to get a bit detailed; the science will always be longer than the slogan.

We will need to look at:

  • The background, mining last century and mining today
  • Where mine water comes from
  • How it is treated
  • The quality of water discharged offsite
  • Monitoring of water quality
  • Where the water goes
  • Groundwater

Background
In the first part of last century it certainly was true that mining did cause pollution. Mines on the Hauraki Goldfields from Coromandel to Thames and Waihi disposed of their waste wherever it was convenient. In Waihi that included disposing of tailings from the Victoria battery straight into the Ohinemuri River. The river was so polluted that it was designated a ‘sludge channel’. An 1895 Act of Parliament allowed for up to 100 tons of tailings a day to be discharged directly into the river and its tributaries. This practice continued up to the closure of the original Martha mine in 1952. Ngati Tamatera in Paeroa remember when the river silted up so badly their traditional food sources all but disappeared.

Modern mining
Today the Ohinemuri River is clean. Clean enough to have been selected as the site of the World Trout Fly Fishing Championships in 2008, until drought conditions causing low river flows meant the event had to be relocated to the South Island.

This is the same river that the new Martha open pit mine has been discharging treated water into since 1987, and the Favona underground mine since 2006.

How can a modern gold mine discharge mine water into a river that is also a world-class fishery? By making sure it is clean.

So what does ‘clean’ mean, and how does this happen? There is a lot of work involved.

Where does mine water come from?
The mines have four sources of water:

  • Mine water: from underground and surface mining
  • Decant: collected from the surface of the tailings pond
  • Underdrainage: seepage from the Waste Rock Embankments is collected in drains and piped to the Water Treatment Plant.
  • Storm water: generated all around the site, treated depending on its quality

How is it treated?
Water on site is treated depending on its source and condition. Some may be cloudy or muddy and needs to spend time in a silt pond before being discharged. Other water may contain iron or manganese. These trace elements occur naturally in the rock and may be released during processing. This water is sent to the Water Treatment Plant.

The water treatment processes destroy any residual cyanide left over from the gold recovery process and remove any trace metals that were originally in the ore that was crushed to extract the gold and silver.

Each day enough water to fill about 10 Olympic sized swimming pools can be treated at the Water Treatment Plant. The plant can treat up to 15,000 cubic metres of non-cyanided water each day and 5,000 cubic metres of cyanided water.

What is the quality of the water discharged from site?
The quality of water discharged from the mine site must be high enough to protect fish, ‘water bugs’ and plants in the river, as well as other downstream users. The conditions of the water consent are based on United States Environmental Protection Agency guidelines. These guidelines are used, as no New Zealand Standard currently exists.

Recently the addition of an $11m Reverse Osmosis Plant has meant that the site can discharge more water as the new process treats water to a very high level of purity, far in excess of that required by regulators. This process forces water containing impurities through a membrane. The membrane filters at the molecular level, holding back anything bigger than a water molecule. This ‘nano-filtration technology’ is also used in the wine and dairy industries. A kidney dialysis machine uses the same technology to purify blood.

How is water quality monitored?
All water on site is monitored, both during treatment and before it is discharged. In addition, the river into which we discharge water and several other streams are also monitored. This monitoring includes a monitoring programme to check fish, ‘water bugs’, plants, algae and sediment. Independent monitoring is also carried out, and external checks are made on the company’s procedures and results. Much of the monitoring is required by consent conditions, but regular monitoring also provides data, which assists in the most efficient operation of the site.

Where does the water go?
Water is discharged into the Ohinemuri River at two approved discharge points. Consents allow up to 26,000 cubic metres a day or during low river flows up to 40% of the river’s total flow. That’s about the same as the amount of water in one Olympic size swimming pool. Flow meters record treated water discharge volumes. River flows are monitored using river level recorders. Linking the maximum allowable discharge to total river flow means that in summer we discharge less water as the river is running low, and more in winter when the river is in high flow. Moderating the mine’s water discharge volumes and ensuring they remain within the consent limits makes sure that we do not affect the natural seasonal variations of the river.

Water is also discharged from one of our tailings storage facilities directly into a tributary of the Ohinemuri River. This tailings facility was decommissioned in 2005 and the water quality is such that a direct discharge was approved by Environment Waikato (Waikato Regional Council) in 2007. The outflow is continuously monitored for water clarity, conductivity and acidity. A wide range of other tests is also used periodically to ensure that water quality remains suitable for discharge.

But what about groundwater?
So gold mines can produce clean surface water, but what about groundwater? Won’t a mine affect groundwater by polluting it or cause problems for locals if the mine is dewatered? Can problems with farm bores be related to the mine? Are district water restrictions during summer related in any way to mining operations?

Once again, let’s use Martha and Favona as examples. There’s a bit of geology involved here.

The Waihi basin is made up of a series of layers created by successive volcanic eruptions. The layers are often different types of rock, and can be capped with layers of clay or soil that have formed during extended periods between eruptions.

Groundwater is contained within these layers, after entering the system as rainfall. Different soils, clays and rock have different levels of permeability. Depending on the rock type, water can move quite quickly, or take hundreds of years to move through an area.

Groundwater doesn’t just occur in one layer or at one level. Often there are two or more separate groundwater systems and the different layers do not interact with each other. The technical name for this separation is an aquiclude. It is possible to dewater at depth and not affect shallow groundwater systems.

We access this shallow groundwater when we sink a farm bore or when it appears on the surface as a spring. Like most groundwater extracted from a volcanic area, Waihi’s water is slightly acidic and contains variable levels of iron and manganese from the host rock. These levels depend on the location and depth of extraction.

Most farm bore problems can be traced to factors such as pump wear, corrosion, iron oxides in pipe work or sediment build up. The Favona vein is hosted within deeper, relatively impermeable andesite. This lack of connection between the upper and lower systems means that the mine does not have a measurable affect on other groundwater users.

Hauraki District Council sources drinking water for Waihi from the Walmsley and Waitete streams. These water sources are charged by rainfall in the hills. The water restrictions the town faces in summer are because of the low flow of these streams. There is no link between mine operations and town drinking water.

What about ‘runoff’ or seepage from underground?
To understand water geochemistry, you need to know your rock. Rock is classified as PAF or NAF; potentially acid forming or non-acid forming. PAF material is built into the walls of the Waste Rock Embankments where it is encapsulated. If oxygen in the air cannot get to it, acid runoff is prevented. All water pumped from underground is treated. When the mine is eventually closed the pumps will be turned off and the mine it will be flooded. No air means no available oxygen, so no acid runoff.

In conclusion
There is no doubt that mining in the first half of last century polluted waterways. This is not the case today. Modern gold mining operations such as Martha and Favona do not result in polluted water. There are strict regulations regarding water quality and discharge rates. Non-compliance can result in the site being shut down. It’s easy to say ‘mining pollutes water’. We work very hard to make sure it doesn’t. It is in our interests to make sure we do not have a negative effect on the environment.

The science is never as interesting as the slogans, and it will always take longer to explain, but despite the opinions expressed in Letters to the Editor it is possible for gold mining and ‘the environment’ to coexist.