Introduction.
The term pH is the standard measure of acidity. The maintenance of a desired pH range has important implications for life processes, agriculture, industry and the environment. Examples of this include:

• for a person to remain healthy, the pH of blood must be maintained within a narrow range
• plant growth can be severely affected if soil pH is too high or too low, and the availability of plant nutrients and non-essential elements may be affected
• pipes and pumping equipment can be corroded in a short time if they carry water that is acidic in nature (i.e. low pH)
• plants and organisms can be adversely affected in rivers and lakes if the water pH is too high or too low.
  
  

The pH Scale.
The pH scale measures hydrogen ion concentration. It indicates the acidity or alkalinity of a solution. Pure water has a pH of 7, acid solutions have a pH less than 7, and alkaline solutions a pH greater than 7.

The following figure illustrates the pH scale by listing some common household substances:

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Fluids in living organisms are strongly buffered. Sea water is an example found in nature. In contrast, rainwater is poorly buffered.

Buffering in water may be increased by adding alkalinity. Substances that contain the bicarbonate ion, the carbonate ion, or the hydroxide ion will increase the buffering of water.

The process of buffering is an important part of acid drainage control. Adding limestone to potentially acid forming (PAF) rock means that rainwater that contacts the rock has a higher degree of buffering. This makes it more resistant to pH changes.

Salts.
When aqueous (water) solutions of an acid and a base are combined, a neutralisation reaction occurs. This reaction is characteristically very rapid and generally produces water and a salt. A salt is made up of a positively charged ion called a cation (other than H+) and a negatively charged ion called an anion (other than OH-). The negative ion is from the original acid and the positive ion is from the original base

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In the mining industry, lime is often added to either neutralise or delay the onset of acidity. The neutralisation of acidity with lime may result in the development of a salt called gypsum (CaSO₄). Gypsum can cause effects such as the scaling of pipelines, and these effects must be monitored and managed, and maintenance carried out where necessary.

The Martha Mine Site
At the Martha Mine, maintenance of a desired pH range is an important part of management in many areas.

Ore Processing.
Lime is added to the ore prior to the addition of cyanide. This ensures that the cyanide remains in solution during the process of leaching of the precious metals. Similarly sulphuric acid is used (with superheated water) to strip the gold and silver from the carbon. At various stages throughout the processing of the ore, the solution is sampled and pH measured. For more information see Processing Plant.

Water Treatment.
Lime is added to water at the Water Treatment Plant to assist in the water treatment process. Many metals are insoluble at high pH and by adding lime, the metals flocculate and precipitate out of the water. The measurement of the pH of the water, at several points within the Water Treatment Plant, is an important means of ensuring that the plant is performing well. For more information see Water Treatment Plant

Soil Acidity.
In an excessively acid soil, one or more of the following effects may be observed:

• some plants simply do not grow well
• the activities of many of the soil organisms are reduced
• elements such as aluminium and manganese become so soluble they are toxic to plants
• essential nutrients such as phosphorus and molybdenum may become insoluble and unavailable to plants.

There is no such thing as a 'correct pH' because different plants have different pH levels at which they grow best.

Soil pH is measured in New Zealand by mixing 10g of soil with 25ml of distilled water.

Virtually all New Zealand soils have pH values between 4 and 8. The vast majority have pH values between 5 and 6.5, and are slightly acid.

In New Zealand agriculture, lime is generally added to raise the pH. The amount of lime that needs to be added depends upon the buffering capacity of the soil. A soil will be more buffered if it has fine texture (eg a clay loam) and/or high organic matter content, and will require more lime to raise the pH.

At the Martha Mine, the maintenance of the soil pH within a desirable range is an important part of rehabilitation and ongoing management of the land. Lime is added as necessary when disturbed areas are rehabilitated, and in subsequent years as recommended following routine soil testing. This practice is no different to normal New Zealand agricultural pasture management.

Acid Drainage Control.
As described in the section A Brief Geological History of the Martha Mine, a proportion of the rock in the open pit contains sulphide minerals (predominantly pyrite, FeS₂). When the pyrite in these rock types is exposed to atmospheric oxygen, it can oxidise according to the following overall reaction:

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Historically there was limited recognition of these problems in the mining industry and there are a number of particularly older mines in the world where acid drainage has resulted in major impacts on the environment. In the last 20 years a much better understanding of the acid drainage problem has evolved. Strategies for preventing acid drainage have been developed and are now being implemented in modern mining operations such as at Martha Mine.

Acid drainage control is a primary objective in the design, construction and operation of the tailings storage facilities. It is important to ensure that there are no significant adverse effects on adjacent rivers and streams.

While the waste rock at Martha comprises predominantly andesite, other materials including volcanic breccia, alluvium, ash and ignimbrite are also present. The andesite and volcanic breccia rock types exhibit various degrees of weathering and are referred to as either oxidised, unoxidised or mixed.

Test work confirms that all oxidised rock, (andesite and ignimbrite) alluvium and ashes are non acid forming (NAF). In contrast, the mixed and unoxidised andesite and volcanic breccia contain variable amounts of sulphide sulphur, predominantly pyrite, with an expected range of 0.5 to 3% sulphur. Much of this rock is potentially acid forming (PAF).

Acid formation is a natural process that occurs over much of the Coromandel Peninsula. This can be seen on road cuttings in the area as iron coloured stainings on the surface of excavations and in open drainage channels. Excavation can accelerate the process because it exposes rock to atmospheric oxygen. Groundwater, rivers and streams can potentially be adversely affected by acid drainage, and the associated increase in the solubility and leaching of metals. There are examples around the world where this has happened, and a local example exists at the old Tui mine near Te Aroha, about 60 kilometres southeast of Waihi..

PAF rock must be identified and managed in such a way as to avoid significant environmental effects, both in the short term and in the long term. Quantities of PAF and NAF rock need to be determined, and considered in mine planning. This is a major consideration at the Martha Mine, and is discussed further in Waste Disposal and Rehabilitation.

At the Martha Mine, the concentrations of many of the environmentally important parameters (including heavy metals such as lead and mercury), are far lower than at many other mine sites, and do not pose a risk to human health or the environment. Details of the specific short and long term controls of acid drainage follow below.

Short term control.
Crushed limestone is added to all PAF rock exposed during construction. This slows the rate of sulphide oxidation and prevents the development of acid conditions. To ensure that sufficient limestone has been added, slurry testing of the PAF rock is carried out.

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Long term control.
In the long term, acid drainage control is achieved by covering the waste rock with a cap that maintains a high degree of saturation, and effectively excludes oxygen. This is described in more detail in the Waste Disposal section. Similarly, acid drainage control in the tailings is described in the Rehabilitation section, as is the expected water quality of the rehabilitated mine lake and tailings ponds.

Maintenance.
Acid waters have the potential to affect pump and pipeline selection and maintenance around the site. The formation of a salt (gypsum) has the potential to affect pump and pipeline performance through scaling, and this process requires management.

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