Case study: Zinc in the environment

We use chemicals every day, and they’re all around us. We know different chemicals come in different forms and that they will behave differently in different situations depending on things like temperature. But do we know what happens to these everyday chemicals when they migrate from the intended application out into the environment?

The Parliamentary Commissioner for the Environment report Knowing what’s out there: Regulating the environmental fate of chemicals sought to answer some of these questions and provide a robust analysis of how New Zealand regulates chemicals. The report has case studies of a selection of chemicals that include the life cycle of the substance, its use and disposal. The case studies were chosen as examples of the different ways the environmental fate of chemicals is managed by New Zealand’s regulatory system.

Four chemicals were selected for this purpose – including zinc.

What is zinc and how do we use it?

Zinc is a chemical element found in the Earth’s crust, including soils. Zinc is an essential trace element for growth and metabolic processes in living organisms.

Humans use a lot of zinc – it is the fourth most widely used metal after iron, aluminium and copper. In 2017, worldwide use of zinc was over 14 million metric tonnes! Its main use is in the steel industry where it is used (sometimes with other metals to form alloys) as a layer to prevent oxidation and rust. Zinc is also used in sunscreens, human and veterinary medications and health supplements, cosmetics, wood preservatives, paint pigments and anti-fouling paints, fertilisers, household products and as an agent in rubber vulcanisation, a process used by the tyre industry.

Environmental fate and exposure to non-target organisms

Despite its natural origin and beneficial properties, zinc can become toxic to organisms at certain doses. At high concentrations, zinc can also outcompete other essential elements (such as calcium and copper) in biological processes. This can result in plants and animals obtaining an altered intake of other nutrients, which can have different impacts. This includes changes in plant growth and photosynthesis, and cellular damage and reproduction problems in animals.

Zinc is a persistent element, this means it does not degrade or break down into smaller molecules called metabolites. It can, however, take different forms (free zinc ions, zinc hydroxide or zinc sulphate) depending on environmental conditions.

The form of zinc will determine its toxicity, with free zinc ions (Zn²⁺) being the most available and toxic form to organisms. Once released in the environment, zinc behaves in different ways depending on the characteristics of the environment:

• In water, the pH among other factors will determine if zinc is in a freely dissolved form or is attached to sediment. In acidic waters where the pH is low, zinc tends to dissolve and be in a free form, making it more available to aquatic organisms such as fish.
• In soils, zinc can be distributed in multiple ways – it can bind to soil particles or porewater (the water between soil particles). Soil particles with a pH of 5 or higher tend to bind with zinc, resulting in low mobility of zinc. However, when the pH is below 5.5, zinc may become more available to plants and microorganisms tends.

So how does zinc move from where it’s meant to be into our environment?

The main sources of zinc in urban areas are worn tyres, painted buildings and galvanised and coated roofs. When it rains enough to create run-off, traces of zinc from these sources get washed out and enter the environment through stormwater run-off. Most of the zinc found in run-off is in a dissolved form (zinc ions), which is the form most available to organisms. Further urban sources of zinc are treated effluents and sludges from wastewater treatment plants.

In rural areas, zinc-based pesticides and animal medicines are key sources of zinc entering soil and surface water. This is because zinc is incorporated into agricultural products such as drenches and sheep dip to treat facial eczema. It is also found in some multipurpose plant pesticides. When the organic compound of the pesticide degrades, the zinc – which does not degrade – gets released. This means it has the potential to accumulate in receiving environments such as soils, sediments and organisms.

So how much zinc is too much?

The toxicity of zinc to flora and fauna is well studied and understood. We also know a lot about how this toxicity varies, including how variable it is across organisms and different ecosystems and geographical regions. For example:

• In freshwater, water fleas are the most sensitive species to zinc among those tested, followed by green microalgae, fish (salmon and brown trout), crustaceans and aquatic snails.
• In the marine space, sea anemones have the highest sensitivity to zinc of those species tested, with reproduction rate impairment observed. Taonga fish species such as tarore (sole), kahawai and pātiki (flounder) have shown higher sensitivity when compared to their similar species in Australia and North America.

The toxicity of a chemical is all in the dose – scientists have different ways to measure toxicity. Some chemicals can cause toxicity at very low doses, so it is important to understand how low doses compare to one another. Parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt) – or mg/L, ug/L and ng/L – are the commonly used terms to describe very small amounts of substances. A lethal concentration or dose of zinc is 2.6 ppm for a kahawai and 2.7 ppm for a pātiki. While a lower dose (e.g. 2.0 ppm) might not kill a kahawai, it could still be severely affected. In contrast, a lethal dose for a sea anemone is 0.017 ppm (or 17 ppb).

Toxicity data from multiple species has helped scientists determine environmental limits for different ecosystems (freshwater and marine) via models. These are often referred to as environmental threshold values or guideline limit values. If the concentration of a chemical in an ecosystem is higher than the limit value, there is a chance it could be harming the organisms living in it.

What do we know about zinc toxicity in the environment in New Zealand?

Worldwide, contamination of soils with anthropogenic zinc has become evident in some agricultural sectors, such as dairy farming and horticulture.

A New Zealand study assessing the environmental risks associated with the use of zinc in pastureland suggests the accumulation of this metal in a large area of the Waikato region over the last 30 years. The study found that around 12% of soils assessed had zinc at levels that were potentially toxic to the functioning of microorganisms when compared with guideline limits.

Additionally, depending on the type of soil, zinc can migrate from farmland to freshwater and groundwater bodies.

The most significant concern for freshwater lakes is related to the ability for zinc to gradually build up in sediments. Over time, the zinc concentration may reach a level toxic to sediment-dwelling organisms such as macroinvertebrates. In the Waikato region three lakes have already recorded zinc levels above the interim sediment quality guideline.

Natural concentrations of zinc in the water of freshwater lakes and rivers are low and range between 0.04 and 1.6 ppb. Comparatively, concentrations of anthropogenic zinc in urban freshwater bodies have been found at much higher levels (5–200 ppb), with higher concentrations peaking during rain events. Stormwater has been identified as one of the main sources of zinc reaching the environment.

A final environmental concern is the strong pattern of co-occurrence between zinc and the development of antimicrobial resistance in microbes in different environments. This is because bacteria, when exposed to zinc, can activate resistance mechanisms that transport harmful compounds outside of the cell. In the process of removing zinc, other toxic substances, including antimicrobials, are removed. The influence of zinc on antimicrobial resistance has been identified in microbes at wastewater treatment plants, in manure and in soils.

Regulation and monitoring

In New Zealand, the use pattern of zinc-containing compounds dictates the type of controls applied and the agencies granting approval. Download the PDF for a summary of zinc regulation and monitoring.

The agencies regulating the use of zinc-based substances can:

• restrict application to ground-based methods and prohibit application into, onto or adjacent to waterbodies
• set a maximum application rate, maximum number of applications and maximum frequency of applications
• set label requirements for maximum application rate and methods of use
• determine disposal, classification, packaging and transport for the substance.

Information about the amount of zinc reaching receiving environments is patchy at best and often lacking.

Knowing what’s out there: Regulating the environmental fate of chemicals

The reporting of the import, manufacturing, sales and exports of zinc compounds in New Zealand is not systematic. Analysis conducted for the report highlighted the absence of mechanisms for regularly collecting and updating industrial emissions and release data at a national level.

Zinc in urban environments

There is limited knowledge of zinc loadings from non-point sources like roofing and flashings, wearing of vehicle parts such as tyres, use of agricultural chemicals or washed-off sunscreen.

A variety of models have been developed to estimate annual loads of zinc in urban areas. However, these do not often adequately account for concentrations of copper and zinc metals in streams, or for the concentrations from one-off events like storms carrying zinc through stormwater systems.