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2 Characteristics of Sheep-dip Contamination

2.1 Dipping practices

In New Zealand, historically most livestock farms had a sheep dip because sheep dipping was required by law. The current estimate is that there are around 50,000 sheep-dip sites across the country. This total is derived from stocking numbers and the number of sheep farm properties, and includes on-farm permanent structures, portable units, communal dip locations and spraying units. In addition, slab areas for applying powder and footbaths (containing, for example, arsenic, copper and zinc) to prevent footrot were often used on a sheep farm.

Sheep are affected by a number of external parasites that inhabit the fleece and feed on the wool, the skin and, in some cases, the flesh of the living animal itself. Sheep have no natural protection against these pests, which can cause considerable suffering, animal mortality and economic loss. Because sheep are protected by an oily fleece the treatment of pests is made more difficult. As a result, the chemicals used have to be potent and the dipping process thorough, in the past often involving complete immersion.

Initially, sheep in New Zealand were dipped to control scab, a small parasite living just under the skin that caused sheep to lose some of their condition. As a result of intensive sheep-dipping efforts, scab was eradicated in the late 19th century, with New Zealand declared free of the disease in 1894. The focus of sheep dipping then moved to controlling lice and keds, which are other parasites infecting sheep.

Figure 2: Dipping in Levin on 2 March 1906

Source: Adkin Collection, National Library.

In the early to mid-1800s the idea of using plunge-type dips was introduced as a dipping technique. These so-called "pot dips" were often shaped as a round bath, sometimes with the addition of an island in the centre. Pot dips were used with a variety of chemicals and were particularly popular for smaller flocks. Swim-through dips were also used from the outset of dipping, often on large stations. These dips always had a large amount of residual dip solution left over. Pot dips and swim-through dips were dug into the ground and lined with timber or concrete, creating a robust, semi-permanent structure.

Please see Appendix 10 for photographs on typical sheep-dip structures.

The invention of the power-spray machine, which made its way to New Zealand by the mid-1940s, was a breakthrough for farmers, because this allowed a new and much faster method of dipping. Sheep did not have to be individually handled, as with previous dips. Instead, the sheep could be left in larger groups in the spray shower and saturation could still be easily achieved. Spray showers were constructed in above-ground structures, which included concrete enclosures, or steel piping and corrugated iron, and an underground sump for recycling dipping liquid. Contamination at these dip sites may not have been as severe compared to plunge or swim-through dips because very little dip was wasted and there was hardly any left-over dip to dispose of at the end.

Tip spraying was used for a relatively short time from the introduction of dieldrin and aldrin in 1955 to their withdrawal in 1961. Tip spraying worked by applying a high concentration of dip at high pressure onto the sheep, usually incorporating a mobile covered race with spray nozzles on either side and along the bottom to ensure that sheep were covered in dip as they ran through the race. The chemical would dissolve in the wool grease and then move towards the skin of the sheep where the parasites were located. Dusting also required the dissolving ability of the organochlorines and worked in a very similar way to tip spraying, except that the nozzles emitted dust and a blower carried the dust onto the sheep.

Jetting was, and is still, being used as additional protection against flystrike, and involves spraying the sheep through a handheld device with a highly concentrated insecticide. Recovery of "off-sheep" spent chemical is improbable. Since the 1980s the pour-on method has become popular to control flies, keds and lice. This method uses an applicator to place insecticide directly along the back of the sheep. The chemical then diffuses through the wool grease of the sheep. Both jetting and the pour-on method use chemicals such as synthetic pyrethroids or insect growth regulators that are of low toxicity for people and sheep, and hence are of lesser concern for the purpose of this guideline. They may, however, pose a risk to aquatic species if they get into waterways.

Antifungal footbaths were also used to prevent footrot, often in a separate location to the sheep-dip sites.

Finally, it should be noted that animal dips are not necessarily confined to sheep: a small number of cattle dips are confirmed in New Zealand.

2.2 Likely pattern of contamination

Due to the persistence of some of the main chemicals that were used historically in sheep dips and footbaths, these contaminants are likely to remain in the soil for years after dipping operations ceased. Some contaminants are highly likely to be present at concentrations that exceed the recommended human health or environmental criteria (see section 5 and Appendix 6). This is usually the case for arsenic, often the case for dieldrin, and occasionally the case for lindane. It is common to find the highest levels of arsenic just below the surface layer due to its slow migration over time. However, generally speaking, on disused sheep-dip sites contamination does decrease with depth.

The migration of sheep-dip or footbath contaminants into aquatic environments may potentially cause adverse affects to humans, animals, aquatic animals and plants, and the wider ecosystem. On islands and properties located close to the foreshore, the dip contents were often discharged directly to the ocean. Humans living close to old sheep-dip sites may also inadvertently come into contact with these chemicals. The scale of historical use of individual dip sites may give an indication of the extent of contamination. For example, compare a communal dip processing 60,000 sheep per week from 1917 to about 1996, with a possibility of as many as 38,000,000 dip events, with a 400-sheep property used from about 1956 to 1960.

When investigating the risks from disused sheep dips, the focus is mainly on the actual site; that is, the location of the dip bath or structure, not the wider property. However, there are other potential areas of concern in the immediate surroundings. For example, the dip solution was often emptied into a burial pit close by, discharged by a pipe over a bank, or pumped out of the bath onto the adjoining yards and allowed to soak into the ground. The sludge from the bottom of the sump, potentially high in accumulated arsenic and organochlorines, was often shovelled out onto the ground alongside the dip (creating a so-called scooping mound).

Therefore, areas of concern include:

  • beneath the dip bath and within the bath
  • the area where the sheep-dip liquid was disposed of
  • around the bath where dip chemicals may have splashed
  • next to the dip bath where the sludge was disposed of
  • the area where the sheep exited the sheep dip (the draining pen)
  • the area where the sheep-dip chemicals were stored.

Depending on local topography and drainage, contamination may have spread some distance from the dip site itself. For example, surface-water run-off and/or groundwater movement may affect areas down-gradient of the former sheep-dip site(see Figure 3).

Figure 3: Sketch of sheep-dip site with associated structures and buildings

The sketch shows concrete dip structures such as the dip itself and the draining pen, the shearing shed as an associated building and fenced areas enclosing the dip area, sheep yards and another draining pen.

Although the focus of this guideline is on farmland, it needs to be stressed that dip sites may also have been located in stockyards, saleyards, railway yards and on other public land. See Appendix 7 for some selected case studies of dip sites on public and private land.

2.3 Chemicals used for sheep dipping

Within New Zealand, a wide range of chemicals has been used historically to control sheep parasites (see Table 1). This guideline focuses primarily on the environmentally persistent chemicals that represent the greatest ongoing risk to human health, livestock and the environment: arsenic, and the organochlorines aldrin, dieldrin, DDT and lindane.

However, for the sake of completeness, brief reference is also made to derris (Rotenone), copper, organophosphate pesticides, synthetic pyrethroid insecticides and insect growth regulators. Appendix 5 contains more detail on the historical application and toxic effects of these chemicals.

Other chemicals such as nicotine, zinc and phenols were used to a much lesser extent in New Zealand.

Table 1: Summary of chemicals used in New Zealand for treating sheep ectoparasites

Chemical [Persistent chemicals of principal concern are highlighted (in bold).]

Period of usage [Years for organochlorines are based on Ministry for the Environment, 1998]

Arsenic

1840s-1980

Nicotine

1840s-mid-1900s

Carbolic acid and potash

1880s

Derris

1910-~1952

Lime sulphur

1849-1891

Zinc

1950s−present

Copper

1950s−present

Organochlorines:

 

  • DDT
1945-1961
  • lindane
1947-1961
  • dieldrin
1955-1961
  • aldrin
1955-1961

Organophosphates

1960s-present

Synthetic pyrethroids

1970s-present

Insect growth regulators

Present

Of the chemicals listed in Table 1, the more recent (organophosphates, synthetic pyrethroids and insect growth regulators) usually readily break down, and so are not marked as persistent chemicals of principal concern. There are exceptions to this in cases where a high level of co-contamination (eg, from copper or arsenic) inhibits microbes that can be involved in chemical degradation.

In general, if there is a reasonable site history, which shows that the dip was used before 1961, it is recommended to test for arsenic and organochlorines (which include dieldrin, lindane, DDT and its primary degradation products DDE and DDD − often referred to as ∑DDT). When investigating a site with a former footrot bath, testing for arsenic, copper and zinc is recommended.

If the dip area is likely to have been used after 1960, it may also be advisable to test for residues of organophosphates and their breakdown intermediates (some may be more harmful to humans than the original compound). For dip sites in use after 1970, an additional test for synthetic pyrethroids may be advisable. However, these analytical suites can be quite expensive, and given the fairly low likelihood of detecting members of either class of compounds at significant levels after more than a few months, these tests might be justifiably carried out on only one or two representative samples collected from the area most likely to be contaminated. Further testing for organophosphates and synthetic pyrethroids would then proceed only if significant levels were detected in the test samples.

Appendix 7 presents a number of case studies that illustrate the behaviour of the chemicals in the environment and the concentrations that may be found in soil and other media.

2.4 Exposure pathways and risks

Exposure pathways

Disused contaminated sheep-dip sites present the following main potential exposure pathways.

  • Pathways for human exposure: the primary pathway consists of ingestion of small amounts of soil or dust. Next most significant is the consumption of home produce if it is grown in contaminated soil, and consumption of drinking water if it has been contaminated. Whānau, hapū, iwi and others who regularly gather and consume aquatic and wild foods may be at risk from contamination of waterways, including sediments. Contamination of farm bore-water supplies near old sheep dips has been documented on several occasions (McBride et al 1998; Hadfield and Smith 2000; Environment Canterbury 2003; McBride 2004). Other minor exposure pathways include dust inhalation and absorption through the skin.
  • Pathways for livestock exposure: the main pathway is ingestion of contaminated soil during grazing (eg, an adult cow may ingest up to 675 grams of soil each day, although variations occur depending on local conditions such as grazing height or the amount of dirt on the foliage), and the consumption of contaminated water.
  • Pathways for exposure of other organisms: the main pathway here is the contamination of nearby stream waters and sediments. Where contaminant levels exceed guideline values (for either water or sediment), there may be some adverse effects on the abundance and nature of freshwater and marine invertebrates that form the base of aquatic food webs.

Figure 4 illustrates the main potential exposure pathways for contaminants present in old sheep-dip sites, and shows how some of the pathways described above are interrelated. The specific pathways and their importance depend on the actual land use at an affected site and should be assessed on a site-specific basis.

Note that disused sheep dips only present a risk to human health and well-being if a complete pathway exists for the uptake of contaminants by the human body.

Figure 4: Exposure pathways from disused sheep-dip sites

Exposure pathways from contaminated soil:

  • Direct ingestion of soil by children
  • Uptake by grazing animals
  • Uptake into and deposition onto food crops

Exposure pathways from contaminated groundwater:

  • Uptake into food crops
  • Contamination of drinking water supplies
  • Irrigation onto food crops

Exposure pathways from contaminated surface water:

  • Impact on aquatic species
  • Contamination of drinking water supplies
  • Irrigation onto food crops

Secondary exposure pathways for people:

  • Consumption of drinking water, animal product and food crops

Movement of contaminants:

  • Erosion into surface water, leaching into groundwater, and re-deposition onto soil.

Health risks

One of the main concerns with contaminated sheep-dip sites is that the chemicals used may pose a risk to people. Risks can be divided into short-term (acute or immediate) risks and long-term, low-level (chronic) risks. The following section is based on health and environmental risk assessments undertaken on a sample range of dip sites in New Zealand (Stage 6b Preliminary Report by Kim 2003). The main acute hazard found was the high arsenic levels at these sites. A typical concentration was 1000−3000 mg/kg, and at some sites the sampled concentrations reached 11,000 mg/kg, compared with the natural soil arsenic content of about 5 mg/kg for that locality. Immediate risks were identified for children and young livestock.

Human health risks

Young children may be at immediate risk from exposure to contaminants in soil when playing in and around an old sheep-dip site, for four main reasons.

  • Concentrations of arsenic at old sheep-dip sites can occasionally exceed 11,000 mg/kg (background concentrations of arsenic in New Zealand soils typically range from 2 to 30 mg/kg).
  • Children ingest more soil and dust than adults due to frequent hand-to-mouth activity.
  • Some children exhibit pica [The word "pica" comes from the Latin word for magpie, a bird known for its large and indiscriminate appetite. Around 25 to 30 percent of children have an eating disorder called pica, which is characterised by persistent and compulsive cravings to eat non-food items.] behaviour, which involves the routine ingestion of significant quantities of non-food items.
  • Children have a lower body weight than adults, meaning that an adult can tolerate ingesting more arsenic in total before being affected.

Disused sheep dips may also pose a physical risk for children if not fenced off. Accidental drowning from falling into dip vats containing liquid have been recorded in New Zealand. For these reasons, the risks posed by an old sheep-dip site to children need to be managed.

When a rural property containing an unrecognised (or unmanaged) sheep dip is subdivided, the relative risk of significant exposure to a small number of children increases even more. This is because one of the house lots will necessarily contain the former sheep dip, and adjacent lots may also contain some soil contamination. For a child living on such a property, the potential frequency and duration of exposure to the contamination is greater than on a farm, where a child is confined to a smaller wandering area. The risk of acute poisoning therefore increases substantially. For this reason, it is particularly important that territorial authorities properly manage old sheep-dip sites on properties that are being subdivided for residential and rural−residential use.

Chronic risk refers to long-term risks (eg, over 30 years) from lower exposure. As noted above, exposure to contaminants from disused sheep-dip sites can occur by several intake routes, which in order of importance are:

  • ingestion of soil
  • ingestion of home produce from around the old dip
  • consumption of contaminated groundwater, surface water, or aquatic and wild foods
  • inhalation (of either soil and dust, or volatilised contaminants)
  • absorption through the skin (dermal absorption).

Usually, inhalation and dermal absorption are insignificant pathways compared with direct soil ingestion. An average child ingests approximately 100 mg of soil and dust per day through normal hand-to-mouth activity. Children with pica condition are particularly vulnerable because they are predisposed to eat soil, with a typical figure being approximately 5000 mg of soil per day. However, it should be noted that due to the potential for bioaccumulation, consuming eggs from chickens raised on-site may pose a greater risk to a child than soil ingestion.

Home-grown produce grown on contaminated soil can be contaminated through direct uptake into the plant from the soil, as well as adhesion of soil particles to the plant. The uptake depends on many factors such as soil pH, the extent of binding of the chemical to soil, or whether the chemical is similar to a nutrient for which the plant has an active transport mechanism. For example, zucchinis, pumpkins and carrots can accumulate organochlorines. In some cases, phytotoxicity may occur with high levels of arsenic contamination. In terms of human exposure at a specific site, ingestion of home vegetables can be a significant intake route. The extent of actual exposure, however, depends on someone eating vegetables or fruit from the contaminated area.

Guideline values for residential soils are designed to provide a good level of protection to ensure that long-term risks are tolerably low. A number of conservative assumptions (eg, taking into account all potential exposure pathways) and a typical exposure period of at least 30 years mean that these guideline values are also sufficient to protect against short-term risk.

Leaching of contaminants into the groundwater can become relevant to residents if their water-supply bores are located on the same or an adjacent property and the water is contaminated to levels above drinking-water standards.

Table 2 gives an overview of the potential human health risks from disused sheep-dip sites in relative terms (high, medium, low). Potential risks to exposed livestock, soil biota, terrestrial plants and aquatic fauna and flora are more difficult to determine on a general basis and have been excluded from the table.

Table 2: Overview of the potential risks to people from disused sheep-dip sites

Land-user group (likely land-use categories)

Main exposure route

Risk

Children
(rural, residential)

Eating contaminated soil; touching and breathing in soil and dust when playing in or around old sheep dips

High
(immediate risk)

Life-style block occupants
(rural, rural−residential)

Eating vegetables grown on a contaminated area; consuming animal products (eg, meat, eggs and milk) from animals kept on a contaminated area; drinking contaminated bore water

High to medium
(longer-term risk)

Residential occupants
(residential)

Touching and breathing in contaminated soil or dust when gardening; eating vegetables grown on a contaminated area (depending on the residential character; eg, a significant number of home gardens)

Medium
(longer-term)

Local iwi and hapū
(rural, parkland, residential)

Eating mahinga kai (aquatic and wild food; eg, freshwater mussels, crayfish, eels, watercress, land-based ferns)

Medium to low
(longer-term)

Farmers and workers
(rural, rural−residential)

Touching and breathing in contaminated soil or dust when working on the farm

Medium to low
(longer-term)

Neighbours of subdivision development (residential)

Breathing in wind-blown contaminated soil particles and dust from site redevelopment for housing

Low

Livestock health risks

Young stock are susceptible to acute poisoning from ingestion of high-to-moderate levels of environmental contaminants. In 1993, two heifers died in the Waikato region from acute arsenic poisoning as a result of ingesting arsenic-contaminated soil within an old sheep dip, and others were chronically poisoned. [MAF Ruakura Animal Health Laboratory Report(Case no. 9335861, dated 14/10/93), a Post Mortem Report to Northern Waikato Veterinary Services. Note the comment in the report: "Arsenic levels in liver greater than 4 mg/kg are considered significant − animal had 15 mg/kg".] In the Kaikoura area, stock deaths have been reported from arsenic poisoning due to grazing near old sheep dips or footbaths (Environment Canterbury, 2003). The probability of livestock becoming poisoned depends mainly on whether:

  • the soil associated with the old dip is high in arsenic (in particular), and the animal ingests a significant fraction of its daily soil from the area immediately around the old sheep-dip site
  • the bore water for livestock is contaminated, or contaminated groundwater flows into surface water used by stock, both from on the property with the dip site and from off the property.

Ecological concerns

Both arsenic and DDT bind strongly to soil, and so leaching into the groundwater is expected to be limited at old disused sheep-dip sites. In some places, however, contaminants in the soil are still progressively increasing in the groundwater after 40 years (see Appendix 7). This demonstrates that gradual leaching through the soil profile depends on the soil type at the site and the leachability of the contaminant, which means this pathway needs to be assessed on a site-specific basis. There may also be instances where surface activities − including remediation − enhance contaminant leaching. For example, arsenic can be mobilised by the addition of phosphate to soil, and organochlorines can be mobilised by organic colloids (eg, if an organic-rich soil amendment is added to the topsoil). It is fairly rare, however, for this low-level discharge to have a significant adverse effect on the wider environment.

Where significant discharge of contaminants to nearby freshwaters or marine water occurs, it poses a risk to aquatic biota. Usually contaminants are adsorbed to the sediment and gradually accumulate as the discharge continues. Surface run-off can also transport contaminated sediment to water in the vicinity of the site, resulting in aquatic flora and fauna being exposed to contaminated sediment.

Terrestrial plants in soils with high arsenic content may show inhibition of growth, photosynthesis and reproduction. Phytotoxic responses typically occur at lower concentrations than toxic effects on soil organisms. Organochlorine insecticides act on the central nervous system of animals and are (not surprisingly) acutely toxic to insects, while higher concentrations are required for acute poisoning of other invertebrates and vertebrates. Environmental concerns about organochlorine insecticide residues arise from their accumulation through the food chain.

Summary of most common concerns for local authorities

In summary, the most common concerns for local authorities relating to the risks of former sheep dips are as follows.

Territorial authorities: ensuring land is suitable for its intended purpose at the time of subdivision or new land-use activities. This essentially means making sure the site has been properly investigated and, if necessary, remediated as part of resource consent approval. Residual concentrations of contaminants at such a site should be equal to, or below, recommended guideline values for the proposed land use.

Regional councils: ensuring that all risks associated with sheep-dip sites are appropriately managed, including discharges to the environment. These may include discharges to groundwater or surface water, or discharges to air or soil associated with the removal or contouring of potentially contaminated soil during redevelopment.

Medical officers of health and health protection officers at district health boards: these usually work closely with local government on contamination issues that cause a nuisance or need to be notified to protect the public. Poisoning due to chemical contamination of the environment is required to be notified on reasonable suspicion (Health Act, Schedule 2).