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7 Best Practice Tips for Common Sheep-dip Scenarios

Three scenarios that may be commonly encountered by local authorities are illustrated below. For each scenario, recommendations are given for the appropriate site investigation, sampling and remediation. However, note that not every former sheep-dip site will fall within these three scenarios, and each site must be assessed on an individual basis. Indications of costs are given as a rough guide. The collection of case studies in Appendix 7 illustrates different dip situations and the specific findings when the sites were assessed. Appendix 10 shows photographs of sheep-dip structures.

The scenarios are as follows.

  • Scenario 1: anecdotal information suggests that a sheep-dip site was previously located on council-owned parkland, but the exact location is unknown.
  • Scenario 2: the location of the disused sheep-dip site is known (ie, associated structures are in place or the sheep-dip location is known) and the site has continued to be used as parkland.
  • Scenario 3: the location of the disused sheep-dip site is known (ie, associated structures are in place) and the site is to be developed for residential use.

7.1 Scenario 1

The council owns parkland that can be accessed by the public. The land was previously owned by a sheep farmer and was acquired by the council in 1975. Information has been provided that a sheep dip was previously located on the land. The location of the sheep-dip site is not known.

Best practice tips

There are benefits in taking immediate action to gather the existing information about the former sheep-dip site (eg, anecdotal information). The council has a primary interest in protecting public health and wants to make sure there are no public health risks arising from a potential sheep-dip site. Also, if the land is later subdivided or there is a change in land use, the local authorities will benefit from having access to recorded information about the site. It also ensures that councils can supply the best information available to future purchasers of the land.

The council contacts previous landowners to ask when they owned the property and what the land use was at the time.

Best practice tips

If the site has been a sheep farm, the council needs to find out if a sheep dip was used (or more than one), and if so, during which period it was operated. Other important questions in this context are:

  • what type of sheep dip was operated?
  • where was the sheep dip located?
  • where were the chemicals stored?
  • where did the discharges go?

The council also accesses old aerial photographs in its database to identify structures associated with sheep dips. Once the potential locations of sheep dips and associated structures are identified, an on-site visit verifies sheep-dip structures above and in the ground.

Best practice tips

An initial visit to the site to see if sheep-dip structures are still present may save some of the research steps above, unless multiple dip sites are suspected.

Another way to locate the old sheep dip is a 'walk over' survey, which would take about a day for an area of 50 x 100 metres. The following surveys are available.

  • A survey with a ground-penetrating radar over the suspected area can identify buried structures, concrete slabs, etc.
  • An electro-magnetic survey over the suspected area looks for areas with significantly high metal content at some depth. This is useful when the site is covered and high arsenic concentrations are suspected, but is no use for organochlorines. Neither can it be used if metal junk is buried, or if the site is covered with reinforced concrete.
  • An X-ray fluorescence survey can be used if contaminants are expected close to or mixed into the surface. Again, it is no use for organochlorines.

Other sources of information the council could use to confirm the location of the historical sheep-dip site are stock and station agents, knowledgeable neighbours or local residents. However, the council should be cautious about potential erroneous or malicious information, and should try to verify any claims made by third parties.

If no anecdotal or other information on the location of the dip site(s) can be found, the council could still consider, as a precautionary action, surface sampling of areas where the public (children in particular) may get in contact with soil or dust. Composite samples (eg, along path-sections or playfields) can help identify whether risks are present, even if the location of the dip is unknown. Given the size of a regular dip site, sampling every 10 m would give a fair chance of including at least one contaminated sample. Skilled park or council staff may do the sampling of surface soils, so the only costs would be the analysis. The laboratory will be able to help with the decision on the number of sub-samples per composite, based on a pre-analysis of a few composites to determine background levels.

If the sampler marks a waypoint on a geographical positioning system (GPS) while going through the park taking samples, and enters the sample number in the GPS at every sampling point, even mapping the results will be very straightforward. The number of each waypoint can later be changed for the analysis result and printed on an overlay of an aerial photograph or topographical map without the need for sophisticated mapping skills.

At this stage, it needs to be emphasised that in some cases, although an area is identified as a previous sheep-dip site location, this does not necessarily mean that the site is contaminated (ie, levels of contaminants above relevant criteria for protection of human health or the environment).

See section 4 for more detail on the identification of former sheep-dip sites.

7.2 Scenario 2

In this scenario, the location of the disused sheep dip is known and the council-owned land is being used as a public park. The council has established that the sheep dip operated between 1940 and 1955.

The council's main concern here is that the site may present a risk to the wider public visiting the park. Before initiating any site investigations, the council gets some quotes for the costs to dig and dump all the contaminated soil, based on estimated volumes. The council is considering hiring an excavator to look at soil profiles and to sample the soil at the same time. Then the council compares the quotes with the approximate costs for developing a conceptual model (an excavator can often be hired for three hours for the cost of one hour of a senior risk consultant's time, and the excavator may be owned and operated by the council) and possible follow-on management measures. This gives the council a rough indication of whether the "dig and dump" option would be more economical than developing multiple reports to assess other options. In this case, the transport and landfilling costs are high because the landfill is a good distance away and the potentially contaminated area is large.

Therefore, the site investigation focuses on:

  • developing a conceptual model that identifies contamination sources and potential exposure pathways
  • undertaking site investigations to delineate the extent of the contamination
  • assessing whether groundwater is at risk, and sampling if risk is identified
  • assessing the risk posed by the site, based on the conceptual model and the sampling results.

The council engages the help of a consultant and develops a conceptual model for this particular site based on the information already gathered at an earlier stage. The model includes the old sheep dip and associated structures as the contaminant sources, the receptors, and the potential exposure pathways (see Figure 4). There are no private or public bores in the 200 m radius of the site and no crops are grown on the land. However, contaminants could potentially have run off into or otherwise entered a little stream nearby and accumulated in the sediments. Direct contact of people with the soil is considered to be the most critical pathway in this scenario, with the most sensitive receptor being children.

Following this evaluation, the site investigation is planned to gather information on the level of contamination at the site and the level of risk the site presents to human health and the environment. It is established that the water table is more than 3 m below the contamination zone and the soil exhibits low permeability and no cracks, resulting in a good barrier between the contamination and the groundwater. Together with the fact that the sheep dip was only operating for 15 years, the council expects that no substantial leaching into groundwater has taken place at this site.

The soil, water and sediment are sampled and submitted for analyses for arsenic and organochlorine pesticides. The results of the soil analysis are compared with Table 4 for soil guideline values under the current land use, which in this case is parkland. The surface water and sediment samples from the stream are then compared with Table 6 to establish any risk to the environment.

The sediment and water samples are all below the soil guideline values. However, the concentration of dieldrin/aldrin in a number of soil samples is above 23 mg/kg, which is the selected soil guideline value for the protection of people for this land use. This means the council has to take measures to mitigate the risk.

Viable management options under this scenario
  1. The contaminants are left buried in the ground. A geotextile (this could be just a wide-woven orange netting, such as old safety barrier netting and does not need to be very costly) is placed over the contaminated site. Then the site is covered with clean soil in the form of a mound. The soil cap should have a minimum thickness of 70-100 cm. The council can then replant the hump and write a management plan that ensures the site is not disturbed in the future. This option is preferable if the park is frequented by children. The cost will be similar to the cost of replanting an area after laying pipes or cables, with the addition of importing some clean soil. The project could be planned when soil elsewhere has to be disposed of. The use of clean surplus soil from a new subdivision could benefit both council and developer.
  2. The area is fenced off from people or larger livestock and the spot is marked on the LIM. A permanent warning sign is erected. Consideration could be given to planting a dense, spiny scrub cover or a rose variety not requiring too much maintenance over the whole site to discourage playing in the enclosed area. This option would be less costly than option 1, and the costs are mainly incurred by the internal activity of the parks department. Since the location of the dip site is known, very limited outside assistance would be needed.
  3. When adverse off-site effects are expected, or the on-site risks are difficult to ascertain (eg, children playing and digging in the park), the council may choose to dig the contaminated soil out to go to landfill after all. A resource consent is likely to be required to do that. An appropriate site validation programme must be implemented following remediation of the dip site. The potential costs of liability may be another reason for the council as owner of the land to choose this option.

7.3 Scenario 3

A property that includes a known former sheep-dip site is to be developed for residential use. Consequently, an approach must be taken to ensure that human health is protected to a level consistent with residential land use.

Best practice tips

An investigation report may be needed as part of a sale and purchase agreement or an application for resource consent. The sampling procedure adopted should follow best practice, and the results must confirm that the site does not represent a risk to human health, a drinking-water source or the environment. If an unacceptable risk is identified some form of risk management needs to be adopted for the site that adequately protects people and the environment from contaminant exposure.

The assumptions under this scenario are as follows:

  • the dip consisted of a concrete plunge bath (approximate depth 2 m)
  • the dip has been backfilled with either surrounding soils or other materials
  • the water table is 4 m below the surface, 2 m below the sheep-dip base
  • the dip is located up-gradient of a waterway (small stream)
  • arsenic and organochlorine chemicals were used in the dip.

Because the location of the old sheep dip is known, the site investigation focuses on:

  • delineating the three-dimensional extent of contaminated soil present
  • identifying any migration routes
  • assessing groundwater quality below hot spots, below depressions and at places where groundwater is shallow (within or close to the contaminated soil layer).

The sampling programme takes into account that the contamination source is at depth (ie, at the base of the former plunge bath), as well as around the splash zones and the drainage platform. Therefore, soil samples are collected from any backfill material, from depths at and beneath the former plunge pool base, and in the underlying natural material. Use of a hand auger or drilling rig allows these soil samples to be collected at the required depth.

Best practice tip

In relation to residential subdivision, the important human exposure pathways are ingestion of soil and dust, and consumption of home-grown produce. These pathways relate to contaminants in soils at or near the ground surface. Systematic sampling is recommended to avoid sampling bias and to provide site coverage.

For this residential scenario, a 7.5 m sampling and validation grid is placed across the area of concern (ie, the former sheep-dip site, including the draining platform and the discharge zone and any areas of chemical storage). This sampling pattern allows samples to be collected from the area where contaminants are likely to be the highest, and also from background areas up-gradient of the plunge bath. The size of the grid depends on the information known about the site and the size of the dip site. Because a surface-water body is located down-gradient of the sheep dip, sediment and surface-water samples are collected. The dip contains liquid (collected rainfall), which is analysed to determine if contaminants are present above relevant water quality guidelines (eg, ANZECC and ARMCANZ, 2000).

Because the sheep dip operated on that location for a number of years, the area in the direct vicinity of the sheep dip has some hard fill. To save sampling time, a small (2-5 tonne) excavator is used to dig small holes (one scoop with a narrow bucket) or shallow trenches in the more central areas. This also allows good inspection of the soil layers, and possible discolorations can be observed with ease. The area is relatively small (50 x 100 m) and one day's work is sufficient.

Groundwater samples are taken from below the former plunge bath, and below two depressions close-by, where infiltration of used liquids may have taken place. Due to the water table being 4 m below the surface and the soil type suggesting a low permeability, it is decided that no other groundwater samples need to be taken unless the preliminary sampling results show unacceptable groundwater contamination. No groundwater contamination is detected.

Elevated contaminant levels for arsenic and lindane are found in a number of soil samples. Usually the depth (and therefore volume) of soil that needs to be excavated for site remediation is determined by contaminant levels that are judged to be comfortably below applicable guideline values for human health.

Viable management options under this scenario
  1. The contaminated soil is removed and relocated beneath a sealed car parking area. As part of a resource consent, a condition may say that prior to occupation the developer needs to vest ownership of this “common area” in a legal entity, such as a body corporate. All residential owners will be shareholders in the upkeep of common property and pay an annual insurance premium against the risk that the contaminated material would need to be excavated and disposed. A bond could be posted in establishing the insurance scheme held against the risk that remedial work is needed at some stage in the future. A risk management plan is written to ensure that the sealed surface is maintained and any disturbances during maintenance works are controlled. The spot is marked on the LIM.
  2. The top 0.5 m of soil is remediated to meet the residential soil criteria. A management plan is put in place and the spot is marked on the LIM.
  3. The soil is excavated to the whole depth of contamination and disposed to landfill. An appropriate site validation programme must be implemented following remediation of the dip site. No management plan is needed. The remediation report and validation report can be issued with the LIM.

7.4 Sampling tips from a professional contaminated land practitioner

  • Overall guidance on sampling can be sought from the Contaminated Land Management Guidelines No. 5 (Ministry for the Environment 2004a).
  • Initially, samples may be composited; sub-samples should preferably be taken from adjacent locations, from similar depths and with a similar site history (eg, the area where a former draining pen is confirmed) to give an idea of where individual analysis would be most beneficial.
  • After the shallow holes are excavated, deeper investigation can be done using a hand auger as the hard surface is taken away.
  • With deeper holes (over 3 m) a drilling rig may be needed if soils are too hard for a hand auger.
  • A small digger with posthole attachment is a good option for sampling in the first few metres, because it is much faster to set up, can drive over uneven ground, sampling from depth intervals will be quite precise as the auger can be taken from the borehole in seconds avoiding smearing and mixing (often the case with auger rigs), and a digger can push soil back in the hole quickly after the completion of sampling. (For more details on costs, see section 7.5).
  • When sampling with large-diametre augers or using an excavator, consider the mixing effect of the backfill in these deeper holes. The deeper layers may be found to be clean, but the surface layer has high concentrations and these are now likely to be found in the fill of these investigation holes, giving rise to the need for remediation of investigation holes later on! When the surface is highly contaminated, consider filling deeper holes with clean fill brought to the site beforehand.

Overall, take care:

  • not to aggravate the contaminant situation by investigation works
  • not to encourage infiltration (in poorly compacted or open investigation bore-holes)
  • not to create cross-flow of shallow water into deeper groundwater.

7.5 Cost estimates for investigation, reporting and remediation

The following subsection may only be relevant for sites that require an extensive assessment and possibly subsequent remediation. Estimated costs are given as of 2006, are GST exclusive, and indicative only.

Investigation costs

These costs are location- and duration-dependent (usually taking between one and four days), and hence the following are rough estimates to assist in the projection of costs only:

  • excavator: $60-$80/hour, plus mobilisation $50-$100
  • sampling by environmental engineer: $70-$90/hour, plus travel expenses
  • hand augering around the centre area and possible migration pathways: one to two persons, $120-$180/hour
  • power augering using a digger plus one person sampling: combined $140-$190/hour
  • placement of some shallow monitoring wells in pre-augered holes: $200−$300 each (this covers 3 to 10 wells per site, but note that surveying the level of the well tops to determine the flow direction may be needed if groundwater flow direction is not evident)
  • analysis of samples (30-80 samples): heavy metal screen $30, organochlorine pesticides screen $90.

Reporting costs (mapping, cross-sections, text)

A rough estimate is $3,000 for phase 1 surface sampling and $8,000 for the final report. However, costs are very dependent on the level of graphics required, the precision of the sample locations (use of a surveyor, or quick GPS record of coordinates, downloaded on to a topographic map or aerial photo overlay), and how elaborate the report is.

Basic reports, which would include a brief text, data mainly in attachments, maps and cross-sections mostly hand-drawn or basic GPS plots, cost between $2,000 and $4,000. Full reports (many pages, well indexed, many CAD maps, diagrams, contour maps, etc) may cost up to $15,000 and more. Full reports may be necessary, for example, when the final plan needs to go through public hearings.

Remediation costs

These are very case-specific, but the following should give some idea of what might be involved.

  • Generally, a 20-tonne excavator per day, given an adequate number of trucks, can excavate 200-300 m3 of soil. A 20-tonne excavator will cost $80-110/hour.
  • Most dip sites will be excavated within one to three days. If a lot of shallow soil contamination is present, a bulldozer and front-end loader to load trucks may shorten the work.
  • In order to know whether to excavate further, sampling results are required, but there is often a hold-up due to the turnaround time of the lab. For this reason, on-site X-ray fluorescence (see Appendix 8) capable of detecting at least half of the desired arsenic concentration is very useful for indicating the extent of the contamination in the field.
  • One way to avoid a third phase is to use a hand auger to take a second sample 0.5 m deeper into the floors and walls of the excavation, in effect creating a second envelope outside the excavated surface being sampled. The extra effort will pay for itself when a third mobilisation can be avoided.
  • There are potential costs from the extra analysis, but not all samples need to be analysed in one go: a further set of samples for analysis is needed only from walls or floor areas of the excavation where the surface is still contaminated.
  • Backfilling and compaction (levelling out shallow excavated parts, less then 400 mm) will usually be part of further development costs of the site, not part of remediation.
  • There may be costs to compact and cap the soil.
  • Pre-treatment of the contaminated soil may become necessary to comply with landfill acceptance criteria - allow for the associated costs.
  • Disposal fees for contaminated soil are likely to make up the majority of the costs of dig-and-dump remediation.
  • Transport costs will be significant when the approved landfill or treatment facility is located some distance away (trucks cost on average $90/hour and can carry 15-25 tonnes of soil). Sometimes trucks can take some clean fill back on a round trip if there is sufficient space on site to stockpile it.

In addition, there might be costs incurred for initial investigations, consultancy fees, erosion controls, validation, peer reviews, consent applications and ongoing monitoring (if required).

Landowners may want to contact their regional council to explore possibilities to apply for funding to investigate or remediate a high-risk site.

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