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5 Site Investigation

5.1 Overview

This section looks at the best practice procedures to be followed in carrying out a detailed site investigation. All sampling programmes for contaminated sites should be undertaken in accordance with the Contaminated Land Management Guidelines No. 1: Reporting on Contaminated Sites in New Zealand (Ministry for the Environment 2003a), and Contaminated Land Management Guidelines No. 5: Site Investigation and Analysis of Soils (Ministry for the Environment 2004a). The following subsection summarises the information contained in this guideline which is appropriate to old sheep-dip sites. Figure 6 summarises the recommended approach to a site investigation.

Figure 6: Recommended approach to a site investigation

The recommended approach to a site investigation follows the following steps:

  1. Set objectives for investigation
  2. Review existing data
  3. Develop conceptual model (gather additional data, back loop to review)
  4. Determine sampling design and strategy
  5. Collect samples
  6. Analyse samples
  7. Interpret data (gather additional information if necessary, back loop to collect samples)
  8. Report data

It is recommended that the landowner contracts a qualified environmental consultant to undertake detailed site investigations that involve sampling. It is unlikely a council would accept a site investigation if it was undertaken by the landowner. During the course of the investigation, the landowner needs to be aware of the risks the site may pose and, where necessary, restrict access to the site until the risks are mitigated to an acceptable level.

5.2 Objectives of the investigation

The first step is to identify the purpose of an investigation and set investigation objectives accordingly. There are many reasons why a local authority or landowner may want to investigate a former sheep-dip site, including:

  • concern that the site represents a risk to the landowner/occupier, employees or wider public
  • potential environmental liabilities need to be determined in order for a site to be sold, purchased or redeveloped
  • the land is proposed to be rezoned and a change in land use will take place in the near future
  • concern that the site represents a risk to crops, livestock or meat quality, or that it may have effects on the terrestrial ecosystem (bacteria, bugs, animals and plants)
  • a down-gradient water source has high concentrations of dip contaminants.

Identifying the issues to be resolved will help to establish clear sampling objectives for the investigation which define why or how samples are being collected. In the context of sheep-dip sites, a common sampling objective will be to establish the nature, degree and extent of contaminant distribution (both vertically and laterally). With regard to off-site effects, locating the sources of contamination will be the first step.

5.3 Data review and conceptual model

Once the objectives for the investigation have been determined, readily available data should be reviewed. Structural evidence or anecdotal information gathered during the identification phase (see section 4) will be useful to include in the data collection. Other sources may include old records of the site, or previous investigation reports.

Based on the existing data, a conceptual model of potential exposure pathways needs to be developed. Experienced contaminated land consultants are usually contracted to develop the conceptual model, set the data quality objectives (such as sampling density and requirements for validation sampling), and carry out the site investigation and sampling. It will be beneficial to both sides if the landowner or council and the consultant work together closely in developing the conceptual model. The model should identify the exposure pathways, including the:

  • contaminant sources (eg, previous sheep-dip location, chemical storage location and methods used to dispose of chemicals, and the likely depth of soil contamination)
  • transport mechanism or exposure route, as illustrated in Figure 4 (eg, drinking contaminated water)
  • receptors (eg, a nearby surface-water body, groundwater well, farm workers, animals).

The model can be improved by gathering additional data, such as from a geophysical survey, an aerial photograph or specialised remote-sensing techniques (eg, Landsat satellite false colour image, or side-scanning radar).

Due to the complexity associated with each individual site, a summary of the recommended procedures for three site scenarios is provided in section 7. Each scenario specifies the recommended way of sampling, assessment of results and selection of remediation options.

5.4 Sampling design and strategy

The sampling programme is developed using the conceptual model. Visual inspection of a site, examination of photographs and maps, and information from local residents can be used to determine the best location for sampling points within an area of investigation. A site assessment may include an investigation of the soil at the site, and of groundwater and surface water within the vicinity of the site. For example, a disused sheep-dip site may contain a permanent concrete trench and an area of soil through which contaminants seep and discharge into a nearby stream. In this case, it will be necessary to sample the dipping vat and adjacent soil and water, soil and sediment in the seepage zone, surface water, and sediments.

Other possible sampling sites within an old sheep-dipping site may include:

  • soil beneath the dip bath and within the bath
  • soil in the splash zone around the dip
  • soil in the disposal/run-off area where sludge and spent dipping chemicals may have been disposed of, and drainage to where the spent or discharged liquid dip flowed
  • soil from yards where freshly dipped sheep were collected before further transport
  • soil in the storage area for chemicals, and soil beneath the woolshed
  • water from one or more areas of the identified site
  • water from one or more groundwater wells in the area
  • water and sediment from one or more areas of a seepage zone
  • water and sediment from one or more areas of a stream or foreshore.

It is worth remembering there is a common pattern of three potential zones of high contamination, at increasing distance:

  • nearby contamination − associated with dipping and dripping (the "splash zone" and the draining platform)
  • one-overarm-shovel distance − contamination associated with digging out the sludge (the "scooping mound")
  • one pipe-length away (pipes found at dip sites include 4- or 6-inch metal irrigation pipes, three-foot concrete pipes, 21-foot galvanised pipes and field tiles) − contamination may be associated with emptying the bath quickly and in such a way that the liquid did not inundate the dip site; for example, dip wastewater may have been emptied down slopes or banks.

Previous assessments of historical sheep-dip sites in New Zealand have used judgemental sampling strategies due to the high costs associated with systematic sampling (McBride 1994; Wilson 1998; McBride et al 1998; Hadfield and Smith 2000; Environment Canterbury 2003). There are a number of different sampling strategies and techniques, and each has certain advantages and disadvantages. Appendix 8 describes the outcomes of a study that compared four different sampling strategies (judgemental, systematic grid, sniffer dog and portable XRF sampling) in order to assess the most appropriate sampling regime for historical sheep-dip sites. It was found that the systematic sampling approach provides current best practice for assessing contamination at old sheep-dip sites.

5.5 Collection of samples

The aim of sampling is to identify the three-dimensional extent of any contamination that exceeds the soil guideline values. The sampling results will provide important information for the development of a remediation plan. When collecting soil samples from an undisturbed sheep-dip site (eg, no previous clean-up, no placement of fill nor development), shallow sampling (0 to 15 cm) may be sufficient to determine the horizontal extent of contamination. For disturbed sites, sampling at varying depths may be required to establish the horizontal extent, because fill material may cover the contamination.

Soil samples should be collected at the depth where contamination is most likely to occur, based on known site conditions. Soil samples need to be collected from inside the base of the sheep-dip bath, and in the natural material underlying any soil back-fill. Soil samples should also be collected within the surface soil of the splash zone and from the exit zone of the dip, in the vicinity of the draining platform or holding pens, and from the area where the sheep-dip chemicals were stored and the spent dipping fluid disposed. Careful sampling procedures need to be used because the actual amount of soil or water analysed by the laboratory is extremely small.

Depending on the purpose of the investigation, it may be appropriate for the soil profile to be classified during soil sampling and a soil log produced to ensure that soil samples are collected at the appropriate soil horizons (eg, within any fill material and in the natural underlying soils). Based on the sampling design, the location of each sample point should be recorded on a site plan, with reference to permanent features and structures, if possible. Sample points or dip features can be recorded accurately in the field using a GPS mapping system. Usually an experienced contaminated land practitioner or consultant is employed to carry out the sampling.

A chain of custody procedure is required to ensure the legitimacy of the samples, and the laboratory chosen for analysing the samples should be IANZ accredited to ISO/IEC guide 17025.

5.6 Data interpretation

Assessing the results

The first step in interpreting analytical results of samples collected from various locations around the site is to examine them for patterns that identify the likely nature and spread of contaminants. The results of chemical analysis of samples taken from soil, sediment or water are typically compared with guideline values to assess whether a site is contaminated. This assessment forms the basis for deciding if further investigations are required, or which remediation or risk management options are feasible for the site. This subsection provides a review of relevant guideline values and how they might be applied.

Appropriate soil guideline values may include those developed to provide protection for receptors living on the site (eg, people, worms, plants) or off the site (eg, fish, algae). Soil guideline values are soil concentrations that are protective of people or ecological receptors, and they are based on generic exposure scenarios.

More specifically, for the protection of human health, soil guideline values are derived by defining some critical receptor (eg, a child of certain age and weight) and defining a tolerable daily intake for that receptor for the particular contaminant. Then, using assumptions for exposure (eg, duration, exposure pathway), the soil concentrations that would equal the acceptable daily intake for the assumed exposure are calculated. As a result, the selection of an appropriate soil guideline value must consider the proposed or current land use. For example, if the site is to be developed for general residential use, the results should be compared to a residential value for protecting human health.

For ecological receptors, soil guideline values are developed to provide a certain level of protection for terrestrial species (plants, soil invertebrates and wildlife) and soil microbial functions. Such guideline values are considered to be most applicable to land uses where a functioning ecosystem is desirable. Soil guideline values for the protection of on-site ecological receptors could also be used to provide target values for long-term soil quality.

Consideration should also be given to the protection of off-site receptors such as surface water and groundwater. For example, if the site is located close to a sensitive ecological receptor (eg, a stream) and a discharge of contaminants is suspected, then it would be appropriate to collect water or sediment samples and compare these results to appropriate guidelines, such as those presented in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC and ARMCANZ, 2000) (see Table 6).

Applying soil guideline values

Soil guideline values for the protection of human health or ecological receptors, or both, have been developed by a number of countries, including New Zealand. Contaminated Land Management Guidelines No. 2: Hierarchy and Application in New Zealand of Environmental Guideline Values (Ministry for the Environment 2003b) was developed to provide consistency across New Zealand when selecting appropriate guideline values for contaminants. Under this guideline, New Zealand risk-based guideline values are the preferred choice. Where these do not exist, overseas risk-based guideline values − especially values derived in a manner similar to those in New Zealand − are the next best choice.

Where international soil guideline values are referred to, it should be noted there are differences in the methods and assumptions used to derive the guideline values, and also differences in the political and legislative backgrounds for contaminated land management in individual countries (see Cavanagh and O'Halloran 2003). This can give rise to different soil guideline values for a given contaminant.

In New Zealand, soil guideline values for a limited number of contaminants are available in three industry-based guidelines: Health and Environmental Guidelines for Selected Timber Treatment Chemicals (Ministry for the Environment and Ministry of Health 1997), Guidelines for Assessing and Managing Contaminated Gasworks Sites in New Zealand (Ministry for the Environment 1997), and Guidelines for Assessing and Managing Petroleum Hydrocarbon Contaminated Sites in New Zealand (Ministry for the Environment 1999). These guideline values are primarily based on the protection of human health, and of on-site ecosystems to the extent necessary to facilitate the use of the land (eg, plant growth and livestock grazing for rural properties). Existing controls based on food safety standards may be more stringent than the proposed soil guideline values. Where food is being produced commercially, standards set by the New Zealand Food Safety Authority (maximum residue limits and maximum permissible levels) will apply, and in situations where sheep farms are converted to dairying, the Fonterra thresholds would become a remediation goal.

Of the contaminants of concern at disused sheep-dip sites, New Zealand soil guideline values are only available for arsenic. This guideline therefore provides additional indicative soil guideline values for selected sheep-dip contaminants of concern (see Table 4). These indicative values will be superseded by any national environmental standards that may be developed in the future. (Refer to www.mfe.govt.nz for the latest information on guidelines and national environmental standards.)

Soil guideline values for selected land uses were derived using an updated method based on that provided in existing industry-based guidelines. An overview of the five land-use categories, exposure parameters for the different land uses, contaminant-specific parameters, and guideline values for individual pathways of exposure are provided in Appendix 6.

Table 4: Soil guideline values for human health (mg/kg)

Chemical

Lifestyle block [Based on the assumption that 50 percent of the produce consumed by residents is grown on-site. Consumption of products (eggs, milk, meat) from animals raised on-site is excluded and should be considered on a site-specific basis.]

Standard residential

High-density urban residential [Based on a residential value with no produce consumption.]

Parks/ recreation

Commercial/ industrial (unpaved)

Arsenic [Values provided in Health and Environmental Guidelines for Selected Timber Treatment Chemicals (Ministry of Health and Ministry for the Environment 1997).]

30

30

100

[No value has been derived for this land use in New Zealand; refer to international guidelines (eg, National Environmental Protection Council 1999; or CCME 2003.)]

500

∑DDTs [Sum of all DDT and DDT metabolites (DDD and DDE).]

8.4

28

70

140

1,700

Dieldrin

0.7

2.7

12

23

190

Lindane

33

140

700

1,400

14,000

For land uses where a functioning ecosystem is desirable (eg, residential land use, parkland), it may also be relevant to consider protecting on-site ecological receptors. No national guidelines have been developed specifically for the protection of on-site ecological receptors at contaminated sites in New Zealand, but several other jurisdictions have developed such soil guideline values (see Appendix 6, table A.6), which may be referred to.

In comparing site investigation results to the appropriate soil guideline value, Contaminated Land Management Guidelines No. 5: Site Investigation and Analysis of Soils (Ministry for the Environment 2004a, p.62) should be followed. If a systematic sampling approach has been used, the 95 percent upper confidence limit of the arithmetic mean of soil contaminant concentrations can be compared to the selected soil guideline value. However, this approach is not appropriate if judgemental sampling has been undertaken, because the sample design is biased.

Guideline values for off-site receptors

Off-site impacts of contaminants may occur as a result of leaching to groundwater, or from the movement of contaminants through surface water run-off or wind. The significance of these discharges is highly site-specific because it depends on a number of factors, including the proximity of the sheep-dip site to the aquatic system of concern, the proximity of a groundwater bore to the sheep dip, the soil type, the slope of the land, or whether earthworks are being undertaken. It is therefore appropriate to assess the potential impact on off-site receptors on a site-specific basis.

The primary concern for contaminants discharged to groundwater is typically whether this would result in contaminants exceeding the drinking-water standards. However, contaminants present in groundwater − particularly shallow groundwater − may also ultimately discharge into surface water systems, meaning that it may also be relevant to consider their potential impact on ecological receptors.

Where surface water or groundwater is used for drinking water for people (primarily groundwater) or for livestock, the results of water quality analyses are most appropriately compared to drinking-water standards or guidelines (Table 5).

Table 5: Guideline values for drinking water

Chemical

New Zealand drinking-water standards [Drinking-water Standards for New Zealand 2005 (Ministry of Health 2005).]
(µg/L)

Livestock drinking-water protection [These figures were derived from Kim (2003) by applying a safety factor of 10 to the approximate minimum concentrations that could correspond to minimum lethal body burdens in stock.]
(µg/L)

Arsenic

10

50

DDT

1

 

Aldrin

0.04 [Combined total concentrations for aldrin + dieldrin.]

-

Dieldrin

77 [Note that the dieldrin limit for stock drinking water is not designed to protect against dieldrin limits being exceeded in any farm produce (milk or meat).]

Lindane

2

-

Comparison of the results of analysing water and sediment samples with guideline values provides a first step in determining whether an impact on the environment may be occurring. Where the protection of ecological receptors is the issue, the results of water and sediment samples are most appropriately compared to the water and sediment quality guidelines provided in Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC and ARMCANZ 2000). Table 6 provides an overview of the recommended surface water and sediment values for the protection of ecological receptors for contaminants associated with old sheep-dip sites.

Table 6: Guideline values for surface water and sediment

Chemical

ANZECC trigger values for surface freshwater 1,2
(µg/L)

ANZECC trigger values for marine water 2,3
(µg/L)

ANZECC sediment quality guidelines4
(µg/kg dry wt)

ISQG-low

ISQG-high

Arsenic (As III/As V)

24/13

2.35/4.55

20,000

70,000

DDT/DDE

0.01/0.035

0.00045/0.00055

1.6/2.2

46/27

Aldrin

0.0015

0.0035

-

-

Dieldrin

0.015

0.015

0.02

8

Lindane

0.2

0.0075

0.32

1

  1. Trigger values for a 95 percent level of protection for freshwater water (ANZECC and ARMCANZ 2000).
  2. The guideline values for water have been listed in µg/L (as provided in ANZECC and ARMCANZ 2000). However, care must be taken because the laboratories may provide results in mg/L.
  3. Trigger values for a 95 percent level of protection for marine water (ANZECC and ARMCANZ 2000).
  4. Sediment quality guidelines (ANZECC and ARMCANZ 2000). ISQG = interim sediment quality guideline. The ISQG-Low is a level below which adverse effects are not expected; the ISQG-High is a level at which significant adverse effects are expected in 50 percent of organisms. Note that these are primarily developed for marine and estuarine systems and should be used only as indicative interim threshold values for freshwater systems.
  5. Low reliability trigger value; this should be used only as an indicative interim working level (ANZECC and ARMCANZ 2000).

Site-specific risk assessment

Site-specific risk assessment is useful for establishing the risk posed by contaminants to people, livestock or ecological receptors currently present on the site. It focuses on modifying the actual exposure of those receptors (eg, time spent on the site, activities occurring). However, where future use of a site is being considered, a given exposure scenario is assumed for a particular land use and there is no basis for modifying the exposure parameters.

It would be appropriate to undertake a site-specific risk assessment if meat, milk or eggs from livestock raised on lifestyle blocks are being consumed by residents on those sites, because organochlorine compounds such as DDT and dieldrin are known to bioaccumulate in the food chain. In addition, site-specific risk assessment would be appropriate for the commercial/ industrial land-use category where paving has been used as a means of reducing exposure to on-site contaminants.

Site-specific assessments may also be relevant where the naturally occurring concentrations of chemicals exceed the relevant guideline values. In New Zealand, some soils have naturally high arsenic concentrations, which may result in the guideline values for arsenic being lower than the natural ranges of arsenic concentrations in surface water, groundwater and sediment. In such instances, it is not appropriate to require remediation to concentrations below naturally occurring concentrations. A site is considered to be above background concentrations when the concentration of a contaminant is clearly higher than its background concentration. Reference can be made to factors such as the upper confidence limit (95 percent UCL) of the background concentration, the number of samples collected and their representativeness, observed or expected variability associated with sampling and analysis, and applicable guideline values.

Soil guideline values typically assume that all of a contaminant measured in the soil by chemical analysis will cause toxicity. However, usually only a small fraction is available for biological uptake. Determination of this bioavailable fraction provides a basis for modifying the exposure of receptors on-site. Presently, there is no generally accepted method for determining the bioavailability of contaminants. The plant uptake factor may be used to derive the generic soil guideline values for a particular site. However, plant uptake of contaminants is highly variable and depends on the species examined and the soil type, among other things.

5.7 Report data

The following checklist (Table 7) outlines the sections that should be included in a detailed site investigation report of a disused sheep-dip site. This information should be collected by a local authority when they are involved in investigations into former sheep-dip sites (eg, during subdivision or change in land use, or otherwise according to rules in a plan). The information contained in this subsection summarises the information given in Contaminated Land Management Guidelines No. 1: Reporting on Contaminated Sites in New Zealand (Ministry for the Environment 2003a), and all sheep-dip site reports should be consistent with the requirements outlined there.

Table 7: Checklist of reporting requirements

Report section

Executive summary

  • Background
  • Objectives
  • Scope of works
  • Summary and conclusions

Scope of works

  • Clear statement of the scope of the works

Site identification

  • Site address
  • Legal description
  • Geographic coordinates
  • Site plan
  • Locality map

Site history

  • List of site owners, including both previous landowners/occupiers and current landowners/occupiers, and previous and current land uses
  • List of contaminants of concern
  • Zoning (present and proposed)
  • Location of ground- and surface-water bodies
  • Location of relevant sheep-dip structures
  • Anecdotal information regarding the site, where possible
  • History of the neighbouring property and site usage
  • Review of aerial photographs, where appropriate

Site conditions

  • Topography
  • Soil
  • Geology
  • Hydrogeology
  • Observations, including site vegetation
  • Access/risk potential - people, livestock, produce, ecosystems

Sampling and analyses plan

  • Sampling and analytical data objectives
  • Rationale for sampling pattern, sampling number and analysis programme

Basis of guideline values

  • Table indicating guideline values used
  • Assumptions and limitations of guideline values

Results

  • Site plan showing sample locations and exceedances of guideline values
  • Tabulated results showing guideline exceedances and other statistical information

Conclusions and recommendations

  • Brief summary of the results
  • Assumptions and uncertainties
  • Recommendations for additional works (if required)
  • List of appropriate site uses