The wide variation of natural parameters, historical dip construction, operation and regulatory oversight demonstrated by the different case studies in this section may help to explain why a site-by-site assessment is necessary in managing the risks from old sheep-dip sites. The guideline aims to provide best practice to achieve good outcomes for future site assessments.
The case studies have been taken from reports produced for the Ministry for the Environment's Sustainable Management Fund Project on sheep-dip sites (including Stage 1 Preliminary Report: A Chronology of Site Discovery and Investigation into Contaminated Animal Dipping Sites in New Zealand 1993-2002 by McBride 2003).
Detailed investigations of selected Waikato sites
An initial study undertaken in 1994 demonstrated that significant contamination of soil by arsenic and organochlorines had occurred at seven selected dip sites. The investigation found no detectable levels of organophosphate compounds in soil, and did not show the distribution of the organochlorine and arsenical contamination. The Waikato Pesticides Awareness Committee (WaiPAC) [WaiPAC is a multi-stakeholder community group that seeks consensus on agrichemical issues in which members' views may initially be very divergent. In particular, the group's strengths and successes are built on an ability to influence parent organisations. Although originally established to address agrichemical issues in the Waikato, WaiPAC is now achieving wider recognition of its work. Raising awareness of environmental issues and protecting human health are key components of WaiPAC's raison d'être.] concluded that more information was needed to determine the risk that old sheep-dip sites pose to the surrounding environment and human health.
In 1997, WaiPAC received funding from Environment Waikato's Environmental Initiatives Fund to further investigate contaminated dipping sites in the region. Study objectives included determining the likely extent of soil contamination around known sheep-dipping sites, and evaluating risks to surface and groundwater, grazing animals and human health. As a result, the study focused on the more persistent chemicals: arsenic and members of the organochlorine family. The project was a collaboration, with HortResearch and the University of Waikato providing technical inputs.
Four historical sheep-dip sites were selected from around the Waikato region to be included in the more detailed study. Site 1 was selected due to indications of dieldrin appearing in the groundwater supply in 1995. Sites 2 and 3 were part of the 1994 WaiPAC scoping study. A crucial factor in selecting sites for this study was the cooperation of the landowners, and keeping the specific locations of each site confidential facilitated this. An overview of the sites included in the 1997/98 works is as follows.
- Site 1: A dip sump on the property was identified as the likely source of contamination during a pesticides survey by Environment Waikato in 1995. It was operated between 1954 and 1961, dipping 500−600 sheep annually, and the spent dip solution was pumped onto land adjacent to the sump. The site is flat and has been extensively cultivated, and is currently covered in a deep-rooted vegetable crop. The farm water well is located approximately 15 m from the dip site and groundwater flow from the dip is towards the well. Soils are free-draining Horotiu sandy loam.
- Site 2: This site was identified by WaiPAC in 1994 as being very heavily contaminated by arsenic, dieldrin and lindane (with some DDT) and was included in this study in order to determine the distribution of the contamination. The site was operated as a sheep farm from around 1932 until 1970, when it was converted to dairying. There is evidence that this dip was in communal use. The old dip was clearly visible, situated on the edge of a small gully.
- Site 3: Environment Waikato brought this site to the attention of WaiPAC in 1993, when two heifers died from arsenic ingestion and others were chronically poisoned. Nothing was known about the use or exact location of old sheep dips on the site, although some broken concrete and rubble was evident at the head of the adjacent gully. Stockyards had been built on a flat area near the edge of a steep gully in which a small spring-fed stream forms. Most of the area was covered in pasture, except for the head of the gully, which was overrun with blackberry and scrub. Significant levels of dieldrin and lindane were discovered in the soil and associated stream.
- Site 4: This site was included because it lies within the urban boundary of Hamilton, and so is a potential target for urban development. An old dip and draining mound were clearly visible. It appears that a woolshed and/or sheep yards may have existed on the site a long time ago and the dip facility has not been used for many years. The relatively flat land is completely covered in pasture. Soils are based on Hamilton clay loam, with areas of Te Rapa peaty sand in a nearby shallow gully.
Sampling and analysis
Soil samples were collected from the four sites using a 25 mm diametre soil corer. At each sampling location three cores to 30 cm depth were taken within an area of 1 m diametre. Cores were sectioned and composited in 10 cm increments. The main sampling took place in May 1997, with additional samples taken from some sites in the period June to August 1998. In addition, deep cores were taken from sites 1, 2 and 4 in June 1998 using a Humax soil sampler (5 cm diametre). Single cores were taken to 125 cm depth at three locations at each of the three sites, with samples comprising the five 25 cm core depth increments. All soil samples were air-dried and ground for chemical analysis. Pasture and surface water samples were collected from June to August 1998. Samples were analysed for arsenic and organochlorines using standard methods.
All four sites close to the original dips were heavily contaminated with arsenic and organochlorine residues. Surrounding areas contained less contamination, but water samples still exceeded the New Zealand drinking-water standards (see Table 5 for values). These residues had persisted for at least 25 years, and it was thought likely that losses by degradation in the soil were proceeding very slowly. At site 1 the dieldrin contamination was severe enough to have led to contamination of local groundwater, even though this chemical is relatively immobile in soil. Contamination by arsenic and organochlorines was extensive at the Hamilton site, with topsoil being above guideline values over an area of at least 20 m by 40 m. At sites 2 and 3, movement of arsenic and organochlorines had occurred down the gullies and into drainage areas. Pasture samples were contaminated, and surface water samples also contained residues well above the ANZECC guidelines. The shallow core sections revealed that contamination at all sites was often uniform to 30 cm depth, particularly for arsenic. The deep core sections showed that the contamination reached greater depths in areas close to the dip baths.
One finding of this survey was that there could be considerable differences in contaminant profiles between sites, and unexpected areas of contamination at some distance from the main dipping area. The following is a more detailed overview of the results for each site.
Soil sampled near the dip sump was contaminated with dieldrin, with levels exceeding 50 mg/kg in the top 20 cm. Lindane was also present at high levels. A deep core taken near the sump showed that dieldrin had permeated the profile, with 23−36 mg/kg found at 0−75 cm, 9.2 mg/kg at 75−100 cm and 0.09 mg/kg at 100−125 cm. Leaching of dieldrin from this area to groundwater at about 4 m depth had occurred. This had led to dieldrin contamination in the well water (15 m away) of up to 0.18 µg/L(compare to New Zealand drinking-water standards of 0.04 µg/L). Five further shallow soil samples at distances of 2 to 10 m from the sump showed a rapid fall-off in dieldrin contamination, from 5 mg/kg to 0.25 mg/kg (mean levels 0−30 cm depth). Arsenic levels were at background levels in all samples, indicating arsenic dips had not been used at this site.
Twenty soil samples showed a broad distribution of arsenic contamination, from as high as 1200−3500 mg/kg just below the dip outfall, to 130−280 mg/kg in the vicinity of the dip and 20−40 mg/kg further down the bank towards the stream. Stream-bed sediments generally had background levels (1−7 mg/kg), but a hot spot 100 m downstream at 90−150 mg/kg showed that contamination may have been more widespread but had been eroded or overlaid by sediment. Lindane (1−10 mg/kg), and to a lesser extent dieldrin (0.05−0.22 mg/kg), residues were also present at the site, but were more localised to the vicinity of the dip. Deep cores showed that the soil strata in these areas were highly contaminated by arsenic (5−3500 mg/kg) and lindane, with significant residues to depths of over 1 m. Pasture samples surrounding the dip contained arsenic (13 mg/kg), dieldrin (0.015 mg/kg) and lindane (0.007 mg/kg), while surface water obtained 20 m from the dip outfall contained 0.5 µg/L of lindane.
Soil samples at the head of the gully leading to the stream were heavily contaminated with arsenic (325−2620 mg/kg) and small amounts of dieldrin (0.04−0.19 mg/kg). The adjacent stockyards and races were also contaminated (arsenic 10−167 mg/kg, dieldrin 0.05−2.1 mg/kg). Soil samples from 20−100 m down the gully showed a generally decreasing trend in arsenic (80−4 mg/kg) and no significant organochlorine residues. Pasture samples from the sheep yards contained mean values of 0.006 mg/kg lindane and 0.02 mg/kg dieldrin. A surface water sample from the spring outfall contained lindane (1 µg/L) and dieldrin (9 µg/L).
The soil samples revealed this site was contaminated by arsenic, dieldrin and lindane over a large area. Adjacent to the large dip bath and race, residues in the 0−30 cm top soil samples were in the range: arsenic 100−2560 mg/kg (with many samples exceeding 300 mg/kg), dieldrin 0.15−3.1 mg/kg, and lindane 0.26−10.6 mg/kg. The contamination gradually decreased with distance from the dip, but samples at 10 m still had significant residues (arsenic 14−125 mg/kg, mean 50; dieldrin 0.01−0.45 mg/kg, mean 0.07 mg/kg), and a sample in a drainage area 20 m distant contained 48 mg/kg arsenic. One area at 7 m contained dieldrin at 16 mg/kg (range 0.9−45 mg/kg). The soil samples indicated there had been physical disturbances in the soil profile in some areas. Herbage samples from beside the dip gave arsenic at 6.5 mg/kg, lindane at 0.67 mg/kg and dieldrin at 0.018 mg/kg. However, blood tests on cattle grazing this property revealed no significant residues (arsenic 1 mg/kg; organochlorines < 0.005 mg/kg).
Investigation of soil and groundwater on the Kaikoura plain
In 2000 Environment Canterbury (ECan) engaged a consultancy to undertake an investigation to determine the extent of arsenic contamination of soil in the Kaikoura area resulting from historical sheep-dipping practices, and to determine whether this was affecting groundwater quality. A sampling programme was designed to collect two or three composite soil samples from 13 sheep dip sites, four footbath sites and a wool-scouring shed.
At all dip and footbath sites, at least one soil sample returned arsenic results above guideline values for arsenic in soils used for residential and agricultural use (30 mg/kg). The concentrations ranged from 30 mg/kg to 4390 mg/kg. Arsenic concentrations were significantly higher at the dip and drip-pad exits compared to the entrance area. Three stream sediment samples collected from the creek adjacent to the wool-scouring site returned arsenic concentrations of 3 and 5 mg/kg.
In spring 2000 ECan staff undertook groundwater sampling from 22 private wells and two springs located on the Kaikoura plains. The aim of the sampling was to determine whether there was a link between the arsenic-contaminated soil found at 18 sites associated with sheep dipping and the arsenic found in groundwater samples in private wells. Of the 37 wells sampled for arsenic in the course of this sampling campaign and an earlier investigation, six yielded samples with arsenic concentrations exceeding the New Zealand drinking-water standards maximum acceptable values of 0.01 mg/L. Two of the six wells were reportedly used for domestic supply, and the well owners were advised that the water in their wells was unsuitable for drinking. The depths of wells with detectable concentrations of arsenic ranged from 4 to 44.5 m. There was no obvious relationship between well depths and arsenic concentrations in the groundwater.
To assess the lateral and vertical extent of the soil contamination and whether these arsenic "hot spots" were the source of the arsenic detected in the groundwater in the Kaikoura area, three sites were selected for further field investigation. Surface soil samples were collected at 2 to 15 m spacings, to a distance of up to 50 m from the sides and exits of the dips. At all three sites, the highest soil arsenic concentrations were recorded at the site itself, with concentrations decreasing with greater distance from the dip. Concentrations of 52 mg/kg were found even in soils 50 m from the former dip structure, which was located within a former sheep-holding yard.
The samples collected from the dip wells generally showed a decrease in arsenic concentrations with increasing depth. At two sites elevated arsenic concentrations were found at depths that were intercepted by the groundwater table (2.4 m at one of the sites).
Monitoring wells at each of the three sites were installed up-gradient, down-gradient and on-site at the three dip/footbath sites. Results showed that groundwater beneath two sites had been affected by elevated arsenic concentrations in soils at these sites: 0.197 mg/L dissolved arsenic under the footbath, and 0.031 at 25 m down-gradient; 0.92 mg/L dissolved arsenic under the dip, and 0.078 mg/L at 30 m down-gradient. Groundwater samples from the up-gradient wells returned results with little or no detectable arsenic. The third site showed little difference in arsenic concentrations between up-gradient, down-gradient and dip wells. The results, however, show that the shallow groundwater in the vicinity of old sheep dips and footbaths is at risk of contamination with arsenic, and wells should therefore be located well away from such sites.
The patterns of arsenic detection in groundwater from private wells did not show a relationship with the locations of the dips or footbaths. In the southern part of the Kaikoura plain, a number of wells yielded groundwater samples with elevated arsenic concentrations, but the patterns of arsenic in groundwater were not consistent with expected plumes of contamination from these sites. For example, arsenic was found in samples up-gradient of identified dip sites, indicating a source other than the dip. In other areas, where shallow wells were located close to dip or footbath sites, no arsenic was found in the groundwater from these wells yet arsenic was detected in groundwater from deeper wells in the area. It is likely the source of arsenic found in the groundwater from private wells is predominantly natural and originating from the greywacke rocks of the Southern Alps, tertiary coal measures and/or discrete and intermittent discharges from hydrothermal springs. These naturally occurring sources of elevated arsenic in groundwater occur along coastal areas such as Woodend, Waikuku and southern Christchurch, where marine sediments are inter-fingered with gravel strata.
In sum, the groundwater quality investigations revealed two separate issues regarding the presence of arsenic in groundwater. Results from sampling of the monitoring wells installed adjacent to the sheep dip/footbath sites indicated localised contamination of the shallow groundwater. The results of widespread sampling of groundwater from private wells located throughout the Kaikoura plain area indicate a naturally derived source of arsenic (Environment Canterbury 2003).
Coromandel residential case study
A number of former sheep-dip sites have been identified and investigated on the Coromandel Peninsula as a result of land-use changes from agricultural to residential. A significant case involved a subdivision that included a former sheep-dip operation, located on the northern coastline of the Coromandel Peninsula. Events that took place at this site will be expanded in more detail as an illustration of a typical process that has been used in dealing with old dip sites during subdivision.
The dip site operated for some 30 years as a sheep dip/shower facility from the mid- to late 1900s, using dieldrin and arsenic-based chemicals as dips and sprays. The land was situated on the coastal section of a 100-acre farm block, which was intended to be subdivided to create a number of smaller sea-side lifestyle/residential blocks. The landowner was directly linked to both the dipping activity and the planned subdivision, and was required by Environment Waikato to undertake an investigation and assessment of potential pesticide contamination on one lot in the top 1 m of soil.
The site investigation process involved an initial screening for organochlorine, organophosphate, organonitrogen and arsenical compounds. The screening process used a composite sampling method aimed at reducing costs and maximising the coverage of the sampling programme. The composite soil analyses broadly defined an area of surface soil dieldrin contamination surrounding the former dip facility. Subsequent characterisation of the soil contamination applying a site-specific sampling method more accurately defined the nature and extent of the residual chemicals. Dieldrin was present in high concentrations (up to 5 mg/kg) in only two samples; aldrin (at a maximum concentration of 0.005 mg/kg) and arsenic were detected only in low-level background concentrations.
The majority of the contamination was limited to the surface layer (0−0.5 m below ground level), with some low-level concentrations (decreasing with depth) between 0.5 and 1 m below ground level. Soils at the site were of a silty clay nature, and no groundwater contamination had resulted from the historical practices. Guideline criteria used to assess the site were based on the proposed residential land use.
Site remediation was carried out in April 2000 using the "dig and dump" approach: excavation and removal to landfill. A total of 125 tonnes of topsoil, sand and concrete were carted to Redvale Landfill in Auckland for disposal. The soils were removed from areas representing the location of the former dip/spray facility, the associated draining platform and the sump. Verification samples were collected after completion of the remedial work and it was recommended that the site be defined as suitable for the proposed residential land use. In late 2000 Environment Waikato confirmed that the lot was "remediated - suitable for residential land-use" on its Selected Land Use database.
Dip Road - case study of a sheep dip in public ownership
WaiPAC was commissioned by the Ministry for the Environment to research this case study as an example of a sheep-dip site in public ownership. It focuses on the various problems local authorities were faced with during the site investigation and remediation phases.
The Kaeo area was apparently the first land in New Zealand to be surveyed into allotments and there is a suggestion that it contains the oldest sheep dip in the country. The original homestead associated with the immediate area was built in 1836.
The council built a communal sheep dip and an animal pound on its property to ensure "dirty" sheep didn't cross the county boundary. These facilities were on State Highway One, which was eventually realigned and the annexed road became formally named Dip Road. There are suggestions that the dip continued operation until 1965. The site is owned by the Far North District Council (FNDC). No council records about the council-operated dip and animal pound have survived. The Dip Road property is about three-quarters of an acre of vacant land and the communal dip was located at the lower end of the section running parallel, and immediately next, to the boundary. The dip was filled from a water tank located on the elevated side of the section. The discharge pipe was located under the boundary fence between the pound and neighbouring private land, and spent dipping fluids and run-off from the dip were discharged down-gradient onto the neighbour's pasture, across swampy land and potentially into a farm drain.
Process of dip-site identification
The down-gradient neighbour negotiated to purchase the council land in 1999, but a visiting family friend (who worked for Waikato District Council) alerted him to the potential for assuming liability of a potentially contaminated site. FNDC requested Northland Regional Council (NRC) to undertake a site assessment of potential risks and a preliminary site investigation was initiated in November 1999.
Arsenic was found in excess of guideline values in soil (30 mg/kg) immediately adjacent to the dip (highest at 98 mg/kg). Occasional run-off and discharge of arsenic to watercourses was also identified as a potential hazard, as was contaminated fluid within the dip structure (only tested for arsenic), which exceeded both the potable water limits and the livestock drinking standards. Organochlorines were not identified (only one initial soil sample was obtained from beside the dip and assumptions made that no further testing for organochlorines was necessary thereafter). NRC's recommendation to FNDC was: (1) "That from a health and safety perspective, the dip be demolished, infilled and levelled to avoid any accidental misadventure" and (2) " that further sampling be undertaken away from the dip to determine the extent of any further on-site contamination".
A further assessment was undertaken by NRC, which found "elevated levels of arsenic up to 5 m away (ie, in the neighbour's property) from close to the dip − but beyond this distance the levels of contamination were low to moderate and within acceptable limits." NRC also recommended that arsenic-contaminated soil be removed from the site and disposed of in the proper manner at an approved landfill.
In February 2002, remedial work was undertaken. Some soil was removed to landfill (18 m3), and the dip structure - the drip pad and upper dip walls − were buried in the dip trench. The dip liquids were disposed of by "punching a hole in the base of the trench to allow drainage". Validation sampling was carried out in March 2002 and further removal of two hot spots on the neighbour's land was undertaken. Vertical mixing of the surrounding soil ("deep discing to a minimum of 300 mm") was used, and clean backfill material was imported. Further vertical mixing well out into the neighbour's paddock followed when results of validation sampling proved unsatisfactory (one sample at 48 mg/kg and one at 103 mg/kg). On 15 May 2002, the remediation consultants stated that the site(s) "was suitable for agricultural or future residential use". In October 2002 FNDC requested NRC to remove both properties from their Register of Contaminated Sites. In response, NRC proposed to remove the dip site (presumably encompassing both properties) from its Selected Land Use Registry as a category V site (verified history of hazardous activity or industry) to a category V2 site (managed remediated).
FNDC, in a letter dated 28 May 2003, stated "Council has paid out the sum of $18,627.35 for the remediation and rehabilitation of the Dip Road property over the past three years". It is unclear which "property" the Council is referring to (either the pound or the neighbour, or both), but one assumes, in the absence of any staff now available who know anything about the FNDC's involvement, that the cost relates to contamination on both lots. The breakdown of the remediation is as follows: NRC testing ($600.00), consultants ($16,671.85 and $915.50), fencing contractor (new boundary fence $250.00) and status check ($290.00). There do not appear to be any FNDC financial charges for its oversight and time spent on the project.
Key problems in the process
(a) Property values
The neighbour felt that his property "saleability" had been seriously constrained as a result of the FNDC's predecessor(s) expelling spent dip fluid over the boundary fence over many decades, and that in fact "he was unable to even sell his property". As a response to this allegation, a valuer assessed the likely implications of reduced property valuation. He was able to demonstrate that there was little or no impairment in saleability value in this particular circumstance as a result of the Council's remedial actions. This, therefore, became a non-issue in terms of getting site closure and resolving the payment of outstanding rates paid "under duress" - a related dispute in which the neighbour sought a reduction in rates due to impinged property values.
(b) Site clean-up
The completion of the remedial action pivots on the assurances of the consultants who supervised the site investigation and clean-up. The neighbour had some scepticism about the effectiveness and completeness of the remediation, as the preliminary assessment of the dip area showed markedly different soil residue levels to the later report. Some of the methods undertaken during the remediation could be debatable in the light of current knowledge. If the clean-up process was flawed, then any assurances by Council become open to future challenge.
(c) Site rehabilitation
There was some frustration evident by the landowner that the completion of the earthworks and associated minor restoration work and re-grassing had not been done. There was also some ongoing confusion as to the location of the original boundary fence (separating the dip outfall and the neighbouring land) versus its true legal position, and the erection of the replacement boundary fence in the incorrect position.
(d) Site closure
The affected neighbour appeared to find the "lack of sign-off" by Council in late 2002 as a significant source of irritation. To achieve closure in this matter, it was suggested that FNDC and NRC write to the neighbour and issue a statement to the effect that "the FNDC consider the risk of contamination has been mitigated and the site(s) is now suitable for continued agricultural use".