3.0 Background

3.1 Site Description

The approximately five hectare site is located in the settlement of Mapua, 15 km west of Nelson (Figure 1). Mapua is located adjacent to the northern end of Waimea Inlet, a large tidal estuary between Rabbit Island and the mainland. The site is located at the base of a small peninsula known as Grossi Point which is bounded to the west by the Waimea Inlet and to the east by the Mapua Channel. The channel leads to the main entrance of Waimea Inlet, approximately 1 km to the north‑east. Aranui Road forms the northern site boundary and Tahi Street bisects the site from north to south. Tahi Street itself was not included in the remediation site.

The overall remediation site is made up of nine separate components (see Figure 1):

• a former landfill area (FCC Landfill) – approximately 0.6 hectares;

• the western portion of the former FCC site (FCC West) – 1.7 hectares;

• the eastern portion of the former FCC site, on the other side of Tahi Street (FCC East) – 1.3 hectares;

• two marine foreshore areas - approximately 0.5 hectares in total; and

• four residential properties to the south of the FCC sites. These include 13, 15, 18 and 20 Tahi Street and make up a total area of approximately 0.8 hectares.

The adjacent site uses are:

• residential to the south along the peninsula;

• residential and undeveloped paddocks to the west; and

• a mixture of commercial and residential to the north beyond Aranui Road.

The former FCC site is currently vacant and almost entirely grassed. The exception to this is an area of compact gravel on FCC East adjacent to Tahi Street. The site is a mixture of flat areas and gentle contours, with the highest points built up about two metres above the general elevation of the surrounding land. An open drain (‘the creek’) runs along the north-west edge of FCC Landfill.

The beach along the southern boundary of FCC Landfill consists of coarse gravel, with tidal mudflats beyond. An engineered embankment covered with cobbles and boulders forms the eastern boundary of FCC East, adjacent to the Mapua Channel. Beyond the embankment the beach exposed at low tide consists of a mixture of gravel, sand and mud, sloping gently towards the adjacent channel.

The tidal range at Mapua is up to 3.3 m and there are strong currents close to the eastern foreshore during the ebb and flow of tides. Measurements show tidal velocities of up to 1.55 m/s at the edge and 1.8 m/s in the centre of Mapua Channel. There is considerably less tidal energy in the mudflats to the south of FCC Landfill as it is protected from the main tidal flows by the position of Grossi Point.

3.2 Site History

3.2.1 General

FCC operated an agrichemical formulation plant on FCC East and West from 1932 until 1988, producing pesticides, herbicides and fungicides that were used throughout the country. The north-eastern portion of FCC East was operated by a subsidiary company, originally known as Lime and Marble Limited and later as Mintech Ltd. The Mintech site was generally used for processing non-toxic minerals but also included the FCC micronising plant² and some biocide preparation. Facilities used for agrichemical formulation and storage were operated on both FCC East and West.

From the 1950s, a number of areas were either in-filled or reclaimed, including: low lying areas of FCC East; the area now known as FCC Landfill, reclaimed from the Waimea Inlet; and the eastern portions of FCC East, reclaimed from the Mapua Channel. The fill material used contained waste material from site operations.

FCC ceased operations in 1988 and by 1996 TDC had either inherited or acquired the FCC portions of the site i.e. FCC Landfill, FCC West and FCC East. FCC Landfill was inherited first, in the early 1990s. In May 1992, TDC installed a clay cut-off wall along the southern edge of FCC Landfill to reduce leachate migration into the Waimea Inlet. From the early 1990s onwards, the site was the subject of a number of environmental investigations and assessments. It was clear from the investigation results that some form of remediation or management of residual contamination at the site was required. Elevated contaminant concentrations were detected in soil on and adjacent to the site, groundwater and in nearby marine sediments. The major contaminants of concern which drove the need for remediation were organochlorine pesticides. Other contaminants included heavy metals, organonitrogen pesticides, organophosphorous pesticides, petroleum hydrocarbons, acid herbicides and elemental sulphur.

The peak soil concentrations were typically found in the vicinity of historical process areas. Marine sediments appear to have been contaminated from site runoff and drainage, including from the landfill, to the nearby estuary and Mapua Channel – see next section.

A decision was made to remediate the site after initial plans for capping the site were set aside. Soil treatment trials to select an appropriate technology were carried out in 1999 – 2000. Resource consents for the remediation were granted in November 2003. The remediation works commenced in October 2004, after a proof of performance stage, and were completed in early 2008. The consents expired in November 2007, four years after they were granted. Since the remediation was completed, the site has remained vacant.

3.2.2 Historic Site Drainage

As noted in the previous section, marine sediments in the adjacent Waimea Inlet and Mapua Channel have been contaminated most likely from site runoff. Stormwater from the site historically discharged as follows:

• a portion of FCC East discharged to an outfall close to the north-east corner of the site;

• the remainder of FCC East and a portion of FCC West discharged to a cylindrical surge chamber located eastern foreshore, approximately halfway along the bund. An outlet from the surge chamber discharged into Mapua Channel below low tide level;

• the majority of FCC West drained to a pipe which discharged to Waimea Inlet; and

• leachate from the landfill was able to drain directly to the estuary.

3.3 Intended Site Uses

The intended site uses at the time of consent were as follows:

• FCC Landfill – recreational / open space. It is envisaged as a green area with benches, plantings and walkways rather than fields to be used for sports;

• FCC West – residential;

• FCC East – commercial and open space;

• the marine sediments adjacent to the site to support an aquatic ecosystem; and

• the four residential properties on Tahi Street continuing in residential use.

3.4 Remediation Overview

This section provides a brief overview of the remediation works. More detail on the various aspects of the works is described as required in the audit sections of this report.

3.4.1 Remediation Management Structure

Over the period of the remediation, a number of parties were involved in a range of roles. This section briefly summarises these roles.

TDC is the site owner and also the consenting authority. MfE initially provided funding for the proposed remediation, along with TDC. In 2001, TDC awarded the contract for the remediation to a joint bid comprising Thiess Services Pty Limited (Thiess) and Environmental Decontamination Limited (EDL). Thiess is a specialist remediation contractor based in Australia. EDL was the supplier of the mechano-chemical dehalogenation (MCD) remediation technology chosen for the reduction of the organochlorine contamination.

Thiess withdrew from the project mid-2004. At that point, MfE took over the management of the remediation and contracted EDL to complete the remediation works. MfE subsequently contracted Effective Management Systems (EMS) as Site Manager. MWH New Zealand Limited (MWH) was appointed by MfE as Site Engineer for the remediation project. MWH subsequently retained EMS as Engineer’s Representative. The main earthworks contractor was initially Hiway Stabilisers Environmental Limited (HSE). However, they were replaced by Taylor’s Contracting Limited (TCL) in October 2006.

Separately, Peter Nadebaum of GHD Pty Ltd (GHD) was appointed for the duration of the remediation works by MfE and TDC as the Site Auditor to provide independent technical advice. As required by a consent condition, a peer review panel of experts was also appointed by TDC. The panel was to meet regularly and advise on progress, monitoring results and other issues as they arose. This panel included the Site Auditor. The appointments of the auditor and peer review panel lapsed with the expiry of the consents in November 2007.

3.4.2 Remediation Preliminaries

As part of the resource consent application for the remediation, a remediation action plan (RAP) was written (Thiess, 2004) and an assessment of potential environmental effects completed (T&T, 2003). A second version of the RAP (MfE, 2005a) and a summary of changes to the RAP (MfE, 2007) were produced during the remediation. The RAP as a whole was never approved by TDC, the consenting authority, despite being condition of the remediation consent. While significant portions of the document were approved by TDC and the peer review panel during the remediation works, overall approval was not given as some sections of the RAP were never finalised.

A total of seven resource consents were granted for the proposed remediation in October 2003. Included as a condition of consent were a set of soil acceptance criteria (SAC). The SAC provide target criteria for soil in residential, open space and commercial areas, and for marine sediments.

The SAC were derived taking into account a number of potential exposure pathways. For the key contaminants of concern (CoCs), the controlling pathway was generally not risk to human health i.e. if only human health was considered, the SAC would have been higher in most cases (Egis, 2001). For example, the residential criterion for one of the key CoCs, DDT (as the sum of DDT and it derivatives), is controlled by the risk to the marine environment of potential discharges of sediment from the site. For the equivalent commercial criteria, the controlling pathway is the effect on groundwater quality, and the risk that the discharge of the groundwater poses to the marine environment after dilution.

3.4.3 Soil Remediation

For the purposes of the remediation, the entire FCC site was divided into a 15 m grid of cells, each with a unique identification. Each cell was divided vertically into a series of layers. The layers were at 0.5 m intervals to 2 m depth and 1 m intervals below that point. This basic reference system was used to label and control the soil remediation.

For practical and logistical reasons, work was carried out in stages involving groups of cells. These groups of cells were called subgrades and were labelled sequentially. Figures 2 and 3 show subgrades for the FCC site overlaid on the main cell grid.

The soil remediation on the FCC sites involved initially characterising soil across the site by laboratory testing and visual assessment into the various categories relating to the SAC i.e. whether the material was compliant with the residential, open space or commercial criteria, or whether it required treatment to make it suitable for one of these categories. Pre-remediation sample results were used as an initial guide, backed up by more detailed sampling undertaken during the remediation. Decisions were then made on a cell by cell basis whether the material could be retained in-situ, used elsewhere on-site, treated and re-used on-site or had to be taken off-site for disposal. Soil was placed in temporary stockpiles relating to each category.

As noted above, the key contaminants of concern were considered to be organochlorine pesticides (OCP). Where OCP concentrations exceeded the commercial SAC and the soil was physically suitable for treatment, the soil was treated to reduce the concentrations below the commercial target criteria. The MCD soil remediation process involves the de-chlorination of organic compounds by the input of mechanical energy to the soil matrix in a ball mill. Additives such as iron, copper and nitrogen compounds were used in the process. Prior to treatment, the soil was screened to remove coarse fractions which were considered to be relatively free of contamination. These were generally recombined with the treated fine fractions prior to reinstatement.

Material removed from the excavation was characterised by laboratory testing, typically by pre-excavation testing of the in-situ soil. As soil was excavated, the extents of the excavation were tested to check for compliance with the relevant SAC. The excavations were then backfilled with material that was considered suitable for the proposed site use for that area i.e. based on the laboratory testing and geotechnical requirements. For example, FCC West was backfilled with soil that complied with the residential SAC. Material was imported to make up shortfalls in certain soil categories. The entire site was finally covered with at least 0.5m of residential quality soil (including the non-residential FCC Landfill and FCC East). The top 0.15 m of this layer consisted wholly of imported topsoil.

The approximate volumes of the various material categories generated or imported during the remediation were as follows (MWH, 2008):

• residential quality soil excavated and reused on site – 18,200 m3;

• marine sediments excavated from the western and eastern foreshores and used as backfill on various parts of the site – 4,800 m3;

• commercial quality soil excavated from site and used as backfill in FCC Landfill and FCC East – 27,500 m3;

• soil treated using the MCD process and used as backfill in FCC Landfill and FCC East – 10,500 m3;

• crushed concrete, buried on site – 2,000 m3;

• imported gravels for backfill of marine sediment excavations – 5,000 m3;

• imported residential soil – 5,800 m3;

• imported topsoil – 5,300 m3; and

• imported clay – 1,000 m3.

As part of the treatment process, a number of reagents were added by EDL to the soil before it passed through the MCD plant. The reagents were made up variously of sand, diammonium phosphate, copper/iron based compounds and urea (PDP, 2007). It would appear that the discharge of the reagents to the site was not specifically covered in the consents granted for the remediation works.

3.4.4 Marine Sediment Remediation

Two areas of foreshore adjacent to the FCC site were included in the remediation (see Figure 1):

• the tidal beach of Mapua Channel, to the east of FCC East; and

• the tidal mudflats in the Waimea Inlet to the south of FCC Landfill, including a tidal channel that crossed the mudflats (the “swale”) . Also included was a section of the tidal creek along the north-west edge of FCC Landfill.

The extent of contamination at these locations was broadly defined by previous investigation results and additional sampling during the remediation works. Based on the pre-remediation results, a surface layer of contaminated sediment was excavated down to the low tide contour in FCC East. In the west, the creek (for most its length adjacent to the site), part of the foreshore and part of the tidal swale were excavated and backfilled. The removal of contaminated sediments was completed in a series of cells, each backfilled with imported gravels after validation sampling from the base of the excavation. The resource consent required that excavated cells were sampled and backfilled within one tide. Consequently, the excavations were backfilled before the validation test results were received.

3.4.5 Groundwater Monitoring

A number of groundwater monitoring wells along the southern and eastern boundary of the FCC site were monitored prior to and during the remediation as a condition of resource consent (see Figure 1). In addition, a number of private groundwater bores located on residential properties to the south were also monitored. The samples collected from the wells were tested for a range of analytes to cover the potential contaminants of concern associated with the site.