Setting standards for cleanfill

Advances in cleanfill management at the Puketutu Island Quarry in Auckland have set the standard for similar operations around the country, writes GRANT HUBBARD.

Winstone Aggregates, the operators of the Puketutu Island Quarry and cleanfill operation, took out the Mimico Environmental Award at the AQA/IOQ conference this year for its cleanfill management on the island.

The quarry, located on an island in the Manukau Harbour near Mangere, presented Winstone with unique environmental challenges that included preventing salt water intrusion to the delicate fresh water aquifer on the island; cleaning up a contaminated sawmill and timber treatment site; and identifying historical/cultural sites and working in partnership with multiple iwi.

Quarrying began on Puketutu Island in the 1950s, along with construction of Auckland Airport and the city’s sewage treatment ponds. The quarry contains basalt rock with scoria cones, and produces high quality chip for the Auckland market. Winstone took over the quarry lease in 1988 and obtained new consents in 1997 to quarry below groundwater level, then backfill with clean fill from other sites.

The original 1993 fill management plan (FMP) has been progressively updated over 16 years to the point where it is now establishing new benchmarks for cleanfill operations around the country.

Quarrying below groundwater level has extended the life of a rock resource right on Auckland’s doorstep. And rather than just filling the hole left by quarrying, Winstone Aggregates is restoring the island’s original profile and contours, and rehabilitating quarried areas into pasture.

Studies into the environmental effects of cleanfilling showed that heavy metals in soils deposited on the island have not had adverse effects on the local environment. Threshold acceptance levels were set for all imported fill. Regular monitoring includes tests for leaching, electrical conductivity, chloride levels, a selection of drinking water parameters, arsenic levels and total hydrocarbons. A network of 32 boreholes has been drilled for monitoring purposes.

The central groundwater aquifer provides residents and guests on Puketutu Island with drinking water. Only fill containing low levels of contaminants is classified suitable for tipping above this aquifer. To maintain water quality, machines are not fuelled above the aquifer, ablutions and amenities have been relocated and water for industry is now taken from other sources, leaving the aquifer for potable purposes.

The FMP sets out fill acceptance criteria and procedures. Winstone’s staff carry out pre-acceptance tests to determine if fill is suitable, visit customer sites, and use XRF on-site testing to check contaminant levels in fill entering the site. A quarantine area is available for non-complying loads. XRF testing instantly identifies contaminant levels in a fill sample using x-rays to plot the unique spectrum of each element. The testers are handheld and provide real time results.

As Winstone staff extended the reach of quarrying operations, they agreed to clean up a sawmill site on the island contaminated with timber treatment chemical CCA (copper, chromium, arsenic). This allowed access to rock resource below the sawmill site.

Mildly contaminated material was excavated and sealed in a clay containment cell, while highly contaminated material was removed and trucked to a municipal waste site. Before the cleanup, groundwater in the area showed contaminant levels 6-25 times the relevant ANZEC criteria for the protection of marine ecosystems. Since the cleanup, groundwater contaminants have been drastically reduced.

Puketutu Island is one of the earliest Maori settlements in New Zealand, and Winstone Aggregate staff regularly liaises with a wide range of iwi. Historic sites are identified and protected from quarrying.

The Mimico Environmental Award came with a $5000 educational grant. Winstone Aggregates will use the grant to increase its contribution to Conservation Volunteers NZ which organises volunteers to carry out conservation projects throughout the country.