Phytomanagement of the Kopu wood-waste pile
New Zealand has 1.6 million ha of Pinus radiata plantations for pulp and timber production. Most timber products are treated with biocides to prevent decay. In the past, pentachlorophenol (PCP) and boron have been used to treat timber. Nowadays copper-chromium-arsenic (CCA) is the treatment of choice. These biocides contaminate the soils near treatment sites and wood-waste disposal sites and pose a risk to ground and surface waters through contaminant leaching. Here we outline the use of phytoremediation to mitigate the environmental risk associated with a timber-industry waste site.
The Kopu timber-waste pile is located at the base of the Coromandel peninsula, North Island, New Zealand (37.2o S, 175.6 o E). The pile has a surface area of 3.6 ha and an average depth of 15 m. Over a 30-year period, from 1966, sawdust and yard-scrapings from timber milling in the region were dumped on the pile. Land around the pile has been engineered so that no surface or ground water enters the pile, and all leachate resulting from rainfall is collected in a small holding pond at the foot of the pile. In the past, vegetation has failed to establish and evaporation from the surface of the pile has been negligible, even in the summer months. This was demonstrated by the presence of saturated material at depths as shallow as 20 mm.
Leachate resulting from the annual rainfall of 1135 mm, as measured at a nearby meteorological station at Thames, regularly caused the holding pond to overflow and enter a local stream. This overflow elevated boron concentrations in the stream to levels that were in excess of 1.4 mg L-1, the New Zealand Drinking Water Standard (NZDWS), especially in the summer months when stream flow was low. In response to these breaches, the local environmental authority placed an order on the forestry company responsible for the site that the problem be remedied.
In July 2000, a one ha trial was established on the Kopu site using 10 poplar and willow clones as well as two species of Eucalyptus. Two Populus deltoides hybrid clones were then chosen as the best candidates for phytoremediation based on survival, biomass production and B uptake. The following year, the remainder of the pile was planted in these two clones at a density of 7000 trees ha-1. Fertilisers were periodically added to the trees and a pump was installed near the holding pond at the foot of the pile for irrigation during the summer months.
We used the Phyto-DSS to calculate the monthly water balance of the pile. Model calculations of leaching are shown in Fig. 2. As expected for such a high rainfall site, the bare pile leaches a considerable amount of drainage water through all months of the year. The impact of trees is to reduce the drainage of water during the summer months when the trees are fully leafed and transpiring at their maximum. The summer months are of greatest concern for contamination of the local waterways because stream flows are lower and there is less dilution of the contaminants. The leaching that occurs during the winter months can be irrigated onto the trees in times of drought during the summer, or alternatively, released into a nearby stream at times of high flow when the risk of exceeding the New Zealand Drinking Water Standard (NZDWS) is minimal.
Poplar leaves sampled from the sawdust pile contained Cu and Cr concentrations that were on average 6.6 and 4.9 mg kg-1 dry mass respectively. Arsenic concentrations were below detection limits (1 mg kg-1). At the end of the growing season, the average leaf-B concentration was nearly 700 mg kg-1 on a dry matter basis, over 28 times higher than the B concentration in the sawdust (40 mg kg-1 dry matter). Bañuelos et al. (1999) has previously reported this B accumulation trait in poplars.
The results indicate that in addition to controlling leaching at the site, poplars may also be able to reduce the B loading by phytoextraction. Unless the trees are harvested, most of the B is returned to the sawdust via leaf-fall. Harvested material could, however, be used as an organic B supplement to trees in orchards that are B-deficient in other parts of the country. The concentrations of other heavy metals in the leaves are unlikely to cause further environmental problems.
The cost of phytoremediation at Kopu is estimated to be NZ$200,000 including a site maintenance plan over 5 years. Half of this total cost was taken up as site assessment, involving scientist time to conduct the plant trial and chemical analysis. The alternative cost of capping the site was estimated by the local environmental authority to be over NZ$ 1.2 M. Capping would also require ongoing maintenance to ensure its integrity.
