The impact of CCA-treated posts in vineyards on soil and ground water

Grapes in Marlborough are typically grown on a vertical shoot positioned trellis system (VSP). For this purpose Pinus radiata posts are treated with CCA, a mixture of copper (Cu), chromium (Cr) and arsenic (As), giving a wood concentration of 1,730, 3,020 and 2,410 mg/kg, respectively on a dry matter basis. The CCA levels around the posts in different soils were investigated and assessed for the potential leaching of CCA into ground water. An initial survey showed leaching of all three heavy metals from the treated posts into the soil surrounding the posts (0.2% of the total vineyard area) compared with the control, depending on vineyard age and soil type. The rate of movement out of the posts was calculated from posts placed in lysimeters. HortResearch's Soil Plant Atmosphere Model (SPASMO) was used to predict the leaching rate of CCA. For As, leaching was found to be 5 mg/post/month, with the Cr rate being about twice that. Further modelling revealed a steady plume of As moving downwards after about 200-300 years. However, long-term hydrogeological modelling showed that sufficient aquifer water flow prevented the accumulation of CCA in the ground water. The modelling approaches are discussed.