Environmental Assessment of the Effects of Leachate Irrigation and Seepage from the Paokahu Landfill, Gisborne, New Zealand.

Abstract

Paokahu landfill operated near Gisborne from 1977 to 2002. About 1 million cubic metres of mixed domestic and industrial (predominantly food processing) wastes were disposed of at the site. The Paokahu landfill is located on the Poverty Bay Flats on low lying flat ground, which was originally the base of a tidal lagoon. A 300m wide band of costal dunes separates the landfill from the Poverty Bay coast. The landfill covers an area of approximately 20 Ha and is unlined but fully capped. Leachate is collected in a cut-off drain which surrounds three quarters of the landfill. Disposal of the leachate is by spray irrigation onto the landfill cap with 13,000 - 15,000 m3 of leachate irrigated annually. The site is currently used for grazing sheep.

The overall aim of this study was to improve our understanding of the effect that Paokahu landfill is having on the environment and to determine if the current management practices are sustainable. Specific objectives were to collate and review all the groundwater and leachate monitoring data held by Gisborne District Council to determine if the landfill was affecting the local groundwater and to investigate the effect of leachate irrigation on the landfill cap's soil and vegetation.

The groundwater monitoring data showed the local groundwater was generally affected by salt water intrusions giving the groundwater high anion and cation concentrations. There was no evidence of a leachate plume originating from the landfill and no conclusive evidence of leachate contamination in any of the groundwater monitoring bores.

The leachate had a high electrical conductivity (mean = 9350 Scm-1) and high soluble salts content (mean values, Na = 845 gm-3, K= 496 gm-3, Ca = 240 gm-3, Mg = 127 gm-3, Cl =1346 gm-3). Heavy metals were present in low concentrations and Semi Volatile Organic Compounds were no longer present in the leachate. Cation and anion concentrations were generally higher in the groundwater than in the leachate. The main risk to groundwater quality from leachate contamination was from ammonical nitrogen and nitrate.

The leachate irrigation had caused an increase in soil cation concentrations particularly Na in the landfill cap. However, the soil Exchangeable Sodium Percentage of 1.5 - 2.9, leachate Sodium Adsorption Ratio (6.5) and Electrical Conductivity of (4317 S cm-1) and silty soil material of the landfill cap indicated that soil swelling and dispersion was unlikely to occur in irrigated areas of the landfill cap. Dispersion index testing supported this conclusion with no significant increase in aggregate dispersion under the irrigated areas of the landfill compared to the non-irrigated areas. Soil metal concentrations were low and there was no significant difference in soil heavy metal concentrations between the irrigated and non-irrigated areas of the landfill cap. Leachate Mn concentrations (mean = 1.39 g m-3) were high enough to be potentially toxic to plants, but the soil pH (7.4) and Ca (46.7 me/100g) content mean that Mn toxicity is unlikely to occur. Leachate irrigation appears sustainable under current conditions.