The Paparua landfill: Hydrogeological, geophysical and hydrogeochemical investigations of groundwater contamination by Leachate, Christchurch, New Zealand

摘要

The impact of leachate generation and migration from unlined municipal solid waste landfills on the Canterbury groundwater resource, was investigated at the largest site, the Paparua County Council Landfill. The 8 ha. landfill is located centrally within the western unconfined zone of the extensive 500 m thick glaciofluvial aquifer system. This unconfined area recharges deeper confined aquifers in the east, which underlie Christchurch City (pop. 300000) providing its sole drinking-water supply. Between its conversion from a gravel pit in 1973 and closure in July 1978, the site periodically operated as a wet landfill and rates a high pollution potential classification (DRASTIC Index = 187, Le Grand +14G). However, leachate is confined to a 200 m x 200 m area in the northeast of the site, between the depths of 6 m and 9 m, by an underlying silty sand aquitard. Landfill stratigraphy was determined from Wenner and Schlumberger soundings. Best-fit sounding models indicated leachate with a 2.3 to 6.7 Ωm layer, underlying unsaturated refuse (19.6 to 30.1 Ωm), and overlying silty sand (50 Ωm to 77 Ωm) and sandy gravels (>300 Ωm). Resistivity profiling at a= 15 m and a=30 m spacings, revealed a low resistivity zone restricted both laterally and vertically to the northeast comer of the site. This zone, taken to represent leachate, exhibited slight southeast migration on the deeper penetrating profile. Down-hole nuclear geophysical logging did not detect the contaminant zone, but the landfill boundary was identified on a seismic traverse by a lack of refraction of the critical ray over refuse infilled land. Beneath the 6 m deep landfill, a 1.5 to 8 m thick silty sand was detected in 4 lithologically logged wells installed at the site. The hydraulic conductivity of this unit at 2.9 x 10-9 to 3.2 X 10-7 m/s is significantly less than the 2.39 x 10-3 m/s of the underlying sediments. The silty sand therefore functions as an aquitard, producing a perched watertable in the overlying refuse and inhibiting downward leachate percolation as illustrated by resistivity pseudosections. In the deeper gravels and sandy gravels, point dilution tests showed that seepage velocities varied from 8.3 x 10-7 to 2.7 x 10-3 m/s and exhibited a strong relationship to uniformity coefficients of the sediments. Laterally continuous high velocity zones within deeper gravels could act as rapid dilution and transport zones for contaminants permeating through the aquitard. Resistivity profiling dictated the installation positions of 16 monitoring wells. The presence of leachate was confirmed with groundwater samples from the northeast corner of the site contaminated with up to 160 mg/l NH4-N, 0.11 mg/l Cu, 270 mg/l CI, 210 mg/l Na, 2.8 mg/l B, high levels of Ca, Mg and high chemical oxygen demands. In these samples, total dissolved solids ranged from 1238 to 1736 mg/l and total organic carbon ranged from 58.8 to 98.7 mg/I. Amines, ketones and terpenes which result from the anaerobic degradation of putrescible refuse were detected. Alkyl benzenes, S- and N- heterocyclic compounds, and xylenes, at 5 mg/m3 in one sample, were the only synthetic organic compounds found. In groundwater beneath the aquitard and downgradient of the northeast zone, these substances were below detection limits. The landfill presently poses no significant threat to the quality of the underlying groundwater resource. However, leachate confinement by fortuitous hydrogeologic factors at the Paparua County Council Landfill cannot be taken as indicative of leachate behaviour at other unlined landfills in Canterbury, nor should the present site conditions be regarded as invariant. This study illustrates the importance of a site-specific investigation in gauging leachate effects, demonstrating the value of an integrated approach using techniques previously unapplied to New Zealand landfills.