Hydrogeologic investigations of the Taylor Pass landfill, Blenheim, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
An engineering geological and hydrogeological investigation has been carried out in the Taylor Pass Landfill area, some 2.5km south-south-west from the township of Blenheim. The primary aims of the study have been to identify the nature and extent of any leachate plume that is being generated from the now-closed landfill site, and to specifically determine the relative contributions (if any) to leachate generation from surface infiltration and groundwater seepage inflows. Investigation methods have included site mapping and trenching, TEM and electrical resistivity surveys, installation of a monitoring network of bores to depths up to 30m, and routine groundwater chemistry sampling and analysis. Leachate plume delineation has relied primarily on water chemistry data constrained by the hydrogeological model determined for the site, as geophysical techniques proved unsatisfactory due to background noise.
The Taylor Pass Landfill is located on the surface of the Taylor Fan, mostly within a former aggregate quarry, and was operational from 1976 to 1995 (although some offal waste is still being disposed of at this site pending remediation of the new Blenheim landfill at Blue Gums). The site hydrogeology is complex, with fan gravels, sands, silts and clays of the Rapaura Formation (<14,000 years BP) underlying the landfill to estimated depths of 15-25m, and a complex interplay of channel migration and overbank deposition being indicated by the available borehole logs and site mapping. Hydraulic conductivities and transmissivities range respectively from 3 x 10⁻⁴ to 8 x 10⁻⁴ m/s and 530 to 1400, with the higher values relating to the channel deposits within the various units of a prograding and migrating alluvial fan. Capping materials for the landfill are variable, although most are loess silts from the nearby Wither Hills, and laboratory-determined permeabilities are typically less than 10⁻⁷ m/s with some materials up to m/s.
Monitoring bores both up-gradient and down-gradient from the Taylor Pass Landfill reveal a relatively saline natural groundwater associated with the low summer flows and high evaporation rates of the southern part of the Wairau Valley. Groundwaters of the Wairau Aquifer occur some 1-1.5km down-gradient of the Landfill site, and are clearly of much higher quality and entirely suited to domestic use, with low ion concentrations, and low to undetectable metal levels. Leachate has been identified by characteristic high bicarbonate, ammonia, manganese, potassium, and calcium, and marginally elevated arsenic, sodium, chloride and sulphate, but interpretation of the plume is complicated by the presence of an older landfill at Brayshaw Park some 1.5m down-gradient from the Taylor Pass Landfill and the fact that there is only limited data available from deeper (>25m) monitoring wells.
It is concluded that the leachate plume from the TP Landfill extends as far north as the Wairau Aquifer, where the substantially higher flows rapidly attenuate the plume without impacting on down-gradient users. A zone of mixing of Taylor Fan and Wairau-derived groundwaters occurs for some 500m up-gradient from the mapped boundary near New Renwick and Alabama Roads, and is considered to account for the apparent plume termination in this vicinity. Additional capping of the Taylor Pass Landfill is not thought to be justified given the interpreted extent of the leachate plume and its rapid attenuation, but further tree-planting and cessation of offal disposal is strongly recommended to reduce further the extent of leachate generation. The drilling of at least three monitoring bores through the centre of the Landfill is also recommended given the absence of reliable data on groundwater levels and fluctuations within the area of refuse.