Morgenstern, U.; Moreau, M.; Rodricks, R. 2026 Assessment of SF6 and CFC contamination plumes in the Heretaunga Plains aquifers: extent, timing, potential sources, water quality and impact on drinking-water supply wells. Lower Hutt, NZ: Earth Sciences New Zealand. GNS Science report 2025/42. 51 p.; doi: 10.21420/XSH7-2K36
Abstract
Groundwater monitoring of drinking-water supply wells in the Heretaunga Plains Aquifer revealed two plumes of sulphur hexafluoride (SF6) and increasing chlorofluorocarbon (CFC-12) concentrations in the central confined aquifer near the coast. These high SF6 and CFC concentrations can only be explained by anthropogenic sources. SF6 and CFCs are human-made gases. CFCs were used historically in a wide range of industrial and commercial applications, but production and use of CFCs was phased out globally in the 1980s due to their ozone-depleting effects. Therefore, currently increasing concentrations in groundwater point to historic sources. SF6 is still used, for example, as an arc quencher in high-voltage switch gear, therefore elevated SF6 concentrations in groundwater may originate from current and/or historic sources. Equipment containing CFC-based refrigerants, spray-can propellants or solvents is commonly found in historic municipal landfills, and the CFC concentrations in the aquifer are consistent with quantities of CFCs that could be expected to leach from a landfill. This indicates that such a source is likely responsible for the increasing CFC concentrations in the Heretaunga Plains Aquifer. The historic Roy’s Hill Landfill is located immediately up-gradient of the part of the aquifer with increased CFC concentrations. This unlined landfill, which is within the recharge area of the aquifer, was closed in 1988 due to concerns that leachate could contaminate the groundwater in the aquifer. High CFC concentrations down-gradient of this landfill imply that this landfill is leaching into the aquifer. This interpretation is supported by a consistent suite of additional water-quality indicators, including elevated ammoniacal nitrogen, electrical conductivity and chloride, together with increased concentrations of boron, sodium and potassium in down-gradient monitoring bores. Collectively, these parameters form a characteristic leachate signature that indicates the Roy’s Hill Landfill to be a persistent source of contamination to the Heretaunga Plains Aquifer since at least the 1970s. While the elevated SF6 and CFC concentrations detected in the Heretaunga Plains Aquifer are not a health concern for drinking-water supplies, other substances co-leaching from historic landfills and industrial sites may pose a risk. Therefore, better understanding of the extent and sources of these leachate plumes was required, together with screening for potential contaminants that could leach into the groundwater from such sites. To address this, Earth Science New Zealand carried out an urgent study to better delineate the spatial extent of the contamination plumes and identify their potential sources. Then, in partnership with Hawke’s Bay Regional Council (HBRC), a fast-response targeted groundwater-sampling campaign was designed to (i) track how CFC and SF6 plumes in the Heretaunga Plains Aquifer have evolved over recent years, (ii) improve data coverage in the aquifer’s recharge zone – considered the most likely source of the contamination – and (iii) investigate the presence of any harmful co-contaminants that may be associated with the plumes. A combination of source tracer, groundwater age and redox data was used to improve understanding of the sources, timing and microbial attenuation of the SF6 and CFC plumes in the Heretaunga Plains Aquifer. This new data confirmed and refined conceptual understanding of groundwater flow through this aquifer, previously developed using these tracers. The two SF6 plumes are well understood through long-term monitoring of drinking-water supply wells. These are unlikely to be related to municipal landfills because CFCs are not significantly elevated in these groundwaters. One of these SF6 plumes, east of Roy’s Hill, was transient. After concentrations peaked around 2018, these have now returned to ambient concentrations. The other SF6 plume, at the northern margin of the Heretaunga Plains Aquifer, has been persistent for at least 10 years. The sources of these SF6 plumes are unidentified. Up-gradient of the area near the coast, where CFC-12 concentrations are increasing, additional data revealed extremely high CFC-12 concentrations, starting at the historic Roy’s Hill Landfill. These observations indicate that the increases observed near the coast are part of a CFC-12 plume originating from this landfill, which is only now arriving at this part of the aquifer. CFC-12 concentrations at the coastal wells are therefore expected to rise significantly in the coming years. Slight increases in hydrochemical parameters recorded through environmental-monitoring programmes are consistent with an influence from the landfill in this area. To assess whether there are any harmful co-contaminants associated with this plume that could pose risks to drinking-water supplies, groundwater samples collected from 33 wells were analysed for major ions, nutrients and dissolved metals, and a further five wells were tested for volatile organic compounds and semi-volatile organic compounds. Despite some detections of slightly elevated concentrations, no elemental or compound exceedances of maximum acceptable values were found under the Water Services (Drinking Water Standards for New Zealand) Regulations 2022. While the plume leaching from the historic Roy’s Hill Landfill into the central Heretaunga Plains Aquifer may require additional monitoring, initial tests for harmful potential co-contaminants found no immediate risk to drinking-water supplies. The SF6 plumes identified at the northern margin of the Heretaunga Plains Aquifer and east of Roy’s Hill, despite their sources being unidentified, can also be considered to pose a low risk of harmful co-contaminants, as these plumes are unlikely to be associated with municipal landfills. While SF6 plumes have not been detected in New Zealand aquifers apart from the Heretaunga Plains Aquifer, high CFC concentrations have been found in drinking-water supply wells across New Zealand, with concentrations up to more than four times higher than the CFC-12 concentrations measured in the Roy’s Hill Landfill monitoring wells. The results from the Heretaunga Plains Aquifer strongly indicate that municipal landfills are the likely source of high CFC concentrations in groundwater. Therefore, it is likely that large fractions of water that have passed through landfills are in these drinking-water supplies. Such wells may require investigation for harmful contaminants potentially co-leaching into drinking-water supply aquifers. CFC-12 appears to be a sensitive tracer to detect contributions of groundwater that have passed through historic municipal landfills. (auths)
