Groundwater quality state and trends in Tāmaki Makaurau / Auckland 2010-2019
Author:Courtney Foster, Kolt Johnson
Source:Auckland Council Research and Evaluation Unit, RIMU
Auckland Council regularly monitors groundwater quality (physical, chemical and microbiological parameters) within some of the region’s aquifers as part of its State of the Environment programme. The primary purpose of the groundwater quality monitoring programme is to regularly assess and report the current state and long-term trends for groundwater quality in aquifers across the region.
This reporting is driven by requirements under the Resource Management Act 1991 and the National Policy Statement for Freshwater Management 2020 (NPS-FM, 2020). Water quality is reported against all relevant New Zealand standards. Results relating to the Drinking Water Standards New Zealand do not comprise a compliance-level assessment but are provided for a comprehensive assessment of the current state of groundwater resources. The results from State of the Environment monitoring are used to inform management of potable drinking water supplies and rural productive land use activities, for evidencing the efficacy of urban stormwater and wastewater management, and to inform the management of the flow-on effects on ecosystem health of streams with high groundwater baseflow.
Sites in the Auckland Council groundwater quality monitoring network are grouped by aquifer type and primary water quality stressor. Urban shallow volcanic aquifers are predominantly affected by stormwater and wastewater, and aquifers in the Franklin (shallow volcanic and shallow and deep sedimentary aquifers) and Kumeu (shallow and deep sedimentary aquifers) areas are predominately affected by rural activities such as fertiliser leaching.
Shallow volcanic aquifers in the Franklin area are affected by high nitrate relating to horticultural land use. The National Policy Statement for Freshwater Management 2020 does not include a National Objectives Framework (NOF) for nitrate in groundwater, however the surface water NOF is relevant to aquifers which provide baseflow to streams. Six groundwater sites in the Franklin area had nitrate levels above the surface water NOF
grade D (‘Acute’), these sites are in shallow oxygenated volcanic aquifers that contribute high baseflow to nearby streams. Long-term trends (10 years, 2010-2019) for nitrate show very likely improving trends at four of these sites and likely to very likely degrading trends at the other two sites. Three further sites in the Franklin area, representing deeper aquifers with less oxygenated waters, have no signs of nitrate contamination.
Two sites in the Kumeu West Waitematā aquifer had generally good groundwater quality. Results showed that groundwater in this aquifer was below guideline values for human consumption and did not pose a risk to ecosystem health. Nitrate was very low (five-year median of 0.01 mg/L); however, one site had a likely degrading trend. Ammonia levels were low and trends in ammonia were very likely improving at both sites.
Nitrate exceeded expected natural conditions in the Three Kings Volcanic aquifer, which suggests land use practices are impacting the aquifer. The Three Kings Aquifer is the baseflow source for Western Springs Lake and Motions Stream and likely to be one source of nitrate contamination in surface waters. E. coli (a faecal indicator bacteria) was present in groundwater samples for the Three Kings volcanic aquifer, most likely linked to stormwater and wastewater leakage in urban areas. The long-term trend in E. coli was likely degrading, which indicates faecal bacterial contamination is likely increasing in this aquifer.
Zinc concentrations in the Three Kings Volcanic aquifer exceeded the Australia and New Zealand Environmental Conservation Council (ANZECC) ecosystem health trigger value for surface water, suggesting that groundwater baseflow to Motions Stream may contribute zinc contamination to the stream. Reporting in 2017 showed no zinc exceedances at this site, suggesting potentially increasing zinc contamination over recent years. Previous reporting showed median zinc values in other Auckland isthmus aquifers (Onehunga Volcanic and Mt Richmond Volcanic) also exceeded ANZECC ecosystem health guidelines for surface water, indicating a link between urban sites and zinc contamination.
Overall, results from the groundwater quality monitoring programme indicate that nitrate is the foremost contaminant of concern for shallow volcanic aquifers in the Auckland region. High nitrate observed in groundwater coincides with both horticultural and urban land uses. The levels of nitrate observed exceeded expected concentrations for natural conditions, New Zealand drinking water standards, and the NOF national bottom line for surface water ecosystem health. Trends in nitrate were predominantly improving in the Franklin shallow volcanic aquifers, but degrading trends were observed in the Bombay Volcanic and Drury Volcanic aquifers. These aquifers provide baseflow to streams, suggesting that groundwater baseflow contributes to nitrate contamination in Franklin streams.
Additional monitoring sites will be added to the programme over the next three years to improve coverage of Auckland’s Aquifer Management Areas and further strengthen our understanding of the effects of land use, climate change, and water abstraction on groundwater quality
Auckland Council technical report, TR2021/03
Published February 2021.
Auckland Council's State of the environment report 2020