Review of groundwater monitoring in the Poverty Bay Flats aquifer system

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Moreau, M.; Tschritter, C.; Murphy, P.; White, P.A. 2017 Review of groundwater monitoring in the Poverty Bay Flats aquifer system. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/40 51 p. + 1 DVD; doi: 10.21420/G24S3D

Abstract: GNS Science and Gisborne District Council (GDC) jointly undertook a review of the long-term groundwater monitoring network for the Poverty Bay aquifer system. The system consists of three shallow aquifer units: Shallow Fluvial Deposit (SFD), Te Hapara Sand (THS), and Waipaoa Gravel (WPG); and two deeper units: Makauri Gravel (MKG) and Matokitoki Gravel (MTK). This review was prompted by GDC’s 2015 proposed regional plan, in response to the 2014 National Policy Statement for Freshwater Management, which requires integrated freshwater management of surface water, groundwater, and coastal water. The objectives of this review were to: 1) ensure that the existing groundwater monitoring objectives for the Poverty Bay aquifer system align with GDC’s current regional plan; 2) consolidate the existing dataset; 3) refine THS aquifer boundaries, identify pressures and report on state and trends, as this aquifer was identified by GDC as vulnerable to seawater intrusion; 4) investigate reduction-oxidation variations in the deeper aquifers; 5) optimise monitoring operations; and 6) provide recommendations to support present and future groundwater monitoring needs, including site redeployment outside of the Poverty Bay aquifer system as required per GDC’s regional plan. The aims of GDC’s current groundwater monitoring programme include characterisation of groundwater, identification of groundwater quality trends over time and to inform sustainable management of groundwater resources including preventing seawater intrusion in the Poverty Bay Flats aquifer system. Analysis of the existing datasets showed there is sufficient information to characterise the five aquifers. These objectives are aligned with GDC’s 2015 proposed regional plan. The groundwater monitoring dataset was consolidated through a review of aquifer attribution at 113 bores using: lithology; spatial distribution; bore depth; and groundwater quality. This resulted in the modification of aquifer attribution at 45 of the bores. At the end of this review, the attribution of 11 bores remained either equivocal or unknown. Consolidation of the dataset also resulted in refinement of aquifer boundaries for the THS aquifer using lithological logs, which has resulted in the Matawhero area now being located outside of the THS aquifer. Aquifer boundaries for the THS aquifer were refined using lithological logs, which has resulted in the Matawhero area now being located outside of the THS aquifer. Identified risks to this aquifer included: contamination through industrial land use; waste disposal (solid and liquid); poor bore construction; seawater intrusion, and the effects of climate change (e.g., sea level rise and decreased rainfall). Groundwater allocations in the THS are currently similar to that of 2012 and in order to maintain environmental low flows in surface waters, are unlikely to increase. Over the 1994-2015 period groundwater elevations in the THS aquifer were measured above sea level (e.g., 4.9 m ASL inland to < 1 m ASL at the coast) and rising at most bores with magnitudes ranging from 0.005 m/yr to 0.044 m/yr. Over the same time period, median conductivity for the THS aquifer ranged from 520 mg/L to 790 mg/L, with low NO3-N concentrations (median value of 0.23 mg/L). For most parameters there was an absence of trend. A review of sampling frequency at each monitoring site was conducted using four parameter selections (1 to 14 parameters) to optimise the monitoring network. Based on a single parameter (NO3-N), optimisation was only achieved for 5 out of 72 sites which could be sampled bi-annually or annually without incurring loss of information. It is likely that a regional nitrate monitoring network would be ineffective in the Poverty Bay due to the low concentrations observed. The monitoring suite may be reduced to the following key indicators: nutrients (nitrogen and phosphorous species); salinity (e.g., total dissolved solids, field conductivity, sodium and Cl concentrations); redox indicators (dissolved Fe and Mn; field-measured dissolved oxygen); and microbial indicators (E. coli, and faecal coliform), provided an appropriate quality assurance procedure is adopted. Oxygenated and highly reduced groundwaters were encountered in both the MKG and the MTK aquifers, with at least one Guideline Value and or Maximum Admissible Value exceeded at most sites, However, no relationships were found between concentrations of Fe, Mn, NH4-N and Cl and bore depth. (auth)