|dc.description.abstract||Groundwater use within the Poverty Bay flats has increased over the past 35 years due to land use changes. As a result, groundwater levels within the Makauri and Matokitoki Gravel aquifers are declining at statistically significant rates. The GDC is investigating whether managed aquifer recharge (MAR) can be used to help achieve the sustainable management of groundwater resources within the Poverty Bay flats. The objective of this research is to identify whether an infiltration basin may be successfully used to artificially recharge the Makauri Gravel aquifer beneath the Poverty Bay flats.
Trend analyses were undertaken to provide updated information as to the current state of groundwater resources. Groundwater levels within the Te Hapara Sands, Shallow Fluvial Deposits, and Waipaoa Gravel aquifers are typically stable. However, further monitoring is needed within these aquifers to increase the spatial coverage of the bores monitored. Groundwater levels within the Makauri and Matokitoki Gravel aquifers are declining at statistically significant rates. These declines are probably due to groundwater abstractions.
Groundwater level changes as a result of rainfall event and associated changes in river stage were examined. Groundwater level changes in the Te Hapara Sands, Shallow Fluvial Deposits, and Waipaoa Gravel aquifer were all dependent on the location of each bore. Groundwater level increases in these aquifers were greatest in close proximity to the Waipaoa River. Groundwater level changes within the Makauri Gravel aquifer could be broken into two groups. In the northern and southern reaches of the Makauri Gravel aquifer groundwater levels increased and declined in conjunction with river stage, indicating little recharge occurs in these areas. In the middle reaches of the Makauri Gravel aquifer groundwater levels increased throughout the monitoring period, indicating that this area of the aquifer is subject to significant amounts of recharge.
A pumping test was undertaken to identify if a hydraulic connection existed between the Makauri Gravel aquifer and the overlying Waipaoa Gravel aquifer in the Caesar Road area. No hydraulic connection was identified. Further research is needed in this area to identify if a hydraulic connection does exist, and if so, to evaluate the vertical exchange of groundwater.
Surface-groundwater interactions were analysed via concurrent Waipaoa River flow gaugings. No consistent gaining or losing reaches were identified. Further gaugings should be undertaken regularly to help characterise surface-groundwater interactions.
Two sites were then identified within the Poverty Bay flats for MAR using infiltration basins using the “HIGGS Index”, which was developed to evaluate the MAR potential of an infiltration basin site. While the two sites showed initial promise via a desktop study, infiltrations tests at each site indicated low rates of saturated hydraulic conductivity due to the presence of sediments with low permeability. Subsequently, the MAR potential of both sites was low.
Hydrogeological modelling was undertaken to ascertain whether an infiltration basin may be successfully used as a MAR technique to artificially recharge the Makauri Gravel aquifer. Modelling indicated that an infiltration basin would result in hydraulic head increases of up to 0.85 m within the Makauri Gravel aquifer. However, infiltration rates must not exceed 0.0425 m/day (10.625 m3/day) to avoid inundation at the ground surface in close proximity to the infiltration basin. Therefore, given the low infiltration rates it is questionable whether the benefits of establishing and operating an infiltration basin within the Poverty Bay flats would outweigh the costs.||