Groundwater and Surface Water Interaction, Wairarapa Valley, New Zealand
Authors:   , Guggenmos Michael 
Published: November 2021

<p>Physical and chemical interactions between surface and groundwater are complex and display significant spatial and temporal variability. However, relatively little is known about the chemical interaction between surface and groundwater; in particular the temporal scales at which this interaction occurs. The aim of this research was to determine if existing and/or potential water chemistry measurements could be used to investigate the interaction between surface and groundwater bodies in the Wairarapa valley, New Zealand and identify specific locations and timescales at which this interaction occurs. Analyses were undertaken at both regional and local scales. The regional scale investigation utilised Hierarchical Cluster Analysis (HCA) to categorise 268 historic surface and groundwater sites from the 3000 km² Wairarapa valley into similar hydrochemical clusters in order to infer potential interaction. Six main clusters were identified, primarily differentiated by their total dissolved solids (TDS), redox potential and major ion ratios. Shallow aquifers, located in close proximity to losing reaches of the upper Ruamahanga, Waiopoua and Waiohine Rivers, were grouped with similar Ca²⁺-HCO₃⁻ type surface waters, indicating (potential) recharge from these river systems. Likewise, rainfall-recharged groundwater sites that displayed higher Na⁺ relative to Ca²⁺ and Cl⁻ relative to HCO₃⁻ were grouped with similar surface waters such as the Mangatarere and lower Waingawa streams. This suggests the provision of this rainfall-recharged signature to river base flow. Deep anoxic aquifers, high in TDS, were grouped together, but showed no statistical link to surface water sites. Results from the regional scale investigation highlight the potential use of HCA as a rapid and cost-effective method of identifying areas of surface and groundwater interaction using existing datasets. A local scale investigation utilised existing quarterly and monthly hydrochemical data from the Mangatarere and Waiohine Rivers and nearby groundwater wells in an attempt to gain insight into temporal variability in surface and groundwater interactions. Time series analysis and HCA were employed, however, the coarse time scales at which data was available made it difficult to make reliable inferences regarding this interaction. To overcome this issue, upstream and downstream surface and groundwater gauging stations were established in the Mangatarere Stream catchment for a 92 day period. Continuous electrical conductivity, water temperature and stage measurements were obtained at three of the four stations, along with one week of hydrochemical grab sampling. The fourth gauging station provided a more limited dataset due to technical issues. The downstream Mangatarere Stream received 30-60% of base flow from neighbouring groundwaters which provided cool Na⁺-Cl⁻ type waters, high in TDS and NO₃‾ concentrations. This reach also lost water to underlying groundwaters during an extended dry period when precipitation and regional groundwater stage was low. The upstream groundwater station received recharge primarily from precipitation as indicated by a Na⁺-Cl⁻-NO₃‾ signature, the result of precipitation passage through the soil-water zone. However, it appeared 2-4 m³/s of river recharge was also provided to the upstream groundwater station by the Mangatarere stream during an extended storm event on JD021-028. Mangatarere surface waters transferred a diurnal water temperature pattern and dilute Na⁺-Ca²⁺-Mg²⁺-HCO₃⁻-Cl⁻ signature to the upstream groundwater station on JD026-028. Results obtained from the Mangatarere catchment confirm the temporal complexities of ground and surface water interaction and highlight the importance of meteorological processes in influencing this interaction.</p>