Characterising the surface and groundwater interactions in the Waimatuku Stream, Southland

Surface water and groundwater interactions are inherently complex, include multiple variables, and occur across time and spatial scales. Understanding the connections and interactions of water resources is critical to sustainable water management in New Zealand and mandated under the National Objectives Framework. The aim of this study was to characterise the groundwater and surface water contribution to the Waimatuku Stream, and identifying the spatial and temporal hydrological pathways to streamflow. In particular, this project investigated whether the Aparima River provided a source of water to the Waimatuku via subsurface flow pathways. The characterisation of the water within the Waimatuku Stream was conducted using a multi-method approach including physical hydrology, isotopic and chemical analysis during stable flows and one period of event flow. Water samples were collected analysed for a suite of water chemistry variables, stable water isotopes (18O and 2H) and the dissolved inorganic carbon isotope (δ13CDIC) over eight months and assessed for dominant chemical identifiers. Cluster and multivariate statistical methods were used to characterise flow path interactions and similarities (or differences) between the water bodies. The calculation of hydraulic gradients throughout the catchment area using stream stage and potentiometric water table heights were employed to assisted in the identification of stream groundwater exchanges and groundwater flow paths. Temporal event flow sampling and examination was used to provide an integrated catchment response to a rainfall event. The Waimatuku Stream was identified through the use of these methods to be connected with groundwater where baseflow sustains flow throughout the catchment and that the stream is a source of groundwater discharge and also a source of groundwater recharge. The base chemistry of the surface water of the Waimatuku Stream had higher solute concentrations compared to the Aparima River and hierarchical cluster analysis separated the wider Waimatuku sites, including surface water and groundwater, into five groups based from their solute concentrations of HCO3, Cl, Ca, Mg, K, SiO2, Na and SO4. Stable isotopes analysis identified that the Aparima and the Waimatuku had different ranges of δ18O, δ2H and δ13CDIC signatures resulting from different source locations. The clear differentiation between the Aparima River and the Waimatuku Stream provided no evidence for an interaction between the two waters. Under normal flow conditions the Waimatuku Stream appears to be fed via groundwater flow derived from precipitation on the land surface, which infiltrates to the water table and then percolates to the stream as suggested by the groundwater flow lines, stable baseflow, geochemical and δ13CDIC signature. The Waimatuku Catchment is groundwater fed from a regional system recharged through the land surface and Bayswater bog. There was no evidence for inter-catchment transfers of water between the Aparima River and the Waimatuku Stream. Future management for the Waimatuku catchment should investigate the hydrology, soil and sediment characteristics of the hyporheic zone to gain understanding of the biogeochemistry, which will aim in further investigation of fluxes of nutrients to the stream and other streams in Southland. To manage the Waimatuku Stream stricter rules on peat land development and nutrient loading to the land surface should be enforced though encouraging better farm practices