Investigation of groundwater-surface water interaction using hydrochemical sampling with high temporal resolution, Mangatarere catchment, New Zealand

Abstract. The interaction between groundwater and surface water is dynamic and is known to show considerable spatial and temporal variability. Generally hydrological studies that investigate this interaction are conducted at weekly to yearly timescales and inadvertently lose information contained at the neglected shorter timescales. This paper utilises high resolution physical and chemical measurements to investigate the groundwater and surface water interactions of the small temperate Mangatarere Stream in New Zealand. Continuous electrical conductivity, water temperature and stage measurements were obtained at two surface water gauging stations and one groundwater station, along with one week of intensive hydrochemical grab sampling. A second groundwater gauging station provided limited additional data. The downstream reach of the Mangatarere Stream received significant base flow from neighbouring groundwaters which provided cool Na⁺-Cl⁻ type waters, high in TDS and NO⁻₃ concentrations. This reach also lost water to underlying groundwaters during an extended dry period when precipitation and regional groundwater stage were low. The upstream groundwater station received recharge primarily from precipitation as indicated by a Na⁺-Cl⁻-NO⁻₃ signature, the result of precipitation passage through the soil-water zone. However, river recharge was also provided to the upstream groundwater station as indicated by the transferral of a diurnal water temperature pattern and dilute Na⁺-Ca²⁺-Mg²⁺-HCO₃⁻-Cl⁻ signature. Results obtained from the Mangatarere catchment confirm the temporal complexities of groundwater and surface water interaction and highlight the benefits of multiple investigative approaches and the importance of high frequency hydrochemical sampling and monitoring for process understanding.