Nitrate-nitrogen dynamics in response to forestry harvesting and climate variability: 4 years of UV nitrate sensor data in a shallow gravel aquifer

Abstract

The leaching of inorganic nitrogen can adversely affect the quality of groundwater and its hydrologically connected streams and rivers. Traditionally, these effects have been assessed using discrete low-frequency water quality measurements. However, it is difficult to characterise the complex biogeochemical processes that control nitrate-nitrogen dynamics in groundwater when using temporally sparse data. In this study, we installed a high-frequency UV nitrate sensor, downgradient of plantation forestry, in a shallow gravel aquifer to understand nitrate-nitrogen dynamics in groundwater. We found that there were two mechanisms of nitrate-nitrogen pulses in groundwater from the upgradient forestry land use, the most prevalent being nutrient loss during winter months, when plant uptake is lower. However, outside of winter months, we observed a higher nitrate-nitrogen concentration (12 mg L−1) after the trees were harvested, compared with 5.9 mg L−1 when there was no harvesting; we attribute this to changing biogeochemical conditions. We used a novel hysteresis approach, comparing nitrate-nitrogen concentrations and groundwater levels after rainfall recharge to understand event-scale variability. First flush events in winter had a larger area (more hysteresis) of 0.65, compared with an average area of 0.35 (less hysteresis) for subsequent events. Peak concentrations occurred earlier in events during 2021 (wetter), compared with 2020 (drier), signifying slower drainage pathways in years with less recharge. Through this analysis we also found evidence that the mobilisation of nitrate nitrogen shifted from rainfall recharge to rising groundwater levels after the surface supply was depleted from successive recharge events. Finally, the nitrate-nitrogen load analysis indicates that leaching and export occur in pulses, which discrete sampling cannot accurately characterise. For example, in 2021, over 80 % of the exported load occurred during a quarter of the year and discharged when there were base flow conditions in the nearby Hurunui River. These findings have implications for forestry land management and the understanding of inorganic nitrogen dynamics in groundwater in response to rainfall recharge. Additionally, these insights may affect nitrate-nitrogen projections under climate change, where periods of drought and storm events are more frequent.