Impact of irrigated agriculture on groundwater resources in a temperate humid region

Highlights: • Despite a temperate climate, the seasonal impact of irrigated agriculture remains acute. • Evolution of mixing processes impacting irrigation water resources. • Quantity and quality of water resources are spatially and temporally variable. • Groundwater pumping has induced mixing with older waters. • Fertiliser use resulted in higher NO₃ concentrations in younger groundwater. The groundwater irrigation expansion, and its multiple potential impacts on the quantity and quality of water resources, is not just restricted to areas that are water limited. In this study we present the seasonal impacts irrigation practices can have on groundwater resources in a temperate humid region, where the average annual rain/PET ratio is 1.0. In this system the irrigation expansion is solely supported by groundwater pumping, but despite this only 5 boreholes are monitored for hydraulic head data. In this study, we compensate the scarce hydrophysical dataset by incorporating environmental tracers (major ions, δ¹⁸O, δ²H and δ¹³C) and dating tracers (³H, CFC, SF₆ and ¹⁴C). Results indicate that at 9 of the 15 irrigation sites investigated, groundwater pumping for irrigation has induced the mixing of recent groundwater (up to < 1 year) with older waters. The origin of the older waters was from either the deeper marl aquifer, or the shallow sand-clay aquifer (SCB) that has a ¹⁴C mean residence time (MRT) of up to 9700 years. Secondly, although high nitrate loads in infiltrating waters were being diverted via the artificial subsurface drainage system, increases in fertiliser loads have resulted in higher NO₃ concentrations in younger groundwater (NO₃: 9–45 mg/L, MRT < 20 years), compared with older groundwater (NO₃ ≤ 9 mg/L, MRT > 20 years). The changes in flow pathways, induced by irrigation, also results in seasonal declines in groundwater NO₃ concentrations due to mixing with older waters. In temperate humid areas, such evaluations of the seasonal evolution of water residence time, mixing process, and agrochemical contaminants are an important contribution to real water resources management in irrigated catchments.