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

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 studywe 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, delta O-18, delta H-2 and delta C-13) and dating tracers (H-3, CFC, SF6 and C-14). 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 C-14 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), comparedwith older groundwater (NO3 <= 9mg/L, MRT N 20 years). The changes in flow pathways, induced by irrigation, also results in seasonal declines in groundwater NO3 concentrations due to mixing with olderwaters. 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. (C) 2017 Elsevier B.V. All rights reserved.