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Abstract. Nitrate (N-NO3) concentration in groundwater, the sole source of potable water in Prince Edward Island (PEI, Canada), currently exceeds the 10 mg L−1 (N-NO3) health threshold for drinking water in 6 % of domestic wells. Increasing climatic and socio-economic pressures on PEI agriculture may further deteriorate groundwater quality. This study assesses how groundwater nitrate concentration could evolve due to the forecasted climate change and its related potential changes in agricultural practices. For this purpose, a tridimensional numerical groundwater flow and mass transport model was developed for the aquifer system of the entire Island (5660 km2). A number of different groundwater flow and mass transport simulations were made to evaluate the potential impact of the projected climate change and agricultural adaptation. According to the simulations for year 2050, N-NO3 concentration would increase due to two main causes: (1) the progressive attainment of steady-state conditions related to present-day nitrogen loadings, and (2) the increase in nitrogen loadings due to changes in agricultural practices provoked by future climatic conditions. The combined effects of equilibration with loadings, climate and agricultural adaptation would lead to a 25 to 32 % increase in N-NO3 concentration over the Island aquifer system. The change in groundwater recharge regime induced by climate change (with current agricultural practices) would only contribute 0 to 6 % of that increase for the various climate scenarios. Moreover, simulated trends in groundwater N-NO3 concentration suggest that an increased number of domestic wells (more than doubling) would exceed the nitrate drinking water criteria. This study underlines the need to develop and apply better agricultural management practices to ensure sustainability of long-term groundwater resources. The simulations also show that observable benefits from positive changes in agricultural practices would be delayed in time due to the slow dynamics of nitrate transport within the aquifer system.

19Nitrate transport in heterogeneous bedrock aquifers is influenced by mechanisms that operate at 20 different spatial and temporal scales. To understand these mechanisms in a fractured sandstone 21 aquifer with high porosity, a groundwater-flow and nitrate transport model -reproducing multiple 22 2010). In these studies, parameters are varied and the amount of nitrate leached below the root 59 zone is monitored and modelled to assess the impact in term of groundwater nitrate contamination. 60The transferability of small-scale results at watershed scale is however rather limited. Distributed 61 physically-based hydrological models are thus commonly used for a more comprehensive 62 4 understanding of nitrate contamination at the watershed scale (e.g., Vaché et al., 2002; Hattermann 63 et al., 2005; Bracmort et al., 2006;Rode et al., 2009). The representation of aquifers in hydrological 64 models is generally over-simplified, with flow and transport processes modelled through simple 65 transfer functions, which for complex aquifer systems may not be adequate to simulate the spatial 66 and temporal distribution of nitrate contamination in groundwater and its transport to downstream 67 surface-water receivers. For this key task, understanding aquifer dynamics, especially groundwater 68 residence times, is crucial to support agricultural and water management plans, given that the 69 contamination intensity and time needed for water quality to change as a result of BMPs both 70 depend to a great extent on hydrogeological characteristics. Therefore, many researchers have 71 taken into account the dynamics of aquifers using numerical groundwater models (e.g., Lunn et al., 72 1996;Vinten and Dunn, 2001;Molénat et al., 2002;Wriedt and Rode, 2006; Dimitriou et al., 2010; 73 Paradis et al., 2016). Numerical modeling of groundwater flow and nitrate transport in aquifers 74 can provide quantitative insight into water quality sustainability as long as model parameters (e.g., 75hydraulic property values and structures) and inputs (e.g., groundwater recharge, nitrate flux) are 76 reasonably well established (Michael and Voss 2009). Estimation of model parameters and inputs 77 can be difficult, notably for large aquifers with limited data. To narrow the range of model 78 uncertainties, model calibration with multiple data sets is generally proposed, with each data set 79 providing different and complementary constraints on the aquifer model (e.g., Kim et al., 1999). 80 81In this paper, the control exerted by the dynamics of a sandstone aquifer on groundwater 82 contamination by nitrate, in a watershed with intensive agriculture, is examined using a 83 groundwater flow and nitrate transport model. The heterogeneous and dual-porosity nature of the 84 aquifer revealed by field and laboratory data were expected to influence water quality, but the 85 5 magnitude and trends of the contamination still remained unknown. To address this shortcoming, 86 the nitrate transport mechanisms within the aquifer were inferred from the grou...

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