Regional Model of Groundwater Management in North Aquitania Aquifer System: Water Resources Optimization and Implementation of Prospective Scenarios Taking into Account Climate Change

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In the multilayered aquifer system of North Aquitania in South-West France, the Oligocene and Eocene aquifers are pumped intensively for 50 years, mainly for domestic water. The huge pumping, which reaches 290 million m3 per year, causes a steady decline in groundwater levels. In particular, given the low recharge by rainfall, the Eocene confined aquifer collapsed with a water level decrease exceeding 30 m near the Bordeaux city. The resource is threatened, both in quantity, since the pumping exceeds the renewal and in quality, due to the reversal of flow gradients, with risk of brackish water invasion from the Gironde estuarine area in which the Eocene aquifer outcrops. The Oligocene is also subject to an increased vulnerability to surface pollution. A mathematical model simulating groundwater flows in this multilayer system has been implemented to quantify the water savings to be achieved, and simulate scenarios trend (population growth, industry and agriculture development) and combined scenarios (savings and substitutions). It is a regional multilayer water resources management model with 15 aquifer layers starting from the Plio-quaternaire aquifer at the top, down to the Bajocian aquifer at the bottom. The model, which has a spatial extension varying from 10,000 to 25,000 km2 according of the aquifer, takes into account 67,000 square cells of size 2 km. It incorporates, at an annual time step, pumping in 3,250 wells and climatic data in five meteorological stations. It is calibrated in transient state using 380 observed time series of water level. The model was used to analyze the ­possibilities of restoring a balanced and safe state, using simulation of pumping scenarios. These simulations include scenarios incorporating trend forecasts of ­population growth, scenarios of economy needs, scenarios of pumping alternative aquifers and climate change scenarios. The climate change scenario selected is the moderate IPCC Arpege A1B scenario from Météo – France.