Optimized approach for groundwater management for drinking water - Application to a 10 million cubic metres /year well field (South of Bordeaux – France).
Résumé
In southwestern France, all drinking water supplying the conurbation of Bordeaux originates from groundwater (springs and well fields). Today, two major aquifers supply this demand: the ‘Oligocene aquifer’ and the ‘Eocene aquifer’; historically, the first one is the main resource for Bordeaux. In recent decades, the proportion of water abstracted from boreholes in the Oligocene aquifer has increased by 10 M m³/year from 1975 to 2005, while since the early 1990s the production from the Eocene aquifer decreased in the same proportions. Considering the increased water demand due to this population increase (8% between 2011 and 2016) and the potential impact of climate change on the flowrate of springs, Bordeaux must increase its drinking-water production capacity while ensuring the sustainability of the groundwater resource.
This study aims at determining the best way for a sustainable exploitation of a well field south of Bordeaux, while minimizing the impact on groundwater resources by using optimization and hydrogeological-surrogate models in addition to the regional hydrogeological multilayer model developed at BRGM with the MARTHE® groundwater-flow software program. The general structure of the model contained 720,093 cells of 100x100 m size in six layers (four aquifers and two confining layers). It will help evaluating the feasibility of using surrogate models for optimizing pumping rates of a well field over a regional multilayer aquifer. This optimization approach investigates a much larger number of combinaitions than the classic trial-and-error approach. It is carried out by the CAPUCINE® software, which follows two strategies:
• Strategy 1: Optimization under constraints considering the management requirements of an existing well field (maximum drawdown, maximum pumping rate of each well);
• Strategy 2: Optimization to study the opportunity of drilling and exploiting new wells while taking advantage of the existing water-supply pipe network.
Though it is usually considered that preserving groundwater quality necessarily implies restricted exploitation, this works shows that other exploitation strategies can reach this objective with a significant gain in the total extracted volume without creating new wells. Well-by-well analysis of the results shows that real-life tests can be carried out, first test show results in accordance with simulations.
Moreover, two alternative configurations, incorporating the creation of new wells, were tested for maximizing the capacity of the well field and optimizing the effective capactity of the existing pipe network. Substantial gains are expected from those configurations, reaching an additional 2.2 to 5.5 M m³/year.