Importance of Aquitard Response Time for Groundwater Management in Multi-Aquifer Systems Subject to Land Subsidence

Groundwater management models have been widely applied to obtain optimal pumping strategies for land subsidence control, but most of them do not explicitly incorporate land subsidence variables (such as cumulative settlement and land subsidence rates) within the model constraints and neglect the transient effect due to aquitard storage. Here, three operating scenarios of a hypothetical multi-aquifer system, which include a highly compressible aquitard, were implemented with the aim of evaluating land subsidence and identifying management schemes with the support of an optimization model for groundwater management. In a 50-year management period, maximizing pumping while restricting drawdown to 10 m after year 25 stabilizes groundwater levels within the aquifer, but land subsidence continues to reach 4.8 m at year 50. The effect of reducing pumping rates and how early in the management period this is implemented is also analyzed. Restricting the pumping rate as early as year 6 leads to reduced land subsidence at year 50 by 17%. If pumping reduction is delayed, larger land subsidence rates occurred in the system (7.9, 8.3 and 9.6 cm/year in the tested cases); however, if the total settlement is evaluated as a proportion of the thickness of the aquitard, values of the order of 10% are presented. Our results highlight the importance of timely decisions for groundwater management based on the response time of the aquitards in multi-aquifer systems.