Reduced Well Yields Caused by Hydraulic Head Declines and Associated Inelastic Aquifer Compression

Abstract:

Reduction in specific storage caused by groundwater pumping that lowers hydraulic heads and causes exceedance of pre-consolidation stress is a globally important phenomenon. It can reduce future well yields, cause interruptions in pumping, and shorten the useful lifespan of wells. Steep hydraulic head declines in many confined sedimentary aquifers worldwide is not only causing land subsidence, it is also causing permanent reductions in aquifer-system compressibility and specific storage as the result of non-recoverable compaction of aquitards and clay inclusions. Initial results are presented from a study of risks to water supply wells from additional drawdown resulting from nonrecoverable compaction, which is quantified and placed in context of regional hydraulic head declines in an unconsolidated sedimentary aquifer system containing multiple aquifers and multiple silt and clay aquitards. While the average drawdowns in this system are expected to increase by no more than 30%, in some cases this additional drawdown will prevent pumping at sustained rates when seasonal irrigation needs are the greatest. This often overlooked aspect of hydraulic head declines should be included in analysis and modeling of regional groundwater-level declines associated with groundwater pumping.