How does compaction affect aquifer response to pumping? The impact of temporally variable specific storage on current and future water availability

Abstract:

Groundwater models are a powerful tool for quantifying future water availability. When projecting future scenarios, basic assumptions can have significant impacts on model results. Very few groundwater models account for the impact of groundwater pumping on land subsidence due to the drainage of clays, or the impact of clay drainage on overall aquifer storage. However, since the specific storage of clays is 1 to 2 orders of magnitude higher than that of sands and varies temporally, drainage from clays is complex and can significantly impact the overall specific storage of the aquifer system.

In this study, we run a series of synthetic MODFLOW groundwater models, some that account for clay drainage and associated compaction, and some that do not. The models, while synthetic, are based on the aquifer system in Parowan Valley, Utah, a predominantly confined aquifer system with subsidence rates of up to 5 cm/year and extensive groundwater pumping. We demonstrate that during episodes of historically high water stress, clay storage buffers the effect of high groundwater pumping on drawdown due to the release of water from storage during inelastic compaction of clays, when the specific storage is ~2 orders of magnitude higher than during times of normal water stress. We also demonstrate that when we do not account for this temporal variation in storage from clays, our models are biased towards higher estimates of recharge and/or hydraulic conductivity, with calibrated parameter estimates that are 38% and 43% higher than our ‘true’ synthetic model, respectively. These biases can result in overly optimistic estimates of future water availability, since storage from clays gradually depletes over time. We also demonstrate that even in regions with very small amounts of clay and low levels of subsidence (<1 cm/year), the effect of clays on aquifer storage can be significant.