Rethinking groundwater-level mapping: The presence of intrinsic vertical hydraulic gradient in confined aquifers

This study investigates the Intrinsic Vertical Hydraulic Gradient (IVHG) and its significant impact on enhancing groundwater-level (GWL) mapping. The IVHG is a fundamental characteristic that is naturally inherent in confined aquifers, reflecting the natural, undisturbed state of vertical hydraulic head variations under equilibrium conditions. Traditionally, GWL mapping has overlooked vertical hydraulic gradients within confined aquifers, assuming that groundwater levels remain constant at any given site within a confined aquifer regardless of well depth. This study identifies an IVHG of approximately 0.07 within the confined Chicot-Evangeline aquifer beneath the Greater Houston area, Texas, and around 0.05 in the underlying Jasper aquifer. This study presents an IVHG-adjusted GWL mapping method that standardizes GWL measurements based on well depth, resulting in more precise and depth-specific GWL contours. This approach offers a critical improvement over conventional methods, providing greater precision in water resource management, conservation planning, and the assessment of land subsidence risks, especially in areas land subsidence has ceased. The IVHG represents the minimum vertical hydraulic head gradient within confined aquifers. In regions where subsidence persists, the IVHG-adjusted method may not fully capture the larger vertical hydraulic gradient, yet it still outperforms conventional mapping by providing a more accurate representation of GWLs at different depths. The findings highlight the need to consider vertical hydraulic gradients in GWL mapping for more precise groundwater assessments and effective management strategies.