Influence of Groundwater Hydraulic Gradient on Bank Storage Metrics

Area of Science:

• Hydrogeology
• Environmental Science
• Water Resource Management

Background:

• The hydraulic gradient between rivers and aquifers is highly variable and significantly impacts bank storage.
• River-aquifer exchange is a complex process influenced by unsaturated zone interfaces, hydraulic conductivity, and penetration depth.

Purpose of the Study:

• To investigate bank storage dynamics under varying hydraulic gradients using a numerical model.
• To determine the factors controlling the proportion of water exchange through river banks versus river beds.
• To assess the accuracy of existing analytical solutions for estimating total exchange.

Main Methods:

• A two-dimensional numerical model of a variably saturated aquifer slice perpendicular to a river was employed.
• The model simulated water exchange at the river-aquifer interface, considering unsaturated zone processes.
• Model outputs were analyzed to understand the influence of hydraulic conductivity, partial penetration, and hydraulic gradient on bank storage.

Main Results:

• Bank storage is maximized without a hydraulic gradient and increases with hydraulic conductivity.
• In the presence of a hydraulic gradient, high hydraulic conductivity does not always lead to maximum exchange flux or penetration.
• Flood wave characteristics do not affect ambient groundwater discharge; high gradients are needed to reduce bank storage during large floods.

Conclusions:

• Existing analytical solutions can estimate total exchange within 50% if unsaturated zone processes are not dominant.
• Accurate measurement of bank storage is challenging due to technological limitations and complex exchange processes.
• Proxies like time series concentration data require further development for quantitative and representative bank storage assessment.