Field and modeling evidence for a “stagnant flow” zone in the upper meter of sandy phreatic aquifers

A stagnant flow zone, where effective hydraulic conductivities are reduced by entrapped bubbles, may be ubiquitous in the upper meter of the saturated zone in phreatic sand aquifers. It is proposed that the stagnant zone acts as a thin semi-aquitard within which groundwater flow paths tend toward the vertical. Once groundwater reaches the underlying active flow zone, the flow paths are mainly horizontal in accordance with the aquifer hydraulic gradients. The presence of stagnant zones in shallow groundwater is inferred from field and modeling data for three sand aquifer sites in southern Ontario, Canada. Field data from an agricultural nitrate monitoring study and the vertical transport of a ground-applied bromide tracer show clear evidence of a stagnant zone influence on solute distributions. More direct evidence is provided by the analysis of data from multi-point tracer tests (akin to borehole tracer tests). The inclusion of a stagnant zone in a numerical simulation of a septic-effluent plume developed in an unconfined aquifer markedly improves the agreement between the simulated and field results. Potential mechanisms for the origin of bubbles in the stagnant zone are discussed. Since many contaminants traverse the upper meter of the saturated zone, an increased understanding of the stagnant zone is important for monitoring and prediction of groundwater contamination.