Chaotic advection describes a flow field in which velocity variations produce radically different flow paths of fluid particles that are initially adjacent to each other, leading to spreading in laminar flows. This spreading can promote reaction by bringing together reactants that were initially separate. Chaotic advection can be generated in aquifers by employing time-varying injection and extraction of water through wells, which has been shown through numerical simulations to enhance degradation during in situ remediation of contaminated groundwater. Naturally-occurring time-varying flows are also present in aquifers. For example, coastal aquifer experience tidal fluctuations, and aquifers that are hydraulically connected to rivers may have fluctuations in flow due to seasonal variations in river stage. In this work, we use numerical simulations to investigate the presence of naturally-occurring chaotic advection in aquifers. We evaluate the chaotic behavior of time-varying velocity fields that are caused by periodic fluctuations in water levels in ocean or rivers that form the hydraulic boundaries of the aquifer. If chaotic advection exists naturally in these systems, it may create hot spots and hot moments of biogeochemical reactions in the aquifer caused by mixing the background groundwater with the groundwater derived from the influx of surface water.
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