Tidal Influence on Remediation Sites: Understanding Predominant Gradients and Flow Inversion Effects on Mass Flux

Many remediation sites across the globe are located in coastal settings or along the banks of tidally influenced rivers. Furthermore, the effects of tides on those nearby surface water bodies manifest themselves in the groundwater flow system. Understanding and quantitatively incorporating tidal influence into the conceptual site model (CSM) is key to predicting groundwater flow, contaminant transport, and mass flux at the site. Without correction for tidal effects, manually measured groundwater monitoring data can appear erratic, both spatially and from one event to the next. Mass discharge estimates can be challenging in these environments, due to daily groundwater flow inversions near the receiving surface water body, which tend to produce significant natural attenuation of plume fronts prior to discharge. Temporary deployment of transducers for continuous water level monitoring in a subset of existing monitoring wells and in the adjacent surface water body, has shown to provide the data necessary to develop correction factors for subsequent manual water level measurement events. Calculation of tidal efficiency and tidal time lag, once quantified, can normalize the hydraulic head data such that a true synoptic potentiometric surface can estimated. Beyond correcting for temporal variation to create snapshots of potentiometric surfaces in time, the continuous monitoring data can be used in conjunction with numerical modeling to understand the transient dynamic nature of tidally influenced systems, including flow inversions, net discharge flow, transient transport and mass flux. This combined approach provides a more comprehensive assessment of site risks and, ultimately, more realistic remediation goals. This presentation reviews site specific data where detailed head measurements and modeling techniques were used to obtain a comprehensive understanding of tidal influenced systems.