Effects of tidal fluctuations and spatial heterogeneity on mixing and spreading in spatially heterogeneous coastal aquifers

Abstract

We study the combined effect of heterogeneity in the hydraulic conductivity field and tidal oscillations on the three-dimensional dynamics of seawater intrusion in coastal aquifers. We focus on the quantification of its impact on solute mixing and spreading of the freshwater-seawater interface. Threedimensional Monte Carlo realizations of log-normally distributed permeability fields were performed, and for each realization, numerical variable density flow and solute transport simulations were conducted. Mixing is characterized by the spatial moments of concentration. The enhanced solute mixing is quantified by an effective dispersion coefficient. The simulations show that heterogeneity produces an inland movement of the toe location along with a significant widening of the transition zone, which is linearly proportional to the product of the arithmetic mean of the correlation lengths in the three spatial dimensions (k a ) and the permeability field variance (r 2 lnk ). We find that once tidal oscillations are included, as the degree of heterogeneity increases, the combined effect of heterogeneity and tidal oscillations on mixing and spreading of the interface reduces. This is explained by the fact that an increase in the log-permeability variance induces an increase in both the effective permeability and the spatial connectivity, which implies a more uniform hydraulic response to tidal forcing and, as a result, the degree of mixing decreases. This study also identifies that the mixing behavior induced by tidal oscillations in heterogeneous coastal aquifers is controlled by the effective tidal mixing number (n e tm ) which depends on the amplitude, the period, the storativity, and the effective horizontal permeability.

Nitrogen attenuation, dilution and recycling at the groundwater – surface water interface of a subtropical estuary inferred from the stable isotope composition of nitrate and water

“…However, a mitigating feature found in many catchments is consistent with findings elsewhere (Pool et al, 2015). The degree of mixing by tidal circulation may be variable 10 in space along the beachface, as suggested by the differences in chloride concentrations at the base of the porewater profiles.…”

Groundwater withdrawal in randomly heterogeneous coastal aquifers

“…Our findings are consistent with previous studies (e.g. Pool et al, 2015) in showing that an analysis relying on the ensemble concentration field tends to overestimate significantly the degree of mixing and spreading of the solute.…”

“…These outcomes (hereafter called rotation effects) are consistent with previous literature findings. In particular, Kerrou and Renard (2010) and Pool et al (2015) observed that the strength of the rotation effect increases with the variance of the log-15 permeability field. Abarca (2006Abarca ( , 2007 showed that a similar effect can be observed in a homogenous domain when considering increasing values of the dispersion coefficient.…”

“…gravity number, permeability anisotropy ratio and transverse and longitudinal Péclet numbers. Enhancement of mixing in the presence of tidal fluctuations and/or FW table oscillations has been analyzed by Ataie-Ashtiani et al (1999) Pool et al (2014) and Lu et al (2015) in homogeneous aquifers and by Pool et al (2015) in randomly heterogeneous three-dimensional systems (under ergodic conditions). A recent review on this topic is offered by Ketabchi et al (2016).…”

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Kuan et al [2012] illustrated through numerical modelling combined with laboratory experiments that in tidally-influenced aquifers where an USP forms, the extent of seawater intrusion into the aquifer is significantly reduced compared to non-tidal conditions. More recently, Pool et al [2014] and Pool et al [2015] conducted numerical analyses to quantify the effect of tides on the saltwater wedge DZ in homogeneous and heterogeneous CUAs. They showed the extent of mixing is influenced by the tidal amplitude, tidal period and hydraulic diffusivity, with the aquifer heterogeneity reducing the impact of tides on the extent of mixing along the DZ.…”