The Impact of the Degree of Aquifer Confinement and Anisotropy on Tidal Pulse Propagation

Abstract

Oceanic tidal fluctuations which propagate long distances up coastal rivers can be exploited to constrain hydraulic properties of riverbank aquifers. These estimates, however, may be sensitive to degree of aquifer confinement and aquifer anisotropy. We analyzed the hydraulic properties of a tidally influenced aquifer along the Meghna River in Bangladesh using: (1) slug tests combined with drilling logs and surface resistivity to estimate Transmissivity (T); (2) a pumping test to estimate T and Storativity (S) and thus Aquifer Diffusivity (D ); and (3) the observed reduction in the amplitude and velocity of a tidal pulse to calculate D using the Jacob-Ferris analytical solution. Average Hydraulic Conductivity (K) and T estimated with slug tests and borehole lithology were 27.3 m/d and 564 m /d, respectively. Values of T and S determined from the pumping test ranged from 400 to 500 m /d and 1 to 5 × 10 , respectively with D ranging from 9 to 40 × 10 m /d. In contrast, D estimated from the Jacob-Ferris model ranged from 0.5 to 9 × 10 m /d. We hypothesized this error resulted from deviations of the real aquifer conditions from those assumed by the Jacob-Ferris model. Using a 2D numerical model tidal pulses were simulated across a range of conditions and D was calculated with the Jacob-Ferris model. Moderately confined (K /K < 0.01) or anisotropic aquifers (K /K > 10) yield D within a factor of 2 of the actual value. The order of magnitude difference in D between pumping test and Jacob-Ferris model at our site argues for little confinement or anisotropy.

Aquifer Diffusivity Estimation Through Joint Inversion of the Amplitude Ratios and Time Lags of Dominant Frequencies of Fluctuating Head

“…Relative to Shuai et al (2017) pumping test results, the most similar estimate of D was from the T5-1A (last interval) pair case where the differences were −1.0%, −10.2%, and −1.1% for D R , D tL and D J respectively (Table 1). Shuai et al (2017) The analysis was implemented for different source and observation point pairs. The independently estimated D is 9.0 × 10 5 m 2 /d (Shuai et al, 2017, estimated from a pumping test).…”

“…Shuai et al (2017) The analysis was implemented for different source and observation point pairs. The independently estimated D is 9.0 × 10 5 m 2 /d (Shuai et al, 2017, estimated from a pumping test). The % difference is relative to the previously reported value by Shuai et al (2017).…”

“…The wells are 15 m deep and spaced 15 m apart from each other perpendicular to the river (Berube et al, 2018). Four wells from this transect were used in this study, and the first three were named as T6, T5, and T1 by Shuai et al (2017), with T1 being the farthest well from the river. An additional nearby well, Veast 1a (or "1A" here for short) was also used in this study.…”

Changes in Physical Properties of Inland Streamwaters Induced by Earth and Atmospheric Tides

“…Also, in the case of groundwater tides, more alternative causes of phase shift were indicated, such as groundwater flow and aquifer characteristics (e.g., rock porosity, fracture planes, and the compressibility of the surrounding aquifers) [33][34][35][36]. The degree of an aquifer confinement and anisotropy impacts the propagation of tides [37]. Overall, the properties of the geologic strata are very important in the behavior of groundwater tides [38].…”

A Search for Freshwater in the Saline Aquifer of Coastal Bangladesh

“…1 and Table 1). Installation was completed using the traditional Bangladeshi hand flapper method-a reverse circulation percussion drilling method that involves the lifting and dropping of the drill tube with circulation to expel mud pit drilling fluid out the top of the drill pipe during the percussion (Horneman et al 2004;Shuai et al 2017). Additional information regarding the drilling method is included in the Supporting Information.…”