title: 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 (DPT); 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 m2/d, respectively. Values of T and S determined from the pumping test ranged from 400 to 500 m2/d and 1 to 5 × 10-4, respectively with DPTranging from 9 to 40 × 105m2/d. In contrast, D estimated from the Jacob-Ferris model ranged from 0.5 to 9 × 104m2/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 (Ktop/Kaquifer< 0.01) or anisotropic aquifers (Kx/Kz> 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.
authors.forename: Pin
authors.initials: P
authors.lastname: Shuai
authors.affiliation: Department of Geology & Geophysics, Texas A & M University, College Station, TX, 77840.
authors.forename: Peter S K
authors.initials: PSK
authors.lastname: Knappett
authors.affiliation: Department of Geology, Dhaka University, Dhaka, 1000, Bangladesh.
authors.forename: Saddam
authors.initials: S
authors.lastname: Hossain
authors.affiliation: Department of Geology, Dhaka University, Dhaka, 1000, Bangladesh.
authors.forename: Alamgir
authors.initials: A
authors.lastname: Hosain
authors.affiliation: Water Management & Hydrologic Science, Texas A & M University, College Station, TX, 77843.
authors.forename: Kimberly
authors.initials: K
authors.lastname: Rhodes
authors.affiliation: Department of Geology, Dhaka University, Dhaka, 1000, Bangladesh.
authors.forename: Kazi Matin
authors.initials: KM
authors.lastname: Ahmed
authors.affiliation: Department of Geological Sciences, The University of Texas at Austin, Austin, TX, 78712.
authors.forename: M Bayani
authors.initials: MB
authors.lastname: Cardenas
mesh_terms: D016880:Anisotropy; D001459:Bangladesh; D060587:Groundwater; D008962:Models, Theoretical; D045483:Rivers; D014872:Water Movements