Terrestrial freshwater lenses in stable riverine settings: Occurrence and controlling factors

Abstract

Rivers in arid and semiarid regions often traverse saline aquifers, creating buoyant freshwater lenses in the adjoining riparian and floodplain zones. The occurrence of freshwater lenses where the river is otherwise gaining saline groundwater appears counterintuitive, given that both hydraulic and density forces act toward the river. In this paper, an analytical solution is presented that defines the extent of a stable, sharpinterface terrestrial freshwater lens (in cross section) in a riverine environment that otherwise contains saline groundwater moving toward the river. The method is analogous to the situation of an island freshwater lens, except in the riverine setting, the saltwater is mobile and the lens is assumed to be stagnant. The solution characterizes the primary controlling factors of riverine freshwater lenses, which are larger for situations involving lower hydraulic conductivities and rates of saltwater discharge to the river. Deeper aquifers, more transmissive riverbeds, and larger freshwater-saltwater density differences produce more extensive lenses. The analytical solution predicts the parameter combinations that preclude the occurrence of freshwater lenses. The utility of the solution as a screening method to predict the occurrence of terrestrial freshwater lenses is demonstrated by application to parameter ranges typical of the South Australian portion of the River Murray, where freshwater lenses occur in only a portion of the neighboring floodplains. Despite assumptions of equilibrium conditions and a sharp freshwater-saltwater interface, the solution for predicting the occurrence of riverine freshwater lenses presented in this study has immediate relevance to the management of floodplains in which freshwater lenses are integral to biophysical conditions.

Plausibility of freshwater lenses adjacent to gaining rivers: Validation by laboratory experimentation

“…The comparison of dimensionless numbers in Table shows that laboratory freshwater lenses have a similar dimensionless lateral extent relative to the River Murray conditions reported by Werner and Laattoe []. However, compared to field conditions, the laboratory experiments involved significantly higher saltwater fluxes relative to the buoyancy force, on the basis of a .…”

“…However, compared to field conditions, the laboratory experiments involved significantly higher saltwater fluxes relative to the buoyancy force, on the basis of a . A wide range of streambed‐aquifer conditions ( b ) are apparent in the data of Werner and Laattoe [], whereas the laboratory experiments have b values in the lower range of field parameters, implying that the laboratory streambed resistance (i.e., the outlet screen) was relatively weak in the face of strong saltwater flows.…”

“…The lens extent can be obtained by combining equation with Werner and Laattoe 's [] equation , to produce: …”

Boundary Condition Nomenclature Confusion in Groundwater Flow Modeling

“…The Robin BC can be applied in groundwater modeling to represent the truncation of aquifers, whereby regions of aquifer that fall outside of the model domain are approximated by C and h ref . Additionally, flow to/from a river (in situations where the groundwater level is higher than the river bed) is often represented using Equation 5, with impedance to flow caused by the river bed included in the parameterization of C (e.g., Werner and Laattoe ).…”

Transient Evolution of Inland Freshwater Lenses: Comparison of Numerical and Physical Experiments

“…While saline groundwater systems in semi-arid and arid environments are generally considered a problem for water resource managers, the occurrence of shallow (<100 m) brackish to saline groundwater promotes the development of localized accumulations of subsurface freshwater referred to as inland freshwater lenses (IFLs) [1][2][3]. IFLs serve as alternative freshwater resources for drinking water, livestock management, and micro-oasis agriculture in several arid and semi-arid regions including the Middle East (e.g., Kuwait, Oman, United Arab Emirates (UAE), Saudi Arabia), Australia (e.g., Murray Basin, Queensland), Central South America (e.g., Paraguay), Central Asia (e.g., Turkmenistan), Africa (e.g., Zambia, Namibia), and elsewhere with similar environmental settings [2][3][4][5][6][7][8][9][10].…”