Evaluating the Effects of River Partial Penetration on the Occurrence of
Riparian Freshwater Lenses
Abstract
Previous studies of freshwater lenses in saline aquifers adjoining
gaining rivers (“riparian lenses”) have so far considered only rivers
that fully penetrate the aquifer, whereas in most cases, rivers are only
partially penetrating. This paper presents a new methodology for
obtaining the saltwater discharge and the shape of a steady-state,
non-disperive riparian lens, where the river is partially penetrating,
combining two previous analytical solutions. The resulting analytical
solution is compared to numerical modelling results to assess
assumptions and the methodology adopted to approximate the “turning
effect”, which is the change in groundwater flow direction (horizontal
to vertical) near the partially penetrating river. A range of conditions
are analysed, constrained by parameters adopted previously for River
Murray floodplains (Australia). Consistency between analytical and
numerical results highlight the capability of the proposed analytical
solution to predict the riparian lens geometry and saltwater discharge
into partially penetrating rivers. The sensitivity analysis indicates
that larger riparian lenses are produced adjacent to the deeper and
wider rivers, as expected. The change in width or depth of the river has
more influence on the saltwater discharge and the horizontal extent of
the riparian lens (and less effect on the vertical extent of the lens
adjacent to the river) for shallower and narrower rivers. This research
highlights the utility of the new method and demonstrates that the
assumption of a fully penetrating river likely leads to significant
overestimation of the saltwater discharge to the river and the riparian
lens horizontal extent and vertical depth.