The impact of aquifer stratification on saltwater intrusion characteristics. Comprehensive laboratory and numerical study

Abstract

Laboratory experiments and numerical simulations were utilized in this study to assess the impact of aquifer stratification on saltwater intrusion. Three homogeneous and six layered aquifers were investigated. Image processing algorithms facilitated the precise calculation of saltwater wedge toe length, width of the mixing zone, and angle of intrusion. It was concluded that the length of intrusion in stratified aquifers is predominantly a function of permeability contrast, total aquifer transmissivity and the number of heterogeneous layers, being positively correlated to all three. When a lower permeability layer overlays or underlays more permeable zones its mixing zone widens, while it becomes thinner for the higher permeability strata. The change in the width of the mixing zone (WMZ) is positively correlated to permeability contrast, while it applies to all strata irrespectively of their relative vertical position in the aquifer. Variations in the applied hydraulic head causes the transient widening of WMZ.These peak WMZ values are larger during saltwater retreat and are negatively correlated to the layer's permeability and distance from the aquifer's bottom. Moreover, steeper angles of intrusion are observed in cases where low permeability layers overlay more permeable strata, and milder ones in the inverse aquifer setups. The presence of a low permeability upper layer results in the confinement of the saltwater wedge in the lower part of the stratified aquifer. This occurs until a critical hydraulic head difference is applied to the system. This hydraulic gradient value was found to be a function of layer width and permeability contrast alike.

Laboratory and Numerical Study of Saltwater Upconing in Fractured Coastal Aquifers

“…The largest mixed concentration volume fraction (11.3%) was yet again observed five minutes after the termination of pumping. This time-lagging between the end of the freshwater abstraction and the peak mixing zone values is in agreement with previous laboratory investigations on SWR in homogeneous [28] and stratified [47] coastal aquifers. The mixing zone data indicate that steady-state was achieved in the fractured-left and fractured-right aquifers 20 min after the cessation of pumping, while an extra 20 min (Figure 8) were needed for the fractured-center case.…”

“…The maximum TL difference between these aquifer pairs was equal to just 0.8 mm, while the maximum deviation in the total non-fresh and the mixed concentration volume fractions were equal to 0.06% and 0.21%, respectively. The permeability contrast between various layers has been identified as defining factor of the saline intrusion dynamics in stratified hydrological systems [47,71,72]. This study expands on this observation, showing for the first time the importance of the permeability contrast on the saltwater upconing mechanism in fractured coastal aquifers.…”

“…Previously documented experimental errors [28,47] could be responsible for the difference between the acquired laboratory and numerical results. In relatively small sandbox setups, such as the one employed in the current study, errors can be potentially introduced by flaws in the sandbox leveling and the adjustment of the hydraulic head difference, as well as slight variations in the density of the saline solution and uneven water flow between the side cylindrical tanks and the main viewing chamber, or the fracture and the glass beads.…”

“…Laboratory sandbox setups offer an idealized physical approximation of groundwater flow and have been successfully utilized over the years to study specific aspects of aquifer SWI, such as the freshwater-saltwater interface [28,29], the impact of tides [30][31][32][33], variations in saltwater density [34] and in applied hydraulic gradients [35], alongside the suitability of various SWI prevention approaches [36][37][38]. It has been established that aquifer heterogeneity significantly affects groundwater flow; thus, saline intrusion in heterogeneous [39][40][41] and more specifically stratified coastal aquifers [36,[42][43][44][45][46][47] has been studied extensively utilizing laboratory-scale experiments. Nevertheless, sandbox investigations of groundwater flow in fractured porous media are relatively rare.…”

Haline Convection within a Fresh-Saline Water Interface in a Stratified Coastal Aquifer Induced by Tide

“…The use of physical models provides an ideal setup for conceptualization, visualization, and measurement of groundwater flow and solute transport through an aquifer [19][20][21][22]. The impact of stratified heterogeneity has been studied on different phenomena [23][24][25][26]. In this study, we used such a physical model, demonstrating the effects of tide and stratification on the FSI width.…”