Laboratory and numerical investigation of saline intrusion in fractured coastal aquifers

Laboratory scale experiments and numerical modelling were employed in this study to investigate saltwater intrusion in fractured aquifers. Saline intrusion was initiated in one homogeneous and six fractured experimental aquifers containing individual discontinuities of varying length and orientation. Automated image processing enabled high precision quantification of three intrusion variables, the toe length of the saline wedge, the width of the mixing zone and the aquifer fraction occupied by saltwater. A discrete fracture matrix model was successfully utilized to recreate the experimental data and expand the study's findings through rigorous sensitivity analysis. The presence of fractures significantly impacted all three intrusion variables under consideration. The length of intrusion was negatively correlated to the horizontal fracture's distance from the systems' seaward boundary. It was demonstrated that for the same fractured aquifer, the presence of a discontinuity can either limit or augment saline intrusion, depending on the applied hydraulic gradient. For gradients steeper than a critical head difference, at which the toe length was the same for both the fractured and the equivalent homogeneous aquifer, intrusion was suppressed further seaward, while for milder ones it intensified. The distance of horizontal fracture from the aquifer's base determined the extend of intrusion in the vertical direction. In general, the longer the discontinuities were, the more significant their impact on groundwater dynamics. In the case of vertical factures, whenever the saline wedges reached their position, the discontinuities contributed significantly in the widening of the mixing zone, while having limited effect on the other two intrusion characteristics. In aquifers with discontinuities adjacent to their side boundaries, a distinct distribution of saltwater concentration was identified, distinguishing them from the rest of the aquifers.