Effectiveness of double-cut-off walls on seawater intrusion and nitrate concentration in unconfined coastal aquifers

Abstract

An effective coastal engineering technique for preventing seawater intrusion is constructing a cut-off wall. Nevertheless, the cut-off walls impact on nitrate concentration in downstream aquifers has not been assessed in the previous research that focused on studying a single subsurface physical barrier. In this work, a numerical model was used to examine the effect and mechanisms of the double-cut-off walls on saltwater wedge length and nitrate concentrations transported downstream of them. SEAWAT code has been implemented to simulate seawater intrusion and nitrate transport in unconfined coastal aquifers. Two cases of homogeneous aquifer (Case-H) and heterogeneous aquifer (Case-LH) were studied. The results showed significantly receded in the saltwater wedge and the spread of nitrate contamination increased due to the heterogeneous conditions. A significant effect on nitrate accumulation and an increase in the pollution area between the double-cut-off walls was observed when the second cut-off wall depth was embedded by more than 50 % of the aquifer thickness due to the weak inflow below the cut-off wall. There was no need to raise the second cut-off depth because there was a significant retraction in the saltwater interface after the first cut-off wall was embedded to a depth of more than 70 % of the aquifer thickness. Raising the second cut-off wall depth to more than 30 % of the aquifer thickness when the first cut-off wall depth ratio was less than 50 % significantly impacted the retreating of the saltwater wedge for the short distance between the double-cut-off walls. When the second cut-off wall depth ratio was less than 50 %, raising the first cut-off wall depth significantly lowered the total concentrations of the nitrate that arrived at the double-cut-off walls downstream. In addition, the total concentrations of the nitrate that traveled toward the sea were reduced to 17 % of the original nitrate concentration by raising the depth of both cut-off walls to 70 % of the aquifer thickness and the distance between them to more than 25 m. Although it was shown that the double-cut-off walls substantially reduced saltwater intrusion and nitrate concentrations, they also created a large dispersion area of nitrate pollution, especially in the heterogeneous aquifer. The retreat of the saltwater interface wedge was significantly impacted by the first cut-off wall depth in the heterogeneous aquifer. This study offers useful information for preventing saltwater intrusion and reducing nitrate concentration downstream of the double-cut-off walls, especially, the double-cut-off walls represent a new study for controlling saltwater intrusion and nitrate pollution in a coastal aquifer. The outcomes of this study can be used for the groundwater resources proper management in coastal aquifers.