Effectiveness and comparison of physical barriers on seawater intrusion and nitrate accumulation in upstream aquifers.

Physical barrier is a coastal engineering widely used to prevent seawater intrusion. However, previous studies have not evaluated the combined influence of type and structure of the physical barrier on nitrate accumulation in upstream aquifers. Furthermore, the mechanisms of nitrate accumulation caused by the physical barriers are still unclear. In this study, numerical simulations were conducted to investigate the mechanisms and influence of the physical barrier on seawater intrusion and NO3- accumulation. The results show that constructing physical barrier can result in the reduction of nitrate discharge to the sea and accumulation of nitrate in upstream aquifers. The accumulation degree is significant if the barrier height is large; the barrier location is near the sea; the nitrate infiltration rate is large; the infiltration nitrate concentration is large; the inflow is weak, and the inflow DOC concentration is low. A cut-off wall is more likely to cause nitrate accumulation than a subsurface dam. It can result in mean nitrate concentration in groundwater upstream increasing by more than 30 %. Because a nitrate accumulation zone is formed behind the cut-off wall where the flow is slow and dissolved oxygen carbon is difficult to be replenished so that the denitrification is weak. Despite the subsurface dam may not result in a significant increase in nitrate concentration, it cannot be applied to the areas where SI has occurred due to the residual seawater problem. The nitrate accumulation in the upstream aquifer is a long-term process that lasts for more than three years to reach a pseudo-steady state. Seasonal variations of inflow and infiltration lead to fluctuation of mean nitrate concentration, thereby the nitrate accumulation rate increased after April and weakened between July and December.