Influence of velocity distribution conditions of coastal aquifer on nitrate contamination and seawater intrusion in the presence of subsurface physical barriers

Abstract

The velocity distribution is an important factor that affects seawater intrusion (SI) and nitrate (NO3−) pollution. However, there are few studies on the impact of subsurface physical barriers (SPBs) on the velocity distribution of the whole aquifer and the impact of velocity distribution on SI and NO3− pollution. Especially, the quantitative method of velocity distribution has not been studied. By the methods of laboratory experiments and numerical simulations, effects of the NO3− concentrations of the pollution source, hydraulic gradients (HGs), the location of the SPB and relative heights of SPBs (HP’) on the SI, NO3− pollution levels and velocity in the presence of SI and SPBs were investigated. The velocity distribution was first quantified to better describe the relationships between the velocity and degrees of SI and NO3− pollution. The results showed that the HG and HP’ were the main factors that affected the velocity, NO3− pollution and SI. The higher the HG, the smaller the HP’, and the decreased SI inferred a more serious NO3− pollution. The influence of SPBs on NO3− pollution and SI was mainly affected by the changes in the velocity distribution in the aquifer. With increasing HGs, for the region with flow rate less than 0.5 m/d (A0.5), the smaller its distribution area is, the smaller the relative area of SI (TLs') is. With an increase in the HG or decrease in the HP’, the relative area of NO3− pollution (Ns') is proportional to the distribution area where the flow velocity is greater than 1 m/d (A1). When the flow velocity distribution condition was A′1 (the relative area of A1) > A′0.5-1 (A′0.5-1 is the ratio of the area where the flow velocities are greater than 0.5 m/d and less than 1 m/d to the total area of the aquifer) > A′0.5 (the relative area of A0.5), NO3− pollution was serious; when the flow velocity distribution condition was A′0.5 > A′0.5-1 > A′1, the levels of NO3− pollution were the lowest.