Abstract
In the coupled estuary–shelf system, plumes originating from the New Hu-Wei and Choshui rivers, consisting of many terrestrial materials, could contaminate the water of the Mailiao industrial harbor. To determine the contribution of the two rivers to pollution, the interaction between river-forced, tide-generating, and monsoon-driven water motions in and around the Mailiao industrial zone harbor was examined by performing a series of numerical model experiments. We used a three-dimensional general circulation model to examine the interplay between Asian monsoon-driven, river-forced, and tide-induced water motions, one of which could primarily affect the plume. The model-derived results for different river discharges revealed that almost all of the ammonium entering the harbor had a slope-positive trend, with oscillations in response to flood–ebb tidal cycles. The ammonium increased with time and flux, except for the 10 m3/s flux. Although the river discharge flux exceeded 200 m3/s, the ammonium entering the harbor was the same as that of the 200 m3/s flux; the ammonium concentration did not increase significantly with time after the flux exceeded 200 m3/s. In addition, irrespective of flood or ebb tidal currents being suppressed by strong Asian monsoons, this mechanism avoided contaminating the water quality of the harbor while northeasterly winds prevailed. By contrast, the southwesterly monsoon drove the geostrophic current northward along the coast; concurrently, the coastal sea level increased to form the surface isobar slope up toward the coast, producing a secondary flow to accelerate geostrophic alongshore currents. The northward geostrophic currents compressed the plumes shoreward, forming a relatively narrow-band plume; the coupling model demonstrated that the southwesterly monsoon-driven current pushed plumes favorably along the west pier into the harbor.
Highlights
Introduction published maps and institutional affilAlong the west coast of Taiwan, a shoal shelf having a width of up to 15 km extends from north to south for more than hundreds of kilometers
Before performing complex numerical experiments, we used the 3D general circulation model with a simple setup to understand the basic features of the region
The model was set to the conditions of no winds, fixed discharge flux at 10 and 10 m3 /s for both CS and New Hu-Wei (NHW) rivers, respectively, and release of ammonium in downstream boundaries
Summary
Two water quality stations with quasi-seasonal, unequal interval time series were deployed inside the harbor to monitor the ammonia concentration to correspond with the EPA environmental regulation. Except for the two peaks of high ammonium concentrations in summer, the other values water quality stations were designed only to sample the downstream river in which the remain stationary, with an average value of 4 μM. Stations confined by a black dashed frame indicate average ammonium concentrations off the coasts of the NHW and CS rivers were 10 and sampling positions at the Choshui (CS) estuary, and stations confined by two red dashed. Off the NHW estuary, the ammonium concentrations at X12, X13, and X14 stations were relatively low (an average value of 1 μM; Figure 3b) This value may be considered a background value around the MIZ region. X10, unexpectedly reached 30 μM, indicating that significant quantities of pollutants and ammonium follow the river water discharge into the NHW river estuary during heavy rainfall. 3 /s m3/s mately in (a) and mm in (a) and 10 m3 /s in (b) in 2015, respectively
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