Analysis of COD Transport in Qinhuangdao Coastal Waters

The transport of pollutant is an important topic for self-purification capability and environment evaluation of ocean. Chemical oxygen demand (COD, determined by potassium permanganate) is selected as a key parameter to indicate the water pollution degree. This study employs field measurements and numerical model of water quality to research the characteristic of COD transport in Qinhuangdao coastal waters. To investigate the seasonal distributions of COD, the field monitoring of water quality at 46 sampling points was implemented in the dry season (April) and the flood season (August) in 2013. The coastal waters were with a relative higher average COD concentration of 1.43 mg/L in the flood season for the large amount of terrestrial COD inputs, compared with the average COD concentration of 1.08 mg/L in the dry season. The simulation of COD transport in Qinhuangdao coastal waters in the flood season demonstrates that COD transport is jointly controlled by the reverse tidal current in SW-NE directions and the discharges of rivers. For the estuary with smaller river discharge, COD transport in the coastal waters is driven dominantly by tidal current. The comparison of COD transport scope in the spring and neap tides indicates that the river input COD was transported farther due to low mixing in neap tide period. The newly-built artificial island near the coastal line hinders the nearshore water exchange, resulting in the deterioration of water quality in the estuary and around the island. For the estuary with large river discharge, COD transport in the estuarine water is greatly influenced by river discharge, for example, there was little change of COD distribution at the estuary of Luanhe River during the spring and neap tides in the same river discharge.