Investigation of Internal Pollutant Loading for a Tide‐Influenced Waterbody

Abstract

Based on laboratory experiments and field investigations, the relationship between internal pollutant release intensity and water velocity was determined for Neijiang, a representative waterbody. A two-dimension, unsteady hydrodynamic water quality model coupled with internal pollutant release was established and validated to assess Neijiang in different, typical years and at different tidal cycles. The internal-to-external pollutant loading ratio (IEPLR) was proposed in this paper for the first time to identify the temporal distribution of the effect induced by internal pollutant loading on the water environment. It was found that the annual internally released amounts of total nitrogen, total phosphorous, and chemical oxygen demand (COD) in the high-water year (1998) was 484.0 Mg, 405.3 Mg, and 1928.3 Mg, respectively, which was 27% greater than levels found in the typical-water year (2001) and the dry-water year (2004). Internal pollutant loading in the three typical years showed marked temporal variation; the high-water year displayed the most asymmetrical distribution. Internally released total nitrogen, total phosphorus, and COD during the tidal cycle of flood season was 1.374 Mg, 1.059 Mg, and 4.456 Mg, respectively, which was approximately 55.9% higher than dry season. External pollutant loading on Neijiang was the significant factor influencing water quality because the mean IEPLRs in the three typical years and the tidal cycles were less than one. However, because of the enhanced intensity of internal pollutant release, there were still 124 d in 1998, 22 d in 2001, and 38 d in 2004 when IEPLRs were greater than one; during these days a sudden algae bloom or water quality saltation might occur in Neijiang.