Contribution of biological processes to self-purification of water with respect to petroleum hydrocarbon associated with no. 0 diesel in Changjiang Estuary and Jiaozhou Bay, China

Mesocosms in 25-m3 ethylene vinyl acetate or 4-m3 polyethylene bags were performed at two sites in China: Changjiang Estuary in spring-summer 1998, and Jiaozhou Bay in autumn 1999 and spring-summer 2000. The experimen- ts were designed to show the contributions of physical, physicochemical and biological processes to self-purification of water with respect to the water-soluble fraction (WSF) of petroleum hydrocarbon associated with No.0 diesel (PHAD). Over the experimental periods, the WSF-PHAD concentration in water declined significantly. A kinetic model for WAS-PHAD distribution in multiphase environments - in this case water (W), suspended sediment (SS), phytoplankton (PPT), zooplankton (ZPT) and atmosphere (ATM) - was developed to estimate the contributions of the various processes. In the model, the key processes responsible for self-purification include (1) physical transport by volatilization from W to ATM; (2) biological transformation through biodegradation; (3) biological transfer involving bioconcentration by PPT and ZPT; and (4) physicochemical transfer through sorption by SS. Model parameters were determined from the mesocosm experiments and parallel laboratory experiments, or taken from the literature. It indicated that volatilization plays an important role in short-term self-purification especially regarding alkanes with relatively low molecular weight, accounting for 82.9±2.5%. After the volatilization stops, bioconcentration by PPT contributes greatly to long-term self-purification especially regarding alkanes of higher molecular weight and polycyclic aromatic hydrocarbons (PHAs), accounting for 0.7 - 17.8% with an average of 7.8±7.3%. The bioconcentration is dominated by biomass of PPT as well as the WSF-PHAD concentration. However, the biodegradation process makes the greatest contribution to both the short- and long-term self-purification, affecting alkanes of both lower and higher molecular weight as well as PHAs. The results suggest that it is necessary to take into account the biologica- l processes of biodegradation and bioconcentration in estimating the self-purification of water with respect to oil pollutants.