The nitrogen isotope fingerprint of wastewater effluents in Hong Kong

Nitrogen pollution in marine waters is directly tied to human development – including agricultural runoff, atmospheric deposition, and urban wastewater effluents. After more than a century of urbanization, Hong Kong faces a major challenge in mitigating marine pollution, particularly nitrogen. Indeed, nitrogen pollution may be one of the primary causes of benthic ecosystem decline as evidenced by a contraction in the historical distribution of corals and seagrasses. Such ecosystems provide vital ecosystem services, not limited to nursery grounds for economically important fisheries. While the Hong Kong Government (HK Environmental Protection Department) has established long-term monitoring of marine nitrogen concentrations, the data obtained on the size of the total nitrogen pool does not indicate the proportion of which is derived from human activities. Stable isotope analysis of nitrogen (δ15N) can provide more accurate information on the source of nitrogen not only in the receiving environment but also in the marine and benthic organisms present. This project critically examines Hong Kong’s nitrogen sources from sewage effluents using stable isotope analysis. Objectives were to investigate (1) the δ15N of sewage effluents sampled across Hong Kong, Kowloon, New Territories, Lantau and Outlying Islands (2) the differences in δ15N across sewage treatment type and (3) the differences in δ15N between the wet and dry seasons with the aim of developing a new approach for detecting and mapping sewage impacts in Hong Kong. Through a 6-month sampling effort across 18 sewage treatment facilities, this study shows that 70% of δ15N values obtained from the sewage effluents were characteristically enriched (δ15N values up to 205 ‰) when compared with naturally occurring nitrogen sources in the biosphere (I10‰ < δ15N < 10‰). Overall, δ15N was more variable in plants with preliminary treatment and chemically enhanced primary treatment (CEPT) than in plants with primary, secondary or tertiary treatment. The mean daily flow rate explained 75.7% of the observed variability in δ15N values of preliminary treatment plants and 85% of the variability in primary treatment plants. This was also the case in plants having CEPT with a negative correlation of 46% between mean daily flow rate and sewage effluent δ15N values. This suggests varying models for the quantity and retention time of wastewater versus δ15N signatures for different treatment types that require further study. Microbial activity by aerobic bacteria and protozoa, ammonia volatilization and the amount of nitrogen present in the effluents are possible causes for δ15N enrichment. Enterococcus counts of three sites sampled indicated that that the municipal sewer system and wastewater treatment plant in Stonecutters Island (56,100 CFU 100 mlI1) and preliminary treatment facilities (57,800 CFU 100 mlI1) in Hong Kong Island are insufficient in preventing bacterial contamination in comparison to US EPA standards for recreational use of coastal marine waters (35 CFU 100 mlI1). These findings indicate that enriched δ15N values directly translate to the presence of anthropogenic inputs and that monitoring δ15N of primary producers is a feasible and important tool for monitoring wastewater management in Hong Kong.published_or_final_versionEnvironmental ManagementMasterMaster of Science in Environmental Managemen