The impacts of airport de-icers on water quality and the growth of undesirable biofilms in rivers

Undesirable river biofilms (URBs, formerly “sewage fungus”) are polymicrobial biofilms that grow in rivers as a response to organic pollution. Despite a continued widespread occurrence, limited research has been conducted in recent decades. Therefore, to reignite research into URBs, this thesis aims to investigate how the delivery of airport de-icer contaminated runoff to receiving waters impacts the growth and composition of URBs. The research conducted can be categorised into four themes: (i) a re-evaluation of “sewage fungus”; (ii) a field study to characterize the composition and drivers of URBs; (iii) microcosm growth experiments to determine how different airport de-icers contribute to URB growth; and (iv) pilot-scale mobilisation experiments to evaluate the mobilisation of airport de-icers in surface runoff. A bimonthly field study conducted over two winters revealed shifts in the bacterial composition of riverbed biofilms with suspected de-icer discharge. The community became dominated by the genera Rhodoferax (19.5 – 56.7% relative abundance) and Sphaerotilus (9.5 – 25.4%) prior to and during a visible URB incident. When aggregated at the family taxonomic level, Comamonadaceae (55.7 – 81.4%) emerged as the key indicator taxon of the significant shift in periphyton and has been proposed as a possible organic pollution biosentinel. The URB had a long-term influence on the periphyton, highlighting the importance of preventing their occurrence. Microcosm experiments of URB taxa growth using common freeze-point depressants as the sole carbon source confirmed de-icers can facilitate URB growth. Sphaerotilus natans growth was stimulated by all de-icers, except formate salts, in suspended culture (assessed by turbidimetric microcosms) and total growth was heavily dependent on carbon concentration. There were some significant differences in the growth kinetics between the active ingredients in de- icers (e.g., maximum growth rate). However, there were only negligible differences between pure active ingredients and their commercial products demonstrating that the additives used in commercial de-icers have negligible influence on microbial growth, despite their potential toxicity to aquatic fauna. Growth experiments with mixed cultures using S. natans and Zoogloea oleivorans were also tested to explore the interactions between URB taxa, including the novel application of timelapse microscopy in URB growth studies and provide a basis for further research. Mobilisation of commercial airport de-icers by rainfall was tested after exposure to various meteorological conditions (sun, wind, temperature) to explore the transport pathways of de-icers in runoff. Runoff was significantly affected by weather between de-icer application and a rainfall event. Sunlight and wind caused less de-icer mobilisation than decreases in temperature, attributed to drying effects. This research has considerable implications to inform surface water winter management plans by airport operators. This research highlights the continued prominence of URBs in rivers, even in countries with advanced wastewater treatment, and has added substantial contributions to knowledge about their composition and growth as a consequence of airport de-icing activities. Through a process-based understanding of de-icer transport and impact, airports can develop more effective and resilient solutions to minimise the ecological impacts in receiving water courses.