Chronic and acute polycyclic aromatic hydrocarbon (PAH) contamination in urban harbours and ports

  • Lesley Carol Chapman-Greig

Student thesis: Doctoral Thesis


This thesis focuses on polycyclic aromatic hydrocarbon chronic (operational, small scale but frequent) and acute (large and isolated event) contamination in urban harbours and ports. An air emission inventory method for marine traffic was extended and adapted to include non-combusted fuel spills and recreational craft. The modified method calculated petrogenic (non-combusted fuel) spills and pyrogenic (incomplete combustion) emissions in Portsmouth Harbour. These calculations were used to determine if urban harbours and ports receive chronic inputs from marine traffic. To confirm if chronic inputs cause chronically contaminated sediments, an environmental seasonal sampling of water and sediment were undertaken. To explore PAH sediment accumulation and loss under chronic and acute conditions a 6-month mesocosm incubation was undertaken. Lastly, to determine if the length of time between contamination and resuspension event impacts on PAH flux to the water column, a 28-day timeline was established. The petrogenic and pyrogenic inventory found clear distinctions between commercial and recreational sectors. The greatest incomplete combustion emitter of particulate matter (PM) and the PAH benzo(g.h.i)perylene (BgP) into Portsmouth Harbour is the passenger, car and continental ferry and cruise ship sector. This sector comprises 64% of total marine traffic volume and produces 80% (6831 ± 87% kg) of the annual emissions. It is the recreational sector that produces 82% (20851 ± 58% litres) of the non-combusted fuel releases into the harbour through inefficient outboard engines, bilge release and bunkering accidents at self-service fuel pontoons. By extrapolating cargo tonnage and release volume of hydrocarbons into Portsmouth Harbour it can be estimated that the total national chronic fuel spill inputs to ports and harbours in 2018 was 3000 m3 ± 58%, while BgP PAH release was over 1000 ±87% tonnes. These figures confirm that chronic operational input is an ‘unseen’ ongoing pollution crisis impacting urban harbours and ports. Despite identifying recreational craft as the source for much of the non-combusted fuel inputs into the harbour, no corresponding peaks in diesel associated PAHs were found solely in marinas. Although dissolved 2-6 ring group PAH distributions did peak in summer at all sites corresponding with the peak in summer marine traffic activity. There was some indication that incomplete combustion inputs were associated with sediment concentrations. Increases of 4 to 6 ring group PAHs were located at commercial ferry sites but these sediment concentrations showed no summer seasonal peak as seen in the water column. The concentrations of 3 to 6 ring PAHs did not change with season but remained stable throughout the year implying that either chronic inputs may result in sediment accumulation or resistance to degradation. From seasonal site sampling a disconnect was found between input, water column and sediment. Dissolved concentrations of PAHs were seasonally dynamic while sediment concentrations remained seasonally stable. The mesocosm incubations were run for 6 months under acute and chronic input conditions. Under both conditions, PAHs were lost from the sediment short term but only long-term loss was observed under an acute input of hydrocarbons. Whereas, in the chronic mesocosm a trend in steady accumulation was seen after 3 months. Within both acutely and chronically contaminated sediments there was an accumulation of 4 and 5 rings that could be the result of HMW > 10 ring PAH partial degradation. This may be a significant driver in the accumulation of PAHs in sediments under chronic conditions as the combination of direct and degradation additions outpace the rate of breakdown. If resuspension place 24 hours after contamination, then the concentrations of all ring groups were removed from the water column but after 24 hrs there was an increased dissolved PAH flux to the water column. This appears to be due to sediment microbial degradation that becomes more important with time. As seen in the mesocosm incubations, PAHs may be accumulating from partial degradation of HMW PAHs into metabolites and lower ringed ‘daughter PAHs’. The degradation reaches a peak at 14 days post-contamination resulting in high concentrations of PAHs in the sediment and in the water column. These fluxes into the water column may not be short lived. In particular, 4 ring influxes into the water column was shown to increase 3 hours after resuspension had stopped. This research has shown the complexity of PAH biogeochemistry and transport in urban harbours and ports. It has shown that any environmental monitoring of PAHs cannot be determined by seasonal sampling of water and sediment at limited sites due to the disconnect between point source, water column and sediment. That is due in part to microbial degradation producing ‘daughter PAHs’ that add to the reservoir of directly deposited PAHs coupled with degradation pulses or peaks that through resuspension will influx the water column to mix with any recently PAH inputs. Further work on developing management policy would be to investigate the environmental sensitivity and environmental capacity of a port and harbour to better inform marine traffic expansion and development. That assessment ought to be based upon the volume of incomplete combustion emissions and non-combusted fuel inputs from all marine traffic. It should incorporate a biogeochemical model linked to resuspension to estimate PAH fluxes to the water column from direct and indirect inputs.
Date of AwardDec 2020
Original languageEnglish
SupervisorFay Couceiro (Supervisor), Steve Mitchell (Supervisor) & John Williams (Supervisor)

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