Modified Faecal Indicator Organism (FIO) decay models for nearshore coastal waters
Authors: Ahmadian Reza , Lam Man Yue 
Published: March 2024

Contaminants in nearshore coastal waters have far-reaching public health and economic implications, such as contaminated food from aquaculture, reduced tourism, and the associated economic losses. The US marine economy annually provides 2.4 million jobs and contributes &#163;312 billion (US$397 billion) to the country&#8217;s Gross Domestic Product. Domestic overnight trips to coastal areas in Great Britain contributed &#163;4.6 billion in year 2022. Faecal Indicator Organisms (FIOs) are a class of contaminants that are highly correlated with illnesses such as gastrointestinal, eye, nose and throat infections, and skin complaints. FIOs are commonly used to indicate pathogen levels in waterbodies and have been routinely monitored in bathing water sites. Numerical hydro-epidemiological models have been developed for water quality prediction and management. FIO decay modelling is an integral part of hydro-epidemiological models to simulate the die-off of FIOs after they have been injected into the waterbodies. While the Stapleton et al. (2007) FIO decay model has been successfully applied for Severn Estuary and Bristol Channel, UK, this research identified two model limitations. They were: (i) the modelled decay rates for dark or highly irradiated environments are not accurate, and (ii) the effect of salinity is not included. The Stapleton decay model was modified by (i) imposing a minimum decay rate (ClipStap model); and (ii) extrapolating the decay rate-irradiation slope at a reference irradiation (260 W/m2) down to lower irradiation regions (ModStap model). The modified models were tested with a TELEMAC-3D hydro-epidemiological model for Swansea Bay, UK. Buoyancy effects due to the salinity difference between river fresh water and saline seawater have been included as the effects are found to be critical for FIO transport. The model was validated and evaluated against the water level, velocity, salinity and FIO concentration data obtained in the &#8220;Smart Coasts &#8211; Sustainable Communities (SCSC)&#8221; research project in year 2011 and 2012. Results showed that while the ModStap model successfully reproduced the reported dark decay rates in the literature, it did not always give better FIO prediction results. In addition, this research demonstrated that the observed diurnal variations of FIO concentrations are caused by the combined action of riverine FIO inflows, tide action, and FIO decay. Given the unsuccessful model prediction, the effect of sediment-FIO interactions (Huang et al., 2015) will be tested with the hydro-epidemiological model. These insights on the effect of irradiation, diurnal FIO variations, and sediment-FIO interactions on bathing water quality are critical for the management of coastal human activities, and nearshore ecology.&#160;Reference:&#160;(i) Stapleton et al. (2007). Link: https://assets.publishing.service.gov.uk/media/5a7c5af4ed915d696ccfc370/scho0307bmef-e-e.pdf; (ii) Huang et al. (2015). doi: 10.1080/15715124.2014.963863&#160;Keywords: Nearshore coastal waters; FIO decay models; irradiation; diurnal variations; hydro-epidemiological models; Swansea Bay