O.2 Inactivation

Sunlight exposure is an important inactivation mechanism for all forms of pathogens and microbial indicators in both fresh and saline waters. In waters of high clarity, it has long been regarded as the most dominant inactivation mechanism. To compute this inactivation flux, the individual inactivation rates due to visible, UVA and UVB light are calculated and combined to compute an overall inactivation rate \(R_{invn}^{pth}\) via Equation (O.2). \[\begin{equation} \left.\begin{aligned} R_{invn}^{pth} = 10^{-6} \times \left(\underbrace{\left[\frac{\left[DO\right]}{K_{DO_{vis}}^{pth} + \left[DO\right]}\times \left[k_{vis}+c_{S_{vis}}S\right]\times PAR\right]}_{\text{Influence of visible light}} + \ldots \\ \underbrace{\left[\frac{\left[DO\right]}{K_{DO_{uva}}^{pth} + \left[DO\right]}\times \left[k_{uva}+c_{S_{uva}}S\right]\times UVA\right]}_{\text{Influence of UV-A light}} + \ldots \\ \underbrace{\left[\frac{\left[DO\right]}{K_{DO_{uvb}}^{pth} + \left[DO\right]}\times \left[k_{uvb}+c_{S_{uvb}}S\right]\times UVB\right]}_{\text{Influence of UV-B light}}\right) \end{aligned}\right\} \tag{O.2} \end{equation}\] \(K_{DO_{vis}}^{pth}\),\(K_{DO_{uva}}^{pth}\), and \(K_{DO_{uvb}}^{pth}\) are the user specified half saturation oxygen concentrations for visible, UV-A and UV-B light inactivation, respectively, \(k_{vis}^{pth}\),\(k_{uva}^{pth}\), and \(k_{uvb}^{pth}\) are the user specified freshwater inactivation rate coefficients for exposure to visible, UV-A and UV-B light, respectively, \(c_{S_{vis}}^{pth}\),\(c_{S_{uva}}^{pth}\), and \(c_{S_{uvb}}^{pth}\) are the user specified coefficients that enhance the inactivation effect of light under saline conditions for exposure to visible, UV-A and UV-B light, respectively, \(S\) is ambient salinity, and \(PAR\), \(UVA\) and \(UVB\) are the photosynthetically available, UV-A and UV-B radiation intensities at the centre of a model cell. The multiplier preceding the calculation is for units conversion purposes.

The inactivation rate \(R_{invn}^{pth}\) from Equation (O.2) is multiplied by free and attached pathogen concentrations to compute the inactivation flux at each model timestep in each model cell via Equation (O.3). \[\begin{equation} \href{AppDiags.html#WQDiagInvn}{F_{invn}^{pth}} = R_{invn}^{pth} \times \left( \left[ PTH_a \right] + \frac{\left[ PTH_t \right]}{2}\right) \tag{O.3} \end{equation}\] \(\left[PTH_a\right]\) and \(\left[PTH_t\right]\) are the concentrations of free and attached pathogens, respectively. If attached pathogens are not simulated then \(\left[PTH_t\right]\) is set to zero for the purposes of the above calculation. The reduced (50%) impact of \(R_{invn}^{pth}\) on attached pathogens reflects the shading of half of all attached pathogens by their associated sediment particles (i.e. those on the underside).