Modelling Quantified Microbial Source Specific Pollution from Domestic Wastewater Treatment Systems during High Flows

The ability to model the transport of source specific faecal bacteria contamination in river networks can equip water resource managers with information of the different pathogens that are present. Such information can be particularly useful in catchment management plans for rivers from which potable water is extracted or where these rivers discharge to coastal zones where bathing or aquaculture is prominent and where the evaluation of human health risks is of primary importance. This paper presents an application and performance assessment of the commercially available MIKE11 Hydrodynamic model for evaluating the fate of faecal bacteria of human origin, Human Gene Marker HF183f, from Domestic Wastewater Treatment Systems within the Dargle catchment. The Dargle is a spate river and the upper catchment is characterised by steep slopes that incorporates peat bogs and land used for forestry and agricultural purposes. Residential dwellings within the area are predominantly single detached units that rely on septic tanks for wastewater treatment. In the context of faecal bacteria of human origin, malfunctioning systems are of concern, particularly in terms of surface ponding, leakage to groundwater and direct discharge to surface waters. The MIKE11 model was calibrated in a two stages process. Firstly, the model was calibrated for prediction of discharge and microbial water quality parameters, namely E. coli and Intestinal Enterococci (IE), using data from a real-time sensor network within the catchment that comprised rain gauges, weather stations and water level recorders, data from which was used to determine flow records from stage-discharge ratings. E. coli and IE concentrations were determined from high resolution sampling during storm events. Following this, water quality samples taken during storm event sampling were used to identify and quantify the human gene marker HF183f using quantitative polymer chain reaction (qPCR) techniques. Results from the qPCR analysis were used to further calibrate the model at sub-catchment level for the transport of microbial bacteria derived from human origins. Using non-compliance statistics from the EPA National Inspection Plan, domestic sources have been calculated based on the percentage number of malfunctioning septic tank units and average daily faecal gene marker concentrations per household. The study highlights issues with how the fate of the human gene marker is modelled using MIKE11, particularly in terms of advection-dispersion inputs and the requirement to associate microbial concentrations with total runoff when modelling surface and groundwater pathways.