HGS HIGHLIGHT – Estimating cumulative wastewater treatment plant discharge influences on acesulfame and Escherichia coli in a highly impacted watershed with a fully-integrated modelling approach
Hwang, H.-T., Frey, S. K., Park, Y.-J., Pintar, K. D. M., Lapen, D. R., Thomas, J. L., Spoelstra, J., Schiff, S. L., Brown, S. J., & Sudicky, E. A. (2019). Estimating cumulative wastewater treatment plant discharge influences on acesulfame and Escherichia coli in a highly impacted watershed with a fully-integrated modelling approach. In Water Research (Vol. 157, pp. 647–662). Elsevier BV. https://doi.org/10.1016/j.watres.2019.03.041
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In this research highlight, researchers used HydroGeoSphere (HGS) to explore the impact of wastewater treatment plant (WWTP) discharge on surface water contamination in a mixed-use watershed in Ontario, Canada. The study focused on tracking acesulfame, a commonly used artificial sweetener, and Escherichia coli (E. coli), a fecal indicator, to understand how these contaminants move between surface and groundwater systems. Understanding the interactions between surface water and groundwater is critical in watersheds where WWTP discharge contributes to regional water quality concerns.
“A three dimensional fully integrated discrete fracture numerical model was constructed using HydroGeoSphere […] for the purpose of simulating spring flow at various elevations for the reference period and future predicted climate scenarios...”
Figure 1. (a) Location of the Grand River Watershed, (b) watershed topographic profile, and (c) the spatially varying finite element mesh within the 3D HydroGeoSphere model (modified after Hwang et al., 2015)
By leveraging HGS, the researchers were able to simulate complex hydrologic interactions, capturing both surface water and groundwater flow dynamics. The HGS model provided detailed insights into how contaminants, such as acesulfame and E. coli, behave under various flow conditions. For example, during low-flow periods, WWTP discharge was a dominant contributor to stream flow, making it more difficult for the system to dilute contaminants. In contrast, high-flow periods allowed for more flushing of contaminants into groundwater systems, leading to complex variations in contaminant levels downstream.
The findings highlight the importance of flow variability in influencing the movement of contaminants from WWTPs. While WWTP discharge was found to contribute significantly to contaminant levels during low-flow periods, the presence of other non-point sources of E. coli, such as agricultural runoff or wildlife, became apparent under higher flow conditions. The simulations demonstrated how hydrologic conditions influence contaminant transport, suggesting that groundwater and surface water interactions must be accounted for in water management strategies to effectively address pollution.
This research highlights the value of integrating HGS modelling with field data to better understand the dynamics of contaminant transport in mixed-use watersheds. By providing a clearer picture of how WWTP discharges interact with natural water systems, the study offers a foundation for improved management of water resources, public health, and environmental protection in regions with significant groundwater–surface water interactions.
Abstract:
Wastewater treatment plant (WWTP) discharge is often considered a principal source of surface water contamination. In this study, a three-dimensional fully-integrated groundwater–surface water model was used to simulate the transport characteristics and cumulative loading impacts of an artificial sweetener (acesulfame) and fecal indicator bacteria (Escherichia coli) from WWTPs within a 6800 km2 mixed-use, highly impacted watershed in Ontario, Canada. The model, which employed 3.5×106 computational nodes across 15 subsurface layers, facilitated a comprehensive assessment of groundwater–surface water interactions under high and low flow conditions; processes typically not accounted for in WWTP cumulative effects models. Simulations demonstrate that the model had significant capacity in reproducing the average and transient multi-year groundwater and surface water flow conditions in the watershed. As a proxy human29 specific conservative tracer, acesulfame was useful for model validation and to help inform the representation of watershed-scale transport processes. Using a uniform WWTP acesulfame loading rate of 7.14 mg person-1 day-1, the general spatial trends and magnitudes of the acesulfame concentration profile along the main river reach within the watershed were reproduced; however, model performance was improved by tuning individual WWTP loading rates. Although instream dilution and groundwater–surface water interactions were strongly dependent on flow conditions, the main reach primarily consisted of groundwater discharge zones. For this reason, hydrodynamic dispersion in the hyporheic zone is shown as the predominant mechanism driving acesulfame into near-stream shallow groundwater, while under high flow conditions, the simulations demonstrate the potential for advective flushing of the shallow groundwater. Regarding the cumulative impact of the WWTPs on E. coli concentrations in the surface flow system, simulated transient E. coli levels downstream of WWTPs in the watershed were significantly lower than observed values, thus highlighting the importance of other sources of E. coli in the watershed.