Integrated Modeling to Support Combined Sewer Overflow Long Term Control Plan

INTRODUCTION Capital Region Water (CRW) is the municipal authority responsible for enhancing, maintaining, and managing the water, wastewater, and stormwater infrastructure in the City of Harrisburg. Combined sewers comprise approximately 60% of CRW's collection and conveyance systems. The combined sewer overflows (CSOs) are discharged into the Susquehanna River and Paxton through CSO outfalls (Figure 1). CRW has developed and submitted an updated City Beautiful H2O Program Plan to formulate a long-term, integrated approach for wastewater and stormwater management. This work includes a technically feasible, water-quality–driven, and financially sustainable long-term CSO control strategy under a Modified Partial Consent Decree (MPCD) involving CRW and the City of Harrisburg, along with the U.S. Department of Justice (DOJ), the U.S. Environmental Protection Agency (EPA), and the Pennsylvania Department of Environmental Protection (PADEP). The key objectives are to control CSO discharges, reduce collection system backups, improve the health of local waterways, protect public health and safety, and achieve compliance with the instream water quality standards. METHODOLOGY A comprehensive water-quality monitoring program and a linked hydrologic, hydraulic, and water-quality modeling framework were implemented to evaluate the alternatives for achieving water quality compliance. The monitoring program targeted storms that will trigger CSOs, included automated sampling at key CSO and stormwater outfalls, and discrete and continuous sampling along Paxton Creek and Susquehanna River. Figure 2 presents the water quality monitoring locations. A SWMM collection system model generated systemwide CSO characteristics and pollutant loads, which were used as inputs into two receiving-water models: a 1D WASP model of Paxton Creek consisting of tributary inflows, stormwater loads from the separate-sewer systems, and CSO loads from CRW's combined-sewer system; and a 2D EFDC model of the Susquehanna River with approximately 3,300 cells and 30 CSO outlets capable of representing near-shore plumes and cross-channel gradients. The models were calibrated and validated using the monitoring data collected in 2023 and 2024. MONITORING RESULTS Water quality monitoring was conducted in 2023 and 2024. A summary of rainfall characteristics and Susquehanna River flows during the sampled events is provided in Table 1. Figure 3 shows the box and whisker plots for measured [i]E. coli[/i] in the Susquehanna River, Paxton Creek, Asylum Run (tributary to Paxton Creek), and the CSO and stormwater outfall locations. The wet weather measurements in the Susquehanna River along the eastern transect, located close to the shoreline, are an order of magnitude higher than the western transect measurements. The reported [i]E. coli[/i] levels in the western transect were below the recreational standards. Figure 4 shows the spatial distribution of [i]E. coli[/i] for the Oct 14, 2023 event. These indicate that the impact of CSO discharge (associated with high bacteria) is limited to the near-shore area of the Susquehanna River. ALTERNATIVE EVALUATION CRW conducted a CSO control technology screening process, whereby a comprehensive list of control technologies was developed and evaluated. Six distinct, feasible mixed-technology alternatives (MTAs) that integrate green stormwater infrastructure (GSI), retention treatment basins (RTBs), targeted sewer separation, satellite storage, and strategic conveyance enhancements were identified. The MTAs were applied throughout the CRW collection system to determine the facility sizes and the cost of implementation to provide a range of CSO control. The integrated models were utilized to evaluate and assess compliance with applicable bacteria water quality standards in the Susquehanna River and Paxton Creek for baseline conditions and the MTAs during the typical year. Model results for the baseline condition show that CSO-driven bacteria plumes are confined to about 150–700 feet from the eastern shoreline in the Susquehanna River. No non-compliance was observed near City Island Beach, an important recreational area, in this scenario (Figure 5). The recommended MTA (MTA-6), with approximately 12 overflows, was simulated to be fully compliant with bacteria standards in the Susquehanna River. For Paxton Creek, the model results show that the baseline CSOs result in non-compliance with [i]E. coli[/i] standards downstream of River Mile (RM) 2.5. For the MTA-6 with 16 overflows to Paxton Creek, the non-compliance is reduced downstream of RM0.5. MTA-6 - featuring a 30 MGD RTB, 200 acres of GSI, and 7 storage facilities - achieves substantial water-quality improvements while improving affordability and constructability. Typical-year CSO volumes decrease from 280 MG to 45 MG and from 513 MG to 96 MG in the Susquehanna and Paxton Creek respectively, with annual overflow frequencies falling to 12 and 16, respectively. SIGNIFICANCES The integrated monitoring and modeling framework provides a robust foundation for CRW's long-term CSO control strategy, by reducing over-design risk through background sensitivity testing and probabilistic compliance analysis while focusing investment in areas with the highest receiving-water benefit. The approach supports adaptive management, enabling continued refinement of infrastructure placement and sizing as additional data and information becomes available.