Identifying Elusive Infiltration: Lessons Learned From A Service Lateral Dye Water Flooding Pilot Program

The Boston Water and Sewer Commission (BWSC) owns and operates a wastewater and stormwater collection system that consists of 1,455 linear miles of sewer, eight pumping stations, 47,413 manholes, 35,934 catch basins, 243 outfalls, and 202 tide gates. In 2014, the Massachusetts Department of Environmental Protection (MassDEP), under 314 CMR 12.00, mandated that municipalities must complete an infiltration and inflow (I/I) analysis of their system and develop a targeted Sanitary Sewer Evaluation Study (SSES) in areas with excessive infiltration and inflow (I/I). BWSC developed a city-wide I/I Analysis Report in May 2017 to achieve compliance with 314 CMR 12.00. BWSC has performed several efforts to improve the overall operation of the system, including sewer separation, combined sewer overflow (CSO) reduction, sanitary sewer overflow (SSO) reduction, and rehabilitation work throughout the system. Since 2001, BWSC has commissioned multiple investigative studies in the Mattapan neighborhood of Boston to locate sources of I/I. These studies have implemented a multitude of field investigation programs including smoke testing, building inspections, dyed water testing, closed circuit television inspection, and manhole inspections. Prior to 2019, nearly 80% of the neighborhood had been smoke tested and every building was inspected for external sources of inflow. Despite these extensive investigations for inflow sources, flow metering conducted as part of the City-wide I/I Analysis Report completed in May of 2017 showed that many sewersheds within the Mattapan neighborhood still have a direct response to rainfall, with R-values greater than 10%. Therefore, in 2020, the BWSC embarked on a SSES to investigate the Mattapan neighborhood as well as portions of Hyde Park, Roslindale, Roxbury, and South Dorchester. The project team devised an alternative SSES strategy to identify previously undocumented sources of infiltration and inflow. As the typical investigative techniques to locate inflow sources, such as building inspections and smoke testing, had already been exhausted, a new and 'outside the box' approach was necessary. A dye water flooding program to investigate the impact of service lateral rainfall induced infiltration and inflow (RDI/I) was included in the Mattapan SSES implementation plan. Dye water flooding involves applying dyed water to the ground surface directly above a service lateral to simulate rainfall as shown in Figure 1. The dye and applied water infiltrate into the ground, potentially into the sewer system as shown in Figure 2. The mainline sewer is monitored at the connection between the mainline sewer and the targeted service lateral for the presence of dye with the use of a CCTV camera. The presence of dye or an increase in flow indicates a hydraulic connection between water percolating through the ground surface and defects in the service lateral. Over a two-week period in October 2021, the project team and its field teaming partner implemented the dye water flooding pilot program with the goal of investigating a total of 50 properties. Access issues, weather, application techniques, and public relations were only some of the difficulties faced during the planning and execution of the program. During the execution of the pilot program, the project team made observations and adjustments in the field to adapt the intended approach to simulate real world conditions while determining the contributions of service laterals to RDI/I. As shown in Table 1, a total of 44 investigations were completed, and approximately 30% of properties exhibited a hydraulic connection between water percolating through the ground surface and the service lateral itself.
Based on the findings of this Mattapan SSES Dye Water Flooding Pilot Program, service lateral dye water flooding has proven to be a worthwhile investigation technique under certain conditions. The purpose of this paper is to discuss the challenges and lessons learned from the implementation of the pilot program. Topics covered will include public outreach, public relations, rainfall simulation calculations, critical success factors, and recommended conditions for the implementation of this innovative investigation technique.