The City of Newton has approximately 271 miles of local sewer lines. Beneath an estimated 75 miles of these sewer lines are underdrain pipes, some of which were installed more than 100 years ago.
by Karl Honkonen
The City of Newton has approximately 271 miles of local sewer lines. Beneath an estimated 75 miles of these sewer lines are underdrain pipes, some of which were installed more than 100 years ago. They were installed to dewater the construction trench while sewer lines were being constructed and often continued to drain groundwater after construction was completed. Over the years, underdrain pipes have been plugged, relocated, abandoned, connected to local drain lines, and in some cases continue to discharge to local receiving waters.
Manhole rehabilitation - spray lining
The sewer and underdrain pipes have also deteriorated over the past 100 years and in many places there is an interconnection between the sewer and underdrain systems. In some locations, this results in underdrain flow, contaminated with wastewater, being discharged to the stormwater system. The City of Newton has identified several stormwater outfalls where water quality is being influenced by its extensive underdrain system.
Identifying and removing the contamination source(s) can be extremely challenging because it often involves three piping systems: sewer, underdrain, and stormwater. A balance must be struck between several competing interests: removing the source of contaminated underdrain flow from the stormwater system; preventing underdrain flow from backing up into the sewer system and exacerbating existing Infiltration/Inflow (I/I) issues; and preventing local groundwater levels from rising and affecting area basements.
Flow in portions of the underdrain system can be significant, in some cases exceeding 500,000 gpd. Preventing this contaminated water from entering the stormwater system, where it has been discharging for 100 years, while simultaneously preventing additional I/I and maintaining local groundwater conditions is a challenge.
Weston & Sampson is approaching the problem by first examining the sewer system. Sewer infrastructure located adjacent to the underdrain and stormwater systems is investigated to identify direct interconnections between the sewer and underdrain system. Structures are also examined for sources of exfiltration between the sewer and the underdrain. Identified sewer system defects are repaired and the underdrain outfall is sampled for contamination. If the outfall meets the appropriate water quality standards, the underdrain is not plugged and is included as part of the city’s regular stormwater sampling program to ensure future compliance. If, however, the underdrain continues to violate water quality standards, it is plugged and local groundwater conditions are monitored. Since the sewer system was repaired prior to addressing the underdrain discharge, increased I/I in the sewer system is less of a concern.
Manhole rehabilitation was an important part of the infiltration and inflow portion of the Newton project.
The City of Newton is looking at improvements to its sewer, underdrain, and stormwater systems as a group. This approach aims to address the concerns of both the EPA (NPDES Phase II Stormwater Requirements) and DEP (reducing I/I in the sewer collection system). The approach also provides economic benefits in the form of reduced sewer system O&M and transportation and treatment costs; environmental benefits in the form of improved water quality; and public relations benefit in the form of improved customer service and city image.
Another benefit of addressing multiple systems is increased funding opportunities. The city is able to use funding from the Massachusetts Water Resources Authority (MWRA) Local Financial Assistance Program (the regional wastewater authority’s I/I reduction funding program) for I/I related sewer system improvements and from its Stormwater Drain Fee.
Stormwater line from manhole
The Stormwater Drain Fee follows a simple, two-tiered rate structure: residential properties pay $6.25 per quarter, or $25 per year; all other properties pay $37.50 per quarter, or $150 per year. It turns out that this simple structure is reasonable and fair, in spite of different lot sizes. Larger homes tend to be on larger lots, with more open space for stormwater to seep back into the ground before leaving the lot and pouring into the city drain system, so the impact on the stormwater system of residential properties is roughly the same. Other properties, such as commercial buildings, tend to have a much higher percentage of their lots paved and covered with impervious surfaces, and thus have a much higher relative impact on the stormwater system.
Newton estimates that its direct annual stormwater program costs are approximately $575,000. This includes costs for a dedicated environmental/stormwater engineer who manages the entire stormwater program; training and new equipment for the stormwater management crew; ongoing maintenance and “good housekeeping” practices such as street sweeping and catch basin cleaning; a public education program; and support for one or more priority capital projects per year.
Newton’s stormwater fee program will be carefully monitored and, if necessary, changes will be recommended to the Board of Aldermen. Some questions that may come up for consideration include whether or not all noncommercial properties should be assessed at the same rate and how to ensure that the fees keep up with the costs of running the program. For those properties that have stormwater management in place or use low-impact development techniques, such as permeable pavement, rain barrels, dry wells or infiltration trenches, an abatement or credit can be requested.
About the Author:
Karl Honkonen is a Project Manager for Weston & Sampson in Peabody, MA, and actively promotes low impact development solutions to stormwater challenges.