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Application of Groundwater Modeling Tools to Evaluate Potential Impacts from Stormwater Infiltration in Philadelphia

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Abstract:

In 1993, the Detroit Water and Sewerage Department (DWSD) started the development and implementation of its Long-Term Combined Sewer Overflow Control Plan (Plan). The Plan utilized a demonstration approach to mitigate 20 billion gallons (BG) of untreated annual CSO discharges to the Detroit and Rouge Rivers from 76 outfalls situated within the city limits of Detroit. To support this approach, DWSD evaluated the collection system transport and wastewater treatment plant (WWTP) capacities, monitored/evaluated in-stream water quality, evaluated CSO control and treatment technologies, and formulated and evaluated CSO control alternatives using complex modeling. Such early CSO program efforts resulted in the construction of: 5 retention treatment basins; 3 screening and disinfection facilities; in-system storage improvements; expansion of the primary WWTP capacity; among other related essential capital investments exceeding 900 million (2008 USD). These initial conventional “grey” treatment projects have contributed to approximately 50%, or 10 BG, of annual CSO discharge reductions to such receiving waters.

However, extended engineering and capital investments undertaken since 2002 to mitigate CSO discharges from 17 outfalls remaining for the upper segment of the main branch of the Rouge River in Detroit that culminated with the start of construction of a 7-mile deep rock CSO capture tunnel project were halted in 2009 due to extreme financial distress DWSD first encountered in 2008. To combat such protracted economic conditions, the Michigan Department of Environmental Quality (MDEQ) and DWSD in partnership with the City of Detroit and Southeast Michigan Council of Governments (SEMCOG), evaluated an array of less costly CSO control alternatives for the 39 square mile upper Rouge CSO tributary area that the nearly 1 billion tunnel project would had otherwise provided, capturing all but one annual overflows, on average, as originally approved by the MDEQ and intended for completion in 2016.

The most cost effective alternative CSO controls evaluated for the upper Rouge tributary area, accepted by the MDEQ and subsequently mandated under provisions of DWSD's June 29, 2011 NPDES Permit modification, provides a 25-year phased timeline. The alternative Plan includes the combination of a “green infrastructure program” in conjunction with nine “right sized” grey treatment facilities within the sub-drainage areas served. The first two such facilities (the first under design) are pilots, consisting of an off-line “first flush” capture tank, in-sewer disinfectant injection piping, and end-of-pipe disposable nets. Upon successful performance evaluations, similar facilities will be constructed for the remaining sub-drainage areas over an extended timeline through 2034 with annual incremental green infrastructure investments, as well, through 2029. Consistent with the hydrologic and land cover parcel analysis performed with various long-term green infrastructure scenarios developed in 2009-2010, DWSD pledged 50 million investment through 2029 in green infrastructure across the upper Rouge tributary area to reduce 10%-20% annual storm water runoff from entering the combined system to thereby further mitigate CSO's.

The aforementioned Green Infrastructure Program is a key component of the Alternative Long Term CSO Control Plan for the Rouge River. Green Infrastructure is a recognized approach to augment wet weather management that is cost-effective, sustainable, and environmentally friendly. Green Infrastructure promotes infiltration, evapotranspiration and related benefits to capture and reuse storm water to maintain or restore natural hydrology in a given watershed. Examples of green infrastructure include tree planting, rain gardens, bioswales, planter boxes, rain barrels, demolition of uninhabitable/dangerous properties and replacement with pervious and vegetative cover. Once the alternative CSO program for the upper Rouge is implemented in 2029, as much as 20% reduction of storm water runoff volume will be removed from the tributary combined system to thereby reduce the size and cost of conventional grey treatment facilities with a projected savings of 600 million to rate payers.

This paper discusses the planning approach utilized during the permit modification negotiations to demonstrate green infrastructure feasibility and includes short-term and long-term implementation strategy and meeting multiple objectives including, storm water runoff reduction, neighborhood stabilization and quality of life enhancements. This paper will also outline how DWSD is working towards achieving the above goals, how ongoing planning efforts are occurring simultaneously with green infrastructure implementation and most importantly, how the collaborative planning and implementation process is unfolding across multiple departments with support from outside partners. Since the program commenced in 2010, citizens are already noticing improvements through demolition of vacant structures, lot greening and significant tree plantings along streets and in parks.

Document Type: Research Article

DOI: https://doi.org/10.2175/193864712811726978

Publication date: 2012-01-01

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