Green Infrastructure Implementation Constraints in Flood Prone Partially-Separated Wastewater Systems

So what happens when green infrastructure infiltrates runoff into the ground in a densely-developed city? Does it disappear and sustain an aquifer and watercourse baseflows? That's the theoretical benefit. Or does it end up in foundations drains (weeping tiles) making its way to the local wastewater treatment plant on low rain days and contributing to sewer back-ups on the high rain days? its the latter. It is surprising how quickly foundation drains respond to surface water inputs - Toronto Water presented at the National Research Council expert panel this week that it takes only 4 minutes for surface water to end up in foundation drains!

Municipal wastewater engineers have know this for a while - that systems with no apparent direct inflows of rainwater or runoff respond quickly to rainfall. My work on a Municipal Class Environmental Assessment in Kitchener, Ontario showed the greatest correlation of wastewater peak wet weather extraneous flows was to the 5-minutes rainfall intensity. Often we expect groundwater collection systems in cities respond slowly to rainfall volumes - they don't. The respond rapidly to short duration rainfall.

So what does this have to do with green infrastructure and the suite of sweet low impact development measures many are going gaga over? Well, its the impact of GI and LID runoff infiltration on wastewater systems. Quick and clear impact. At the NRC expert panel this week I summarized a list of documents that expressed concern for GI and LID wastewater system impacts including aggravated basement flooding due to this infiltration. This is the list:

1) Water Environment Association of Ontario (July 17, 2017 memo) 

Identifies concerns of interference with wastewater systems (flood impacts), water distribution systems (chlorides/corrosion), human health impacts to drinking water distribution (compliance with Procedure F-6-1), excessive costs:

2) City of Ottawa (February 23, 2017 letter)

Indicates "While the intent appears to be not to "make things worse or better" (specifically with respect to current condition runoff volumes), there should be recognition of situations in older neighbourhoods (with partially separated sewers, for example) where increased infiltration should be avoided given the cumulative impacts over time that could raise groundwater levels leading to increased risk of basement flooding, increased I & I to sanitary sewers, etc.”:

3) City of Barrie (July 10, 2017 letter) 

Identifies financial impacts, capital cost increase of 200%, lifecycle cost increase of 550%, concerns for “damage to private properties and excessive sewer infiltration”, LIDs “highly susceptible to failure due to sand accumulation” :

4) City of Guelph (June 28, 2017 memo)

"Guelph downtown stone rubble masonry heritage buildings are prone to flooding with raised groundwater elevation; any additional infiltration measures using LIDs may aggravate basement flooding due to leaky masonry walls and severe impacts on the buildings structural stability; in addition, impacts on aged infrastructures such as, watermain corrosion, potable water quality interference (F-6-1) and enhanced sanitary infiltration can be anticipated.”

5) City of Markham (July 14, 2017 memo)

Identifies sanitary infiltration impacts, adjacent property impacts, excessive capital cost based on completed tenders, high soft cost, high lifecycle costs, chlorides/watermain corrosion:

6) Ministry of the Environment / Workshop on Stormwater Quality Best Management Practices (1992)

Identifies impacts of on-site infiltration source controls called Best Management Practices (BMPs):
 " - basement leakage problems related to infiltration near housing
   - surcharging of sanitary sewers by short circuiting of infiltrated water”
Therefore the impacts of green infrastructure have been long-known in Ontario. Unfortunately, green infrastructure is often cited as a panacea for water resources challenges in Ontario, often when only narrow view of is taken that ignores existing municipal infrastructure systems and practical constraints associated with the infiltration of large quantities of chloride and contaminant-laden stormwater runoff.

7) US Transportation Research Board / Evaluation of Best Management Practices for Highway Runoff Control, Issue 565 (2006)

Identified inflow and infiltration (I/I) risks with infiltration green infrastructure (BMPs) in urban areas:
“In urban areas, unrestricted infiltration may exacerbate infiltration and inflow (I/I) problems in both separate and combined systems”

7) City of Seattle / Street Edge Alternatives Project (city web site)

Identified bioretention groundwater impacts to adjacent properties through engineering analysis, indicating that green infrastructure introduces property flood risks:

"Our original hope for retaining flows and allowing infiltration into the native soils throughout the length of the block was not possible because some homes had an existing groundwater intrusion problem. To limit the potential for stormwater to adversely impact the residences of concern, our geotechnical engineers identified some swales that needed an impermeable liner.” :

We could go on and on. The InfraGuide on inflow and infiltration, the CSA guideline on IDF curves - these all note the issue with infiltration stresses on wastewater systems.

Green energy in Ontario gave us smart metres that did not improve the flow of money. Green infrastructure can give us smart swales that will not improve the flow of groundwater .. OK, in partially-separated sewersheds.

Urban groundwater flow has been called the darkest of the dark arts, making hydrology look like algebra - call wastewater systems groundwater infiltration analysis something fancy like the RTK method and it sounds scientific, but there is really just a mysterious system of 'urban karst' of hidden pathways and problems lurking under the surface. A black box. Those of us in the industry who have worked closely on wastewater smoke and dye tests know that adjacent laterals 'speak to each other' and the subsurface flow is erratic and unpredictable.

As Jeff Goldblum said in Jurassic Park, 'Nature Finds a Way', and the natural flow of infiltrated water finds a way too ... right into foundation drainage and overtaxed wastewater systems. As Adrienne Barbeau said in The Fog "Be afraid, be very afraid" ... as there is huge fog surrounding green infrastructure and low impact development measures - we should re afraid of what these things will do to our wastewater systems.