2005 Spill Dumps a Million Litres of Sewage Into Riparian Area
by Dave Burkhart
BLESS members and directors who attended the photo monitoring tour of Big Lake last July may remember encountering construction workers and machinery working on a sewage leak at the north end of 215 street near Big Lake.
The incident report submitted to Alberta Environment on July 27 2005 by the Alberta Capital Region Wastewater Commission (ACRWC), the entity responsible for the line that leaked, stated 1,100,000 litres of sewage had been released approximately 500 metres from Big Lake.
Sewage being pumped from Spruce Grove and Stony Plain had escaped from a hole in the sewage line running along the south shore of the lake. The leaking sewage eventually worked its way to the surface and then flowed overland into a forested riparian area. The leak was reported by a resident of the nearby farm at 9:00 p.m. on Friday July 22, 2005. The ACRWC report stated it was difficult to determine how much of the released sewage, if any, entered Big Lake.
Gord Thompson, professional engineer and general manager at ACRWC said the sewage line running along the south shore of Big Lake is gravity fed west of 231 Street. From there it is pressurized by the pumping station on the shore of Big Lake, at a line pressure of approximately 100 psi, to lift the sewage uphill to its next destination -- an ACRWC treatment facility at 127 Avenue and 195 Street.
Mike Darbyshire at ACRWC said the Big Lake pumping station was commissioned in 1985. The station operates continuously 24 hours a day seven days a week. It is not manned or physically monitored on a regular basis but does have a SCADA (Supervisory Control and Data Acquisition) system that enables remote monitoring. There is an alarm system within the station that activates when doors are opened or wastewater is released. The facility has its own backup generator, regularly tested and maintained according to Darbyshire, to power the pumps and operate valves in the event power is lost in an emergency. The pumping station incorporates an overflow structure that can divert raw sewage directly to Big Lake. On a typical day the Big Lake pumping station moves roughly ten million litres of raw sewage uphill through the line between 231 Street and Edmonton.
Overnight on July 22, crews who were first on the scene shut down the pumping station to reduce the amount of sewage flowing from the hole in the force main. Intense rainfall later that night forced the crew to re-energize the pumps to cope with the increased inflow backing up the system.
Saturday morning, July 23, the same day the BLESS monitoring tour arrived on the scene, crews were making plans to dig a sump to contain the overland flow and use septic trucks to haul away any sewage the sump might accumulate. Around 3:00 p.m. Saturday, however, the flow suddenly stopped, likely from a blockage within the force main. A septic truck remained on standby through the night in the event the flow of sewage began again.
Sunday morning, the sewage entering the line was diverted to the old Spruce Grove lagoons on Century Road and efforts to repair the line began. Crews began excavating to find the source of the leak which was finally located on Wednesday, July 27th.
Brian Lacey was the investigator from Alberta Environment Compliance and Monitoring who responded to the incident. He visited the scene and said the sewage had been leaking from a hole in the pipe two- to three-inches long and approximately half an inch wide. The hole appeared to have been caused by corrosion that most likely began because of a tear in the plastic covering of the pipe, incurred when the line was first installed.
Repair crews welded a patch over the hole in the pipe, installed a sacrificial anode and repaired the outer coating. The line was pressure tested Thursday morning, July 28th and restored to service that afternoon. The ACRWC report stated it was not possible to recover the spilled sewage because of steep topography in the area. The report also said recovery would be disruptive to the riparian zone where the sewage ended up.
In a telephone interview, Thompson said another leak had occurred near the same location in 1992 but it had been caused by a different mechanism. In that case the pipe had shifted because of movement in the underlying thalweg and had developed a crack. A thalweg is the lowest longitudinal profile of a stream or valley. At the time, the crack was repaired, horizontal drains were drilled and the site was regraded.
Because of steep terrain, the section of sewage line that leaked was originally installed by boring rather than trenching, the method employed to install the majority of pipe along the south shore of the lake. Most of the pipe in the line is "Hyprescon", a steel cylinder with a concrete liner manufactured with high tensile wire wrapped around the outside and sprayed with a gunite covering.
The pipe that leaked is, however, different. That section of the line has steel pipe with a polyethylene jacket and is installed roughly 12 metres below the surface. Thompson said this is the only section of pipe along the south shore that was installed by boring.
Because it removes, for treatment elsewhere, the sewage from two growing municipalities that at one time flowed into Atim Creek, Thompson said the line is part of the solution to environmental degradation -- in spite of the two leaks that have occurred over 20 years of operation.
Big Lake has a problem with nutrient loading. The west bay of the lake is designated by Alberta Environment as being eutrophic and the east bay is hyper-eutrophic. Thompson said the nutrient loading of the million litres of sewage released in the July 2005 spill was equivalent to a bag of fertilizer.
According to Thompson, the sewage line between Spruce Grove/Stony Plain and Edmonton has a capacity of 0.7 cubic metres per second and the ability to handle 60 million litres per day during high rainstorm events. ACRWC is able to divert sewage to the Spruce Grove lagoons whenever capacity is exceeded. They also have the ability to divert raw sewage directly into Big Lake at the pumping station if necessary. Thompson said the line installed in the mid 1980s is expected to have a life of 100 years.
When questioned about line maintenance, Thompson said the line is visually inspected annually. Slopes where the sewage line runs are checked for stability. The line is not normally pressure tested. He said leaks such as the one experienced in July 2005 cannot be identified by monitoring flow or pressure drops.