The Ministry of the Environment Conservation and Parks (MECP) has adopted legislation that requires the removal of up to 80% of the total suspended solids (TSS) present in urban stormwater. This is critical to stormwater management.
In many urban areas throughout Ontario, this legislation has led to the implementation of different types of stormwater management facilities (SWMFs), to reduce stormwater runoff pollutant loads. Civica evaluated sediment removal through a combination of wastewater monitoring, flow monitoring, and rainfall monitoring in addition to TSS sampling at nine SWMFs using enhanced forecasting technology and more innovative techniques of TSS sampling.
The goal was to determine if the stormwater management of ponds are providing the desired sediment removal efficiency.
Monitoring Design and Protocols for Sediment Removal Efficiency
The nine SWMFs to be evaluated were located in the Red Hill Creek Corridor in the City of Hamilton. The proposed included the assessment of sediment removal efficiency, which required time series flow measurements and the creation of TSS pollutographes (e.g., plots of the concentration of TSS as a function of time) at the inlet and outlet of each pond. The following sections details the monitoring approach.
Data loggers equipped with area-velocity sensors were installed on the inlet and outlet pipes of the SWMFs to capture the rising and falling hydrograph for the duration of each storm event during the monitoring period.
These loggers can store all necessary measurement parameters (water depth and velocity) to calculate the flow rate in full and partially full pipes. The data stored on the loggers were queried by real time data access (RDA) system, DataCurrent. Depth and velocity data were collected at five-minute intervals for each pond inlet and outlet in addition to level data for each pond. The following equipment were installed:
- 18 area velocity flow monitors with telemetry
- Nine level loggers (1 per SWMF)
Rainfall monitoring was an essential component of this project to quantify the storm events as well as to validate flow monitoring results. Rainfall data was leveraged to determine when to start sampling as well as to provide information to calculate rainfall characteristics such as volumes and intensities. A tipping bucket rain gauge with a telemetered data logger was set up on site to record rainfall events, in addition the City operates several rain gauges within the City that could be cross referenced.
To evaluate the TSS concentration of the water entering the SWMF, grab samples were taken at the inlet pipe to the facility’s forebay. To evaluate the TSS concentration leaving the facility, grab samples were collected at the outfall pipe of the facility. The trigger for deploying staff to complete grab samples were rain events equal to or greater than 25 mm over a 24-hour period. The DataCurrent software is capable of forecasting and alarming precipitation 48 hours in advance from three (3) global climate models. The forecast information available includes the volume, intensity, and the duration of the storm. In addition to this automated system, weather forecasts were regularly monitored to make informed decisions on when to deploy staff to collect the necessary grab samples.
The proposed sampling protocol was based on the Urban Stormwater BMP Performance Monitoring manual (USEPA, 2002). During qualified rainfall events, grab samples were taken at the pond inlet and outlet and kept in one-litre polypropylene bottles. The rough rain event duration estimate was used to establish the time interval at which the one-litre samples were taken in order to ensure that the samples were representative of the entire runoff period. On-site observations of the rain intensity and runoff flow were used to adapt the previously determined time intervals between samples. Collected grab samples were analyzed at a lab accredited for TSS analysis.
TSS Removal Efficiency Analysis
The efficiency ratio (ER) method was used to evaluate the TSS removal efficiency. This approach estimates the treatment efficiency and is primarily aimed at wet weather flows. ER is defined in terms of the average event mean concentration (EMC) of constituents over some time period.
The term EMC is a statistical parameter used to represent the flow-proportional average concentration of a given parameter during a storm event. It is defined as the total constituent mass divided by the total runoff volume.
Number of Samples
Sampling for this project included a total of 24 samples per pond per storm—12 at the inlet and 12 at the outlet. Typical sampling protocols included a customized sampling interval for the upstream (inlet) of each pond that was based on the comparison of upcoming rainfall data (e.g., forecast comparison of DataCurrent and local weather networks) and previous event response. Downstream (outlet) monitoring was based on a 36-hour drawdown that was typical for most sites, with some customization for events/ponds based on past event responses.
Monitoring and Sampling Locations
The monitoring locations were selected to meet the following criteria:
- Stations should be located where field technicians can safely access to during dry and wet weather conditions (e.g., where surface visibility is good and traffic hazards are minimal)
- Proposed locations are accessible especially during storm events
- Monitoring locations have the lowest potential for health and safety hazards
- Site hydraulic conditions are suitable for flow monitoring and meet the assumptions on which the flow measurement method is based
- Locations likely to be affected by backwater and tidal conditions should be avoided since these factors can complicate the reliable measurement of flow and the interpretation of data
TSS samples were collected during qualified storm events from the one upstream and one downstream location near where inflows and outflows are measured. Upstream TSS is indicative of concentrations and pollutant loads that would be observed downstream without SWMFs. It is important to monitor only waters that flow into the BMP to be able to use the resultant data to compare upstream water quality with downstream locations.
Monitoring stations established downstream of an SWMF were used to monitor TSS of flows that are removed by the SWMF. Monitoring stations were located immediately downstream of the SWMF so that effluent was sampled before it is introduced into the receiving waters or is exposed to factors that may affect constituent concentrations.
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