3.1. Stormwater Quality
A summary of hydrological parameters for the seven sampled storm events is shown in Table 2
. Sampled storms had average rainfall intensities that ranged between 0.6 and 11.3 mm/h and storm durations between 2.83 and 11.75 h. The 2-year average rainfall intensities were calculated using local IDF curve equations [19
]. The highest intensity storm event over the evaluation period occurred on 25 June 2014, when 28.2 mm of rain fell over a 2.42 h period (Table 2
). Based on measured rainfall depth at the Milne Dam location and intensity-frequency-duration curves for the nearest climate station with 20 years or more of historical data (Environment Canada, 2013-Toronto Buttonville Airport climate station), this storm was approximately equivalent to the 2 year return period storm events of 5 min, one hour and two hours duration. Flows from this event were fully captured by the active storage component of the USDC (i.e.
, did not raise water level to the elevation of the overflow outlet pipes).
Descriptive statistics for EMC data is provided in supplemental materials (Table S1)
. TSS concentrations were observed to decrease through the USDC (TSS: Inlet > Forebay > Outlet). The forebay was found to be an important component in the TSS removal. Overall, 50% of the suspended particles were removed between the inlet and forebay with remaining particles being reduced again by 50% after traversing the permanent pool. The range of values at the outlet was observed to be much more regular than at the inlet, indicating that the pollutants at the outlet are fines that will not settle regardless of detention time.
The average removal efficiency of TSS by the USDC was 82%. Under provincial stormwater quality criteria the USDC is providing an “enhanced level of protection”, i.e.
, the longterm removal of 80% of suspended solids. With this removal rate the USDC is achieving similar TSS removal performance as compared to SWM ponds [8
]. Another important feature of the South Unionville USDC is that due to changes in catchment development and layout the USDC is significantly oversized. A smaller sized USDC would be expected to provide lower rates of TSS removal.
3.2. Stormwater Temperature and Turbidity
Temperature was monitored at the inlet and outlet to determine what effect USDCs have on the thermal properties of urban runoff. The time series record is provided in Figure 3
and a probability plot of the temperature data is shown in Figure 4
. The gap in recorded inlet data between 24 October and 11 November 2014 was caused by a temporary malfunction of the HOBO temperature sensor. In total ninety discrete rainfall events were observed over the six month period generating forty-three measureable thermal events.
Throughout the summer and early fall (June–September) the temperature of water at the outlet of the USDC was consistently cooler than inlet waters. During the summer months peak inlet temperature regularly exceeded 18 °C during runoff events. At the outlet, the USDC water temperature increases steadily from 8 °C to its highest around 13 °C in October. Water temperatures then steadily decreased as the year progressed into November. The inlet temperature varied significantly more than outlet temperatures. Runoff is thermally enriched as it flows over warmed impervious surfaces (e.g., asphalt) to the sewer system and into the USDC. The water stored inside the USDC is not exposed to energy inputs including solar radiation and atmospheric temperature. As the concrete structure of the USDC is installed 1.4 m below ground, it is well insulated from above ground air temperature fluctuations. Results shown in Figure 3
illustrate that effluent from the USDC remains cool during warm summer months and does not exhibit diurnal patterns in water temperature as would be seen in a SWM pond. This is likely due to heat exchange between the warm inflowing runoff and the cool permanent pool water and concrete structure itself.
Over the time period noted by Circle 1 in Figure 3
, a sharp drop and rise in the inlet temperature, and a sharp drop in the outlet temperature was observed. The drop was caused by a storm on 24th November that occurred during the night when air temperature was cooler than the permanent pool water temperature. Instead of being warmed by the impervious surfaces, the runoff entering the USDC was warmed as the stormwater flowed through the chambers. Outlet water temperatures were frequently warmer than inlet temperatures throughout November.
and Figure 6
present box plots of minimum, mean and maximum inlet and outlet temperatures during summer and fall runoff events, respectively. Relative to previous case studies of a USDC [1
] the observed outlet temperatures were very cool. For summer storms the most significant thermal effect is a pronounced reduction in event maximums. On average, outlet maximum temperatures were 5 °C cooler than inlet maximum temperatures. Additionally, event mean and minimum temperatures were typically reduced by 4 °C and 2.7 °C respectively. It is important to recognize that the cool outlet temperatures observed by at the South Unionville USDC are influenced by the oversizing of the USDC. The temperature of the outflow eliminates the release of thermally enriched runoff and would be appropriate for discharge into coldwater streams [22
]. For the general protection of aquatic life, daily mean temperatures of 18 °C and a maximum daily temperature of 19 °C is recommended [24
]. Over the study, two events had inflow temperatures that exceeded the daily mean temperature guideline. More significantly, ten events had inflow temperatures that exceeded the daily maximum guideline. Outflow from the USDC never exceeded 13.5 °C demonstrating that the system provided a significant thermal buffering benefit.
During the fall (October and November) inlet and outlet water temperatures were very similar; averaging 13.8 °C and 13.2 °C during runoff events, respectively. Outlet temperatures were extremely uniform during the fall while inlet temperature were more variable ranging from 7 °C to 17 °C. Cold water species (e.g., brown trout, brook trout) require water temperatures of 13 °C or cooler for spawning during October and November [24
]. Therefore the USDC mitigated thermal enrichment during rain events over these months also, particularly in October, to such an extent that outflow water could be released to a downstream receiving waterbody where coldwater fish spawning habitat exists.
The turbidity of inlet and outlet flows was also continuously monitored over the summer and fall (Figure 7
, Figure 8
, Figure 9
and Figure 10
). In total thirty-eight turbidity events were observed.
In monitoring studies TSS and/or suspended solids (SS) [5
] has been reported more often than turbidity [20
]. During runoff events, the mean turbidity at the inlet was 33 NTU which is higher than inlet turbidities reported in several pond monitoring studies (e.g., 17 NTU [20
], 9–21 NTU [26
]). During the data collection several areas within the South Unionville USDC remained under active construction with un-stabilized soils. Therefore, it is not entirely surprising that runoff delivered to the USDC was often highly turbid. During runoff events, the mean turbidity at the USDC outlet was 8.8 NTU which is also higher than levels reported in some of the SWM pond literature (1 NTU [20
Turbidity at the USDC outlet, in both the summer and fall, was an order of magnitude smaller than inlet levels. Provincial Water Quality Objectives (i.e.
, local drinking water standards) recommend a turbidity of less than 5 NTU [28
]. Outlet turbidity was above this objective during summer runoff events but was achieved during some fall events. For the protection of aquatic life, it is recommended that high flow water with background levels greater than 80 NTUs should not experience more than a 10% increase during short term events [29
]. For the majority of runoff events this target was achieved.
3.3. Vertical Profiles
Vertical measurements of temperature, TDS (calculated from specific conductivity measurements) and DO (Figure 11
) show that DO stratification was present within the USDC permanent pool water column at times during the study period. Water temperatures and TDS only slightly increased (≤1.6 °C and <2 g/L, respectively) with depth. Therefore the USDC appears to have limited stratification.
DO levels were found to decrease with depth and exhibited some stratification (Figure 10
). The DO stratification is likely due to accumulation and decomposition of fine particulate organic matter in the permanent pool. For the protection of warm water aquatic life it is recommended that DO concentrations remain above 6 mg/L [28
]. The water in the USDC was found to be below the minimum acceptable DO concentration for protection of aquatic organisms at all depths in the 10 March 2014, 2 December 2014 and 18 February 2015 profiles and for the majority of the 4 May 2015 profile (Figure 10
). The low DO levels are an important drawback of USDC and should be considered seriously by designers when selecting underground detention systems as a stormwater control and treatment approach. To gain a better understanding of dissolved solids and DO stratification in USDCs and how it compares to SWM ponds, further sampling is required to capture conditions after a longer period of the USDC being in service, during winter, spring and summer months, and over the course of individual storm events.