Department Urban Water Management

DIMES II - Diffuse micropollutant emissions from urban areas

A first Swiss-wide assessment of micropollutants in wet-weather discharges indicated that many combined and separate sewer overflows can be critical at a local assessment level.

Model-based screening for critical wet-weather discharges related to micropollutants from urban areas

Wet-weather discharges contribute to anthropogenic micropollutant loads entering the aquatic environment. Thousands of wet-weather discharges exist in Swiss sewer systems, and we do not have the capacity to monitor them all. We consequently propose a model-based approach designed to identify critical discharge points in order to support effective monitoring.
We applied a dynamic substance flow model to four substances representing different entry routes: indoor (Triclosan, Mecoprop, Copper) as well as rainfall-mobilized (Glyphosate, Mecoprop, Copper) inputs. The accumulation on different urban land-use surfaces in dry weather and subsequent substance-specific wash-off is taken into account. For evaluation, we use a conservative screening approach to detect critical discharge points. This approach considers only local dilution generated onsite from natural, unpolluted areas, i.e. excluding upstream dilution.
Despite our conservative assumptions, we find that the environmental quality standards for Glyphosate and Mecoprop are not exceeded during any 10-min time interval over a representative one-year simulation period for all 2500 Swiss municipalities. In contrast, the environmental quality standard is exceeded during at least 20% of the discharge time at 83% of all modelled discharge points for Copper and at 71% for Triclosan. For Copper, this corresponds to a total median duration of approximately 19 days per year. For Triclosan, discharged only via combined sewer overflows, this means a median duration of approximately 10 days per year. In general, stormwater outlets contribute more to the calculated effect than combined sewer overflows for rainfall-mobilized substances. We further evaluate the Urban Index (Aurban,impervious/Anatural) as a proxy for critical discharge points: catchments where Triclosan and Copper exceed the corresponding environmental quality standard often have an Urban Index >0.03.
A dynamic substance flow analysis allows us to identify the most critical discharge points to be prioritized for more detailed analyses and monitoring. This forms a basis for the efficient mitigation of pollution.
Mutzner, L.; Staufer, P.; Ort, C. (2016) Model-based screening for critical wet-weather discharges related to micropollutants from urban areas, Water Research, 104, 547-557, doi:10.1016/j.watres.2016.08.003, Institutional Repository

Data for micropollutants in wet-weather discharges is still scarce. Therefore, we aim to monitor and understand the occurrence of micropollutants (e.g. pharmaceuticals, biocides, pesticides) from urban areas by studying selected catchments with defined land uses. We surveyed two sites in the urban water observatory with conventional active water sampling and passive samplers to assess the applicability of passive samplers for rain-event based sampling in sewers.

 

Monitoring of 20 sewer overflows with passive samplers

Passive samplers to quantify micropollutants in sewer overflows: accumulation behaviour and field validation for short pollution events

Contaminants in sewer overflows can contribute to exceedances of environmental quality standards, thus the quantification of contaminants during rainfall events is of relevance. However, monitoring is challenged by i) high spatiotemporal variability of contaminants in events of hard-to-predict durations, and ii) a large number of remote sites, which would imply enormous efforts with traditional sampling equipment. Therefore, we evaluate the applicability of passive samplers (Empore styrene-divinylbenzene reverse phase sulfonated (SDB-RPS)) to monitor a set of 13 polar organic contaminants. We present calibration experiments at high temporal resolution to assess the rate limiting accumulation mechanisms for short events (<36 h), report parameters for typical sewer conditions and compare passive samplers with composite water samples in a field study (three locations, total 10 events). With sampling rates of 0.35–3.5 L/d for 1 h reference time, our calibration results indicate a high sensitivity of passive samplers to sample short, highly variable sewer overflows. The contaminant uptake kinetic shows a fast initial accumulation, which is not well represented with the typical first-order model. Our results indicate that mass transfer to passive samplers is either controlled by the water boundary layer and the sorbent, or by the sorbent alone. Overall, passive sampler concentration estimates are within a factor 0.4 to 3.1 in comparison to composite water samples in the field study. We conclude that passive samplers are a promising approach to monitor a large number of discharge sites although it cannot replace traditional stormwater quality sampling in some cases (e.g. exact load estimates, high temporal resolution). Passive samplers facilitate identifying and prioritizing locations that may require more detailed investigations.
Mutzner, L.; Vermeirssen, E. L. M.; Mangold, S.; Maurer, M.; Scheidegger, A.; Singer, H.; Booij, K.; Ort, C. (2019) Passive samplers to quantify micropollutants in sewer overflows: accumulation behaviour and field validation for short pollution events, Water Research, 160, 350-360, doi:10.1016/j.watres.2019.04.012, Institutional Repository

Passive samplers in sewers and rivers with highly fluctuating micropollutant concentrations – Better than we thought

Considerable pollutant loads can enter surface waters during rain events. Three factors challenge quantification of these pollutant fluxes using traditional sampling methods: (i) concentration fluctuations; (ii) unknown event duration; and (iii) placement, operation, and maintenance of equipment. Passive samplers offer the advantage of sampling in a continuous mode without power supply. However, variable uptake rates due to environmental factors and desorption in the case of fluctuating concentrations can affect the accuracy of time-weighted average (TWA) concentration estimates. While uncertainties related to environmental factors could be accounted for with additional effort, we can neither control nor quantify the concentration variability. We present measured and modelled concentration profiles at high temporal resolution and provide a systematic approach to assessing deviations from true TWA concentration due to fluctuating concentration profiles. We evaluate sampling of sewer overflows (0.3–14 h) with Chemcatcher and 1-week sampling in rivers. The uncertainty due to fluctuating concentrations is small, and other factors such as chemical analyses and sampler calibration have a similar or higher impact. The uncertainty due to fluctuations clearly increases with the sampling duration, particularly when exceeding the half-life of equilibrium. We conclude that passive sampling can also be used in wastewater systems with potentially high concentration variations.
Mutzner, L.; Vermeirssen, E. L. M.; Ort, C. (2019) Passive samplers in sewers and rivers with highly fluctuating micropollutant concentrations – Better than we thought, Journal of Hazardous Materials, 361, 312-320, doi:10.1016/j.jhazmat.2018.07.040, Institutional Repository

In 2017, we monitored 20 wet-weather discharge sites across Switzerland with the efficient passive sampler approach. With the collected monitoring data we aim to identify differences between urban catchments and find correlation between land use and micropollutant discharge.