| Date | November 2019 | Marks available | 3 | Reference code | 19N.1.SL.TZ0.4 |
| Level | Standard Level | Paper | Paper 1 | Time zone | Time zone 0 |
| Command term | Describe | Question number | 4 | Adapted from | N/A |
Question
Figure 10: Untreated sewage release into the St Lawrence River
Montreal:
In November 2015, the City of Montreal discharged between 5 and 8 billion litres of untreated sewage into the St Lawrence River.
- It was called “FlushGate” in the Canadian news.
- Citizens were warned to avoid contact with the water.
- River pollution levels returned to normal within 4 to 10 days.
- The Mayor of Montreal described it as the “most environmentally-friendly solution”.
Quebec:
In November 2016, the City of Quebec discharged 110 million litres of untreated sewage into the St Lawrence River.
Nationally:
- 25 % of Canadians do not have access to sewage or wastewater treatment centres.
- 205 billion litres of untreated sewage are released into Canadian rivers and oceans each year.
[Source: adapted from www.cbc.ca/news]
Figure 11(a) and (b): Coliform bacteria concentrations (in units/100 mL) in the
St Lawrence River near Montreal
Figure 11(a): Before the release of untreated sewage
Figure 11(b): One day after the release of untreated sewage
[Source: adapted from CBC news, © OpenStreetMap contributors www.openstreetmap.org/copyright and © Mapbox.
Data adapted from Ville de Montréal the open data portal http://donnees.ville.montreal.qc.ca/dataset?q=intercepteur
and licensed under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/legalcode]
Using Figure 11(b), estimate the highest concentrations of coliform bacteria (in units/100 mL) found in the St Lawrence River one day after the untreated sewage was released.
Outline an environmental problem that may result from the release of untreated sewage into a river.
With reference to Figures 10, 11(a) and 11(b), describe a method to monitor the impact of the release of untreated sewage into the St Lawrence River ecosystem.
Markscheme
120 000 (units/100 ml);
Units are not required.
eutrophication / algal bloom;
… due to high levels of nitrates and phosphates/nutrients;
hypoxic conditions;
… due to high oxygen demand;
rotten egg smell / production of hydrogen sulphide;
… due to anaerobic decomposition;
increase in micro-organisms/pathogens within shellfish;
… due to pathogens being filtered out of the water;
ill health in people / increase in waterborne disease;
...eating shellfish/fish contaminated with pathogens/bacteria (from sewage);
...from swimming in water contaminated with pathogens/bacteria (from sewage);
...drinking water contaminated with pathogens/bacteria;
death/loss of benthic species
… due to particulates blocking feeding/respiratory systems;
decrease in photosynthesis;
… due to increase in turbidity (reducing light penetration).
Accept any other reasonable response. Answer must have the named problem and the associated outline for both marks.
Do not accept ‘thermal pollution/increase in temperature’.
Do not accept just ‘loss of biodiversity / water unsuitable for human use’.
can use indirect or direct measures of pollution;
direct measurements of dissolved oxygen using a probe / light and dark bottle method to measure BOD / direct counts of coliform units using microscopes / Secchi disc to monitor turbidity / titration to measure changes in nitrates/phosphates;
freshwater invertebrates can be used as indicator species;
compare with historic data / take measurements before the release of the untreated sewage (for ‘normal’ measurements);
take measurements along the length of the river, from the point source downstream (to determine area impacted);
take measurements over a period of time, to observe changes in water quality.
