| Date | May 2010 | Marks available | 1 | Reference code | 10M.3.HL.TZ1.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 7 | Adapted from | N/A |
Question
Release of sewage in marine waters is a common practice but it can cause water contamination with pathogens. A series of experiments were conducted to compare inactivation rates of two different groups of microbes with different sunlight exposures. One group were fecal coliform bacteria and the other were coliphage viruses. Experiments were conducted outdoors using 300-litre mixtures of sewage-seawater in open-top tanks.
A two-day experiment was carried out with untreated sewage added to seawater. Both days were sunny with no clouds. The figure below shows the inactivation of the microbes in seawater as a function of the cumulative amount of sunlight and time. The survival curves of the two microbes are plotted against sunlight exposure (lower x axis) during daylight periods and against time during the overnight period (upper x axis). The y axis gives counts of bacteria and viruses per 100 ml.
Identify the time at which fecal coliform bacteria counts fell below 1 unit per 100 ml.
Deduce, using the data in the graph, the effect of sunlight on fecal coliform bacteria.
Deduce, using the data in the graph, the effect of sunlight on coliphage viruses.
For an accidental sewage spill, suggest, giving a reason, which of the two microbes may be most useful as a fecal indicator two days after the spill.
Markscheme
28 hours after untreated sewage added (Allow answers in range 27–29 hours)
sunlight reduces counts of fecal coliform bacteria significantly;
fecal coliform bacteria fall below 1 count per 100 ml on day 2;
no reduction during dark period;
significant drop on day 1 / bacteria count drops from 105 per 100 ml to less than 102 per 100 ml on day 1;
sunlight causes small reduction of coliphage viruses;
coliphage virus counts never fall below 102 counts per 100 ml;
no reduction during dark period;
coliphage viruses because they are less affected by the sun / numbers do not decrease much in two days
Examiners report
Most were able to correctly read the graph to identify the time as 28 hours after sewage was added.
Many were able to correctly deduce the effect of sunshine on (i) coliform bacteri.
Many were able to correctly deduce the effect of sunshine on (ii) coliphage viruses.
Only some were then able to correctly suggest that coliphage viruses were the most useful fecal indicator as they were less affected by the sun.
