Define biochemical oxygen demand (BOD).

Outline how turbidity changes after the raw sewage discharge point in Graph B.

Suggest how the population growth curve for algae in Graph C would appear if the pollutant had been nitrates and phosphates from fertilizer run-off.

Outline why point source pollution is often easier to manage than non-point source pollution.

a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity.

OWTTE.

[1 max]

after the point of sewage discharge the turbidity levels rise until about 150m (accept 125-175m) downstream where levels peak/plateau and thereafter steadily decline;

turbidity will increase at the point sewage enters the water, as the pollutant is particulate/coloured;

turbidity will increase after the outlet of sewage as bacteria grow rapidly as they consume/decompose the sewage;

turbidity remains high as algae now rapidly grow as there are nutrients from the sewage decomposition available;

turbidity decreases once the nutrients levels fall to the pre-sewage levels and the algae growth declines;

turbidity increases as algae decrease, and then decrease as algae increase. 

Award 1 max for a clear description of change in turbidity levels along the stream.

[2 max]

increase in nutrients will lead to rapid algae growth /could cause algal bloom;

algae levels will decline as nutrient levels become more restricted downstream from the source;

algae would look like the microorganisms curve in the diagram/ rapidly go up right after the pollution outlet;

algae would also decline, like the microbe curve, as the nutrients run out and algae start to die;

microbes would grow and they could shade out the light for algae causing a further drop in the curve;

the trend observed would be opposite to the current growth curve for algae (on graph C). 

Only credit responses that refer to algae levels in Graph C, do not credit descriptions of eutrophication.

[3 max]

point source can be clearly identified;

so pollution can be more easily monitored;

solutions should be more easily applied / pollution can be stopped directly;

with non-point pollution, source is widespread/dispersed and difficult to identify;

solutions also have to be widespread/dispersed, so there are increased costs/difficulties of monitoring;

compliance is difficult to ensure with non-point pollution because of widespread/dispersed nature of sources. 

[2 max]

Very few students were able to provide a comprehensive definition of biochemical oxygen demand.

Most students achieved a mark for describing changes in turbidity level along the stream. Few candidates were able to explain the cause of turbidity or give reasons for the changing levels.

Most candidates achieved some marks for this question but few achieved the full 3 marks. Many responses overly discussed eutrophication without reference to the curve.

This question was answered well by most students.