11 Your task

The key objective of the investigation is to determine the presence/level of organochlorine pesticides (OCPs) in the Chinese coastal bay shown in Figure 8. The bay covers an area of approximately 380 km² and is fed by a number of rivers running from mainly agricultural land, where the use of OCPs has, in the past, been widespread.

Figure 8 Map of the bay and surrounding area with reference grid and sampling locations. The bay is of variable depth; the area highlighted as shallow open water does not exceed 6 m in depth at low tide. © The Open University

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The diagram is a bird's eye view map of a bay and surrounding area. A grid with three columns (A to C) and six rows (one to six) overlays the map. The grid contains eighteen cells, with each cell ten kilometres wide. Thirteen red stars, denoting the location of sampling, are placed within the grid at locations C1, A2, B2, C2, A3, B3, C3, A4, B4, C4, B5, B6, and C6. Marsh/salt ponds are located at, or close to, cells A1-A5 and B1. Urban areas are located within cells A4-A6, B1, and C1-C6. Aqaculture is located at C1. Inter-tidal mud flats are located at A1-A5, B1-B2 and C1. Shallow open water is located at A2-A5, B2-B6, C1-C6. Deep water is located at B3-B6, C2-C4 and C6. Annotation: The map includes the following coloured keys: inter-tidal mud flats (light blue), shallow open water (cyan), deep water (dark blue), urban area (grey), marsh/salt ponds (yellow), aquaculture (light blue with grey stripes).

Figure 8 Map of the bay and surrounding area with reference grid and sampling locations. The bay is of variable depth; the area ...

The area surrounding the bay is densely populated; the eastern shore is a largely built up, urban and industrial area with a major port. The bay is tidal with a small opening in the south into the Yellow Sea. The northern shore consists of intertidal flats, which are submerged at high tide but exposed at low tide, salt ponds and a number of ponds used for the cultivation (aquaculture) of shellfish. The bay, like many coastal regions in China, suffers from algal blooms due to high nutrient levels

In order to determine the levels of OCPs in the bay you need to decide which sites to sample. To help you do this most efficiently first think of a hypothesis you would like to test. It is important to compare results with a ‘null hypothesis’. For example, suppose that we want to evaluate pollution from a point source (such as a river or a sewage pipe) into a lake. An example of a null hypothesis would be that there is no change in concentration of a pollutant across the lake, irrespective of its distance from the point source. To test this you might take one sample at the point source and another one or two for comparison at a distance from the point source

• Discuss and form a hypothesis about the concentration of pesticides in the tidal bay area. Use the map and information which follows it, as well as the information given in Table 2 about the particular sampling location for each grid area to help you.

  To help you refer to the map and table together it will be useful to copy the table into a Word document. Highlight Table 2 and press Ctrl-C, then open a Word document and press Crtl-V. You may need to adjust the zoom which is in the ‘View’ menu in Word.

• Assume that this is an initial study and you are only able to sample a limited number of sites within the bay. Develop a sampling plan that will test your hypothesis.

  Think about:

  • Where are the best sites for the sampling?
  • How many samples should be taken?
• Write down your hypothesis, null hypothesis and sampling plan.