Portail Géomatique UniLaSalle

Satelite view of algae bloom over lake Erie

QE001 – Pollution of surface water bodies by diffuse contaminants

1. About this exercise

  • GIS expertise: intermediate
  • Duration: 3 hours
  • GIS software: QGIS 3.40
    • Consult this  section  to switch to English language interface.
  • Last update: 30/07/2025
  • Requested tools or skills :
    • Selection of features by two ways:
      • By attributes
      • By location
    • Clip shapefiles
    • Dissolve features
    • Design layout of the map
  • Elements of vocabulary:
    • Area of Interest (AOI) : zone d’études
    • Plots: parcelle agricole
    • Watershed: bassin versant

2. Context

You are asked to identify the agricultural plots which are close enough to the river network. Hence, they may contribute to water pollution due to agricultural contaminants (e.g. fertilizers or pesticides). This work will be useful to promote best agricultural practices in order to mitigate the transfer of contaminants to the environment.

About diffuse contamination

The diffuse contamination refers to the release of potential pollutants from a range of activities that, individually, may have no effect on the water environment, but, at the scale of a watershed, can have a significant effect. (SEPA , 2019).

The fertilizers and pesticides spread on the agricultural plots may reach the water bodies (rivers, lakes, aquifers) and affect the water quality status. High concentration of fertilizers leads to eutrophication and algal bloom (cf. introductive picture).

The image displays a tractor spraying fertilizers on a field
Tractor spraying on a field.

3. Instructions

Design a map layout presenting the following features:

  • the plots located at max. 200m of the rivers and which are cropped with wheat,
  • the area of interest (AOI) is “Le Thérain” sub-watershed, located in the Seine watershed,
  • the river network flowing inside the AOI,
  • the OpenStreetMap basemap,
  • a small frame displaying the AOI extent within the Seine watershed,
  • the usual map layout (scale bar, North arrow, etc.)

Export the map in PDF

a. Expected results

The picture represent a layout with 2 maps : a small that indicates the location of the area of interest, a large one with the plots that may contaminate the rivers with fertilizers
Example of expected map

b. About the final map layout

The small map (global scale) will be realized in first, then the detailed map (local scale).

4. Dataset

a. Download the dataset

The dataset is available  here . Paste the ZIP file in your personnal folder and unzip it.

Follow the guidelines below to organise your dataset. All the generated files must be placed in the ‘Output‘ folder.

One folder contains all the elements of the project. Within this folder, there is an "input" folder containing input data and an "output" folder containing output files such as datasets, projects, and exported map layouts.

b. Metadata review

Please fill the ‘metadata.XLSX’ Excel file (stored within the dataset).

You may find information by accessing the  QGIS documentation .

5. Spatial analysis

Step 1: Design the global map (map 1)

Step 2: Clip the dataset to the AOI extent

Step 3: Select the wheat plots

Step 4: Select the plots close to the river network

6. Design the map layout

7. To go further

In France, some water ways are required to be protected by buffer strips. These buffer strips are strips of grassy vegetation of at least 5m in width. Their role is to slow down the water runoff and hopefully catch contaminants before they reach the waterway.

You can find the protected waterways for the AOI  here .

All the waterways protected by BCAE 4 measures are available for download  here .

Plan

→ Index