Electrical Conductivity (EC) has been used to analyze the content of dissolved salts in water. EC refers to the ability to transmit electricity (Ikeda et al. 1991). Pure water has low values of EC because the electricity is conducted by the ions in solution; therefore, the greater the concentration of ions in water, the greater the value of EC. This tutorial explains how to analyze the EC of a river using the Profile Tool plugin in QGIS 3.0.
To begin with, the plugin must be installed by clicking on Plugins/Manage and Install plugins and search Profile Tool and click on Install/Upgrade plugin
Add the vector layer that contains the information about the rivers
Add the point layer that contains the information about the electrical conductivity of the river samples
Now, interpolate the EC point data to obtain a raster with the information of the EC of the study area. The Inverse Distance Weighted (IDW) Interpolation tool will be used. Go to the Processing toolbox and search IDW interpolation. The vector layer will be the Electrical Conductivity shapefile and the interpolation attribute will be the column containing the EC values. Click on the green plus sign and click on the three points next to the Extent option:
Click on select extent on canvas and select the extent of your study area to obtain a raster with the same extent and click on Run in Background afterwards:
Open the Profile Tool by going to Plugins/Profile Tool/Terrain profile
Then, select the interpolated raster and click on Add Layer in the Profile Tool window:
Then go to options, Selection and choose Selected Polyline
In Layers, select the one that has the information of rivers:
And click on the river of interest, the river will turn to yellow because it means that it was selected:
The EC profile through the course of the river will be shown in the profile window ad a graph. The X axis is the location of the river and the Y axis the EC values in micro Siemens/cm. The EC profile can be saved by clicking on the Save as button, select the folder where you want to save it and click on Save.
For this example, what can be seen is that the EC in the intersection of the rivers is much higher than the one upstream. The characteristics of the river are different throughout its course. At 0, we assume that there is a natural spring with low contents of dissolved ions and when it intersects with the next river, a higher concentration of ions appears. There are several reasons for this to happen, the first one could be higher evaporation rates in the intersection of the river, if there is a lower volume of water, the ions concentrate; the second one could be a potential source of contamination, the intersection of the rivers is close to a mine tailing but we do not have enough information to guarantee this assumption, and the third one could be due to different geological processes, since we do not have information about the geology profile we cannot dismiss the option of rock dissolution happening in the river. This exercise allows identifying potential processes involved in the variability of dissolved ions in water and it is an excellent way to visualize the spatial variation of the chemistry of water in the area.
You can download the input files for this tutorial here.
Ikeda, S. et al., 1991. Ion concentrations in river water and their estimation from electric conductivity. Analytical sciences, 7(Supple), pp.1077–1080.