# Determine the direction of groundwater flow using borehole water levels

I am working on a project that focusses on groundwater of the Marico Biosphere Reserve in South Africa and I would like to determine the general direction groundwater flows through the study area. Using the South African National Groundwater Archive, I requested information for 3000 boreholes within and surrounding the study area:

For the selected boreholes in the above image, I have the depth of the latest water level reading as well as the elevation of the borehole itself. Using either QGIS or Global Mapper, what would be the best way to determine the general flow of groundwater?

• gis.stackexchange.com/questions/194511/… Commented Mar 12, 2019 at 18:51
• Add a field for water elevation in the boreholes, using the field calculator and an expression like `"borehole_elevation" - "water_depth"`. Interpolate the points to create a smooth surface of groundwater elevation, then create a slope raster. Those last two steps are explained in the link @cyril provided.
– csk
Commented Mar 12, 2019 at 19:53
• Of course, this all assumes that groundwater flows uniformly from areas where the water table is high, to areas where the water table is low. Is that a valid assumption? Are there other variables to consider? You might want to ask about that on Earth Science SE.
– csk
Commented Mar 12, 2019 at 19:58

## 1 Answer

1. Reduce the water levels to give RLs i.e., reduced levels to a common chosen unique horizontal line. (Simply choose a datum line))
2. Select three holes that form a reasonable triangle with substiantial differences in RLs
3. Use the the highest and lowest water level Rls from the 3 holes and interpolate the water level RL that matches the intermediate intermediate RL (THIRD RL)
4. Draw a line from the point of interpolation to the THIRD RL. This line is called an equipotential line (line of equal total pressure)
5. The direction of flow is perpendicular to the equipotential line and towards thebline of the lowest pressure (THIRD RL).

Repeat the above for other selected 3 bores to eliminate errors in data acquisition and well construction.

Hope this helps

Edem ([email protected]) Retired Geotechnical Engineer