Three monitoring wells are used (as shown below) to determine the direction of groundwater flow in a confined aquifer. The piezometric heads in the wells are found to be 70 m in well 1, 80 m in well 2 and 90 m in well 3. Determine the direction of flow and hydraulic gradient.

Background material

Groundwater-level data can be used to determine direction of groundwater flow by constructing groundwater contour maps and flow nets. To calculate a flow direction, at least three observation points are needed. First, relate the groundwater field levels to a common datum – map datum is usually best – and then accurately plot their position on a scale plan. Second, draw a pencil line between each of the observation points, and divide each line into a number of short, equal lengths in proportion to the difference in elevation at each end of the line (Figure 7). The third step is to join points of equal height on each of the lines to form contour lines (lines of equal head). Select a contour interval that is appropriate to the overall variation in water levels in the study area. The direction of groundwater flow is at right angles to the contour lines from points of higher head to points of lower head (Figure 8).