In order to define groundwater flow directions and rates through aquifers, individual measurements of hydraulic head are combined to generate contour maps of water level – or potential energy (Figure 29). These maps define the potentiometric surface, which is much like a topographic contour map but defines the distribution of potential energy in the groundwater system. Each contour, or equipotential, represents a line of equal hydraulic head.
Video: Slag Heap Experiment (3:46)
Transcript: Slag Heap Experiment
Text On Screen: Slag Heap Experiment
Text On Screen: What would happen to groundwater if it rained on the slag heap?
Presenter: Now that we've thought about how and why groundwater is moving through this system, we want to use the groundwater model to make some predictions about how a contaminant would move through the groundwater system. So let's imagine our model represents a geologic cross-section under the East Helena smelter site and we want to think about how contamination from the slag heap would move through the groundwater system. So if the slag heap was rained on, arsenic and selenium from the slag would leach into the groundwater system. How do we think that contamination will move through this system and why? Take a minute to think about how and why contamination will move through this groundwater system.
Text On Screen: Where do you predict contamination will go if it rains on the slag heap? Why?
Presenter: Okay, so now we're going to add some dye at the location of the slag heap and see where that contamination moves in our model of a groundwater system.
Text On Screen: Watch in 8X fast motion
Text On Screen: Discuss: What happened and why? How was what happened in the model similar to the real world? How was what happened in the model different from the real world?
To first approximation, groundwater flows down-gradient (from high to low hydraulic head). As is the case with surface water, or a ball rolling down a hill, the water flows in the direction of the steepest gradient, meaning that it flows perpendicular to equipotentials. There are exceptions to this – for example, if the hydraulic conductivity of the aquifer is much higher in one direction than another, or dominated by fractures with particular orientations, then these can redirect groundwater flow askew to the maximum gradient.
The potentiometric map also provides clues about the rate of groundwater flow. If you think back to Darcy material and our in-class activity from last week, you will recall that groundwater flow rate depends on the head gradient (i.e. the hydraulic gradient) and hydraulic conductivity. In a simple one-dimensional Darcy tube experiment, the head gradient is just the difference (h1-h2)/L. In two dimensions, the head gradient is defined by the slope of the potentiometric surface – just as the slope of the land surface is defined on a topographic map. The path that water takes in the aquifer, defined as a continuous line tracing the maximum gradient on a map of the potentiometric surface, is known as a flowline.
