In this video, we'll use a sand box to explain several concepts related to groundwater flow. In addition, we'll see how we measure groundwater levels in the field. A sandbox is a Plexiglas box that's filled with sand. In this case, quite coarse sand. It also has 10 tubes. These are wells stuck to measure the groundwater levels. In addition, there are two reservoirs that we can use to maintain a constant water level. If we look at the sand, we see that it's light and dry at the top while the bottom of the sand is a lot darker. If we start digging into the sand, we see that it becomes more cohesive. The further down we go and the wetter the sand becomes. At some point, we reach the water table. We see that the water level in the hole that we dug is the same as the water level that's indicated in our groundwater wells. If we start raining on our sand box, then, the water level goes up. Now let's go outside and see how we measure groundwater levels in the field. We measure groundwater levels in wells. Wells are pipes that have holes throughout their entire length, so that the water can flow in and out of the pipe. We can measure the water level inside the well with a beeper. The tip of this beeper makes a sound once it touches the water. At the moment, the water level is 145 centimeters below the top of the well. Since the well is 20 centimeters sticking out above the ground, this means the water level is 125 centimeters below the ground surface. In order to see how ground water levels change over time, we use water level loggers. They measure and record the water level every five minutes. Here we see a measured groundwater time series. The groundwater levels went up when it was raining, and then declined during extended dry periods. The Federal Office of the environment, monitors groundwater at more than 600 locations across Switzerland. We can also use the groundwater box to see how groundwater moves through an aquifer. In this case, we have a situation with two lakes. The water level in the lake on the left side is quite high, and the water level in the lake on the right side is much lower. Therefore, the groundwater levels are much higher on the left side than on the right side, and groundwater moves from the left side to the right side. We will now insert a color tracer, so that we can more clearly see how the water moves from one side of the box to the other side of the box, or how the groundwater moves through the aquifer. You can also think of the tracer as a pollutant that moves through our aquifer. When we inject a tracer below the lake, the water moves downward and sideways. This is the groundwater recharge area. The plume of the tracer spreads as it moves through the aquifer. Some of the tracer arrives at the lake quickly, while the majority of the tracer is still in the aquifer. Near the lake on the right side, the water in the tracer flow upwards. This is the groundwater discharge area. Because groundwater flows below the ground, we normally can't see it. But we hope that this video with the sand box has helped you to understand, and see how groundwater flows through aquifers.