An aquifer is an underground layer of water-bearing rock. Water-bearing rocks are permeable, meaning that they have openings that liquids and gases can pass through. Sedimentary rock such as sandstone, as well as sand and gravel, are examples of water-bearing rock. The top of the water level in an aquifer is called the water table. An aquifer fills with water from rain or melted snow that drains into the ground. In some areas, the water passes through the soil on top of the aquifer; in others, it enters through joints and cracks in rocks. The water moves downward until it meets less permeable rock. Aquifers act as reservoirs for groundwater. Water from aquifers sometimes flows out in springs. Wells drilled into aquifers provide water for drinking, agriculture, and industry. Students in Ms. Mieja's class constructed this model of an aquifer. They were curious about the effects of fertilizers and toxins on the quality of the ground water. They wanted to modify their model to show how fertilizers travelled in this system. What would their next step be in modifying their model?