Aquifer


An aquifer is an underground layer of water-bearing material, consisting of permeable or fractured rock, or of unconsolidated materials like gravel, sand, or silt. Aquifers vary greatly in their characteristics.
Wikipedia

The term aquifer originates from the Latin, "water-bearing" or "water bearer" from aqua=water and ferre=carry. An aquifer is any layer of rock containing water that can flow through the rock at a certain speed, so it must have cavities that are connected to each other. This is obviously strongly connected with karstification, and this process creates connected voids, not only caves but also smaller crevices and cracks. So all karst areas have an aquifer, but there are other rocks which contain aquifers like porous sandstones, or unconsolidated sediments.

The water is connected, which means that the water surface is at the same elevation all over the aquifer, this is a direct result of the law of communicating vessels. At least it would be, except for another fact: the water body is in general not static, there is outflow and recharge. Water goes into the aquifer through rainfall which then vanishes in dolines, sinkholes, and cracks in the rock. Also loosing streams, especially at the border of the aquifer to insoluble rock, flow into the groundwater through sinks. Outflow is another word for spring, in this case a karst spring, where the water flows out of the aquifer. The groundwater table therefore has a gradient from the inflow to the outflow, whereby the spring itself is naturally part of the groundwater table. In other words, the water table of the spring extends into the rocks and is then called groundwater table. Depending on the geography, the groundwater table may have different forms. When all rivers flow in on one side and the springs are on the other side, the water table forms a slightly slanted plain. If the inflow is from rain falling in a certain area, and there are springs on all sides, the water table forms a hill with the summit at the point which most distant from a spring.

The gradient of the water table influences the energy of the movement and thus the speed. If there are heavy rains, the water table rises where the rain falls, as a result, the water flows much faster to the spring. There is enough space in the cave system, and the water flows as a flood, and soon the flood arrives at the spring which has a much higher yield. This is a typical behaviour of a karst aquifer, and different from other aquifers, which have a limited size of pores and so the throughput is limited and the yield oncreases only slightly.

The aquifer also provides a way to classify caves into three groups. Above the water table of this aquifer, the caves do not have water, they are called dry or fossil, the scientific term is vadose cave. Of course, they are not dry, there is dripping water and high humidity, they even may have standing lakes, but they do not have a cave river. Below the water table the caves are completely filled by water and are called underwater cave, the scientific term is phreatic cave. Then there is the area around the water table, caves in this area are typically filled with air but have a river flowing through the cave. Those caves are called river caves, the scientific term is epiphreatic cave. As the water table may go up and down periodically, those river caves may be dry during the dry season, and may be reactivated or even flooded during the wet season.

However, these different types of caves can also be viewed as a chronological sequence. Cave formation takes place in four phases: Dissolution of the limestone underwater, erosion by a cave river, disappearance of the cave river and formation of speleothems and finally reaching the surface, collapse of the ceiling and disappearance of the cave. The last two phases are vadose caves. This cave development described here is a simple model that can be observed in many cave areas. The obvious cause of this development is the fact that the receiving water, the valley with the spring, cuts deeper and deeper, turning the underwater cave into a river cave and finally into a fossil cave. The development shifts downwards floor by floor, creating cave systems with five, six or seven floors.