The Nature of Rocks Against Ground Water
According to Krusseman (Bakri, 2003) in terms of the nature and behavior of rocks to ground water, especially physical properties, structure and texture, the rocks can be divided into 4 (four) types:
1. Aquifer is a layer of rock that has an arrangement such that it can store and drain significant groundwater such as sandstone, and limestone
2. Aquatic mud is a layer of rock that can store water but cannot drain ground water in significant quantities such as clay, shale, fine tuff
3. Iitarum is a layer of rock that can store water but can only drain ground water in very limited quantities such as basal scoria, shale, marl, and claystone
4. Aquiflug is a layer of rock that cannot store and drain groundwater such as igneous rock and metamorphic rocks and even if there is water in the rock layer, it is only found in solid or fractured rock.
When viewed from the nature and stratigraphy of rocks in nature, the aquifer layer can be distinguished, including (Bedrock types on the composition of ground water):
1. Unconfined aquifer (free aquifer) is an aquifer where the ground water level is the upper boundary plane of the water saturated zone.
Ground water contained in the aquifer layer is called unconfined ground water where the ground water level is called the pheartic ground water level
2. Confined aquifer is a aquifer where the ground water is located under a waterproof layer and has a pressure greater than atmospheric pressure. This groundwater is limited by the waterproof layer on the top and bottom. The artesian ground water surface is due to drilling, so the ground water level will move upwards to near the surface of the ground or radiate to certain conditions.
3. Leakage aquifer (semi-confined aquifer) is an aquifer layer where the ground water is located in a semi-impermeable layer and the position of the rock is located between free aquifer and depressed aquifer
4. Ferced aquifer (aquifer hang) is an aquifer where the ground water mass is separated from the mother ground water by a relatively impermeable layer of water that is not very wide and is located in the unsaturated zone.
Aquifers are underground layers of porous or sand rock between non-porous layers of rock (sandstone, gravel, or cracked limestone or granite). Many people tend to think of aquifers as "underground lakes" which do not occur because water is held between rock particles. Water infiltrates into the soil through pores, fissures, and other places until it reaches the saturation zone where all space is filled with water (not air). The saturation zone occurs because infiltration of ground water reaches the impermeable layer of rock so that it is unable to penetrate further into the earth (an impermeable layer known as "aquitard" or "akiklud").
The water held in this aquifer is known as ground water. The top of the saturation zone is known as the water table. Tables usually follow the topographical form of the above soil. The depth of the water level is usually greater in areas with low rainfall than in areas with high rainfall. The water table can increase in a few wet years and fall in the dry season.
Two main forces drive the movement of ground water. First the water moves from a higher altitude to a lower elevation due to the influence of gravity. Second, water moves from the high pressure area to the low pressure area. Together the two forces form the driving force behind moving groundwater known as a hydraulic head.
Water has the potential to move through four types of rocks: unconsolidated rocks, porous sedimentary rocks, porous volcanic rocks, and cracked rocks. In unconsolidated materials, particles are not attached to each other in a coherent manner (for example, sand will be consolidated but sandstone will be consolidated). Water can move through the space between particles. Common gravel and aquifer sand. Because there is more space between particles when particles are larger, water moves faster through layers of large particles (for example, gravel) than it does through layers of small particles (for example, clay).
Carbonate stones, such as limestone, are fragile so they tend to be broken (fault) and this fracture allows some water movement. More importantly, because water will dissolve carbonate rocks after water is able to enter the fault, the opening in the stone becomes larger allowing more water to move. Limestone which has large openings dissolved in it is known as karst. Volcanic rocks such as basalt are produced as a result of volcanic activity. If the rock cools quickly, fractures can be produced on the rocks allowing significant water movement. Metamorphic and crystalline rocks such as granite, quartzite and slate are basically water-resistant. However, faults (faults) that occur in rocks can allow water movement. The amount of movement through the cracked rock depends on the frequency and linkages of the fracture.