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The specification states that you need to be able to: Define and explain the following terms: Porosity Permeability hydrostatic pressure hydraulic gradient Aquifers water table. Porosity and Permeability What is meant by the terms porosity and permeability? Porosity: Amount of pore space between grains: Porosity = Volume of pore space x 100% Total volume of rock Porosity and Permeability 2 Factors influencing porosity: Grain shape Packing Amount of cement and smaller grains between grains. Sedimentary rocks have a greater porosity as a rule than igneous and metamorphic rocks that are made of interlocking crystals. Some examples: Limestone Sandstone Shale Clay Sand Gravel Granite Basalt 10% 18% 18% 45% 35% 25% 1% 1% Permeability: Definition: The ability of a liquid to pass through a rock. Permeable = water can pass through, varies from high to low. Impermeable = Cannot pass through. The larger the pore spaces the more permeable the rock will be especially if the pore spaces are well connected. A moderately porous rock may not necessarily be permeable, as the pore spaces may not be connected. E. g. shale has a porosity of 18% similar to sandstone but clay minerals hold onto the water and so it is impermeable. Post Depositional Changes: 1. 2. 3. 4. These can affect porosity and permeability: Faulting (increase) Jointing (increase) Cementation (decrease) Metamorphism will crystallise the rock and reduce the porosity. Hydrostatic Pressure Normal water pressure exerted at any point in a water body at rest.” This increases with depth. Hydraulic Gradient Forms when areas of low and high hydrostatic pressure are connected e.g. within an artesian well. The water in the high-pressure zone will flow towards the lowpressure zone (at the surface). In artesian systems the well is dug into the aquifer below the water table. This creates a strong hydraulic gradient and water keeps flowing. This will become clear when we have covered artesian basins. Aquifers The water table tends to follow the surface topography. Water Table: Surface of the saturated groundwater zone. If it happens to occupy a position above land then water seeps out forming lakes, marshes that fill until they reach the water table level. Where a steep surface and the water table meet then springs will occur. This is particularly common in limestone, which are porous and can dissolve in water forming cave systems. Springs will occur where the permeable aquifer meets an impermeable rock below. Aquifers 2 There are different types of aquifer: Confined aquifers: This is an aquifer between 2 impermeable rocks. The 2 sandwiching rocks are called aquicludes. Unconfined aquifers: Not covered by impermeable rock. Water table level may vary depending on the season. Aquifers 3 Perched aquifers: Smaller aquifers that hold amounts of water higher than the regional water table e.g. valley sides. These can often lead to the formation of springs. Water can be extracted from these aquifers in a number of ways: WELLS These are dug or drilled down into an unconfined aquifer and must extend below the water table. The water is either pumped out mechanically or drawn out by hand. The main problem is that over time if used excessively output may exceed the main meteoric input and water table level can drop locally. ARTESIAN WELLS A well is dug or drilled into a confined aquifer. This time no pumping is needed as water in the aquifer is under considerable pressure. The well acts like a pressure release valve and water flows out under it’s own pressure. E. g. the Great Australian Artesian basin in Queensland. Artesian Wells 2 In artesian systems the well is dug into the aquifer below the water table. This creates a strong hydraulic gradient and water keeps flowing. When the aquifer is folded producing a broad synform then an artesian basin is formed e.g. the Australian basin covers 1.5 million km2. Water is picked up in the mountains in the east and the water slowly moves through the rock west towards an arid region, where wells up to 1.5 km deep.