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examBoard: Cambridge
examType: IGCSE
lessonTitle: Through-flow and Ground Water Flow
When rain falls on the Earth's surface, not all of it runs straight into rivers. A significant amount soaks into the ground and moves in two important ways: through-flow and groundwater flow. These processes are vital parts of the water cycle that help supply our drinking water, support ecosystems and shape landscapes.
Key Definitions:
Through-flow happens when water moves horizontally through the upper soil layers. After rain falls, water soaks into the soil (infiltration) and then moves sideways, following the slope of the land. This usually occurs in the top layers of soil where there are plenty of spaces between soil particles. Through-flow is faster than deeper groundwater movement but slower than surface runoff.
Groundwater flow occurs when water moves deeper into the ground, filling the spaces between rocks and sediments. This water continues to move downward (percolation) until it reaches an impermeable layer that it cannot pass through. It then flows horizontally through the permeable rock layers, often very slowly, sometimes taking thousands of years to travel just a few kilometres.
The way water moves underground depends greatly on the types of soil and rock it encounters. Some materials let water pass through easily, while others block its path.
This is how easily water can flow through a material. Highly permeable materials like sand and gravel let water pass through quickly. Clay has low permeability and slows water down.
This refers to the amount of empty space (pores) in a material. Materials with high porosity can hold more water. Sandstone is porous with many spaces, while granite has very few pores.
When all the spaces in soil or rock are filled with water, it becomes saturated. Water can't infiltrate saturated ground easily, which is why flooding often happens after prolonged rain.
An aquifer is a layer of permeable rock or sediment that contains or transmits groundwater. Aquifers are incredibly important for human water supplies, with many towns and cities around the world relying on them for drinking water.
These are sandwiched between impermeable layers (like clay) above and below. Water in confined aquifers is often under pressure. When a well is drilled into a confined aquifer, water may rise up without pumping – this is called an artesian well.
These have an impermeable layer at the bottom but not at the top. They're recharged directly by rainfall infiltrating from above. The upper surface of the saturated zone in an unconfined aquifer is called the water table.
The water table is the upper level of the zone where all the spaces in the soil and rock are completely filled with water (the saturated zone). Above the water table is the unsaturated zone, where some air still fills the spaces.
The water table isn't flat – it roughly follows the land surface above it, rising under hills and dipping under valleys. It also changes height with the seasons:
When groundwater emerges at the surface, it creates springs. This happens where:
Groundwater also provides baseflow to rivers and streams. This is the steady flow of water that keeps rivers running even during dry periods when there's no rain. Without groundwater contribution, many rivers would dry up completely between rainstorms.
The Great Artesian Basin is one of the largest groundwater systems in the world, covering 22% of Australia. Water enters the basin through rainfall and infiltration at the eastern edge, then slowly travels underground at about 1-5 metres per year. Some of this water has been underground for over 2 million years! The basin provides essential water for farming, mining and communities in the arid interior of Australia. However, excessive extraction has caused pressure declines, with some natural springs drying up. Since 1999, the Australian government has been capping free-flowing bores and installing controlled systems to reduce water waste, showing how careful management of groundwater resources is essential.
Humans affect groundwater in several important ways:
Pumping water from wells for drinking, irrigation and industry can lower the water table. If we take water out faster than it's naturally replaced, this leads to groundwater depletion.
Chemicals from farming, industry and landfills can seep into groundwater. Once groundwater is polluted, it's very difficult and expensive to clean up.
Covering land with buildings and roads prevents infiltration, reducing groundwater recharge. Deforestation can increase infiltration initially but may lead to soil erosion and reduced water quality.
To ensure groundwater remains available for future generations, we need to manage it carefully:
In parts of southeast England, water companies are using Managed Aquifer Recharge (MAR) to help balance seasonal water availability. During wet winter months when river flows are high, excess water is pumped into chalk and sandstone aquifers for storage. During dry summer months when demand is higher, this stored water can be extracted. This approach helps prevent flooding during wet periods while providing a reserve for droughts. The North London Artificial Recharge Scheme can store up to 18 million cubic metres of water in the chalk aquifer beneath London, enough to supply about 150,000 homes for a year.
These underground water movements are crucial for:
Understanding through-flow and groundwater flow helps us protect these vital water resources and use them sustainably. As climate change affects rainfall patterns and human demand for water increases, managing groundwater wisely becomes even more important.
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