Groundwater

• Water that seeps into the soil is called soil-water recharge. This percolation process replenishes any lost groundwater.

  • This water exists in the unsaturated zone where pores in the soil are filled with air.
  • Eventually, water will seep into the saturated zone where pores are predominantly filled with water. This then becomes groundwater.

• During periods of high evapotranspiration, there is soil-water use such that groundwater is drawn from the soil.

  • Even though the soil may still have water, plants may not be able to access it due to the molecules adhering to the rocks. This last amount of water storage is the wilting point.
  • When the soil is at its wilting point, no additional water can be drawn easily.

• The water table defines the boundary between saturated and unsaturated rock.

  • It typically follows the topography of the land above but more subdued. However, human activity can modify this structure.
    • As long as topography does not intersect the water table, the groundwater will remain at depth, generally flowing toward low elevations.
    • Where the water table intersects the surface, groundwater can flow out onto the land in the form of springs.
  • The water table generally slopes from higher to lower areas.
  • It is generally deeper below the surface under mountains than under lowlands
  • Groundwater just below the water table flows down the slope of the water table.
  • A high part of the water table separating parts sloping in opposite directions is called a ground-water divide Groundwater flows in opposite directions on either side of a divide into a drainage basin.
  • Streams do not necessarily coincide with the water table.
  • The water table is not necessarily continuous. In fact, there can be perched water tables that sit above the main one and separated from the latter by impermeable rock.

• Soil can only hold a certain amount of water. Its field capacity

Groundwater Flow §

• Groundwater flows in the subsurface or re-emerges in the surface as springs or lakes.

• Ground water flow can provide a water source even during dry season.

• The flow rate of groundwater is primarily controlled by the permeability of the soil High permeability means fast flow rate.

• To a lesser extent, it is also driven by the slope of the water table — its hydraulic gradient

• An aquifer is a large body of permeable saturated material through which groundwater can flow in significant volumes.

  • This necessitates the material the aquifer is made of having high permeability.
  • An unconfined aquifer is one where the water bearing unit is not impeded by impermeable rocks to the surface.
  • A confined aquifer is one separated from the surface by low permeability rock.

• An aquiclude is a large body of impermeable material that blocks water flow.

• A well is a hole dug or drilled deep enough to intersect the water table. If the well is within an aquifer, the water will fill the open space to the level of the water table which allows the water to be drawn out.

  • A well dries up when groundwater has been used up or the water table drops.
  • An artesian well is one where water is forced above the water table level via water pressure.

• A spring is a place where groundwater flows out of the ground onto the surface.

  • In most springs, the water table intersects the surface.
  • Hot springs get their temperature from hot rocks or magma.
  • A geyser is a kind of hot spring that erupts sprays of hot water and steam.
  • Many springs are related to limestone aquifers or the boundary between two different rock units.

• Most lakes occur where the water table intersects the ground surface.

  • Some lakes, however, are transient and only exist due to precipitation or snowmelt or are reliant on water inflow.
  • In the cases where the water table intersects the surface, the water rises above and forms the lake.
  • Many lakes are along the bottoms of valleys as this is where the water table and surface tend to be closest.
  • Wetlands form at the same level as the water table. Typically, the soil has low permeability which traps precipitation into the soil.

• A stream is said to be gaining when water flowing through the stream increases downstream. This happens when the river coincides with a basin on the water table (i.e., they are at a lower elevation to the water table around them)

• A stream is said to be losing when water flowing through the stream decreases downstream. This happens when the river coincides with a divide on the water table (i.e., they are at a higher elevation to the water table around them).

  • Some losing streams disappear (all water on the surface eventually percolates) when they cross from hard, less permeable rocks to softer, more permeable rock.

Human Impact §

• Overpumping a well causes the water table to form a depression. This causes water to flow towards the depression.

  • The depression can bring contaminated water to the well
  • It can also dry up existing wells, alongside streams and lakes.
  • They can also cause the surface above to subside due to the sediment compacting (since water no longer exists to fill in the spaces).
  • If the pump is near the coast, if the well dries up, it will eventually start pumping salty or brackish water.
    • The freshwater prior to pumping acted as an interface between the pump and the salt water since it is less dense. A lack of it, thus, causes saltwater incursion where the interface moves so that the saltwater coincides with the pump.

• Some sources of water contamination:

  • Chemical elements released via natural weathering.
  • Mining activities which spread contaminants.
  • Chemical spills.
  • Landfills, especially those containing chemicals that can seep into the soil.
  • Human waste.
  • Fertilizers and Herbicides
  • Manufacturing
  • Polluted water that travels downstream.
  • Construction

• Water contamination moves along the flowing groundwater which causes it to spread via diffusion

Links §

• Exploring Physical Geography by Reynolds