Groundwater Problems

 

There are four basic problems affecting groundwater resources that must be managed.

  • Groundwater Pollution
  • Depletion
  • Land Subsidence
  • Salt Water Intrusion

In figure 3 water abstraction is taking place at three different types of well. The first is simply a bore-hole which has been drilled into the surface to tap the natural water table saturated zone. This is classed as an unconfined aquifer and is sometimes referred to as the water table it consists of the groundwater being stored or transmitted on top of an impervious layer. Often, the water table will be a mirror image of the topography of the overlying land, i.e. follow the shape of the hills and valleys above. This water is at greatest risk of contamination by the activities of man. Contaminants from various industries and materials can be washed through the top layers of soil and directly into the aquifer.

The second well is known as an Artesian well, this is where water is being removed from a confined aquifer. In a confined aquifer the groundwater is trapped between layers of impermeable material, often in the form of clays or shale. The water in this well will rise to the level of the local recharge area, in this particular example the water needs to be actively pumped to obtain the water. However if the recharge area is higher than the drilled well then the extra pressure causes the water to flow naturally out of the well this is shown in the third well in figure 3 often pressure valve devices are needed to control this type of well.

Groundwater pollution
This is generally considered to be the greatest threat to freshwater supplies in the future.
One of the major concerns associated with aquifers is the time taken for the aquifer to become "decontaminated". Groundwater generally travels fairly slowly, so that contaminants seeping into the aquifer may take a long time to disappear. Moreover contaminants travelling through the soil now may therefore still be contaminating the aquifer in years to come.

Depletion of ground water
Time also plays a role in the depletion of aquifers. Even in areas of high rain full aquifers can become depleted because human pumping may remove the water at a faster rate than it is able to travel through the recharge zone. We add to this problem by building roads and houses etc over recharge zones thus restricting the flow back into the aquifer.
The problem becomes even more serious in drought areas where aquifer water is drawn from great depths. This water was probably laid down thousands of years ago when the area was wetter, and may never be fully re-charged again with the dryer climate and increased human use.
Task 2: A Case Study: The High Plains Ogallala aquifer is one of the largest known underground freshwater aquifers in the world, what are the current problems facing this resource and why? List some of the management options.
Land Subsidence
This can happen when water is removed from an aquifer far more quickly than it can be replenished. The water table becomes lower and the voids and air spaces between the earth/soil particles close together. The volume of the area is reduced and the overlying land subsides. In Mexico City, (which was built on an ancient lakebed) this has progressed to the point where some older buildings can only be entered by the second floor. Similarly Tokyo has subsided 4m over the last 100 years. The lowering of the water table also often means that new wells have to be constructed for water removal to occur, this lowers the water table even further if the rate of abstraction is not controlled adequately. In addition even if water is introduced artificially into the aquifer through pumping water in, the damage caused to the particles and void space is often too severe for the aquifer to recover its original capacity. Falling water tables from the over-pumping of groundwater are now ubiquitous in parts of China, India, Thailand, the western United States, North Africa, and the Middle East.
Task 3. What are sinkholes? How are they caused?
Salt water Intrusion
This is a significant problem in many coastal cities. Aquifers near the coast tend to be "layered", i.e. salt water from the sea concentrates at the base of the saturated zone and freshwater from rain that has percolated from the surface lies at the top of the saturated zone. If over pumping occurs however the saltier water is drawn into the upper areas and this can cause more saltwater to be drawn in. The cycle becomes self perpetuating, as water is drawn out of the upper area some salts will remain and become concentrated which in turn draws water up from the lower regions and so on.