Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. A formation of rock/soil is called an aquifer when it can yield a useable quantity of water. The depth at which soil pore spaces become saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology.

Groundwater can be a long-term 'reservoir' of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years). The figure shows how deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle. Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. It is estimated that the volume of groundwater is fifty times that of surface freshwater; the icecaps and glaciers are the only larger reservoir of fresh water on earth.

Usable groundwater is contained in aquifers, which are subterranean areas (or layers) of permeable material (like sand and gravel) that channel the groundwater's flow. Aquifers can be confined or unconfined. If a confined aquifer follows a downward grade from a recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump. The top of the upper unconfined aquifer is called the water table or phreatic surface, where water pressure is equal to atmospheric pressure.

Typically groundwater is thought of as liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is believed to provide lubrication and buoyancy which allow thrust faults to move. Nearly any point in the Earth's subsurface has water in it, to some degree (it may be very dry or mixed with other fluids). Groundwater is not confined only to the Earth, either; subsurface water on Mars is believed to have given rise to some of the landforms observed there.

Groundwater makes up about twenty percent of the world's fresh water supply, which is about .61 percent of the entire world's water supply. (Environment Canada Website)

Problems with groundwater

Groundwater overdraft

Groundwater is a highly useful and abundant resource, but in arid or semi-arid regions it cannot renew itself as rapidly as it is being withdrawn by humans. If groundwater is extracted intensively from water wells, as for irrigation or municipal use in arid or semi-arid regions, it may not recover to its pre-development state. The most evident problem that may result from this is a lowering of the water table beyond the reach of existing wells. Wells must consequently be deepened to reach the groundwater; in some places (e.g., California, Texas and India) the water table has dropped hundreds of feet due to well pumping. A lowered water table may, in turn, cause other problems such as subsidence.

Subsidence

In its natural equilibrium state, the groundwater in the pore spaces of the aquifer supports some of the weight of the overlying sediments. When groundwater is depressurized or even removed from aquitards, where the materials are very compressible and pore pressures can be high, compaction may occur. This compaction, typically called subsidence, may be partially recoverable if pressures rebound, but much of it is not. Thus the aquifer is permanently reduced in capacity, and the surface of the ground may also subside. The city of New Orleans, Louisiana is actually below sea level today, and its subsidence is partly caused by removal of ground water from the various aquifers beneath it.

Seawater intrusion

Generally, in very humid or undeveloped regions, the shape of the water table mimics the slope of the surface. The recharge zone of an aquifer near the seacoast is likely to be inland, often at considerable distance. In these coastal areas, a lowered water table may induce sea water to reverse the flow toward the sea. Sea water moving inland is called a saltwater intrusion. Alternatively, salt from mineral beds may leach into the groundwater of its own accord.

Mining groundwater

Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries (see figure above). When the usage of water is greater than the recharge, it is referred to as mining water (the water is often called fossil water, due to its geologic age). Under those circumstances it is not a renewable resource.

Groundwater pollution

Not all groundwater problems are caused by over-extraction. Pollutants released to the ground can work their way down into groundwater. Movement of water within the aquifer is then likely to spread the pollutant over a wide area, making the groundwater unsafe to use.

Areas of karst topography on limestone bedrock are especially vulnerable to surface pollution. Sinkholes and underground caverns allow direct groundwater flow without the filtering effect of a permeable aquifer. See environmental engineering and remediation. Water table conditions are of great importance for drinking water supplies, agricultural irrigation, waste disposal (including nuclear waste), and other ecological issues.

One example of widespread groundwater pollution is in the Ganges Plain of northern India and Bangladesh where severe natural pollution by arsenic affects 25% of water wells in the shallower of two regional aquifers. The pollution occurs because aquifer sediments contain organic matter (dead plant material) that generates anaerobic (an environment without oxygen) conditions in the aquifer. These conditions result in the microbial dissolution of iron oxides in the sediment and thus the release of the arsenic, normally strongly bound to iron oxides, into the water. As a consequence, arsenic-rich groundwater is often iron-rich, although secondary processes often obscure the association of dissolved arsenic and dissolved iron.