Wells Groundwater and Hydrology

The term Hydrology comes from the Greek “hydro” meaning water and “logos” which means study. Therefore the science of hydrology involves the study of water and all of its interactions with the environment.

Hydrologists can use their knowledge of this science to ensure the correct siting of wells in order to utilise groundwater resources safely and economically. Groundwater is literally water that is found within the ground as opposed to water found in lakes or rivers. The job of the hydrologist, in exploiting groundwater, is to provide a suitable water supply at the lowest possible expense.

On earth water is re-cycled through the hydrological cycle. Throughout the cycle water can be returned to the air by either evaporation or transpiration by plants. Water leaves the atmosphere falling to the ground as rain, snow or hail. On the ground water may soak into the soil, filling any voids, this is known as infiltration while the remaining water forms surface runoff. Runoff moves by gravity, down slope, to any body of water such as a lake or river. The amount of precipitation which eventually reaches a river or lake will depend on vegetation cover, the slope of the land, the type of soil and how much of the land is covered by man made structures. The where there are a number of man made structures such as roads and car parks less water can reach the soil to infiltrate so there is more runoff. Some water within lakes and rivers can infiltrate the surrounding rocks, while infiltrated water may flow out of the rock into rivers or lakes. This forms a dynamic equilibrium, the net direction of water flow depending on the type of surrounding rock and the amount of water it already contains at any time.

In addition to knowledge of the hydrological cycle an understanding of the properties of rocks and soils is also a very important part of hydrology. Rocks and soils which are porous and have voids able to contain water are called aquifers. Impermeable rocks and clays are called confining beds or aquatards and these can restrict the flow of water into and out of aquifers.

The surface layer of soils, through which water infiltrates, is known as the unsaturated zone, in this zone voids are filled with a mixture of water and air. Below this is the saturated zone in which the voids are completely filled with water. The saturated zone is one source of groundwater which can be tapped using a well. The dividing line between the saturated and unsaturated zones is known as the water table. The water table level will change over time depending on the amount of precipitation permeating the soil. It has been found that the water level immediately around a working well is usually below the water table of the surrounding soils. The water table surrounding a working well forms a funnel shaped cone of depression. The profile of this cone will depend on the rate of water abstraction from the well and the speed at which the water can be replaced by infiltration. When a well ceases being used the cone of depression is filled by infiltration and the water level in the well returns to the same level as the water table.

The other source of groundwater that can be tapped using a well is a confined aquifer. This is a layer of water saturated porous rock with a confining bed above it. Such aquifers are recharged in areas away from the wellhead. By drilling a well through the confining bed at an appropriate place this valuable water resource may be tapped. Water to recharge a confined aquifer can come from a variety of sources, possibly from a river that cuts through the confining bed or from a higher mountainous region where the confining bed has been eroded allowing precipitation to enter the aquifer.

In deciding whether an aquifer provides a suitable water source a number of factors must be considered.

First how porous is the rock making up the aquifer. This gives an estimate of how much water can it hold.

Next the hydraulic conductivity of the rock. This is an indicator of how easily water can travel through an aquifer. It is measured as the hydraulic constant of a rock or soil. Clay is a poor aquifer and has a low hydraulic constant. Sands and gravels are both good aquifers with high hydraulic constants.

Finally there is the hydraulic gradient. This is a measure of how fast the water flows with gravity from the recharge point of the aquifer to the wellhead.

The hydrologist will also need to know the expected abstraction rate from the well. If an aquifer cannot be recharged quickly enough or does not contain sufficient water to meet the required abstraction rate then an alternative water source must be found.

When siting a well the hydrologist must also be aware of any groundwater contamination that may have occurred in the surrounding area. If such contamination has occurred then abstraction from the well can cause the contaminants to migrate through the aquifer to the wellhead. Depending on the type of contaminant and the final use of the water being abstracted this could make the water unusable without expensive purification.