Concerns with Groundwater Quality and Quantity

In general, groundwater is not used as extensively as surface water [1]. This is primarily due to the lack of understanding and knowledge of groundwater. In addition, there are two main areas of concern with groundwater usage: maintaining quality and ensuring a sustainable quantity.

Groundwater Quality Issues  

Just like surface water, groundwater quality can be diminished due to the effects of human use. Groundwater quality can be diminished from contamination of chemicals seeping into the ground, sewage contamination leaching into aquifers or from saltwater infiltration where saline water enters a freshwater aquifer.

Contamination

Water quality is a major concern for groundwater wells because contamination can come from a variety of sources. It is common for run-off from agricultural lands to percolate into the ground. This water may contain high concentrations of chemicals, including arsenic, ammonia, acids or nutrients like nitrates and phosphates from pesticides and fertilizers. 

The amount of pesticides in groundwater in Alberta is of increasing concern[2]. In general, in order to produce the same amount of crops from year to year, farmers must use a higher dose of pesticide to have the same effect that it did a few years prior (also known as the law of diminishing returns) meaning the amount of chemicals reaching groundwater is increasing [3]. In Canada there are strict water quality regulations that determine the maximum allowable concentration of a compound in potable water. If the well is producing water with concentrations higher than the regulated amount of any chemical, then the well can no longer be used without treatment. Sophisticated filtration or purification systems are needed to make the well water potable. Fortunately, for the most part, pesticide content in Alberta has been below the maximum allowable concentration for drinking water [4]. 

Sewage Contamination

In areas with older septic tanks, sewage waste can leach into the aquifer rendering nearby wells useless. Leaching of these contaminants is not limited to unconfined aquifers near the surface as contaminants can percolate deep into confined aquifers. If a water source becomes contaminated, it must undergo a remediation process in order to be usable. Remediation is a process of removing contaminants and making the source of water clean. Remediation of aquifers is more difficult than remediation of surface water sources, and the deeper the aquifer the more difficult the process becomes.

Saline Infiltration

Saline infiltration is another complication that can negatively affect aquifer quality. Although saline infiltration is most often a concern with deeper aquifers, it can also be problematic in aquifers at the surface. Saline infiltration can occur when an aquifer is being pumped too intensely, resulting in a low pressure zone forming very close to the deepest part of the aquifer. This low pressure zone draws water from the surrounding area and if the unit below the aquifer contains saline water, this is what will be drawn into the pumping well. Once saline water infiltrates the producing aquifer, the well is no longer useable.

Groundwater Quantity Concerns

Another concern for groundwater users is maintaining a usable quantity of groundwater. It is important to note that when water loses its quality, the overall quantity of useable water is decreased however; water quantity can be impacted directly through withdrawing groundwater. 

Extensive groundwater withdrawal can result in overdrafts. Once a well begins to be pumped, a cone of depression is formed around it. When two wells are near each other their cones of depression can intersect and form an interference zone. In regions where overdrafts are widespread, land subsidence can occur.

Withdrawal

A well needs to be installed in order to get water out of the ground. A hole is drilled down into the ground until the water table is reached. The hole will extend deeper into the water table so that it can take in more water. Older wells will be operated manually (imagine a bucket on a string collecting water). However, most modern wells are automated, meaning that a pump will move the water out of the well.

Overdrafts

Overdrafts refer to groundwater that is withdrawn from the aquifer and that will not be replaced by recharge. Essentially, when this water is removed, it is not replaced naturally. In areas where groundwater is pumped heavily, overdrafts cause the water table to move lower in the aquifer.

Overdrafts can occur especially during times of drought, when alternative water sources are required. Drought can exacerbate groundwater quantity challenges if withdrawals are not managed properly. 

Cone of Depression

Before pumping begins, the water table has a smooth, flat surface. After pumping however, there is a sizable hole around the pumping well and an unsaturated zone which extends deeper around the well. This is called a cone of depression because of the cone shape caused by pumping. In many areas, there will be multiple wells pumping from the same aquifer. In Figure 1 there are two wells drawing from the same aquifer. Each well has its own cone of depression.

Interference Zone

An interference zone is a large unsaturated zone in the area in between two wells where their cones of depression meet. If two wells drawing from the same aquifer are close enough, their cones of depression will overlap creating an interference zone. These interference zones are quite complex. In some cases, an interference zone results in overdrafts. When the overdrafts become larger, the wells will no longer be able to pump groundwater because the water table has been lowered below the bottom of the well.

Figure 1: Water withdrawal from an aquifer. This figure shows two wells pumping from the same aquifer. The upper diagram shows the cones of depression if only one of the wells was pumping, while the bottom diagram shows the combined cone of depression when both wells are pumping. This is known as well interference [5].

Subsidence

When groundwater overdrafts are widespread in a region, subsidence may occur. Subsidence is when the ground is compacted because the aquifer loses water. The water in an aquifer creates pressure that helps it keep its structure. When this water is removed, the pressure is released and the aquifer loses its structure. As a result the land begins to sink. 

In San Joaquin Valley, a region in California where there is significant agricultural activity, water has been pumped out of the aquifer at a rate that was higher than the aquifer could accommodate. As a result, the land subsided, or compacted at an astonishingly rapid rate. From 1925 to 1977, a period of just 27 years, the ground subsided by more than 28 feet [6].

 

 

  

Sources:

[1] Alberta Environment and Sustainable Resource Development (ESRD). 2014. Groundwater Use. Accessed from the web from: http://esrd.alberta.ca/water/programs-and-services/groundwater/documents/FocusOnGroundwaterUse-Aug06-2014.pdf 

[2] Alberta Agriculture and Rural Development. 2008. Use of Fertilizers, Manures and Pesticides for Sustainable Farm Management. Accessed from the web from: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex12413 

[3] BioEthics Education Project. Unknown date. Biodiversity: agriculture – fertilisers. Accessed from the web from: http://www.beep.ac.uk/content/338.0.html  

[4] Alberta Agriculture and Rural Development. 2008. Use of Fertilizers, Manures and Pesticides for Sustainable Farm Management. Accessed from the web from: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex12413 

[5] Institute for Water and Watersheds. “Groundwater and Wells.” Orgeon State University. Accessed online at http://wellwater.oregonstate.edu/groundwater-and-wells.  

[6] Galloway, D. and Riley, F. (n.d). San Joaquin Valley, California: Largest human alteration of the Earth’s surface. Accessed from the web from: http://pubs.usgs.gov/circ/circ1182/pdf/06SanJoaquinValley.pdf