Ground Water - An Important Rural Resource, Managing the Quantity of Groundwater Supplies
|Publication Date:||December 2015|
|Last Reviewed:||November 2018|
|History:||Replaces OMAFRA Factsheet 06-113 of the same name|
|Written by:||H. Simpson, Ph.D.|
PDF Version - 140 KB
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Table of Contents
- Groundwater Supplies
- Pumping a Supply Well
- Managing the Groundwater Resources and Well
- Other Resources
All Ontarians can play a role in protecting groundwater quality and quantity. The second of four in a series that will help Ontario's farmers and rural residents learn more about groundwater, this Factsheet discusses the availability of groundwater for rural use, the effect of pumping on aquifers and how to manage the use and impact on groundwater.
Other OMAFRA Factsheets in this series include:
- Understanding Groundwater
- Protecting the Quality of Groundwater Supplies
- Private Rural Water Supplies
Groundwater is a valuable resource for rural families and businesses - in some situations it may be the only water source. When living in a rural area, it is important to understand what can be done to conserve water usage, reduce contamination and ensure its future abundance.
A water supply well is a hole that is drilled, dug or bored into the ground or underlying bedrock from which groundwater is drawn. Groundwater is also tapped where it naturally flows to the surface in the form of springs. Both springs and wells have been known to dry up on occasion. A lack of groundwater affects not only well supplies, but also the lakes, rivers and wetlands that depend on groundwater contributions.
Aquifers are permeable formations at or below the ground surface that yield useful amounts of water when pumped for water supplies. The amount of groundwater that is pumped from an aquifer over time depends on its size (storage capacity), geological composition (ability to transmit water) and water balance (amount of water gained and lost).
A water balance accounts for the water entering an aquifer through recharge and underground flow, the amount that leaves the aquifer through pumping or underground flow, and the amount that remains in the aquifer. For most untapped aquifers, the amount of water entering the aquifer (recharge) during an average year nearly balances or equals the amount leaving (discharge). As a result, the amount of water stored in the aquifer does not change much - it will increase slightly in rainy years and decrease slightly in drier years.
Aquitards are materials that prevent a significant flow of water. Water moves extremely slowly through aquitards. Aquifers and aquitards, and other groundwater concepts, are discussed in more detail in the OMAFRA Factsheet, Understanding Groundwater.
Groundwater is just one part of the global water cycle. Precipitation that seeps and infiltrates into the ground becomes groundwater that will then move underground toward a lake, stream or ocean where it discharges to become part of the surface water. Water completes the cycle by evaporating into the atmosphere to become precipitation again.
Although many aspects of the water balance and water cycle (e.g., rainfall) recharge the availability of groundwater supplies, well owners can affect the water balance in an aquifer by how the well is pumped. In some cases, an aquifer can be over-pumped to the point that it can no longer produce water. This is a particular problem where aquifers are small, perched, or pumped excessively or too rapidly. Perched aquifers are discussed in more detail in the OMAFRA Factsheet Understanding Groundwater.
Figure 1. An unconfined aquifer before pumping.
Pumping a Supply Well
In an unpumped aquifer, water infiltrating the subsurface seeps down to the water table and then flows horizontally towards a discharge area (Figure 1). An unconfined aquifer is where the top of the aquifer is also the water table. When a well is pumped in an unconfined aquifer, water is removed from the aquifer, and the water table in the aquifer is lowered. This lowering or drawdown of the water table is greatest close to the well and gets smaller in all directions as the distance from the well increases (Figure 2). This pattern of drawdown in the aquifer is referred to as the cone of depression or the drawdown cone. The size and shape of the cone of depression will grow and shrink as the rate and duration of well pumping changes.
Pumping, and the cone of depression it creates, causes water that would normally flow past a well on its way through the aquifer to the stream (as part of the water cycle) to be captured by the pumping well and brought above ground for use. Pumping can create groundwater "divides" or dividing lines that separate areas of groundwater that flow back and are captured by the well from areas that are not. In Figure 2, water recharged to the left of the "divide" will flow towards the well, and water recharged to the right of the "divide" will flow towards the stream. The groundwater removed from the aquifer may eventually be replaced by rain and snowmelt that seeps into the ground and aquifer. However, the amounts may not exactly balance or recharge at the same time as the withdrawals, and the amount of water stored in the aquifer will change.
In most cases, the pumping well does not capture water from the stream (Figure 2). However, if the well is pumped at a high enough rate or pumped long enough, the flow divide will move towards the stream. This will affect the flow of groundwater into the stream within the zone of capture of the well. In this situation, some surface water could also be drawn into the aquifer as induced infiltration and eventually pumped out of the well. This is not usually a problem with private domestic wells but may be encountered with larger capacity wells such as those used by a municipality or industrial operation.
Figure 2. An unconfined aquifer showing the water table during pumping.
Make sure pumping does not cause too much water to be captured or diverted from nearby streams, ponds or wetlands. In addition to impacting these surface water features, surface water may contain contaminants (e.g., pathogens) that can impair the quality of groundwater pumped from a well and make it unsuitable for drinking without treatment. This can be avoided by locating new wells away from surface water features.
Managing the Grounwater Resource and Well
There are three main causes of running low on water:
- insufficient volume of water (or a sufficiently high water table) in the aquifer
- pump rate faster than the water can move through the aquifer to get to the well
- well construction insufficient for supplying the volume of water needed
When considering a significant increase in water use, it is important to know in advance if the well, and the aquifer it taps, can supply the amount of water needed. The first step is to measure (e.g., using a water meter) or estimate the current and projected water usage. Next, contact a licensed well professional or professional geoscientist (hydrogeologist) to determine if the well and aquifer can meet the increased demand, and if the well has the proper pump size. Professionals can help determine the well yield, which is the maximum rate that can be pumped from a well and still have the water level in the well remain above the pump intake.
To ensure an ongoing supply of groundwater, consider the following actions:
- reducing the volume of water used by maintaining the system to avoid leakage, and installing water conserving equipment
- reducing the size of the pump, and possibly adding storage, to reduce the drawdown during pumping
- modifying the system to increase storage by installing a larger pressure tank (this is important if the well has a limited depth of water inside the casing during pumping (limited available drawdown) or if the aquifer is shallow or not very productive.
- cleaning or rehabilitating the well to improve its performance (using a licenced well professional)
Water levels in the aquifer and the well can drop to the point that water can no longer be pumped. This drop occurs when the demand (pumping) is greater than the supply, when extended dry periods happen or when neighbouring wells have pumped down the water level in the aquifer. Figure 2 illustrates a deep well that has been pumped and has lowered the water level in a shallow well. The Water Well Book, OMAFRA Best Management Practices Series, contains information on calculating the available drawdown and yield of a well.
There may be some cases where a new larger well, or several new smaller wells, are needed to provide an adequate water supply. Information on different types of wells and the management of highly vulnerable water supplies is provided in the OMAFRA Factsheet Private Rural Water Supplies.
It may be necessary to try to find a larger and more productive supply (such as a deeper aquifer) that can be tapped for water supply needs. Consult a licensed well professional or professional geoscientist to find out if there is access to a deeper aquifer below the property. Consider tapping into an aquifer that has a lower vulnerability to contamination. The importance of aquifer vulnerability is discussed in more detail in the OMAFRA Factsheet Protecting the Quality of Groundwater Supplies.
Groundwater is a valuable resource for rural families and businesses. When living in a rural area, it is important to understand the effect pumping has on aquifers, what can be done to conserve water usage and how to ensure a future abundance of water.
Ministry of the Environment and Climate Change. Green Facts: Managing Your Water Well in Times of Shortage. PIBS 3784E.
This Factsheet was updated by Dr. Hugh Simpson, Program Analyst, OMAFRA, Guelph; Jim Myslik, JPM Consulting, Marden; Dr. Brewster Conant, Department of Earth Sciences, University of Waterloo. It was reviewed by Tim Brook, Water Management Engineer, OMAFRA, Elora; Rebecca Shortt, Water Quality Engineer, OMAFRA, Simcoe; and H.J. Smith, Environmental Management Specialist, OMAFRA, Guelph.
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