August 22, 2014
By: Alejandro Baladron Julian
True or False. Do you know your groundwater? Or are you fooled by the common misconceptions?
How did you do? Find the answers in the blog.
Just 200 years ago, groundwater was a mysterious phenomenon. Today, thanks to hydrology laws explaining the movement of water combined with our knowledge of geology we understand the nature and occurrence of water in the ground. However, the fact that we can only see groundwater in a few notable exceptions – when water seeps from a hillside or when water comes up after digging a well – may explain why many people are confused about how water occurs, moves, and is stored in the ground. Even today, most of the public who rely on this resource know almost nothing about it. There is a tremendous amount of mythology and confusion surrounding the sources of groundwater. This may explain why techniques like dowsing, using supernatural abilities to find water below the ground, are still used today instead of the most basic principles of groundwater science.
Some widespread misconceptions: groundwater and surface water are separate, groundwater flows in underground rivers, and that groundwater is a non-renewable resource. But probably the most common hydrology misconception is to assume that groundwater is any water occurring beneath the earth’s surface.
Not all the water in the ground is groundwater
It is common to think that groundwater is any water occurring in the ground. This oversimplification of hydrology processes is accepted when we are communicating information about hydrology science to non-scientific groups, but incorrect when used in water resources decision-making. Well informed decisions necessitate making a distinction between groundwater and subsurface water.
Voids, or the spaces between grains of sand and cracks in rocks near the earth’s surface, allow water to move beneath the land surface under the force of gravity. Water moving in soil through void spaces is referred to as underground water.
Underground water occurs in two different zones. One zone, the unsaturated zone, is located immediately beneath the land surface in most areas, and contains both water and air in the voids. This zone is also known as the zone of aeration and vadose zone. The unsaturated zone is almost always underlain by a second zone in which all voids are full of water, the saturated zone (Figure 1).
Depending on the flow path that underground water takes, it will be considered subsurface water or groundwater. Subsurface water flow occurs in the unsaturated zone when rain falls faster than it can infiltrate downwards. On the contrary, groundwater is found in the spaces between soil particles and cracks in rocks underground located in the saturated zone – the result of precipitation which seeps down through the land surface until it reaches the saturated zone. Groundwater slowly moves in the saturated area and eventually seeps into streams, lakes, and oceans (Figure 2).
To summarize, there are two different types of water occurring beneath the soil’s surface: subsurface flow, which moves in the unsaturated zone located close to the earth’s surface, and groundwater, which occupies the saturated zone. Only the water occupying the saturated zone can be truly considered groundwater.
Why differentiate groundwater from subsurface water matters?
It is important to make a distinction between groundwater and subsurface water because their properties are different and so are their impacts in landscapes. Groundwater typically moves very slowly, creates habitats with low oxygen concentration, affects the chemistry of the ground, and is the source of streams in the absence of rainfall.
On the contrary, subsurface water flowing in the unsaturated zone can move faster than groundwater flow, allows medium to high presence of oxygen, and lacks the potential to feed streams, springs and seeps during the dry season.
So, what does this mean for development projects?
Is it subsurface flow or groundwater what you see seeping out of that hill?
Recently, Great Ecology studied the hydrological conditions on a property to determine if development plans could move forward after the property owners observed saturated patches of soil. As the puddles were located in the proposed development area, they needed to identify the origin of water to understand how these puddles would affect their plans. The study results indicated that the puddles were created by a temporal subsurface flow which was generated by a large and frequent number of rainfall events, the presence of large slopes, and impervious soil layers beneath ground surface…not by groundwater inputs. Why does this matter? As the seeps and puddles were caused by temporal subsurface flow rather than by groundwater inputs, they are just hydrological nuisances and will disappear soon after rainy periods are done and will likely not interfere with the area planned for development. However, had the source been groundwater, it would indicate a constant and consistent water source, forcing the property owners to take additional development actions, which may increase project costs, require moving the proposed development location, or be unable to complete the project.
Understanding the hydrology of a project site is just as critical as choosing the appropriate plants for repairing damaged landscapes. Long-term success of a development depends on accurate diagnosis of hydrological conditions, which requires a specialist’s knowledge, equipment, and ability to manipulate predictive formulas.
Smerdon, Brian and Todd Redding. Groundwater: More Than Water Below the Ground! Streamline Watershed Management Bulletin. Vol. 10/No. 2. 2007.
Hall and Associates, Ruth Dight, and Applied Geotechnology, Inc. Ground Water Resource Protection: A Handbook for Local Planners and Decision Makers in Washington State. King County Planning Division. Washington State, Department of Ecology. 1987.
Schwalbaum, W Jesse. Understanding Groundwater. Nova Science Publishers, Inc. Commack, New York. 1997.
Winkley, Steven. Dispelling Common Ground Water Misconceptions. New York Rural Water Associations. 2005.