Geology - Groundwater Systems
Posted On: 2020-08-15
Geology 101 - Gale Martin - Class Notes
Only a portion of the water that falls as precipitation returns to the ocean as fluvial runoff. As rain strikes the surface of the Earth, some of it soaks into the ground and seeps into the soil . Here it becomes part of the soil moisture, often used by plants and small organisms. If the water continues deeper into the underlying regolith it can collect in cracks and crevasses in rock or between the pores within the rock itself. This body of water, referred to as groundwater , flows beneath the surface on its return course to the oceans.
Water can be found in any rock that has openings or pores. Cracks and fractures are common in many rocks, but unconsolidated sedimentary rocks have the greatest porosity . Small voids are common between the loose, round grains of a clastic sediment. If water completely fills the pores, the body of rock is referred to as the zone of saturation . Above this is the zone of aeration . Here water may coat individual grains or 'stick' between close grains (known as capillary action ) but the voids are not completely filled. The imaginary "line" the separates the two zones of rock is known as the water table .
The water table is complex and flexible; moving up and down as the volume of water in the pores fluctuates. This is a dynamic balance between recharge by infiltration and discharge by several means throughout the system. The type of rock and the climate of the region can greatly influence the depth at which the water table will occur. Where there is sufficient precipitation, the water table often mimics the topography: elevated under hills and lower, often flowing, into valleys. Streams, lakes and ponds are often surface expressions of where the water table reaches the Earth's surface. Such streams, referred to as effluent in nature, flow year round with groundwater as a primary source. In areas where rain is less frequent, water tables are typically much deeper and more flat in configuration. Here the streams are commonly influent : water drains down through the stream bed toward the water table and stream flow is intermittent in nature.
The flow of water through the subsurface is governed by the permeability of the rock. Not all rock allows groundwater to flow through it. The behavior of groundwater is governed by complex interactions of many factors, including the characteristics of the rock, the influence of gravity and the friction of water as it moves between grains. Calculations, using Darcy's Law, can be used to determine groundwater flow. If the pores within the rock are interconnected by sufficient openings, water easily flows through the rock. Low permeability occurs if the interconnections are too small or if clay particles block openings preventing effective flow. If there are no connections within the rock, permeability can be nonexistent.
Based on it's usefulness to mankind, rock formations are separated into several types of groundwater systems. Zones of saturation that have high permeability are commonly referred to as aquifers . Flow rates are sufficiently high to yield water for use by mankind. If the rate of flow is too low, the rock body is confining beds, allow very little flow to occur. If water encounters such a formation, flow can diverted, blocked, or " perched " above the main water table until an access route can be found around the impermeable bed.
Types of Groundwater Discharge
Groundwater reaches the Earth's surface by many means. Springs and seeps are common in areas where aquifers intersect the surface at the side of a hill or where folded rock is exposed and confining layers eroded away. Faults and cracks act as weaknesses in rock where water can easily flow from underlying aquifers to the surface. Hot springs occur when the groundwater flows through hot rock beneath the Earth's surface, quickly surfacing at temperatures above expected levels. In areas of geologically recent igneous activity, groundwater quickly picks up heat from deep seated magma chambers and becomes "superheated" quickly. This heated groundwater may arise as hot springs or, if the water "flashes" into steam, geysers .
When the water table is relatively close to the surface, a hole, or well , can be dug and water drawn to the surface. Deep wells require pumps to draw the water upward. Under special circumstances, wells can deliver water under pressure. Commonly referred to as artesians , these wells consist of a confined system. The aquifer in an artesian is bounded above and below by confining beds which restrict the direction of flow. The aquifer is tilted; the area of recharge being higher in elevation than the discharge zone. When water infiltrates into the upper reaches of the system it can only flow in one direction. This results in a pressure build up or hydraulic head. If the artesian is breached, either by a well hole or by intersecting the surface naturally, the excess pressure allows the water to flow above the land surface without pumping.
Erosion in Groundwater Systems
Unlike fluvial systems, groundwater is limited in how it can erode rock. Hydraulic erosion is uncommon because flow rates are slow relative to the strength of the rock. The openings through which groundwater moves restrict the size of particles that can be carried by the water, thus limiting the abrasive capability of most groundwater systems. (It is this very reason that well, spring and artesian water is considered so "pure" -- the water appears clear because it lacks suspended sediment loads.) The most common means of eroding rock in groundwater systems is by chemical dissolution of rock through which the water flows. Soluble rocks types are most susceptible to groundwater erosion. Karst topography commonly develops in regions underlain by limestone, marble and evaporite formations.
Rainwater that percolates through soil picks up compounds from decaying organic matter, making groundwater even more acidic than usual. This groundwater quickly dissolves away any soluble rock it encounters. Cracks in limestone and marble enlarge and widen as water flows through them. Large networks of crisscrossing chambers, known as caves and caverns , develop along pre-existing fractures in the rock. As the cave grows larger, the overlying rock may become too heavy to support and the roof may collapse into the empty void. These collapsed structures appear on the Earth's surface as sinkholes . The sinkholes occur in straight lines that follow previous fracture systems. Streams that flow across the surface of karst regions appear erratic in character: they commonly disappear into sinkholes, flow beneath the surface and reappear in springs several miles away.
As the cave system increases in size, the roof fails along most regions of the cave network. Sinkholes become connected and produce solution valleys with flat bottoms and step sides. As the surface network enlarges and connects, tall spires of carbonate rocks are left behind as erosional remnants in the predominantly collapsed karst topography.
Deposition in Groundwater Systems
As the groundwater chemically dissolves more and more carbonate rocks, it becomes "saturated" with calcite ions. If the water flows through a cave system that contains open spaces with air pockets, the water drips off the ceilings and drizzles down the sides of the caverns. The calcite beings to precipitate out as flowstone , or travertine , a layered deposit that accumulates by growth of carbonate crystals. The flowstone takes on many shapes as it drips, splatters and drops, the most common are called stalagmites and stalactites .
The carbonate ions can precipitate out in many environments, including the cement that lithifies sedimentary rock. One form of deposition, known as tufa , occurs around springs, especially hot springs and geysers. When groundwater is heated, its "corrosive" nature increases and it becomes more capable of chemically weathering rock. Upon reaching the surface, the groundwater quickly cools and minerals precipitate near the spring. Pools of warm water create "steps" around most hot spring sites, and numerous colors, produced by organisms sensitive to selective temperature ranges, are often prominent (ex.: Yellowstone National Park). Sulfur rich minerals are common in tufa surrounding hot springs and geysers. The most common minerals, however, are carbonates.
A form of carbonate precipitation, hardpan or caliche , is associated with water infiltration in desert regions. Rain falling on carbonate rich mountains chemically dissolves the rock. When the water runs off into the catchment basin, typically a desert valley, it begins to soak into the ground. The intense heat of the region evaporates the water before it can percolate into the groundwater system. This results in the ions within the water precipitating as crystals in the top layer of sediment in the valley. Through repetition, an accumulation of minerals "cement" the sediment together. This prevents further percolation of water through the layer and creates a solid rock layer (hardpan) only a few feet below the surface.
Misuse of Groundwater Sources
Mankind has found the presence of groundwater to be a convenient water source. They have tapped and used the source, almost at will, with little regard for the limits of the system. There are two general ways that groundwater sources can be misused: quantitatively and qualitatively. Qualitative misuse, or contamination, of groundwater sources is a large field often covered in environmental courses. Dumping of chemicals, locations of landfills, feedlots and waste disposal sites and improper use of toxic and non toxic chemicals should be everyone's regard. Groundwater easily filters out solid waste particles due to the small interstices it flows through. But these opens can become clogged and restricted easily. The greatest danger is that groundwater retains dissolved contaminants for extended periods of time and, therefore, great distances. Remediation of groundwater sources is an expensive and difficult task.
Quantitative misuse of groundwater is an important issue in many areas due to the limited nature of groundwater flow. It takes years for a groundwater system to regard itself; yet mankind approaches many systems as endless sources of water. Let's briefly cover some of the common difficulties encountered with overdrawing groundwater sources.
When a well is sunk, the casing must go deep enough to penetrate into the zone of saturation even during times of lower water table levels, ex. droughts. The rate at which a person draws on the aquifer should always be lower than the rate at which the groundwater flows through the system. As a person draws on a well, the water around the casing can be pulled out of the aquifer faster than the water can flow back into the openings of the sediment. This produces a cone of depression in the water table around the well casing. If you stop pulling water out of the well, the water will eventually flow back into the cone of depression and the water table will recover from this temporary depletion.
If you continue to pull on the well at too fast a rate, the region around the well "goes dry", i.e., the cone of depression completely surrounds the well region. (This is the same effect as when you are drinking a slush cone and you suck on the straw too fast.) This situation can also be temporary. Let the groundwater recover (set the slush cone aside) for a long period of time. The amount of time depends on the flow rate of that system, but the water will eventually flow back into the depleted zone from higher regions in the aquifer.
Unfortunately, the typically response to a well "going dry" is not to "set it aside" and come back after it has recovered. People usually "dig it deeper" or "pump on it faster" to force the well to pull more water from the ground. When a well is continuously overdrawn or multiple wells are overdrawn in a single region, the water table level can drop on a regional basis. Recovering from this regional depression can take years, if it's even possible to do.
Often, when there is a regional drop in water table level, the aquifer permeability can be affected. The openings in the aquifer are supported by the buoyancy of the water between the grains. When the water is withdrawn, the overlying weight of material can compact the aquifer and the openings are collapsed. What was once flowing aquifer system can become an aquiclude as the permeability decreases and flow rates drop. This compaction of the aquifer results in land subsidence of the overlying surface. Several groundwater systems have been restricted in their use or completely determined "off limits" due to the land subsidence resulting from their overuse (others should quickly pay heed!).
When the groundwater system is in areas of carbonate rocks, the withdrawal of water results in unique problems. The dissolution of the carbonates in the system forms caverns in the aquifer. Excessive withdrawal removes the roof support supplied by the groundwater and results in the catastrophic collapse of the aquifer producing sinkholes . (Note: these aquifers are also more prone to contamination: the large cavities do not filter particles as do clastic dominated aquifer systems.)
Regions with multiple formations that contain groundwater may have unique problems involved with excessive withdrawal from one of the systems. The most common of these types, involves salt water encroachment . Fresh water aquifers "float" above denser salt water systems. When the groundwater is removed from the fresh aquifer, salt water easily flows into the aquifer to replace the withdrawn water. This results in brackish and contaminated wells. Though this may be more prominent in coastal regions, "mixing" problems are possible wherever two or more types of groundwater systems occur. If one of the systems is "contaminated", the mixing can raise important issues in groundwater management.