Watersheds Primer Part 2 - Ground Water Basics
Ground Water: A Hidden Resource
Ground water is the water located underground that saturates the spaces between alluvial material (sand, gravel, silt, clay) or the crevices or fractures in rocks. The ground water is very important as it provides approximately 60% of the water that flows in streams and it directly provides water to approximately 40% of the population in the study area. Thus, it is important that watershed management begin from the "geology up".
Ground water may exist near to the land surface (less than 10 feet down) or it may occur at depths of up to 100 to several hundred feet. The underground rock units in which the ground water occurs are called "aquifers". These aquifers are large and deep expanses of rock that lie below the land surface. Water collects within the cracks, fissures and open spaces of these rock units. An easy way to visualize an aquifer is to look at rock walls along roadsides where the rock is exposed and water maybe seen seeping out of the cracks of the rock unit. This rock wall represents an "outcrop" (or location where the rock intersects the land surface) of an aquifer. The point where the ground water emerges into view is called a "seep" or "spring". As that water flows down the rock face and into a receiving drainage channel below, it begins a "stream".
In the study area, the aquifers consist of several complicated geologic rock types. Much of the area is underlain by crystalline rock (similar to granite) where the ground water exists only within cracks and fractures. Very hard sedimentary rocks underlie some areas where ground water exists in voids (or spaces) between grains or in cracks, fractures and bedding planes between layers of rock. The Chester Valley and some other localized areas are underlain by carbonate rocks (limestone and marble) where ground water exists in cracks and fractures that over time become larger as the water dissolves the carbonate rock along its flow paths.
Underground, ground water exists in very large volumes. However, because it occurs only in the cracks, fractures and solution openings of the aquifers, this water may be difficult to find as it does not exist in a continuous manner across the region. Although it does not occur in a continuous manner across the area, ground water is readily available throughout the study area watersheds. This is evident from the widespread use of wells for residential, agricultural, commercial, industrial, turf irrigation, and public water supply purposes.
Ground water moves very slowly but continuously through the interconnected openings of the aquifer. It moves generally from areas of high elevation (or high water pressure) to areas of lower elevation (or pressure). In the study area, ground water movement tends to follow the topography. As illustrated in the figure on ground water movement, ground water moves by gravity in a downhill direction, until it reaches a point where it intersects the land surface (i.e., at a seep or spring, wetland, or stream bottom).
Ground water movement showing infiltration and ground water flow.There are several important terms that relate to ground water that help to understand how ground water occurs. Moving from the land surface downward into the aquifer, one finds the following conditions:
(Adapted from Winter et. al., 1998 – Modified from Dunne, T. et al., 1978)
(Adapted from Winter et. al., 1998 – Modified from Dunne, T. et al., 1978)
Unsaturated zone: This zone lies immediately between the land surface and above the water table. It is known as the unsaturated zone (or zone of aeration, or the vadose zone) because while this zone is moist, it is not fully saturated with ground water. Water in the soil of the unsaturated zone is referred to as soil moisture. Spaces between soil, gravel and rock of this zone are filled with some water (suspended) and air.
Capillary water: This occurs immediately above the water table, in the unsaturated zone. Capillary water moves upward from the water table by capillary action into the overlying sediments or rock fractures. This water can move slowly in any direction, from a wet particle to a dry one. While most plants rely on moisture from precipitation that is present in the unsaturated zone, their roots may also tap into capillary water or into the underlying saturated zone.
Saturation zone: This zone is completely saturated with water. The upper limit of this zone is known as the water table (or phreatic surface).
Aquifer: ground water is found in aquifers or underground layers of rock that are saturated by water percolated downward from the overlying land surface or from other geologic rock units that are sloping toward it. Aquifer capacity is determined by the porosity (or amount of inter-connected cracks, fractures, and other open space in the rock) and its area (the depth and lateral extent of the aquifer). Under most of the United States, there are two major types of aquifers: confined and unconfined.
Confined aquifers: also known as artesian or pressure aquifers, confined aquifers exist where the ground water system lies between layers of clay, dense rock or other materials with very low permeability that create a cap or seal over the aquifer. Water in confined aquifers may be very old, arriving many years ago. It's also under more pressure than unconfined aquifers. Thus, when tapped by a well, water is forced up, sometimes above the soil surface. This is how a flowing artesian well is formed.
Unconfined aquifers: most of the ground water in the study area occurs in unconfined aquifers. These do not have an over-lying low-permeability layer above it. Water in unconfined aquifers may have arrived recently by percolating through the land surface. This is why water in unconfined aquifers is often considered very young, in geologic time. In fact, the top layer of an unconfined aquifer is the water table. It's affected by atmospheric pressure and changing hydrologic conditions. Discharge and recharge rates in unconfined aquifers depend upon the amount of rainfall and infiltration that occurs in the areas above them. The recharge area of unconfined aquifers includes the entire overlying land surface.
How Ground Water & Surface Water Connect
Ground water and surface water are fundamentally interconnected. This is why one can contaminate the other. To better understand the connection, take a closer look at the various zones and actions. A way to study this is by understanding how water recycles in the hydrologic (water) cycle. The source of ground water (recharge) is from precipitation that percolates downward. In southeastern PA, approximately 28% of annual precipitation is recharged to ground water. Left untouched, ground water naturally arrives at a balance, discharging about the same volume of water as it receives from recharge, depending on hydrologic conditions.
As rain or snow falls to the earth's surface,
- Some water runs off the land to rivers, lakes, streams and oceans (surface water).
- Water entering the soil can infiltrate deeper to reach ground water that can discharge to surface water or return to the surface through wells, springs and marshes. Here it becomes surface water again. And, upon evaporation, it completes the cycle.
Ground water interacts closely with streams, often flowing (or discharging) into a stream, wetland or lake. An unconfined aquifer that "feeds" a stream is called a "gaining stream". In fact, ground water is responsible for maintaining the flow in streams, springs, lakes, wetlands and marshes of most of the study area. This is particularly important during dry weather when no overland runoff occurs. This is why protection of a particular stream, lake or other surface waterbody should also address protecting the contributing aquifer.
In a few areas, water seeps through the stream bed and infiltrates into an underlying aquifer. These are called "losing streams".
Watershed & Aquifer Boundaries
Surface watersheds are defined by a simple process of delineating points of the highest elevations in land (or ridge lines) that drains to the common water body (i.e. lake, pond, stream, estuary, etc.). Watersheds are all shapes and sizes, ranging from just a few acres to several million acres. Many smaller watersheds can be "nested" inside a larger watershed. Larger sizes - ranging from the entire Delaware River Basin to a small drainage of one creek - can be delineated using topographic maps such as those published by U.S. Geologic Survey (USGS).
The term "watershed" is typically used to describe the land area contributing drainage to a common stream or water body. The same concept can apply to aquifers. However, the "watersheds" (or contributing areas) of aquifers are more difficult to delineate and are not as routinely mapped as watersheds. It requires an understanding of the aquifer, the geology, and the nature of ground water flow in that area. The "watershed" of an unconfined aquifer often extends to approximately the limits of the overlying surface water body’s "watershed" boundary (i.e. the surface watershed of the overlying lake, stream, or estuary). For example, in the study watersheds that are underlain by carbonate rocks, the surface water watershed may not (and usually does not) coincide with the ground water watershed (Sloto, 1994). The contributing "watershed" of a confined aquifer (such as occurs in the Coastal Plain province) may be limited to the outcrop of the aquifer unit that is located nearby or many miles away. Semi-confined aquifers may receive water from both outcrop areas and overlying aquifers. Delineating the aquifer recharge areas can be complex.