The link between groundwater and surface water
(Note: This article is an excerpt from Water Primer: Part 3 Groundwater, K-State Research and Extension publication MF-3022, by Danny Rogers, Professor, Biological and Agricultural Engineering; Morgan Powell, Professor Emeritus, Biological and Agricultural Engineering; and Kerri Ebert, Extension Assistant, Biological and Agricultural Engineering. The full publication can be accessed online at: http://www.bookstore.ksre.ksu.edu/pubs/MF3022.pdf/ -- Steve Watson, Agronomy eUpdate Editor)
Groundwater and surface water are often one closely interrelated system. Groundwater feeds springs and streams. Surface water recharges aquifers. The interaction of groundwater and surface water affects water quality and quantity.
Groundwater can be contaminated by polluted surface water, and surface water can be degraded by discharge of saline or other low-quality groundwater. Streams and their alluvial aquifers are so closely linked in terms of water supply and water quality that neither can be properly understood nor managed by itself. The total combined stream-aquifer system must be considered.
Water movement in soil and geology
Gravity causes excess soil water to percolate (seep) through the soil and subsoils until an impervious layer is reached. Permeable soils, such as sand, hold only a small amount of water and allow quick percolation. Clay soils have extremely small openings between microscopic particles that significantly slow water movement. Many saturated clay soils are virtually impermeable. Depending on the geology of the soil or rock material, percolation rate may range from a fraction of an inch to a few feet per day.
Groundwater movement is also affected by the contour of the surface land (topography). Groundwater flows from a high-water surface elevation to a low-water surface elevation unless barriers are encountered.
Since groundwater moves toward lower elevations, a well driller often looks for water near the bottom of a hill. However, water quality should be protected by locating a well as far up the slope as possible to be away from sources of contamination such as a septic system or livestock area.
As water moves through the soil, it passes through an area above the water table known as the unsaturated, or vadose, zone (see Figure 1). This layer may be moist or wet, but the pores are only partially filled with water, leaving space for air. As water percolates through the soil it eventually reaches a zone where all the interconnected pore openings are filled with water.
This is called the saturated zone. The top of the saturated zone is the water table.
Figure 1. Water movement from the soil surface to groundwater.
The layer directly above the water table is the capillary fringe. In this layer groundwater moves upward from the water table by means of the capillary action of the pore spaces. Capillary action is caused by the surface tension of water. The water table rises and falls according to the season of the year, the amount of recharge, and the removal of groundwater. If groundwater is removed by pumping from wells, the water table lowers, which may change the direction of groundwater flow. As a result, sections of streams that once received groundwater decrease, and therefore, stream flow decreases (Figure 2).
Figure 2. Stream-aquifer interaction. (Source: Sophocleus and Sawin, 1997.)
Sophocleous, Mario A. and Robert S. Sawin. Safe Yield and Sustainable Development of Water Resources in Kansas. Kansas Geological Survey Public Information Circular 9. October 1997
Groundwater storage and recharge: Confined and unconfined aquifers
The storage systems for groundwater are the spaces between particles of soil, sand, gravel, or rock, as shown in Figure 1. When saturated with water that can be removed by a pump, these formations are called aquifers or groundwater reservoirs.
People often envision an underground river or lake when thinking about aquifers, but in most areas of the world, and specifically in Kansas, water is stored in the small openings between the soil and rock material and is more like a wet sponge. Aquifers may be as small as a few acres or larger than a state.
The volume of contained water depends on the size, thickness, and porosity of the aquifer as well as the recharge. Storage volume is commonly known as the number of acre-feet per acre or feet of water. An acre-foot is the volume of water (325,851 gal.) it takes to cover an acre to a depth of 1 foot.
Aquifers are classified as either unconfined or confined. Unconfined aquifers lack a restrictive layer above the water table, as shown in Figure 3. Locations where water percolates down to the groundwater system are called recharge areas. The recharge that supplies these aquifers infiltrates the soil directly above the aquifer.
Figure 3. Well, aquifers, and the water table.
Unconfined aquifers, especially shallow ones that are covered by porous (sandy) soils, are the most susceptible to contamination. These unconfined aquifers are often close to the surface, with their only protection being overlying soil and geologic material. Figure 3 illustrates both an unconfined and a confined aquifer.
A confined aquifer is a water-bearing layer between two impermeable, or confining, layers. The confining layers may be clay or rock, such as shale. The recharge area can be far away from the geologic formation penetrated by a well, but regardless of distance the two are connected.
The greatest recharge tends to occur in areas of coarse-textured or sandy soil with low water-holding capacity. Some recharge even occurs in clay soils because when heavy rains follow dry periods, cracks in the clay may allow water to enter deeply into the soil past the root zone. Recharge also occurs where seeps, springs, wetlands, streams, lakes, or other surface water percolate down to the aquifer. Areas of recharge can be small and localized or very extensive, covering hundreds of square miles.
Water in a confined aquifer is contained or restricted similar to water in a pipe. The water can become pressurized if the confined aquifer slopes away from the recharge area. When the pressurized water rises above the top of the confining layer, a condition called an artesian well is created — illustrated in Figure 3 by Well 1. A flowing artesian well occurs where the pressure is high enough to force water to the surface, causing water to flow freely from the well without using a pump. The water table rises and falls according to the season of the year, the amount of recharge, and the removal of groundwater. If groundwater is removed by pumping from wells, the water table lowers, which may change the direction of groundwater flow. As a result, sections of streams that once received groundwater decrease, and therefore, stream flow decreases.
Water quality in confined and unconfined aquifers
The material overlying the aquifer protects the water quality of an aquifer by slowing the movement of water that may carry potential contaminants and allowing biological and chemical processes to remove some contaminants. These processes, however, can be overwhelmed by pollution or contamination that cannot be easily broken down or are applied in quantities in excess of the process capacity to break them down. Many older wells are either poorly constructed or poorly maintained and allow direct entry of surface water into the aquifer.
Because aquifers are not exposed to air or sunlight, they do not have the self-oxygenating, cleansing properties of surface waters. Therefore, when groundwater becomes contaminated, the concentration (plume) of pollutants moves slowly with the aquifer water, allowing for minimal dilution or dispersion. By the time a plume reaches a well site, it may be difficult to determine where it originated, when it was released into the groundwater, and how long it will affect the well site.
Confined aquifers have greater protection from surface contamination, as long as the recharge area is protected from pollutants, because they are between layers of relatively impermeable materials. They are typically deeper underground than unconfined aquifers and have an increased measure of protection from contamination from the ground surface. However, confined aquifers can be contaminated by pollutants in the recharge area or when confining layers are penetrated by test holes, poorly constructed wells, abandoned wells, or another breach of confinement that allows pollutant entrance.
Source: Water Primer: Part 3 Groundwater, K-State Research and Extension publication MF-3022: http://www.bookstore.ksre.ksu.edu/pubs/MF3022.pdf/