B Ground water

Many Natural Resources Conservation Service (NRCS) programs deal with the development, control, and protection of ground water resources. The planners of agricultural waste management practices should be familiar with the principles of ground water. NRCS references that include information on ground water include National Engineering Handbook (NEH) Section 16, Drainage of Agricultural Lands; NEH Section 18, Ground Water; Engineering Field Handbook (EFH) Chapter 12, Springs and Wells; and EFH Chapter 14, Drainage.

Generalized map of areas of karst and analogous terrains in the conterminous United States

Karst terrain

|| I I 11 Terrain or lava analogous to karst or II I I 11 karst buried beneath deep soil cover

| | Terrain analogous to karst resulting from 1 1 deep-seated piping (erosion by water)

Legend

Karst terrain

|| I I 11 Terrain or lava analogous to karst or II I I 11 karst buried beneath deep soil cover

| | Terrain analogous to karst resulting from 1 1 deep-seated piping (erosion by water)

0 100 200 300 400 500 Miles

B"

Chapter 7 Geologic and Ground Water Part 651

Considerations Agricultural Waste Management

Field Handbook

(1) Zones of underground water

All water beneath the surface of the Earth is called underground water, or subsurface water. Underground water occurs in two primary zones: an upper zone of aeration called the vadose or unsaturated zone, and a lower zone of saturation called the phreatic or saturated zone. The vadose zone contains both air and water in the voids, and the saturated zone is where all interconnected voids are filled with water (fig. 7-3). The term ground water applies to the saturated zone. Ground water is the only underground water available for wells and springs.

The vadose zone includes the soil-water zone, the intermediate zone, and the capillary fringe. The soil-water zone extends from the ground surface to slightly below the depth of root penetration. Water in this zone is available for transpiration by plants or direct evaporation. This zone is usually at less than saturation except during rainfall or irrigation. Water held by surface tension moves by capillary action. Excess water percolates through the soil-water zone by gravity. An intermediate zone may separate the soil zone from the capillary fringe. An intermediate zone does not exist where the water table (described later)

Figure 7-3 Zones of underground water (AIPG 1984, Heath 1983, and Todd 1980)

Soil moisture

Figure 7-3 Zones of underground water (AIPG 1984, Heath 1983, and Todd 1980)

Soil

Air spaces

Rock (weathered)

Rock (fractured)

Intermediate zone

Capillary zone

Water table

Sand and gravel

Zone of fracture concentration

Soil moisture

Soil particles

Soil particles

Ground water Soil particles

Ground water Soil particles

Ground water

Ground water

Creviced rock

approaches the ground surface. Regions in the intermediate zone may be hundreds of feet thick. Water in the intermediate zone cannot move back up to the soil-water zone by capillary action. Intermediate zone water moves either downward under gravity or is held in place by surface tension.

Directly above the water table is a saturated zone, the capillary fringe. This zone occurs in fine to medium grained soils and in rocks with fractures less than 1/8 inch wide. Water in the capillary fringe is under less than atmospheric pressure. It rises a few inches to more than 10 feet above the water table, depending on the earth materials (sand, low; clay, high). Surface tension and capillary action cause water in this zone to rise. Capillary rise increases as the pore spaces decrease.

In the saturated zone, water is under hydrostatic pressure and occupies all pore spaces. The upper surface of the saturated zone is called the water table. The elevation of the water table is at atmospheric pressure. The saturated zone extends from the plane of the water table down to impermeable geologic material.

(2) Aquifers

An aquifer is a geologic unit capable of storing and conveying usable amounts of ground water to wells or springs (fig. 7-4). When siting any agricultural waste management component, it is important to know:

• What type(s) of aquifers may be present and at what depths.

• What the aquifer use classification is, if any.

Aquifers occur in many types of soil or rock material. Productive aquifers include sand and gravel alluvial deposits on flood plains of perennial streams; glacial outwash; coarse-grained, highly porous, or weakly cemented sedimentary rocks (some sandstones and conglomerates); and karst topography. An aquifer need not be highly productive to be an important resource. For example, there are millions of low-

Figure 7-4 Aquifers (from AIPG 1984)

Figure 7-4 Aquifers (from AIPG 1984)

Ground Water The Line Country

yielding (less than 10 gpm) private domestic wells throughout the country. In upland areas, often the only aquifer available for a ground water source is fractured rock occurring near the surface (up to 300 feet deep).

An aquifer may be unconfined, confined, or perched. An unconfined aquifer, or water table aquifer, has no upper confining layer (fig. 7-5). Hence, the upper surface of the saturated zone is under only atmospheric pressure. It is, therefore, free to rise and fall with recharge or pumping. Recharge generally occurs locally. The static water level in a well in an uncon-fined aquifer is the elevation at which water stabilizes after pumping ceases. Unconfined aquifers are the type most commonly experienced in NRCS work.

Some unconfined aquifers result in flowing artesian wells. This occurs when the water table locally rises above the ground surface. Topography is the primary control on most flowing wells in major valley bottoms. The valleys serve as ground water discharge areas. Because hydraulic potential increases with depth in valley bottoms, deep wells frequently tap a hydraulic head contour with a head value greater than that of the land surface, and therefore, will flow (fig. 7-6).

A confined aquifer is overlain by a confining layer of lower permeability (fig. 7-7). The surface of ground water under confined conditions is often subject to higher than atmospheric pressure because it is con fined by impermeable layers bounding the aquifer. A well in a confined aquifer that has higher than atmospheric pressure is called an artesian well. The poten-tiometric surface is the level to which ground water rises in a tightly cased well penetrating a confined aquifer. Recharge areas are typically remote from any given well location. The classic model of a flowing artesian well (see fig. 7-4) is the case where an aquifer crops out (that is, is exposed at the Earth's surface) and receives recharge in an upland area. Low permeability materials (aquicludes) lying above

Figure 7-6 Cross section through stream valley showing ground water flow lines and flowing (artesian) well from unconfined aquifer (from Fetter 1980)

\ Flow line Unconfined aquifer

//////////, Impermeable boundary///// ///////

Figure 7-5 Unconfined aquifer (from AIPG 1984)

ill!

f

\ Pumping well n a t 1 ^— Ground surface

-

^— Water table

\

/ ' ' Cone of drawdown

Unconfined aquifer

Figure 7-5 Unconfined aquifer (from AIPG 1984)

and confining the aquifer generate hydraulic heads greater than the surface elevation head. The confined aquifer, therefore, produces flowing artesian wells.

A perched aquifer is a local zone of unconfined ground water occurring at some level above the regional water table. An unsaturated zone separates the perched aquifer from the regional water table. A perched aquifer generally is of limited lateral extent. It forms in the unsaturated zone where a relatively impermeable layer, called a perching bed (for example, clay), intercepts downward-percolating water and causes it to accumulate above the bed (fig. 7-8). Perched aquifers can be permanent or temporary, depending on frequency and amount of recharge. Perched aquifers can present dewatering problems during construction if not discovered during investigation of the site.

The United States Environmental Protection Agency (EPA), under the provisions of the Safe Drinking Water Act, has the authority to designate sole source aquifers. A sole source aquifer is an aquifer that provides the principal or sole source of drinking water to an area. No Federal funds can be committed to any project that EPA finds would contaminate the aquifer and cause a significant health hazard.

A state may have designated use classifications just as surface water resources have. A state may have designated use classifications to protect aquifers for future use by a municipality, for example. Some aquifers may be regulated against overdraft or ground water mining.

(3) Porosity

Most earth materials within a few hundred feet of the Earth's surface contain solids and voids. Downward percolating water collects in voids and becomes available for wells and springs. Porosity is defined as the ratio of the volume of voids to the total volume of a soil or rock mass, expressed as a percentage.

Volume of voids in a given soil mass (L3

Volume of given soil mass (L )

Figure 7-8 Perched aquifer

Figure 7-8 Perched aquifer

Perching bed -Regional water table -

Sand and gravel

Perching bed -Regional water table -

Sand and gravel

Figure 7-7 Confined (artesian) aquifer (from AIPG 1984)

-=!

^— Ground surface

Original pressure level

~~ —.

___ -—

\

^ —'If Impermeable clay confining bed

^— Cone of pressure decline Confined aquifer

The two main types of porosity are primary and secondary (fig. 7-9).

Primary porosity refers to openings formed at the same time the material was formed or deposited. An example of primary porosity is the voids between particles in a sand and gravel deposit. Primary porosity of soil depends on the range in grain size (sorting) and the shape of the grains. Porosity, however, is independent of particle size. Thus, a bathtub full of bowling balls has the same porosity as the same tub full of bb's. This assumes the arrangement (packing) is the same for balls and bb's. However, the tub full of a mixture of bowling balls and bb's will have a lower porosity than either the bb's or the bowling balls because bb's will occupy space between the bowling balls.

Secondary porosity refers to openings formed after initial deposition or formation of a material. Processes that create secondary porosity include physical weathering (freeze-thaw, wetting and drying, heating and cooling), chemical or biological action, and other stresses that produce fractures and joints. Secondary porosity is extremely common in most geologic materials near the Earth's surface. This type of porosity enables contaminants to move with little attenuation (reduction) or filtration.

Figure 7-9 Porosity—how ground water occurs in geologic materials

Figure 7-9 Porosity—how ground water occurs in geologic materials

Rings of capillary water (not ground water) surround contacts of rock particles

Approximate level of the water table

All openings below the water table are full of ground water

Gravel

Primary porosity

Gravel

Primary porosity

Rings of capillary water (not ground water) surround contacts of rock particles

Approximate level of the water table

All openings below the water table are full of ground water

Creviced rock

Secondary porosity

Creviced rock

Secondary porosity

(4) Specific yield

Specific yield is the ratio of the volume of water that an unconfined aquifer (soil or rock) releases by gravity drainage to the volume of the soil or rock mass. A material that has high porosity, such as clay, does not necessarily yield a high volume of water if the material also has low permeability (see section, Permeability of aquifer material). Such a material has low specific yield. See table 7-1 for comparison of porosity and specific yield of some geologic materials.

volume of water drained (L3)

volume of geologic material (L3)

Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment