Aquifer, Aquitard or Gas pipe?

[IMMORTAL HOWDEN ENDER]

I think it is about time that we had a public debate about this. I am reliably informed that a recent Highland News Poll split the local community on this issue. Is the fourth stand being delayed because of an aquifer or a gas pipe? Apparently 47% felt that it was an aquifer 32% felt that it was a gas pipe and 21% were unsure.

If you were one of the pollsters or you have something valuable to add to this debate please join in. We, the ICT supporters, deserve to know the truth.

Ah, I hear you all say. What is an aquifer? OK i will start off the debate.

Aquifers versus aquitards

Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). Aquitards (sometimes, if completely impermeable, called an aquiclude or aquifuge) are saturated regions, which due to lower hydraulic conductivity, do not yield a sustainable amount of water in an economic fashion (e.g., clay, silt or fresh bedrock often form aquitards). Economically feasible is a relative term; for example, an aquifer that is quite adequate for local domestic use, as in a rural area, might be considered an inadequate aquitard for industrial, mining, or urban water supply.

In non-mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium. They are typically composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, due to the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), while the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas - sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.

For more details on ranges of parameters in aquifers and aquitards, see the hydraulic conductivity and storativity articles

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Confined versus unconfined

There are two end members in the spectrum of types of aquifers; confined and unconfined (with semi-confined being in between). Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. Unconfined aquifers usually receive recharge water directly from the surface, from precipitation or from a body of surface water (e.g., a river, stream, or lake) which is in hydraulic connection with it. Confined aquifers have the water table above their upper boundary (an aquitard or aquiclude), and are typically found below unconfined aquifers. A "perched aquifer" occurs when the porous, water-bearing segment of rock is located on top of a layer of non-porous rock.

If the distinction between confined and unconfined is not clear geologically (it is not known if a clear confining layer exists, or the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10-5), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically then called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content).

For more details on parameters related to the confined vs. unconfined distinction, see the porosity or storativity articles.

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Human dependence on groundwater

Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. Fresh water aquifers, especially those with limited recharge by meteoric water, can be over-exploited and, depending on the local hydrogeology, may draw in non-potable water or saltwater (saltwater intrusion) from hydraulically connected aquifers or surface water bodies. This can be a serious problem especially in coastal areas and other areas where aquifer pumping is excessive.

Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers.

Some aquifers are "riparian aquifers". These are related to rivers, fluvial deposits, or unconsolidated deposits along river corridors, and are usually rapidly replenished by infiltration of surface water. Some municipal well fields are specifically designed to take advantage of induced infiltration of surface (usually river) water, leaving them potentially vulnerable to water quality problems in the surface water body (chemical spills, petroleum spills, and bacteriological problems).

Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface (within a couple of hundred meters) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolate into the aquifer through overlying unsaturated materials.

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Subsidence

In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress due to the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence. Unfortunately, much of the subsidence due to groundwater extraction is permanent (elastic rebound is small). Thus the subsidence is not only permanent, but the compressed aquifer has a permanently-reduced capacity to hold water.

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Examples

An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer [2] in central Texas. This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas.

One of the largest aquifers in the world is the Guarani Aquifer, with 1.2 million km² of area, from central Brazil to northern Argentina.

Aquifer depletion is a global problem, and is especially critical in northern Africa; see the Great Manmade River project of Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States.

The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted for growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contain primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid portions of the aquifer, is estimated to total only about ten percent of annual withdrawals.

The Mahomet Aquifer supplies water to some 800,000 people in central Illinois and contains approximately four trillion US gallons (15 km³) of water. The Mahomet Aquifer Consortium [3] was formed in 1998 to study the aquifer with hopes of ensuring the water supply and reducing potential user conflicts.

The Great Artesian Basin is one of the largest groundwater aquifers in the world. It plays a large part in water supplies for remote parts of South Australia.

[f*ballers wife]

oh my!!!

howdi.... i got the bit about "I think it is about time that we had a public debate about this. I am reliably informed that a ...." then you kinda lost me babe! :006:

[tm4tj]

You are right cap'n Ender, the ground is wet.

[IMMORTAL HOWDEN ENDER]

To be fair I should offer the alternative argument

202.01 Description. This work shall consist of the removal, wholly or in part, and satisfactory disposal of all buildings, fences, guardrails, structures, old pavements, abandoned pipe lines, storage tanks, septic tanks, privy vaults, and any other obstructions which are not designated or permitted to remain, except for the obstructions to be removed and disposed of under other items in the contract. It shall also include the salvaging of designated materials and backfilling the resulting trenches, holes, and pits. When specific pay items are not listed on the plans for removal of structures and obstructions as set forth in this section, such work shall be performed under 203 Roadway Excavation and Embankment.

202.02 Construction Requirements. The Contractor shall raze, remove, and dispose of all buildings and foundations, structures, fences, guardrails, old pavements, abandoned pipe lines, storage tanks, septic tanks, privy vaults, and other obstructions any portion of which are on the right-of-way, except utilities and those items for which other provisions have been made for removal. All designated salvageable materials shall be removed, without unnecessary damage, in sections or pieces which may be readily transported, and shall be stored by the Contractor at specified places within the project limits. Unusable material shall be destroyed or disposed of outside the limits of view from the right-of-way with written permission of the property owner on whose property the material is placed. Copies of all agreements with property owners shall be furnished to the Engineer. Basements or cavities left by structure removal shall be filled to the level of the surrounding ground, and if within the area of construction, shall be compacted in accordance with 203.

When existing sewers are encountered in removal operations and are determined by the Engineer to be inactive or are to be abandoned, they shall be plugged or sealed at the ends where broken into before backfilling operations proceed. Plugging and sealing shall be accomplished by furnishing and placing approved precast stoppers or masonry bulkheads.

202.03 Bridges, Culverts and Other Drainage Structures Removed. Bridges, culverts, and other drainage structures in use by traffic shall not be removed until satisfactory arrangements have been made to accommodate traffic.

The substructures of existing structures, including piling, shall be removed down to the proposed stream bottom and those parts outside the stream shall be removed to a minimum of 0.3 m (1 foot) below proposed ground surface. Where such portions of existing structures lie wholly or in part within the limits for a new structure, they shall be removed as necessary to accommodate the construction of the proposed structure.

When specified, all structural steel, timber, and other reusable materials shall be carefully dismantled, and when specified, steel members shall be match marked as directed by the Engineer. Specified salvaged materials shall be considered as the property of the state and such materials shall be stored as specified in 202.02. Where alteration of an existing structure requires removal of portions of the structure, such removal shall be performed with sufficient care as to leave the remaining portion of the structure undamaged. In case of damage to the existing structure, repair or replacement shall be made at the Contractor's expense and to the approval of the Engineer.

202.04 Pipe Removed. This section provides for "Pipe removed for reuse or storage" and "Pipe removed." For both types of removal, the work under this section shall include excavating all material necessary to permit removing the pipe; disposing of excavated material, including broken pipe; sealing openings left in walls of manholes or catch basins that are to remain in place; and removing and disposing of pipe headwalls.

(a) For "Pipe removed for re-use or storage," the work shall include removing, cleaning when re-used, transporting, and storing the pipe. All pipe shall be carefully removed and every precaution taken to avoid breaking or damaging the pipe. Pipe to be relaid shall be removed and stored when necessary, so that there will be no loss or damage before relaying. The Contractor will be required to replace sections lost from storage or damaged by negligence or by use of improper methods at no additional cost to the State.

(b) For "Pipe removed," the pipe becomes the property of the Contractor and shall be disposed of in accordance with 202.02.

© Excavating. Where the plans call for pipe to be removed for re-use or storage, a section of pipe line shall be removed sufficient in length to permit determining the quality of pipe and the possibility of removing it without damage. If the Engineer determines that the pipe is worth salvaging and can be salvaged, the Contractor shall perform the remainder of the excavation in a manner that will not damage the pipe. If the Engineer determines otherwise, the pipe shall be removed under "Pipe removed," and the original item shall be non-performed.

Where caving occurs, the caved material shall be excavated before the trench is backfilled.

All excavated material shall be used or disposed of in accordance with the provisions of 203.

(d) Backfill. The trench resulting form the removal of pipe shall be backfilled in accordance with the provisions of 203 except when the trench lies within the limits of subsequent excavation. Any trench resulting from pipe removal under proposed pavement or paved shoulder shall be backfilled in accordance with the requirements of 603.08 for backfilling conduit Type A or B.

202.05 Pavement, Walks, Curbs etc. Removed. When designated for removal, an existing wearing course, concrete base course, concrete pavement, bituminous wearing course on brick and/or concrete base, concrete, walks, concrete steps, concrete gutters, stone or concrete curbs, concrete traffic dividers, etc., shall be removed and disposed of as follows:

(a) Materials to be salvaged shall be carefully removed and stored on the right-of-way at locations determined by the Engineer.

(b) Materials that are not to be salvaged or that are not suitable for re-use shall be disposed of in the same manner as excavation, 203.05.

© Where only a portion of an existing walk, pavement, etc. is to be removed, a neat joint shall be sawed or otherwise cut at the removal limit if it does not occur at an existing joint.

[Guest Lauraness]

oh for feck sake i didnt read none of that!!!!!

too long looking!!!

but i have to be on my grandas side

yup