residents have been utilizing their shallow aquifer fordecades.Prior to developm
ID: 111966 • Letter: R
Question
residents have been utilizing their shallow aquifer fordecades.Prior to development,the sandand gravel aquifer had anaverage saturated thickness of about 100feet.The average porosity of the
Group is about 22%.The average precipitationin Mountains is about10-25 inches per
year,but on average,only a small percentage(10to30%)of this precipitation recharges
the aquifer.assume that thewater level in the aquifer is about 3 feet lower than the river water level,onaverage.the agricultural drains are 8 feet lower than thewater table,so that the aquifer is losing water to the drains.Assuming upstream subsurface aquifer inflows are balanced bydownstream outflows we can write the water budget for the Group of aquifers of the Basin asfollows:
ATh
t
= QMR +QRS +QIR QDR QET QPUMP
whereQMR ismountainfrontrechargeinft^3/year,QDR is drain seepage losses inft^3/year,
QRS is river seepage inflows in ft3/year, QIR isirrigation see page that recharges the aquifer
in ft^3/year, QPUMP is groundwater pumping inft^3/year, QET is riparian zone forest
evapotranspiration losses in ft^3/year, AT is area ofthe Basinaquifer system in
ft^2, is aquiferporosity,h is average aquifer water levelin feet,and t is time in years.One
acre-foot equals 43,560ft^3.
Next, you take very careful flow measurements in the agricultural drain, again one mile
apart. At the upstream cross section you measure 32 cubic feet per second (cfs). At the
downstream point you measure 43 cfs. As before, assume that your measurements are
representative of the whole drainage ditch as well as the whole year. If the ditch gains 11
ft3 every second over each mile of flow distance, how much water seeps from the aquifer
into the ditch annually over the full 92 mile length (it’s slightly less sinuous than the river)
that flows through theBasin?
Explanation / Answer
An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted using a water well. The study of the water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer and aquiclude, which is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer.
Aquifers may occur at various depths. Those closer to the surface are not only more likely to be used for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Part of the Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon ranges between Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra Nevada and neighboring ranges in the United States' Southwest, have shallow aquifers that are exploited for their water. Overexploitation can lead to the exceeding of the practical sustained yield; i.e., more water is taken out than can be replenished.
Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).
Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero-gauge-pressure isobar) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure.
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