residents have been utilizing their shallow aquifer for decades.Prior to develop

Question

residents have been utilizing their shallow aquifer for decades.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.

t

he 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 by

downstream 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 isriparianzoneforest

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.

What was the volume of water stored in the Santa Fe Group Aquifer prior to explosive growth? Assume that the basin is roughly 90 miles long and 25 miles wide, for a total area of 2250 mi2. Assume the thickness is 100 ft, as described in the first paragraph. Be sure to account for porosity in your estimate.

Explanation / Answer

Here total porosity is 22%

now porosity = pore volume / total volume

so pore volume = 22 * 2250

= 49500/100

= 495 so the porosity of the given aquifer is 495

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