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Figure 9-1.Static water level before pumping.

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surface area by the average depth. The answer is cubic feet. Multiply by 7.5 to obtain gallons at the water  source.  An  example  of  this  is  a  pond  with an  average  depth  of  7  feet  and  a  surface  area  of 2,864  square  feet.  It  is  calculated  as  shown below: Q=  Ax  D  x  7.5 Q  =  2,864  x  7  x  7.5 Q  =  150,360  gallons Lakes  and  ponds  are  usually  located  within the  water  table,  and  the  hydraulics  of  the  water feeding  the  lake  or  ponds  are  similar  to  that  of wells.  Therefore,  a  drawdown  test,  using  a method  similar  to  the  one  described  below  for wells,  may  be  used  to  calculate  the  quantity  of water.   To   perform   the   test,   you   should   draw down the static water level to 1 or 2 feet and then record  the  recovery  time.  Also,  devise  a  method to  discharge  the  water  being  pumped  so  it  does not return to the source during the test. To calculate the quantity of water that can be supplied   from   newly   constructed   or   existing wells,   you   must   make   a   drawdown   test.   To perform  this  test  properly,  you  must  understand the hydraulics of a well. Before being pumped, the level of water in a well is the same as the level of the water table in the water-bearing formation in which the well is completed.  This  is  called  the  static  level  in  the well  and  in  the  foundation.  (fig.  9-1).  The  depth from the ground surface to the static water level should  be  measured  and  this  distance  used  to describe its position. Thus if the water in the well Figure 9-1.—Static water level before pumping. 9-2 is  25  feet  below  ground,  the  static  water  level  is said  to  be  25  feet  for  this  well.  Elevation  of  the static water level above mean sea level can also be used to describe its position. When a well is pumped, the water level drops. After several hours of pumping at a constant rate, it stabilizes itself in a lower position. This is called the  pumping  level  or  dynamic  water  level  for  this rate  of  pumping  (fig.  9-2). The  distance  the  water  is  lowered  by  pumping is called the drawdown. It is the difference between the   static   level   and   the   pumping   level.   The drawdown   in   the   well,   resulting   from   pumping, lowers   the   water   pressure   in   the   well,   but   the surrounding   water-bearing   formation   retains   its original  pressure.  In  response  to  this  difference  in pressure,   water   flows   out   of   the   pores   of   the formation into the well. The  water-bearing  formation  does  not  furnish its water all at once to the well being pumped. The flow  of  water  into  the  well  is  held  back  by  the frictional resistance offered by the formation to the flow   of   water   through   its   pores.   The   resistance varies in each formation and is developed in direct proportion  to  the  rate  of  movement  or  velocity  of the  water  in  the  formation.  The  rate  of  flow, resulting from a given pressure difference, depends on the fictional resistance to flow developed in the formation.   The   term   used   to   describe   this characteristic of a porous material is  permeability. For  a  particular  type  of  well,  the  yield  of  the well  for  any  given  drawdown  is Figure  9-2.—Pumping  level.



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