Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 94

Recharge area Discharge area

Pumped well

Unconfined
aquifer

Confining bed

Confined
aquifer

Centuries

Confining bed

Confined
aquifer

Millennia

FIGURE 3.33
Range in groundwater residence time. (From Focazio, M.J. et al., Assessing Ground-Water Vulnerability to
Contamination: Providing Scientifically Defensible Information for Decision Makers , United States Geological Survey
Circular 1224, Denver, CO, 2001.)

TABLE 3.4
Estimated Depth and Residence Time of the World's Water Supply

Parameter Equivalent Depth (m) Approximate Residence Time

Oceans and Seas 2,500 4,000 years

Lakes and Reservoirs 0.250 10 years

Swamps 0.007 1-10 years

River channels 0.003 2 weeks

Soil moisture 0.130 2 weeks to 1 year

Groundwater 120 2 weeks to 10,000 years

Ice caps and Glaciers 60 10-10,000 years

Atmospheric water 0.025 10 days

Biospheric water 0.001 1 week

Source: Environment Canada, Aquifers and wells, http://www.ec.gc.ca/water/

images/nature/gedwtr/a5f3e.htm, 2009.

3.5.10 Groundwater Withdrawal

Withdrawal of groundwater is accomplished by pumping a well installed within the aqui-
fer. As groundwater is pumped from the aquifer, the water level in the aquifer begins to
decline as water is removed from storage. A hydraulic gradient is created as the water level
falls below the level of the surrounding aquifer. Water then begins to migrate from the
aquifer into the well. The water level will continue to decline within the aquifer, and the
flow rate of water into the well will increase until the inflow rate is equal to the withdrawal
rate. This process occurs only when the pumping rate does not exceed the capacity of the
aquifer formation to transmit groundwater to the well. In this case, the diameter of the
well may be too small, and the pump should be placed lower in the well, or the withdrawal
rate should be decreased.