Discharge Ground surface
Pump
Static water level
Drawdown
Cone of depression
Pump water level
Zone of influence
FIGURE 5.13 Hydraulic characteristics of a well. (From Spellman, F.R. and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)
The curve of the line extends from the pumping water level to the static water level at the outside edge of the zone (or radius) of influence (see Figure 5.13). Note: The shape and size of the cone of depression is dependent on the relationship between the pumping rate and the rate at which water can move toward the well. If the rate is high, the cone will be shallow and its growth will stabilize. If the rate is low, the cone will be sharp and continue to grow in size. 5. Zone (or radius) of influence — The distance between the pump shaft and the outermost area affected by drawdown (see Figure 5.13). The distance depends on the porosity of the soil and other factors. This parameter becomes important in well fields with many pumps. If wells are set too close together, the zones of influence will overlap, increasing the drawdown in all wells. Obviously, pumps should be spaced apart to prevent this from happening. Note: Two important parameters not shown in Figure 5.13 are well yield and specific capacity. 1. Well yield is the rate of water withdrawal that a well can supply over a long period. Alternatively, this is simply the maximum pumping rate that can be achieved without increasing the drawdown. The yield of small wells is usually measured in gallons per minute (liters per minute) or gallons per hour (liters per hour). Š 2003 by CRC Press LLC
For large wells, it may be measured in cubic feet per second (cubic meters per second). 2. Specific capacity is the pumping rate per foot of drawdown (gallon per minute per foot), or Specific Capacity =
Well Yield Drawdown
(5.18)
EXAMPLE 5.14 Problem: If the well yield is 300 gal/min and the drawdown is measured to be 20 ft, what is the specific capacity?
Solution: Specific Capacity =
300 20
= 15 gal min ft of drawdown
Specific capacity is one of the most important concepts in well operation and testing. The calculation should be made frequently in the monitoring of well operation. A sudden drop in specific capacity indicates problems such as pump malfunction, screen plugging, or other problems that can be serious. Such problems should be identified and corrected as soon as possible.
5.9.2 WET WELL HYDRAULICS Water pumped from a wet well by a pump set above the water surface exhibits the same phenomena as the groundwater well. In operation, a slight depression of the water surface forms right at the intake line (drawdown), but in