Water Well Interference NT
Explanation of water well interference
Water wells may interfere with one another due to the composite drawdown of closely spaced wells. Figures 1 and 2 illustrate this concept. Figure 1 is an exaggerated diagram that shows drawdown for a single well. In the case of a surficial aquifer containing the water table, drawdown is the drop in water table elevation below the original elevation before the water well pumping began. There is a corresponding drop in water level in the well. In three dimensions, this drawdown looks like a cone and is often called the "cone of depression." Figure 2 illustrates the case where two wells are added nearby. If the two wells are pumped at the same rate as the first well, they each produce the same amount of drawdown as the first well; and the total drawdown is the sum of the drawdowns produced by each well. The total drawdown with three wells pumping is illustrated in Figure 2. Since the drawdown produced by each well increases the drawdown in the other wells, they may be said to interfere with one another. Figure 2 illustrates how the pump would have to be lowered in the first (middle) well to accommodate the effect of the other two wells. The degree of this interference depends on the following factors:
- The spacing of the wells (how far apart they are),
- The pumping rate of the wells,
- The permeability of the aquifer.
Reduced spacing and/or increased pumping rate increases interference. Lower permeability increases well interference, because lower permeability increases drawdown.
Figure 1. Diagram of drawdown produced by pumping one water well.
Figure 2. Diagram of drawdown produced by pumping three water wells.
Figures 1 and 2 are greatly exaggerated relative to domestic water well effects. A single domestic water well has a small average pumping rate and usually produces small drawdown in an aquifer, but multiple water well pumping of many closely spaced wells can produce significant drawdown and well interference because the small drawdowns are added to one another.
Simulation of water well interference
I have simulated the effect of pumping numerous domestic wells in a subdivision to illustrate such interference. The simulation utilized a technique called analytic element modeling that calculates the drawdown caused by each well at a large number of points in the area around the wells. It then adds the drawdowns at each point to get total drawdown. For a water table aquifer, it subtracts the drawdowns from the original water table elevation to get the pumping water levels, then contours the results. The contours are lines of equal elevation of the water table, just as a contour map of the land surface shows the configuration of the topography. In cases where the hydrology and geology is complex and/or more realistic modeling is needed, numerical modeling may be used instead of the relatively simple analytic element method
The simulation was for a subdivision that would ultimately contain 300 individual domestic wells, each serving one house. The aquifer was simulated as being 2600 feet thick having a permeability consistent with sandstone. In the absence of better data, rough estimates of permeability in subdivisions may be obtained from data on water well driller's completion reports (specific capacity data). The wells were simulated as penetrating 300 feet below the original water table. The original water table was given a slope of 10 feet per 1000 feet. The original elevation of the water table is contoured in Figure 3 as feet above the base of the horizontal aquifer. The pumping rate applied to each well was 0.37 gallons per minute. A succession of simulations were made by adding wells at randomized locations until the subdivision was populated by 300 wells.
Figure 3. Original elevation of water table in subdivision well interference simulation.
Adding one well made no perceptible difference in the contours. Ten wells caused some contours to bend slightly, reflecting the effect of drawdown on the slope of the water table (Figure 4). In groundwater jargon, the contours are said to bend "upgradient."
Figure 4. Elevation of water table after 10 wells were pumping in the subdivision.
After 50 homes were present and 50 subdivision wells were pumping, the upgradient deflection of the contours is obvious (Figure 5). This deflection is produced in the computer program by subtracting drawdown from the original water table elevation as described above.
Figure 5. Elevation of water table after 50 wells were pumping in the subdivision.
After the subdivision was half developed and 150 wells were pumping, the deflection of the contours due to decline in water table elevation (drawdown) was more pronounced (Figure 6).
Figure 6. Elevation of water table after 150 wells were pumping in the subdivision.
After the subdivision reached buildout and 300 wells were pumping, the deflection of the contours due to drawdown was very pronounced (Figure 7).
Figure 7. Elevation of water table after 300 wells were pumping in the subdivision.
Figure 8 shows the drawdown that produced the contours in Figure 7. The elevation of the water table was reduced 36 feet at four centrally located wells. More than 6 feet of drawdown occurred at all subdivision wells. Minimum well depths in the built out subdivision would need to accommodate this drawdown.
Figure 8. Drawdown after 300 wells were pumping in the subdivision.