WATER WELL INTERFERENCE - ANALYTICAL ELEMENT GROUNDWATER MODEL

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.  In an area where wells are added over time, such as a real estate subdivision, the early wells may not have been constructed to accommodate the decreasing water levels.  In such cases, pumps may have to be lowered, and wells may have to be deepened or replaced.  In the worst case, there may not be enough available drawdown for aquifers with adequate water quality to accommodate all of the wells, and some well owners may find themselves without an adequate water supply.  Such problems may be avoided by practicing good groundwater management, which may involve estimating aquifer safe yield using groundwater models that can simulate the effects of water well interference.

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.

Water Well Interference References

Fetter, C. W. Jr. (1980): Applied Hydrogeology, Chrles E. Merrill Publishing Company.

Heath, R. C. (2004): Basic Ground-Water Hydrology; U. S. Geological Survey Water-Supply Paper 2020 (revised). 🔗

Todd, D. K., (1959): Ground Water Hydrology, John Wiley

Posted March 19, 2018. Revised March 1, 2025.  

Revised May 30, 2019, November 2,2021