What influences the size and shape of the cone of depression of the water table surrounding a well?

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DER requires a preliminary assessment of the radial extent of the cone of depression and pumping rates and times for all dewatering operations in ASS areas.

Empirical methods to calculate the radial extent of the groundwater cone of depression and pumping rates and times for dewatering operations are outlined in Appendix E of

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1. In the absence of site-specific information, the saturated thickness of superficial aquifers may be obtained from: Perth Groundwater Atlas, Second Edition (Department of Environment, 2004) for sites in the Perth metropolitan region; and from information held by the Department of Water (for sites elsewhere in the State). https://www.water.wa.gov.au/maps-and-data/maps/perth-groundwater-atlas
2. Adapted from Davidson, W.A., 1995, Bulletin 142: Hydrogeology and Groundwater Resources Of The Perth Region, Western Australia, Geological Survey Of Western Australia.
3. Hemond,H.F. and Goldman, J.C., 1985, On non-Darcian water flow in peat. Journal of Ecology, 73, 578-584

Cone of depression/Area of influence
When a public supply well is pumping, groundwater flow changes direction in a portion of the watershed. Instead of moving toward the natural discharge area, the groundwater within the influence of the pump flows toward the well from every direction. The pumping well creates an artificial discharge area by drawing down (lowering) the water table around the well. This area of drawdown is called the cone of depression. Except for wells in bedrock, the cone of depression of a private residential well is usually very small; the cone of depression of a public supply well, however, can extend thousands of feet from the well.

The cone of depression is most easily illustrated by a diagram that shows a cross-section of the well and the cone. However, to make this concept clear in relation to the land surface, it is useful to visualize the area from above. From this perspective, the cone of depression is termed the area of influence. Although, technically, the terms refer to different views of the same phenomenon, they are often used interchangeably.

The cone of depression changes size
When the amount of groundwater that is withdrawn by the pumping well is equal to the amount of groundwater recharge within the area of influence, the cone stops expanding. However, the cone of depression does not always remain the same size. If there is no precipitation to recharge the aquifer, and the well keeps pumping, the pump will pull water from a greater distance, and the cone of depression will get deeper and wider. After heavy precipitation, with good recharge, it will get smaller.

Land use can change the size and shape of the cone of depression and the ability of the aquifer to supply water. If impermeable surfaces (such as parking lots) cover a portion of the area of influence or its upland recharge area, and the runoff from those surfaces flows overland to streams instead of recharging the groundwater, the cone of depression for the pumping well will have to expand to compensate for the lost groundwater recharge. If there is no porous, permeable land within reach of the pumping well that can provide the recharge needed, the yield of the well may decrease. If enough of the potential recharge area is covered with impermeable surfaces, or if nearby surface waters are diverted for other purposes, the yield can be reduced so drastically that the well must be abandoned.

Limit of the well's influence
A well draws water from only a portion of the watershed, specifically, the cone of depression and upland recharge areas. Outside these areas, collectively termed the areas of contribution, groundwater does not move toward the well. Instead, it moves in its normal pattern from the recharge area down to the discharge area.

Induced recharge Most public supply wells in Massachusetts are located in buried valley aquifers that are associated with a nearby stream or river. Most of those wells draw surface water from the stream in a process called induced recharge. Induced recharge occurs when the cone of depression reaches as far as the stream, thereby lowering the water table beneath it. If there are no impermeable barriers such as clay or thick deposits of organic muck in the streambed, the pump will pull water from the stream down through the aquifer and into the well. Under these conditions, polluted surface water can enter the well and degrade the quality of the water supply. In Massachusetts, induced recharge probably occurs in all but a few public supply wells located in valley aquifers.

Four areas that should be protected
There are four areas significant to groundwater supplies that must be identified and protected to prevent contamination. They are all a part of a watershed.

Aquifers -- Aquifers are geologic formations that are capable of yielding a significant amount of water to a well or spring. In Massachusetts, buried valley aquifers are the sites of most public supply wells. Coastal outwash plains in southern Plymouth County, Cape Cod, Martha's Vineyard, and Nantucket are also excellent aquifers.

Cone of depression -- This is the area around the well where the water table is lowered when the well is pumped. Since water is withdrawn from this area to supply the well, it should receive utmost protection. Contamination that enters groundwater within the cone of depression will eventually reach the pumping well.

Recharge areas -- Recharge areas are porous, permeable geologic deposits (usually sand and gravel) that can absorb precipitation and allow it to percolate down to the water table and flow into the aquifer. These areas usually include the land surface directly above the aquifer and the porous, permeable areas adjacent to the aquifer. Although the most important recharge areas are those that replenish the portions of an aquifer that supply a well, all aquifer recharge areas should be protected, especially if there is a potential for developing new wells in the aquifer in the future.

Surface water - When the cone of depression intersects a lake, river, or stream, surface water may be drawn into the well via induced recharge. This occurs commonly in Massachusetts because most public supply wells are located in valley aquifers near rivers and streams. In these cases, both the quality and quantity of the surface water can affect the well. Therefore, it is important to protect the surface water that contributes to recharge. To do that, it is necessary to control land use in the watershed so that contaminants will not reach the river or stream, and to ensure that upstream use of the water does not decrease the quantity required to supply the well.

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