Difficulties arise in determining the timing, location, and magnitude of the impacts. The degree to which ground water pumping depletes surface water supplies is dependent on several features of the particular basin. Considerations include: 1) the degree to which the river and aquifer are interconnected, 2) the distance between the river and the pumping source, 3) the rate of pumping, and 4) the physical characteristics of the aquifer. These factors are discussed in the following paragraphs.
The degree of river and aquifer interconnection is of great importance in controlling the amount of surface water depletion resulting from ground water pumping. If a river is perched above an aquifer, ground water pumping has no effect on river flow. If the river is not perched, but sediments have accumulated in the riverbed, or the river only slightly penetrates into the aquifer, then the hydraulic communication between the river and aquifer may be limited. Examples are shown in the following illustrations: partially penetrating river with silt deposition and fully penetrating rivers. Spring discharge will nearly always be impacted by nearby ground water pumping from the same aquifer.
The distance between a surface water body and a pumping location strongly affects the timing and degree that pumping will impact stream depletion. Pumping near an interconnected surface water body will have a nearly immediate impact on the surface water source. The impact may be nearly equal to the rate of ground water pumping. At greater distances, the effects of pumping will be distributed over longer time periods and may be shared with other hydraulically connected surface water bodies.
The rate of stream depletion associated with pumping from a given location is normally proportional to the rate of ground water pumping. If the rate of pumping from a given well is doubled, then the rate of stream depletion resulting from pumping that well also doubles. Stream depletion will be proportional to pumping rate unless aquifer water levels change so dramatically that springs are dried up, streams become perched, or aquifer properties change.
Ground water that is pumped, but not consumptively used (for example, industrial pumping that is discharged to seepage ponds), may return to the aquifer from which it was extracted and have little or no impact on surface or ground water supplies outside the immediate vicinity. Similarly, ground water pumped in excess of the amount required for crops to grow may return to the aquifer and have little or no quantitative impact on the surrounding resource. It is the amount of water that is permanently extracted from the aquifer and consumptively used that is of significance.
Aquifer physical characteristics also affect the timing and magnitude of stream depletion from pumping. Aquifer layering, water transmission, and storage properties may have a strong influence on the direction and rate of propagation of pumping effects. Wells completed in deeper layers may have a more disbursed and delayed impact on surface water bodies than wells completed in upper layers of an aquifer. Highly transmissive aquifers with limited water storage capacity will transmit effects more rapidly than aquifers of lower permeability or higher storage capacity.
A common misconception is that impacts of ground water pumping may be projected along estimated flow paths through an aquifer. If this were true, then only down-gradient streams and springs would be affected by up-gradient pumping. In fact, the effects of ground water pumping propagate radially in all directions (assuming aquifer properties are uniform), regardless of the direction of ground water flow. This means that pumping effects are felt upstream, laterally across the gradient, and downstream, making conjunctive water rights allocation extremely difficult. In the case of the Snake River Plain aqufier, this means that even down-gradient pumpers have some impact on the upper river reaches. Snake River Reach maps, which demonstrate the effects of long-term pumping on various reaches of the Snake River, are presented for example.