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I recently measured groundwater levels using a tape in three different boreholes for where the holes are in same area. Each hole had a different level of water. Holes A and B are just 4 meters apart and hole C is about 70 m from holes A and B.

The water depth in A was 12 ft (3.7 m), in B it was 9 ft (3 m) and in C it was about 71 ft (22 m). How can the water depth be different for holes so close to one another in the same area?

I asked professor about this and he replied, it is due to different heights of each borehole and asked me to Google it. I assumed that since water in equilibrium has the same water level, so the water in each borehole will be at same depth. But the readings doesn't match with my assumption ? What could be the reason?

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  • $\begingroup$ I would naively think localized changes in elevation wouldn't be matched with large changes in the absolute level of the water table, and so would mean the groundwater is at different depths relative to the surface. That the forces that set groundwater height aren't just in equilibrium vertically, but also horizontally. But not a topic I know much about. $\endgroup$ Commented Oct 15, 2023 at 23:12
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    $\begingroup$ Is there another large well in the area that significantly lowers the water table at well C? Are wells A and B close to a river or stream? $\endgroup$ Commented Oct 15, 2023 at 23:50
  • $\begingroup$ @JeopardyTempest sorry this works drive.google.com/… $\endgroup$ Commented Oct 16, 2023 at 2:13
  • $\begingroup$ Presumably, you're asking about some specific data from the paper that you've linked to. Where exactly is this in the paper? $\endgroup$ Commented Oct 16, 2023 at 3:02
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    $\begingroup$ Things to consider: are the holes on flat ground or a slope? Is the geology of the area uniform? Porosity & permeability affect ground water movement. Is hole C near a depletion source? Is there more than one water table, is there a perched water table in the area? $\endgroup$
    – Fred
    Commented Oct 16, 2023 at 8:16

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All things being equal your assumption that boreholes in close proximity to one another would have the same water level is correct. However there are a number of factors that could alter this (assuming you have corrected for any difference in surface elevation and none of the 3 boreholes are pumped).

One possibility is that the holes have been drilled, or cased, to different depths, and are therefore connected to different aquifers. For instance A and B could be shallow and completed in a perched aquifer, and C drilled deeper into a regional aquifer. A similar picture would emerge if A and B are drilled deeper into confined semi-artesian aquifers, and C is the shallow borehole. The latter seems likely in the context of a paper that is discussing confined aquifers. Interpretation of measurements in confined aquifers can be complicated as the observed pressure at a particular depth depends not only on the aquifer water content, but also on the mechanical properties of different layers and the water content of overlying layers.

Even in a homogeneous aquifer you can expect to see variation in pressure (which will be seen as variation in level) in boreholes that are cased to different depths if there is some vertical flow - so in a recharge area heads may reduce slightly with depth , whereas in a discharge area they would increase with depth.

Disparate water levels in boreholes that are close to each other can also often be seen in aquifers where almost all the permeability is from fractures. If the fractures aren't interconnected the water level may vary depending which fracture happens to be intercepted. If there is another well pumping from one of the interconnected fractures, even at considerable distance, you can see wildly different water levels. An extreme version of this can occur when a borehole hasn't actually intercepted any fractures at all, so effectively operates as a cistern and is disconnected from the groundwater system.

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