Title
Discussion of the influence of hydrogeology on measured changes in groundwater elevation. (Staff Presenting: Amy Woodrow)
Report
SUMMARY:
At the May 2025 meeting of the Basin Management Advisory Committee (BMAC), staff was asked to prepare an informational presentation on the differences between confined and unconfined aquifers, with a focus on how groundwater levels respond in each type of aquifer as a result of changing conditions in the water balance such as groundwater pumping or recharge. In response, Monterey County Water Resources Agency (Agency) staff is providing the following summary and an accompanying presentation that will be reviewed at the July 9, 2025 BMAC meeting.
DISCUSSION:
The potential of a geologic formation to store and yield water as a productive aquifer depends on the type of geologic material (lithology) and layering of the geologic material (stratigraphy) within the distinct geologic unit (formation). Properties of the sediment or rock constituting the aquifer, as well as the presence and properties of any confining layers, will change how the aquifer stores and transmits water, how it recharges, and how its water table or potentiometric surface responds to pumping.
A confined aquifer is one in which the aquifer below land surface is completely saturated with water. A confined aquifer has layers of impermeable material above and below it, causing it to be under pressure. When a confined aquifer is penetrated with a well, groundwater in the well will rise to a level that is above the top of the aquifer surface, though not necessarily above the land surface. The level to which water rises in a well that is screened (i.e., open to receiving groundwater) in a confined aquifer is referred to as the potentiometric surface.1,2
An unconfined aquifer, sometimes referred to as a “water table” aquifer” is one in which the upper water surface is at atmospheric pressure.1 The aquifer materials of an unconfined aquifer are only partially ...
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