What characterizes an unconfined aquifer?

An unconfined aquifer is characterized by the absence of an overlying impermeable layer, which allows water to flow freely from the surface into the aquifer. This type of aquifer is typically recharged by precipitation and surface water infiltration, making it more susceptible to contamination from surface sources. The lack of an impermeable layer permits groundwater to fluctuate significantly in response to changes in precipitation and extraction, as it allows for a direct connection with the surface environment.

In contrast, an overlying impermeable layer, as mentioned in the first choice, would define a confined aquifer rather than an unconfined one. Definitions regarding partial filling and connections to surface water also pertain more specifically to aquifer types. An unconfined aquifer can vary in its saturation levels, but it does not have the same implications as being only partially filled, which could occur in different contexts. Additionally, unconfined aquifers may indeed be connected to surface water sources, unlike what is suggested in the last choice, which generally applies to confined aquifers. Therefore, the defining trait of an unconfined aquifer remains the lack of an overlying impermeable layer, which facilitates a dynamic hydrological interaction with the surface.