What defines a free aquifer?

A free aquifer, also known as an unconfined aquifer, is defined by the condition that the water surface is at atmospheric pressure. In this type of aquifer, the water table is exposed to the atmosphere, allowing it to fluctuate freely based on factors such as rainfall and evaporation. Since the water in a free aquifer is not confined by any impermeable layers above it, it is directly affected by atmospheric pressure. This characteristic is crucial as it differentiates free aquifers from confined aquifers, where water is trapped beneath layers of impermeable rock or sediments and, therefore, experiences pressure that is greater than atmospheric.

In the context of the other options, those elements do not accurately describe the characteristics of a free aquifer. The presence of saltwater is irrelevant, as free aquifers can either be fresh or saltwater, depending on their location. When discussing water under constant pressure, this condition relates more to confined aquifers which are under artesian pressure, rather than free aquifers. Additionally, while access to water in aquifers may involve pumps, the defining feature of a free aquifer is not that water is only accessible through pumping; rather, it can often be accessed easily through wells or surface water infiltration.