Which statement correctly describes an unconfined aquifer?

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Question: Which statement correctly describes an unconfined aquifer?

Option a: It is isolated from recharge and cannot discharge to streams.
- This is incorrect for an unconfined aquifer. Unconfined aquifers are open to the surface and receive recharge directly from precipitation and surface water; the idea that they are isolated from recharge contradicts their defining characteristic. They also commonly discharge to streams, springs, and other outlets, not remain isolated.

Option b: It is overlain by an aquitard and water is under pressure, often artesian.
- This describes a confined (artesian) aquifer, not an unconfined one. An aquitard thickly overlain would create hydraulic head and pressure (water under pressure). Unconfined aquifers lack a confining layer above the water table, so this statement mischaracterizes their structure and pressure conditions.

Option c: It is always saline and located below sea level.
- This is incorrect. Unconfined aquifers are not inherently saline, nor are they necessarily below sea level. They can be fresh groundwater and occur at various depths and elevations depending on geology and hydrology. Salinity or submergence under sea level is not a defining feature of unconfined aquifers.

Option d: It has a water table at atmospheric pressure and is recharged from above.
- This is the correct description. An unconfined aquifer has its upper boundary as the water table, which is exposed to atmospheric pressure at the surface. It is recharged from above by precipitation and surface water infiltrating downward, making it directly open to the atmosphere. Groundwater in the unconfined zone responds to surface conditions more readily than in confined systems.

Option e: It is perched above the regional water table due to a clay layer.
- A perched aquifer is a local, smaller secondary aquifer sitting above the regional water table, usually separated by a relatively impermeable layer such as clay. While it is a real concept, a perched aquifer is not the general description of a standard unconfined aquifer. The statement would describe a specific local situation, not the general unconfined aquifer.

In summary, the defining traits of an unconfined aquifer are that its upper boundary is the water table at atmospheric pressure and that recharge occurs from above through infiltration. Other options describe characteristics of confined, saline, or perched systems, which do not apply to the standard unconfined aquifer.

Which statement correctly describes an unconfined aquifer?

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