Perched aquifers in groundwater hydrology are typically located:

Introduction / Context:
A perched aquifer is a localized saturated zone separated from the regional (main) water table by an unsaturated interval. It forms when infiltrating water accumulates atop a relatively impermeable lens within the vadose zone.

Given Data / Assumptions:

• Heterogeneous stratigraphy including discontinuous clay or silt lenses.
• Infiltration from precipitation or irrigation.
• Regional water table exists at greater depth.

Concept / Approach:
The defining characteristic is a perched saturated zone above the main phreatic surface due to a low-permeability layer that impedes vertical percolation locally. Thus, perched aquifers are “above” the main water table, not coincident with it.

Step-by-Step Solution:

Verification / Alternative check:
Observation wells may show shallow saturation in local zones while nearby deeper wells reflect a lower regional water table; perched water disappears laterally where the lens pinches out.

Why Other Options Are Wrong:
At the surface implies surface water; below the regional water table describes the phreatic zone; “everywhere” disregards the local nature; “none” is invalid given the correct description.

Common Pitfalls:
Confusing perched aquifers with seasonal water tables; assuming perched conditions extend basin-wide.

Final Answer:
Below the ground surface but above the regional water table, resting on a local impervious lens.