Groundwater contouring terminology: isopiestic (isopiastic) lines In groundwater mapping, “isopiestic lines” on a plan refer to contours drawn to represent which hydraulic quantity?

Introduction / Context:
Groundwater maps visualize spatial variations of hydraulic head. The term “isopiestic” (sometimes written “isopiastic”) refers to equal-pressure or equal-head lines. Correct interpretation is essential for inferring flow directions and gradients for well siting, dewatering, and contaminant control.

Given Data / Assumptions:

• Confined conditions are common when using piezometric heads; unconfined cases use water-table contours.
• Heads are referenced to a common datum (e.g., mean sea level).

Concept / Approach:

On a plan map, a contour labeled with head values connects points of equal hydraulic head measured in observation wells. These contours are isopiestic lines and represent the projection of the piezometric surface onto the horizontal plane.

Step-by-Step Solution:

Verification / Alternative check:

Cross-sections reconstruct the 3D piezometric surface, while plan-view isopiestic lines provide 2D visualization consistent with measured heads.

Why Other Options Are Wrong:

• Water-table elevation only: Correct for unconfined aquifers, but the term isopiestic typically emphasizes piezometric head (confined).
• Piezometric surface itself: That is a 3D surface; the map shows contours of that surface (isopiestic lines).
• Stream stage contours / None: Not the definition.

Common Pitfalls:

• Mixing water-table (unconfined) and piezometric contours (confined).
• Using inconsistent datums across wells.

Final Answer:

Piezometric head (hydraulic head) values.