Where is gauge (atmospheric) pressure zero in groundwater terms? Identify the location(s) at which the pore water pressure equals atmospheric pressure (i.e., zero gauge pressure) in hydrogeologic terminology.

Introduction / Context:
Understanding the reference surfaces for pore water pressure is essential in seepage, slope stability, and earth pressure problems. The terms “water table” and “phreatic surface” are often used to describe where pore water pressure equals atmospheric pressure.

Given Data / Assumptions:

• Static groundwater conditions for definition.
• Gauge pressure is measured relative to atmospheric pressure.
• Terminology from soil mechanics and hydrogeology is used.

Concept / Approach:

The water table is the top surface of the saturated zone where pore water pressure equals atmospheric pressure (u = 0 gauge). “Phreatic surface” is a synonymous term for this free surface in seepage flow fields. Below this surface, u > 0 (positive), and above it, pores may be partially saturated with negative pore pressures (suction).

Step-by-Step Solution:

Verification / Alternative check:

Seepage net construction marks the phreatic line as the zero-pressure line; piezometers terminating at this surface read atmospheric pressure.

Why Other Options Are Wrong:

Sea level/mean sea level are elevation datums, not inherently locations of u = 0 in soil pores unless they coincide with the groundwater surface in situ.

Common Pitfalls:

Confusing hydraulic head datum with pressure boundary; assuming zero gauge pressure at arbitrary elevations.

Final Answer:

Both the water table and the phreatic surface