Influence of vertical flows in wells on groundwater sampling

Area of Science:

• Hydrogeology
• Environmental Science
• Water Resource Management

Background:

• Traditional groundwater sampling assumes horizontal flow, yielding permeability-weighted samples.
• Vertical flows in wells, driven by ambient vertical head gradients, can disrupt this assumption.
• These vertical flows are significant in wells with longer screen intervals (3-10m) and lower pumping rates.

Purpose of the Study:

• To investigate the origin of groundwater samples under ambient vertical head gradients.
• To analyze the impact of pumping rates and wellbore flow on sample representativeness.
• To determine conditions under which a permeability-weighted sample can be obtained.

Main Methods:

• Numerical flow and particle transport modeling were employed.
• Simulations explored various pumping rates, durations, and pump intake positions.
• The study analyzed sample provenance under differing vertical gradient conditions.

Main Results:

• Sample origin is highly sensitive to pump intake position, pumping rate, and duration when vertical gradients exist.
• Even extended pumping may not ensure the sample represents the entire screened interval.
• Sample bias can occur even when ambient vertical flow is less than the pumping rate.

Conclusions:

• Ambient vertical flows in wells can introduce unrecognized or underestimated sampling bias.
• Accurate interpretation of groundwater sampling data requires consideration of vertical flow dynamics.
• Higher pumping rates, potentially exceeding low-flow recommendations, may be necessary for permeability-weighted samples.