The use of multilevel sampling techniques for determining shallow aquifer nitrate profiles

Area of Science:

• Environmental Science
• Hydrogeology
• Analytical Chemistry

Background:

• Nitrate is a pervasive global pollutant in aquifers.
• Shallow aquifers often exhibit vertical nitrate stratification, peaking near the water table and decreasing with depth.
• Effective monitoring techniques are essential for understanding this stratification.

Purpose of the Study:

• To compare the efficacy of three open-hole multilevel sampling techniques (packer system, dialysis membrane samplers, bailer) for detecting nitrate vertical stratification.
• To evaluate the advantages and limitations of each method in shallow aquifers with (sub)horizontal flow.
• To assess the impact of sampling time and technique on hydrochemical profiles.

Main Methods:

• Literary review to select three multilevel sampling techniques.
• Field deployment of packer systems, dialysis membrane samplers, and bailers in a shallow aquifer piezometer in Northern Italy over three seasons.
• Evaluation of optimal purge and equilibration times, and water volume loss for each method.
• Analysis of nitrate concentration profiles.

Main Results:

• All multilevel techniques successfully highlighted vertical nitrate stratification, consistent throughout the year.
• Nitrate concentrations generally decreased with depth, but varied significantly between sampling campaigns.
• Dialysis membrane samplers provided the most accurate hydrochemical profiles, essential for detecting profile discontinuities.
• Bailer and packer systems yielded similar nitrate profiles, with the bailer being easier to use.

Conclusions:

• Multilevel sampling is effective for characterizing nitrate stratification in shallow aquifers.
• Dialysis membrane samplers offer superior accuracy for detailed hydrochemical profiling.
• The bailer presents a practical, user-friendly option for routine monitoring despite slightly lower accuracy compared to dialysis samplers.