Wellbore-wall compression effects on monitored groundwater levels and qualities

Area of Science:

• Geotechnical Engineering
• Hydrogeology
• Environmental Science

Background:

• Wellbore construction methods can impact subsurface hydrological monitoring.
• Clayey Andisol aquifers present unique challenges for accurate groundwater assessment.

Purpose of the Study:

• To investigate the effects of wellbore-wall compression from rough excavation on groundwater levels and quality.
• To compare monitoring results from wellbores constructed under wet versus dry aquifer soil conditions.

Main Methods:

• Nine wellbores were constructed using dynamic cone penetrometry in a shallow clayey Andisol aquifer.
• Five wellbores were created under wet soil conditions, and four under dry conditions.
• Groundwater levels and quality were monitored for 15 months.

Main Results:

• Wellbores in wet conditions showed delayed and narrower groundwater level fluctuations compared to dry conditions.
• Hydraulic conductivity was 1-2 orders of magnitude lower in wellbores constructed under wet conditions.
• Significant nitrate removal was observed in one wellbore constructed under wet conditions, suggesting heterotrophic denitrification.

Conclusions:

• Wellbore-wall compression from rough excavation in wet clayey soils substantially reduces hydraulic conductivity, leading to unreliable groundwater monitoring.
• Accurate groundwater level and quality data can be obtained by constructing wellbores under dry, hard aquifer soil conditions.
• The findings are particularly relevant for clayey Andisols and highlight the importance of construction conditions.