Purging and other sampling variables affecting dissolved methane concentration in water supply wells

Area of Science:

• Environmental Science
• Hydrogeology
• Water Quality Analysis

Background:

• Distinguishing natural groundwater methane from anthropogenic sources (oil and gas) is challenging due to variability.
• Understanding factors influencing methane concentration in residential wells is crucial for accurate assessment.

Purpose of the Study:

• To evaluate the impact of sampling points, purged volumes, and temporal variations on dissolved methane concentrations in groundwater wells.
• To identify correlations between methane variability and other water quality parameters.

Main Methods:

• Analyzed dissolved methane and water quality parameters from 10 residential wells in Northeastern Pennsylvania.
• Varied sampling points within the well system and water volumes purged before collection.
• Monitored natural variations in methane concentrations over time.

Main Results:

• All wells exhibited dissolved methane concentrations above 1 mg/L.
• Maximum methane concentration change within a single well was 3.2-fold; eight wells showed <100% change.
• Methane variability correlated strongly with salinity indicators (sodium, chloride, conductivity, TDS), suggesting fluid source changes.

Conclusions:

• Significant groundwater methane variability is primarily driven by shifts in the influx of sodium-rich fluids, not sampling artifacts.
• Minimal benefit in purging large water volumes; recommend purging only standing water in pressure tanks and lines.
• Refined sampling and data interpretation methods are needed for accurate methane source determination.