On-Site Vapor-Phase Analysis as a Novel Approach for Monitoring Groundwater Wells

he results of comprehensive field testing of on-site vapor-phase-based groundwater monitoring methods are presented to demonstrate their utility as a robust and cost-effective approach for rapidly obtaining volatile organic compounds (VOCs) concentration data from a monitoring well. These methods—which rely on sensitive, commercially available field equipment to analyze vapor in equilibrium with groundwater—proved easy to implement and can be tailored to site-specific needs, including multilevel sampling. During field testing, low-flow groundwater concentrations could be reasonably estimated using submerged passive vapor diffusion samplers or field equilibration of collected groundwater (R2 = 0.85 to 0.96). These two methods are not as reliant on in-well mixing to overcome vertical stratification within wells as simpler headspace methods. The importance of well and aquifer-specific factors on concentration data (and therefore method selection) is highlighted, including the effect of changing in-well patterns due to seasonal temperature gradients. Results indicated that vertical stratification was relatively limited within the set of wells included in these studies, resulting in similar performance for short depth-discrete passive vapor diffusion samplers (constructed from 40-mL vials) and longer samplers (2.5 to 5 feet in length) designed to cover a larger portion of the screened interval. A year-long, multi-event evaluation demonstrated that vapor-phase-based monitoring methods are no more variable than conventional groundwater monitoring methods, with both types subject to similar spatial and temporal variability that can be difficult to reduce. Vapor sampling methods represent a promising approach for estimation of groundwater concentrations by reducing the cost liabilities associated with monitoring while providing a more sustainable approach.