Understanding Passive Diffusion Bags (PDBs)
More than 20 years ago Passive Diffusion Bags (PDBs) were introduced as a more streamlined, cost-effective solution for sampling for Volatile Organic Compound (VOC) concentrations in groundwater monitoring wells. Passive diffusion samplers have become widely used in the years since their introduction, leading to the development and use of samplers using membranes with different pore sizes that can accurately sample for all contaminants, including, metals, inorganics, ionic compounds, SVOCs and emerging contaminants such as 1,4 Dioxane and PFAS.
PDBs are typically 18 to 28-inch-long flexible, tubular “bags”, made from, semi-permeable, polyethylene membranes. They are filled with deionized water, sealed, and deployed on a weighted tether into the saturated screen interval of 1-inch and larger monitoring wells.
The key principle behind PDBs is that contaminant molecules dissolved in water will naturally diffuse or “flow” from areas of higher concentration to lower concentration if there is a pathway through which the molecules can move. When PDBs are deployed, contaminants in the groundwater diffuse through the microscopic pores in the membrane and equilibrate with the deionized water inside the bag over about 2-3 weeks’ time.
Once equilibrium is reached, the concentrations in the PDB continually adjusts to the surrounding groundwater concentrations so that the sampler always represents the constituents of the last several days of residence time.
That means PDBs can be installed at one sampling event and left in-place, indefinitely, until the next sampling event and then recovered with a representative sample. No bailing, no pumping, no waiting for parameters to stabilize. Simply pull out the sampler and discharge into a sample container.
Ideal Applications of PDBs
PDBs are particularly well-suited for specific types of environmental sites:
Contaminated Sites: Ideal for monitoring pollutants, PDBs can provide accurate readings of contaminant levels over time, essential to assist in tracking contaminant migration and remediation efforts.
Risk Assessment Sites: PDBs can be deployed to evaluate whether there is groundwater contamination flowing through a site’s underground aquifer. PDBs are with wide-ranging contaminant capability are selected for application at solid waste locations, UST sites and monitoring networks across all industries.
Large Sites with Many Wells: Substantial cost-savings roll-up as more wells convert to passive sampling and reduce the time/labor demand.
Remote and High-Traffic Sites: Because Passive Diffusion Samplers are small, lightweight, and don’t need cumbersome support equipment, it’s easier to get to remote sites and in-and-out of high traffic areas in minutes, adding a safety component to passive sampling.
Slow-Recharge Wells: No waiting for wells to recover from being pumped down and no concern about aerated samples in re-charging wells that have been pumped. Simply install at one event and recover at the next.
Benefits of Using PDBs
Time Efficiency: PDBs significantly reduce the time required for sample collection and processing. They can be left in situ for extended periods and maintain a dynamic equilibrium with the surrounding aquifer, allowing for accurate, representative data from the period when sample collection occurs.
Cost-Effectiveness: With fewer resources needed for sample collection and processing, PDBs offer a more economical solution compared to traditional groundwater sampling methods.
Ease of Use: PDBs are straightforward and easy to deploy and retrieve, making them accessible even for teams with limited experience in passive sampling methods.
Reduced Risk of Contamination: The closed system of PDBs minimizes the risk of sample contamination, ensuring reliable results.
Environmental Friendliness: PDBs are a more sustainable option, generating less waste and requiring fewer consumables than traditional methods.
Best Practices for Using PDBs
To maximize the effectiveness of PDBs in your groundwater sampling projects, consider the following best practices:
Sampler Selection: PDBs can be made with membranes of different porosity, allowing for accurate sampling of different types of compounds. Ensure the sampler you choose for your project aligns with your site’s contaminants of concern.
Proper Deployment: Ensure that PDBs are correctly placed in the monitoring wells and at the right depths to sample the targeted contaminants.
Deploy After Retrieval: Eliminate duplicate mobilizations at long term monitoring sites with multiple sampling events, by deploying your next event’s samplers into your wells after you’ve retrieved your prior event’s PDBs. The PDBs can stay in the wells uncompromised and will be ready for quick retrieval at your next event.
Conclusion
Passive Diffusion Bags are a proven technology in groundwater sampling. Their ability to provide accurate, cost-effective, and efficient sampling makes them an invaluable tool for environmental consultants.
As the industry continues to evolve, embracing innovative technologies like PDBs will be crucial in staying ahead and delivering superior environmental monitoring services.
For more information read The Ultimate Guide to Passive Groundwater Sampling.
Frequently Asked Questions
The deployment of Passive Diffusion Bags (PDBs) can be optimized for various environmental sites by considering specific site characteristics. In urban areas, where space and access might be limited, PDBs are advantageous due to their minimal setup requirements. They can be easily deployed in monitoring wells situated in confined spaces, which is often a challenge in densely built-up areas. For rural sites, PDBs are beneficial in reducing the frequency of site visits, which can be logistically challenging and costly due to remote locations.
When considering groundwater depth, PDBs are versatile. For shallow groundwater monitoring, they provide an efficient way to sample without extensive pumping, which can disturb the water table and produce large volumes of contaminated wastewater. In deeper wells, PDBs should be accurately placed at specific depths to target the contaminants of interest, considering factors like groundwater flow and contaminant concentration gradients. This targeted deployment is crucial in mining sites or areas with stratified contaminant layers.
In both urban and rural settings, it’s essential to understand the hydrogeology of the site. Knowledge of the groundwater flow, contaminant types, concentration variations, and sample volume requirements, can guide the optimal type and placement of PDBs in the wells.
While PDBs offer several advantages for groundwater sampling, there are limitations and challenges to consider. One of the main limitations is the amount of volume that can be sampled at one sampling event. With any passive sampler, including PDBs, you are limited to sample the water that is within the fully saturated length of the well screen. PDBs can be built to sample up to 1-L per sampler, and multiple PDBs can be deployed on a single tether to obtain more sample volume. Users should understand that there are cases where saturated screens are too short to provide adequate sample volume for some laboratory methods.
Another challenge is the potential for biofouling or sediment buildup on the bags, especially in wells with high microbial activity or sedimentation. This can affect the accuracy of the samples. Regular maintenance and monitoring of the PDBs can help identify and mitigate these issues.
Integrating data collected from PDBs with other environmental monitoring technologies can provide a more comprehensive understanding of a site’s condition.
Geospatial technologies, such as Geographic Information Systems (GIS), can be used to map and analyze the spatial distribution of contaminants. Combining PDB data with GIS allows for the visualization of contaminant plumes and identification of trends over time.
Additionally, integrating PDB data with real-time monitoring systems, like automated water quality sensors, can provide a more dynamic picture of groundwater conditions. These sensors can continuously monitor parameters like pH, temperature, and conductivity, offering immediate insights into changes in the groundwater environment. By combining PDB data with other monitoring tools, environmental consultants can gain a more holistic view of the site’s condition, enabling more informed decision-making for remediation and management strategies. This integrated approach can be key to effective environmental monitoring and management.