2013 OIL & GAS AWARDS
Environmental Initiative Of The Year
Region of Nomination: Midcontinent
Contact Information:
Robert L. Sherrill, PG
Sage Environmental Consulting, LP
4611 Bee Caves Road, Suite 100
Austin, Texas 78746
Technology:
Use of Polyethylene Diffusion Bags for Sampling/Monitoring Groundwater Quality
Description:
A standard, off-the-shelf Polyethylene Diffusion Bag (PDB), designed for 2-inch wells, consists of
a 1.75-inch diameter by 24-inch long, sealed, 4-millimeter thick, low density
polyethylene (LDPE) bag filled with 350-milliliters of laboratory-grade deionized water.
In some cases, depending on the age and materials of the inplacement well, a protective
mesh can be used to cover and protect the bag. A 36-inch long by 0.75-inch diameter
version is also commercially available for use in 1-inch wells. The LDPE bag material acts
as a semi-permeable membrane allowing hydrocarbon compounds such as total
petroleum hydrocarbons (TPH) or volatile organic compounds (VOCs) to pass through.
The bag is suspended in the well at the desired sampling depth and left for a
deployment time of usually 10-days (minimum). The bag is retrieved from the well
following the appropriate deployment time, the corner of the bag is cut and the internal
water transferred to standard laboratory analytical containers for the required
analytical testing under existing methods. Figure 1 shows the deployment configuration
of the PDB with the deployment line, PDB, and stainless steel weight used to prevent
Figure 1
robert.sherrill@sageenvironmental.com
Office: 512-327-0288 ext. 1004
Cell:
512-470-8710
Fax:
512-687-1225
buoyancy.
How It Works: While the laboratory-grade deionized water filled PDB is suspended in the well, hydrocarbon analytes
will diffuse through the LDPE and be concentrated in the internal water. This occurs due to Fick’s Law of Diffusion (see
Figure 2) which states that “Compounds tend to migrate from an area of high concentration to an area of low
concentration until equilibrium is achieved.” The results of analyzing
Figure 2
the collected deionized water will be the analyte concentration in the
groundwater at the depth setting (along the PDB’s length) during the
duration of the deployment period. As such, both time-sensitive and
depth-sensitive results are achieved.
The minimum deployment time for the PDB is dependent on the
constituents to be analyzed and the equilibration dynamics of the
groundwater flow. As a general rule, hydrocarbon constituents
achieve equilibrium within a minimum of 180 hours; however, it is
recommended that the PDB be deployed for a minimum of 10-days in
order to allow for “worst-case” slow aquifer equilibration.
Innovative Use: Although this technology was first developed in the late 1950s for kidney dialysis machines, use in the
environmental field became apparent in the late 1990s following field trials at USEPA superfund sites. The use of PDBs
has recently received wide acceptance within the regulatory world especially by the USEPA and State Environmental and
Oil and Gas regulatory agencies. The innovative deployment of diffusion bags in groundwater monitoring wells has
proven itself to result in a more accurate picture of groundwater impact, vertical stratification of impact, reproducibility
of data, and significant cost savings. The use of PDBs not only gives an accurate definition to zones of impact but also
allows for the design of groundwater remediation systems that minimize volume production while removing a higher
volume of contaminant mass over a shorter period of time. Since the groundwater quality is specific to the 2-foot
interval of the PDB setting, in cases of groundwater remediation several PDBs can be deployed in a single well in order
to identify the specific interval of contaminant flow allowing for an intake design that maximizes contaminant mass
removal while minimizing fluid volume production.
Advantages Over Current Sampling Technologies: Passive sampling methodology has several advantages over the more
traditional purge and sample methodologies, such as bailing or low-flow purge and sample. Because it is a passive
methodology, it eliminates the need to generate, manage, and dispose of purge water. Water-quality sample
advantages over bailing or pumping water include eliminating flow-weighted averaging of the concentration results,
false positives due to induced active water transport, gas exchange and mixing, sediment mobilization, and phaseseparated hydrocarbon mobilization. Sampling implementation advantages include highly reproducible data, no crosscontamination from water column disturbance, no equipment decontamination time (all sampling equipment in contact
with the groundwater is either dedicated or disposable), decreased labor costs due to rapid deployment/retrieval time,
no purge time, ease of use with no moving parts requiring minimal training, and no depth limitations. In addition, since
no maximum time requirements are incurred, the next sampling events PDB can be set in place during each event thus
eliminating the deployment time wait.
Sustainable Cost Advantages: At fields, with ongoing traditional sampling programs meeting regulatory-mandated
groundwater monitoring requirements, where the PDB has been deployed, the cost of monitoring has been reduced by
as much as 70%. These cost savings are primarily due to minimal setup time, no purge time, no purge water
management, no decontamination time, the relative low cost of the PDB units (~$28 each), and minimal sampling time
(~15 minutes). At one field involving the sampling of 38 wells of depths ranging from 15-feet to 145-feet, labor time was
reduced from 41 man-hours to 12 man-hours.
Long-Term Benefits: The long-term benefits of using PDB technology involved in groundwater monitoring and
remediation, which in most cases involve years of implementation, are practical, strategic, and economic. The
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Significant reduction in field sampling labor costs and field equipment costs freeing up financial assets for actual
cleanup;
Eliminates the generation and disposal of contaminated purge water waste;
Public visibility is minimized due to the rapidity of implementation;
Eliminates turbidity from samples which cause false positives leading to unnecessary environmental efforts (no
filtering required, eliminates possible non-dissolved elevation of metals and total hydrocarbons);
Simple to use with minimal training and no specialized knowledge;
Excellent reproducibility and comparability of data through time allowing for accurate strategic decision making
and a timely determination of completion and closure;
PDBs are disposable, inexpensive, and require no decontamination;
Capable of collecting discrete sample depth intervals within a single well and well screen;
Capable of collecting very discrete sample depth intervals which reflect the concentration only in the length of
the PDB in the well screen allowing for effective and efficient design and implementation of a groundwater
remediation strategy or design;
No depth limitations (have been used at depths 10-feet to 860-feet); and
PDBs can be placed in wells during the previous sampling event and collected during current sampling event no
matter the time interval (quarterly, semiannually, annually, etc.) thus limiting setup or non-sampling time.
Reason for Submittal: Although used in the environmental field for only a short time, the use of PDB technology to
maximize determination, delineation, and remediation of impacted groundwater from Oil and Gas operations as a
strategic tool is rapidly growing from, primarily, the Western and Southwestern US to across the country. Papers and
presentations at professional organizations are informing the industry of this simple tool that allows for a more timely
and cost-effective way of improving and protecting the environment.