In the past few years, especially since risk-based corrective action programs were implemented, regulators have been more focused on VAP to assure that site characterization is more accurate due to its critical closure.
Benefits of the New Technology
In dealing with groundwater contamination, migration of the chemicals through varying types of sediments can cause extensive vertical migration of the contaminant plume. This is especially important when dealing with chlorinated solvent plumes that may migrate vertically, sometimes against the groundwater flow direction, based on relatively small changes in hydraulic conductivity of the geologic unit. Good practice and regulations require VAP to extend to a depth where an aquitard sufficient to limit any further migration is encountered, or analytical data indicates that the maximum depth and extent of impact has been defined.
What Does VAP Accomplish?
Vertical aquifer profiling is an effective and efficient tool for groundwater investigation. Utilization of Simulprobe®:
In dealing with groundwater contamination, migration of the chemicals through varying types of sediments can cause extensive vertical migration of the contaminant plume.
Reduces the number of permanent monitoring wells and the long-term cost by better defining the area/level of impact;
In many cases, eliminates the need for nested monitoring wells to determine initial plume characteristics (eliminating nested monitoring wells reduces well installation, sampling and analytical costs, as well as long-term monitoring);
Allows complete definition of vertical distribution of contaminants through the aquifer;
Allows for optimal placement of the monitoring well screen in cases where there is a vertical component to the impact;
Defines the stratigraphy and homogeneity of the aquifer; and
Develops the data necessary for modeling required relevant regulations.
Traditionally, VAP has been conducted by using either the top-down or the bottom-up profiling method. Common agency guidance requires sampling every 10 feet vertically. In the top-down method, groundwater samples are collected as the augers are advanced down through the aquifer. Typically the groundwater samples are collected from temporary wells installed ahead of, or inside of, the augers. The well is then developed (i.e., pumped to remove fine sediment) and the sample is collected. The well materials are then removed from the auger, and the augers are advanced to the next predetermined interval. The procedure is repeated until a basal confining unit is encountered. In many cases, the associated soil material can not be collected as part of the VAP due to drilling equipment limitations.
With the bottom-up method, the augers are advanced to the bottom of the aquifer, a well is installed, and the augers are pulled back allowing the formation to collapse around the well screen. After the well is developed, the groundwater sample is collected. The well is then pulled back toward the surface to the predetermined interval and the procedure is repeated until the top of the aquifer is reached.
Both of these standard procedures produce large volumes of potentially contaminated groundwater that require handling, characterization and proper disposal. In addition, a lot of time is spent installing, developing, removing and decontaminating well materials.
In the traditional methods of VAP, several rounds of installing well screens and casing up and down the well are required, along with decontamination. The Simulprobe® system provides advantages over this method.
How Does Simulprobe Work?
The Simulprobe® is a patented system for simultaneously sampling soil, soil gas and groundwater. During the drilling, the borehole is advanced to the desired depth using a standard hollow-stem auger. The innovative probe is pressurized with nitrogen to prevent groundwater flow from entering the probe. Covered in a large rubber sheath, the probe is then lowered down through the augers and advanced beyond the augers using a standard drive hammer. Once the probe has reached the desired sample interval, it is pulled back two to three inches to expose a screen, and the nitrogen is bled off. Groundwater then enters the probe under ambient hydrostatic pressure. After the sample has been collected, the probe is re-pressurized with nitrogen to seal the water container and prevent cross contamination. The probe is then retrieved to the surface. Once at the surface, the groundwater sample is transferred from the Simulprobe to the sample containers via a single-use Teflon™ straw.
Advantages of Using the Simulprobe®
According to Virgil Stearns of Stearns Drilling, Dutton, Mich., 'We have found that we can save 33 percent of time using the Simulprobe over traditional methods and collect both soil and water samples from the same interval with no additional work.'
Reduces the number of permanent monitoring wells and the long-term cost by better defining the area/level of impact.
Mr. Noah Heller, the inventor of the Simulprobe®, confirms that almost 175 probes are in use in California and Arizona, and another 25 are in use in Michigan, New York, Japan and Germany. Mr. Heller indicated that the greatest advantage of the system is the ability to collect a single, discrete sample that cannot be cross-contaminated like some other similar systems. An economic analysis of the new probe indicates that on projects requiring VAP, the savings in time, materials and purge water can reduce the project cost by 10 to 40 percent. The more sample locations, the greater the savings.
Perchloroethylene (PCE), also know as tetrachloroethylene, was released during former plant operations in an urban brownfield area. During investigations in the late 1980s and early 1990s, investigators documented groundwater impact in the area. Limited VAP conducted at the site documented an area of approximately three acres of PCE-impacted groundwater underlying the site. Based on the results from the initial VAP, an interim remedy groundwater purge system was installed and operated from 1991 through June 2000. In 1999, site conditions were assessed relative to new site characterization methods and standards. The environmental consultants determined that due to the relatively low levels of PCE (all 20 wells below 50 parts per billion (ppb)), closure could be achieved to meet regulatory agency criteria by restricting the use of groundwater for drinking. All other cleanup criteria (e.g. volatilization to indoor air) had long been met. The first hurdle was to document to the agency that the plume had be en adequately defined. A total of four VAP locations with groundwater samples to be collected from 10 to 57 feet below ground surface (bgs) were proposed. An economic analysis indicated that use of Simulprobe reduced the VAP cost by 30 percent over the traditional casing pull back method.
The VAP work documented that the client's plume was very small, dilute and probably already meeting regulatory limits. However, at least two additional VOC plumes from off-site sources were documented as being at least partially captured by the operating purge well.
Based on the results of the VAP and progress toward an area-wide restriction on drinking the groundwater (the city already had such a restriction in the building code to meet regulatory agency requirements), the regulatory agency allowed the purge well to be shut down. Shutdown of the purge well over a 12-month period has saved this company over $60,000 in system operations costs. Currently, additional VAP is being done to fine tune the groundwater characterization for closing out the site.
American Society for Testing and Materials (ASTM) D 5784-95 - Guide for use of Hollow Stem Augers for Geoenvironmental Exploration and the Installation of Subsurface Water-Quality Monitoring Devices.
ASTM D 6001-96 - Guide for Direct-Push Water Sampling for Geoenvironmental Investigations.
Michigan Act 451 of 1994, Part 201 (as amended), Section 14
Michigan Department of Environmental Quality (MDEQ) Hydrogeologic Study Guidance Document
Benefits of the New Technology
- Collects true, discrete groundwater samples that are representative of aquifer conditions;
- Minimizes cross contamination;
- Allows the geologist to better characterize the aquifer by actually viewing the sediments that produced the groundwater sample, eliminating 'blind drilling' or logging cuttings;
- Eliminates production of large quantities of development and purge water associated with the temporary well and screened lead auger method, which eliminates handling and disposal costs and concerns for the development and purge water;
- Increases productivity by collecting soil and groundwater samples from the same depth interval at the same time; and
- Increases productivity by eliminating the time and costs associated with setting and decontaminating materials used for the temporary monitoring wells.
Although analytical results from saturated soil samples are not officially accepted by regulatory agencies, obtaining soil sample from an aquifer is essential in characterizing the stratigraphy of a unit.
Limitations of the New Technology
- The instrument can't be used in bedrock;
- Clayey sediments may take a long time to produce water, which is true for the traditional methods as well; and
- Driving the Simulprobe may take longer than a regular split spoon.