Gasoline floating on groundwater, vinyl chloride in saturated sands, hexavalent chromium leaching into a creek, perfluorooctanesulfonic acid (PFOS) heading towards a drinking water well, trichlorethylene (TCE) dense non-aqueous phase liquid (DNAPL) in weathered bedrock.
If so, you probably have been around a site with soil and/or groundwater contamination or you’ve picked a really unusual hobby. If you are a responsible party, a stakeholder, or an environmental manager responsible for handling the regulatory aspects of a contaminated site, the goal is generally to reach the cleanup finish line (closure) with the best possible results for the least possible cost.
Consider this analogy. We know three things about fingerprints. 1) They are found on fingers, 2) they are generally arranged on the surface of the finger tips, and 3) each fingerprint is different from all others in the fine detail. Remediation can be viewed much the same. We know there is an impact to soil and/or groundwater, the limits of the contamination are generally defined, and the source is generally known. The fine details may have many commonalities between sites; however, any single variant can eliminate a viable technology.
A preliminary screening process should be conducted with the stakeholders fully engaged. Contamination type, soil type, permeability, depth to impacted soil and groundwater, hydraulic conductivity, geochemical and physical subsurface conditions, source zone characterization, dissolved phase contamination delineation, site surface cover and structures, subgrade utility conflicts, access to utilities, available capital, risk tolerance, regulatory clean up levels, and regulatory mandates are just some of the variables that we can use to filter through available technologies.
Remediation technologies range from barely emerging to very mature and each has their place. We view each of the site-specific considerations as a filter and each technology as a possible solution. During technology selection, the filters are layered together and the technologies are poured across the filters. Most will be caught somewhere in the filter stack and a few will make it through. Deeper evaluations are made prior to pilot testing. Sometimes, one technology might not be the best solution. Part of that evaluation is to look at the specific shortcomings of each technology and determine if they can be technically and cost-effectively overcome with combinations of technologies. Combined technologies can open new options for stagnant sites and may offer significant synergistic improvements.
Example: DOD Portfolio, TX
A well-funded and risk-tolerant site had screened out many technologies to address a residual but persistent low level TCE and vinyl chloride (VC) plume and initially selected an emulsified vegetable oil product. During pilot testing two things became clear; the soft, plastic clays would not accept injection of sufficient material, and the product was extremely effective in the limited locations where half of the design volume was successfully administered. Knowing this, a second evaluation was made and pneumatic fracturing was identified as a possible supplemental technology to overcome the injection volume and distribution challenges. A second pilot test was performed and found to increase the injection radius of influence from an unpredictable maximum of seven feet to a consistent thirty feet. Injection volumes increased from a maximum of 50 gallons of product injected at a time up to 6 times that volume with little to no daylighting. Additionally, Injection flowrates increased from approximately 1.5 gallons per minute up to greater than 150 gallons per minute. The disadvantages for injection combined with pneumatic fracturing are high daily contractor costs (three to five times that of the average injection crew) and a linear start/stop process where tasks cannot be performed concurrently. However, in this instance, with all the site-specific conditions these disadvantages were outweighed by the significantly reduced number of injection points due to the increase in pumping speed and reach. The project was completed for roughly 25% of the cost of the next best method. Post injection monitoring later indicated that site closure is imminent.
Similar combinations should be evaluated if a prior or current technology is close but not quite there for your needs.
To learn more about proper technology selection and evaluation for contaminated sites, join us on May 30th for our webinar on Soil and Groundwater Remediation Technology Selection in which W&M remediation expert David McAlister will further discuss how site-specific conditions can affect the technology selection process in soil and groundwater remediation projects as well as best methods to reach your site remediation goals.