In situations when light non-aqueous phase liquid (LNAPL) is discovered in groundwater, free product recovery is required. The NJDEP recommends several different methods for free product recovery including LNAPL skimming, bioslurping/enhanced fluid recovery, water/fluid injection and excavating for mass LNAPL recovery.
JM Sorge recently used excavating for mass LNAPL recovery as an effective method for containing the free product in groundwater at a former trucking repair facility. Since site conditions permitted large excavations for an extended period of time, JMS was able to delineate the LNAPL in groundwater by expanding the excavations based on observing the entry of free product on the groundwater surface.
Four total excavations were conducted at the site to recover as much free product as possible. Pumps and sorbents were placed in each of the excavations to recover the free product. A large 20,000-gallon “frac tank” was positioned on site with hosing to the pumps in each excavation to recover the LNAPL.
Typically, the collected LNAPL and contaminated groundwater in the frac tank would be removed by a vacuum truck and disposed of at an offsite facility, but JMS was able to install a water filtration system so that the contents of the frac tank could pass through to a sanitary sewer line on site, resulting in significant monetary savings. The disposal of the frac tank contents into the sanitary sewer line cost approximately 2 cents per gallon, whereas removal by a vacuum truck for offsite disposal would cost approximately 75 cents per gallon. Since approximately 500,000 gallons of contaminated water was disposed of on site, this disposal configuration was extremely cost-effective.
These excavations were left open for several weeks to allow as much LNAPL to be collected as possible. Precipitation events allowed for further dilution in the groundwater before mass recovery into the frac tank. Post-excavation groundwater sampling indicated a significant decrease in contaminant concentrations and no free product was further encountered.
The specific method of LNAPL recovery to utilize varies from site to site, but mass LNAPL recovery through excavations has proven to be cost-effective and successfully addresses the presence of LNAPL free product in shallow groundwater. To view the NJDEP’s LNAPL Guidance with a list of LNAPL recovery methods click here: http://www.nj.gov/dep/srp/guidance/srra/lnapl_guidance.pdf