An Assessment and Treatment Tool
SITE
Former Retail Petroleum Site in Milton, FL
CONTAMINATION
Dissolved gasoline range organics migrating offsite to the east
OBJECTIVE
One age old issue for proper site characterization is inaccessibility. Most sites have some type of building or structure in the way of completing an easy assessment. Some buildings lend themselves to gathering limited data where others make it very difficult to get any data whatsoever. Data gaps can end up causing significant problems leading to prolonged remediation and higher costs. Even though assessment is a hot topic and High Resolution Site Characterization (HRSC) is making great advances in more complete more
accurate assessments, this problem is not addressed.
BACKGROUND
Elevated hydrocarbons were identified in the onsite compliance wells in 1991. The six (6) underground storage tanks (USTs) were removed and the tank pit was over excavated. Assessment activities were completed in 1993 and a remedial action plan (RAP) was developed and implemented in 1994. Several different remedial strategies were implemented from 1994 to 2017 with limited success. The data gaps associated with a city document storage warehouse, measuring 50 ft wide by 100 ft long, located directly downgradient from the former retail petroleum site, appeared to be the limiting factor to the success of the remedial strategies. The dissolved hydrocarbon plume from the former gas station had migrated beneath the adjacent warehouse. Two vertical monitoring wells had been installed in the warehouse and identified a hotspot under the building. Vertebraetm well systems were utilized to aid in filling the data gaps under the building.
Vertebrae is a multipurpose nested well installed by Horizontal Directional Drilling (HDD). Vertebraetm wells were installed and sampled providing a more accurate representation of the plume, thus allowing a surgical treatment plan to move forward. The provided data has the clarity and spacing like HRSC tools.
PRE-INSTALLATION DATA
- Two monitoring wells (MW-17B and 18B), screened from 3’-8’ and 4’-9’, respectively, characterize the plume under the building.
- The most impacted well reports a concentration of total xylenes at 10,000 ppb at MW-18B.
LITHOLOGY
- Fine to medium grained organic sand underlain by medium to coarse grained sand reside in the top 15 ft.
- The lithology transitions to a sandy clay layer below 15 ft bls.
VERTEBRAETM ACTIVITIES
- Three HDD bores were advanced to install a total of 15 well segments under the building.
- The northern and the southern wells were installed at 11 ft bls and the center well was installed at 7 ft bls.
- These wells were developed and sampled after the development water cleared and insitu parameters stabilized.
The data collected is invaluable. Most of all, from the collected data, the true ‘hot’ zone under the building has been identified, as well as a better estimate of mass present. This data also provides clear iso-contours of the plume.
VERTEBRAETM BENEFITS
- The wells are installed from outside the building, but precisely where they are needed under the building.
- The horizontal spacing can be engineered. Wells could be installed 10 ft on center to allow increased precision similar to HRSC data collection. In this case study, the wells were approximately 25 ft on center.
- The wells are multi-purpose. The original intent of the installation at this site was for treatment. The sampling performed after installation confirmed screen placement for future treatment. The screens will allow excellent ChemOx treatment at the precise depth it is needed with ample control to target and remedy the site quickly. However, if speed is not an issue, treatment can be bio or extraction of the groundwater. The wells are truly multipurpose.
- Unlike most HRSC tools, these wells can be resampled again and again. This allows monitoring of the site conditions instead of having to recomplete a HRSC event. In instances such as this site, not all the wells have to be used for treatment; some can be reserved for sampling only.
- The installation occurs congruent to the lithology and plume shape. It allows better treatment and less overall drilling. This is valid even when a building is not the issue.
- Vertebrae simply provide a better overall solution over vertical wells.
- Only one vault is needed per well group minimizing unsightly well pads.
- The cost was reduced using these wells, not increased.
The benefits are extraordinary for Vertebrae. This is changing the way the industry thinks
about data gaps.
