Formatted Title

Barrier Reinjection of Emulsified Vegetable Oil within a Bedrock Aquifer and Innovative Monitoring Methods

Background/Objectives

The site is a 226-acre trichloroethene (TCE) and cis-1, 2-dichloroethene (DCE) plume that is approximately 0.5-miles wide by 1-mile long impacting an estimated 180 million gallons groundwater. The plume originated from a former unpermitted dump site where waste was deposited into ravines that were later backfilled. The former unpermitted dump has since been converted to residential property with significant open space. The plume underlies residential neighborhoods and has contaminated two bedrock aquifers.  Three public water supply wells and as many as 12 residential drinking water wells have been affected. The site remedy utilizes in situ bioremediation barriers to treat groundwater.

In 2016 an emulsified vegetable oil (EVO) barrier was installed within the bedrock aquifer via injection wells to address TCE contamination. A cost-effective performance assessment monitoring regimen was established using passive sampling devices and downhole meters. The performance assessment data obtained by the passive samplers were used to evaluate in situ geochemical conditions, contaminant destruction, and reactivity of the barrier.

Approach/Activities

EVO was injected in 15 wells installed in the right of way to form a 275-foot-long permeable reactive barrier in the TCE groundwater plume core downgradient of the source area.

Monitoring of this barrier was conducted using nontraditional methods.  Typically, low flow sampling is the preferred method, which can be time consuming (1 to 1.5 hours per well), collects a single groundwater sample from the middle of the screen, and generates investigation derived waste (IDW) in the purge water.  To minimize expense and waste generation, and yield vertical contaminant profiles, sampling was conducted using passive methods.  Geochemical parameters were collected in-well using a downhole meter, reducing sampling time and IDW.  Wells are vertically profiled at three to four intervals using passive diffusion bags for volatile organic compounds and at the middle interval using dual membrane samplers for metals, nitrate, sulfate, and total organic carbon (TOC). The wells are 4 inches in diameter, so at the middle sample interval multiple 1.75-inch diameter passive samplers are installed.  Following sample retrieval, a down hole meter is used at two depth intervals (top of the water table and bottom of the water table) to collect real-time field parameters (pH, dissolved oxygen, conductivity, and temperature).  Total sample time per well for multiple intervals and parameter measurements is approximately 45 minutes. The down hole meter was also lowered into injection wells to evaluate the health (e.g., reactivity) of the barrier.

Results/Lessons Learned

Results provided information regarding zones of high mass flux zones within the aquifer and assessment of barrier performance.  PDBs provided vertical profile which showed that the highest concentrations of TCE in groundwater migrated at the base of the aquifer at the interface between aquifer and basal confining layer. VOC data are managed in tables with graphs by micrograms per liter and micromoles per liter over time. Similarly, pH, ORP, arsenic, iron, manganese, nitrate, sulfate and TOC trends are graphed to assess the data. Sampling data over time indicated the barrier was very effective; however, over time additional injections of EVO were needed to maintain reactivity.  Measurements of pH decline triggered adjustment with sodium bicarbonate.

Based on the performance assessment results, two additional injections were completed - one in 2017 and another in 2020.  Each subsequent injection required more reconditioning of the wells.  The initial injection was successful with EVO generally gravity draining into the wells with little to no pressure required.  The second injection was more difficult, with the wells needing reconditioning with hot water and surfactant followed by bailing and surging.  This removed the residual byproducts (ferrous sulfide, biomass and organics) from the well and well screen. The second injection was forced under pressure and with the use of well packers. After the second injection, several wells became clogged with EVO.  This made it difficult to obtain downhole measurements at each injection point.  Additionally, when a third injection was completed additional reconditioning efforts were required to unclog two wells.  This third injection required hydro-jetting with steam and surfactant followed by injection under pressure and left many injection wells unusable.