Sustainable Solution for Arsenic Remediation
Red Barn Cattle Dip Vat Site • Brighton, FL
Solution Overview
Client:
The Seminole Tribe of Florida, a federally recognized sovereign nation which employs 2,000+ personnel with $24M in purchasing power.
Business Challenge:
Remediate groundwater contaminated with arsenic from historic cattle dip vat operations.
Solution:
Apex Companies professionals with comprehensive understanding of aquifer biogeochemistry used in‑situ chemical oxidation (ISCO) to stimulate naturally occurring bacteria as part of the remedial process. We used non‑toxic, sustainable materials to accelerate in-situ remediation.
Results:
- The 5-month pilot test resulted in a 95 percent reduction of arsenic in groundwater.
- Savings up to $1M over expensive pump and treat (P&T) system.
- Green, sustainable technology.
- Use of common, local materials.
Challenge
The Seminole Tribe of Florida (STOF) discovered remnants of a historic cattle dip vat operations on its Brighton, Florida Reservation. The initial site assessment by other consultants revealed the presence of elevated arsenic concentrations in soil and groundwater associated with the former cattle dip vat operation. The STOF contracted other consultants to remove impacted soil and to conduct groundwater arsenic remediation using P&T methods combined with surface flocculation of arsenic using hazardous chemicals. Years of P&T operation failed to measurably reduce the groundwater arsenic concentration. Apex was retained to conduct a pilot study for an in‑situ chemical oxidation (ISCO) of arsenic in groundwater associated with a former cattle dip vat in Brighton, Florida.
Solution
Apex designed and implemented the scope of work for full-scale remediation using the previously excavated source area backfilled with lime rock and gravel as an infiltration gallery. Outside of the source area, Apex installed 630 linear feet of trenches, excavated to six feet deep, installed on 15 feet centers and equipped with 4-inch diameter septic field drainpipe. The remedial design stimulated naturally occurring and ubiquitous sulfate reducing bacteria (SRB) to generate hydrogen sulfide through natural respiration processes. This technique reduces iron and forms a stable precipitate with dissolved arsenic. The primary mechanism for arsenic removal from groundwater entailed physical precipitation of the arsenic with iron and other inorganic compounds, especially those associated with the reduction of sulfate to form arsenopyrite. Sodium lactate was injected into four injection wells followed by 1,000 gallons (per well) of magnesium sulfate solution. We conducted post‑remediation groundwater monitoring using select pilot test area wells.
This process used non-hazardous amendments such as locally grown sugar cane and/or orange processing byproducts.
Results
SRB testing and DNA analysis reported sulfate reduction was estimated to be approximately 20-fold higher in one of the injection wells. This evidence confirmed that the introduction of sulfate and lactate to the aquifer successfully stimulated the SRB population to grow and thrive. Further post injection evidence of SRB activity was observed in black staining in soil cores near injection wells and the black discoloration of sampled groundwater from the injection wells, indicated the generation of iron sulfide from SRB respiration processes and binding to dissolve elemental iron.
Apex conducted one year of quarterly groundwater monitoring to track the progress of remediation. After one year of full-scale remediation, arsenic concentrations were reduced by more than 50% throughout the full‑scale treatment area.
The Red Barn is a historic landmark and consequently has cultural significance to the STOF. Apex coordinates field activities with the STOF Historic Preservation Office and has given presentations to Tribal committees to inform them of Apex’s work at the site and positive impact to the land.