Graph comparing PHREEQC model output and soil column flushing results.
Orion teamed with a technical expert to conduct modeling and perform a SOIL COLUMN STUDY to evaluate in situ aquifer flushing for remediation of cadmium-impacted soil and groundwater.
Orion was retained to study the feasibility of in situ flushing for removing the heavy metal cadmium from a contaminated aquifer. Orion’s goals were to evaluate the efficacy of aquifer flushing for achieving remedial objectives and estimate critical design parameters for potential full-scale remediation. Orion partnered with Accord Environmental (Accord) of Long Beach, California, to design, construct, and perform the soil column study.
Accord performed PHREEQC geochemical modeling to simulate chemical reactions and transport processes for various potential flushing reagents. In addition, the model was used to predict the potential effect of geochemistry on reagent effectiveness and the number of pore volume flushes required to meet remedial objectives for cadmium. Accord modeled seven flushing solutions, four cation exchange agents (CIX), and two chelating agents. Modeling results indicated that the CIX agent calcium chloride and the chelating agent ethylenediaminetetraacetic acid (EDTA) would reduce dissolved cadmium concentrations to below remedial objectives within a reasonable period of time. As a result, these reagents were selected for subsequent bench-scale soil column testing.
In preparation for column testing, groundwater and undisturbed soil cores were shipped from the project site to Orion’s office. Orion and Accord designed column testing apparatuses around the intact soil cores to maintain the physical properties of native soil and replicate the field conditions as closely as possible. Before testing, soil columns were equilibrated with a 1 part per million (ppm) cadmium solution to conservatively test the effectiveness of flushing reagents. The soil columns were subsequently flushed with either 100 ppm calcium chloride or 85 ppm EDTA at an average flow rate of 2 milliliters per minute.
EDTA and calcium chloride reduced cadmium concentrations to near the remedial objective within 10 and 24 pore volume flushes, respectively. Cadmium concentrations rebounded during both flushing scenarios. The rebound in cadmium concentrations may have been caused by preferential flow pathways or other physical variabilities that can influence cadmium mobility.
The modeling and column test results indicated the following conclusions:
- Both EDTA and calcium chloride are viable flushing agents, but EDTA would likely be more effective in a field application.
- Higher concentrations of EDTA could potentially increase the cadmium flushing rate based on PHREEQC modeling and published results of soil flushing with EDTA.
- Flow rates in a field application should be limited to increase flushing solution contact time with soil and allow for more diffusion of cadmium from less mobile pore water.
- Cadmium concentrations would likely continue to decrease in a field application as the system equilibrates after completing active EDTA flushing.
Ultimately, Orion and Accord concluded that aquifer flushing was a viable remedial action alternative for treating the cadmium-impacted soil and groundwater. Orion subsequently used the soil column study results to estimate project lifecycle costs assuming aquifer flushing with EDTA as the full-scale remedial technology.
Soil column testing configuration.