White Papers

Approach for Determining the Presence of Zero-Valent Iron Powder Within the Subsurface Using Magnetic Separation

During a recent Feroxsm (Zero-Valent Iron Powder Injection) project in Oklahoma, soil cores were collected after the injections and analyzed to qualitatively delineate and confirm the emplacement of Feroxsm powder in the subsurface. Pneumatic Fracturing and iron-injections were performed in 7 boreholes targeting a depth of 12 to 31 feet below ground surface (bgs). A total of 4050 kg of iron powder was injected into the formation.

The geology at the site can be described as clays, silty clays, and clayey silts with inter-bedded lenses of fine grained sands. At some depths, these sands were strongly cemented forming very hard layers. Subsequent to the injection, approximately 28.5 feet of soil cores were collected within the targeted area using hollow stem augers and 6-inch stainless steel California sample barrels within a split spoon. These cores were taken approximately 10 to 15 feet from the injection locations and from a depth of 12 to 35 feet bgs.

At previous technology applications, the visual identification of iron within soil cores is not always straight forward. This is due to the nature of the iron powder injections and the small particle size of the iron powder. Therefore, a novel magnetic separation test was devised to assist in the identification of iron powder within the soil cores. Samples from each 6-inch California sample sleeve were removed and allowed to dry. They were then crushed and spread over a magnet that was wrapped in plastic. The distribution of Feroxsm powder within the subsurface was made relying upon the magnetic properties of the individual zero-valent iron particles, as well as mixed iron and soil that adhered to the magnet.

Figures 1 and 2 show magnetically separated soil particles from depths of 17.5 to 18, and 22 to 22.5 ft bgs, respectively. In Figure 1, the particles are moist and fairly large. In Figure 2, the sample was allowed to dry for a longer duration, which allowed the soil to be crushed finer. This resulted in significantly more iron-rich soil adhering to the magnet.
 
figure #1

In addition to the magnetic separation test, an iron filled fracture was identified in a soil core located at 17.9 feet bgs during the analysis (Figure 3). This fracture was horizontally orientated and bisected the core. The presence of iron at this location was confirmed using the magnetic separation test.

In summary, the results from the magnetic separation tests indicate that Zero-Valent iron was found in over 50% of the soil samples collected, and was identified at depths from 12 to 31 feet bgs. The magnetic separation test not only provided direct evidence of the injected iron powder in the subsurface, but also helped delineate the distribution of iron within the treatment zone. Furthur research and testing on this field method is required to better standardize the test so that reliable comparisions between different site applications in different geology can be made.

 


figure #2
  figure #3

 

back