Environment
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Measuring Carbon-based Contaminant Mineralization Using Combined CO2 Flux and Radiocarbon Analyses
Chapters
Summary October 21st, 2016
A protocol is described wherein CO2 mineralized from organic contaminant (derived from petroleum feedstocks) biodegradation is trapped, quantified, and analyzed for 14C content. A model is developed to determine CO2 capture zone's spatial extent. Spatial and temporal measurements allow integrating contaminant mineralization rates for predicting remediation extent and time.
Transcript
The overall goal of this combined Carbon Dioxide Flux Radiocarbon content and Zone of Influence modeling method is to track in situ petroleum-derived contaminant degradation using the depleted radio carbon signal in the carbon dioxide, a final degradation product. This method can help answer key questions in the bioremediation and environmental assessment fields. Such as, how much containment is being degraded on site?
Either through natural attenuation or in conjunction with engineered remediation approaches. The main advantage of this technique is that it relies on measuring the carbon deriving from the actual containment backbone, rather than making multiple indirect measurements as lines of evidence. The method allows calculating the actual in situ biodegradation rate.
For the field instillation prepare the air pumps as follows. First, drill a hole into the pump housing for a short piece of 1/16 inch diameter plastic gas impermeable tubing. Insert a five to six inch length of tubing into the hole.
Then, seal all external parts of the pump with marine sealant. Followed by a coat of silicon sealant. Next, pressure test the sealed pumps.
Gently blow in the tubing while blocking the outflow. Next, prepare a carbon dioxide trap for each collection well and a control. Weigh out 25 grams of sodium hydroxide in a 100 milliliter serum bottle.
Cap the serum bottle with a septum and crimp it tightly. Now, attach the electrical leads. In the field, running power to each well requires long lines to each pump.
Inside the field wells, cap their output using modified caps with two 1/16 inch holes for gas lines. In the well, the lowered gas line is set slightly above the ground water level and the return line is set just below the cap. Then, seal the hoses and cap gasket with vacuum grease so that all surfaces provide a gas tight seal.
From the well, route the lower tubing into the pump inlet, and route the gas line from the pump to the carbon dioxide trap. Using a 16 gauge needle to pierce the septum. Then, using another needle, route a return line from the trap to the gas line and in just below the cap.
Now, operate the setup to make an initial collection of 30 well volumes. Later, discard the initial traps. Pull out the needles from the septum and put them in a new bottle.
Then, log the start time of the collection. To analyze the collected gas, measure the trapped carbon dioxide in the water samples using coulometry. First, transfer triplicate one milliliter sub samples to 40 milliliters serum vials capped with septa.
Then, acidify each subsample with one milliliter of 50%volume by volume phosphoric acid. Feed the line into a magnesium perchlorate trap. And next use a silica gel trap with a 60 angstrom mesh.
Then, bubble the gas stream into the coulometric cell and use the measured coulombs to quantify the carbon dioxide levels. Two weeks to two months after starting the collection, collect the traps. Shut down the power to the pumps and remove the needles.
If needed, seal the traps for longterm storage. To analyze the trap, dissolve any remaining unspent solid sodium hydroxide with water. Sparge the water with carbon dioxide scrubbed helium gas to remove the dissolved carbon dioxide.
Then, combined the fully liquified trap contents and measure the total volume. Next, transfer sub samples of five to 10 milliliters into 40 milliliter vials with septa. Then, acidify the sub samples with 50%volume by volume phosphoric acid.
Now, sparge and analyze the resulting gas stream by coulometry as previously described. Later when you make your calculations for this sampling period be sure to account for the equilibrium kinetics by manually subtracting the lowest collection rate from the collection rate measured on the other wells. Use MT3DMS coupled with MODFLOW 2005 via the ModelMuse interface to simulate carbon dioxide diffusion in equilibrium associated with the well screen.
Configure ModelMuse with the MODFLOW program location. To do so, click Model menu then select MODFLOW Program Locations. Then point the program to the MODFLOW 2005 program instillation directory.
Under the same dialog, configure ModelMuse with the MT3DMS program location. Next, configure MODFLOW Packages and Programs under the Model menu. Under the option flow, make sure the Layer Property Flow package is selected.
Under Boundary conditions, select Specified head. Then select CHD Time-Variant Specified Head package. Next, select MT3DMS and BTN Basic-Transport package.
Then, set the Mobile Species to carbon dioxide. Now, configure the MODFLOW options. From the Model menu, select MODFLOW Options and set the model units.
Next, configure MODFLOW Time. From the Model menu, select MODFLOW Time. A 360 length stress period will run the simulation for 15 days.
Next, configure the MODFLOW data sets. From the Data menu, select Data Sets. Then, enter the data from the site of interest.
For Hydrology, enter k values in three dimensions. MODFLOW Initial Head and MODFLOW Specified Head. For MT3DMS, enter the diffusion coefficient, carbon dioxide, Initial Concentration carbon dioxide and Longitudinal Dispersivity.
Lastly, edit the global variables. From the Data menu, select Global Variables and enter the carbon dioxide collection rate used at the site and enter the initial carbon dioxide concentration. Now run the simulation.
Press the green arrow on the top icon bar to start the simulation. Then, save the input files when prompted, and the simulation will run. After the run, compile and export the data by selecting the file menu, then export and then the MT3DMS input files.
To observe and output the model's results, click the visualize icon. Then select the simulation and output the Zone of Influence boundary values in 3D coordinates. At the test site, historical chlorinated hydrocarbon contamination has been highest within the central well cluster.
The measured carbon dioxide production ranged from zero to 34 milligrams per day and was lowest in a central well cluster where historical contamination was the highest. Site geochemical parameters, such as PH calcium and other cation concentrations did not indicate any carbonate dissolution, which might bias radio carbon measurement interpretation. Two two-week dry season measurements were averaged for subsequent calculations.
Respiration and radio carbon measures did not vary considerably between the two week periods. Using previous reports for the ground water hydraulic in the carbon dioxide solute properties, a Zone of Influence model was generated. Using estimates from the Zone of Influence model, the mass chlorinated hydrocarbon removal at each well was calculated.
Ultimately, the wells near the site periphery had the highest chlorinated hydrocarbon degradation and carbon dioxide production, and the models supported this observation. While attempting this procedure, it's important to remember to minimize the contamination with atmospheric carbon dioxide by capping quickly and minimizing contact with the atmosphere. All diluent should be sparged with inert gas to remove dissolved CO2.
Following this procedure, other methods, like determining site biogeochemical conditions can be preformed in order to answer additional questions, like what controls in situ degradation rates?
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