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.
A method is described which uses the absence of radiocarbon in industrial chemicals and fuels made from petroleum feedstocks which frequently contaminate the environment. This radiocarbon signal — or rather the absence of signal — is evenly distributed throughout a contaminant source pool (unlike an added tracer) and is not impacted by biological, chemical or physical processes (e.g., the 14C radioactive decay rate is immutable). If the fossil-derived contaminant is fully degraded to CO2, a harmless end-product, that CO2 will contain no radiocarbon. CO2 derived from natural organic matter (NOM) degradation will reflect the NOM radiocarbon content (usually <30,000 years old). Given a known radiocarbon content for NOM (a site background), a two end-member mixing model can be used to determine the CO2 derived from a fossil source in a given soil gas or groundwater sample. Coupling the percent CO2 derived from the contaminant with the CO2 respiration rate provides an estimate for the total amount of contaminant degraded per unit time. Finally, determining a zone of influence (ZOI) representing the volume from which site CO2 is collected allows determining the contaminant degradation per unit time and volume. Along with estimates for total contaminant mass, this can ultimately be used to calculate time-to-remediate or otherwise used by site managers for decision-making.
Environmental cleanup costs are staggering, with numerous contaminated sites in the US and abroad. This makes innovative treatment and monitoring strategies essential to reaching Response Complete (RC) status (e.g., no further action needed) economically. Traditionally, lines of converging evidence have substantiated in situ bioremediation, abiotic contaminant conversion, or other forms of natural attenuation. Lines of evidence cannot be used to absolutely confirm degradation or to gather contaminant degradation rate information under in situ conditions1. Collecting a wide array of data to predict remediation timescale(s) has often been recommended, but linking these data cost-effectively to absolutely confirm remediation has been problematic2-4. Obtaining the most realistic and complete site conceptual model data with as little cost as possible is an ultimate site-management goal. Moreover, regulator and stakeholder demands represent additional drivers for obtaining the most timely, valuable and cost-effective information. Relatively inexpensive methods capable of providing compelling evidence for contaminant turnover rates offer the most value for meeting cleanup goals.
Because very distinct isotopic signatures are available in carbon-based contaminants, carbon isotopes have been recently applied to understanding contaminant attenuation processes at field sites5-13. Stable carbon isotopes can be used to determine if a source is attenuating based on Rayleigh distillation kinetics (c.f. 5,6 for reviews). This methodology, while convenient, may be limited when contaminants are from mixed sources — or do not represent an isotopically-unique "starting" spill (from which initial stable carbon isotope ratios can be derived). Natural abundance radiocarbon analysis represents an alternative (and perhaps complementary) isotopic strategy for measuring carbon-based contaminant degradation to CO2. Fuels and industrial chemicals derived from petroleum feedstocks will be completely devoid of 14C relative to contemporary (actively cycling) carbon, which contains 14C created by cosmic radiation reactions in the atmosphere. Radiocarbon analysis is not subject to fractionation as is stable carbon isotope analysis, and 14C decay is not significantly impacted by physical, chemical or biological processes. Moreover, the 14C signal — or lack thereof — in petroleum-derived materials is evenly distributed throughout the contaminant pool making it a fully miscible tracer. The technique described here relies on the observation that any CO2 generated from a fossil derived contaminant will be devoid of 14C while CO2 generated from microorganisms degrading NOM will contain easily-measurable amounts of 14C. Measuring 14CO2 also allows one to directly link full contaminant degradation (i.e., mineralization) to a harmless end product.
14CO2 analysis has been used to follow fossil fuel-derived contaminant degradation products7-13. This is due to the analytical resolution between end members (fossil and contemporary) which is roughly 1,100 parts per thousand (‰). Generally, accelerator mass spectrometry (AMS) is used to resolve natural abundance radiocarbon. Atmospheric CO2 (~ +200‰) living biomass (~ +150‰) and soil organic matter-derived CO2 (~ -200–+100‰) are all analytically distinct from fossil-derived CO2 (-1,000‰). This is due to the complete decay of all 14C, which has a half-life of roughly 6,000 years. Fuels and industrial chemicals derived from petroleum feedstocks, which are millions of years removed from active carbon cycling, have a distinct radiocarbon signature (-1,000‰ ≈ 0% modern — meaning no detection on AMS). The measurement is straightforward and in terms of sample contamination, almost all potential biases are toward the conservative (contaminating the sample with modern CO2). For instance, atmospheric CO2 getting into a sample would increase the radiocarbon isotopic signature and thus cause underestimating the degradation rate.
CO2 evolved from fossil-fuel based contaminant degradation will be radiocarbon-free. At a background site with no contamination, CO2 respired from natural organic matter (NOM) will be age appropriate to the NOM. Within the plume or at the fringes, contaminant-derived CO2 will have 0% modern carbon. CO2 from NOM sources and CO2 derived from fossil sources can be distinguished with a two end-member mixing model11. It is thus possible to estimate the proportion of the entire CO2 pool (respired carbon) attributable to the contaminant. Using solely this proportion, fossil-hydrocarbon or industrial chemical oxidation at field sites has been confirmed7-13. This proportion of contaminant derived CO2 can then be coupled with total CO2 mineralization rate (all CO2 collected per unit time and volume) to determine intrinsic contaminant mineralization rate. Assuming this attenuation rate would continue at given site conditions, one could then estimate time needed for site closure.
Techniques are available for determining soil horizon CO2 fluxes with methods having open- or closed-system designs14. Closed-system flux chambers and gas flux models have been used to determine net respiration in contaminated soils12,13,15-17. In these studies, spatial measurements directly associated with a contaminant plume and with background areas showed enhanced biodegradation of organic contaminants. Various modeling methods were used to scale vertical flux measurements to site volume. The goal of this study was to develop methods for collecting ample CO2 for AMS analysis (~1 mg) without influence from atmospheric CO2 contamination (sealed wells) while using the collection rate to determine contaminant respiration. Finally, modeling a zone of influence (ZOI) to ultimately scale the measurement to 3 dimensions (volume) allowed determining the chlorinated hydrocarbon (CH) conversion on a per unit volume and per unit time basis. The ZOI allows one to determine how much volume the respiration and radiocarbon measurements are taken from. The method consists of trapping evolved CO2 by recirculating well headspace gas through a NaOH trap, measuring the radiocarbon content of the collected CO2, using a two end-member model to apportion the CO2 collected to contaminant origin, then scaling the measurement to a volume calculated by a site-specific groundwater model. The well headspace gas is recirculated so that only equilibrium processes "pull" CO2 from the adjacent ZOI.
A protocol is described which aims to combine rate measurements, proportion mineralization from contaminant(s) and ZOI to determine overall site contaminant degradation. The critical components are, measuring CO2 production (mineralization when corrected) over time, concurrently collecting the respired CO2 in sufficient quantity (~ 1 mg) for AMS radiocarbon analysis providing amount derived from contaminant degradation, and, creating a ZOI model to relate the captured CO2 to a known volume of soil or groundwater (or both). These three main components are combined to arrive at an overall calculation at each sampling point for amount of contaminant degraded per unit volume per unit time (g m-3 d-1, for instance). Scaling the calculations, through repeated and geographically separated measurements (wells covering a site subsampled over longer time-scales), will allow site managers to estimate spatial and temporal degradation dynamics and respond appropriately to regulators and stakeholders.
The described protocol uses recirculating pumps or long-term deployed passive samplers (a strategy currently under development) to trap out CO2 from well headspace gas. The reason is several fold. Primarily, sufficient CO2 must be collected in order to obtain radiocarbon measurements (~1 mg). Respiration rates can be measured using surface soil:air exchange traps or by using soil respiration instruments (Licor flux chamber for instance). These methods suffer from the need to asynchronously gather sufficient CO2 for radiocarbon analysis — thus perhaps biasing the measurement. For instance, a flux chamber can be outfitted to measure soil:air CO2 exchange while accounting for influx of atmospheric CO217. Unless respiration rates are high, ample CO2 for radiocarbon measurements may not be trapped. In this case, samples can be taken from large soil gas samples or from groundwater (with DIC)12. Furthermore, measuring CO2 flux at the soil:air surface is subject to influx from the atmosphere lateral to the flux chamber or trap. Sampling well headspace "isolates" the signal to the region of contamination (depending on well installation to some degree) but is suitably removed from atmospheric influx (and atmospherically-generated modern 14CO2). The main difficulty is sampling from the well without having to open it in order to change traps (for temporal sampling).
Using recirculating pumps allows one to sample well headspace and change CO2 traps at regular intervals without having to expose the sample location to atmospheric 14CO2. It also allows one to sample considerable CO2 which can then be analyzed for flux and natural radiocarbon content. The recirculation protocol is not without difficulty. A major problem is delivering ample power to run pumps continuously in the field. For the initial experiment (described here), solar panels provided enough energy to run pumps for each two-week period. Voltage logs showed that after several days, solar power could not keep up with the needed power and pumps were not operational for several hours each day. This was immaterial to the flux modeling and overall collection, but highlights the difficulty in providing ample power to field-deployed hardware. In currently-running collections, power to pumps has been interrupted by ground crews mowing in the monitoring well field. Several power lines have been severed. We are currently evaluating headspace-deployed passive CO2 traps which could be lowered into the well and retrieved at a later date with absorbed CO2. A risk-benefit analysis is underway (the risk mostly derived from having to open the well head and allow in atmospheric 14CO2).
The technique's main limitations are not being able to distinguish the exact respiration source in mixed contaminant systems and not being able to account for intermediate carbon-based degradation products (i.e., DCE, VC, methane). For instance, at the current site, there was historical fuel hydrocarbon contamination in addition to CH contamination. CHs are almost exclusively made from petroleum feedstocks. At the described site, CH is primarily isolated in the region studied – while some residual petroleum evidently exists to the North. No petroleum was found in wells sampled for this work. However, at a mixed contaminant site, the overall mineralization rate might be difficult to tie to one individual or class of contaminants. Using this method, one can quantify the complete CH degradation (to CO2). If, the contaminant carbon is converted to CH4 (anaerobic conditions), the CH4 may be "lost" if it diffuses away from the ZOI. That carbon will likely be converted to CO2 within oxic portions in the vadose zone. If this does not occur within the ZOI, the reported method will not account for it. In this case, the described method can be considered a conservative estimator, which from a regulatory perspective, is desirable. Additionally, the ZOI modeling is not without uncertainty. Simulations are based on "singular" values such as porosity and bulk density which are measured in subsamples assumed to be homogenous — but in reality are heterogeneous at the macro- and microscales. A perceived limitation may be the analysis cost for natural abundance radiocarbon (which can be as much as $600 per sample). The definitive nature of the information gathered from radiocarbon makes the cost very low in reality. With several well-chosen samples, one can determine if substantial remediation is occurring. If, for instance the CO2 associated with a contaminant plume is radiocarbon-depleted relative to a background site10. A site with low ambient pH (> ~4.8) and considerable limestone (CaCO3) may be a poor candidate for applying this technique. Ancient carbonate deposits might dissolve in low pH and bias the analysis.
The technique's significance is considerable, as a sole measurement type (natural abundance radiocarbon) can immediately be used to confirm in situ conversion of contaminant to CO2. This analysis is definitive. Radiocarbon cannot become depleted except through radioactive decay – which is constant despite physical, chemical or biological alteration of any starting material. Static radiocarbon measurements (for instance DI14C) can be made on batch samples and immediately confirm if 14C-depleted CO2 is prevalent at a site (irrefutably indicating contaminant mineralization to CO2). This information alone is incredibly valuable to site managers as without it, they are required to use numerous indirect lines of evidence to infer that contaminant mineralization is occurring. No other single measurement can provide a concrete connection between carbon-based contaminant and the carbon-containing CO2 produced through complete degradation.
Future applications are currently underway in which our group will increase sampling temporal resolution to encompass an entire year. By collecting CO2 and determining the mineralization rate(s) over the spatial extent of the site, we will be able to refine models for contaminant degradation over time. This information is critically needed by site managers in order to most effectively manage contaminated sites. In limited use, regulators at three sites where the technique has been applied have recognized the methods definitive results. This has led to cost savings and helped to guide remedial alternatives.
The authors have nothing to disclose.
Financial support for this research was provided by the Strategic Environmental Research and Development Program (SERDP ER-2338; Andrea Leeson, Program Manager). Michael Pound, Naval Facilities Engineering Command, Southwest provided logistical and site support for the project. Brian White, Erika Thompson and Richard Wong (CBI Federal Services, Inc) provided on-site logistical support, historical site perspective and relevant reports. Todd Wiedemeier (T.H. Wiedemeier & Associates) provided documentation, discussion and historical site perspectives.
Air pump; Power Bubbles 12V | Marine Metal | B-15 | |
Marine Sealant | 3M | 5200 | for sealing pumps |
Silicone Sealant | Dap | 08641 | for sealing pumps |
Tubing for gas recirculation | Mazzer | EFNPA2 | |
Stopcocks (for gas lines) | Cole-Parmer | 30600-09 | for assembling gas lines |
Male luer lock fittings | Cole-Parmer | WU-45503-00 | for assembling gas lines |
Female luer lock fittings | Cole-Parmer | EW-45500-00 | for assembling gas lines |
4" Lockable J-Plug well cap | Dean Bennett Supply | NSN | 2" if smaller wells |
HOBO 4-Channel Pulse Data Logger | Onset | UX120-017 | Older model no longer available. Use to monitor pump operation |
Serum bottles 100 mL (cs/144) | Fisher Scientific | 33111-U | For CO2 traps |
Septa (pk/100) | Fisher Scientific | 27201 | For CO2 traps |
Coulometry | |||
Anode solution | UIC, Inc | CM300-001 | |
Cathode solution | UIC, Inc | CM300-002 | |
For IC analysis | |||
Dionex Filter Caps 5 ML 250/pk | Fisher Scientific | NC9253179 | Caps for IC |
Dionex 5 mL vials, 250/pk | Fisher Scientific | NC9253178 | Vials for IC |
If using solar power | |||
Renogy Solar Panel kit(s) | Renogy | KT2RNG-100D-1 | Bundle provides 200W |
VMAX Solar Battery | VMAX | VMAX800S | For energy storage |