Surficial CO₂ efflux surveys have been used to delineate hydrocarbon source zones in contaminated aquifers and provide estimates of hydrocarbon biodegradation rates. This approach requires distinguishing between CO₂ derived from petroleum degradation and CO₂ produced from natural soil respiration. To this end, radiocarbon has been used to differentiate between ¹⁴C–depleted CO₂ from hydrocarbon degradation and ¹⁴C–enriched CO₂ from natural soil respiration to effectively quantify the contribution of each source to total CO₂ efflux, and by deduction natural source zone depletion (NSZD) rates. In this study, a systematic method comparison has been conducted to evaluate available approaches for collecting CO₂ gas samples for radiocarbon analysis used to correct total CO₂ efflux measurements for quantifying natural source zone depletion rates. Gas samples for radiocarbon analysis were sampled from (i) dynamic closed chambers (located at ground surface), (ii) static chambers (also at ground surface), (iii) shallow soil gas probes (0.3 m bgs), and (iv) soil gas monitoring wells (~0.6 m below ground surface) during a CO₂ efflux survey conducted at the site of a historical pipeline rupture near Bemidji, MN. The mean fraction of radiocarbon (F¹⁴C) obtained from samples overlying the source zone were (i) 0.93 ± 0.01, (ii) 0.73 ± 0.03, (iii) 0.71 ± 0.04, and (iv) 0.41 ± 0.06, for the four methods respectively. These F¹⁴C values were used to apportion total CO₂ efflux measurements into contributions of contaminant–derived CO₂ efflux and natural soil respiration to evaluate natural source zone depletion processes. Results suggest that the method of radiocarbon sampling has a significant effect on the calculated fraction of the CO₂ efflux originating from contaminant-related soil respiration, with contributions ranging between 27% and 59% of total soil respiration. Results indicate that radiocarbon sampled from static chambers and shallow soil gas probes methods offer the best compromise between CO₂ sample yield and sample representativeness, providing the most reliable estimates of CO₂ effluxes originating from contaminant degradation. However, the results also show that for this study, all methods agree within a factor of <2.3 regarding the inferred NSZD rates.

已使用表面CO ₂外排调查来描绘受污染含水层中的烃源区，并提供烃生物降解率的估算值。这种方法需要区分之间CO ₂从石油降解和CO衍生的₂从天然土壤呼吸产生的。为此，放射性碳已被用于区分来自烃降解的¹⁴ C贫乏的CO ₂和来自自然土壤呼吸的¹⁴ C富集的CO ₂，以有效地量化每种来源对总CO ₂的贡献外排，并通过扣除自然源区耗竭（NSZD）率。在这项研究中，已进行了系统的方法比较，以评估收集用于放射碳分析的CO ₂气体样品的可用方法，这些方法可用于校正总CO ₂外排测量值以量化自然源区的枯竭率。从（i）动态密闭室（位于地面），（ii）静态室（也在地面），（iii）浅土气体探头（0.3 m bgs）和（iv）采样进行放射性碳分析的气体样品在明尼苏达州贝米吉附近的历史管道破裂现场进行的CO ₂外流调查中，对土壤气体监测井（距地面约0.6 m）进行了监测。放射性碳的平均分数（F ¹⁴对于四种方法，从覆盖源区的样品获得的C）分别为（i）0.93±0.01，（ii）0.73±0.03，（iii）0.71±0.04和（iv）0.41±0.06。这些F ¹⁴ C值用于将总的CO ₂外流测量值分配到污染物衍生的CO ₂外流和自然土壤呼吸的贡献中，以评估自然源区的耗竭过程。结果表明，放射性碳采样方法对计算出的CO ₂分数具有显着影响。外排源于与污染物有关的土壤呼吸作用，占总土壤呼吸作用的27％至59％。结果表明，从静态腔室和浅层土壤气体探针方法中采集的放射性碳在CO ₂样品产量和样品代表性之间取得了最佳折衷，从而提供了最可靠的估算值，该估算值是源自污染物降解的CO _2外排量。但是，结果还表明，对于本研究，关于推断的NSZD率，所有方法均在小于2.3的范围内。