A viable means of quantifying the rate of natural source zone depletion (NSZD) at hydrocarbon contaminated sites is by the measurement of carbon dioxide (CO₂) and methane (CH₄) effluxes at the surface. This methodology assumes that gas effluxes are reflective of actual contaminant degradation rates in the subsurface, which is only accurate for quasi-steady state conditions. However, in reality, subsurface systems are highly dynamic, often resulting in fluctuations of the water table. To quantify the effects of water table fluctuations on NSZD rates, a simulated biodiesel spill in a 400 cm long, 100 cm wide and 150 cm tall sandtank was subjected to lowering and raising the water table, while soil-gas chemistry and surface CO₂ and CH₄ effluxes were measured. Results show that water table fluctuations have both short-term (perceived) and long-term (actual) effects on NSZD rates, interpreted using surface efflux measurements. When the water table was lowered, surface effluxes immediately increased up to 3 and 344 times higher than baseline for CO₂ and CH₄ effluxes, respectively, due to the liberation of anaerobically produced gas accumulated below the water table. After this immediate release, the system then reached quasi-steady state conditions 1.4 to 1.6 times higher for CO₂ than baseline conditions, attributed to increased aerobic degradation in the broadened and exposed smear zone. When the water table was raised, quasi-steady state CO₂ and CH₄ effluxes declined to values of 0.9 and 0.4 times baseline effluxes, respectively, implying that contaminant degradation rates were reduced due to submergence of the smear zone. The findings of this study show that the dynamic effects of water table fluctuations and redistribution of the contaminants affect surface effluxes as well as short-term (perceived) and long-term (actual) contaminant degradation rates. Therefore, water table fluctuations need to be considered when quantifying NSZD at contaminated sites using sparse temporal rate measurements to estimate NSZD rates for extended periods of time (e.g., annual rates).

量化碳氢化合物污染地点自然源区消耗率（NSZD）的可行方法是通过测量表面的二氧化碳（CO ₂）和甲烷（CH ₄）外流。该方法假设气体外流反映了地下的实际污染物降解速率，这仅对准稳态条件准确。然而，实际上，地下系统是高度动态的，经常导致地下水位的波动。为了量化地下水位波动对NSZD速率的影响，对模拟的生物柴油泄漏物（长400厘米，宽100厘米，高150厘米）进行了降低和升高地下水位的研究，同时研究了土壤气体化学作用和表面CO ₂和通道₄测量流出。结果表明，地下水位波动对NSZD速率具有短期（感知）和长期（实际）影响，这可以通过地表外排量测量来解释。当地下水位降低时，由于释放在地下水位以下积聚的厌氧产生的气体，地表通量立即迅速增加，分别比基线高出CO ₂和CH ₄排放量三倍和344倍。立即释放后，系统达到了准稳态条件，CO ₂的浓度比基线条件高1.4到1.6倍，这归因于在扩大和暴露的涂片区中有氧降解的增加。升高地下水位时，准稳态CO ₂和CH ₄外排量分别下降到基准外排量的0.9和0.4倍，这意味着由于涂片区的浸没，污染物的降解率降低了。这项研究的结果表明，地下水位波动和污染物的重新分布的动态影响会影响表面外排以及短期（感知）和长期（实际）污染物的降解速度。因此，当使用稀疏的时间速率测量值来估计长时间内的NSZD速率（例如，年速率）时，在受污染地点量化NSZD时，需要考虑地下水位波动。