University of British Columbia, Earth, Ocean and Atmospheric Sciences, Vancouver, British Columbia, Canada.
University of British Columbia, Earth, Ocean and Atmospheric Sciences, Vancouver, British Columbia, Canada; BGC Engineering Inc., 500-980 Howe St, Vancouver, British Columbia, Canada.
J Contam Hydrol. 2021 Mar;238:103771. doi: 10.1016/j.jconhyd.2021.103771. Epub 2021 Jan 9.
A viable means of quantifying the rate of natural source zone depletion (NSZD) at hydrocarbon contaminated sites is by the measurement of carbon dioxide (CO) and methane (CH) 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 CO and CH 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 CO and CH 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 CO than baseline conditions, attributed to increased aerobic degradation in the broadened and exposed smear zone. When the water table was raised, quasi-steady state CO and CH 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).
一种可行的量化受烃污染场地自然源区衰减（NSZD）速率的方法是测量地表二氧化碳（CO）和甲烷（CH）的逸出通量。该方法假设气体逸出通量反映了地下实际污染物降解速率，这仅在准稳态条件下是准确的。然而，在现实中，地下系统高度动态，经常导致地下水位波动。为了量化地下水位波动对 NSZD 速率的影响，在一个 400cm 长、100cm 宽和 150cm 高的沙箱中模拟了生物柴油泄漏，并降低和升高地下水位，同时测量土壤气体化学和地表 CO 和 CH 逸出通量。结果表明，地下水位波动对 NSZD 速率有短期（感知）和长期（实际）的影响，可以通过地表逸出通量测量来解释。当地下水位降低时，地表逸出通量立即增加，CO 和 CH 逸出通量分别比基线高 3 倍和 344 倍，这是由于地下水位以下积累的厌氧产生的气体释放所致。在这种即时释放之后，系统达到准稳态条件，CO 比基线条件高 1.4 到 1.6 倍，这归因于在拓宽和暴露的污斑区中增加的好氧降解。当地下水位升高时，准稳态 CO 和 CH 逸出通量分别下降到基线逸出通量的 0.9 和 0.4 倍，这意味着由于污斑区被淹没，污染物降解速率降低。本研究的结果表明，地下水位波动的动态效应和污染物的重新分布会影响地表逸出通量以及短期（感知）和长期（实际）污染物降解速率。因此，在使用稀疏时间速率测量来估算受污染场地的 NSZD 时，需要考虑地下水位波动来估算较长时间段（例如，年速率）的 NSZD 速率。
