Chemical extraction results of aquifer sediments for concentrations of iron and arsenic in different redox zones at the crude-oil spill site near Bemidji, Minnesota

In a crude-oil-contaminated sandy aquifer at the Bemidji site in northern Minnesota, biodegradation of petroleum hydrocarbons has resulted in release of naturally occurring As to groundwater under Fe-reducing conditions. This data set was collected for a study that used chemical extractions of aquifer sediments collected in 1993 and 2011-2014 to evaluate the relationship between Fe and As in different redox zones (oxic, methanogenic, Fe-reducing, anoxic-sub-oxic transition) of the contaminated aquifer over a twenty year period. The data presented showed that 1) the aquifer has the capacity to naturally attenuate the plume of dissolved As, primarily through sorption; 2) Fe and As are linearly correlated in sediment across all redox zones, and a regression analysis between Fe and As reasonably predicted As concentrations in sediment from 1993 using only Fe concentrations; 3) an As-rich “iron curtain,” associated with the anoxic-sub-oxic transition zone, migrated 30 m downgradient between 1993 and 2013 as a result of the hydrocarbon plume evolution; and 4) silt lenses in the aquifer preferentially sequester dissolved As, though As is remobilized into groundwater from sediment after reducing conditions are established.

位于美国明尼苏达州北部贝米吉（Bemidji）站点的一处受原油污染的砂质含水层中，石油烃类的生物降解作用在铁还原条件下，促使天然存在的砷（As）释放至地下水中。本数据集源于一项研究，该研究针对1993年与2011-2014年采集的含水层沉积物开展化学萃取分析，以解析受污染含水层在20年周期内，不同氧化还原带（有氧、产甲烷、铁还原、缺氧-亚氧过渡带）中铁与砷的关联关系。本次发布的数据表明：1）该含水层主要通过吸附作用，天然衰减溶解态砷污染羽流；2）在所有氧化还原带的沉积物中，铁与砷均呈线性相关关系，仅通过铁浓度开展的铁-砷回归分析，可较为准确地基于1993年的铁浓度预测当年沉积物中的砷浓度；3）与缺氧-亚氧过渡带相关的富砷"铁帘"，在1993年至2013年间因石油烃污染羽流的演化，沿地下水流向向下游迁移了30米；4）含水层中的透镜状粉砂层会优先固存溶解态砷，但当地层形成还原条件后，沉积物中的砷会重新活化进入地下水中。