Attenuation of nitrate in aquitard sediments of southern Ontario
Attenuation of nitrate in aquitard sediments of southern Ontario
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DOI：
10.1016/0022-1694(95)02885-4
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发表时间：
1996-05
影响因子：
6.4
通讯作者：
W. Robertson;B. Russell;J. Cherry
中科院分区：
文献类型：
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作者：
W. Robertson;B. Russell;J. Cherry
为了检查硝酸盐的持久性，在安大略南部的五个地点使用岩心挤压水和压力计样本进行了详细的地球化学剖面分析，这些地点的地下水正在富含淤泥的弱透水层沉积物中以16至超过20厘米的速度向下移动。年-1。农业活动导致浅层地下水中NO3−-N含量升高（5-50 mg 1−1），但在氧化还原层（表层风化（棕色）沉积物和下层未风化（灰色）沉积物之间的分界线），NO3−-N含量一直在降低，通常降至非常低的水平（< 0.05 mg 1−1-N）。氚测年表明，地下水在氧化还原层深度（3-5米）的补给在1970年和1980年之间，因此，N 03消耗似乎是生物降解反应的结果，因为在此期间没有发生重大的土地利用变化。硝酸盐贫化带与氧化还原跃层的密切联系，特别是沉积物硫含量突然增加，孔隙水SO 42 −水平也增加，表明主要的衰减反应是利用未风化沉积物中存在的还原硫化合物作为电子供体的自养反硝化作用。质量平衡计算表明，由于NO3−污染增加了硫的消耗，在这些地点，氧化还原跃层向下迁移速率的增加仅约为1 mm/年。对文献的回顾表明，大多数富含粉砂和粘土的沉积物的硫含量与本文所研究的相同或更高，因此，在大多数情况下，如果含水层被几米或更长的未风化的封闭沉积物覆盖，则可能会提供高度的保护，使其免受表面活性剂NO3−污染。
To examine nitrate persistence, detailed geochemical profiling, using core-squeezed water and piezometer samples, was carried out at five sites in southern Ontario where groundwater is moving downward in silt-rich aquitard sediments at rates of 16 to more than 20 cm year−1. Elevated levels of NO3−-N (5–50 mg 1−1) that occur in the shallow groundwater as a result of agricultural activity, were found to be consistently attenuated, generally to very low levels (< 0.05 mg 1−1-N), at the ‘redoxcline’, the horizon marking the boundary between the surficial weathered (brown) sediments and the underlying unweathered (grey) sediments. Tritium dating suggests that groundwater at the redoxcline depths (3–5 m) was recharged between 1970 and 1980, thus the N03 depletion appears to result from biodegradation reactions since no major landuse changes have occurred during this period. The close association of the nitrate depletion zones with the redoxcline, where, in particular, sediment sulphur contents increase abruptly, and where also porewater SO42−levels increase, suggests that the dominant attenuation reaction is autotrophic denitrification using reduced sulphur compounds present in the unweathered sediment as the electron donor. Mass balance calculations suggest that the increase in the downward rate of migration of the redoxcline, owing to added sulphur consumption from NO3−contamination, is only about 1 mm year−1at these sites. Review of the literature indicates that most silt- and clay-rich sediments have S contents in the same range, or higher, than those investigated here, thus, in most cases where aquifers are overlain by several metres or more of unweathered confining sediments, it is likely that a high degree of protection is afforded from surficial NO3−contamination.