Sediment fingerprinting in agricultural catchments: A critical re-examination of source discrimination and data corrections
Sediment fingerprinting in agricultural catchments: A critical re-examination of source discrimination and data corrections
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DOI：
10.1016/j.geomorph.2013.08.003
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发表时间：
2014
期刊：
影响因子：
3.9
通讯作者：
H. Smith;W. Blake
中科院分区：
文献类型：
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作者：
H. Smith;W. Blake
精细沉积物来源指纹识别技术已在农业河流流域得到广泛应用。农业环境中成功的源辨别取决于一个关键假设，即印在流域土壤上的土地利用源特征可以从影响土壤和沉积物性质的其他景观因素所产生的特征中辨别出来。在本研究中，我们通过调查（i）源辨别的物理和化学基础以及（ii）可能混淆农业流域中源非混合的潜在因素（包括颗粒大小和有机物对示踪剂特性的影响）来重新审视这一关键假设。该研究位于塔马河，这是英格兰西南部的一个主要农业流域（920 平方公里），也受到采矿的影响。来源歧视主要集中在牧场和耕地的利用以及河道库上。在 12 个月的时间内，每月收集七个流域的时间积分悬浮沉积物样本。在源土壤和沉积物中测量的物理和化学特性包括沉降放射性核素（137Cs、过量210Pb）、主要和次要元素地球化学成分、总有机碳和颗粒尺寸。来源辨别完全取决于表层和地下土壤之间示踪剂特性浓度的差异。这是基于表面升高的沉降放射性核素浓度，而许多地球化学特性由于风化和成土作用而在表面耗尽，尽管表面土壤污染可以扭转这种趋势。然而，研究流域中的来源歧视受到以下因素的限制：（i）耕地和牧场的轮换导致这两个来源之间的差异减少，以及（ii）许多示踪物特性的耕地来源特征类似于牧场和河岸来源的混合。此外，废弃历史矿山的金属污染和高地泥炭土沉积物的有机富集共同导致了几个流域中某些示踪剂特性的非保守行为。源土壤的颗粒大小和有机碳含量的差异可以解释下游沉积物中这些特性的大部分变化，而不是选择性传输效应。粒径、有机碳和示踪剂特性浓度之间的不一致关系进一步破坏了对示踪剂数据集进行广泛应用的校正的基础。敏感性分析表明，针对有机质的差异校正源示踪剂数据可能会对源贡献估计产生巨大的变化，而这种变化是不合理的，因此不应使用此类校正。与不良来源辨别和非保守行为相关的混杂因素很可能影响许多农业流域的沉积物指纹研究。因此，许多指纹识别研究中对来源贡献的估计可能包含重大的未量化错误。
Fine sediment source fingerprinting techniques have been widely applied in agricultural river catchments. Successful source discrimination in agricultural environments depends on the key assumption that land-use source signatures imprinted on catchment soils are decipherable from those due to other landscape factors affecting soil and sediment properties. In this study, we re-examine this critical assumption by investigating (i) the physical and chemical basis for source discrimination and (ii) potential factors that may confound source un-mixing in agricultural catchments, including particle size and organic matter effects on tracer properties. The study is situated in the River Tamar, a predominantly agricultural catchment (920 km2) in south-west England that has also been affected by mining. Source discrimination focused on pasture and cultivated land uses and channel banks. Monthly, time-integrated suspended sediment samples were collected across seven catchments for a 12-month period. Physical and chemical properties measured in source soils and sediment included fallout radionuclides (137Cs, excess210Pb), major and minor element geochemical constituents, total organic carbon and particle size. Source discrimination was entirely dependent on differences in tracer property concentrations between surface and sub-surface soils. This is based on fallout radionuclide concentrations that are surface-elevated, while many geochemical properties are surface-depleted due to weathering and pedogenetic effects, although surface soil contamination can reverse this trend. However, source discrimination in the study catchments was limited by (i) rotation of cultivated and pasture fields resulting in reduced differences between these two sources, and (ii) the cultivated source signature resembling a mix of the pasture and channel bank sources for many tracer properties. Furthermore, a combination of metal pollution from abandoned historic mines and organic enrichment of sediment from upland areas of peaty soils resulted in the non-conservative behaviour of some tracer properties in several catchments. Differences in the particle size and organic carbon content of source soils could explain much of the variation in these properties in downstream sediment, rather than selective transport effects. Inconsistent relationships between particle size, organic carbon and tracer property concentrations further undermined the basis for the use of widely applied corrections to tracer datasets. Sensitivity analysis showed that correcting source tracer data for differences in organic matter can produce large changes to source contribution estimates that cannot be justified, and such corrections should not be used. Confounding factors related to poor source discrimination and non-conservative behaviour are highly likely to affect sediment fingerprinting studies in many agricultural catchments. As a result, estimates of source contributions in many fingerprinting studies may contain significant unquantified errors.