There is no such thing as ‘undisturbed’ soil and sediment sampling: sampler-induced deformation of salt marsh sediments revealed by 3D X-ray computed tomography

Abstract

Within most environmental contexts, the collection of ‘undisturbed’ samples is widely relied upon in studies of soil and sediments. However, the impact of sampler-induced disturbance is rarely acknowledged, despite the potential significance of modification to sediment structure for the robustness of data interpretation. In this study, 3D computed X-ray microtomography (μCT) is used to evaluate and compare the disturbance imparted by four commonly used sediment sampling methods within a coastal salt marsh. Paired sediment core samples from a restored salt marsh at Orplands Farm, Essex, UK, were collected using four common sampling methods (push, cut, hammer and gouge methods). Sampling using two different area-ratio cores resulted in a total of 16 cores that were scanned with μCT to identify and evaluate sediment structural properties of samples that can be attributed to sampling method. 3D qualitative analysis identifies a suite of sampling disturbance structures including gross-scale changes to sediment integrity and substantial modification of pore space, structure and distribution, independent of sediment strength and stiffness. Quantitative assessment of changes to pore space and sediment density arising from the four sampling methods offers a means of direct comparison between the impact of depth sampling methods. Considerable disturbance to samples results from use of push, hammer and auguring samplers, whilst least disturbance is found in samples recovered by cutting and advanced trimming approaches. In many environmental studies involving sediment recovery through coring or other depth sampling, there is no such thing as an undisturbed sediment sample. The novel use of μCT scanning of sealed sediment cores has enabled the identification and evaluation of the nature and extent of sample disturbance resulting from four common types of sediment recovery methods. Depth sampling and coring methods remain key tools for understanding sediments and soils, but referring to undisturbed sediment sampling is no longer tenable without supporting evidence.