Towards high temporal and in situ estimations of suspended sediment sources

Abstract

Natural soil erosion and sediment transport processes are important in shaping the Earth’s critical zone. However, excess soil erosion and sediment delivery may pose different problems related to soil health, surface water quality and the safety of human living environments. To obtain robust information on the location of sediment source areas and to quantify their contributions to the sampled in-stream suspended sediment (SS) is important to guide the implementation of targeted management measures. Sediment fingerprinting is a widely applied approach to obtain such information, relying on the comparison of chemical and/or physical properties (i.e., fingerprints) between potential soil sources and target SS. However, there are several limitations and challenges associated with this approach. One of the major limitations relates to the available resources, which are often prohibitive in the context of research budgets. Given the relatively high costs and workloads involved in conventional source and SS sampling, and the subsequent laboratory fingerprint analysis procedures, repeat source and target SS sampling campaigns, or long durational studies, are limited. This situation remains despite the fact that it is widely known that catchments are rather dynamic, causing different source areas to be activated and deactivated over time. To this end, the work described in this thesis aims to develop new fingerprints (i.e., absorbance measurements at the UV-VIS wavelength range, and SS particle size distribution) that allow for increased temporal observations (i.e., up to minutes), by testing instruments that could directly obtain these fingerprints from water samples, and eventually measure in situ at high temporal resolution. Both fingerprints were tested at two scales in proof-of-concept studies: (i) in a laboratory scale setting, using artificial mixtures with known soil sample contributions to evaluate un-mixing model soil sample apportionment outcomes, and (ii) in a catchment scale setting, comparing un-mixing model source apportionment results with source apportionment results through sediment source budget estimations. The laboratory scale experiments showed rather small mean deviations to the known soil sample contributions (i.e., 15% and 7%, using absorbance and particle size distribution, respectively), comparable to other SS fingerprinting studies using artificial mixtures to evaluate un-mixing model results. Catchment scale experiments showed more variable outcomes, indicating the need for careful evaluation of the un-mixing model source apportionment results. Using absorbance; mean deviation between model results and sediment budget was 18%, though deviations were shown to reach up to 52%. Using particle size distribution; relatively low mean deviations (19%) were observed between model outcomes and sediment budget at relatively high discharge values (which were exceeded 12% of the time during the 5 month study period, during which 82% of the total SS load was transported). Overall, results presented the potential usability of both fingerprints, allowing for increasing high temporal resolution source ascription due to easy and rapid measurements that could be obtained directly from water samples.

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Identifiers

ORCID for Niels Fedde Lake: orcid.org/0000-0002-5909-2005

ORCID for Peter Shaw: orcid.org/0000-0001-9044-1069

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