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A long-term study of stable isotopes as tracers of processes governing water flow and quality in a lowland river basin: the upper Thames, UK

Darling, W. George; Bowes, Michael J.. 2016 A long-term study of stable isotopes as tracers of processes governing water flow and quality in a lowland river basin: the upper Thames, UK. Hydrological Processes, 30 (13). 2178-2195. 10.1002/hyp.10779

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Abstract/Summary

A long-term study of O, H and C stable isotopes has been undertaken on river waters across the 7000 km2 upper Thames lowland river basin in the southern UK. During the period, flow conditions ranged from drought to flood. A 10-year monthly record (2003–2012) of the main River Thames showed a maximum variation of 3‰ (δ18O) and 20‰ (δ2H), though inter-annual average values varied little around a mean of –6.5‰ (δ18O) and –44‰ (δ2H). The δ2H/δ18O slope of 5.3 suggested a degree of evaporative enrichment, consistent with derivation from local rainfall with a weighted mean of –7.2‰ (δ18O) and –48‰ (δ2H) for the period. A tendency towards isotopic depletion of the river with increasing flowrate was noted, but at very high flows (>100 m3/s) a reversion to the mean was interpreted as the displacement of bank storage by rising groundwater levels (corroborated by measurements of specific electrical conductivity). A shorter quarterly study (October 2011 – April 2013) of isotope variations in 15 tributaries with varying geology revealed different responses to evaporation, with a high inverse correlation between Δ18O and baseflow index (BFI) for most of the rivers. A comparison with aquifer waters in the basin showed that even at low flow, rivers rarely consist solely of isotopically unmodified groundwater. Long-term monitoring (2003–2007) of carbon stable isotopes in dissolved inorganic carbon (DIC) in the Thames revealed a complex interplay between respiration, photosynthesis and evasion, but with a mean inter-annual δ13C-DIC value of –14.8 ± 0.5‰, exchange with atmospheric carbon could be ruled out. Quarterly monitoring of the tributaries (October 2011 – April 2013) indicated that in addition to the above factors, river flow variations and catchment characteristics were likely to affect δ13C-DIC. Comparison with basin groundwaters of different alkalinity and δ13C-DIC values showed that the origin of river baseflow is usually obscured. The findings show how long-term monitoring of environmental tracers can help to improve the understanding of how lowland river catchments function.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1002/hyp.10779
CEH Sections: Rees (from October 2014
ISSN: 0885-6087
Additional Keywords: GroundwaterBGS, groundwater, stable isotopes, baseflow index, dissolved inorganic carbon, rainfall, river response, specific electrical conductivity
NORA Subject Terms: Hydrology
Date made live: 04 Jul 2016 12:34 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/513900

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