From soil to cave: Transport of trace metals by natural organic matter in karst dripwaters

• Published in: Chemical geology 2012-04, Vol.304-305, p.68-82
• Main Authors: Hartland, Adam, Fairchild, Ian J., Lead, Jamie R., Borsato, Andrea, Baker, Andy, Frisia, Silvia, Baalousha, Mohammed
• Format: Article
• Language: English
• Subjects: binding capacity; binding sites; carbon; cobalt; Colloids; Complexation; copper; Hydrology; hydrophobicity; iron; karsts; leachates; manganese; Nanoparticles; nickel; organic matter; particulates; rain; soil; Soil organic matter; soil quality; solutes; Speleothems; zinc
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2012.01.032

This paper aims to establish evidence for the widespread existence of metal binding and transport by natural organic matter (NOM) in karst dripwaters, the imprint of which in speleothems may have important climatic significance. We studied the concentration of trace metals and organic carbon (OC) in sequentially filtered dripwaters and soil leachates from three contrasting sites: Poole's Cavern (Derbyshire, UK), Lower Balls Green Mine (Gloucestershire, UK) and Grotta di Ernesto (Trentino, Italy). The size-distribution of metals in the three soils was highly similar, but distinct from that found in fractionated dripwaters: surface-reactive metals were concentrated in the coarse fraction (>100nm) of soils, but in the fine colloidal (Co) for sites in colloidal or dissolved NOM. Large shifts in Cu:Ni in dripwaters coincided with high fluxes of particulate OC (following peak infiltration) and showed increased similarity to ratios in soils, diagnostic of qualitative changes in NOM supply (i.e. fresh inputs of more aromatic/hydrophobic soil organic matter (SOM) with Cu outcompeting Ni for suitable binding sites). Results indicate that at high-flows (i.e. where fracture-fed flow dominates) particulates and colloids migrate at similar rates, whereas, in slow seepage-flow dripwaters, particulates (>1μm) and small colloids (1–100nm) decouple, resulting in two distinct modes of NOM–metal transport: high-flux and low-flux. At the hyperalkaline drip site PE1 (in Poole's Cavern), high-fluxes of metals (Cu, Ni, Zn, Ti, Mn, Fe) and particulate NOM occurred in rapid, short-lived pulses following peak infiltration events, whereas low-fluxes of metals (Co and V>Cu, Ni and Ti) and fluorescent NOM (< ca. 100nm) were offset from infiltration events, probably because small organic colloids (1–100nm) and solutes (