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

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 (> 100 nm) of soils, but in the fine colloidal (< 100 nm) and nominally dissolved (< 1 nm) fractions of dripwaters. The concentration of Cu, Ni and Co in dripwater samples across all sites were well correlated (R-2 = 0.84 and 0.70, Cu vs. Ni, Cu vs. Co, respectively), indicating a common association. Furthermore, metal ratios (Cu:Ni, Cu:Co) were consistent with NICA-Donnan n(1) humic binding affinity ratios for these metals, consistent with a competitive hierarchy of binding affinity (Cu > Ni > 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 (> 1 mu n) and small colloids (1-100 nm) 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. 100 nm) were offset from infiltration events, probably because small organic colloids (1-100 nm) and solutes (< 1 nm) were slower to migate through the porous matrix than particulates. These results demonstrate the widespread occurrence of both colloidal and particulate NOM-metal transport in cave dripwaters and the importance of karst hydrology in affecting the breakthrough times of different species. Constraints imposed by soil processes (colloid/particle release), direct contributions of metals and NOM from rainfall, and flow-routing (colloid/particle migration) are expected to determine the strength of correlations between NOM-transported metals in speleothems and climatic signals. Changes in trace metal ratios (e.g. Cu:Ni) in speleothems may encode information on NOM composition, potentially aiding in targeting of compound-specific investigations and for the assessment of changes in the quality of soil organic matter. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.