Natural organic matter (NOM) is an important constituent of soils and surface waters for environmental and engineering processes. In aquatic systems, dissolved organic matter (DOM) exerts control on the transport and speciation of trace metals (e.g., mercury), the depth of the photo zone, and the formation and stability of particles. In soils, the organic-rich surface layer can accumulate metals. Predicting the influence of disturbances (e.g., forest fire, mining operations, changes in water use) on the cycling and bioavailability of trace metals relies on our understanding of how NOM influence these processes.

CWEST Participants: Brett Poulin, Joe Ryan

What is natural organic matter?

Dissolved Organic Matter

Dissolved organic matter (DOM) across a spectrum of quality from aliphatic (left) to aromatic (right). The quality of the DOM tells us about source materials, degree of degradation, and the role of DOM on environmental and engineering processes. Photo by Brett Poulin.

Natural organic matter (NOM) is ubiquitous in terrestrial and aquatic environments. The source of NOM is primarily plant materials and cellular components. Individual molecules of NOM may exist as the original macromolecule structure (e.g., cellulose components, proteins, amino acids, carbohydrates), or as partially degraded products of these larger macromolecules. The wide range of NOM sources and various levels of degradation generates a vast number of individual organic molecules. This continuum of molecular size and structure contribute to bulk NOM properties critical to environmental and engineered treatment processes.

Trace Metal Binding by Natural Organic Matter

Toxicity of metals in surface water

DOM-metal interactions can affect the bioavailability and toxicity of metals in surface water. Photo by Jack P. Webster.

Natural organic matter contains functional groups (e.g., carboxyl, phenol, amine, thiols) that form complexes with trace metals. In soils, these interactions are responsible for the accumulations of trace metals in organic-rich layers. In aquatic systems, the strength and abundance of metal binding sites in dissolved organic matter (DOM) can influence metal toxicity and transport. For example, numerous studies of streams and rivers observe a strong positive correlation between the concentrations of total dissolved mercury and DOM (e.g., Brigham et al., 2009: Dittman et al., 2010). For mercury and copper, the strength of the metal-DOM complex is a function of the metal-to-DOM concentration ratio (Haitzer et al., 2002; Craven et al., 2012). Quantitative models predicting metal bioavailability and toxicity to aquatic life use this information. For instance, the strength of copper-DOM complexes were used to evaluate the potential toxic effects of copper to salmon populations in the Koktuli River and Talarik Creek, which reside in the watershed that could be impacted by the proposed Pebble Mine in Alaska.

Surface Reactivity of Dissolved Organic Matter

Beyond serving as a ligand, dissolved organic matter (DOM) also exerts control on particle formation under conditions of mineral supersaturation. DOM slows particle growth rates and stabilizes particles at the nano-scale (Aiken et al., 2011). Recent studies with mercury demonstrate that nanocolloidal mercury sulfide (β-HgS) formed in the presence of DOM is bioavailable to methylating bacteria (Graham et al., 2012); this is a previously unrecognized source of bioavailable mercury. Properties of the DOM, namely the aromatic carbon content, prove as important factors controlling the surface reactivity of DOM.

NOM Bioavailability Map

In aquatic environments, DOM reacts with a continuum of dissolved metals, polynuclear clusters, nanoparticles, and colloids. These interactions influence metal bioavailability. (From Aiken et al., 2011)

Forest Fires and Mercury Cycling

One of the largest sources of mercury remobilization from terrestrial landscapes is biomass burning. In addition to atmospheric emissions, wildfire leads to soil and debris destabilization and erosion in watersheds. Eroded material can facilitate the transport of mercury associated with organic debris. Following wildfire, downstream water bodies exhibit increased production of methylmercury (Caldwell et al., 2000), a potent neurotoxin and the form of mercury that bioaccumulates in organisms and biomagnifies up aquatic food webs. Some of our current research is aimed at elucidating the role of wildfire in generating methylation “hotspots” and the potential controls on post-wildfire mercury burdens in surface water ecosystems.

Forest Fires and Mercury Recycling

Following wildfires, loss of vegetation and protective litter layers in watershed soils may lead to increases in erosion and debris transport. Debris and associated contaminants are then deposited in waterways and reservoirs. Photos by Jack P. Webster.

Mercury Cycling in the Florida Everglades

Florida Everglades

Water is delivered to Everglades National Park through a combination of sheet-flow through Water Conservation Areas and channel flow through canals. The high concentrations of dissolved organic matter in these systems, responsible for the yellow/red coloration of the water, plays an important role in ecological and biogeochemical processes. Photos by Brett A. Poulin.

Efforts to improve water quality in Everglades National Park highlight the complex biogeochemical cycle of mercury. Here, mercury is deposited from wet and dry atmospheric deposition. Microbial populations in anaerobic sediments facilitate the conversion of inorganic mercury to methylmercury. Other important factors that influence mercury cycling are the introduction of sulfate from agricultural runoff and high concentrations of DOM originating from the degradation and leaching of peat soils. Questions remain about the attenuation of sulfate in the Everglades and the consequent effects on mercury bioavailability. An improved understanding of the biogeochemical controls on mercury and sulfur cycling in the Florida Everglades will improve the management of this ecosystem.

USGS Video - Organic Carbon and the World Around Us

References

Aiken, G. R., Hsu-Kim, H., & Ryan, J. N. (2011). Influence of dissolved organic matter on the environmental fate of metals, nanoparticles, and colloids. Environmental science & technology, 45(8), 3196-3201. DOI: 10.1021/es103992s

Brigham, M. E., Wentz, D. A., Aiken, G. R., & Krabbenhoft, D. P. (2009). Mercury cycling in stream ecosystems. 1. Water column chemistry and transport. Environmental science & technology, 43(8), 2720-2725. DOI: 10.1021/es802694n

Caldwell, C. A., Canavan, C. M., & Bloom, N. S. (2000). Potential effects of forest fire and storm flow on total mercury and methylmercury in sediments of an arid-lands reservoir. Science of the total environment, 260(1), 125-133. DOI: 10.1016/S0048-9697(00)00554-4

Craven, A. M., Aiken, G. R., & Ryan, J. N. (2012). Copper (II) binding by dissolved organic matter: Importance of the copper-to-dissolved organic matter ratio and implications for the biotic ligand model. Environmental science & technology, 46(18), 9948-9955. DOI: 10.1021/es301015p

Dittman, J. A., Shanley, J. B., Driscoll, C. T., Aiken, G. R., Chalmers, A. T., Towse, J. E., & Selvendiran, P. (2010). Mercury dynamics in relation to dissolved organic carbon concentration and quality during high flow events in three northeastern US streams. Water Resources Research, 46(7). DOI: 10.1029/2009WR008351

Drexel, R. T., Haitzer, M., Ryan, J. N., Aiken, G. R., & Nagy, K. L. (2002). Mercury (II) sorption to two Florida Everglades peats: Evidence for strong and weak binding and competition by dissolved organic matter released from the peat. Environmental science & technology, 36(19), 4058-4064. DOI: 10.1021/es0114005

Gerbig, C. A., Kim, C. S., Stegemeier, J. P., Ryan, J. N., & Aiken, G. R. (2011). Formation of nanocolloidal metacinnabar in mercury-DOM-sulfide systems. Environmental science & technology, 45(21), 9180-9187. DOI: 10.1021/es201837h

Graham, A. M., Aiken, G. R., & Gilmour, C. C. (2012). Dissolved organic matter enhances microbial mercury methylation under sulfidic conditions. Environmental science & technology, 46(5), 2715-2723. DOI: 10.1021/es203658f

Haitzer, M., Aiken, G. R., & Ryan, J. N. (2002). Binding of mercury (II) to dissolved organic matter: the role of the mercury-to-DOM concentration ratio. Environmental science & technology, 36(16), 3564-3570. DOI: 10.1021/es025699i