It is increasingly becoming known that mercury transport and speciation in the terrestrial environment play major roles in methyl-mercury bioaccumulation potential in surface water. This review discusses the principal biogeochemical reactions affecting the transport and speciation of mercury in the terrestrial watershed. The issues presented are mercury-ligand formation, mercury adsorption/desorption, and elemental mercury reduction and volatilization. In terrestrial environments, OH-, Cl- and S- ions have the largest influence on ligand formation. Under oxidized surface soil conditions Hg(OH)2, HgCl2, HgOH+, HgS, and Hg0 are the predominant inorganic mercury forms. In reduced environments, common mercury forms are HgSH+, HgOHSH, and HgClSH. Many of these mercury forms are further bound to organic and inorganic ligands. Mercury adsorption to mineral and organic surfaces is mainly dictated by two factors: pH and dissolved ions. An increase in Cl- concentration and a decrease in pH can, together or separately, decrease mercury adsorption. Clay and organic soils have the highest capability of adsorbing mercury. Important parameters that increase abiotic inorganic mercury reduction are availability of electron donors, low redox potential, and sunlight intensity. Primary factors that increase volatilization are soil permeability and temperature. A decrease in mercury adsorption and an increase in soil moisture will also increase volatilization. The effect of climate on biogeochemical reactions in the terrestrial watershed indicates mercury speciation and transport to receiving water will vary on a regional basis.

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Many complex models that utilize continuous simulation (SWMM, HSPF, SLAMM, SIMPTM, etc.) require information pertaining to the accumulation rate of pollutants on the land surfaces. This is one of the most perplexing issues in stormwater modeling. A representation of the accumulation rates is usually obtained through trial and error during calibration, with little, if any, actual direct measurements. Historically, direct measurements have been misapplied in modeling applications, resulting in unreasonable model predictions. Many modelers therefore forego accumulation rate data, preferring to back into values from outfall observations. This approach makes it very difficult to correctly predict the sources of stormwater pollutants in urban areas and to make reasonable stormwater management decisions using source area controls. This dilemma has come about due to a major misinterpretation of previously collected field data: the assumption that street dirt loadings are zero after most rains. With the correct understanding and modeling of the washoff process, the vast amount of historically collected accumulation data becomes an important modeling resource. This Chapter presents a summary of this useful information. This information has been used in Pitt and Voorhees' Source Loading and Management Model (SLAMM) and variations have been used in Sutherland's Simple Particulate Transport Model (SIMPTM) to more accurately predict these important source area processes. Relatively simple modifications can be made to other continuous models that utilize accumulation and washoff functions for more accurate and complete stormwater control predictions.