MATERIALS AND METHODS

Techniques and procedures for water sample collection and analysis have been described elsewhere (Reddy and others, 1996). Mercury analyses were from Hurley and others (1996). Constants used to calculate the distribution of mercury species have been determined experimentally and can be found in various tables of stability constants. Representative values given below are taken from Dyrssen and Wedborg (1991).

Table 1. Mercury ion pair formation reactions

Equation	Dryssen & Wedborg	Log K - WHAM	PHREEQC
Hg2+ + 2Cl- = HgCl2	13.22	13.98	13.21
Hg2+ + Cl- = HgCl+	6.74	7.21	6.68
Hg2+ + 2OH- = Hg(OH)2	22.23	21.83	21.90
Hg2+ + OH- = HgOH+	10.67	10.6	10.6

WHAM considers the concentration of DOM to be approximately twice the DOC. Further, it is assumed that 50% of the DOM is active "fulvic acid". In effect WHAM uses the DOC concentration as equal to the fulvic acid concentration. This estimate of 50% active fulvic acid in DOM is the midpoint in a range of measured factors (35% to 65%). Calculation of mercury species distribution in the absence of DOC was done using WHAM (Tipping 1994) and PHREEQC (Parkhurst 1995) (Figure 1). Speciation calculations for solutions containing DOC were done using WHAM (Figure 2).