Big Dam West Lake Mercury Mass Balance

BIG DAM WEST LAKE MERCURY MASS BALANCE 6.1. Introduction 

While several researchers have attempted mass balances for mercury in rivers and lakes [74,75,93,94], none have attempted to collect quantitative mercury measurements with a whole-ecosystem approach. The purpose of the study was to examine a complete set of mercury fluxes in the BDW Lake basin. A detailed explanation of the mass balance model, mass flux equations and the calculation of error is available in other publications [86,92]. 

Our conceptual model divided the BDW lake basin into three land cover types (based on remote sensing data) (i) terrestrial, (ii) wetland, and (iii) lake (Fig. 3). Average annual mass movements for total mercury were calculated using the collected data [86]. Wet precipitation was the only source of mercury inputs considered to the terrestrial system accounting for 184 g ( r = 20.2 g) of mercury deposited over land. The total outputs from the terrestrial system accounted for 372 g (o- = 36.7 g), of that, 35% (132 g, tr = 0.0 g) was incorporated into vegetation and, 13% (49 g, o-=0.0g) was volatilized from the soil surface. Although we were unable to measure mercury runoff directly, 191 g would be necessary in order to balance the inputs and outputs of the wetland component. 

In comparison to our findings, Henry et al. [94] performed a mass balance on Onondaga Lake, NY, and found that annual total mercury inputs accounted for 14.116 kg. Of the total inputs, 96.3% (13.6 kg) was due to terrestrial inflows, 3.1% (0.44 kg) was atmospheric deposition, 0.4% (0.056 kg) was sediment flux, and 0.1 % (0.02 kg) was groundwater. Of the 13.916 kg of outputs, sedimentation accounted for 79.8% (11.1 kg), outflow 20.1% (2.8 kg), and volatilization 0.1% (0.016 kg). These results compare well with what was observed in this study with inflows contributing the majority of the total mercury, followed by 

The area within the BDW drainage basin is largely dominated by the terrestrial ecosystem. Of the 31.18 km in the BDW watershed, 88.7% is terrestrial, 5.9% is wetland, and 5.5% is lake surface. The 207 g of mercury deposited in precipitation follows a similar distribution. The terrestrial catchment dominates the mass movement of total mercury in the BDW watershed. This mercury is then available for methylation as it moves through the wetland portions of the catchment. O Driscoll et al. [92] reported the highest % of MeHg (30-40% of the total Hg) occurring in the wetland areas of this site. 

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