A mass Budget for Mercury in the Willamette River Basin, Oregon, USA

In Oregon’s Willamette River Basin (Basin), health advisories currently limit consumption of fish that have accumulated methylmercury. Under the Clean Water Act, these advisories create the requirement for a mercury Total Maximum Daily Load (TMDL), which, in turn, created the need to better characterize the cycling of mercury in the Basin. This paper presents: (a) a mass balance model relating the processes, reservoirs, and fluxes of the Basin’s mercury cycle, (b) first-approximation estimates of reservoir masses, flux rates, and turnover times, and (c) an assessment of the impact of anthropogenic mercury sources on this cycle. Deposition from the atmosphere to land is estimated to be the largest (587 kg yr−1) single flux; with much (≈54%) of this deposited mercury returned to the atmosphere via volatilization. Combined local anthropogenic (360 kg yr−1) and global emissions (390 kg yr−1) substantially overshadow all other anthropogenic point-source inputs. Runoff of mercury from native soils (particulate and dissolved phases) was estimated as the largest (≈70%) single source of Hg to surface water; contributions from anthropogenic air emissions deposited on land and then transported as runoff were ≈17%. All other currently identified and quantifiable non-atmospheric anthropogenic point sources made a small (≈5%) contribution to total loadings. These first-approximation estimates suggest that management of soil erosion should have a higher priority than reductions in local anthropogenic air emissions, with the caveat, however, that the degree of linkage between any such reductions and that of methylmercury levels in fish is presently unclear. Additional work will be required to develop the better parameterized and calibrated model needed to predict the outcome of Hg loading reduction alternatives under consideration within the TMDL process.