AVAILABILITY OF ATMOSPHERICALLY DEPOSITED MERCURY TO RUNOFF AND RECEIVING WATERS
Abstract:Current practice in estimating mercury runoff from the watershed in TMDL analysis assumes much of the atmospherically deposited mercury is available for transport to receiving waters; such estimates are overly conservative, and do not reflect the complex nature of mercury surface reactions. The time-dependence of mercury washoff is critical in assessing the variable availability of mercury to the receiving waters. This review illustrates the importance of the relationship between mercury deposition and runoff efficiency for common watershed surfaces. With such fundamental information, better quantitative estimates of mercury loading to marine/freshwater bodies, design of runoff control practices, and land use planning can be developed.
The recent emphasis on mercury depositional impacts on water quality has taken a number of forms including: generating of mercury TMDLs, strengthening a variety of Clean Air Act mercury source regulations, and developing relatively extensive depositional monitoring and modeling programs. However, there has been very little research examining the role of the terrestrial watershed in terms of determining the availability of atmospherically deposited mercury to receiving waters. More specifically, how the biogeochemical characteristics of terrestrial surfaces and dry/wet weather events effect the temporal variation of mercury concentration and chemical form in surface runoff. On the contrary, most TMDLs make the assumption that all mercury deposited on the land surface will be transported through the watershed and into receiving waters. Mercury is deposited onto the watershed through both wet and dry deposition processes and as both reactive gaseous mercury and particulate mercury. To assume uniform availability of all forms of deposited mercury ignores photochemical and biogeochemical processes that will impact mercury fate in the watershed. Transformation processes occurring within the watershed will certainly modify the temporal patterns between mercury deposition onto the watershed and mercury loading into the receiving water. A mass balance approach that accounts for terrestrial transformation processes to determine how much deposited mercury actually washes off into surface water is needed.
Characterization of stormwater runoff has indicated that even hard surfaces (such as pavements and concrete), may retain a significant fraction of heavy metals and prevent transport in runoff (Pitt, 1987). This lack of availability is especially true for particulate bound toxicants. As vegetated or permeable surfaces are considered, it is possible for the watershed to act as a permanent sink for a portion of the deposited mercury.
The purpose of this review is to illustrate that the availability of atmospherically deposited mercury is a function of watershed characteristics including: terrestrial sorption properties, surface water chemistry, rainfall intensity, antecedent dry weather periods, and photochemical reactions. Using this information and applying it to the interaction of surface types (i.e., paving and vegetation) with deposited mercury during wet/dry weather cycles will allow an estimate of mercury runoff from land cover to receiving waters in a multi-use watershed.
Document Type: Research Article
Publication date: 2002-01-01
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