Watersheds mass balance studies

Accurately estimating the total amount of acid deposited on a receptor, such as a lake, or a forest, is problematic because acids are deposited by difficult-to-quantify dry processes as well as wet processes. The wet deposition of both sulfuric and nitric acids is believed to account for only about half of the total deposition of these acids on surface waters, soils, and vegetation. Data from whole watershed mass balance studies (e.g., Table 4-13) support the hypothesis that total deposition of sulfate considerably exceeds what is measured in the form of wet deposition alone. A significant amount of H2S04 is deposited as sulfate aerosols, such as ammonium sulfate ](NH4)2S04]. The direct absorption of S03, followed by oxidation of S03 to H2S04 at the absorbing surface, is another deposition mechanism. Dry deposition of nitric acid includes sorption of nitric acid vapor onto surfaces, as well as deposition... 

Geochemical mass balance studies (also known as input-output budgets) invoke a simple conservation-of-mass principle. If the flux of any element leaving a watershed (e.g., via streams), and the flux of that element into the watershed (e.g., via atmospheric precipitation) are known, the difference between the two can be calculated, and this difference must be due to the sum of all reactions and transformations involving that element which took place within the watershed. Pioneering mass balance studies on weathering profiles and/or small watersheds include those of Garrels and Mackenzie (,51, 52) and Cleaves and Bricker and their... 

Rates estimated in the above studies are shown in Table I. Watershed-scale geochemical mass balance studies yield calculated feldspar weathering rates one to three orders of magnitude slower than rates determined in laboratory experiments. 

The primary objectives of mass-balance studies are (i) quantify the mass fluxes into and out of watershed systems (ii) interpret the reactions and processes occurring in the watershed that cause the observed changes in composition and flux (iii) determine weathering rates of the various minerals constituting the bedrock, regolith, and soils of the watershed and (iv) evaluate which mineral phases are critically involved in controlling water chemistry to help develop models of more general applicability (i.e., transfer value). 

Yuretich R. F. and Batchelder G. L. (1988) Hydrogeochemical cycling and chemical denudation in the Fort River watershed, central Massachussets an appraisal of mass-balance studies. Water Resour. Res. 24, 105-114. 

The input of airborne lead to the Forest ecosystems has been studied at the Hubbard Brook Experimental Forest in New Hampshire. The small catchment approach has been used to study the lead biogeochemical cycle since 1963 (Likens et al., 1977 Driscoll et al., 1994). By monitoring precipitation inputs and stream output from small watersheds that are essentially free of deep seepage, it is possible to constmct accurate lead mass balance. The detailed study of soil and soil solution chemistry and forest floor and vegetation dynamics supplemented the deposition monitoring. 

A more powerful and flexible method, and one that is widely used in watershed weathering studies, relates changes in the chemistry of input-output waters to reactions between the waters and solid phases in the system. This approach, termed chemical mass balance (method (iii)), is considered the best way of making quantitative estimates of rates of elemental transfers in the Earth s surface environment (Clayton, 1979). In method (iii), the hydrochemical method developed by Garrels and Mackenzie (1967), mineralogy and composition of the primary and secondary phases must be well characterized. The composition and volume of the initial water and the final water must be... 

The export of solutes released by weathering from watersheds exhibits a strong relationship to rock type (Bricker and Rice, 1989). The same relationship had been demonstrated by Velbel (1985) in a study of weathering at the Coweeta Hydrologic Laboratory in North Carolina. He employed the mass-balance formalization of Plummer and Back (1980) to determine the mass flux from the watersheds, to interpret the mineral... 

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