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This keyword data block is used to modify the definition of a previously defined surface-assemblage composition. New surfaces and surface-site types may be added, and the quantity of each surface-site type may be changed. In addition, it is possible to change the elemental composition of the surface and the elemental composition in the diffuse layer of the surface. The format of the data block is the same as the SURFACE_RAW data block except that the data block need not be complete. The SURFACE_MODIFY data block can be used selectively to change data items. This Example data block lists a subset of identifiers that can be used in SURFACE_MODIFY data blocks.
-only_counter_ions --Excludes co-ions in diffuse layer. See Line 7 in Explanation 1 of the SURFACE data block.
-thickness --Thickness of diffuse layer. See -Donnan and -diffuse_layer in the SURFACE data block. Optionally, thickness or -t [ hickness ].
-debye_lengths --A factor for the Debye length is defined to determine the thickness of the diffuse layer. See -Donnan in the SURFACE data block. Optionally, debye_lengths or -de [ bye_lengths ].
-DDL_viscosity --When considering multicomponent diffusion in the diffuse layer of surfaces, fraction is the viscosity in the diffuse layer relative to the viscosity in the free pore space. See -Donnan in the SURFACE data block. Optionally, DDL_viscosity or -DD [ L_viscosity ].
-DDL_limit --If debye_lengths are used to define the thickness of the diffuse layer, limit is the maximum fraction of the total water that can be in the diffuse layer. See -Donnan in the SURFACE data block. Optionally, DDL_limit or -DDL_l [ imit ].
-component --Identifier that indicates that information for a surface site will be defined. The identifier -component is required to precede the other identifiers that define information for the surface site. Optionally, component or -c [ omponent ].
-Dw --Diffusion coefficient for the surface site, which is used only if -multi_D is true in a TRANSPORT simulation. Optionally, Dw or -D [ w ].
-charge_component --Identifier that indicates that information on surface charge for a surface will be defined. The identifier -charge_component is required to precede the other identifiers that define information for the surface charge. Optionally, charge_component or -ch [ arge_component ].
-specific_area --Specific area for the surface. See the SURFACE data block. Optionally, specific_area or -s [ specific_area ].
-grams --Mass of the material containing the surface. See the SURFACE data block. Optionally, grams or -g [ rams ].
-charge_balance --Charge of the surface, including the charge in the diffuse layer. If -Donnan or -diffuse_layer are defined for the SURFACE, then the charge balance for the surface will be near zero, such that the charge in the diffuse layer will be equal to the sum of the charge for all species for all surface sites for that surface. Optionally, charge_balance or -c [ harge_balance ].
-capacitance0 --Capacitance for the 0-1 planes in the CD-MUSIC formulation. See -capacitances in the SURFACE data block. Optionally, capacitance0 or -ca [ pacitance0 ].
-capacitance1 --Capacitance for the 1-2 planes in the CD-MUSIC formulation. See -capacitances in the SURFACE data block. Optionally, capacitance1 or -capacitance1 .
-diffuse_layer_totals --Identifier begins a block of data containing the moles of elements in the diffuse layer for a surface. Values are present only if -Donnan or -diffuse_layer are defined for the surface. Optionally, diffuse_layer_totals or -d [ iffuse_layer_totals ].
The SURFACE_MODIFY data block allows modification of a preexisting surface assemblage, but care is needed for any modifications because data items have interdependencies. Although it is possible to add a new surface or surface site to a surface assemblage, defining consistent data for the new item is difficult. It is important that the -totals data block is consistent with the -charge_balance for surface components and that the -diffuse_layer_totals data block is consistent with the -charge_balance for surface charge components. If charge balance is not preserved, unexpected pH and redox conditions will result.
One use of the SURFACE_MODIFY data block might be in transporting surfaces, for example in conjunction with a sediment transport model. Another use might be for diffusive transport in the diffuse layer of surfaces. These transport calculations need to transport the charge [component -charge_balance and (or) charge-component -charge_balance ], elements [component -totals and (or) charge-component -diffuse_layer_totals ], and possibly other quantities, such as -grams and -specific area . SURFACE_MODIFY data blocks could be used to update surface-assemblage definitions after the transport calculations.
A SURFACE_MODIFY data block modifies only the data items specifically defined in the data block. Any data items in the surface assemblage definition not modified by the data block remain unchanged. Note specifically, that the -totals data block modifies all elements included in the data block, but the number of moles of any element related to the surface site that is not listed in the data block will remain unchanged.
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