Many hydrogeochemical models have appeared in recent years for simulating subsurface solute transport. The hydrological transport of solutes can be described by a set of linear partial differential equations, and the chemical equilibria are described by a set of nonlinear algebraic equations. Three approaches are currentiy used to formuiate the problem' (i) the mixed differential and algebraic equation (DAE) approach, (2) the direct substitution approach (DSA), and (3) the sequential iteration approach (SIA). An extremely important consideration in any approach is the choice of primary dependent variables (PDVs). Six types of PDVs have been employed in the existing models' (1) concentrations of all species, (2) concentrations of all component species and precipitated species, (3) total analytical concentrations of aqueous components, (4) total dissolved concentrations of aqueous components, (5) concentrations of aqueous component species, and (6) hybrid concentrations. Because of many possible combinations of PDVs and approaches, many hydrogeochemical transport models for multicomponent systems have been developed. This paper critically evaluates and discusses these models. The discussion and evaluation are conducted in terms of (1) how severe can the constraints be that a model imposes on computer resources, (2) which equilibrium geochemical processes can a model include, and (3) how easily can a model be modified to deal with mixed kinetic and equilibrium reactions. The use of SIA models leads to the fewest constraints on computer resources in terms of central processing unit (CPU) memory and CPU time' both DAE and DSA models require excessive CPU memory and CPU time for realistic two-and three-dimensional problems. Only those models that use the first three types of PDVs can treat the full complement of equilibrium reactions simultaneously. DAE and SIA models can be modified with reasonable ease to handle mixed chemical kinetics and equilibria. DSA models require strenuous efforts to modify for treating mixed chemical kinetics and equilibria. Therefore SIA models using the third type of PDVs are recommended for their practicality and flexibility. DSA and DAE models should remain research tools for one-dimensional investigations.
Immunoblastic sarcoma has been observed in association with, or subsequent to, chronic immune stimulation, connective tissue disorders, and immunoblastic lymphadenopathy. A case of Waldenström's macroglobulinemia progressinng after a few years into immunoblastic sarcoma is reported. Splenectomy led to disappearance of hemolytic anemia and of pulmonary infiltrates, as well as to marked reduction of macroglobulins. Although immunoblastic sarcoma usually terminates fatally within two or three months, complete remission was induced by combination chemotherapy. It is speculated that Waldenström's macroglobulinemia, immunoblastic lymphadenopathy, and immunoblastic sarcoma are related disorders reflecting a clonal nature of immunoblastic lymphoma cells and the plasmacytic cells.
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