Structure of Geochemical Models

Zero-dimensional geochemical models are concerned with the contents of solutions of an aquatic subsystem (e.g. pore water, ocean water, precipitation, groundwater), the equilibria between the various dissolved species as well as their adjoining gaseous and solid phases. A computer model by the name of WATEQ (derived from water-equilibria) was published for the first time by Truesdell and Jones (1974). Unfortunately, this was done in the computer language PLl which has 

All these programs share the common feature that they start out from water analyses which were designed as comprehensive as possible, along with conceiving the measured values as cumulative parameters (sums of all aquatic species), only to subsequently allocate them to the various aquatic species. This was done by applying iteration calculus that accounts for the equilibria related to the various aquatic species. 

For instance, the measured value of calcium in an ocean water sample containing 10.6 mmoles/1 is allocated to 9.51 mmol/1 Ca, 1.08 mmol/1 CaSO , 0.04 mmol/1 CaHCOC, and to a series of low 

Based on the activities of the non-complex aquatic species (e.g. [Ca +] or [SO/ ], the next step consists in calculating the saturation indices (SI) of each compound mineral  

In this equation lAP denotes the Ion Activity Product (in the example of gypsum or anhydrite these would be ([Ca ] [SO/ ]). Kj,p is the solubility product constant of the respective mineral. A saturation index SI = 0 describes the condition in which the solution of the corresponding mineral is just saturated, SI 0 describes the condition of supersaturation of the solution, SI 0 its undersaturation. The activity [A] of a substance is calculated according to the equation  

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