The process of evaporation, including transpiration (evaporation from plants), returns to the atmosphere more than half of the water reaching the Earth's land surface; thus, it plays an important role in controlling the chemistry of surface water and groundwater, especially in relatively arid climates. Geochemists study the evaporation process to understand the evolution of water in desert playas and lakes as well as the origins of evaporite deposits. They also investigate environmental aspects of evaporation (e.g., Appelo and Postma, 1993), such as its effects on the chemistry of rainfall and, in areas where crops are irrigated, the quality of groundwater and runoff.

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation.

Springs and saline lakes of the Sierra Nevada

We choose as a first example the evaporation of spring water from the Sierra Nevada mountains of California and Nevada, USA, as modeled by Garrels and Mackenzie (1967). Their hand calculation, the first reaction path traced in geochemistry (see Chapter 1), provided the inspiration for Helgeson's (1968 and later) development of computerized methods for reaction modeling.