Meteoric waterline

This equation, known as the global meteoric waterline (GMWL), is based on precipitation data from locations around the globe. The slope and intercept of the local meteoric waterline (LMWL) for rain from a specific catchment or basin can be different from the GMWL. The deuterium excess (d excess, or d) parameter has been defined to describe these different meteoric waterlines (MWLs), such that... 

Highly saline water of distinctive isotopic composition is often found in environments of such depth and low permeability that flow rates must be extremely low or zero. These waters are often characterized by Ca " -Cl compositions and stable isotope composition above the meteoric waterline (see Chapter 5.17). They apparently result from water-rock equihbration over very long periods of time. Geochemically, they have little influence on waters in active circulation systems, but mobile isotopes of the noble gases diffusing upward from this environment can be a powerful tool for understanding the flow systems into which they move. The noble-gas isotopes can also provide clues to the histories of these nearly static waters. 

The most noticeable isotopic difference between saline waters from crystalline rocks and sedimentary formation waters is their position above the meteoric waterline. This is postulated to be due to mineral hydration reactions in a very water-depleted environment (Fritz and Frape, 1982). Several recent smdies have suggested that hydration reactions in low water to rock environments can occur and result in increasing salinity. The incorporation of OH into primary silicate such as amphiboles and phyUosUicates (where OH crystal lattice sites are part of the mineral structure) is suggested as one mechanism for controlling solute concentration (KuUemd, 2000). The formation of secondary OH containing mineral phases such as zeolites and clays can also continue to consume water molecules and concentrate the residual fluids both chemically... 

Figure 19 6 H versus 6 0 aKr values of reverse osmosis desalted seawater and the global meteoric waterline (Craig, 1961). The anthropogenic line lies on a possible mixing relationship between natural waters along the meteoric waterline (arrow) and the desalted seawater. Future formation of a large volume of desalted seawater expects to cause infiltration (e.g., leakage, reuse) of freshwater with high S H and 6 0 values relative to natural replenishment.

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