Groundwater from igneous rocks

Deep groundwaters from igneous rocks are usually saturated with respect to CaC03, which limits the maximum Ca2 -concentration and thereby also indirectly determines the F"- and H2P04"-concen-trations due to the limited solubility of CaF2 and Ca-phosphates. Both F" and H2PO4" are usually found in the groundwater due to the presence of accessory minerals like fluorite and apatite in the rock. 

Garrels and Mackenzie wanted to test whether simple evaporation of groundwater discharging from the mountains, which is the product of the reaction of rainwater and CO2 with igneous rocks, could produce the water compositions found in the saline alkaline lakes of the adjacent California desert. They began with the mean of... 

The Redox Potential. The groundwaters at great depths in igneous rocks are essentially free from dissolved oxygen ( ). The redox potential is determined and buffered by the presence of redox couples, mainly Fe(111)/Fe(11). For the reaction... 

The element is very widely distributed in Natme. It is found in igneous rocks (13 ppm), in sedimentary rocks (1 25 ppm, but concentrations as high as 400 ppm have been reported), and in virtually all soils (0.1 40 ppm). In waters, it occurs in rivers, lakes, streams, groundwaters, and in the seas and oceans. Concentrations vary greatly, from 0.1 ppb in Lake Superior to as high as 276, 000 ppb in the Waiotapu Valley rivers of New Zealand. 

Suppose one holds an igneous rock specimen in which all of the mineral grains crystallized at very nearly the same time, for which the crystallization time was short compared to the period since solidification, and which has not experienced any reheating or alteration since initial crystallization. One could then measure some time-dependent property of the specimen (e.g., the amount of " °Ar accumulated from the decay of " °K) and infer a reasonably well-defined crystallization age for the rock. Unformnately, an analogous set of conditions is quite unlikely to hold rigorously for any sample bottle full of groundwater. As previously emphasized by Davis and Bentley (1982), the dynamic nature of groundwater systems renders such simplistic scenarios unlikely. This review has emphasized that, due to the... 

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