Final answer:
The pH of subsurface glacial till beneath playas in Nevada increases with depth due to the interaction between atmospheric CO2 and groundwater. This leads to the dissolution of limestone and the release of calcium ions and carbonate ions, resulting in an increase in pH. The formation of caves and the process of evaporation and precipitation also contribute to the increase in pH.
Step-by-step explanation:
The increase in pH of subsurface glacial till beneath playas in Nevada with depth can be attributed to various factors. One major factor is the interaction between atmospheric CO2 and groundwater. When atmospheric CO2 dissolves in the groundwater, it forms a weak acidic solution. This solution then reacts with the glacial till, which contains minerals like limestone (CaCO3). The acidic solution dissolves the limestone, releasing calcium ions (Ca2+) and carbonate ions (CO3^2-), which results in an increase in pH.
Additionally, the formation of caves in the subsurface can also contribute to the increase in pH. As groundwater containing CO2 enters microscopic cracks in limestone deposits, the CaCO3 dissolves, resulting in the formation of caves. Water from the surface seeps into these caves and evaporates, causing CaCO3 to precipitate and form stalactites and stalagmites. The process of evaporation and precipitation shifts the equilibrium towards the deposition of CaCO3, leading to an increase in pH.
It is important to note that the specific geological composition and hydrological conditions in Nevada play a crucial role in determining the pH changes in subsurface glacial till. The presence of CO2-rich groundwater, the type of minerals in the glacial till, and the overall flow dynamics of water through the subsurface all contribute to the observed increase in pH with depth.