What do aqueous geothermometers really tell us?

Abstract

The application of chemical geothermometry to shallow groundwaters or spring discharge assumes that there is minimal mixing or re-equilibration of water as it travels from depth to the surface. In this study, we examine the potential for mixing and re-equilibration by examining heat and fluid flow along crustal-scale faults in tectonic geothermal systems. Numerical modeling results indicate that maximum in situ temperatures could be underpredicted by up to 30% due to mixing of fluids that enter the fault at different depths. This, coupled with the depression of isotherms by downward groundwater flow in the hanging wall, could cause underestimates of maximum circulation depth of greater than 80% in extreme cases. Kinetics does not favor re-equilibration in the shallower portions of faults due to low temperatures and higher fluid velocities. However, in areas of deeper circulation or higher heat flow such reactions are possible.

Significance of silica in identifying the processes affecting groundwater chemistry in parts of Kali watershed, Central Ganga Plain, India

“…Silica geothermometry was applied on thermal groundwaters in the Gulf of Suez region, Egypt (El-Fiky 2009) and the results showed that the sub-surface temperatures range between 29.4 and 104.5°C. Ferguson et al (2009) studied the potential for mixing and re-equilibration of water by examining heat and fluid flow along crustal scale faults along tectonic geothermal systems and found that significant mixing with shallow groundwater will occur resulting in an under-prediction of maximum circulation temperature by 10-35 % using silica and chalcedony geothermometers. Silica was correlated with TDS and Cl - (Khan and Umar 2010) to study the effect of various geogenic and anthropogenic processes on groundwater chemistry.…”

Modeling thermal convection in supradetachment basins: example from western Norway

“…These results agree well with similar conclusions drawn by Ferguson et al . () regarding significant depression of isotherms in the hanging walls of crustal‐scale faults due to groundwater flow.…”

Effect of an 860-m thick, cold, freshwater aquifer on geothermal potential along the axis of the eastern Snake River Plain, Idaho

“…The following assumptions are made when using geothermometers: (1) the relevant hydrothermal minerals in the reservoir are in equilibrium with the geothermal liquid; (2) the pore fluid pressure in the reservoir is fixed by coexistence of liquid and steam; (3) the geothermal liquid cools, either conductively or adiabatically, through steam separation at 100 °C; (4) the geothermal liquid does not mix with cold, shallow waters during the ascent towards the surface; (5) the geothermal liquid does not precipitate any relevant minerals along the upflow path (Marini 2004). In most situations it is difficult to prove that these assumptions are met (Ferguson et al 2009).…”