Response to Concept 2, One of Many Possible Responses

It is likely that the density driven convection in a shallow brine system is generated by an unusual event, such as an infrequent, long, intense rainfall, in contrast to continuous or periodic events as might be expected in a large geothermal system. Consequently, it is challenging to capture and record an overturn event while it is in progress. In the Abu Dhabi sabkha system discussed in this book, halite and other extremely soluble salts that accumulated on the surface over 10 to 20 years were dissolved in a rainfall event lasting nearly a week, creating a dense solution on the surface with sufficient head to initiate recharge. The surface brine was denser than the aquifer brine because dissolution of minerals is not controlled by humidity, so brines created by dissolution can have a higher total solute concentration (thus higher density) than brines created by evaporation.

The most direct method of capturing a density driven convection event is by repeated geophysical measurements that can sense the change in electrical conductivity with both time and depth. That is, by establishing a network of fixed measurement locations that can be revisited after potential overturn events to measure any change from background conditions. This typically results in many uneventful recordings because an overturn event (where the surface solutes are transported to the bottom of the aquifer) may occur once per decade or less frequently. After a triggering event, several days or weeks may pass before an event reaches the stage at which it can be measured and it can disappear in a few months. The time between the triggering event and when it can be measured is dependent on many factors including hydraulic conductivity, the magnitude of the recharge event and other factors. If it is important to gain information about an event, it might be worth the significant cost of automating the system to capture readings after potential triggering events if the site is remote and difficult to access.

Owing to the difficulty of capturing an event by direct geophysical measurements, it is possible to capture the long-term chemical and isotopic effects of overturn events if the system has different oxidation-reduction conditions between its top and bottom, perhaps due to influx of a reducing underlying deep-basin brine. If oxidation-reduction conditions differ with depth, an overturn event may significantly alter isotopes and concentrations of redox-sensitive major elements such as carbon, sulfate, and nitrogen. That is, on reduction the new condition may cause them to change phase to either solid or gas and be lost as a solute. The change of phase is always accompanied by an isotopic change, and this might be detected by isotopic analysis of groundwater samples. Whereas capturing the effects of density driven convection may not be as intellectually satisfying as direct measurement, it may be sufficient to provide data consistent with a density driven convection that are difficult to explain by other processes. Furthermore, the capture of long-term chemical and isotopic effects might be useful in selecting a site for direct measurement because it is an indication that density driven convection is likely to be occurring at that location.

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A Conceptual Overview of Surface and Near Surface Brines and Evaporite Minerals Copyright © 2021 by Warren W. Wood. All Rights Reserved.