6 Fluid Dynamics of Saline Systems
Natural density–driven free convection is important in meteorology and oceanography and has been studied for over a century in theoretical and laboratory settings. However only recently has density-driven free convection been identified as an active process in groundwater hydrogeology (Van Dam et al., 2009; Van Dam et al., 2014; Stevens et al., 2009). Studies have shown the critical importance of this process for contaminant transport (Mao et al., 2006; Liu and Dane, 1996; Zhang and Schwartz, 1995; Koch and Zhang, 1992; Schincariol and Schwartz, 1990), hydrothermal ore deposition (Coumou et al., 2008; Garven et al., 1999), carbon sequestration (Riaz et al., 2006), nuclear waste disposal (Yang and Edwards, 2000), and brine reflux beneath saline lakes (Holzbecker, 2005; Wooding et al., 1997; Rogers and Dreiss, 1995).
Electrical resistivity tomography (ERT) was used to document the density-driven free convection of convective fingers following a significant rainfall event on a sabkha 60 km northwest of the city of Abu Dhabi, United Arab Emirates. The convective fingers are visible in Figure 20a (Van Dam et al., 2009). In this sabkha system, free convection occurs when high-density surface water, generated by dissolution of surface salt by rainfall, recharges the aquifer. This observation is supported by analysis of the same system a year after the rain event. It revealed that the convection fingers had dissipated leading to the decay of the convective instabilities with time (Figure 20b).
Figure 20 – Density driven free convection in the United Arab Emirates sabkha illustrated by inverted resistivity images from dipole-dipole ERT (electrical resistivity tomography) surveys. a) Rainwater dissolved salts on the surface creating a dense solution that recharged the sabkha aquifer as revealed by fingering of dense salt water which is indicated by high (red) conductivity (S/m, Siemens per meter) observed along three different compass azimuths shown as profiles a-a’, b-b’, and c-c’ (modified from Van Dam et al., 2009). b) Using the same electrode position, resistivity meter, and analysis method; data collected a year later on profile a-a’ revealed that the unstable convective flow had dissipated such that the high conductivity fingering had largely disappeared along profile a-a’ (modified from Van Dam et al., 2014).
This interpretation of the dissipation of the convective fingers is consistent with the ubiquitous presence of tritium throughout the aquifer, additional analysis of the aquifer halite budget, and hydrodynamic modeling of the transient character of the fingering instabilities (Van Dam et al., 2014). Numerical modeling results show that the transient dynamics of the gravitational instabilities (their initial development, infiltration into the underlying lower-density groundwater, and subsequent decay) agree with the timing observed in time-lapse ERT measurements. Experimental observations and modeling results are consistent with the hypothesis that density-driven free convection is occurring in this system (Van Dam et al., 2014). This is further supported by evaluation of the isotopes of nitrogen (Wood and Böhlke, 2017).