4.2 Chemically Open Leaky Systems

If a system is completely open to solutes, there will be no accumulation of solutes or evaporite minerals. In many systems some, but not all groundwater solutes leak from the system so that it is partly open or “leaky.” Surprisingly, the leakage ratio (mass out/mass in) is in most cases more important in solute/evaporite evolution than the input solute ratios (Wood and Sanford, 1990; Sanford and Wood, 1991). As a result, it is possible to develop a large thickness of one or two minerals instead of the many minerals observed in a totally closed system. In a steady-state water flux condition, a leaky system permits a certain solute flux as the evolved solution escapes. This is referred to as the leaky ratio (Q_o/Q_i) where Q_o is total outflow flux and Q_i is total inflow flux (Figure 11).

Figure 11 – Conceptual mass-balance model for a steady state leaky basin where the leakage ratio (Q_o/Q_i) has a significant control on the solute chemistry and the mineral development in the basin (Wood and Sanford, 1990).

The solute loss from a leaky system has profound control over the suite and thicknesses of evaporite minerals formed in the basin and the remaining solutes. For a conservative constituent (i.e., one that does not react), the relationship between leakage ratio (Q_o/Q_i) and concentration ratios (C_o/C_i) for different basin volumes evaporated (where volumes evaporated essentially represent time) is illustrated in Figure 12. The concentration ratio C_o/C_i is the concentration of output solutes to the concentration of input solutes. As shown in Figure 12, the C_o/C_i ratio reaches a constant maximum at different numbers of basin volumes for different leakage ratios Q_o/Q_i. Importantly, after the C_o/C_i becomes constant, the minerals generated are constant and this results in large deposits of a few minerals.

Figure 12 – The steady state C_o/C_i ratio for a conservative constituent as a function of the number of evaporated basin volumes t_o for leakage ratios (Q_o/Q_i) of 10, 30,100, and 300. C_o is solute concentration of the groundwater outflow and C_i is the input concentration. (modified from Sandford and Wood, 1991).

The impact of leakage ratio is illustrated by the difference between Figure 13a and Figure 13b which have the same input solute chemistry but different leakage ratios. In Figure 13a, the leakage ratio is 0.001 indicating it leaks more than the system shown in Figure 13b with a leakage ratio of 0.0001. Systems with a smaller the leakage ratio have mineral development that is more similar to a closed basin.

Figure 13 – Comparison of simulated minerology and thickness of deposits in systems with the same input water but different leakage ratios. a) An example of the mineralogy and thickness associated with a leakage ratio of 0.001. b) The different mineralogy and thickness developed in a system with less leakage, represented by a ratio of 0.0001. The water input to both systems has the same Southern High Plains groundwater solute chemistry. The solute changes follow a similar pattern, but the lower leakage ratio results in formation of layers comprised of minerals that form later in the precipitation process (from Wood and Sanford, 1990).

In the more open system (Figure 13a) with a leakage ratio of 0.001, only three minerals develop. In the less leaky basin (Figure 13b) with a leakage ratio of 0.0001, eight minerals form including halite, hexahydrite, and polyhalite, which are not present in the leakier system. The smaller the leakage ratio, the closer the system is to being a closed basin and the more similar the mineral development is to minerals formed in a closed system. In contrast, a very leaky basin might form only low solubility calcite or gypsum.

Solute evolution as a function of the number of basin volumes evaporated (which is a measure of the length of evolution time) with the input water chemistry (that of sea water) is illustrated in Figure 14 for two different leakage ratios, 0.001 and 0.01.

Figure 14 – Graphs showing the difference in solute evolution resulting from different leakage ratios: a) solute evolution as driven by evaporation from a basin with less leakage, Q_o/Q_i = 0.001 as compared to a system with more leakage as shown in b) where Q_o/Q_i = 0.01. Modified from Sanford and Wood (1991).

Inclusion of a leaky system conceptual model, in contrast to only an open or closed system, resolves many of the problems encountered when using a closed system model to explain mineral development observed in field settings.