Is brackish water present in all aquifers? How does it affect groundwater?

We see groundwater as a water reservoir inside the earth crust. It is defined that the top of the groundwater reservoir is called water table, but we rarely ever ask ourselves how deep groundwater is or about its quality. A complete vision of the hydrogeological system requires the understanding of the current state of groundwater quality and quantity.

Recent studies show the presence of brackish water in groundwater systems in almost the whole territory of the United States [1]. We do not have that kind of information about South American aquifers, but we could infer that there are also sceneries of deep high salinity in our countries.

Predicted depth to brackish groundwater, in feet below land surface. Source: USGS [1]

What is brackish groundwater?

Brackish groundwater is water located under the surface which is more saline than freshwater but less saline than seawater. This kind of water can be found in wetlands or deep aquifers.

Brackish water is defined as groundwater with a concentration of total dissolved solids (TDS) in a range from 1000 to 10000 mg/L. A description of the different water classifications in relation to its salinity is shown below.

Groundwater classification from multiple publications. Source: USGS [1]

The presence of brackish water can affect the availability and the cost of use of water. The concentration of chemical species associated to brackish water can exceed the Environmental Quality Standards, impacting irrigation, transport and storage.

Why does freshwater become brackish water?

It is important to see groundwater as a reservoir, or as a glass. In this glass, freshwater has been stored for years and the ions related to salinity, which are heavier than water molecules, tend to go to the deeper horizons because of gravity.

To understand salinity we need to take into account that water is the universal solvent, and as the water moves in the groundwater systems, it will dissolve the minerals of the porous media and its amount of total dissolved solids will increase as the residence time is higher.

In the following figure we can see the effect of geologic time in the groundwater distribution in arid aquifers and the high residence times involved.

Hydrogeological settings to illustrate the occurrence of essentially non-renewable groundwater resources. Source: UNESCO [2]

When talking about groundwater it is important to mention that the time scales considered can vary from years and so on (decades, centuries, millenniums). These processes affect the chemical composition of water.

Salinity and TDS data with respect to depth for each pool with data in eight counties across California. Source: Kang et Jackson [3]

Some horizons may be more productive and present better conditions for exploitation, however, the presence of freshwater aquifers intercalated with brackish water aquifers, limit its potential of being a permanent freshwater source.

Distribution of Oxigen 18, chloride and tritium vs depth. The impact of droughts in aquifer salinity and the trend of more salinity with depth can be observed. Source: Unesco [2]

Conclusions

We do not know about the distribution of salinity with depth in most of the aquifers in our region. This limits the understanding and evaluation of potential freshwater source aquifers. Human activity and climate change sceneries present challenges in aquifer management which need to involve the global state of water quality and distribution of groundwater.

References

1. Stanton, J.S., Anning, D.W., Brown, C.J., Moore, R.B., McGuire, V.L., Qi, S.L., Harris, A.C., Dennehy, K.F., McMahon, P.B., Degnan, J.R., and Böhlke, J.K., 2017, Brackish groundwater in the United States: U.S. Geological Survey Professional Paper 1833, 185 p., https://doi.org/10.3133/pp1833.
2. Non-renewable Groundwater Resources, 2006, Foster et al., IHP-VI Series on Groundwater Nr. 10, url: http://unesdoc.unesco.org/images/0014/001469/146997E.pdf
3. Salinity of deep groundwater in California: Water quantity, quality, and protection, Kang and Jackson, 2016, Stanford University, url: http://www.pnas.org/content/113/28/7768.full?tab=author-info