I wish to differentiate the contribution of evaporation and seawater intrusion in the groundwater salinity (coastal aquifer), arid/semi-arid region using groundwater quality data. I don't have isotope data. any good recommendation/suggestions?
Piper plot combined with ionic rations can be a solution for this purpose. Piper plot will help to identify the origin of the salt in the ground water. And the ionic rations can be correlate with evaporation. I have little knowledge of this field. I hope it will give you a good in site.
Dear Subhajit Dey,
thanks for your reply. Piper plot may be useful to identify the water types, ion exchange reactions and seawater mixing processes and not evaporation. i used it earlier in my papers.
I used a tool to measure surface salt water intrusion into a wilderness area stream. It was a water quality sonde made by Insitu company. I programmed it to take readings at 15 minute intervals. It measured water level (which had to be adjusted for atmospheric pressure), salinity, conductivity, and a few others. This could be used in wells you are monitoring to record changes in salinity and water level with time. You could compare these to tidal records and evaporative demands calculated from temperature, relative humidity, soil moisture and if needed, plant water pressures. If aquifer is confined, evaporation may not be issue. Also water demand from pumping for irrigation or other uses can depress water table and be a reason to consider if there is salt water intrusion.
Dear Hansen
thanks for your details regarding the coastal aquifer monitoring.
Your best option is using conservative tracers in combination. Chloride/Bromide mass ratios will help identify sea water and mixed seawater-groundwater sources. The ratio does not change with evaporation unless salt begins precipitating. Panno, S. V., K. C. Hackley, H. H. Hwang, S. E. Greenberg, I. G. Krapac, S. Landsberger, and D. J. O’Kelly, 2006, Characterization and identification of Na-Cl sources in ground water: Ground Water, v. 44, no. 2, p. 176–187, doi: 10.1111/j.1745-6584.2005.00127.x.; The next useful tracer to detect evaporation are stable isotope ratios for hydrogen vs. oxygen in water. Multiple samples collected seasonally will define the slope of an evaporation line. If evaporation is a dominant feature, the slope will be significantly different than that of the local meteoric water line. Gat, J. R., 1996, Oxygen and hydrogen isotopes in the hydrologic cycle: Annual Review of Earth and Planetary Sciences, v. 24, no. 1, p. 225–262, doi: 10.1146/annurev.earth.24.1.225.
My advice is not to use the Gibbs diagram.
"Groundwater chemistry and the Gibbs diagram"
Andres Marandi, Paul Shand,
https: //doi.org/10.1016/j.apgeochem.2018.07.009
Dear Gani
Thanks for your suggestion. I know, Gibbs diagrams are created only for surface water. But you can differentiate the high saline and low saline waters, ion exchange reactions, etc based on TDS and Na/Ca ratio. Diagram will give only basic information.
You can quantify the contribution of evapotranspiration to chloride in groundwater if you have info about precipitation and the evapo-concentration factor of your study area. Then assuming no other source of salinization, the remaining chloride content in salinized groundwater is attributed to saltwater intrusion.
More elaboration on the evapo-concentration factor appears on page 1887 of Khadra and Stuyfzand 2014.
Khadra WM, Stuyfzand PJ, 2014. Separating baseline conditions from anthropogenic impacts: example of the Damour coastal aquifer (Lebanon), Hydrological Sciences Journal, 59:10, 1872-1893, https://doi.org/10.1080/02626667.2013.841912.
Article Separating baseline conditions from anthropogenic impacts: e...
Regards
I would go with plotting water chemistry of the groundwater on a trilinear plot and compare with seawater. If it is close, I might try inverse geochemical modeling with something like Netpath.
In addition to Cl/Br, the Ca/Cl, Na/Cl and B/Cl ratios can be diagnostic.
Seawater loses Na and B and gains Ca when intruding freshwater aquifers (ion exchange), so these element ratios can distinguish between evaporation and SW intrusion, as well as telling whether intruded SW is being flushed from and aquifer (Ca loss, Na+B gain).
For B, Na, see Ravenscroft & McArthur Applied Geochemistry 19 (2004) 1413–1430.
As an update on Panno et al. for Cl/Br, see McArthur et al. Science of the Total Environment 437 (2012) 390–402
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