Hydrogeochemical Tool to Identify Salinization or Freshening of Coastal Aquifers Determined from Combined Field Work, Experiments, and Modeling

Abstract

This study proposes a hydrogeochemical tool to distinguish between salinization and freshening events of a coastal aquifer and quantifies their effect on groundwater characteristics. This is based on the chemical composition of the fresh-saline water interface (FSI) determined from combined field work, column experiments with the same sediments, and modeling. The experimental results were modeled using the PHREEQC code and were compared to field data from the coastal aquifer of Israel. The decrease in the isotopic composition of the dissolved inorganic carbon (delta(13)C(DIC)) of the saline water indicates that, during seawater intrusion and coastal salinization, oxidation of organic carbon occurs. However, the main process operating during salinization or freshening events in coastal aquifers is cation exchange. The relative changes in Ca(2+), Sr(2+), and K(+) concentrations during salinization and freshening events are used as a reliable tool for characterizing the status of a coastal aquifer. The field data suggest that coastal aquifers may switch from freshening to salinization on a seasonal time scale.

Identification of the Hydrogeochemical Processes in Groundwater Using Classic Integrated Geochemical Methods and Geostatistical Techniques, in Amol-Babol Plain, Iran

“…The coastal area on the north side of the Amol-Babol Plain is under increasing human pressure from population growth and the increase in the development of agricultural activities. Groundwater resources along the shoreline could be threatened by the mixing mechanism of seawater intrusion into the freshwater, due to the over abstraction of groundwater [11, 12]. …”

The effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from column experiments under aerobic and anaerobic conditions

“…This flow rate is equivalent to a velocity of about 200 m·y -1 , which is one order of magnitude higher than the general groundwater velocity in the Israeli coastal aquifer. However, a previous experimental study (Russak and Sivan, 2010) in similar settings showed that the geochemical process of cation exchange is not significantly affected by such differences in flow velocities. This flow rate was chosen in order to make the duration of the experiments reasonable and avoid long-term "column effect" (such as preferential flow of the water in the side wall of the column).…”

“…This indicates that the observed depletion in Ca 2+ (due to adsorption, e.g., Appelo et al, 1990;Russak and Sivan, 2010) should have triggered the CaCO 3 dissolution resulting in the observed ALK enrichment.…”

“…Cation exchange is a well-known process in the FSI, controlling the major cations (e.g., Appelo et al, 1990;Russak and Sivan, 2010). Regarding the DIN system, it was shown that NH 4 + tends to adsorb in freshwater more than in saline water (e.g., Appelo et al, 1990;Seitzinger et al, 1991).…”

Electrical Conductivity as a Proxy for Groundwater Density in Coastal Aquifers

“…It is reasonable to assume that groundwater in coastal aquifers will obey a similar relation because mixing of fresh water and sea water is usually the most important process controlling the amount of dissolved substances. Deviations from the UNESCO 1980 equation of state are expected to occur, however, due to geochemical reactions by which the relative proportions of individual substances will differ from those of an ideal mixture of fresh water and ocean water (Poisson et al 1981; Millero 1999; Russak and Sivan 2010). Also, the presence of uncharged species such as dissolved silica (H 4 SiO 4 ) may contribute to the density, but have no effect on EC.…”