METHODS OF SOIL-CHLORIDE EXTRACTION FOR RECHARGE ESTIMATION USING THE CHLORIDE METHOD

In humid environments, recharge to ground water systems is typically estimated using a water-balance approach, which yields a single recharge rate to represent a basin area usually hundreds to thousands of square miles.  An approach to determining  recharge rates that potentially can more effectively capture the spatial variability of recharge within a watershed in a humid region is the chloride method, a method commonly used in arid environments.  One of the required inputs to the chloride method is the chloride concentration within soil in the vadose zone.  To this end, soil elution of numerous, consecutive soil samples is performed to develop a vertical profile of chloride concentration.  The published procedures for soil elution are variable, but may require as many as three rinses for each soil sample – this can become very labor intensive if there are numerous samples.  We have investigated chloride extraction by comparing the standard soil elution method to  two additional techniques using: (1) an anion exchange (Na₂SO₄) solution, and (2) slow column filtration under N² pressure.  All methods were conducted using three rinses and the extraction solutions were analyzed by ion chromatography.  The total mass of chloride extracted by all three methods was comparable.  The quantity of chloride extracted in each successive rinse decreased systematically; however, the chloride mass extracted using the anion exchange and column filtration was qualitatively greater after each successive rinse than that extracted by the corresponding standard elution rinse.  The extracted chloride mass from the column extractions showed the greatest deviations and, as such, the values for successive rinses were not significantly different from those obtained using the standard elution method.  The results from the anion exchange solution are potentially more consistent; however, more replicate analyses need to be completed.  The results of this research suggest that more effective and efficient chloride extraction might be possible using methods other than standard elution and rinse techniques.  These improved methods should result in more accurate and precise recharge estimates using the chloride method.