SECTION 7 



TRACE ELEMENTS ASSOCIATED WITH EVAPORITES 



Introduction 



Trace element concentrations in evaporite minerals forming in agricultural evaporation 

 ponds were examined by dissolving and analyzing evaporite minerals collected from Peck Pond 

 cells 2 and 3. The mineralogy of the salt crusts was found to be dominated by thenardite, a Na^SO^ 

 mineral. The evaporites displayed a number of differing morphologies including fine-grained, 

 slabs, and large crystals. 



Methodology 



The elemental compositions of 7 representative evaporite deposits were determined by 

 dissolving 1 gram of the mineral in lOOmL of distilled deionized water. The evaporites were 

 observed to dissolve completely except for a dark colored residue consisting of particulate organic 

 material which was present in some of the samples. The solutions were all filtered through a 0.45 

 ^m membrane filter prior to chemical analysis. The chemical analyses for the dissolved salts are 

 shown in Table 7. 1. 



Molybdenum was analyzed using a Perkin -Elmer 2100 Graphite Furnace (GFAAS) with 

 deuterium background corrector and a palladium hydroxylamine matrix modifier. Prior to 

 analysis, samples were acidified to pH 2 using nitric acid. To ensure accurate performance of the 

 instrument, replicate samples were run every ten samples eind recovery tests every twenty 

 samples (Loya, 1989). Arsenic and selenium were analyzed by Hydride Vapor Generation Atomic 

 Absorption Spectrophotometry (HVGAAS), while boron was analyzed using Inductively Coupled 

 Plasma Spectrophotmetry (ICPS). 



Results 



Sodium and sulfate were the major chemical constituents comprising the salts which 

 confirm the results of the x-ray diffraction analysis. To examine if the trace elements become 

 enriched or depleted in the solid-phase versus their concentration in the pond waters, the ratio 

 of SO^ to trace elements in both the solid-phase and solution-phase were plotted (Figure 7.1). The 

 chemical composition of the solution phase was taken as the mean concentrations in Peck Pond 

 Cell 2 SE. The diagonal line shown in Figure 7.1 represents chemical compositions where the 

 ratio of SO^ to trace element in both the solid phase and solution phase are equal. Above this Une 

 is a region where the solid phase is depleted relative to the solution phase. Below this line, the 

 trace element is enriched in the sohd phase relative to the solution phase. The diagram shows 

 that As, B and Se are depleted in the solid-phase, while Mo concentrations were nearly equal, 

 to slightly enriched in the solid-phase. 



Table 7.1 Trace elements associated with pond evaporites (values in nmolea/L) 



piige 7.1 



