The following evaporites were found in all three ponds: thenardite (NajSO^), polyhalite, 

 tychite, halite (NaCl), nahcolite (NaHC03), and nesquehonite (MgC0^'3Hfi). 



Since the influent waters to ponds are characterized as Na^SO^, NaCl-Na^SO^ or Na^SO^- 

 NaCl type waters, the predominant evaporites formed are thenardit* and halite. In 

 addition, the presence of calcium and carbonates in these waters also produce evaporites 

 such as gypsum and calcite (CaCOj) in copious amounts. 



Seven evaporite samples were obtained from Peck pond and subjected to mineral 

 identification and chemical analyses of redissolved salts. These samples were domi- 

 nated by thenardite (Na^SO^) with morphologies ranging from fine-grained minerals 

 found along shorelines to large crystals and slabs found in drying to dried pond bottoms. 

 A representative water sample was also collected and chemically analyzed. 



The evaporite samples were dissolved in distilled deionized water (1 gram evaporite in 

 100 ml water) and analyzed for several trace elements (Se, As, B, Mo), major solutes (SO^, 

 CI, Na, Ca, Mg, K), and DOC (Dissolved Organic Carbon). 



Based on the above chemical analyses the association of trace elements in the evaporites 

 were ascertained. The molar ratio of 80^ to a given trace element in the evaporite was 

 compared to the ratio of SO^ to a given trace element in the pond water. The results show 

 that B, Se, and As were depleted in the evaporite (solid phase) as compared to the pond 

 water (solution phase) while Mo was enriched in the evaporite. 



Additional studies are needed in other pond facilities to ascertain this relationship as 

 well as the mechanisms oftrace element adsorption to evaporites and occlusion (trapped) 



and co-precipitation oftrace elements in evaporites. 



▲ Magnitude of Salt Load 



• An overall assessment was made on the 27 evaporation ponds with a total surface area 

 of 7,070 acres that annually receive 31,900 ac-ft of subsurface drainage from about 

 56,500 acres of tile-drained fields containing 810,000 tons of salts (TDS). 



• The above data were transformed into unit values. For example, 



o Each acre of tile-drained field required 0.125 ac of pond. 



O About 0.6 ac-ft/ac-yr of tile-drained effluents were collected and disposed 



into ponds. 



O About 4.5 ft/yr of tile effluents were disposed into ponds. 



O The concentration of TDS in tile effluents obtained from the field was 



about 14.3 tons/ac-yr. 



O The concentration of TDS in tile effluents disposed into ponds was about 



25.4 ton&/ac-fl. 



O The mass of TDS disposed in ponds was about 115 ton&'ac-yr. 



• The 31,900 ac-fVyr of tile effluents discharged into ponds is nearly twice that discharged 

 from drains in the Grasslands Subarea to the San Joaquin River. 



pfige 1.13 



