precipitation, the salts may either form a surface-covering crust or suspended particles which 

 settle and accumulate on the pond bottom. 



Since salt crusts on the surface of the pond reduce the evaporation rate significantly, 

 surface salt crusts should be minimized or removed to ensure maximum efficiency of the 

 evaporation. 



One method of removing salt crusts might be to control the flow of effluent using gates 

 between adjacent cells. If differences in water level are maintained throughout cells, salt crust 

 can be removed through addition of effluent from a more dilute cell. 



Another suggestion is to carry out salt removal during the night because cooler 

 temperatures generally decrease the solubility product and hence, more salts will be available 

 for removal by some mechanical means. 



Sadan Proposal 



Abraham Sadan and Cominco Ltd has a patent (Swinkel et al., 1986) to separate and 

 purify salts in a non-convective solar pond. They contend that a brine consists of combinations 

 ofhigher hydrated and lower hydra ted or anhydrous forms of salts. The patent claims that under 

 saturated conditions it is possible to crystallize salt in a higher hydrated form, dehydrate it to 

 a lower hydrated form in a non-convective solar pond, and recover the salt from the bottom of a 

 pond in solid, pure form essentially free from other salts in the brine. 



Sadan (Sadan, 1987) presented to the Westlands Water District a proposal to recover 

 high purity anhydrous Na^SO^ using an example of reducing 8,000 ac-fi/yr of tile effluents to 

 14.25 ac-fl (52,000 tons) of anhydrous Na^SO^, 4.25 ac-fl (12,000 tons) of NaCl and 20.0 ac-ft 

 (36, OCX) tons) of MgCl, bitterns. The proposal requires a 1,500 acre preconcentration pond from 

 which CaCOj and CaSO^»2HjO would precipitate leaving 180 ac-fl of concentrated brine. The 

 brine goes to a 16 acre deca pond in which Na^SO^'lOHjO (mirabilite) would precipitate out 

 leaving a sulfate brine of 80 ac-fl. A 16 acre winter cooling pond is also required in which 

 Na^SO^'lOHjO would precipitate and dissolve. The cooled brine of 50 ac-fl is transferred to a 

 12 acre pond to precipitate NaCl. The remaining 20 ac-ft of bitterns is stored in a 36 acre non- 

 convective pond from which Na,SO^ may precipitate. 



In the above process, selenium was assumed to remain in the dissolved state and 

 evapoconcentrate in the brines. The example given estimated Se would increase from 0.31 to 

 15.08 mg/liter in the preconcentration pond, from 15.08 to 33.46 mg/liter in the deca pond, from 

 33.46 to 53.89 mg/liter in the winter cooling pond, and from 53.89 to 136.20 mg/liter. 



Our assessment is that the process outlined above would require extremely close controls 

 on salinity levels to preferentially precipitate out Na^SO/ lOH^O, NaCl and Na,SO^. This may 

 be possible in an industrial processing plant but probably not in agricultural evaporation ponds. 

 Moreover, the assumption that Se would evapoconcentrate in a conservative manner and not be 

 reactive is contrary to our observations in evaporation ponds. 



Best Design to Sustain Evaporation Rate and Precipitation 



Although the current design of evaporation ponds is based on United States Department 

 of Agriculture-Soil Conservation Service (1982) design criteria, the best design considered here 

 is based on having the highest efficiency and the least detrimental effect on the environment. The 

 best design will take into account the suggestions mentioned previously. 



Size 



Evaporation ponds should have enough capacity to satisfy the maximum storage 

 expected or total inflow minus the total outflow. 



Total inflow = drainage from the field 

 + rainfall 



+ perimeter drainage (drainage collected by interceptor drain) 

 Total outflow = evaporation -t- seepage 



pagt 9.3 



