Magnitudes Discharged into Ponds 



The 31,900 ac-ft/yr of subsurface drainage discharged into ponds is nearly 1.9 

 times the 17,000 ac-fVyr of groundwater discharged from drains in the Grasslands 

 Subarea into the San Joaquin River (CH2M HILL, 1988). The 810,000 tons/yr of TDS 

 disposed into ponds is about 5.8 times the 139,400 tons/yr of TDS in the ground water 

 discharged from drains in the Grassland Subarea into the San Joaquin River. CH2M 

 HILL (1988) gives smother estimate of 743,800 ton&'yr disposed into ponds. 



The 810,000 tons/yr of TDS disposed in ponds is about 26% of the estimated 

 3,100,000 tons/yr of salt accumulation in the San Joaquin Valley's west side (CH2M 

 HILL, 1988). 



In addition, the quantities of trace elements disposed into ponds are about 595 

 tons/yr of boron, 4,340 Ibs/yr of selenium, 7,900 Ibs/yr of arsenic, and 44 tons/yr of 

 molybdenum. 



Accumulation in Ponds 



As noted in the interim report, the average measured seepage rate from the 

 evaporation ponds is about 1.0 ac-ff ac-yr. 



Net Volume Evaporated = 31 900 ac-ft/yr - (1.0 ac-ft/yr )(7 070 ac) (8.7) 



= 24 830 ac-ft/yr 



This annual net volume of water evaporated in the ponds is about 77.8% of the 

 total drainage influent if seepage losses are accounted for. 



Net TDS Accumulation = g'^i 9 qq g^'.^y^ 810 000 tons/yr = 630 500 tons/yr (8.8) 



This is the annual net weight of salts accumulating in the ponds when seepage 

 losses are considered. 



In order to estimate the volume of TDS accumulating in the pond, density of the 

 salt precipitates must be assumed. Because the pond waters are predominantly of the 

 Na2S04 -type water, thenardite (Na2S04) is the principal evaporite mineral formed in 

 these ponds. The density of pure crystalline thenardite is 2.66 g/cm (Sonnenfeld, 1984) 

 The density of other predominant evaporites identified in the salt deposits are 2.71 g/cm 

 for calcite (CaCOa), 2.32 g/cm^ for gypsum (CaS04»2H20), and 2.23 g/cm for bloedite 

 (Na2S04»MgS04»5H20). 



Assuming a density of 2.66 g/cm^ for salts precipitated in ponds, the annual 

 volume of salts accumulating is estimated to be about 164,500 cubic yds or an average 

 deposition thickness of 0.17 in/yr. 



In contrast, simulation runs by C-Salt as given in the interim report assumed 

 that the density of precipitated minerals to be identical to the calculated density of the 

 brine from which precipitation took place. The density of the brines when precipitation 

 was occurring ranged from less than 1.1 to 1.4 g/cm as pond waters were 

 evapoconcentrated 50-fold. This lower density is more representative of the shoreline 

 salt deposits. , , /• , 



Assuming a density of 1.28 g/cm"" gives an estimated annual volume of salt 

 accumulation of about 342,000 cubic yds or an average deposition thickness of 0.36 in/yr. 



The above range of estimates on annual salt deposition in ponds indicate huge 

 amounts available for possible salt harvesting or for disposal. The potential for 

 commercial salt harvesting is, however, constrained by the evel of punty, distance t^ 

 the market and economics of worldwide markets (Personal communication E. Lee). 

 Moreover the presence of toxic elements such as selenium, arsenic, boron, molybdenum 

 and uranium may constrain how these salts are to be ultimately disposed. 



page 8.2 



