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VZ and V, must be measured at both the beginning and end of the measurement period in each of the five ponds. This may be accomplished by recording the depth of the water in the center of each pond with a staff gauge. Since the surface area of the pond is known, the i volume of the water in the pond may then be calculated. It will be important to perform the measurements in precisely the same place each time because the bottom of the pond may be irregular. In this way, even though the actual volume calculation of water in the ponds is off slightly, due to irregularity of the bottom surface, the calculation will be off by the same amount for each measurement. The important consideration is to calculate the j change in storage between the two measurements. e The resulting calculations should provide an approximate amount of water infiltrating through the bottom of the ponds. Each of the above variables will contain a certain percent- age of error. For example, evaporation rates measured at Tracy's weather station may vary slightly from the actual evaporation rates at the holding ponds. However, the calculations should allow a reasonable evaluation of the amount of water infiltrating through the bottom of the ponds. Total Dissolved Solids Mass Balance Study As the City has conducted its quarterly monitoring program in the holding ponds, it has observed that the TDS concentrations in the ponds appear to increase as the water levels in the ponds decline. This indicates that evaporation has tended to concentrate the TDS E remaining in the water. A TDS mass balance study conducted at the same time as the water budget study may reveal whether the TDS concentrations in the ponds are increasing over time as water is evaporating from the ponds; or, whether TDS concentrations are remaining constant because of infiltration of pond water into the groundwater. Other processes may also be occurring in the ponds that would affect the TDS concentrations in the pond water. For example, TDS in cannery wastewater may be subject to biodegradation in the ponds. `> A TDS mass balance study will provide some insight on the amounts of dissolved solids entering the ponds, being stored in the ponds, and possibly being removed from the ponds. The basic relationship describing the TDS mass balance is similar to the relationship describing the water budget: TDS mass into ponds = TDS mass out of ponds + change of TDS mass in ponds The TDS mass dissolved in a volume of water may be calculated by multiplying its concentration in the water by the volume of water. Thus, the above relationship may be expanded as follows : [TDSJI.ur = TDSout + ([TDS2]VZ [TDS1]VJ where : [TDS;] = concentration of TDS in the inflowing water TDSOUt = mass of TDS leaving the ponds a [TDSJ = concentration of TDS in the ponds at the end of the study period [TDS,] = concentration of TDS in the ponds at the beginning of the study period e SAC/SAC36357/041.WP5 13 8/94