SECTION 6 



TRACE ELEMENT ACCUMULATION IN POND WATERS 



Introduction 



During the evaporation of agricultural drainage waters from evaporation ponds, the 

 solutes are separated from their solvent, water. Some solutes are subject to solute transport 

 which physically carries them away from the evaporation pond usually to the groundwater table, 

 but the distribution of the majority of solutes is due to chemical partitioning. The chemical 

 partitioning of a solute is related to its suite of reaction mechanisms and relative reactivity. The 

 elevated concentrations created by evapoconcentration is conducive to driving many reactions. 

 For some reactions, however, a favorable concentration gradient may not be enough. 



This set of calculations investigates the general reactivities of certain solutes (arsenic, 

 selenium, boron and molybdenum) which have been highlighted as priority toxicants. Their 

 reactivities are referenced against chloride ions which are assumed to be non-reactive conservative 

 constituents of the evaporation pond waters. Though the study does not pinpoint specific 

 reactions, it is important to determine which solutes causing toxic concern arebeing retained in 

 the water column and hence, pose an exposure risk to wildlife and waterfowl. 



Evapoconcentration 



This is the term given to the process by which the ratio of solute to water solvent is 

 increased by the removal of the solvent and retention of the solute. The change in the ratio is 

 termed the Evapoconcentration Factor (ECF) and may be calculated for changes over time or 

 progressive cells. 



ECF Formulae 



The ECF formulae have been derived to provide an estimate of the levels of an element 

 in reference to chloride which is assumed to be a non-reactive component of the solution. 



Figure 6.1 shows the equations that are used to calculate predicted values. The Time- 

 Dependent ECF (TDECF) applies to changes in the degree of salinity which occur over time. The 

 Multi-Cell ECF (MCECF) applies to differences in the degree of salinity which occur in multi-cell 

 evaporation ponds. The notation m^^ generally represents molar concentration (M) but if the 

 volume of water is assumed constant, then it may be expressed in number of moles. 



Calculations with the TDECF and MCECF which are reported here are applied to data 

 obtained between fall 1986 and summer 1988. By virtue of the assumptions in building the 

 formula, the TDECF calculation only provides values which are independent of conditions in- 

 between the two time points of interest. In contrast, the MCECF calculation utilizes averaged 

 data over the time period of interest. Although Pryse comprises two cells, the second cell was 

 saturated with respect to solid phases too often to allow the assumption of non-reactive chloride 

 ions. Hence, the MCECF formula was applied only to Peck pond. 



(1) Time-Dependent ECF 



(2) Multi-Cell ECF 



ECFd) 



%«..« 



= ECF(t) X m^djj) 



m 



ECF(n) 



Cl,n 



m. 



m 



predpcn 



Cl,n=0 

 = ECF(n) X m 



x/i=£) 



Figur« 6.1. Formulae for calculating predicted concentrations during evapoconcentration. 

 (n = cell number, 7 = element of interest, t = time) 



pttge 6.1 



