pollution. One source may be direct 

 output from industrial (or municipal) 

 effluents. An example of this type of 

 pollution is discharge of Hg from chlor- 

 alkali plants. Local metal input into 

 the water also results from dredging. 

 Metals already present in the sediments 

 may be released into the water during 

 dredging by changing conditions of Eh 

 (redox potential) and pH. 



PROCESSES CONTROLLING CONCENTRATION 

 AND AVAILABILITY OF HEAVY 

 METALS IN ESTUARIES 



The total concentration of a heavy 

 metal in an estuary or marsh may not 

 determine its availability or toxicity 

 to organisms. For example, occluded 

 metals (enclosed within crystalline min- 

 eral particles) may not be biologically 

 available. On the other hand, metals 

 dissolved in the water or loosely sorbed 

 to particles are directly available to 

 plants and other organisms. Intermediate 

 in availability are metals bound in or- 

 ganic material which can be made avail- 

 able as the result of decomposition or 

 ingestion of the organic material (Gibbs 

 1973). 



The biological role of a metal in a 

 salt marsh estuarine system is influ- 

 enced by its distribution in the system. 

 Several processes interact to determine 

 metal distributions. If a system is in 

 equilibrium, thermodynamic processes 

 determine the distribution of the metal 

 between the solid, liquid, or gaseous 

 phases. The form of a metal which is 

 most thermodynamically stable can be 

 predicted from relative solubilities of 

 various forms of the metals under vary- 

 ing conditions of Eh and pH (Garrels and 

 Christ 1965). 



Equally important in most natural 

 systems are kinetic processes which con- 

 trol the physical, chemical, and biolog- 

 ical distribution of the metals in the 

 marsh estuarine system. Physical pro- 

 cesses are responsible for the transport 

 of metals in the water column and for 

 the removal of particulate material 

 (e.g., trapping of particulate material 

 in the salt marshes). Chemical and 

 biological mechanisms may control the 



distribution of the metals between sol- 

 uble and particulate material (Figure 

 1). Types of chemical reactions influ- 

 encing metal distribution include pre- 

 cipitation, oxidation-reduction, sorp- 

 tion, and complexation. These kinetic 

 reactions can occur when a system in 

 equilibrium is disturbed. 



For example, during dredging opera- 

 tions, normally reduced sediments are 

 exposed to oxygen and a series of reac- 

 tions affecting heavy metal concentra- 

 tions can occur. Windom (1973) studied 

 the heavy metal concentrations in the 

 water of collected dredged spoils in 

 closed containers and the change in 

 concentrations with time after discharge 

 (Figure 2). After dredging, the concen- 

 tration of Fe in the overlying water 

 fell below that typically found in the 

 water column over the sediment. It 

 remained low for 10 days and then showed 

 a rapid increase in the water. Total Cu 

 and Pb followed similar patterns, but 

 Cu++ ion remained consistently low. An 

 explanation is that Fe++ was oxidized 

 upon contact with G2 and precipitated as 

 Fe(0H) 3 which sorbed the Pb and com- 

 plexed Cu compounds. 



After 1C days in the closed con- 

 tainer, the system became anoxic and 

 under reducing conditions the Fe re- 

 turned to solution as Fe++ or metastable 

 iron sulfide, thus freeing other metals 

 associated with the Fe (0H)3 precipi- 

 tate. Cu++ ion apparently was not asso- 

 ciated with the precipitate and did not 

 markedly change in concentration. Zn 

 and Hg showed the opposite trend. 

 Relatively high concentrations were 

 observed over the first few days when 

 oxygen was apparently present, but the 

 levels in solution decreased when the Fe 

 concentration increased. It appears, 

 therefore, that disturbing sediments in 

 a dredging area causes short-term ef- 

 fects on metal distribution and activity 

 but has little long-range effect. 



Biotic influence on the fluxes and 

 fates of heavy metals in a salt marsh 

 can be expected due to the high biologi- 

 cal activity of the system. Plants may 

 affect movement of metals by (1) remov- 

 ing them from solution or (2) transfer- 

 ring the metals from one compartment to 

 another (e.g., from the sediments to the 

 water column). Metals may be taken up 

 from the sediments or water and stored 



127 



