PREFACE 
\ 
The Maryland Water Resources Administration received a 
grant from the U.S. EPA Chesapeake Bay Program to conduct a 
study concerning oyster mortality in the Chester River, a sub- 
estuary of the Chesapeake Bay. The State of Maryland contracted 
with the University of Maryland to perform a three—part study to 
evaluate the reported oyster mortality. Due to the fact that 
this was a retrospective study, the evaluation of past large 
instananeous inputs of chemicals and the resulting potential 
impact was not made. The results of the University of Maryland's 
research follows. 
The results of the three studies involving aquatic bioassays 
and arlalysis of phthalate esters, tin and organotin compounds in 
various environmental media did not show conclusive cause-and- 
effect relationships concerning the oyster mortality. 
The bioassay study showed no significant point source or 
ambient acute aquatic toxicity to organisms tested; however, 
chronic stress was indicated by growth reduction of oysters 
during the study period and by occasional low levels of dis¬ 
solved oxygen in the lower estuary. The point source bioassay 
observations showed mortality of organisms as well as controls; 
however, the controls and point source tests were not signifi¬ 
cantly different. 
The analysis of phthalate esters in alluvial sediments 
showed a decreasing trend downstream from a point source. A 
pond receiving an industrial discharge showed extremely high 
concentrations of phthalate esters. The levels found in the 
vicinity of the pond warrant further consideration. Levels of 
phthalate esters in the estuarine sediments are low. The tox¬ 
icity of the levels found is unknown, especially to oysters. 
A review of current literature shows no documentation of the 
toxicity and dynamics of phthalates to exposed oysters. The 
oyster concentration data in this report are the first published 
data available for di (2-ethylhexyl) phthalate (DEHP) in the 
American oyster. Although the toxicity of phthalates to the 
oyster would be expected to be low, because of the economic 
importance of the oyster, further laboratory bioassays are 
warranted to determine acute-and chronic-effect concentration 
levels. 
Other modeling efforts conducted by state and Federal 
- workers have estimated that the major transport pathway of 
IV 
