Clam shells

[Mojave]

A couple of weeks ago I visited one of the local reservoirs and due to the low water level I was able to gather a good size bag of freshwater clam shells, I think they're clams anyway. I am moving soon and want to start preparing my new patch of ground for the Spring 2013 growing season. I'm hoping broken up clam shells will provide a natural source of slow release calcium.

Here's the problem. Uvas Reservoir is notorious for it's high levels of mercury (due to past mercury mining in the area) and there are warnings posted about pregnant women are not to eat the fish at all and everybody else severely restrict the amount of fish they eat from the reservoir. Does anyone know if shellfish can absorb mercury into their shells? I'd really hate to import some toxic heavy metal into my future garden.