The article is a little reactionary but begs the question, why take any chances? It says that a 12-foot section contains 250 lethal doses but doesn't tell you if its a 2 x 4, 4 x 4, etc. Clearly they are trying for maximum impact.
If you knew how much wood you had in a dock and could determine the amount of arsenic in the wood, you could divide the arsenic mass by the number of gallons in the pond to determine the worse case arsenic water concentration. Most states have criteria for surface water and of course drinking water. In Michigan the drinking water criteria is 50 micrograms per liter and for surface water its 150 micrograms per liter.
That means if you had a pond with 4-acre feet of water, you would need about 1/2 pound of pure arsenic added to the water to exceed the drinking water standard and about 1.5 pounds to exceed the surface water standard.
The good news is that not all of the arsenic in the wood would leach out, and if it did leach, not all would remain in the water. Arsenic has a strong affinity to organic sediments. The actual amount of arsenic needed to exceed the water criteria would be higher because some of the arsenic would be bound to sediments, etc.
Arsenic in Michigan occurs naturally in soils on the east side of the state at concentrations that are on the average 6 times the concentration on the west side. The soil concentration on the east side commonly exceeds the health-based standard. The difference is due to the presence of shale bedrock on the east side (remember the organic affinity). If you are still concerned about arsenic, eliminate or paint the treated wood. You can take a soil sample to an environmental lab for an arsenic analysis for about $10. Many health departments will provide a free arsenic analysis for drinking water wells.
Hope that helps
More arsenic info
