I just figured I'd drill hole in the wood and sink it.
The pieces are not the same size, especially with random holes
being drilled. I was more concerned about the chemicals in treated wood
Most treated wood is CCA (chromated copper arsenate) treated. Here's what Wikipedia has to say about it:
(I edited some of the nonrelevant "filler" text out in order to make it easier to read)
"Chromated copper arsenate (CCA) is a wood preservative that has been used for timber treatment since the mid-1930s. It is a mix of chromium, copper and arsenic (as Copper(II) arsenate) formulated as oxides or salts, and is recognizable for the greenish tint it imparts to timber. CCA is known by many trade names and is the world’s most widely used wood preservative. It is manufactured to national and international standards depending on the country of intended use.
The chromium acts as a chemical fixing agent and has little or no preserving properties; it helps the other chemicals to fix in the timber, binding them through chemical complexes to the wood's cellulose and lignin. The copper acts primarily to protect the wood against decay, fungi, and bacteria, while the arsenic is the main insecticidal component of CCA, providing protection from wood attacking insects including termites and marine borers. It also improves the weather-resistance of treated timber and may assist paint adherence in the long term. Releases to the environment
Over time small amounts of the CCA constituents, mainly the arsenic, may leach out of the treated timber, according to the United States' Environment Protection Agency website. Arsenic is also found naturally in the soil, food and water. The amount and rate of arsenic leaching from CCA varies considerably depending on numerous factors, such as local climate, acidity of rain and soil, age of the wood product, and how much CCA was applied. Many studies in less aggressive soil types show leaching to be as low as 0.5 ppm (red pine poles in service,) or up to 14 ppm (treated pine in garden beds). Research in Volume 36 of Wood and Fibre Science shows that soil contamination due to the presence of CCA-treated wood after 45 years is minimal. Should any chemicals leach from the wood they are likely to bind to soil particles, especially in soils with clay or soils that are more alkaline than neutral.
A more serious risk than leaching is presented if CCA-treated timber is burnt in confined spaces such as a domestic fire or barbecue, and the smoke is inhaled."
So basically, what leaches out into the water probably won't have much effect in a large body of water like a lake. I would be more concerned if it was used in a small pond or tank. Just don't burn the scrap, especially indoors