Taken from the Simpson Strong Tie web site
Waterborne, Creosote, and Oil-borne (penta) are the three broad classes of preservatives typically used when pressure-treating wood.
Wood treated with waterborne preservatives is typically used in residential, commercial and industrial building structures. Creosote is primarily used for treating railroad ties, guardrail posts, and timbers used in marine structures. Oil-borne (penta) is most often used for treating utility poles and cross arms.
Several typical waterborne preservatives used in building applications include: Chromated Copper Arsenate (CCA-C), Alkaline Copper Quat (ACQ-C, ACQ-D, ACQ-D Carbonate), Micronized Copper Quat (MCQ), Copper Azole (CBA-A & CA-B) and Sodium Borates (SBX/DOT).
These treatments are often referred to by trade names such as: Wolmanized Natural Select™ (Copper Azole), Preserve and NatureWood® (ACQ), MicroPro™, Smart Sense™ (MCQ), and Advance Guard® (Borate). Each preservative usually has a number of variations available so care should be exercised when specifying treated wood.
Some different oil-borne preservatives that are used are Chlorpyrifos/IBPC, Copper Naphthenate and Pentachlorphenol. One advantage of these treatments is that they do not create swelling in the wood, but there is generally an added cost over water-borne treatments as well as availability in some regions.
I believe this information is correct. For some perspective, note that this information is said to be current. Therefore, depending on the type of treatment, you could have a wide variety of chemicals in pressure treated wood, including pentachlorophenol (penta), napthalene, copper, chromium, arsenic, chreosote, boron, sodium, carbonate and other less well known substances. The only way to tell what is in your pressure treated wood is by checking the manufacturer's data sheet on the wood. Since all of these processes are apparently still in use, it is incorrect to state that arsenic is no longer used.
If you are concerned about chemicals leaching out of the wood into the groundwater, you can always use naturally durable wood without preservatives.