Good Morning, 

Thanks for your question.

 We are volunteer Extension Master Gardeners who answer yard and garden questions. This question is out of my realm of expertise. Having said that, I did some research on the issue. Please click on the following link from the National Pesticide Information Center: https://npic.orst.edu/faq/cca.html

This information states: 

What are the environmental risks from CCA-treated wood?

The chromium, copper, and arsenic in CCA-treated wood may leach onto the wood’s surface, and into the surrounding soil and water. The age, size, and type of the wood, moisture in the area, and type of soil all affect how much CCA will leach. Wood sealants may reduce or prevent leaching.

Once chemicals are in the soil, they may change their form, a process known as speciation. The final chemical form can affect the chemical’s toxicity and how easily it moves in the environment. Factors such as soil pH, temperature, soil type, and soil moisture can affect whether and how speciation occurs.

The chemicals in CCA-treated wood mostly accumulate in soil closest to the wood. Under some conditions, they can travel farther. In general, they move least in organic soils, slightly more in clay soils, and the most in sandy soils and water. CCA movement in soils can range from less than 6 inches to up to 8 feet from the source.

This leads to another link: https://npic.orst.edu/factsheets/treatedwood.html

If you scroll down on this fact sheet, it states: 

Can plants take up ingredients from treated wood?

If compounds leach from treated wood, they may be available for uptake by garden plants. However, it is difficult to predict the amount of plant uptake. Materials used in treated wood that leach into soil may be bound to soil. They also may change into different forms that plants cannot take in through their roots. Information about plant uptake is limited.

Plant uptake may depend on many factors, including plant type, soil type, and distance to the treated wood. For some components of treated wood, levels may be higher in the roots and fibrous plant parts.¹⁹ Other studies suggest accumulation in the leaves.²⁰ The amount of root uptake can also vary by plant or crop, even when grown in the same soil.¹⁶

^{I cannot advise you on health and safety issues. Here is a link about growing rhubarb: https://extension.umn.edu/vegetables/growing-rhubarb  You might consider moving the plant away from the site or securing a new plant to put in an area away from the site.} 

^{You could plant non-edible plants in the area like shrubs or flowers.} 

^{Good Luck!}