Get sand, gravel and cinder/cement blocks or ceramic piping. When it's time to close shop smash the bricks and mix them with the sand and gravel. Neutralize your chems and pour them into the bags. Tie up, allow to evaporate for days and drop off. Infested building materials don't raise eyebrows. You can add to the kayfabe by perfuming it adequately. Lots of compounds out there to sell the idea of septic system renovations and because of the sheer amount of questionable materials in our bodies they're not going to just throw it in with the compost.

EDIT: Also. There's a fair amount of ways to turn most genuinely suspicious things into something else or hide it in something else. A lot of controlled precursors are miscible in other petrochemicals so just get a highly rated scrapyard to take care of it, nicely hidden inside the oil in the tank you're selling them at a fair but profitable price.

They're gonna drain it out, dispose of it and both parties are happy.

PS: Thank you for being cool. Harm reduction is harm reduction for everyone. It's not the schmuck down river's fault that the government disapproves of the vast majority of mind alteration.

PPS: Rent a flatbed truck to transport the goods. It confers the idea that you know what you're doing and are there on legitimate business. Use woven fabric bags rather than trash bags obviously.