to make food or drink less pure by adding another substance to it (Oxford)
to make food or drink weaker or to lower its quality, by adding something else (Cambridge)
The water supply had been adulterated with chemicals from the soil. (Oxford)
There were complaints that the beer had been adulterated with water. (Cambridge)
water supply had been adulterated with chemicals
adulterated with water
Contaminate
Cheapen
Cleanse
Purify
Adulteration