Legal Question in Landlord & Tenant Law in Missouri

What your written lease says about utilities and who is responsible for lawn maintenance will likely rule the outcome of your question. If it is silent on the subjects, then you may be able to abate your rent payment(s) for the amount of additional water being used. (Assuming you pay the water bill for your side of the duplex) You will probably have to accurately determine the cost of the extra water.

Good luck