I’m sorry but I’m not going to be able to give you much good news.
The fact that you may have bought the property 13 years ago and this problem existed at that time, is not relevant. You buy the property inheriting its problems.
If this were to have drained across the car park for 20 years, there were, then you could claim an easement for drainage under the Prescription Act.
However it has changed direction for whatever reason, it is your pond and you are responsible for debating the nuisance.
However even though you may be able to claim an easement, it must not cause nuisance which is what they are suggesting.
You would also be responsible for any damage caused by the water drainage or for the removal of any algae for any injury if anyone slips on the algae.
I don’t know what they are asking you to contribute one third towards.
Can I clarify anything else for you?
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If the water crosses your property from somewhere else, then you may have a claim against wherever the water is coming from. It depends on whether anything on your property (the pond) is responsible for diverting the water so that it causes nuisance.
It depends on the reason why the direction of the water has moved away from the drain as to whether you are liable for that or not.
If the water has been draining into their drain for more than 20 years, then you have acquired an easement under the Prescription Act to allow the drainage to continue as previously.
I’m glad to help. Best wishes.