If it was the *Tesco path weed killer this is known as and manufactured by DOFF (http://www.doff.co.uk/) Long lasting path clear.

It will stay resident and active in the soil for 3 - 6 (maybe even longer) months. Best to strip off the top 4 to 6" of top soil and replace or wait a good 12 months before you plant/seed/re-turf.

You cannot 'water' this out as you will just get a diluted solution causing 'run off*'.

PRODUCT DESCRIPTION

Tesco Path Weed killer.

MAFF CODE: M12754
Actives: 2,4-D "AMITROLE DIURON" (AD)
More info he
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Intended manufactured field of use: HOME GARDEN WEEDKILLER ONLY.
Manufactured and approval holders: Doff Portland Ltd
Authorised (UK) outlets/retailer(s) (amongst others): DOFF Portland, Asda, Tesco, Wilkinson, B&Q and Focus Stores Limited.

If you have had the heavy rains the Amitrole Diuron may/will have been washed past the 6" top soil mark and possibly much deeper

You may even have a diluted '*run off' to other areas of your garden. This will stunt or slowly kill plants coming into contact with it.

This is why you should always read the label/instructions with any form of chemicals/feeds/bug control/cleaners etc., you are going to apply to your home or garden - the affects, as seen, can be very destructive and hugely expensive to rectify.