Theo - True, a bio swale is a very good idea if you have the space. The trick is to keep it shallow - there are some good design guides on the web but most would not be concerned about minimising evaporation, which could be an issue for you. Algal blooms could develop but so long as they are not toxic - like blue-green "algae" (which is actually a bacteria) - that should be fine. If you are concerned about toxic algae you can add a pond aerator - even a solar powered one. One thing to be careful of is to ensure the water you take out is filtered or drippers will clog. I suggest putting a coil of slotted drain pipe in the bottom of the pond and covering it with ~100mm of sand and then connecting the outlet/return pipe to that - making sure there are no spots where unfiltered water can get in. Keep an eye on the salinity - you can use a cheap hand held conductivity probe to monitor that. If you have seasonal surplus rainfall let that also flush out the bio swale to wash out the dissolved salts/chemicals. Finally, you might need to talk to your local/state government about environmental regulations/approvals - especially if the volumes are large. Let me know how you go. :)

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Theo - True, a bio swale is a very good idea if you have the space. The trick is to keep it shallow - there are some good design guides on the web but most would not be concerned about minimising evaporation, which could be an issue for you. Algal blooms could develop but so long as they are not toxic - like blue-green "algae" (which is actually a bacteria) - that should be fine. If you are concerned about toxic algae you can add a pond aerator - even a solar powered one. One thing to be careful of is to ensure the water you take out is filtered or drippers will clog. I suggest putting a coil of slotted drain pipe in the bottom of the pond and covering it with ~100mm of sand and then connecting the outlet/return pipe to that - making sure there are no spots where unfiltered water can get in. Keep an eye on the salinity - you can use a cheap hand held conductivity probe to monitor that. If you have seasonal surplus rainfall let that also flush out the bio swale to wash out the dissolved salts/chemicals. Finally, you might need to talk to your local/state government about environmental regulations/approvals - especially if the volumes are large. Let me know how you go. :)