A separation tank having an inlet and an outlet and forming a low flow path and a high flow path, both conveying water from the inlet to the outlet. At normal, low flows, the water enters the inlet and passes through the low flow path having treatment chambers to remove, respectively, heavy materials and floatable materials. At high flows such as with heavy storm runoff, water exceeding a predetermined flow rate is conveyed from the low flow path into the high flow path where there are baffles to treat the water to remove both heavy materials and floatable materials. In optional embodiments, there is a filter/recharge chamber that may contain a filter and/or a groundwater conveyance system such that filtered or unfiltered water can be discharged from the low flow path through a groundwater conveyance system to groundwater, to the normal outlet, or both.