I live in a drought stricken part of south Texas and am looking for ways to use all resources for watering my garden and my yard. Most pump the "runoff" from the well back into the brackish part of the Nueces River, which is adjacent. Would using this runoff build up any metals that would later be detrimental to the soil and would it be suitable for the watering the garden?
Is the Overflow to Pumped and Filtered Well Water Harmful
Certified GKH Gardening Expert
As long as it is from the same source you drink from, it should be safe for your plants. It is very common for people in drought areas to turn to using "grey" water, which is basically the water they just used for washing dishes, bathing, etc in the house. This sounds like it is no different and should not be harmful to the plants.
The only thing I can think you will want to check is when you say filtered, is the water also softened? Softened water can cause issues with plants. This article can explain: