Water runoff from rainfall can be a bigger problem to our environment in certain areas than you can imagine. In area where there is a lot of pavement, either asphalt or concrete, the water that runs into the drainage or off the edges of a road or parking lot can carry contaminates like metals, oils and even pesticides to name only a few. Eventually these pollutants will ending up into a natural water supply, mainly from storm water drainage systems.
Creating a natural organic rain garden can help this problem. Plants and the soil can absorb and clean many of these toxins naturally. Plants can help purify by absorbing toxins and a good healthy organic soil is full of bacteria that can keep the harmful nitrates from reaching the water supplies by turning them into a nitrogen gasses.
When building a water garden, they only need to be a shallow depression, they are not deep ponds. The soil in your area will determine the size needed. You don't want water to stand in the garden for a long period of time, 48 hours should be the maximum time it sits before it drains into the soil, evaporates or is taken up by the plants.
Organic gardening is a great method to choose for your garden. It is the same method that nature uses to grow plants naturally and to clean the air we breath along with water we drink. When using these practices in your home garden, you will be doing your part in helping to keep these natural resources clean.