Ultra Violet Light
Ultra Violet lights or UV Lights can be installed on wells that continually fail for Coliform bacteria. The way that a UV light works is that as the water passes through the light the bacteria are sterilized. It will not clean a well of bacteria, like a chlorination but instead sterilizes the bacteria.
UV lights, generally, require a lot of maintenance and can be troublesome. Most counties will require that you perform at least 2 forced chlorinations before installing a UV light.