In wellbore construction it has become environmentally and economically practical to treat flow back and produced water at the well site. Typically the flow back and produced water is initially contained in an open air pond. The water is then run to a treatment system to remove particulates and then deposited in a second clean pond. The water in the second clean pond may be allowed to sit in the pond for weeks before it is pumped out to be taken and disposed of or most likely to be used as a water source in the construction of a different well. Unfortunately even though the water may have initially been treated with the biocide due to the open environment to water is usually re-contaminated with both nutrients and both aerobic and anaerobic bacteria. The anaerobic bacteria usually include sulfur reducing bacteria which in turn produces hydrogen sulfide dissolved in the water. It has been found that an effective way to treat the clean pond to prevent contamination of the clean pond by sulfur reducing bacteria and its consequent hydrogen sulfide is to initially treat the second pond with a short-term but quick kill biocide as well as with the hydrogen sulfide scavenger that may then act as a long term biocide.