A biocidal additive includes a blend of a biguanide compound and a polymeric ammonium compound. A method of treating a water system includes adding a biocidal additive to the water system to achieve a minimum inhibitory concentration or greater in the water system. In exemplary embodiments, the water system is a well treatment fluid for oil and gas applications, the blend is a synergistic blend, the biguanide compound is polyhexamethylene biguanide, and the polymeric ammonium compound is poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio)ethylene dichloride].
