I would suggest using a separate system for the arsenic filtration. A good filter to use would be the Pentek AS-CK Arsenic Cartridge Kit is designed for use in the Pentek AS-301E Arsenic Reduction System. Kit includes two filters that collectively remove iron, manganese, hydrogen sulfide, Arsenic III and Arsenic V. Part number 255401-43. FiltersFast also carries the Pentek AS-301E 3-Stage Arsenic Reduction System for those filters.
http://www.filtersfast.com/Pentek-AS...Filter-Kit.asp
http://www.filtersfast.com/Pentek-AS...ter-System.asp
The AS-CK is for treatment of water containing no more than 50 ppb total arsenic.