Arsenic can be a very serious contaminant and recent regulations have lowered the max. contamination level from 50ppb to only 10 ppb (billion!).
A lead/led ssytem is often recommened as an effective and monitorable way to deal with arsenic. Determine if the As is AsIII or AsV.
The K5 system does have an As filter that is certified for 500 gallons (depending on the influent level) butthat is for drinking purposes only. For a whiole house application, the system has to be able to not only remove the As, but also provide adequate flow rate---and, not be affected by other contaminants such as iron, extreme pH levles and so on.
Lead/lag system
http://www.wqpmag.com/Arsenic-Remova...ns-article7624
Here is a detailed report;
http://www.epa.gov/nrmrl/pubs/600r05001/600r05001.pdf
Andy Christensen, CWS-II