As arsenic treatment systems for groundwater are being designed and installed, it is becoming apparent that many co-occurring anions can be present along with arsenic. Geological formations containing arsenic are often enriched with other trace metals, including uranium, antimony, tungsten, vanadium, and silica. With exception of silica, U, Sb, W, and V have all been implicated with various adverse human health effects. New regulations mean that many small utilities still do not know how much uranium is present in their waters. While silica does not pose a health effect, its presence detrimentally impacts arsenic removal by iron adsorbents. Elevated phosphate is less likely to occur in conjunction with elevated arsenic, but we have data from several sites with significant P concentrations. There is also the potential for chemical contaminants (e.g., perchlorate) to occur with arsenic - and systems designed to remove ClO4 - generally do not remove arsenic. Therefore data from multiple groundwaters across the US were studied for simultaneous removal of arsenic and co-occurring anions.