At low pH values (4<pH<7), As(V) is retained on an anion exchange resin, while
As(III) is not.

Therefore, you can split a sample. Keep one part for total As determination.
Run the other part through an anion exchange column and keep the effluent.
Analyze both samples for total As by atomic absorption, ICP-MS, or any other
suitable method.

As(V) is total As minus As(III). I doubt that any of the field kits are
sufficiently precise or accurate to do As speciation in the field. See W. H.
Ficklin, Talanta 30:371(1983) and M. Edwards et al., J. Am. Water Works Assoc.
90:103(1998).