Matt,,are you always concerned by the topic? Have you been in touch with Clement from KLEARIA? They have very reliable practical sansors tha will match your needs, in the field or on line.
Dear Matt, We have developped innovative metal monitoring solutions for heavy metals in water (portable or in line instrument)
Our solution is already validated by a world leader company in bottle water so we demonstrated the robustness of the system (precision equivalent to an ICPMS).
There are several institutions in UK dealing with this aspect especially, Atkins International who did a wonderful job in India for Thermal Power Plants. You may please contact them. All the best.
If it's a deep groundwater well, the arsenic will be on a constant stady level. So it's either there or not. Regular testing (yearly) looks like a more feasably option..