Testing water samples for arsenic contamination has become an important water quality issue worldwide. Arsenic usually occurs in very small concentrations, and a sensitive analytical method is needed. We present here a 1-day laboratory module developed to introduce Earth Sciences and/or Chemistry student undergraduates to key aspects of this topical issue. In this practical session, students were first introduced to the worldwide problems of arsenic contamination in groundwaters as a motivation of the experimental work. The latter consisted of quantification of arsenic levels in surface and drinking water at trace levels (nM, ppb) using the electroanalytical technique of anodic stripping voltammetry and the method of standard addition. Results were discussed with respect to water quality guidelines and geology. The complexity of data interpretation in this exercise can be tailored to a range of abilities and subject areas suited to the students and the course.