Testing water samples for arsenic (As) 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. This latter consisted in the quantification of As levels in surface and drinking water at the trace level (nM - ppb) using the electroanalytical technique of anodic stripping voltammetry and the method of standard addition. Results were then 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.