The use of an electrochemically based probe for in situ determination of heavy metals in groundwater is described. The in situ probe, consisting of a microlithographically fabricated iridium ultramicroelectrode array sensor (UMEA), a custom-built miniaturized potentiostat and microcontroller, is connected to a laptop computer for control and data acquisition. The utility of this probe for rapid on-site screening of heavy metals was shown by conducting a proof-ofconcept field demonstration at a metals contaminated landfill site located at Hanscom Air Force Base in Bedford, MA. Using the electroanalytical technique of square wave anodic stripping voltammetry (SWASV), the sensor was used to measure both the labile and total pH <2 forms of copper, lead, cadmium, and zinc at the parts-per-billion level. The on-site and in situ measurements were compared to laboratory-based electroanalytical measurements and to contract laboratory U.S. EPA Method 200.7 ICP analysis. The results indicate that in situ electroanalytical measurements of labile metal species are feasible, providing a rough estimation of metals concentrations and trends in their behavior. The in situ and on-site results agree to within an order of magnitude with the laboratory determinations. This study demonstrates that such electroanalytical-based devices may be of value for an initial, rapid, low-cost, screening of heavy metal contaminated sites.