SOURCE IDENTIFICATION AND NATURAL ATTENUATION OF ARSENIC CONTAMINATED GROUNDWATER IN NORTHEASTERN OHIO

Elevated level of As is found in groundwater located downstream from a possible contamination source in northeastern Ohio. The aquifer in the study area is Sharon conglomerate of Pennsylvanian age. It is partially confined. The groundwater is Ca-Mg bicarbonate type but could become Ca-Mg chloride/sulfate type in some of the shallow wells.

Arsenic together with Cr and Fe are determined by instrumental neutron activation analysis of TDS extracted from groundwater samples collected from 39 drinking wells. Arsenic concentrations in groundwater vary from less than 5 to 36 ppb. The lowering of As action level down to 10 ppb by the US EPA in 2006 raises concern over the quality of drinking water in the area. The strong correlations between As and Cr implies that As pentoxide was introduced with chromic acid in pesticide as wood preservative. The selective leaching from treated woods under neutral pH condition could produce an As/Cr ratio of 5. Arsenic could also form co-precipitate with Fe as FeAsO₄ or scorodite. However, As concentrations in groundwater of the study area are well below the solubility product.

Concentration gradients of As, Cr, Fe and groundwater flow pattern indicate that the source is from the north of the study area. The contamination plume appears to have been significantly attenuated naturally over the years. In addition, the surface flow in communicating with groundwater aquifer has dissected and removed portions of the plume as concentration contours of As, Cr and Fe all possess similar patterns. It, therefore, implies that the contamination source is no longer releasing these metals.