Mapping A Biogenetic Oxidation-Reduction Plume Using Spontaneous Potential At A Field Site In Oyster, Virginia

Abstract

At a contaminated site near Oyster Virginia, spontaneous potential (SP) measurements correlate closely with dissolved oxygen (DO) data from wells, indicating that a contaminant plume can be identified by this geophysical method. The field site is a 200 m X 190 m grassy field underlain by electrically homogeneous, Pleistocene, semi­consolidated sands bearing oxidized groundwater. At this site, DO measurements from 28 wells revealed a zone of reduction created by the biodegradation of buried tomato waste leaching from trenches. SP was measured using two sampling techniques: along a grid of 16 transects at 3 m intervals; and at 5 m intervals along 7 selectively oriented transects between significant wells. Spatial distribution of SP values compared favorably with the patterns formed by the DO data. This correlation was confirmed by the non-parametric Spearman rank correlation analysis, along two transects perpendicular to the redox boundary, which revealed a strong negative correlation between DO and SP at the 99% confidence level. Other potential sources of SP were ruled out by the use of resistivity, magnetometry, and ground penetrating radar, thus isolating redox potentials as the sole cause of SP. Therefore, SP can be used to map zones of reduction present in oxidized subsurfaces when all other sources of SP can be eliminated.