Evolution of Dissolved Organic Matter Along a Septic System Plume: Evidence of Sustained Biogeochemical Activity in the Groundwater Zone

Abstract

AbstractDissolved organic matter (DOM) is an important energy source for biogeochemical reactions. However, DOM is often assumed to be recalcitrant in the groundwater zone due to extensive microbial processing in the overlying soil zone and long groundwater residence times. Consequently, further heterotrophic microbial processes proceed at inherently low rates. Septic systems provide an ideal opportunity to study the evolution of groundwater DOM due to the development of confined and easily identifiable plumes of known age. Here we use measures of DOM composition (size‐exclusion liquid chromatography, ultraviolet absorbance, and C:N) to quantify DOM evolution along an exceptionally well‐characterized septic‐impacted groundwater plume. Elevated concentrations (5 to 8 mg C/L) close to the tile bed decrease to values below 2 mg C/L with lower UV‐absorbing components normalized to overall DOM concentration (specific ultraviolet absorbance) along the six‐year‐old plume flow path. The humic substance fraction (HSF) comprises the largest percentage of DOM and decreases in concentration along the septic plume. HSF properties also change with travel time becoming lower in molecular weight, specific ultraviolet absorbance, and C:N. Denitrification continues along the length of the plume concomitant with changes in DOM and HSF composition. Thus, HSF, typically considered recalcitrant in subsurface environments, is actively cycled by microorganisms on multiyear time frames in accordance with the recent paradigm shift for soil organic matter evolution. Lastly, measures of DOM composition indicate changes in DOM that are not evident from measures of concentration alone.