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

Dissolved 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. Plain Language Summary On-site wastewater disposal in septic systems is a known source of groundwater pollution, including large amounts of organic matter. Dissolved organic matter (DOM), made up of thousands of different molecules, plays a number of important roles within the environment. For instance, DOM is an energy source for microbes. Generally, groundwater DOM can be difficult to use as an energy source as easily degradable components have been lost as slow groundwater transport leads to enhanced physical, chemical, and biological processing. However, recent studies find continuous reworking and recycling of DOM occurring on longer time scales. Septic plumes provide an ideal environment to study this as they provide a known source of easily degradable DOM that can be tracked. Our objective was to measure changes to the concentration and composition of DOM along an extensively studied septic-impacted plume. This allowed us to link DOM evolution with known groundwater ages and known processes. We found that as DOM degraded, most of the change over time came from the humic substance fraction. We were also able to calculate one of few rates for wastewater DOM in groundwater. Results from this study illustrate how DOM continues to change on long time scales in ways that are not observed by measuring only its concentration.