THE MEMORY EFFECT: IN SITU ELECTRON DONOR BIODEGRADATION RATES AS A FUNCTION OF EXPOSURE HISTORY IN A SHALLOW GROUNDWATER SYSTEM

Microbial-mediated redox reactions in groundwater systems can result in significant degradation or immobilization of contaminants that are harmful to the environment and human health. Groundwater conditions can be perturbed to stimulate native microbial communities via the addition of electron donors, electron acceptors, and/or nutrients in order to increase the rates of contaminant transformation.

The degradation rates of an electron donor are well known to vary widely between study sites depending on several site-specific factors including the physical characteristics of the solid subsurface media, the chemical species and concentrations of electron acceptors, and the structure and function of the microbial community. Recent scientific studies have suggested that the electron donor degradation rate can vary significantly within a single monitoring well based on the short-term exposure history of the groundwater system. Moreover, groundwater remediation practitioners suggest that the electron donor degradation rate can increase as a function of exposure history even after the physical and chemical conditions of the groundwater system have returned to their initial, pre-electron donor addition state; we refer to this as the “memory effect”.

The objective of this study is to rigorously characterize and control the physical and chemical groundwater system conditions before, during and after repeated electron donor exposures in order to scientifically describe the memory effect. The experimental field site is located at the Oak Ridge Field Research Center in Oak Ridge, Tennessee where previous scientific studies focused on electron donor additions for reduction and immobilization of radionuclides. The most recent electron donor addition was in 2005 and the site has since been under natural conditions.

This study will use the single well push-pull method to repeatedly expose a shallow and unconfined aquifer to ethanol (electron donor) and nitrate (electron acceptor). The biodegradation rate of ethanol under nitrate-reducing conditions will be compared between groundwater wells with and without a short-term exposure history. The memory effect described here may have broader implications on degradation rates of other electron donors that are contaminants such as crude oil and its refined products.