Real-time soil and groundwater monitoring via spatial and temporal resolution of biogeochemical potentials.

Oxidation-reduction potentials (ORP) govern the transformation of organic compounds in water and soils. Standard methods for measurements of ORPs in subsurface setting are deeply flawed due to heterogeneous samples from wells, failure to capture weakly poised redox couples, and biases with ex-situ measurements. In this study, we developed a real-time in-situ ORP sensor system that continuously measures biogeochemical electrical potentials using vertically distributed point sensing electrodes in direct contact with the soil. Three hundred thousand data points, providing a full range of aqueous ORP values (+ 600 to - 600 mV vs. Ag/AgCl) were collected over 513 days to spatially and temporally resolve subsurface biogeochemical processes at a former petroleum refinery. Water quality and microbial community data support the validity of the ORP data. In locations impacted by petroleum light non-aqueous phase liquids (LNAPLs), barometric pumping and ebullition events drive near-daily cycles of ORP changes in the vadose zone of 400 mV. When only dissolved phase hydrocarbons are present, near-daily redox cycles are absent and values for ORP indicate methanogenic conditions immediately about the water table. When hydrocarbons are not present, redox conditions are more oxidizing by + 400 to + 700 mV. The embedded electrodes revealed variations in hydrocarbon biodegradation in time and space that cannot be resolved by collection and analysis of conventional samples of groundwater and soil gas.

A Tiered Approach to Evaluating Salinity Sources in Water at Oil and Gas Production Sites.

A suspected increase in the salinity of fresh water resources can trigger a site investigation to identify the source(s) of salinity and the extent of any impacts. These investigations can be complicated by the presence of naturally elevated total dissolved solids or chlorides concentrations, multiple potential sources of salinity, and incomplete data and information on both naturally occurring conditions and the characteristics of potential sources. As a result, data evaluation techniques that are effective at one site may not be effective at another. In order to match the complexity of the evaluation effort to the complexity of the specific site, this paper presents a strategic tiered approach that utilizes established techniques for evaluating and identifying the source(s) of salinity in an efficient step-by-step manner. The tiered approach includes: (1) a simple screening process to evaluate whether an impact has occurred and if the source is readily apparent; (2) basic geochemical characterization of the impacted water resource(s) and potential salinity sources coupled with simple visual and statistical data evaluation methods to determine the source(s); and (3) advanced laboratory analyses (e.g., isotopes) and data evaluation methods to identify the source(s) and the extent of salinity impacts where it was not otherwise conclusive. A case study from the U.S. Gulf Coast is presented to illustrate the application of this tiered approach.