In-Situ, Inc.

Real-Time surrogate monitoring of Groundwater pollutants at Oil and Gas production sites

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Courtesy of In-Situ, Inc.

Using surrogates to monitor groundwater quality in real time
The development of oil and gas resources, especially by hydraulic fracturing, has increased concerns about potential groundwater contamination (Figure 1).Typically, grab sample collection and expensive analytical tests are required to determine whether or not groundwater quality has been compromised.Time-consuming analytical methods may be expensive and difficult to perform. Site operators, stakeholders, regulatory agencies, and the public often need information instantly in order to meet regulatory requirements and to prevent contamination and exposure.

Real-time data can keep interested parties informed and able to quickly address potential contamination of groundwater supplies. But real-time monitoring of criteria pollutants such as methane and hydraulic fracturing fluid is not practical with currently available technologies. However, by identifying reliable pollutant indicators or surrogates, operators and regulatory agencies can monitor sites efficiently and respond to groundwater quality changes that may indicate potential breakthrough of contaminants.

Real-time water quality monitoring networks can be established by using telemetry systems, robust water quality sensing technologies, web-based data centers, and mobile devices that can receive alarm notifications (text, email, phone call). Before such a monitoring network can be established, pollutant-surrogate relationships must be established.

A surrogate is something that replaces or acts as a substitute for another, and surrogate measurements can be monitored continuously and used to estimate concentrations of a certain water-quality parameters for which continual data are not available or not practical. Using surrogates reduces grab sampling and analytical testing costs, decreases manual data analysis, and provides real-time information on a system's physical properties (Christensen et al 1999). For example, significant research has been conducted on using conductivity as a surrogate for chloride (Granato et al. 1999; Hem 1992). Conductivity sensors offer stable operation and are not sensitive to drift.

For oil and gas operations, the potential exists to use oxidation-reduction potential (ORP) or dissolved oxygen (DO) as a surrogate for methane and to use conductivity/ total dissolved solids (TDS) as a surrogate for hydraulic fracturing (fracking) fluid (Table 1). Like conductivity sensors, ORP sensors offer a stable, low-cost solution for monitoring groundwater quality in real time.

Establishing a pollutant-surrogate relationship
Every site has unique geochemical properties, which means that the use of real-time monitoring of a surrogate may not be possible in all locations.

To establish a pollutant-surrogate relationship, groundwater quality profiles of several monitoring wells throughout an oil/gas development site must be established. Representative samples are collected from monitoring wells throughout the site, and a certified lab conducts a bench-scale study to establish a correlation between the pollutant and the potential surrogate. A standard curve can be developed and used to estimate the in-field pollutant concentration through real-time measurement of the surrogate.

An established regression equation may be used for all monitoring stations in an area (Cain 1987). Regression models can have disadvantages in that the predictive ability of the regression model is a function of the number and distribution of available measurements from the population being studied (Granato et al. 1999). Once a good correlation has been established between a pollutant and surrogate, a real-time water quality monitoring network can be set up.

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