• Home
• Contact
• Applications
• Products
• About Kavlico

RSS Print

Creating Optimal Conditions for Hydrocarbon Remediation
Jerome C. Vernon, Datawrite Research Co.; Bob Arkell, Kavlico Corp.

Bioventing has gained recognition as a nondisruptive method for dealing with many petroleum spills. But making it work requires dynamic monitoring for the continuously changing soil conditions.

Aggressive approaches to cleaning up hydrocarbon spills have been highly successful. But they have met with strong criticism. Such approaches can involve great cost, inconvenience, and interruption of normal activities in the immediate area, and sometimes cause further harm to people and other organisms. Even the removal of contaminated soil can cause problems because it often endangers wildlife that survived the spill.

In recent years, interest has increased in less disruptive approaches to the cleanup of hydrocarbon spills. Of these, bioventing has received the most attention. This approach involves indigenous microorganisms (actinomycetes, algae, bacteria, fungi, and protozoa) in the soil that use petroleum compounds as food energy and, in the process, produce carbon dioxide and hydrogen sulfide. Bacteria, the most important of these microbes, use up to 35 lb. of oxygen in degrading 10 lb. of petroleum products. Midweight petroleum products—including gasoline, jet fuel, kerosene, and diesel fuel—respond best to bioventing.

Stimulating a remediation site’s indigenous bacteria to digest these products involves introducing oxygen to the soil. The goal is to increase the oxygen content enough to encourage natural biodegradation for the purpose of eliminating contaminants, but not so much as to cause the contaminants to volatilize. This can be achieved naturally either through barometric pumping or through introducing air into the soil by injection or vacuum pressure.

Barometric Pumping

Barometric pumping depends on a pressure differential between the atmosphere and the soil. Such a differential occurs during barometric pressure changes primarily due to diurnal (day/night) influence, weather condition, and ground-water level [1]. Although the pressure differential levels (peaks and valleys) are site specific, differentials of up to 0.7 in. w.c. can exist when the top layer of the soil has low permeability (e.g., clay), while the lower layers are more permeable (e.g., lightly packed sand or gravel). During high land pressure periods, air is forced through the vadose zone until it reaches the water table and the pressure equalizes. When surface pressure decreases, the air in the soil escapes to the atmosphere. This pressure cycle typically consists of eight hours of inhalation (night), eight hours of exhalation, and a pair of four-hour transition periods.

Precise monitoring of this dynamic phenomenon is critical to determining the adequacy of the air supply in the soil and the progress of natural bioattenuation. A tool to accomplish this monitoring consists of a low-range differential pressure sensor that monitors the diurnal pressure cycle, and an in-situ oxygen detector that senses changes in soil oxygen level that directly affect biological activity.

Monitoring subsurface pressure changes and oxygen levels necessitates either digging a new monitoring well into the vadose zone or using an existing one. A monitoring well is most often constructed of 2 in. PVC pipe with slit-PVC pipe well screen (see Figure 1).

An oxygen detector with an internal vapor/pressure sample cavity and tubing can be installed in the well near a filter pack (sand pack screen zone). Alternatively, the oxygen detector can be buried directly in sand (filter pack) and top-sealed with grout at the surface. While the latter method provides the best results, it precludes the oxygen detector from being retrieved and reused at other locations. With either installation option, the well must be sealed completely in order to prevent vertical air movement.

Many existing monitoring and injection/ extraction wells have long screen areas that can easily act as conduits for gas movement, causing concentration fluctuations where a significant diurnal variation is present. To adequately assess biological activity in wells with more than one screen zone, oxygen detectors may be deployed at several locations to monitor different soil layers.

The differential pressure sensor and dataloggers are placed within the monitoring well vault at the surface. The positive port of the pressure sensor is attached to 1/8 in. i.d. tubing, which extends directly to the subsurface oxygen detector’s vapor collection cavity. This tubing also allows in-situ calibration of the oxygen sensor by pumping atmospheric air into the sensor cavity. The negative port of the pressure transducer vents to local atmospheric pressure, also through 1/8 in. i.d. tubing connected to a hydrophobic filter that waterproofs the sensor. The pressure sensor is attached with Velcro strips to the inside cavity of the well case; the pressure ports are oriented vertically so that they point to the bottom of the well. Electrical leads from the pressure and oxygen sensors are attached to their respective dataloggers by means of weatherproof three-position connectors (see Figure 2).

The temperature-compensated differential pressure transducer operates over a wide temperature range and provides 0.25-4.0 VDC output. The ceramic capacitive technology used to sense low pressures provides the accuracy and long-term stability that is so critical for reliable pressure readings. The temperature-compensated oxygen detector operates over a range of -3C to 60 C and provides an output of 2.5-43 mV (normal ambient level). The oxygen detector maintains a high degree of responsiveness throughout its seven-year life span.

Although the datalogger can take readings at intervals of 5 s to 24 hr, measurements are typically taken every 2 hr. Microprocessor-controlled circuitry turns the sensors on for several milliseconds, takes a reading, stores the data, and then shuts off the power until the next reading. The maximum number of readings is 500. Data are downloaded into a PC or HP Palmtop via the micrologger software and saved in a common format that allows easy transfer to spreadsheets for charting and graphing. Integral software calibration procedures in each datalogger provide accurate readings in engineering units desired. A 9 VDC transistor battery provides power for more than one year of unattended operation.

Bring in the Air

Where circumstances are not favorable to the use of barometric pumping, air is introduced into the soil either by creating a vacuum condition (vapor extraction) or by direct injection. The specific pressure range of the transducers used in this type of application depends substantially on source pressures applied, permeability, and distance from the source. Pressure normally declines the further the monitoring points are placed from the center of the radius of influence. Monitoring soil pressures at several points is recommended, as this can reveal barriers and pathways of flow. The locations and number of monitoring points depend on the type of soil found at the site. For fine-grained soils, around fringe or inside plume, monitoring points can be as close as 15 ft, while for coarse soils or background data comparisons they can be up to 100 ft apart.

The level of biodegradation activity from bioventing or vapor extraction systems can be determined through enumeration of the overall microorganism population and/or that of a compound-specific population. Field respirometry measurements are done dynamically by monitoring oxygen levels before, during, and after bioventing soil treatment to determine the process of bioremediation [2, 3]. With this method, a steady-state level, Css, at a monitoring well location is subtracted from source oxygen level, Cin (~20.9%), over a given time, t. This is expressed as:

An evaluation of time can be approximated as the time when the oxygen level reaches the average of the initial and steady-state concentrations:

For example, for an initial steady-state oxygen (Co) level of 3% to reach half (~7%) of its full resulting steady-state condition of 14% takes 33 hr. The respirometry rate at this location is ~5% per day, determined as:

References
1. D.C. Foor et al. Nov. 1994. “Passive Bioventing Driven by Natural Air Exchange,” Batelle, presented at Petroleum Hydrocarbons and Organic Chemicals in Groundwater, sponsored by NGWA, Houston, TX.
2. D.X. Li. 1995. “Bioventing Feasibility Assessment and System Design Using Subsurface Oxygen Sensors,” J Air Waste Management Assoc., 45: 1047-3289.
3. R.E. Hinchee and S.K. Ong. 1992. “A Rapid In Situ Respiration Test for Measuring Aerobic Biodegradation Rates of Hydrocarbons in Soil,” J Air Waste Management Assoc., 42: 1005-1312.

Jerome C. Vernon is president and CEO, Datawrite Research Co., 2140 S. Church St., Visalia CA 93277; 209-739-1003. Bob Arkell is applications manager, Kavlico Corp., 14501 Los Angeles Ave., Moorpark, CA 93021; 805-523-2000, fax 805-523-7125, www.kavlico.com.

Published in October 1998 Issue of Sensors Magazine