SEQUENCE for Visualizing and Evaluating
Natural Attenuation

Introduction to SEQUENCE

SEQUENCE (U.S. Patent Pending) is a revolutionary new software package that provides an innovative approach for visualizing the effects of natural attenuation based on a modified radial diagram method. Given the simplicity and effectiveness of this visualization approach, SEQUENCE visual aids are ideally suited for preparing presentations of natural attenuation to both technical and nontechnical audiences.

Specifically, SEQUENCE visual aids may be used to simultaneously show spatial and temporal trends for multiple organic pollutants on one site map. For some sites, this one radial diagram map may provide the primary line of evidence necessary to support natural attenuation as a site remedy. In addition, the unique functionality and flexibility of SEQUENCE allows for the preparation of SEQUENCE-Redox Maps which are now being used to identify trends in geochemical indicators (including electron acceptors and metabolic by-products). These trends which are clearly identified from SEQUENCE-Redox Maps provide an important secondary line of evidence supporting the occurrence of natural attenuation in groundwater.

Click here for background information on natural attenuation processes.

Here's How It Works...

SEQUENCE is based on a modified radial diagram method and may be used to plot radial diagrams representing measured concentrations or calculated molarity values for one or more chemicals. Each radial diagram represents chemical data corresponding to one monitoring well location and consists of multiple axes (one for each chemical) extending radially around a uniform origin (see above figure). Each axis can plot multiple measured values thus allowing you to easily compare sampling data from multiple sampling events and/or compare downgradient contaminant levels vs. background concentrations.

Identifying Trends in Geochemical Indicators Using SEQUENCE-Redox Diagrams
The following figure is an example of a SEQUENCE-Redox radial diagram. This diagram depicts the concentrations of typical electron acceptors (oxygen, nitrate, sulfate) and metabolic by-products (manganese, iron, methane) measured in groundwater. The outer data series represents background concentrations of these
redox indicators that may be measured in an aerobic aquifer. The inner data series represents the concentrations of these redox indicators that may be measured at a monitoring well situated in a portion of the aquifer where extensive biodegradation reactions have occurred.

The axes of a SEQUENCE-Redox diagram are aligned in the same 'sequence' that these geochemical indicators are influenced during the biodegradation of organic compounds. For example, oxygen is the most preferred of the available electron acceptors in groundwater because it provides the greatest amount of energy for microbial metabolism. When the oxygen has been depleted, the next electron acceptor in the sequence to be used in biodegradation reactions is nitrate. Once the dissolved nitrate has been depleted, microorganisms in the soil will preferentially reduce the solid manganese and iron coatings on soil particles resulting in the dissolution of manganese and iron into groundwater. As the manganese and iron in the soil become depleted, the microorganisms will preferentially reduce sulfate during the biodegradation of the organic compounds. The most reducing conditions during biodegradation will occur when the supply of all of these electron acceptors has been depleted and the microorganisms begin reducing carbon dioxide in groundwater to produce methane (this process is referred to as methanogenesis).

In summary, the concentrations of the dissolved electron acceptors (oxygen, nitrate, sulfate) will decrease during biodegradation, and the concentrations of the dissolved metabolic by-products (manganese, iron, methane) will increase. SEQUENCE has been specifically designed with the unique graphical capabilities required to prepare these specialized SEQUENCE-Redox diagrams for analyzing the natural attenuation trends by:

• Orienting the axes of the radial diagram in the same sequence that these indicators are influenced during biodegradation.
   
• Specifying scale and directions for each axis (i.e., concentrations increase radially away from or towards the origin), based on whether the indicator is an electron acceptor or a metabolic by-product.
   
• Plotting both the background concentrations and measured well concentrations on the same radial diagram for a simple and quick comparison of biodegradation effects on each of these indicators.

Evaluating Degradation of Multiple Organic Contaminants
The following figure is an example of a SEQUENCE BTEX-CAH radial diagram map. This diagram depicts the concentrations of the primary contaminants measured in the groundwater (BTEX, TCE, DCE, VC and Cl). The outer data series (indicated by the purple line) represents the concentration levels for each of these contaminants as they are measured at the source of contamination. The inner data series (indicated by the red line) represents the concentrations of these contaminants measured at the monitoring wells located downgradient from the contaminant source.

These diagrams, when plotted on a site map, clearly demonstrate decreasing concentrations of BTEX, TCE and DCE downgradient of the contaminant source. This trend of decreasing contaminant levels provides a defensible primary line of evidence that natural attenuation processes are actively degrading the contaminant levels in the groundwater. In addition, the increased concentrations of degradation by-products such as VC and Cl in Wells B, C and D provide further evidence of intrinsic biodegradation breaking down the BTEX, TCE and DCE in the groundwater. The formation of degradation by-products provides a secondary line of evidence to support the occurrence of TCE natural attenuation at the site. The decreasing concentrations of vinyl chloride at Station F, as shown on the above BTEX-CAH radial diagram map, suggests that this potentially harmful daughter product is also being attenuated farther downgradient from the source area. SEQUENCE can also be used to evaluate and illustrate temporal trends at selected sampling locations for multiple sampling events. This type of analysis can be used to clearly illustrate decreasing (or increasing) concentrations of multiple contaminants at multiple locations over time. Compare this analysis method with typical analysis methods requiring multiple contour maps for each contaminant of concern and you will immediately see the advantages of using SEQUENCE to prepare a clear and concise presentation of the relevant data.

The SEQUENCE Data Management System

The input data required by the SEQUENCE data management system includes the following:

• Station ID (name of the sampling location)
• Station coordinates (X, Y location)
• Chemical species being analyzed
• MCLs for each chemical species (optional)
• Number of sampling events
• Chemical species concentrations at each station for each sampling event (flexible concentration units)

SEQUENCE Radial Diagram Options

• Line thickness and color
• Axes range and tick marks
• Symbol size and color
• Label fonts and colors
• Size of radial diagrams plotted on the map
• Import and overlay of AutoCAD .dxf site maps
• Plot titles
• Much more

Clearly, SEQUENCE is a 'must have' tool for evaluating and visualizing natural attenuation processes. The built-in data management system together with an easy-to-use interface and comprehensive graphical tools make it the ideal choice for analyzing any site with any number of sampling points.

Documentation

SEQUENCE is accompanied by a comprehensive user's manual containing easy-to-follow instructions and a step-by-step tutorial. SEQUENCE also comes with detailed descriptions of the natural attenuation processes and helpful guides on identifying trends and presenting data.

Hardware Requirements

• PC Pentium
• 32 MB RAM
• 25 MB free disk space
• SVGA display and mouse
• Windows 95/98 or NT installed