Tactic AM-1: Delineation and Sampling of the Spill Area
It is important to mark the spill area so that its boundaries can be located at a later date, especially if the site is snow covered. Airborne Forward-Looking Infrared (FLIR) photography can be used to identify the spill area even if the site is snow covered (Fig. 123). Delineation should begin as soon as possible after the spill has been contained. Correct the boundary location as needed. The contrast between clean and contaminated snow is especially useful for visually delineating affected areas. Even relatively clear fluids such as diesel, methanol, and produced water, can cause dramatic changes in the color and physical characteristics of snow.
To delineate large spill areas (>1,000 square feet), two workers walk the perimeter of the spill in opposite directions from a common starting point, and place markers every 50 to 100 feet to provide a visible boundary. The two workers should meet midway around the perimeter of the spill area, and then retrace each other’s routes to confirm the delineation. While walking, they look for visible impacts, including spilled substance on the ground; discoloration of plants or soil; sheen on standing water or foliage; and dead or damaged vegetation. For smaller spills, a single worker may perform the delineation. Aerial photographs are of great value for identifying and mapping site features and spill boundaries.
A scaled map of the site probably will be required for planning, monitoring, and reporting purposes and will be most useful if prepared using professional surveying methods (Figs. 124–125). As soon as practical after containment, a sampling system should be implemented, to be used for monitoring (Tactics AM-2 and AM-4). The preferred method is systematic sampling at nodes on a grid system, which facilitates the unbiased selection of sampling locations (Fig. 126). Depending on the shape of the affected area, the grid should be a square or rectangle that is large enough to encompass the containment area and some adjacent unaffected (reference) tundra. Vegetation monitoring plots should be located at the same locations where samples were collected.
For affected tundra areas that are ~0.5 acre (~150′ x 150′) in size, a grid with 15-ft spacing would create 100 nodes where lines intersect. Typically, samples are collected at a subset of nodes, chosen using an unbiased selection method. This approach can be used for a site of any size simply by expanding the grid. For larger sites (> 1 acre), the distance between nodes can be changed to create a reasonable number of potential sampling locations, or to meet specific sampling objectives. For example, the spill area may be subdivided into areas with high, medium, and low concentrations of contaminants (Fig. 127). If the sampling plan stipulates that 10 samples should be collected from each area, the size of the grid can be adjusted to provide at least 10 potential sampling locations in each area. When the affected tundra includes patterned ground, the grid distances should be less than the average polygon diameter to avoid sampling bias among topographical features (e.g., polygon centers, rims, and troughs).
Two methods exist for establishing the grid system at a site. Importantly, neither method interferes with cleanup operations, because permanent stakes or markers are not needed to locate sampling stations within the spill area. Nevertheless, sample stations can be relocated with good precision. The first method uses permanent markers (e.g., survey nails, wooden stakes, or rebar) that are driven into the ground in two parallel rows on opposite sides of the spill, and that are separated by the appropriate distance between grid nodes. Each marker is labeled with a row number (e.g., 1 through 10), letter (e.g., A, B, C), or distance from a corner of the grid (e.g., 0, 2, 4, 6 for a 2-ft grid). Individual sampling points are then located by stretching a tape between the corresponding end stakes and sampling is done at specified distances along the tape. The second method uses a “virtual grid” created with computer software (e.g., AutoCAD). The coordinates of selected sample stations are then uploaded to a field computer, which is used to navigate to the sample location (Fig. 128). The use of a virtual grid generally requires contracting the services of a professional surveyor, although other personnel with special training and equipment can also establish a virtual grid.
A variety of maps probably will be needed and should include at least the following elements:
- Location of the spill source.
- Boundary of the affected area.
- Areas of low and high concentration of spilled substance.
- Adjacent roads and structures.
- Tundra types within affected area.
- Sensitive areas and habitats (identification may require special training or additional work).
- Nearby drainages or water bodies, most likely direction of water movement, location of culverts in road.
- Slope and topography (e.g., elevation contours).
- Location of monument used to control survey locations and elevations.
- Sampling grid that can be overlaid on the site map.
- Sampling locations (including background samples), preferably at nodes on sampling grid.
- Vegetation study plots, transects, or photo-plot locations (include direction of photo).
- North arrow, scale and approximate latitude and longitude of the site.
Considerations and Limitations
- Technical literature (e.g., http://www.epa.gov) is available to help design a plan for sampling and data collection.
- The area of an uncontained spill will expand with time on all types of tundra.
- The boundaries of spills of saline or water-soluble substances are difficult to delineate visually, especially when snow is absent. These spills tend to spread rapidly except in winter, when the fluids mix with snow and freeze. If salts or other water-soluble compounds are present in high enough concentrations, the vegetation may die or show signs of stress (wilting, discoloration, loss of foliage) in affected areas.
- Seasonal frost action in the soil may push wooden stakes out of the ground over time (i.e., frost-jacking). Wooden stakes may also be disturbed by winter vehicle traffic in the area. Metal rebar may pose a physical hazard. Survey pins (e.g., 9-inch nails) with bristles are preferred because they do not pose a safety hazard, they are less affected by frost-jacking, and they can be relocated with a metal detector.
- Plywood boardwalks may be needed to protect tundra from trampling.
- Considerations for site assessments used by the Alaska Department of Environmental Conservation are found in 18 AAC 78.090.
- This tactic has been adapted from Tactics T-1 and T-2 in the Alaska Clean Seas Technical Manual (http://www.alaskacleanseas.org/techmanual.htm).
Equipment, Materials, and Personnel
- Permanent markers (9-inch nails or wooden lath stakes) (1 or 2 workers) – to mark spill perimeter and grid system.
- Handheld GPS unit (1 operator) – to provide coordinates for initial site delineation.
- Professional survey equipment and personnel (variable) – to permanently mark grid layout, sampling locations, and to provide a scaled drawing.