The Global Positioning System (GPS) is a surveying method used to precisely locate a three-dimensional position (latitude, longitude, and elevation) anywhere on the surface of the Earth. The system can be used at any time under all weather conditions. The GPS has replaced traditional survey methods as a low-cost, high-accuracy technology. It is also a very quick survey method to use, which on hazardous waste sites, serves to minimize personnel exposures to a variety of potential hazards.
GPS is based on a constellation of 24 non-geosynchronous satellites orbiting 21,000 kilometers above the Earth. The GPS satellites transmit coded radio frequency signals which are used to accurately determine the position of ground-based receivers. The range or distance to the satellites is determined by measuring the time the signal is sent from the satellite minus the time it arrives at the receiver, multiplied by the speed of light. By simultaneously measuring the distance to four satellites, one receiving position can be determined to an accuracy of 30 to 50 meters. If more than one receiver is used, the coordinates of a roving receiver can be determined relative to the other, fixed receiver to within an accuracy of a few meters to a few millimeters, depending on the type of receivers used, distance between the receivers, and the accuracy to which the satellite positions are known. High-precision GPS receivers, along with precise satellite orbits, can be used for high accuracy applications, while hand-held systems can be used for environmental mapping, for gathering data for geographic information systems, and for resource inventories.
GPS Advantages
GPS offers a cost-effective, accurate, and rapid means of determining the locations of points (e.g. soil sample locations and monitoring wells), lines (e.g. streams, roads, or utility lines), and areas (e.g. contamination plumes, wetlands, or buildings). Its numerous advantages for hazardous waste investigations and other environmental studies include:
- GPS does not require line-of-sight like conventional survey methods.
- The level of accuracy can be varied according to project needs. The 1- or 2-meter accuracy of hand-held units satisfies most applications at a very low cost. Millimeter accuracy can be provided for traditional survey applications.
- GPS positions and site characteristics can be converted into most common geographic information system (GIS) formats, eliminating time-consuming intermediate conversion steps.
- Site physical, chemical, and biological characteristics can be readily fixed to GPS coordinate files and databases. For example, the location of a monitoring well can be tied to its depth, diameter, and water level.
- GPS software can rapidly calculate the linear distance and area of surface features. For example, the area of a disposal pit can be determined by walking around the perimeter with a GPS receiver.
- GPS data can be taken from static (survey bench mark) or moving platforms (boat, plane, or car).
- GPS can be used to navigate and accurately relocate key locations in fields, on lakes, or in non-roaded areas for resampling, surveying, or other site investigations.
- GPS has a virtually unlimited number of applications in the conduct of environmental studies.
Some of the environmental applications include:
Environmental Resources: Mapping the aerial extent of wetlands and wildlife habitats; Determining the location and areal extent of forest stands; Mapping and locating cultural resource sites; Mapping and characterizing chemically-stressed areas
Hydrogeology: Mapping the extent and flow path of ground and surface water contamination; Determining the elevation of groundwater monitoring wells; Mapping boundaries of fluvial channels for erosion studies; Assessing acreages of fire damage
Hazardous Waste Site Investigations: Mapping monitoring well, sampling, and surface and subsurface feature locations; Installing and mapping sampling grids; Coordinating with real-time surveys such as soil gas testing to delineate contaminant plumes and guide subsequent placement of monitoring wells.
Surveying Landfill 04
GPS technology was used in a drum removal and disposal operation at Landfill 04, Eielson Air Force Base (AFB), Alaska. Landfill 04 is a former Army landfill used in the 1950s and 1960s which reportedly received general refuse, small quantities of waste oil and spent solvents, and possibly small amounts of munitions and spent cartridges. More recently, the landfill was used as an emergency ordnance demolition area, where small munitions were incinerated in a burning kettle.
The landfill contained approximately 800 drums at or near the surface in 10 drum piles scattered over a square-mile area. The objectives of the drum removal action were to locate, inspect, sample, and remove the exposed drums. Individual drums and drum piles were located in heavily wooded terrain, and often were buried or overgrown with vegetation and were difficult to see. Access to the drums was further complicated by uneven terrain and steep hillsides. For these reasons and because we wanted to minimize personnel exposures to hazardous materials and conditions, GPS was selected as the survey method for the project.
Buildings, roads, drum piles, perimeter fencing and other site features were surveyed using a base receiving station and a backpack-mounted roving receiver. The base receiver was positioned on a tripod at an existing survey marker with known coordinates at Eielson AFB. Survey points for the roving receiver were entered into the GPS data logger, and the survey data downloaded to a notebook computer at the end of the day. The survey data were generally accurate to within 1 to 2 meters. The site map generated from the GPS data was provided to a drum removal subcontractor who then used the map and location data to ensure that all drums were accounted for and removed from the site.
Using GPS for the Landfill 04 drum removal operation minimized time personnel spent in a potentially hazardous area. Another advantage was that GPS did not require line-of-sight reference to a base point. GPS operators could simply walk to a survey point and determine their coordinates regardless of their location in dense vegetation or on steep hillsides. It also proved to be a cost-effective technology for surveying the drum pile locations. The GPS survey was completed in two days. Furthermore, the electronic data acquired were easily translated into a site map for use by a drum removal subcontractor. The use of GPS coordinates made it relatively easy for the removal contractor to locate the drums in the dense underbrush.
GPS was also used at Eielson AFB for locating abandoned underground storage tanks, groundwater wells, waste oil pits, and areas with petroleum contaminated soils as well as World War II and Cold War historical sites and other abandoned military sites that had been overgrown with vegetation. These sites were initially identified using aerial photographs, older base maps, or during annual surveys of the base boundaries by air. The original roads or entry points were no longer obvious. Once identified and located, the site positions were recorded using GPS. The coordinates from GPS were again used to create site maps and the instrument made it easier to return to the sites without the use of reliable landmarks, trails, or other distinguishing features. Eielson AFB subsequently was able to conduct preliminary assessments and site investigations to determine whether environmental cleanups were required.
Timothy Mustard, CIH, is a health and safety manager for Parsons Engineering Science in Denver, Colorado. He was the Project Manager for the Eielson AFB drum removal action. He serves as project manager or safety manager for hazardous waste operations nationwide.
Robin Stankoff is an environmental engineer at Eielson AFB, Alaska. She served as the government's project manager for the Eielson drum removal action.