70 « The Containment of Underground Nuclear Explosions The monitoring stations are extremely sensitive; they can detect changes in radiation exposure due to changing weather conditions. For example. during periods of low atmospheric pressure, gamma exposure rates are elevated on the order of 2 to 4 uR/hr because of the natural radioactive products being drawn out of the ground. To inform the public, data from the community monitoring stations are posted at each station and sent to local newspapers (figure 4-8). In addition to the 18 community monitoring stations, 13 other locations are used for the Air Surveillance Network (shown as circles in figure 4-7) to monitor particulates and reactive gases. The air surveillance networkis designed to coverthe area within 350 kilometers of the Nevada Test Site, with a concentration of stations in the prevailing downwind direction. The air samplers draw air through glass fiber filters to collect airborne particles (dust). Charcoal filters are placed behind the glass fiber filters to collect reactive gases. These air samplers are operated continuously and samplesare collected sures to a specific individual. By measuring exposures at fixed locations, it is possible to determine the maximum exposure an individual would have received had he or she been continually present at that location. In addition, about 50 people living near the test site and all personnel who work on thetest site wear TLD’s. All TLD’s are checked every 3 months for absorbed radiation. Radioactive material is deposited from the air onto pastures. Grazing cows concentrate certain radionuclides, such as iodine-131. strontium-90, and cesium-137 in their milk. The milk therefore becomes a convenient and sensitive indicator of the fallout. The Environmental Protection Agencyana- lyzes samples of raw milk each month from about 25 farms (both family farms and commercial dairies) surrounding the test site (figure 4-11). In addition to monthly samples, a standby milk surveillance network of 120 Grade A milk producers in all States westof the Mississippi River can provide samples in case of an accident (figure 4-12). Samples from the standby network are collected annually. three times a week. The Air Surveillance Networkis supplemented by 86 standby air sampling stations located in every State west of the Mississippi River (figure 4-9). These stations are ready for use as needed and are operated by local individuals or agencies. Standby stations are used 1 to 2 weeks each quarter to maintain operational capability and detect long-term trends. Another potential exposure route of humans to radionuclides is through meat of local animals Samples of muscle, lung, liver, kidney. blood. and bone are collected periodically from cattle purchased from commercial herds that graze northeast of the test site. In addition, samples of sheep. deer. horses, and other animals killed by hunters or Noble gas and tritium samplers are present at 17 of the air monitoring stations (marked with asterisk in figure 4-7). The samplers are located at stations close to the test site and in areas of relatively low altitude where wind drains from the test site. Noble gases, ike krypton and xenon, are nonreactive and are sampled by compressing air in pressure tanks. Tritium, which is the radioactive form of hydrogen, iS reactive but occurs in the form of water vapor in air. It is sampled by trapping atmospheric moisture. The noble gas and tritium samplers are in continuous operation and samples are recovered and analyzed weekly. analyzed for gamma-emitters. Bone and liver are analyzed for strontium and plutonium: and blood/ urine or soft tissue is analyzed for titum. To monitor total radiation doses, a network of approximately 130 TLDs is operated by EPA. The network encircles the test site out to a distance of about 400 miles with somewhatof a concentration in the zonesofpredictedfallout (figure 4-10). The TLD network is designed to measure environmental radiation exposures at a location rather than expo- accidents are used (figure 4-13). Soft Ussues ae A human surveillance program is also carned out to measure the levels of radioactive nuclides in families residing in communities and ranches around the test site (figure 4-14). About 40 families living near the test site are analyzed twice a vear A whole-body count of each person is made to assess the presence of gamma-emitting radionuclides. GROUNDWATER About 100 underground nuclear tests have been conducted directly in the groundwater. In addition. many pathways exist for radioactive material from other undergroundtests (tests either above or below the water table) to migrate from the test cavities to the groundwater. To detect the migration of radioactivity from nuclear testing to potable water sources. a long-term hydrological monitoring program is