Int. J. Environ. Res. Public Health 2009, 6 178 fallout), was some 300 km long by 30-50 km wide. The contaminated area was estimated at 15,000- 20,000 km2, with approximately 100 km2 being defined as constituting a serious radiation hazard to humans (>7.4 MBq m-2 90Sr: MBq = 106 Bq). The third contamination event occurred between 10 April and 15 May, 1967, when desiccated, contaminated sediments from Lake Karachay were dispersed by wind up to 50-75 km away from the Mayak PA site. An estimated 22 TBq was deposited over 1,800 km2, leading to contamination concentrations in the range 11-210 kBq m-2 137Cs [1, 9]. Caesium-137 was the predominant long-lived radionuclide dispersed and may have accounted for as much as 75 % of the total radioactive inventory [10]. The natural and artificial lakes which have been utilized at the site for storing liquid radioactive wastes are shown in Figure 2. Measurements taken in 1993 gave concentrations in water of 70 and 100 MBq l-1 for 90Sr and 137Cs, respectively, in Reservoir 9 (Lake Karachay, R9). Corresponding values for R17 were 300 and 150 kBq l-1, respectively [11]. More recent discharges in R9 are documented to be about 50, 27 and 24 PBq in 1994, 1995 and 1996, respectively [11]. Corresponding discharge values into R17 are 21, 4 and 5 PBq of β-emitters. Pollution of groundwater from R9 is a recognised problem. The plume containing R9 contaminates was reported to cover some 10 km2 and be spreading at 80-100 m yr-1 [1, 11]. Plans exist to fill in and cap R9 with hollow concrete blocks, gravel, and soil and clay layers in an effort to stabilise the site and stop the groundwater contamination plume. Only partial in- filling is believed to have occurred to date. The Techa River Cascade (TRC, Figure 2) is still used for disposal of liquid low-level liquid waste (LLLW). The TRC is comprised of four reservoirs (R3, R4, R10 and R11). The dam for R3 was built in 1951; the dam for R4 already existed but was heightened in 1956; the dam for R10 was built in 1957 and the final R11 dam was constructed in 1964 [1]. Table 1 presents details of the TRC reservoirs. The water level in the R11 has fluctuated and generally increased in recent years. This is thought to increase the amount of 90Sr seepage through dam 11 and into the Techa [12]. Sr-90 concentrations in water samples collected downstream of dam 11 have increased from around 30 Bq/l in 1982 to 90 Bq/l in 2002 [13]. In addition, Asanov Swamp soils and sediments that were contaminated by the early discharges are another main source of radioactivity to Techa waters. The average 90Sr concentration in Techa river water sampled at Muslyumovo village (40 km downstream of dam 11) was 6 times the Russian intervention levels in July – August 2004, such that living in this settlement is seen as potentially hazardous to health by the Russian Federal Medical-Biological Agency (FMBA) [13]. Table 1. TRC areas, volumes and estimated inventories in 1991 [14]. Values in parentheses are JNREG estimates [1]. Reservoir 3 4 10 11 Area (km2) 0.5-0.8 1.3 18-19 44 Volume (m3 ×106) 0.78 4.1-4.3 76.64 215.74 Estimated inventory (TBq) 666 222 4070 (1200) 1443 (1300) 85 70 5 (25) 40 (60) Percentage total activity in sediments (%)