Industrial and low-level radioactive liquid wastes at the National Reactor Testing Station (NRTS) in Idaho have been disposed to the Snake River Plain aquifer since 1952. The movement and distribution of these wastes have been monitored. The aquifer is extremely large and has a high transmissivity. The total discharge to the aquifer at NRTS has averaged about 1 × 10⁹ gal/year and contained relatively small quantities of tritium, strontium-90, cesium-137, cobalt-60, chloride, hexavalent chromium, various acids and bases, and heat. Tritium and chloride have dispersed over a 15-sq-mi area of the aquifer in low but detectable concentrations and have migrated as much as 5 mi downgradient from discharge points. A remarkable degree of lateral dispersion has rapid y diluted and spread the waste products. The movement of cationic waste solutes, particularly strontium-90 and cesium-137, has been significantly retarded due to sorption phenomena, principally ion-exchange.

Digital modeling techniques have been applied very successfully to the analysis of this complex waste-transport system by numerical solution of the coupled equations of groundwater motion and mass transport. The model includes the effects of convection transport, flow divergence, two-dimensional hydraulic dispersion, radioactive decay, and reversible sorption. The 20-year transport and distribution history of waste chloride and tritium has been successfully simulated by the model.

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The less conservative cationic solutes also have been successfully modeled. The modeling results indicate that hydraulic dispersion (especially transverse) is a much more significant influence than has been previously suggested by earlier studies. The model may be used to project future waste migration patterns for varied hydrologic and waste conditions.

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