Hydrologic conditions at the Idaho National Engineering Laboratory, Idaho, emphasis: 1974-1978

Abstract

The Idaho National Engineering Laboratory (INEL) site covers about 890 square miles of the eastern Snake River Plain and overlies the Snake River Plain aquifer. Low concentrations of aqueous chemical and radio-active wastes have been discharged to shallow ponds and to shallow or deep wells on the site since 1952. The regional water table ranges from about 200 feet to more than 1,000 feet below land surface within the INEL boundaries. The gradient of the water table averages about 4 feet per mile to the south-southwest. During the latest period of record, 1974 through 1978, the position of the water table has shown a net decline that ranges from 0.2 foot near the northern boundary of the INEL to more than 10 feet in the central and southern parts of the site. Recharge from surface water has been minimal or non-existent during the latter part of this period. A large body of perched ground water has formed in the basalt underlying the waste disposal ponds in the Test Reactor Area. This perched zone contains tritium, chromium-51, cobalt-60, strontium-90, and several nonradioactive ions. Tritium is the only mappable waste constituent in that portion of the Snake River Plain aquifer directly underlying this perched zone. Low concentrations of chemical and low-level radioactive wastes enter directly into the Snake River Plain aquifer through the Idaho Chemical Processing Plant (ICPP) disposal well. From 1974 through 1978, this 600-foot well was used to discharge a total of 1,861 million gallons of waste water which contained 1,697 curies of radioactivity, ninety-five percent of which was tritium. Tritium has been discharged to the well since 1953 and has formed the largest waste plume, about 28 square miles in area, in the regional aquifer, and minute concentrations have migrated downgradient a horizontal distance of 7.5 miles. Other waste plumes south of the ICPP contain sodium, chloride, nitrate, and the resultant specific conductance. These plumes have similar configurations, flow southward, and the contaminants are in general laterally dispersed in that portion of the aquifer underlying the INEL. Other waste plumes, containing strontium-90 and iodine-129, cover small areas near their points of discharge because strontium-90 is sorbed from solution as it moves through the aquifer and iodine-129 is discharged in very low quantities. Cesium-137 is also discharged through the well but it is strongly sorbed from solution and has never been detected in a sample of ground water at the INEL. Radionuclide plume size and concentrations therein are controlled by aquifer flow conditions, the quantity discharged, radioactive decay, sorption, dilution by dispersion, and perhaps other chemical reactions. Distributions of non-radoactive chemical wastes are subject to the same processes except for radioactive decay.