USGS Groundwater Studies at the Idaho National Laboratory

From a U.S. Geological Survey fact sheet, September 2017.

The USGS has studied and monitored groundwater at the Idaho National Laboratory (INL) since 1949. Its monitoring networks and studies describe how waste disposal has affected water in the eastern Snake River Plain aquifer, and how much water is available for long-term use. The work, done with the U.S. Department of Energy's Idaho Operations Office, is essential to how the INL and the State of Idaho manage and use the aquifer. Reports are public at https://id.water.usgs.gov/INL/Pubs/index.html.

The aquifer

The INL lies in the west-central part of the eastern Snake River Plain, a northeast-trending basin about 200 miles long and 50–70 miles wide, bounded by faults to the northwest and by downwarping and faulting to the southeast. It has filled with basalt lava flows layered with land-laid sediments, which together form the eastern Snake River Plain aquifer — one of the most productive aquifers in the United States and the region's main source of groundwater.

• Flow: groundwater generally moves from northeast to southwest and comes out at springs along the Snake River below Twin Falls, about 100 miles southwest of the INL. In water year 2015, those springs discharged 3.45 million acre-feet.
• Recharge: mainly from irrigation water and streamflow soaking in, groundwater from neighbouring mountain basins, and precipitation. At the INL, the Big Lost River, which drains more than 1,390 square miles of the Lost River and Pioneer Mountains, sinks into the aquifer along its channel and at the playas where it ends. Its average flow below Mackay Reservoir over water years 1905–2015 was 216,700 acre-feet a year. Since 1965 the DOE has diverted excess runoff reaching the INL to spreading areas in the southwest of the site, to prevent flooding.
• Discharge: mainly pumping for irrigation and springs along the Snake River; spring flow varies with water use, irrigation practice and precipitation.
• Depth and speed: depth to water ranges from about 225 feet below land surface in the north of the INL to more than 900 feet in the southeast. Much of the water moves through the upper 200–800 feet of basalt, where transmissivity ranges from 1.1 to 760,000 square feet a day. The hydraulic gradient is 2 to 10 feet per mile (4 on average), and measured flow velocities of 2–26 feet a day mean water beneath the INL takes about 50–700 years to reach the springs near Twin Falls.

The rocks

Beneath the INL the aquifer is olivine tholeiite basalt, interlayered with thin wind- and water-laid sediment, laid down in a sinking basin over the past 10 million years; it is about 445 to 1,200 feet thick. Most groundwater moves through the fractured tops, sides and bases of lava flows, which link into networks; a series of thin flows passes water more easily than one thick flow, and fine sediment and dense, thick basalt can block it, while coarse sediment may help it along.

Gamma logs from 333 wells and many cores were used to build a two-dimensional framework and, in 2006, a conceptual model for groundwater-flow and contaminant-transport modelling. New coring, rock testing, mapping of surface vents and three-dimensional modelling with paleomagnetic measurements and age dates are refining it.

Chemistry

Chemical and radiochemical constituents in the groundwater come from natural processes and human activity. Water reaching the aquifer at the INL comes from several sources — surface water and irrigation return flow, tributary valleys, the aquifer upstream, industrial waste discharge and geothermal water — each with its own chemistry. Where they mix, at the Birch Creek Playa, the Little Lost and Big Lost River playas, the Big Lost River channel and spreading area, and the southeastern INL, reactions change the water further. Work to understand the groundwater's chemistry and how wastewater moves through the system continues.

Models

The DOE and the State of Idaho need to understand how contaminants move, to minimise risks and plan any cleanup. Numerical models built from the conceptual model estimate hydraulic properties, flow paths, travel times and plume movement. Two kinds of three-dimensional flow model were developed: a steady-state model, in which inflows equal outflows and nothing changes over time, and a transient model, in which flows vary with climate and water use, changing storage and the direction and speed of flow. They were described in 2010 and are being revised to assess contamination risk for future INL facilities.

Monitoring

Since 1949 the USGS has sampled water quality and measured water levels in a network of more than 200 wells, mostly open to the aquifer over their full depth below the water table — good for detecting when a plume arrives and how far it spreads.

• Water quality, from 143 wells as of 2017: some combination of tritium, strontium-90, cesium-137, plutonium-238, plutonium-239 and -240, americium-241, gross alpha and beta radioactivity; chromium, sodium, chloride and sulfate; nutrients; volatile organic compounds; and specific conductance, pH and temperature.
• Water levels, from 213 wells as of 2017, monthly to annually, with eight recording continuously.

A USGS hydrologist sampling a well at the INL. USGS.

A USGS technician measuring a well's water level. USGS.

The data go into the USGS National Water Information System (https://water.usgs.gov/nwis/).

Streamflow

From 1984 through 2009 the USGS ran eight streamgages, six crest-stage gages and a lake-stage gage in the Big Lost River Basin; in 2009 the USGS and the DOE cut the network to seven streamgages, all with satellite telemetry for real-time data. Two of the three highest gages have recorded continuously since the early 1900s, the third since the 1940s. The network measures snowmelt runoff, seepage losses from the channel and diversions, and infiltration in ponded areas, supports flood-control studies, and lets the INL check flow and storage at once. Each year the USGS also samples the Big Lost River at four sites, and the Little Lost River, Birch Creek and Mud Lake at one site each, for the baseline chemistry of water recharging the aquifer.

The USGS streamgage (site 13132513) measuring flow into the INL spreading areas. USGS.

Sources

• U.S. Geological Survey, U.S. Geological Survey Geohydrologic Studies and Monitoring at the Idaho National Laboratory, Southeastern Idaho, Fact Sheet 2017–3070, September 2017. https://doi.org/10.3133/fs20173070 — three photographs recovered from the PDF, and its units (square miles, square feet a day), which the import had lost.
• The fact sheet draws on Ackerman (1991), Ackerman and others (2006, 2010), Bartholomay and others (1997, 2017), Mann (1986) and others.
• Rewritten in hubnx's own words.