40 years of plutonium production at Hanford created large amounts of
radioactive and chemically hazardous waste.
The most hazardous liquid wastes were pumped into underground storage tanks.
The remainder - an estimated 440 billion gallons of contaminated liquid -
was dumped into the soil in ponds, trenches, and ditches, and sometimes
injected directly into the groundwater. Leaking storage tanks and unplanned
spills added to the contaminated liquids in the soil. As a result, there is
extensive contamination of groundwater beneath the Hanford site.
Some of the contaminated groundwater reaches
the Columbia River, causing localized contamination where groundwater plumes enter the river.
Thankfully, there is minimal effect on water quality downriver. However,
because the
Hanford Site
is just 35 miles from the Oregon border, our focus is to ensure cleanup
efforts are protective of the river.
Hanford's Groundwater
Groundwater is found underground in the cracks and spaces in soil. It is
stored in, and typically moves slowly through, layers of soil, sand, and
fractured rocks called aquifers. Aquifers contain large connected spaces
that allow water to flow. The speed at which groundwater flows depends on
the slope of the water table, the size of the spaces in the soil or rock,
how well the spaces are connected, and the rate at which new water enters
the aquifer.
Hanford's sub-surface is very complex. Ice-age floods repeatedly deposited
layers of soil and rock, so the direction and speed of groundwater flow
around Hanford can vary widely.
Hanford's central plateau, where 177 underground waste storage tanks are
located, sits about 200 feet above the groundwater, roughly 7-12 miles from
the Columbia River. The tanks in the central plateau hold most of the liquid
waste. Groundwater from this area takes several years to reach the river. In
other areas of Hanford, groundwater contaminants can reach the river in just
weeks or months.
A Mix of Contaminants
Hanford's groundwater includes chemicals such as carbon tetrachloride,
chromium, and nitrate. It also includes radioactive materials such as
uranium, strontium 90, technetium 99, tritium, and iodine 129.
More than 70 square miles of groundwater is contaminated above regulatory
standards. Of the contaminants, chromium, nitrate, uranium, technetium,
tritium, and strontium have all reached the Columbia River. These materials
can be harmful to people and the environment.
The Tri-Party Agreement, which sets milestones for Hanford cleanup actions,
includes target dates and deadlines related to groundwater cleanup.
Treatment Methods
Hanford has operated groundwater pump-and-treat systems since 1994, and has
greatly increased capacity since then. These systems pump contaminated
groundwater out of the ground, and remove contaminants using different
treatment processes. The cleaned water is then injected back into the
ground.
Five pump-and-treat systems are operating at three of Hanford's reactor
areas along the Columbia River. There is also a very large treatment
facility in Hanford's central plateau. These pump-and-treat systems have
greatly reduced the size and concentration of many of the groundwater
plumes, and have reduced the amount of contaminants that are getting into
the river.
One particular contaminant of concern is hexavalent chromium, which was used
extensively at Hanford's nine reactors to purify the water used to cool the
reactors. Huge amounts of hexavalent chromium - which is toxic to fish - was
leaked in each of the reactor areas, in close proximity to the Columbia
River. The pump-and-treat systems in the reactor areas are designed
primarily to remove the hexavalent chromium from the groundwater. Hanford
workers also chased chromium contamination in the soil at two of the reactor
areas, digging 85 feet deep to groundwater and removing tons of contaminated
soil.
Between 1999 and 2014, the plumes of hexavalent chromium have shrunk thanks
to various treatment efforts (click photo at right to enlarge).
Hanford has also experimented with a number of other technologies
Chemical Barriers
An underground chemical barrier was created near Hanford's D Reactor to
convert hexavalent chromium into a less mobile and less toxic form as water
flowed through the barrier. Part of the barrier is working to reduce the
chromium hazards, but another part of the barrier has failed. Hanford is
using a pump-and-treat system to augment the part of the barrier that isn't
working.
A calcium phosphate barrier was formed near Hanford's N Reactor. As
radioactive strontium in groundwater flows through the barrier, it binds to
the calcium phosphate. This barrier shows promise, and Hanford plans to
expand the barrier when more funds are available.
Biostimulation
Hanford also tried a process called biostimulation, using molasses and
vegetable oil to feed tiny microorganisms (bacteria) in the soil, which
consume oxygen in the groundwater. This alters the chemistry of soil and
groundwater, changing chromium to a less mobile and less toxic form.