Hg-Contaminated Industrial and Mining Sites in N. America
95
Table 1. Mass balance of mercury at Oak Ridge Y-12 plant (Data from uec 1983)
Environmental media
Mercury (Mg)
Percent of inventory"
Lost to air (1950-1963)
35
0.32
Lost to creek (I 950- I 993)
109
1.0
Lost to soil/rock under facility
194
1.8
Lost to headwater pond sediment
6.8
0.06
Not accounted forb
590
5.4
Total
935
8.6
"Total inventory charged to facility = 10 886 Mg.
bCharged inventory minus accounted for inventory plus amounts lost or spilled.
emission was well above background, it was small compared to other
atmospheric Hg sources in the region, including coal-fired power plants.
The Oak Ridge site has been intensively investigated over the past 15 years with
the result that a vast amount of environmental information is now available (e.g.
Table 2). Corrective actions at this site have focused on identifying and
eliminating point sources of mercury, as well as on eliminating human health
concerns associated with high mercury level (up to 3000 ~tg g-') in floodplain
soils downstream from the site. Actions included filling and capping an onsite
pond (1988), renovation and replacement of subsurface storm and process drains
(1986-1993), treatment of basement sump waters (largely composed of groundwater) using activated carbon systems (1993-1995), and bypassing site drainage
waters around a small (l-ha) impoundment (1996). As a result of these and other
actions, mercury in drainage waters leaving the site has been substantially
reduced (from more than 100 g day-' in 1985 to about 15 g day-' in 1996) over
the past decade, but concentrations (Fig. 3) still exceed the interim target
regulatory limit (200 ng 1-'), which was predicted to yield downstream fish
concentrations less than 0.4 ~tg g -, (limit selected for protection of human
health). The state and federal agencies mandate a final instream limit of 12 ng 1-'.
Whether this limit can be met is the subject of current discussion and the focus
of treatment technology evaluations. Over $30 million (US) has been expended
over the past decade in characterizing and treating continuing releases of waterborne mercury. While concentrations of mercury in water at the facility
boundary have declined, mercury in fish (redbreast sunfish) collected from this
location has also declined significantly, but in fish collected further downstream
it has either remained constant or shown increases in concentration over the past
decade (Fig. 4). It is suspected that as point sources of mercury at the facility
have been reduced or eliminated the relative importance of instream sources
(sediments, floodplain deposits) has increased and prevented recovery at
downstream reaches. The temporal increase 17 km downstream from the facility
(Fig. 4) is more enigmatic, but may be related to changes in water quality which
enhance the production and/or bioaccumulation of methylmercury. Additional
corrective actions are in progress or planned.
Floodplain soils containing approximately 80 t of mercury, distributed over
25 km of creek and 200 ha, were subjected to intensive characterization between
1982 and 1995 with a final decision to excavate all soils over 400 pg g -, for
95
Table 1. Mass balance of mercury at Oak Ridge Y-12 plant (Data from uec 1983)
Environmental media
Mercury (Mg)
Percent of inventory"
Lost to air (1950-1963)
35
0.32
Lost to creek (I 950- I 993)
109
1.0
Lost to soil/rock under facility
194
1.8
Lost to headwater pond sediment
6.8
0.06
Not accounted forb
590
5.4
Total
935
8.6
"Total inventory charged to facility = 10 886 Mg.
bCharged inventory minus accounted for inventory plus amounts lost or spilled.
emission was well above background, it was small compared to other
atmospheric Hg sources in the region, including coal-fired power plants.
The Oak Ridge site has been intensively investigated over the past 15 years with
the result that a vast amount of environmental information is now available (e.g.
Table 2). Corrective actions at this site have focused on identifying and
eliminating point sources of mercury, as well as on eliminating human health
concerns associated with high mercury level (up to 3000 ~tg g-') in floodplain
soils downstream from the site. Actions included filling and capping an onsite
pond (1988), renovation and replacement of subsurface storm and process drains
(1986-1993), treatment of basement sump waters (largely composed of groundwater) using activated carbon systems (1993-1995), and bypassing site drainage
waters around a small (l-ha) impoundment (1996). As a result of these and other
actions, mercury in drainage waters leaving the site has been substantially
reduced (from more than 100 g day-' in 1985 to about 15 g day-' in 1996) over
the past decade, but concentrations (Fig. 3) still exceed the interim target
regulatory limit (200 ng 1-'), which was predicted to yield downstream fish
concentrations less than 0.4 ~tg g -, (limit selected for protection of human
health). The state and federal agencies mandate a final instream limit of 12 ng 1-'.
Whether this limit can be met is the subject of current discussion and the focus
of treatment technology evaluations. Over $30 million (US) has been expended
over the past decade in characterizing and treating continuing releases of waterborne mercury. While concentrations of mercury in water at the facility
boundary have declined, mercury in fish (redbreast sunfish) collected from this
location has also declined significantly, but in fish collected further downstream
it has either remained constant or shown increases in concentration over the past
decade (Fig. 4). It is suspected that as point sources of mercury at the facility
have been reduced or eliminated the relative importance of instream sources
(sediments, floodplain deposits) has increased and prevented recovery at
downstream reaches. The temporal increase 17 km downstream from the facility
(Fig. 4) is more enigmatic, but may be related to changes in water quality which
enhance the production and/or bioaccumulation of methylmercury. Additional
corrective actions are in progress or planned.
Floodplain soils containing approximately 80 t of mercury, distributed over
25 km of creek and 200 ha, were subjected to intensive characterization between
1982 and 1995 with a final decision to excavate all soils over 400 pg g -, for