Mercury Subsurface Maxima in Sediments
473
field into 4-8-cm increments and subsamples were sent to Daniel J. Steck,
Schaefer Environmental Radiation Lab., st. John's University, Collegeville,
Minnesota, for counting CS-137 and other radioisotopes. Sediment strata
measuring the highest and second highest counts were assigned 1954 and 1964
dates, respectively, based on nuclear weapons testing activities (MPCA 1996). To
the bottom of the core was assigned the date of reservoir construction, and to the
top, the year of sampling.
The Lake Superior core sediments were dated using paleomagnetic (Johnson
and Fields 1984) and carbon-13 (Huber 1993) methodology.
2.2.4
Benthos and Associated Surface Sediments
A benthic population survey was conducted in the spring of 1995 (May 31 to June
6) near the sediment study sites. Benthic organisms were qualitatively sampled
using Ekman dredges and processed on site by screening and washing using a
standard bottom sieve bucket (Wildco, Wildlife Supply Co., Saginaw, Michigan).
Two types of 15 x 15 x 15 cm Ekman dredges were used. For water depths less
than 1.5 m, a handle-mounted stainless steel Ekman dredge was used; for deeper
water, a brass Ekman dredge on a nylon cord was employed. Both were operated
such that the top 10 cm (approx.) of sediment was sampled. Between 5 and 22
dredge samples were collected per site depending on the abundance of benthic
biomass. A small portion of each dredged sediment sample was subsampled near
the middle of the dredge (using a stainless steel spoon), from the top 0-4 cm, and
transferred to a labeled ziploc bag for storage at 4 °C until analyzed for mercury
content. The remaining sediment sample was sieved for organisms on site using
the general procedures used for benthos screening and separation by taxa as
found in USEP A (1990).
Benthic samples were placed in labeled ziploc bags and kept on ice until
reaching the lab, where they were kept at 4 °C until identification, final screening,
and washing were completed no later than 2 days after sampling. Processing
included depuration of live organisms in water for 24 h to remove stomach
contents. For each sampling site, organisms were separated by taxa, weighed, and
stored frozen in plastic weighing boats until analyzed. The primary taxa of
interest for mercury content in this study was Hexagenia. Selected taxa from
samples with sufficient biomass were analyzed for mercury to determine uptake
for that trophic level.
2.2.5
Contamination Prevention Measures
All reservoir sampling was done using clean sampling techniques. This involved
the use of clean, mercury-free latex gloves by personnel handling core and
benthos samples, mercury-free sample containers protected using ziploc storage
473
field into 4-8-cm increments and subsamples were sent to Daniel J. Steck,
Schaefer Environmental Radiation Lab., st. John's University, Collegeville,
Minnesota, for counting CS-137 and other radioisotopes. Sediment strata
measuring the highest and second highest counts were assigned 1954 and 1964
dates, respectively, based on nuclear weapons testing activities (MPCA 1996). To
the bottom of the core was assigned the date of reservoir construction, and to the
top, the year of sampling.
The Lake Superior core sediments were dated using paleomagnetic (Johnson
and Fields 1984) and carbon-13 (Huber 1993) methodology.
2.2.4
Benthos and Associated Surface Sediments
A benthic population survey was conducted in the spring of 1995 (May 31 to June
6) near the sediment study sites. Benthic organisms were qualitatively sampled
using Ekman dredges and processed on site by screening and washing using a
standard bottom sieve bucket (Wildco, Wildlife Supply Co., Saginaw, Michigan).
Two types of 15 x 15 x 15 cm Ekman dredges were used. For water depths less
than 1.5 m, a handle-mounted stainless steel Ekman dredge was used; for deeper
water, a brass Ekman dredge on a nylon cord was employed. Both were operated
such that the top 10 cm (approx.) of sediment was sampled. Between 5 and 22
dredge samples were collected per site depending on the abundance of benthic
biomass. A small portion of each dredged sediment sample was subsampled near
the middle of the dredge (using a stainless steel spoon), from the top 0-4 cm, and
transferred to a labeled ziploc bag for storage at 4 °C until analyzed for mercury
content. The remaining sediment sample was sieved for organisms on site using
the general procedures used for benthos screening and separation by taxa as
found in USEP A (1990).
Benthic samples were placed in labeled ziploc bags and kept on ice until
reaching the lab, where they were kept at 4 °C until identification, final screening,
and washing were completed no later than 2 days after sampling. Processing
included depuration of live organisms in water for 24 h to remove stomach
contents. For each sampling site, organisms were separated by taxa, weighed, and
stored frozen in plastic weighing boats until analyzed. The primary taxa of
interest for mercury content in this study was Hexagenia. Selected taxa from
samples with sufficient biomass were analyzed for mercury to determine uptake
for that trophic level.
2.2.5
Contamination Prevention Measures
All reservoir sampling was done using clean sampling techniques. This involved
the use of clean, mercury-free latex gloves by personnel handling core and
benthos samples, mercury-free sample containers protected using ziploc storage