TRACE METALS ASSIMILATION IN TREATMENT WETLAND SEDIMENTS

摘要

Sediment metals concentration data from the Sacramento Regional County SanitationrnDistrict's Constructed Wetland Demonstration Project has been analyzed for spatial andrntemporal trends. The sediment monitoring program covers a five-year period, startingrnwith baseline sampling in December 1993 and extending through 1998. The principlernsampling studies include core sampling in vegetated areas, a sediment partitioningrnstudy and an accretion study. Of the nine trace metals measured, six metals (silver,rncadmium, copper, mercury, lead and zinc) consistently exceed apparent massrnremoval rates of 60 percent. Apparent concentration and mass removal rates for threerntrace metals (arsenic, chromium and nickel) have steadily declined over thernmonitoring period and currently display negative removal rates. Analysis of thernsediment trace metals data indicates an apparent trend towards increasing sedimentrnconcentrations over time for arsenic, chromium, copper, nickel, and zinc. However,rnfor the other trace metals, no consistent observable trend towards increasingrnsediment concentrations is observed. Analysis of wetland cell sediment samplesrnindicates an apparent longitudinal trend toward elevated trace metals concentrationsrnwithin the initial wetland cell quadrant. However, there are no statistically significantrndifferences observed between the first and second quadrants, or the second quadrantrnand the second half-cell. Detrital layer trace metals concentrations are on average fourrntimes greater than the underlying mat and clay layers, a trend that is statisticallyrnsignificant for all metals but chromium and nickel. Of the five trace metalsrndemonstrating an increasing trend over time, only chromium and nickel are observedrnin concentrations approaching or exceeding the freshwater sediment Probable EffectrnLevels (PEL). Chromium and nickel are identified as metals for which removal rates arernlow or even negative, and are the only metals for which accumulated sediment metalsrnconcentrations increase over distance. Water column speciation indicates that nickelrnenters the wetland in a predominantly dissolved and strongly organic-complexed form.