DGT is a passive sampling technology

Transcription

1 DGT is a passive sampling technology Assessment of bioavailable methylmercury in San Francisco Bay using Diffusive Gradient in Thin films (DGT) collects chemical compounds without provision of energy from external source devices are deployed for defined period of time, retrieved and contaminant is quantified often used for monitoring (mostly organic contaminants) measures dissolved fraction of contaminant (bioavailable fraction?) can be widely deployed may identify localized sources of contaminants Holger Hintelmann, Brian Dimock Trent University, Water Quality Center, Peterborough, ON Katie Harrold, Ben Greenfield requires calibration biofouling may alter accumulation rate, need to know deployment temperature provide time weighted average concentration, may not identify pulses (but will sample and integrate the pulse) San Francisco Estuary Institute, Oakland, CA 1 piston cover nylon filter difusive gel resin gel support gel piston base DGT (Diffusive Gradient in Thin film) M C =D b At!g Cb = Diffusive coefficient of MeHg-Li species Measured in the 25 ºC Based on the Fick s First law of diffusion F= 2 M!g DAt MeHg-Cl MeHg-OH F : flux M : mass of the specie accumulated by the resin t : gel exposure time A : diffusive gel area D : Diffusive coefficient Cb : specie concentration in solution!g : diffusive gel thickness MeHg-cysteine D = cm2 s-1 D = cm2 s-1 MeHg- humic acid D = cm2 s-1 After retrieval, resin gel is peeled. Species bound to the resin are eluted and analysed. With M measured, it is easy to calculate Cb Depends on hydrodynamic size of MeHg species 3 4

2 Mercury speciation Hg T Hg(O) Hg(II) MeHg Me 2 Hg Speciation results [MeHg] T = [MeHg] labile + [MeHg-DOM] + [MeHg] inert Epilimnion A" t M = $ D " MeHg L MeHg # L " # [ i i ]! g i MeHg speciation change with depth MeHg + MeHg - Cl MeHg - thiol MeHg - DOM Toxicity Bioaccumulation Operational tool to define MeHg speciation Metalimnion Hypolimnion New information on MeHg mobility and reactivity Mainly dominated by DOM except for the metalimnion and the bottom of the lake (high sulfides, high DOM) 5 6 Assembling and disassembling DGTs in the field water sediment pore water 7 8

3 Study objectives Design for marine deployment of DGTs Can we employ DGT tools for identify sources of bioavailable MeHg in the bay? How does the information provided by DGTs compare to results from the small fish sampling program? Each sample is actually the average of three DGT devices (water) 9 1 MeHg accumulation in DGT over time mass MeHg on DGT in pg accumulation rate = 1.1 pg/day r 5 2 = deployment time in days location: Martin Luther King Regional Shoreline MeHg accumulation in DGT over time MeHg in pg/l ± deployment time in days 11 12

4 Spatial overview of MeHg collected on DGTs DGTs in the Bay area 5 pg MeHg on gel 13 MeHg collected on DGTs by category 14 Comparison of DGT and fish (MISI) data HgT in fish (MISI) in ng/g 3 WWTP Hg mine Ind. watershed Legacy sediment non-embayment Embayment Alviso Kirker Creek 5 Pt Isabel marsh Wetland MeHg on DGT in pg Long term monitoring 5 pg MeHg on gel 15 16

5 Comparison of DGT and fish (TOSM) data 18 Alviso HgT in fish (TOSM) in ng/g Sediment DGTs 12 1 Pt Isabel marsh MeHg on DGT in pg Spatial overview porewater MeHg (by DGT) MeHg accumulated on sediment gels Site Site code water MeHg pg/l sediment MeHg pg/l diffusive flux MeHg ng/m2/year Bay off Menlo Park 61OTH China Camp CHINA Petaluma R. N WE Mission Creek MISCK Benicia State Park BENPK Water Treatment Plant WWTP Kirker Creek KIRKE Guadalupe River NALMA porewater water sediment: porewater integrated over 3 cm subsurface water: integrated over 2 cm above surface 5 pg MeHg on gel 19 2

6 Spatial overview MeHg flux potential Comparison above surface vs open water sediment DGT water DGT 25 1 pg MeHg per m2 and year MeHg in pg/l 21 Summary DGT technique offers an alternative (complementary?) tool to identify hot spots of MeHg production: Alviso, Pt Isabel (Kirker Creek) measure diffusive fluxes across sediment/water interface can be deployed almost anywhere DGT and fish data do not correlate at every site: DGTs were deployed for one week only need to carefully check for other factors (salinity, DOC, exact deployment location,...) 23 22