Measurement of gaseous elemental mercury (GEM) in diffuse soil emission using the static closed-chamber method

Why diffuse GEM released from the soil is of interest for Earth Sciences? A reliable evaluation of the input to the atmosphere of gaseous elemental mercury (GEM) is critical for the development of environmental regulations and controls of this highly toxic air pollutant. Notwithstanding, the measurement of GEM discharged from mercury-enriched soils in natural (e.g. volcanic/hydrothermal systems, wetlands) and anthropogenic (e.g. mine wastes, landfills, sewage sludge amended soils) lacks of specific protocols.

The «most» classical approach for fGEM measurement Dynamic flux chambers (DFCs) and micrometereological methods (e.g. Kim et al., 1995; Marsik et al., 2005; Fritsche et al., 2008). Systematic experiments tested the influence of different chamber design/operating conditions and materials on DFC measurements (Eckley et al., 2010, and references therein): an unified interpretation of the optimal parameters is still a challenge.

Accumulation chamber: a possible application for fGEM measurement(?) fGEM were recently measured at the Solfatara crater using accumulation chambers (Bagnato et al., 2014).  fCO2 were also measured with the same type of apparatus and the spatial distributions of both these parameters were constructed.  Size and material of the chamber can be varied, according to fluxes, interaction with sunlight 

The static-closed chamber (SCC approach): what’s old and what’s new The “static closed-chamber” (SCC) method (Rolston, 1986; Livingstone and Hutchinson, 1995), has widely been applied for the measurement of diffuse CH4 soil fluxes in geothermal and volcanic environments (Klusman and LeRoy, 1996, Etiope, 1999; Klusman et al., 2000; D'Alessandro et al., 2009; Castaldi and Tedesco, 2005); rarely used for diffuse soil fluxes of hydrocarbons (Tassi et al., 2013)… …but never used for fGEM measurements

Materials The SCC is a polyethylene cylinder with a basal area of 201 cm2 and an inner volume of 1,810 cm3. The camera is equipped with a piercable rubber septum on its top. The SCC is used in combination with a Lumex® (RA-915M) that measure Hg0 concentrations from 2 to 50,000 ng/m3. Thus, with this method the fGEM fluxes are measured directly in the field!

Materials The Lumex® inlet port is equipped with a three-way Teflon valve to allow the free entrance of air. This peculiar device was placed to prevent possible noise signals related to flux changes during the gas injection. A carbon trap set at the free air entrance minimized the baseline instability related to the variable GEM concentrations in air.

Procedure At each measurement point, a gas sample were collected from the SCC when it was positioned on the ground and after fixed time intervals (1, 2 and 3 min) using a 60 cc syringe inserted in the SCC through the pierceable rubber positioned on its top. The gas of the syringe was immediately injected in the Lumex® through the injection port device.

Calibration of the inject device In the lab, a calibration curve, measuring known amounts of GEM saturated gas at a controlled temperature, was constructed using the Lumex® injection apparatus. peak area injected Hg (ng)

Computation of fGEM from GEM concentrations in the ST The amount of GEM collected in the syringe (KSYR, in ng) was calculated on the basis of the calibration curve. The correspondent GEM amount in the SCC (KSCC, in ng) was computed by multiplying the KSYR values for the ratio between the volumes of the syringe and the SCC. GEM fluxes are proportional to the increase of the KSCC values during the sampling time-series (dKSCC), thus they can be calculated, as follow: fGEM = (dKSCC/dt)/A where A is the basal area of the SCC and dt is the sampling time interval (1 min).

Results (repeatibility test)   1 2 3 4 5 17945 11690 20819 271431 50639 17556 13383 22126 289328 53282 17294 13253 20427 281337 53812 18338 21211 274161 51833 18991 12731 278997 55799 6 17816 13513 21603 281284 56196 7 18208 12993 20689 281592 53545 8 18729 13904 280605 55795 9 21734 271227 51563 10 18599 13122 21733 292349 56328 mean 18221 13188 21316 280231 53879 SD% 3.07 4.87 2.96 2.47 3.88 The test was carried out at 5 sites (10 replicates) at Solfatara crater. Data from each site were extremely consistent (SD% from 2.47 to 4.87) independently on the fGEM values (ranging of more than 1 order of magnitude).

Other measurements The fCO2 values were measured usign the accumulation chamber method. The temperature of the soil at 7 and 15 cm depths was measured using a portable Tersid thermocouple (dynamic range from -20 to 1,150 °C; uncertainty ±0.1 °C).

Results (Solfatara crater) Up to 214 fGEM measurements were carried out at the Solfatara crater (Campi Flegrei, southern Italy), a hydrothermally altered tuff cone characterized by an anomalous diffuse soil emission of GEM-rich geogenic gases.   n. min max mean median fGEM 214 1296 1957500 79719 22385 fCO2 19.8 117951 4578 1320 T (7 cm) 10 95 31 25.5 T (15 cm) 12 39.5 32 Both fGEM and fCO2 varied of 3 orders of magnitude and their median values are 1/3 of the mean ones (right skew).

Results (Solfatara crater) The spatial distribution of fCO2 evidences several zones of anomalously high emissions. The same zones seem to be also highlighted by fGEM and soil temperatures.

fGEM vs. fCO2 Notwithstanding, in the binary diagrams the fCO2 and fGEM values are poorly correlated. …a better correlation is shown by fCO2 and soil temperature.

fGEM vs. soil temperatures Binary diagrams shows that the fGEM values are poorly correlated with the soil temperatures (both at 7 and 15 cm depths). This is quite surprising, since GEM mobility is known to be strongly dependent on temperature. Moreover, at Solfatara the diffuse release of hot, CO2-(and GEM-)rich deep fluids is strongly controlled by local permeability.

Concluding remarks: the method The SCC approach provides a reliable method for fGEM measurements in different environments This SCC method provide fGEM values in the field (the classical SCC methods carry out the analysis in the lab) External standard calibration provides a high accuracy A satisfactory reproducibility is obtained through simple field operations Low detection limits can be easily obtained by varying the accumulation time in the SCC and/or the chamber dimensions

Concluding remarks: the results GEM and CO2 have a common origin but a different behavior during fluid uprising fGEM cannot be computed on the basis of fCO2 Dependence of fGEM on soil temperature is masked by the effect of other chemical-physical parameters