Supporting Information

Transcription

1 Supporting Information Submarine groundwater discharge of total mercury and monomethylmercury to central California coastal waters Frank J. Black *, Adina Paytan, Karen L. Knee, Nicholas R. de Sieyes, Priya M. Ganguli, Ellen Gray, and, A. Russell Flegal WIGS Laboratory, Department of Environmental Toxicology University of California, Santa Cruz; Santa Cruz, CA Institute of Marine Sciences University of California, Santa Cruz; Santa Cruz, CA Department of Geological & Environmental Sciences Stanford University, Stanford, CA Department of Civil and Environmental Engineering Stanford University, Stanford, CA * Corresponding author, tel.: (609) fblack@princeton.edu Current address: Department of Geosciences Princeton University, Princeton, New Jersey The Supporting Information for this article consists of 9 pages and includes information on trace metal cleaning protocols, the collection and analysis of nutrient and suspended particulate matter samples, description and results of tests for artifactual MMHg formation during sample distillation, and sampling description and discussion of results related to temporal variability and tidal influence on Hg T and MMHg concentrations in groundwater and adjacent surface waters. One table presents relevant data for a comparison of Hg T and MMHg fluxes in SGD to other major sources. Also included are two figures; Supporting Information Figure 1 presents plots of Hg T versus MMHg concentration in groundwater and surface water, while Supporting Information Figure 2 presents time series data of HgT and MMHg concentrations in groundwater and adjacent surface waters at one Elkhorn Slough site. S1

2 Trace metal cleaning protocol Sample bottles and tubing were cleaned by a one week soak in Micro detergent, followed by thorough rinsing with deionized water, a one week soak in 3N HNO 3 (trace metal grade), followed by thorough rinsing with Milli-Q water, a one week soak in 3N HCl (trace metal grade), followed by a final thorough rinsing with Milli-Q water (Millipore). Cleaned bottles were then double bagged in polyethylene zip-lock bags until used, and were only handled with gloved hands using trace metal clean techniques (1). Collection and Analysis of Nutrient and Suspended Particulate Matter Samples Filtered (0.45 µm) nutrient samples were collected in acid cleaned high density polyethylene bottles, and frozen immediately in the field with dry ice. Dissolved nutrient concentrations were measured with a Lachat QuickChemR 8000 Series Automated Ion Analyzer using methods A for amorphous silica (SiO 2 ), A for ammonium (NH 4 + ), C for nitrate plus nitrite (NO NO 3 - ), A for nitrite (NO 2 - ), and A for orthophosphate (PO 4 3- ) (2). Nutrient concentrations were measured in this study not with the goal of estimating nutrient fluxes via SGD, as such estimates have already been made for both Elkhorn Slough (3) and Stinson Beach (4) using more comprehensive data sets. Rather we were interested in identifying potential correlations between groundwater concentrations of nutrients and those of Hg T and/or MMHg. Unfiltered suspended particulate matter samples that had been collected in acid cleaned 500 ml polyethylene bottles were measured by vacuum filtration using a 0.45 µm filter (5). Measurements of general water parameters (temperature, ph, salinity, conductivity) were made in the field using a hand-held probe (YSI, Yellowsprings). S2

3 Artifactual Formation of MMHg The potential for artifactual formation of MMHg during sample distillation (6,7) was evaluated by adding 100 ng L -1 of inorganic Hg(II) to randomly selected surface water samples and all groundwater samples before distillation, and then comparing the resulting MMHg values to the same sample with no added inorganic Hg(II). A recent study has demonstrated that seawater samples with a relatively high ratio of dimethylmercury (DMHg) to MMHg should be sparged of DMHg prior to acidification and MMHg analysis due to the conversion of DMHg to MMHg under acidic conditions (8). Although our study did involve the collection of seawater samples, we did not sparge them prior to acidification and MMHg analysis. This was because seawater samples collected in this study were from the surf zone, where constant wave action facilitates the exchange of gases and any DMHg present in offshore waters is likely to quickly evade to the atmosphere, thus not poising a problem for MMHg analysis. The difference in the concentration of MMHg measured in all surface water samples spiked with 100 ng L -1 inorganic Hg(II) and the same samples not spiked with Hg(II) was within the analytical error, demonstrating no artifactual formation of MMHg in the surface waters collected. However, artifactual formation of MMHg was observed in 32% of the groundwater samples, which were all tested. In those groundwater samples in which conversion of inorganic Hg(II) to MMHg was measured, on average 0.3 ± 0.2% of the Hg(II) was converted to MMHg during distillation. This artifactual formation of MMHg in some of the high DOM samples, but not in any of the low DOM surface water samples, is consistent with previous findings (6,7). In those samples in which artifactual formation of MMHg was observed during distillation, the measured concentration of MMHg was corrected using the ambient concentration of Hg(II) measured for each sample and the percentage of Hg(II) converted to MMHg when spiked with S3

4 100 ng L -1 inorganic Hg(II). Such a reduction in the value of MMHg never represented more than a 10% decrease relative to the measured MMHg concentration. Temporal variability and tidal influence on Hg T and MMHg concentrations Temporal variability in Hg T and MMHg concentrations was assessed at Stinson Beach by sampling under similar tidal regimes (high tide) roughly 8 months apart (both during the dry season). A comparison of the results from Oct. 31, 2007 and July 7, 2008, demonstrated no significant difference (p>0.1, paired t-test) between the two dates in filtered or unfiltered Hg T or MMHg concentrations in groundwater or surface waters (main text Figure 2). However, we note that our data set (only a few transects on two dates) may not be sufficient to quantitatively assess variability over timescales of this magnitude. The effect of daily tidal cycle on Hg T and MMHg concentrations in groundwater and adjacent surface seawater was assessed by sampling two cross-shore transects at Stinson Beach on July 7, The first transect was sampled at low tide (-0.05 m at 10:00) and the second at high tide (1.18 m at 16:20) on the same day. There was not a significant difference in the dissolved concentration of Hg T (p=0.31, paired t-test) or MMHg (p=0.12, paired t-test) when comparing the two transects (main text Figure 3). The only site for which the dissolved concentration of MMHg changed substantially between transects was in groundwater at well MW-07 (decreased from 2.9 pm at low tide to 0.79 pm during high tide). The MMHg concentrations in this well were consistently higher than the other wells (main text Figure 3) and water from this well was suboxic at the time of sampling. The lack of a measurable difference in MMHg concentration in groundwater at the other wells between low and high tide suggests that the decrease in MMHg at MW-07 was due to factors other than tidal cycle. S4

5 Temporal variability on the scale of hours and the effect of tide elevation at Elkhorn Slough were assessed by sampling groundwater and adjacent surface water during a 7 hour time series at site ES-2 (Supporting Information Figure 2). No trends were discernible for the Hg T in groundwater, nor for the filtered or unfiltered Hg T or MMHg concentrations in the surface waters. There was a pronounced increase in the dissolved MMHg concentration in groundwater (from 0.68 to 3.1 pm) during the morning and early afternoon, then decreasing to 2.4 pm in late afternoon (Figure 5). This trend does not appear to correlate to the daily tidal cycle (-0.21 m low tide at 6:15, 1.10 m high tide at 13:10, 0.86 m low tide at 17:00). This could be due to a lag between the tidal change and SGD discharge, or it is possible that the temperature in the area around the pit increased during the afternoon, leading to an increase in microbial activity and an associated increase in the production of MMHg. That latter possibility is consistent with the absence of a corresponding temporal trend in Hg T concentrations in the groundwater or MMHg concentrations of adjacent surface waters, which both remained relatively constant throughout the same period (Supporting Information Figure 2). S5

6 Supporting Information Figure 1. There is a statistically significant relationship between the dissolved concentration of Hg T and MMHg in groundwater (p=0.003, simple linear regression), but not in adjacent surface waters (p=0.47, simple linear regression). Filled circles denote Stinson Beach samples while open circles those from Elkhorn Slough. S6

7 Supporting Information Figure 2. Time series of HgT and MMHg concentrations in filtered and unfiltered groundwater (GW) and adjacent surface waters (SW) at Elkhorn Slough site ES 2 on June 19, The solid vertical line at 13:10 denotes time of high tide, while dashed vertical line at 17:00 denotes low tide. S7

8 Supporting Information Table 1. Comparison of the major sources and fluxes of Hg T and MMHg to coastal surface waters to SGD fluxes estimated in this study and previous studies. Source of Flux Hg T flux (nmol m -2 d -1 ) MMHg flux (nmol m -2 d -1 ) Atmospheric deposition 1, Benthic fluxes due to diffusion and bioirrigation only Benthic fluxes due to diffusion and advection SGD at Waquoit Bay, MA SGD at Elkhorn Slough, CA (This study) 3.0 ± ± 0.21 SGD at Stinson Beach, CA (This study) ± 160 nmol m -1 d ± 12 nmol m -1 d -1 1 Hg T atmospheric flux for San Francisco Bay, references 56 and 57 in main text. 2 MMHg atmospheric flux (average annualized via wet deposition) for Monterey Bay, CA, Conaway et al., unpublished data. 3 Estimated from concentration gradients between pore waters and overlying waters or using laboratory based flux chambers employing sediment cores, references 31, 45, 46, 48 51, 63 in main text. 4 Measured using in-situ benthic flux chambers, references 31, in main text. 5 Reference 12 in main text. 6 Fluxes in SGD at Stinson Beach were normalized to shoreline length (m -1 ) rather than area (m -2 ) because there were insufficient data to accurately define the area of the seepage face at this open ocean facing beach. S8

9 Supporting Information Literature Cited (1) Flegal, A.R.; Smith, G.J.; Gill, G.A.; Sanudo-Wilhelmy, S.; Anderson, L. Dissolved trace elements cycles in the San Francisco Bay estuary. Mar. Chem. 1991, 36, (2) Ranger, C.; Diamond, D. QuikChem Methods. Lachat Instruments, Milwaukee, WI., (3) Breier, J.A.; Nidzieko, N.; Monismith, S.; Moore, W.; Paytan, A. Tidally driven chemical fluxes across the sediment-surface water interface in Elkhorn Slough, California: evidence from a coupled geochemical and hydrodynamic approach. Submitted to Limnol. Oceanogr (4) de Sieyes, N. R.; Yamahara, K. M.; Layton, B. A.; Boehm, A. B. Submarine discharge of nutrient-enriched fresh groundwater at Stinson Beach, California is enhanced during neap tides. Limnol. Oceanogr. 2008, 53, (5) Greenberg, A.E.; Clesceri, L.S.; Eaton, A.D. Standard methods for the examination of water and wastewater. American Public Health Association, Washington, DC., (6) Bloom, N. S.; Colman, J. A.; Barber, L. Artifact formation of methyl mercury during aqueous distillation and alternative techniques for the extraction of methyl mercury from environmental samples. Fresenius J. Anal. Chem. 1997, 358, (7) Hintelmann, H.; Falter, R.; Ilgen, G.; Evans, R. D. Determination of artifactual formation of monomethylmercury (CH 3 Hg + ) in environmental samples using stable Hg 2+ isotopes with ICP-MS detection: Calculation of contents applying species specific isotope additions. Fresenius J. Anal. Chem. 1997, 358, (8) Black, F.J.; Conaway, C.H.; Flegal, A.R. Stability of dimethyl mercury in seawater and its conversion to monomethyl mercury. Environ. Sci. Technol. 2009, 43, S9