Task Determine total mercury and methylmercury in surface sediments of different Delta ecosystems

Transcription

1 Task Determine total mercury and methylmercury in surface sediments of different Delta ecosystems Wesley A. Heim, Mark Stephenson, Autumn Bonnema, and Kenneth Coale Introduction For this task the major effort is to conduct studies of spatial and temporal distribution of methylmercury (MMHg) and total mercury (Hgt) in sediment and water to determine the role of habitat and season in controlling concentrations. This information will be used to identify habitat types and locations of hot spots in the Delta and will be a framework for future mercury monitoring projects. Two approaches were used in this monitoring effort: (1) study many sites a few times per year and; (2) study a subset of four sites monthly over multiple years. The synoptic study, conducted in phase one of our Calfed work ( ), investigated mercury distribution across the Delta and was conducted only during winter months. Hgt in sediment increased westward from the Delta and MMHg concentrations were higher (3 ng g -1 ) in the central Delta than surrounding tributaries (<1 ng g -1 )(Heim, Coale et al. 27). Hgt in sediments showed no apparent seasonal trend while MMHg increased from 1 to 6 ng g -1 during late spring and summer at four of six locations studied (Heim, Coale et al. 27). Our current work focused on identifying key habitat types and sampling them repeatedly over all seasons to better identify the range in Hgt and MMHg concentrations present in the Delta. Furthermore, this study aims to verify and quantify seasonal variations of MMHg in sediments and in the water column. We present the following hypotheses: (1) Hgt and MMHg concentrations in Delta sediments are spatially variable relative to habitat type and the distribution remains relatively constant year to year; (2) MMHg concentrations in Delta sediments increase during late spring through early summer as a result of increased Hg methylation in the sediment; (3) Within the Delta, wetland and marsh regions are major sites of MMHg production and; (4) Distribution of MMHg is related to the distribution of Hgt in Delta sediments. Methods Sampling Locations and Dates Page 1 of

2 In order to determine how MMHg and Hgt distribution in sediments relates to habitat type, 25 sites were sampled five times during this study. The sampling locations were representative of the broad range of habitat types found in the Delta that are incorporated into the National Wetlands Inventory Arc View GIS layers (Heim 23). Figure 1 shows the sampling locations for the Spatial Study. Sediment samples were collected from across the Delta system with specific landscape features targeted within each geographic area (Table 1). Figure 1. Map of the Delta showing sediment sampling locations for the Spatial Study. Table 1. Key to Spatial Study sampling plan showing geographic area, site code, and landscape feature sampled within geographic area. Landscape Features Open Water (fine) Open Water (sand) Mudflat Marsh Geographic Area Code Seasonal Wetland San Pablo Bay SPB X X X X X X Suisun Bay SB X X X X West Delta WD X X X X Central Delta CD X X X Northwest Delta NWD X X X Cosumnes River CR X X X South Delta SD X X X Salt Pond Surficial sediments (topmost.5 cm) were collected monthly at four locations from June, 23 to August, 25, to assess temporal trends in MMHg and total Hg. Sediment samples were collected from Browns Island (Photo 1), Franks Tract (Photo 2), Sycamore Slough (Photo 3), and Snodgrass Slough (Photo 4) (Figure 2). Page 2 of

3 In Addition, nineteen monthly water grab samples were collected for unfiltered MMHg, and filtered MMHg at all four seasonal sites. These sites were chosen to represent a variety of conditions and habitats found in the Delta. The Browns Island site is located in a small open water area at the center of a tidal marsh. Browns Island, as the western most seasonal site was the only site that experienced a range of salinity from mostly fresh during high flow season to ~4 during low flow season. Franks Tract is a large open water area surrounded by fringe tule marsh. The tract was flooded in the late 193s and prior to flooding was a peat soil farming field. Franks Tract is representative of what surrounding farm tracts would transform into once flooded. Sycamore Slough is a tidally influenced dead end rip rapped offshoot of the South Mokelumne River. It is surrounded by agricultural fields on the landward side of the levees. Fringe marsh habitat dominates the upper Sycamore Slough channel at the study site. Snodgrass Slough is the northernmost site located upstream of the Delta Cross Channel, east of the Sacramento River. The study site at Snodgrass Slough is at the dead end of a shallow channel with a woodland riparian strip. The site is thick with submerged aquatic vegetation. The Sacramento River has greater influence over this site during times when the Delta Cross Channel Gate is open. Figure 2. Map of the Delta showing sampling locations for the Seasonal Study. The topmost.5 cm of sediment was collected using a sampler designed and built, at Moss Landing Marine Laboratories (Heim 23). For Spatial Study sites in open water with a sandy substrate a Van Veen Grab sampler was used and the topmost.5 cm of sediment was retrieved from the sampler. In marsh habitat 2 hand Page 3 of

4 cores were used and the top.5 cm of sediment was extracted. Sediment samples were transferred into 6 ml wide-mouth borosilicate glass jars, with Teflon lined polyethylene caps using established ultra clean handling protocols (Gill and Fitzgerald 1985). Samples were placed on dry ice for transport back to the laboratory and kept frozen prior to analysis. The following ancillary measurements were made in the field: water temperature, sediment temperature, electrical conductivity, salinity, ph, turbidity, and dissolved oxygen. In addition, samples were collected for total suspended solids, chlorophyll a, grainsize, and loss on ignition (LOI). Photo 1. Browns Island seasonal sampling site. Photo 3. Sycamore Slough seasonal sampling site. Photo 2. Franks Tract seasonal sampling site. Photo 4. Snodgrass Slough seasonal sampling site. Laboratory analysis Hgt Analysis Page 4 of

5 Sediment samples were digested by adding 4. ml of concentrated HCl to 1. g of wet sediment and swirling. Next, 1. ml of concentrated HNO 3 was added, swirled, and samples were loosely capped and digested in a fume hood at room temperature overnight (Bloom 2). After complete digestion, samples were diluted up to 4 ±.5 ml with high purity deionized water (DI, 18 megaohm), capped tightly, shaken vigorously, and allowed to settle until the supernatant was clear. Hgt was measured by aqueous-phase reduction with stannous chloride solution followed by atomic absorbance detection using an automated PerkinElmer flow injection mercury system (Heim 23). The method detection limit (MDL), defined as three times the standard deviation of nine determinations of sand, known to be low in Hg and spiked with 6 ng Hg g -1 dw sediment, was 1.5 ng Hg g -1 dw sediment. MMHg Analysis Sediment samples for MMHg analysis were processed by the KBr and CH 2 Cl 2 extraction procedure described by Bloom et al. (Bloom, Colman et al. 1997). Briefly,.5-1. g of wet sediment was digested with acidic KBr solution and extracted into 1 ml of CH 2 Cl 2 in a 35 ml Teflon centrifuge tube. A 2. ml aliquot of CH 2 Cl 2 was then back extracted into DI water by purging out CH 2 Cl 2 with high-purity nitrogen gas. Extracts were analyzed for MMHg by aqueous-phase ethylation, trapping on a carbotrap column, gas chromatography separation, thermal decomposition to elemental Hg, and detection by cold vapor atomic fluorescence spectroscopy ((Liang, Hovart et al. 1994)). The MDL defined as three times the standard deviation of nine determinations of low MMHg content sand, spiked with.6 ng MMHg g -1 dw sediment, was.19 ng MMHg g -1 dw sediment. Results and Discussion Spatial Study Figure 3 shows MMHg sediment concentrations for samples collected from select habitat types within seven geographical areas of the Delta as listed in Table 1 and illustrated in Figure 1. Marsh habitat generally had higher MMHg concentrations than other habitat types sampled and also had greater variability. Surprisingly, seasonal wetland habitat in San Pablo Bay had MMHg concentrations much lower (median = 1.33 ng g -1 ) than the marsh habitat. Salt pond habitat was sampled only in San Pablo Bay and was comparable to marsh habitat with a median MMHg concentration of 6.46 ng g -1 (Data not shown). Open water sand substrate habitat had the lowest MMHg concentrations of all types sampled. Page 5 of

6 25 2 MMHg ng/g dry SPB Marsh SPB Mudflat SPB OWF SPB OWS SPB SW SB Marsh SB Mudflat SB OWF SB OWS WD Marsh WD Mudflat WD OWF WD OWS CD Marsh CD OWF CD OWS NWD Marsh NWD OWF NWD OWS CR Marsh CR OWF CR OWS SD Marsh SD OWF SD OWS Figure 3. Methylmercury (MMHg) sediment concentrations in samples collected from San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). Samples were collected from the following habitats: marsh, mudflat, open water fine grain substrate (OWF), open water sand substrate (OWS), and seasonal wetland (SW). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Marsh habitat in the Delta with the highest MMHg concentrations were found in San Pablo Bay, west and south Delta, and Cosumnes River compared to Suisun Bay, central Delta, and northwest Delta. The median MMHg concentrations of sediment samples collected from San Pablo Bay, west Delta, south Delta, and Cosumnes River marsh habitat were 11.5, 4.92, 4.16, and 1.99 ng g -1 respectively. Suisun Bay, central Delta, and northwest Delta marsh sediment MMHg median concentrations were.75,.8, and.72, ng g -1 respectively. MMHg in sediment decreased from west to east with the exception of Suisun Bay. Marsh habitat in the central Delta had lower concentrations than marsh to the northeast (Cosumnes River) or south (south Delta). MMHg concentrations in marsh habitats of the central delta and northwest delta (Prospect Slough area) were similar. Page 6 of

7 25 MMHg ng/g dry SPB SB WD CD NWD CR SD Marsh Habitat Figure 4. Methylmercury (MMHg) sediment concentrations (ng g -1 dry) in samples collected from marsh habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. MMHg ng/g dry SPB SB WD Mudflat Habitat Figure 5. Methylmercury (MMHg) sediment concentrations (ng g -1 dry) in samples collected from mudflat habitat of San Pablo Bay (SPB), Suisun Bay (SB), and West Delta (WD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Mudflat habitat was sampled in San Pablo Bay, Suisun Bay, and west Delta. Figure 5 shows MMHg concentrations in mudflat habitat were similar across the Delta. MMHg concentrations (median) in mudflat habitat at San Pablo Bay, Suisun Bay, Page 7 of

8 and west Delta were.35,.27, and.3 ng g -1 respectively (Figure 5). The greatest variability in MMHg concentration was observed at the west Delta site. MMHg ng/g dry SPB SB WD CD NWD CR SD Open Water Fine Habitat Figure 6. Methylmercury (MMHg) sediment concentrations (ng g -1 dry) in samples collected from open water fine grain substrate habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. MMHg concentration in open water fine grain substrate increased moving east from San Pablo Bay to the central Delta and north and south from the central Delta into Cosumnes River and south Delta (Figure 6). Median MMHg concentrations in open water fine grain substrate habitat of Cosumnes River, south Delta, and central Delta were 2.27, 2.7, and 1.36 ng g -1 respectively. MMHg concentrations were lower at west Delta and northwest Delta open water fine grain substrate habitat (median.75, and.77 ng g -1 ). The lowest MMHg concentrations of open water fine grain substrate were found at San Pablo Bay (.32 ng g -1 ) and Suisun Bay (.36 ng g -1 ). In each geographical location across the Delta the open water sand substrate habitat had the lowest MMHg concentrations compared to other habitat types sampled. Figure 7 shows MMHg concentrations in sand substrat habitat moving west to east from San Pablo Bay into the central Delta were less than.2 ng g -1. The northwest Delta, Cosumnes River, and south Delta had MMHg concentrations of.42,.18, and.22 ng g -1 respectively. Page 8 of

9 2. MMHg ng/g dry SPB SB WD CD NWD CR SD Open Water Sand Habitat Figure 7. Methylmercury (MMHg) sediment concentrations (ng g -1 dry) in samples collected from open water sand substrate habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Figure 8 shows all data collected during the spatial study grouped by habitat type. Marsh habitat had the highest MMHg concentrations and also the greatest variablility. The median MMHg concentration of all samples (n= 34) collected from marsh habitat was 2.4 ng g -1 and half the samples had concentrations between 2 and 5 ng g -1 of MMHg. Seasonal wetland had the second highest median MMHg concentration (1.33 ng g -1 ) with half the samples having MMHg concentrations between.4 and 3 ng g -1. However, the sample size for this habitat is small (n =5) and as a result uncertainties are larger than other habitat studied. Open water fine grain substrate had the third highest median MMHg concentration (.83 ng g -1, n = 34) and less variation than marsh or seasonal wetland habitat. Half of the samples collected from open water fine grain substrate habitat had MMHg concentrations between.4 and 1.56 ng g -1. The lowest median MMHg concentrations were found in mudflat (. ng g -1, n = 14) and open water sand substrate habitats (.17 ng g -1, n = 33). Mudlfat habitat had the least variation of all habitat types, but it should be pointed out that this may be a result of samples being collected from a smaller spatial area as mudflat habitat was found in only three of seven sampled geographic areas of the Delta. Page 9 of

10 25 2 MMHg ng/g dry Marsh Seasonal wetland Open water fine Mudflat Open water sand Figure 8. Sediment methylmercury (MMHg) concentrations measured in samples collected from marsh, seasonal wetland, open water fine grain substrate, mudflat, and open water sand substrate. The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Figure 9 shows Hgt sediment concentrations for samples collected from select habitat types within seven geographical areas of the Delta as summarized in Table 1 and illustrated in Figure 1. Cosumnes River had the highest Hgt Concentrations in the Open water fine grain substrate habitat. The lowest Hgt concentrations were observed in the open water sand substrate habitat of the west Delta, central Delta, and south Delta. Generally, Hgt sediment concentrations across the Delta were 1-3 ng g -1 dry weight. Salt pond habitat was sampled on three occasions only in San Pablo Bay. Salt Pond habitat in San Pablo Bay had a median sediment Hgt concentration of 195 ng g -1 (data not shown), which is similar to open water sand substrate habitat in San Pablo Bay (Figure 9). Page 1 of

11 SPB Marsh SPB Mudflat SPB OWF SPB OWS SPB SW SB Marsh SB Mudflat SB OWF SB OWS WD Marsh WD Mudflat WD OWF WD OWS CD Marsh CD OWF CD OWS NWD Marsh NWD OWF NWD OWS CR Marsh CR OWF CR OWS SD Marsh SD OWF SD OWS Hgt ng/g dry Figure 9. Total mercury (Hgt) sediment concentrations in samples collected from San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). Samples were collected from the following habitats: marsh, mudflat, open water fine grain substrate (OWF), open water sand substrate (OWS), and seasonal wetland (SW). The median concentration is represented by x. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. A trend of decreasing Hgt concentration moving east from San Pablo Bay into the central Delta was observed in marsh habitat (Figure 1). The median Hgt concentration of marsh sediment in San Pablo Bay was 326 ng g -1. Similar to MMHg results, median Hgt concentration of seasonal wetland habitat (78 ng g -1 ) in San Pablo Bay was much lower than the median MMHg concentration of marsh habitat. In Suisun Bay and the west Delta the Hgt median concentrations were 232 and 24 ng g -1 respectively. The concentration dropped to 89 ng g -1 in the central Delta. Hgt concentrations were also higher at northwest Delta (22 ng g -1 ), Cosumnes River (274 ng g -1 ), and south Delta (16 ng g -1 ) than in the central Delta marsh habitat. Page 11 of

12 6 5 HgT ng/g dry SPB SB WD CD NWD CR SD Marsh Habitat Figure 1. Total mercury (Hgt) sediment concentrations (ng g -1 dry) in samples collected from marsh habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. 4 Hgt ng/g dry SPB SB WD Mudflat Habitat Figure 11. Total mercury (Hgt) sediment concentrations (ng g -1 dry) in samples collected from mudflat habitat of San Pablo Bay (SPB), Suisun Bay (SB), and West Delta (WD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Page 12 of

13 In mudflat habitat Hgt also showed a decreasing trend moving from San Pablo Bay east to the west Delta. Median Hgt concentration in San Pablo Bay mudflat habitat was 39 ng g -1 and decreased to 237 ng g -1 in Suisun Bay, and was the lowest in west Delta mudflat habitat (12 ng g -1 )(Figure 11). Hgt concentrations in open water fine grain substrate habitat showed a similar trend to mudflat and marsh habitat types. Hgt concentration dropped from 251 ng g -1 in San Pablo Bay to 137 ng g -1 in the central Delta. Suisun Bay and west Delta open water fine grain substrate habitat had median Hgt concentrations of 2 and 252 ng g -1 respectively (Figure 12). Open water fine grain sediment concentrations were similar in the northwest Delta (19 ng g -1 ), south Delta (151 ng g -1 ) and central Delta (137 ng g -1 ). The highest Hgt concentration in this habitat type was found at Cosumnes River (495 ng g -1 ). This may be explained by the fact that the Cosumnes River is the only river in this watershed which has been unobstructed by a dam. Historic mining activities may have a greater influence on Cosumnes River than other locations sampled. Hgt ng/g dry SPB SB WD CD NWD CR SD Open Water Fine Habitat Figure 12. Total mercury (Hgt) sediment concentrations (ng g -1 dry) in samples collected from open water fine grain substrate habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. As in other habitat types mentioned above, open water sand habitat Hgt concentration decreased from west to east; Hgt concentration decreased from 163 ng g -1 in San Pablo Bay to 42 ng g -1 in the central Delta. Suisun Bay and west Delta open water sand substrate habitat had median Hgt concentrations of 94 and ng g -1 respectively (Figure 13). Within the open water sand substrate habitat the northwest Delta had the highest median Hgt concentration (24 ng g -1 ) of all locations sampled. Open water sand substrate habit at Cosumnes River had a low median Hgt concentration (2 ng g -1 ) in contrast to the open water fine grain Page 13 of

14 substrate at the same location (495 ng g -1 ). Expectedly, Hgt concentrations in open water sand substrate habitat were generally lower than other habitat types within the same geographical area (Figure 9). 5 4 Hgt ng/g dry SPB SB WD CD NWD CR SD Open Water Sand Habitat Figure 13. Total mercury (Hgt) sediment concentrations (ng g -1 dry) in samples collected from open water sand substrate habitat of San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. Figure 14 shows Hgt concentrations in sediments for all habitat types sampled. Marsh, mudflat, and open water fine substrate had higher Hgt concentrations than seasonal wetland and open water sand substrate. Open water fine substrate had the greatest variability followed by marsh, open water sand substrate, mudflat, and seasonal wetland (seasonal wetland only sampled in San Pablo Bay). Median concentrations were similar for open water fine, mudflat, and marsh habitats but highest concentrations of Hgt were found in open water fine substrate. Page 14 of

15 7 6 5 Hgt ng/g dry Marsh Mudflat Open water fine Seasonal wetland Open water sand Figure 14. Sediment total mercury (Hgt) concentrations measured in samples collected from marsh, mudflat, open water fine grain substrate, seasonal wetland, and open water sand substrate. The median concentration is represented by *. Vertical lines represent maximum and minimum concentrations. Boxes are bracketing lower and upper quartile. The transformation of inorganic Hg to MMHg is termed methylation and the degree to which this occurs is termed (net) methylation efficiency. Methylation efficiency has been estimated in aquatic sediments using the concentration ratio of MMHg to Hgt as a proxy (Krabbenhoft, Hurley et al. 1999). Furthermore, a study done in Sweden has shown percent MMHg in surface sediments may be used as a proxy for the rate of methylation, across a range of sites from different environments (Drott, Lambertsson et al. 28). In this study, we use the above mentioned concentrations of MMHg and Hgt to calculate the percent MMHg and use the percent MMHg to characterize the methylation efficiency of the habitat types and environments studied. Figure 15 shows the percent MMHg for select habitat types within seven geographical areas of the Delta as summarized in Table 1 and illustrated in Figure 1. The range in percent MMHg observed in this study (median ratio = ) is similar to values reported by Drott et al. (28) in a study of Swedish small lakes, small freshwater dominated estuaries, and larger brackish water estuaries all subjected to heavy, local Hg contamination. Generally, percent MMHg was less than.5 for many locations and habitat types sampled (Figure 15). However, there was also clear differences in percent MMHg observed in different habitat types and spatially across the Delta. For example, marsh habitat in San Pablo Bay, west Delta, and south Delta typically had elevated percent MMHg (Figure 15). In the Page 15 of

16 case of San Pablo Bay marsh (percent MMHg = 2.8) we observed high concentrations of MMHg and Hgt in the sediments. West Delta marsh had a percent MMHg of 2.4 as a result of fairly high MMHg concentrations and lower Hgt concentrations. The marsh habitat of south Delta had a percent MMHg of 2.84 which is comparable to the value found in marshes of San Pablo Bay. Marsh habitat in Suisun Bay, northwest Delta, central Delta, and Cosumnes River was less favorable for MMHg production than locations mentioned above. Suisun Bay marsh had low MMHg concentrations and fairly high Hgt concentrations resulting in one of the lowest percent MMHg (median =.32 %) measured; northwest Delta similarly had a low percent MMHg (.33 %). Central Delta marsh had low MMHg concentrations and low Hgt concentrations resulting in a percent MMHg of 1. (Figure 15). Marsh habitat at Cosumnes River had fairly high MMHg concentrations but high Hgt concentrations drove the percent MMHg down (.77 %). Salt Pond habitat in San Pablo Bay had a percent MMHg of 3.32 (data not shown) which is considerably higher than most of the habitat types sampled across the Delta. The moderate concentrations of inorganic Hg found in Salt Pond habitat in San Pablo Bay yielded high concentrations of MMHg; salt ponds appear to be a habitat type efficient at Hg methylation. In summary, Hgt concentrations were typically higher in marsh, mudflat, open water fine grain substrate habitats than in seasonal wetland and open water sand substrate habitat. Low concentration of Hgt in sandy habitat is not surprising as Hgt is often correlated to grainsize and is preferentially bound to clay particles (Conaway, Squire et al. 23). Hgt concentrations were higher in San Pablo Bay and decreased into the central Delta. This confirms observations made by (Heim, Coale et al. 27)) who reported a similar finding. The elevated sediment Hgt concentrations in San Pablo Bay may be a result of a system starved for sediment and as a result the reintroduction of historically deposited Hg into surface sediments due to erosion and redistribution. Cosumnes River had the highest Hgt concentrations measured in this study. Coincidently, the Cosumnes River is the only river which flows unobstructed by any dam into the Delta. Most of the habitat types sampled in the Delta had fairly low MMHg sediment concentrations. In San Pablo Bay the marsh, salt pond, and seasonal wetland habitat was significantly higher than mudflat and open water sites. Similarly, in Suisun Bay and west Delta only marsh habitat had elevated MMHg concentrations. Heim et al. (27) reported MMHg sediment concentrations in the central Delta to be 1-3 ng g -1 and less than 1 ng g -1 in the perimeter waterways such as west Delta, northwest Delta, and Cosumnes River. We report similar concentrations of MMHg in the central Delta and northwest Delta. However, MMHg concentrations measured in this study at Cosumnes River, West Delta and south Delta are higher than those reported by Heim et al. (27). This is likely due to time of collections. Heim et al. (27) conducted a synoptic survey of sediments during winter wet season, when MMHg concentrations are typically lower. This study was conducted during both wet and dry seasons and the result is a clearer picture of MMHg distribution in sediments which considers habitat, location, and time. Page 16 of

17 SPB Marsh SPB Mudflat SPB OWF SPB OWS SPB SW SB Marsh SB Mudflat SB OWF SB OWS WD Marsh WD Mudflat WD OWF WD OWS CD Marsh CD OWF CD OWS NWD Marsh NWD OWF NWD OWS CR Marsh CR OWF CR OWS SD Marsh SD OWF SD OWS Percent MMHg Figure 15. Percent of total mercury (Hgt) as methylmercury (MMHg) in sediment samples collected from San Pablo Bay (SPB), Suisun Bay (SB), West Delta (WD), Central Delta (CD), Northwest Delta (NWD), Cosumnes River (CR), and South Delta (SD). Samples were collected from the following habitats: marsh, mudflat, open water fine grain substrate (OWF), open water sand substrate (OWS), and seasonal wetland (SW). The median percent is represented by x. Vertical lines represent maximum and minimum percentages. Boxes are bracketing lower and upper quartile. MMHg as a percent of the total Hg present in sediment generally followed the pattern of MMHg sediment concentration. The mudflat and open water habitats appeared to have limited ability to convert inorganic Hg to MMHg, with the exception of open water in south Delta. Marsh, salt pond, and seasonal wetland habitats in the Delta showed great efficiency at converting inorganic Hg to MMHg. Seasonal Study Figure 16 shows average MMHg concentrations in sediment collected monthly from Browns Island. MMHg concentrations in surficial sediments at Browns Island ranged ng g -1. MMHg concentrations were variable month to month and year to year but no clear seasonal trend was observed. Total Hg measured monthly at Browns Island averaged 248 ± 35 ng g -1. Figure 17 shows MMHg concentrations in filtered and unfiltered water collected by surface grabs at Browns Page 17 of

18 Island January, 24 to August, 25. Unfiltered MMHg concentrations ranged from.8 to.59 ng L -1. A clear seasonal trend was observed with highest concentrations of MMHg occurring late winter and spring each year at this site. Filtered MMHg concentrations covaried with unfiltered MMHg (r 2 =.96) and were ~2 percent lower than unfiltered MMHg concentrations. When looking at Figure 16 and 17 it appears that water-column and sediment MMHg concentrations may correlate over time at Browns Island during spring and summer but not during winter of 4-5. The maximum MMHg concentrations occurred March and May for both water and sediment at this site. This could be interpreted to suggest benthic production is important to water-column MMHg during spring and summer with another source dominating during fall and winter. However, the data did not support this observation when correlation analysis was performed. Possibly, the time series and sampling interval was too short to elucidate this apparent trend MMHg (ng/g) dry Aug-5 Jun-5 Apr-5 Feb-5 Dec-4 Oct-4 Aug-4 Jun-4 Apr-4 Feb-4 Dec-3 Oct-3 Aug-3 Jun-3 Figure 16. Average methylmercury (MMHg) concentrations (ng g -1 dry weight sediment) in surficial sediment (topmost.5 cm) collected from Browns Island June, 23 to August, 25. Page 18 of

19 .6 MMHg ng/l.4.2 Jul-5 May-5 Mar-5 Jan-5 Nov-4 Sep-4 Jul-4 May-4 Mar-4 Jan-4 Figure 17. Methylmercury (MMHg) concentrations in filtered ( ) and unfiltered ( ) water, collected by surface grabs, at Browns Island, January, 24 to August, 25. Figure 18 shows monthly average MMHg concentrations in sediment collected form Franks Tract. MMHg concentrations ranged from ng g -1. With the exception of the June, 25 sampling event, MMHg concentrations were fairly uniform during this study and showed no seasonal trends. Heim et al. (27) observed an increase in MMHg concentration from 2 to 6 ng g -1 during spring the second year of a two year study. In that study it was suggested the spring increase was not observed in the first year of the study due to sampling interval. We observed a similar increase June, 25. Unexpectedly, the increase was not repeated year to year. In this study the sampling interval was sufficient to capture an event and confidence is high that MMHg concentrations remained unchanged through spring of 22 and 23. The average total Hg sediment concentration at Franks Tract during this study was 15 ± 25 ng g -1. Figure 19 shows MMHg concentrations in filtered and unfiltered water collected by surface grabs at Franks Tract January, 24 to August, 25. In contrast to MMHg in sediment water concentrations showed a clear seasonal trend. Unfiltered MMHg concentrations ranged from.4 to.15 ng L -1. Highest concentrations of MMHg occurred February each year at this site. A second seasonal trend observed is a lesser peak in unfiltered MMHg concentration occurring July and August. Filtered MMHg concentrations covaried with unfiltered MMHg (r 2 =.88) and were ~2 percent lower than unfiltered MMHg concentrations. As noted for Browns Island above, it appears that water-column and sediment MMHg concentrations covary during spring and summer. However, no significant correlation was found. Again, we conclude additional data is needed to improve the understanding of the relationship between sediment and water column MMHg concentrations at this site. Page 19 of

20 6 5 MMHg (ng/g) dry Aug-5 Jun-5 Apr-5 Feb-5 Dec-4 Oct-4 Aug-4 Jun-4 Apr-4 Feb-4 Dec-3 Oct-3 Aug-3 Jun-3 Figure 18. Average methylmercury (MMHg) concentrations (ng g -1 dry weight sediment) in surficial sediment (topmost.5 cm) collected from Franks Tract June, 23 to August, 25. Concentrations of MMHg in sediment collected monthly from Sycamore Slough are shown in Figure 2. MMHg concentrations varied from.69 to 7.23 ng g -1. A clear seasonal trend in MMHg sediment concentrations was observed at Sycamore Slough during this study. MMHg concentrations were higher during summer than other seasons. Total Hg sediment concentrations averaged ± 33 ng g -1. MMHg concentrations in filtered and unfiltered water collected by surface grabs at Sycamore Slough January, 24 to August, 25 are shown in Figure 21. Unfiltered MMHg concentrations ranged from.4 to.17 ng L -1. Clear seasonal differences were observed at Sycamore Slough withhighest concentrations of MMHg in water occurring during winter. Filtered MMHg concentrations covaried with unfiltered MMHg (r 2 =.78) and were ~4 percent lower than unfiltered MMHg concentrations. Page 2 of

21 .2 MMHg ng/l.1. Jul-5 May-5 Mar-5 Jan-5 Nov-4 Sep-4 Jul-4 May-4 Mar-4 Jan-4 Figure 19. Methylmercury (MMHg) concentrations in filtered ( ) and unfiltered ( ) water, collected by surface grabs, at Franks Tract, January, 24 to August, 25. Snodgrass Slough MMHg sediment concentrations are shown in Figure 22. The range of MMHg measured in sediments from Snodgrass Slough was 1.9 to 5.98 ng g -1. Concentrations of MMHg in surficial sediments at Snodgrass Slough were higher during spring and summer compared to winter and fall measurements. The average Hgt sediment concentration of all samples collected at Snodgrass Slough was 21 ± 26 ng g -1. Figure 23 shows MMHg concentrations in filtered and unfiltered water collected by surface grabs at Snodgrass Slough January, 24 to August, 25. Unfiltered MMHg concentrations ranged from.8 to.3 ng L -1. At Snodgrass Slough, concentrations of MMHg in water were highest March to June. Filtered MMHg concentrations covaried with unfiltered MMHg (r 2 =.64) and were ~38 percent lower than unfiltered MMHg concentrations. Water collected from Browns Island had the highest MMHg concentrations of the four sites studied. Snodgrass Slough had the second highest MMHg concentrations in water, followed by Franks Tract and Sycamore Slough. Similarly, MMHg sediment concentrations were highest at Browns Island followed by Snodgrass Slough, Sycamore Slough and Franks Tract. There appears to be a general relationship between water and sediments with sites having higher sediment MMHg concentrations also having higher MMHg concentrations in water. However, as discussed above, the relationship is not significant. At Browns Island and Franks Tract dissolved MMHg accounted for ~8 percent of the total MMHg compared to ~6 percent at Sycamore and Snodgrass. Gradients in organic matter, as measured by percent loss on ignition (LOI), between sites followed the same trend as MMHg concentrations; The highest percent LOI was 19.3 ± 3.8 measured at Page 21 of

22 Browns Island, followed by Snodgrass Slough (14.3 ± 1.9), Sycamore Slough (9.9 ±.4), and the lowest percent LOI was measured at Franks Tract (8.3 ±.9). Grainsizes at all four sites were similar and averaged 61.5 ± 3.7 percent silt and 22.2 ± 3.4 percent clay. Chlorophyll a, measured as a proxy for primary production, showed a typical seasonal pattern at all four sites. However, the magnitudes differed with higher concentrations measured at Franks Tract and Sycamore Slough (see Appendix) than at Browns Island and Snodgrass Slough. Chlorophyll a concentration and MMHg water concentration at Browns Island and Snodgrass Slough showed a weak positive relationship while at Franks Tract and Sycamore Slough there was a weak negative relationship. These differences may be related to water residence time which is likely longer at Browns Island and Snodgrass Slough due to both sites being enclosed with relatively slow moving water compared to Franks Tract and Sycamore Slough, which are open and influenced by stronger currents and wind. There was a positive relationship between Hgt and MMHg in sediments (r 2 =., all seasonal sites combined). The low r 2 value illustrates the importance of Hg speciation with respect to microbial conversion of Hg to MMHg; Hgt is one factor controlling MMHg concentration MMHg (ng/g) dry Aug-5 Jun-5 Apr-5 Feb-5 Dec-4 Oct-4 Aug-4 Jun-4 Apr-4 Feb-4 Dec-3 Oct-3 Aug-3 Jun-3 Figure 2. Average methylmercury (MMHg) concentrations (ng g -1 dry weight sediment) in surficial sediment (topmost.5 cm) collected from Sycamore Slough June, 23 to August, 25. Page 22 of

23 .2 MMHg ng/l.1 Jul-5 May-5 Mar-5 Jan-5 Nov-4 Sep-4 Jul-4 May-4 Mar-4 Jan-4 Figure 21. Methylmercury (MMHg) concentrations in filtered ( ) and unfiltered ( ) water, collected by surface grabs, at Sycamore Slough, January, 24 to August, 25. Page 23 of

24 7 6 5 MMHg (ng/g) dry Aug-5 Jun-5 Apr-5 Feb-5 Dec-4 Oct-4 Aug-4 Jun-4 Apr-4 Feb-4 Dec-3 Oct-3 Aug-3 Jun-3 Figure 22. Average methylmercury (MMHg) concentrations (ng g -1 dry weight sediment) in surficial sediment (topmost.5 cm) collected from Snodgrass Slough June, 23 to August, 25. Page 24 of

25 .4.3 MMHg ng/l.2.1 Jul-5 May-5 Mar-5 Jan-5 Nov-4 Sep-4 Jul-4 May-4 Mar-4 Jan-4 Figure 23. Methylmercury (MMHg) concentrations in filtered ( ) and unfiltered ( ) water, collected by surface grabs, at Snodgrass Slough, January, 24 to August, 25. Factors Controlling Distribution of MMHg As discussed above sites with higher percent LOI also had higher MMHg sediment concentrations. Figure 24 shows percent LOI and MMHg sediment concentrations for all samples collected during the Seasonal and Spatial studies. A significant positive relationship was observed between percent LOI and MMHg concentrations in sediment (r 2 =.45, p <.1, n = 115). A first order estimate of MMHg concentration may be obtained from organic matter distribution in the Delta. The relationship between total concentrations of Hg and MMHg in sediments was significantly positive (r 2 =.1, p <.1, n = 263) for a data set comprising all sites of the Spatial and Seasonal Studies, Figure 25. The relationship between total Hg and MMHg was weaker when a full coherent treatment of the data set was used compared to four sites over time as stated above (r 2 =., only Seasonal Study sites). It is clear Hgt is one factor contributing to MMHg production in the Delta. This observation is in agreement with previous reports (Heim et al. 27;(Hammerschmidt and Fitzgerald 24); (Benoit, Gilmour et al. 1998); (Drott, Lambertsson et al. 28) of sites contaminated with diffuse and local sources of Hg. The relationship between Hgt and MMHg is generally explained in terms of Hg availability to methylating bacteria. Drott et al. (28), report an inevitable link between Hgt concentration and its bioavailable forms over long terms and data sets with large variations in Hg. Drott et al. (28) state, locations where the solubility of Hg is regulated by equilibria involving surface complexation generate a positive relationship between dissolved Hg and total sediment Hg. On the other hand, if HgS(s) or mixed Hg/FeS(s) phases control Hg solubility, the concentration of Page 25 of

26 15 MMHg (ng/g) % Loss on Ignition Figure 24. Methylmercury (MMHg) versus percent loss on ignition (LOI) in suficial sediments collected from all Spatial and Seasonal Study sites (y =.21x.15, r 2 =.45) MMHg (ng/g) Hgt (ng/g) Figure 25. Methylmercury (MMHg) versus total mercury (Hgt) concentrations in suficial sediments collected from all Spatial and Seasonal Study sites (y =.1x +.3, r 2 =.1). Page 26 of

27 bioavailable, neutral mercury sulfides in pore water will be independent of total Hg concentration if dissolved inorganic sulfides are below 1 mm. Hgt concentrations will have less of an impact on MMHg production in locations where sediment have solid HgS phases (Drott et al. 28). We suspect many locations in the Delta have HgS(s) phases and this may explain the low r 2 value for the relationship between Hgt and MMHg. Prospect Slough and Cosumnes River sites exemplify the importance of Hg speciation with respect to Hg methylation. Bloom (21) reported Cosumnes River sediments had very little HgS(s) compared to Prospect Slough. There is a positive relationship between total Hg and MMHg in a subset of Cosumnes River data (r 2 =.45) in contrast to a subset from Prospect Slough where no relationship was observed. This illustrates the importance of Hg speciation in addition to the total amount of Hg as a control on MMHg production. In much of the Delta microbial conversion of inorganic Hg occur independent of the total amount of Hg present. Summary and Conclusions (1) Concentrations of MMHg in sediments varied by habitat and were highest in samples collected from marshes followed by seasonal wetland, open water fine grain substrate, mudflat, and open water sand substrate. (2) Hgt concentrations were typically higher in marsh, mudflat, open water fine grain substrate habitats than in seasonal wetland and open water sand substrate habitat. (3) Hgt concentrations had a west/east trend with higher concentrations in San Pablo Bay which decreased moving eastward into the central Delta. (4) MMHg concentrations in water varied by season at all sites, however, sediment MMHg concentrations varied by season only at Sycamore and Snodgrass Slough sites. (5) Sites with highest MMHg sediment concentrations also had highest MMHg concentrations in water. (6) A first order estimate of MMHg concentration may be obtained from organic matter distribution in the Delta. (7) The relationship between total concentrations of Hg and MMHg in sediments was significantly positive, however, in much of the Delta microbial conversion of inorganic Hg occurs independent of the total amount of Hg present. Page 27 of

28 References Benoit, J. M., C. C. Gilmour, et al. (1998). "Behavior of mercury in the Patuxent River estuary." Biogeochemistry 4: Bloom, N. (21). Solid phase Hg speciation and incubation studies in or related to mine-site runoff in the Cache Creek watershed (CA). Assessment of ecological and human health impacts of mercury in the Bay-Delta watershed. Monterey. Bloom, N. S. (2). Preparation of Solids Samples for Total Mercury by Cold Aqua-Regia Digestion. Appendix D in Quality Assurance Project Plan for the CALFED Project: "An Assessment of Ecological and Human Health Impacts of Mercury in the Bay-Delta Watershed". Bloom, N. S., J. A. Colman, et al. (1997). "Artifact formation of methyl mercury during aqueous distillation and alternative techniques for the extraction of methyl mercury from environmental samples." Fresen. J. Anal. Chem. 358(3): Conaway, C. H., S. Squire, et al. (23). "Mercury speciation in the San Francisco Bay estuary." Mar. Chem. 8: Drott, A., L. Lambertsson, et al. (28). "Do potential methylation rates reflect accumulated methyl mercury in contaminated sediments." Environ. Sci. Technol. 42: Gill, G. A. and W. F. Fitzgerald (1985). "Mercury sampling of open ocean waters at the picomolar level." Deep-Sea Res. 32(3): Hammerschmidt, C. R. and W. F. Fitzgerald (24). "Geochemical controls on the production and distribution of methylmercury in near-shore marine sediments." Environ. Sci. Technol. 38: Heim, W. A. (23). Methyl and total mercury in surficial sediments of the San Francisco Bay-Delta. Moss Landing Marine Laboratories. San Jose, California, San Jose State University at Moss Landing Marine Laboratories. Heim, W. A., K. H. Coale, et al. (27). "Spatial and habitat-based variations in total and methyl mercury concentrations in surficial sediments in the San Francisco Bay-Delta." Environ. Sci. Technol. 41(1): Page 28 of

29 Krabbenhoft, D. P., J. P. Hurley, et al. (1999). The Aquatic Cycling of Mercury in the Everglades (ACME) Project; a process-based investigation of mercury biogeochemistry in a complex environmental setting. Proceedings of South Florida restoration science forum. S. Gerould and A. Higer. Boca Raton, FL, United States, U. S. Geological Survey program on the South Florida ecosystem. Proceedings of South Florida restoration science forum: Liang, L., M. Hovart, et al. (1994). "An improved speciation method for mercury by GC/CVAFS after aqueous phase ethylation and room temperature precollection." Talanta 41(3): Page of