Environ. Sci. Techno/. 1995, 29, 1867-1875
lfluences of Watershed Characteristics on Mercury Levels in Wisconsin Rivers JAMES P. HURLEY,*ft,* J A N I N A M. B E N O I T , * CHRISTOPHER L . B A B I A R Z , * MARTIN M. SHAFER,* A N D E R S W. A N D R E N , * J O H N R. S U L L I V A N , § RICHARD HAMMOND,s AND D A V I D A. WEBB5 Bureau of Research, Wisconsin Department of Natural Resources, 1350 Femrite Drive, Monona, Wisconsin 53716, Water Chemistry Program, University of Wisconsin, 660 North Park Street, Madison, Wisconsin 53706, and Bureau of Water Resources Management, Wisconsin Department of Natural Resources, 101 South Webster Street, Madison, Wisconsin 53707 ~~~
~
Total and monomethyl mercury were measured at 39 river sites in Wisconsin during fall 1992 and spring 1993. Using a Geographic Information System (GIs), w e delineated watersheds with unique and homogeneous physical characteristics. Mean unfiltered total Hg (HgT) was higher in spring (7.94 ng L-l) than in fall (3.45 ng L-l). Major differences in HgT yields were observed among various land-use groupings. In wetland/forest watersheds, elevated HgTflUXeSwere associated with the filtered phase, while in agricultural watersheds, increased HgT fluxes were due to particle loading . Mono methyl mercury (Me Hg) yields from wetland/forest sites were higher than agricultural/forest sites and agricultural only sites. Percent wetland surface area was positively correlated with MeHg yield. These results identify the importance of land use and land cover in influencing Hg concentrations, speciaton, and transport in rivers.
Introduction Although numerous studies have focused on Hg cycling mechanisms in lake and oceanic systems (1- 3,relatively little research has been conducted on Hg transport mechanisms in flowing rivers. As in earlier lake studies, clean sampling techniques (8, 9) were not commonly used for trace metal sample collection and analyses of river waters. Studies by both Windom et al. (10) and Benoit (11) have characterizedthe unreliability of data sets for Eastern United States rivers in which samples were obtainedwithout strict adherence to trace metal clean techniques. From a regulatory standpoint, accurate assessment of trace metals in rivers using clean protocols is essential to evaluate potential impacts of contaminants in receiving waters. It is common for effluent discharge permits to be based on either an assimilative capacity of a receiving water or a background trace metal level in a river, a level which the discharger is not allowed to exceed. In recent studies which utilized updated Hg sampling and analytical techniques, Hg levels in U.S. rivers were reported to be in the same range as those reported for lakes (genarally < 10ng L-l). In cases of contaminated effluents, however, HgT levels approached 50 ng L-’ (4, 12,13). In Wisconsin, previous work by Babiarz a n d h d r e n (14)found unfiltered HgT in rivers from major drainage basins of the state to all be below 10 ng L-’ with a slight positive relationshipbetween Hg and suspended particulate matter (SPM) levels. It is difficult to directly assess mechanisms controlling Hg mobilization and transport in rivers. Although rivers and lake watersheds receive similar fluxes of atmospherically derived Hg inputs on an areal basis, transformation processes and bioavailabilityof deposited Hg may be quite different. Prior to reaching the river, Hg may be incorporated in soil matrices (15) and slowly leached into groundwater (16). Samples obtained from flowing rivers represent water derived from numerous and sometimes geologically distinct regions of the larger watershed. Complex watersheds characterized by various land uses and soil types thus make it difficult to delineate individual transport processes. For example, in an erosional watershed, sorption to suspended particle loads may be the dominant control for transport of Hg. In a high chloride watershed, anionic complexation might govern mobilization. Differences in watershed type may exert a strong influence on Hg levels, so it is important to limit the complexity of the systems studied if one is to evaluate individual processes from the results. In this study, we chose 39 river sites from a variety of individual watershed types in an attempt to identify important mechanisms controlling fate and transport of Hg in rivers. Sites were selected to represent “relatively homogeneous units” (RHU[fromU.S.Geological Survey’s National Water Quality Assessment (NAWQA)terminologyl) reflective of individual characteristicsin a given watershed. * Corresponding author e-mail address:
[email protected]; Telephone: (608)262-3979; Fax: (608)262-0454. + Bureau of Research. 4 Water Chemistry Program. $ Bureau of Water Resources Management.
0013-936W95/0929-1667$09.00/0
0 1995 American Chemical Society
VOL. 29, NO. 7,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1807
TABLE 1
Study Site Characteristics and Unfiltered HBT levels
watershed
latitude, longitude
land use/land cover, % composition, sum fall spring map (Fig 1) watershed wet1992 1993 designation area ( k d ) urban agric forest water land barren (ng/L) (ngA)
East Fork Black River Moose River Popple River Upper Tamarack River Black River (MI) Brule River Upper Eau Claire River Nemadji River ( M N ) Sand River Thornapple River Wolf River at Lily Wisconsin River at Conover
44"25'07", 90'3814" 46"01'13", 9 0 ' 5 8 5 0 45O47'43, 88"45'28 46"08'37", 92'17'40 46"3953", 90"02'38 46"42'21", 91"3611" 46°13'25,91"43'57" 46"31'03,92"23'26 46"54'04",90"57'25" 45'39'36, 90'58'44 45"20'10", 88'53'23'' 46'05'26, 89"15'49
Wetland and Forest WF1 756 WF2 138 WF3 95 WF4 233 WF5 707 WF6 438 WF7 337 WF8 315 WF9 71 WFIO 262 WF11 842 WF12 220
Duck Creek Milwaukee River at Batavia Otter Creek S. Br. Pensaukee River Rush River Rattlesnake Creek Sheboygan River at Dotyville
44'27'58", 88"13'08 43"33'44", 88O03'08 42"40'58", 90'03'19' 44"45'07", 88'1639' 44"50'54", 92"24'05' 42"4630,90"55'23 43"45'21", 88"1602"
Agriculture Only A01 247 A02 135 A03 126 A04 83 A05 173 A06 116 A07 42
Fish Creek Big Eau Pleine River S. Fork Grand River E. Br. Pecatonica River Pigeon Creek Big Rib River Chaffee Creek Kickapoo River Tomorrow River Ten Mile Creek
46"34'39", 90"57'44" 44°52'56,90"10'44 43'40'29", 89"05'39" 42"53'23", 89O54'03" 91'09'35'' 445'61 l", 45'1 1'08", 90"05'09" 43"57'00", 89'2050" 43'4630, 90'33'27'' 44'31'33, 89'20'23 44"15'51", 89O48'32"
Agriculture and Forest AF1 498 AF2 292 AF3 49 AF4 137 AF5 92 AF6 266 AF7 120 AF8 142 AF9 114 AFIO 190
Sheboygan River at Marsh
43'50'59',
88"02'35'
0.0 0.0 0.0 0.0
14.W 0.0 0.1 2.4
67.9 55.3 51.8 73.4
0.7 16.5 0.0 44.8 0.3 47.9 0.4 23.9
0.0 0.0 0.0 0.0
0.2 0.3
2.6 1.4
85.0 89.6
1.7 7.6
10.5 1.4
0.0 0.0
0.0 0.0 0.3 2.1
5.6 0.6 6.3 0.5
93.0 71.0 70.8 63.4
0.0 0.0 5.2 19.4
1.4 28.2 17.7 13.7
0.0 0.2 0.0 0.8
5.06 10.21 8.41 9.05 6.72 8.25 4.58 6.71 4.23 6.24 1.00 4.55 0.78 3.09 2.50 12.51 0.93 21.05 7.29 4.68 2.24 4.22 1.70 1.93
0.6 1.2 0.1 0.2 1.6 0.0 1.1
89.2 87.3 98.3 87.8 95.4 99.3 93.0
4.7 6.0 1.6 3.8 2.9 0.7 0.0
0.0 0.3 0.0 0.0 0.2 0.0 0.0
5.0 5.1 0.0 8.1 0.0 0.0 3.5
0.6 0.2 0.0 0.0 0.0 0.0 0.2
1.52 5.00 2.23 6.69 2.73 23.69 1.84 3.29 0.69 3.72 1.87 29.63 1.24
2.0 0.8
32.9 85.3
55.8 12.7
0.8 0.0
8.8 1.3
0.0 0.0
1.1 1.5 0.3 1.4 0.3 0.3 0.6
86.8 76.7 24.4 41.6 65.8 57.9 63.2
12.2 21.7 65.0 50.4 34.0 31.2 21.9
0.0 0.2 0.9 1.1 0.0 1.4 0.1
0.0 0.0 9.0 5.3 0.0 9.4 13.9
0.0 0.0 0.4 0.4 0.0 0.0 0.3
9.10 4.37 7.00 7.79 3.67 2.19 26.59 1.51 22.51 3.53 5.36 5.05 9.73 0.74 18.06 3.08 2.15 4.65 7.39
Agriculture and Wetland 1.474.4 AW 345
4.9
1.0
18.1
0.1
0.98
0.0 0.0 1.7
0.0 0.0 1.6
0.0 0.0 1.4
0.1 0.0 0.6
1.04 42.70 2.89 8.05 1.48 7.85
4.42
Urban Kinnickinnic River 42"59'58, 87"55'13" Lincoln Creek 43'05'51", 87'5819" Milwaukee River at Estebrook 43"0600", 87'54'32''
UR1 UR2 UR3
Black River at Hemlock Chippewa River at Durand Fox River at Wrightstown Mississippi River at Trenton Wolf River at Shiocton Wisconsin River at Plover
IN1 IN2 IN3 IN4 IN5 IN6
a
44"49'32", 44"37'40", 44"1932", 44"3559", 44"2916", 44"27'53",
90O3642" 91"58'10" 88'09'52'' 92'33'51'' 88"3648" 89'35'04''
57 25 1,803 Integrator 1,339 23,340 15,669 120,952 3,170 13,455
93.0 97.6 40.1
7.0 2.5 54.7
3.17 3.92 7.12 6.20 2.40 4.58
6.30 5.59 4.49 6.48 5.92
Agricultural area predominantly cranberry bogs.
Our comparison among sites for discussion focuses on differencesin Hg cycling among GIs-determined land use/ land cover patterns. Following strict Hg clean sampling and analytical techniques, we show distinct differences in HgT and MeHg content and transport among these watershed types.
Materials and Methods Site Selection. Samplinglocations were chosen based upon
the characteristics of the physical watershed area upstream of a given sampling point. Thirty-three of the sites (Table 1) represent watersheds with RHUSof contrasting physical and geological characteristics in Wisconsin. The primary information used for site selection were land use and surficial deposits. Secondary consideration was given to bedrock type and water table depth in order to enhance 1868 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 7 , 1995
the specificity of site selections. A matrix approach was used to account for as many combinations of land use and surficial deposits as feasible. A Geographic Information System (GIS) was used in conjunction with ARClINFO software to quantify specific watershed characteristics. Data used for quantifymg watershed characteristics were contained in the GIS ARC/ INFO coverages (data layers). After the sampling site was plotted within the delineated watershed, the applicable ARClINFO coverages were overlaid on the watershed. The data coverages consist of a series of polygons, which represent the different land use types, etc. After the data coverages were overlain on the watershed, the coverage was made to fit the exact coordinates of the delineated watershed upstream of the sampling point. The data polygons, digitally output to a spreadsheet, were then
quantified to represent absolute areas and percentages of the total watershed area. Although our study enabled the comparison of five types of watershed characteristics (land use/land cover, bedrock type, surficialdeposits,water table depth, and Ecoregion),we limit this discussion to land use/ land cover comparisons. The land use/land cover information was derived from the US. GeologicalSurvey (USGS) data set at 1:250000 scale. Land surface features were interpreted by the USGS using NASA high-altitude photographs and National High-AltitudePhotography Program photographs at 1:60000 scale. Source photos were from 1971 to 1982. Most sites are contained in watersheds that have had minimal human development since 1982. Land use/land cover information was important in choosing 33 indicator sites, representative of specific watershed characteristics. Where GIs data was unavailable, we translated USGS topographic maps. In addition, six integrator sites were chosen on major rivers that collect drainage from a variety of indicator subwatersheds. These integrator sites are a subset of rivers sampled during the 1991-1992 study (14)andthusmaintainacontinuousdata set. Field Sampling. Stream Dischurge. Standard USGS flow measurement protocols were used (17). Wadable stream flows were determined with a pygmy or AA (Price) current meter. Stream discharges of larger rivers were taken from rating curves at USGS gaging stations. Hg Sampling. At each site, strict low-level trace metal clean techniques (8) were used during sample collection, preservation, and storage. By following these stringent protocols, previous work in Wisconsin has demonstrated that Hg levels in lake waters (0.5-2 ng/L Hg for unfiltered epilimnetic samples,9) were similar to those levels observed in remote ocean sites (IS). Mercury samples were collected using an all-Teflon tubing weight, Teflon FEP tubing, and a Geotech pumping system equipped with precleaned C-Flex pumphead tubing. The filtration scheme was modified between fall 1992 and spring 1993. In the fall, we retained particles on a 0.45pm Calyx filter capsule (particulate phase was calculated by difference from unfiltered and filtered samples). After sampling during this base flow period, we observed that at several sites, particulate HgT was 10% of unfiltered HgT. It was felt that in order to obtain better information on particle concentrations (ng g-’ HgT), we needed to directly analyze particles for Hg. Therefore, in the spring, we retained particles on a quartz fiber filter (0.7 pm nominal cutoff)housed in an all-Teflon filterholder (filteredfraction calculated by difference 19). We assume that the filtered fraction from each sampling period was comparable despite the slight particle size cutoff difference. This is especially true under high SPM levels in the spring, where quartz fiber filters probably retain particles at sizes 20%) and greater than 10% forested regions. Agricultural only sites contain >85% agricultural and ‘10% forest zones. One site, the Sheboygan Marsh is grouped separately because of its unusual land use composition of agricultural and wetland zones. It is also a calcareous wetland, unlike the others in the study (Figure 1). Urban sites contain ’30% urban areas and represent a high potential for point-source contamination. Our urban sites are limited to Milwaukee; therefore, our results for this grouping may not be a true indicator of urban sites statewide. Finally, integrator sites contain a mixture of the aforementioned subwatershed types. HgT concentrations exhibit strong seasonal responses within watershed groups and amongwatershed types (Table 3). During the base flow period of fall 1992, mean Hgl concentrations were highest at wetlandlforest sites and agricultural/forest sites, followed by agriculture only and urban sites. Integrator sites, which represent a mixture of many watershed types, exhibited mean HgT values of 3.45 ng L-I during the fall, similar to the wetland/forest and agriculture/forest sites. Under base flow conditions, mean percent filtered HgT levels were similar for agriculture only and wetland/forest sites, with slightly higher particulate phase association in agriculture/forest and urban sites. Mean HgT concentrations at all watershed types increased during high flow in the spring 1993. Unlike during the fall, mean HgT levels at urban sites (19.5 ng L-l) were the highest of any grouping, yet if one were to consider the Kinnickinnic value as an outlier, mean HgT levels would be more similar to wetlandlforest and agricultural/forest sites. In other watershed groupings, mean HgT concentrations increased approximately 2-fold and were similar to the integrator site means. Mercury concentrations can be combinedwith discharge measurements and watershed surface area to establish a
92'
910
90"
89'
88"
92'
91'
900
890
88"
FIGURE 1. Site location map. See Table 1 for symbol description.
watershed yield, WY (mg km+ d-9 by
where [HgJ is the measured concentration of a Hg species (HgT,MeHg) at time i, Vis the volumetric flow (m3d-9 at time i, and A is the watershed area (km2). Yields were calculated on a per day basis since sites were only sampled twice during the annual cycle. Yields calculated in Table 4 may be biased because they are based on two-point sampling. Comparisons are made to other studies with detailed sampling over an annual cycle. However, our sampling dates most likelyrepresent extremes in flow. One might expect that mean annual yields from our data to be between the values for spring and fall. In general, the yield rates calculated for HgT in this study are of similar magnitude to those of other studies. Differences among watershed groupings become more apparent by comparing yields rather than concentration alone (Table 4). During base flow periods in fall 1992,mean HgT yields were lowest in agriculturaland urban areas (0.14 and 0.45 mg km-2d-l, respectively);however, during high
flow (spring 19931,urban sites, as expected, contributed a significantly greater yield (53.9mg km-2 d-l) than other site groupings. However, the urban mean value is highly influenced by the high at the Kinnickinnic River. If removed, yields are quite similar to wetland/forest sites. Among nonurban groupings,wetlandlforest sites exhibited the next highest yield (28.1 mg km-* d-9 during high flow, followed by integrator sites (15.1 mg km-2 d-9 and agriculturallforest sites (14.7 mg km+ d-9. A detailed assessment of yields at every site was beyond the scope of this study. Our subsequent work has focused on characterizing detailed annual yields at seven of these sites. The 7-fold increase in HgT yield from wetland sites was not accompaniedby a significant increase in the percentage associated with particles. This observation reveals an important contrast with HgT transport in agricultural sites, which showed increased particle association during the spring. During high flow periods, pore waters in wetlands are likely displaced, transporting DOC-bound HgT downstream. In agricultural zones, by contrast, HgT is likely complexed with particulate organic matter in soils (15). During baseflow, HgT in rivers was low, reflecting both a retention by soils and relatively little transport through VOL. 29, NO. 7, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1871
TABLE 3
H ~ levels T and Percent as Filtered for Individual Watershed Types YO as filtered
HaT concn
watershed Qpe
range
mean
SD
median
range
mean
SD
median
Wetland and Forest fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
0.93-8.41 0.28-7.36
3.88 3.16
2.61 2.25
4.23 3.71
36-99
75
19
78
1.70-21.0 0.77-15.8
7.61 5.95
5.31 4.10
6.72 5.84
34-92
75
16
78
19-99
64
32
73
59
28
64
Agriculture and Forest fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
0.74-9.73 0.49-3.64
3.92 2.09
2.92 1.17
3.53 1.98
3.08- 18.1 0.90-6.06
7.51 3.56
4.67 1.86
6.18 3.18
5.1-84
Agriculture Only fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
0.69->2.73* 0.54-2.73 3.29-6.69 2.33-2.58
22.73 1.58
0.66 0.78
1.84 1.51
60-99’
78
16
77
4.68 2.52
1.53 0.24
4.36 2.44
38-71
58
19
59
1.48 0.81
30-76
54
23
55
8.05 5.56
35-71
54
18
57
60
33
68
60
34
71
Agriculture and Wetland fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
0.98 0.87 4.42 4.27
Urban fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
1.04-2.89 0.79-0.88 7.85-42.7 2.80-24.2
1.80 0.83 19.5 10.9
0.97 0.05 20.1 11.6
Integrator fall 1992 unfiltered HgT filtered HgT spring 1993 unfiltered HgT filtered HgT
2.40-6.20 0.35-3.50
3.45 2.24
2.46 1.64
2.89 1.86
4.49-6.48 1.23-5.41
5.76 3.84
0.79 2.10
5.92 4.18
5.6-90.2 19-96
a Samples were lost during fall 1992 for particulate HgT on Rattlesnake Creek, Otter Creek, and Milwaukee River; therefore, mean unfiltered HgT was calculated based on partitioning of similar samples (78%) from the subgroup.
groundwater to the receiving stream. The increase in spring yield, accompanied by high SPM, reflects erosion of organic soils and transport of the particle-bound HgT downstream. Monomethylmercury in River Waters. Distinct differences in MeHg concentrations were apparent among the two land use groupings under study (Table 5). Mean and median MeHg concentrations for wetlandlforest sites were higher than those of agricultural/forest sites. We also calculated MeHg as a percent of HgT to compare this distribution with atmospheric deposition. Typically, atmospheric sources have been found to contain 1-2% or less of HgT as MeHg (12, 36). Our mean values in Table 5 significantly exceed these percentages, possibly suggesting methylation at these sites. Methylation appears to be more important at wetland/forest sites than in agricultural/forest watersheds. Methylmercury concentrations were higher in the fall than in the spring in wetland/forest watersheds (unlike Hg1-1.Elevated fall levels may be due to increased microbial 1872
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 7 , 1 9 9 5
activity and methylation during warmer periods preceding sampling. Ramlal et al. (37) observed this response for methylation rates in sediment cores from lakes in Canada. If sulfate-reducing bacteria are important in the transformation of inorganic Hg to MeHg (38,391, our results suggest the presence of these microorganisms in both anoxic soils and wetland pore waters. Mean percent MeHg levels even exceed atmospheric deposition distributionsin agricultural/ forest zones, suggestingeither methylation in soil horizons, groundwater,or streambeds or perhaps that nonmethylated species of Hg are preferentially retained in soil matrices. Methylmercury yields (WYM~H~) calculated for the two sampling periods during this study compare well with those of other studies (Table 5). Our mean values for wetland/ forest sites, which contain a mix of wetland and upland zones (0.15 and 0.51 mg km-2 d-l, spring and fall, respectively), compare more closely with St. Louis et al.’s (27) wetland site values than their uplundlwetland values. However, our study compared a number of different
TABLE 4
Watersked Yields of H@ from Rivers and Streams and Comparisons to Similar Studies yield (mg k w 2 d-') watershed type
unfiltered Hm range
fall spring springs
0.13-9.44 0.91-168.0 0.91-32.4
fall spring
0.25-7.71 3.99-19.3
fall spring
0.09-0.18 3.83-19.9
fall spring wetland, N. Wisconsin (40) forested Swedish watersheds 29 5 43 exp lakes area, Canada (27) upland uplandhetland headwaterlwetland lake
filtered H ~ range T
filtered H ~ mean T
96 as filtered
0.06-6.66 0.52-125.9 0.52-26.5
1.60 21.8 12.3
79 78 80
1.oo 8.46
41 57
0.1 1 4.32
78 49
0.082 4.25
88 97
Wetland and Forest ( n = 12) 2.01 28.1 15.4
Agriculture and Forest ( n = 9) 2.43 0.18-2.42 14.7 0.99-18.3 Agricultural Only ( n = 7) 0.14 8.85
0.09-0.14 2.97-7.48
Agriculture and Wetland (n = 1) 0.093 4.39
fall spring fall spring spring6
unfiltered H ~ maen T
0.23-0.74 25.4-110.2 25.4-26.1 0.74-2.69 5.03-31.4
Urban ( n = 3) 0.45 53.9 25.8
0.17-0.23 8.84-62.4 8.84- 18.5
0.20 29.9 13.7
44 55 53
Integrator ( n = 6) 2.00 15.1
0.10-1.85 1.41-26.9
1.14 10.9
57 72
Other Studies 1.2 1.6-13.9 2.19- 16.2 6.30-9.59 5.56 5.28-6.25 3.34 0.84
Excludes Sand River. Excludes Kinnickinnick River.
wetland and forest types, and the range of yields among sites is quite large. For instance, the lowest WYbIeHg for wetlandlforest sites was 0.010 mg km-2 d-l at the Upper Eau Claire River (spring 1993). Although this site contained about 90% forest, nearly 8% of the surface area of the watershed is a lake (Table 11, to which a large fraction of flow can be attributed. Methylmercury yields calculated from basins which contain lakes are low (27).By contrast, the greatest MeHgyield (1.24mgkm-' d-l) fromtheMoose River (spring1993)was derived from awatershedwith nearly 45% of its area classified as wetland (Table 1). Our yields are lower than Krabbenhoft et al. (40) from a site which was predominantly wetland. The effects of wetlands on MeHg yields are shown in Figure 2. A positive correlation exists between percent wetland area in the watershed and MeHg yield. Seasonal regressions remain strong and unchanged despite a near uniform increase in absolute yield between fall and spring (6 = 0.58 and 0.54 for fall and spring, respectively). Additional sampling at greater frequencies would enable calculations of annual yields and perhaps an even stronger relationship would result. Watershed Transport Flftlciency for Mercury. Although our survey represented only two sampling points during a water year, the data produced by this study can be used to compare relative inputs and outputs on a seasonal basis. If one assumes that atmospheric deposition of Hg is the major external transport vector to the watersheds and that flow past our sampling site is the sole output from the
watershed, then an watershed transport efficiency can be calculated as (3) where is the watershed transport efficiency (unitless),WY(w)Hg(~) is the watershed yield determined from eq 2, and AD(w)Hg(I) is the atmospheric deposition rate (mg d-l) of a given Hg speciesfor a given watershed. Several assumptions are made when using the W E comparison. Values for AD for HgT and MeHg were obtained from a study sitelocated in north-centralWisconsin (36).Seasonal differences in atmospheric deposition are not considered. We also do not account for dry deposition or uptake of HgO by plant material in watersheds. We assume that this deposition rate is transferable statewide,mainly due to lack of data from other parts of the state. We do not account forvolatilization of Hgwithin a watershed, and we recognize that it may be important, especially in watersheds where lakes and reservoirs constitute a significant portion of the watershed. For WTE calculations,we did not include urban or integrator sites in the comparison since the potential for substantial point sources exist in these watersheds. Table 6 summarizes mean W E values for wetland and forest, agricultural and forest, and agricultural only watersheds. Major differences in WTEs exist between spring and fall sampling and among watershed classifications.In wetland and forest areas, WTEs suggest high retention of HgT during low flow in fall and nearly complete transport VOL. 29, NO. 7, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
1873
TABLE 5
TABLE 6
MeHg Concentrations, Percent of H ~ as T MeHg and Watershed MeHg Yields
Watershed Transfer Efficiencies for Hgl and MeHf
watershed type
range
mean
SO
watershed type
med
Wetland and Forest fall MeHg concn ( n g L-’) % of HgT as MeHg yield ( m g km-2 d - l ) spring MeHg concn (ng L-’) % of HgT as MeHg yield ( m g km-2 d - l l
0.020-0.870 1.6-1 1.1 0.003-0.778
0.291 0.284 0.200 6.4 3.2 5.8 0.148 0.217 0.050
0.015-0.390 0.4-6.7 0.010-1.242
0.194 0.130 0.164 3.2 1.9 3.1 0.507 0.426 0.388
Agricultural and Forest fall MeHg concn (ng L--’) 50.016-0.235 50.089 0.090 % of HgT as MeHg 50.2-9.3 53.0 3.1 yield ( m g km-2 d-’1 50.005-0.150 50.043 0.052 spring MeHg concn (ng L-’) 0.036-0.200 0.095 0.056 0.5 -6.5 2.1 1.9 % of HgT as MeHg yield ( m g km-2 d-’) 0.041-0.487 0.150 0.140 Agricultural Only fall MeHg concn (ng L-’) 50.016-0.069 50.030 0.021 Yo Of HgT as MeHg 50.5-4.5 12.2 1.7 yield ( m g km-2 d - l ) 50.003-0.009 50.005 0.002 spring MeHg concn (ng L-’) 0.025-0.158 0.085 0.048 % of HgT as MeHg 0.9-3.7 2.3 1.3 yield ( m g k m - 2 d - l ) 0.033-0.202 0.125 0.060
0.035 1.6 0.014 0.081 1.4 0.097
0.027 1.8 0.005 0.074 2.4 0.130
MeHg Yields from Other Studies N. Wisconsin wetland (40) 0.7-1.4 ELA, wetland (27) 0.263 ELA, upiwetland (27) 0.088-0.096 ELA, upland (27) 0.019 Swedish forest 0.38-0.52 catchment (43) 1.0
I
-
o
0.8
Fall
U
-I Iji
5
0.6
P
E 0.4 I”
,
Spring
+
G /
r’
02 I
0
0.0 0
10
20
30
40
50
Percent Wetland Area in Watershed
FIGURE 2. Methyl mercury yield vs percent wetland area for wetland/ forest sites.
during spring (almost 90%). Similarly in agricultural and agricultural/forestwatersheds, WTEs are greater during high flow in spring than in fall. Overall, the efficiencies in agriculturally influenced watersheds were lower than in wetland/forest watersheds. Methyl mercury WTEs are consistently greater than HgT values for each watershed type. W E values greater than 1for MeHg may suggest formation of methylmercurywithin a watershed. In agricultural/forest systems, values for spring and fall remain below 1, although in spring more efficient transfer is observed. In wetland sites, the fall WTE value is similar to that of the spring in agricultural/forest 1874
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 7, 1995
wetland and forest fall spring springb agriculture and forest fall spring agricultural only fall spring
WEtigr
range
WEngr WEM~H~ WEM~H~
mean
range
mean
0.01-0.30 0.029-5.38 0.029-1.06
0.06 0.90 0.49
0.07-3.23 0.04-5.15 0.04-5.15
0.61 2.11 2.04
0.03-0.25 0.13-0.62
0.08 0.31
0.02-0.62 0.13-2.02
0.18 0.62
0.003-0.006 0.12-0.64
0.005 0.01-0.04 0.29 0.14-0.84
0.02 0.52
”Atmospheric deposition rate for Hg, (11.4 ug m - 2y-’ = 31.2 mg k r r 2 d-’1 and MeHg (0.088pg m2yr-’ = 0.241 mg km-* d-’1 from ref 36. Excludes Sand River.
value. However, in spring, the meanWTEMvieHs is 2.11,which suggests wetlands as a significant source of MeHg. Spring values for individual sites with significant wetland components, such as the Moose River (5.2) and the Upper TamarackRiver(3.71,suggest substantial methylation within their watersheds.
Conclusions The Hg content of rivers is controlled by a number of complex reactions and transport processes within a watershed. In the absence of direct geological sources, our results show that once deposited on terrestrial systems, atmospherically derived Hg is processed via differing mechanisms among sites and between seasons. Our interpretations were strengthened by the use of a GIS to delineate individual watershed subunits and by trace metal clean sampling techniques for low-level Hg analyses. The use of a GIS provided a powerful tool for comparison among distinct watershed types from both a water management and a water research perspective. Correlation of Hg levels and watershed characteristics upstream of a sampling location gives water resource regulators a valuable tool for managing resources with respect to mercury contamination. In Wisconsin, many streams and lakes that receive no significant point sources ofmercurycontainfishwithHglevels exceedinganadvisory threshold. Based on the results of this study, we have found that characteristics such as land use and land cover can influence the mobility of diffuse nonpoint sources of mercury to “unimpacted” water bodies. Furthermore, delineation of watersheds by a GIS may provide a predictive tool for regulating discharge of Hg into rivers in watersheds where “clean” Hg data are unavailable. Our results also support a hypothesis regarding global Hg cycling and the effects on terrestrial systems. In a conceptual model of global Hg cycling, Mason et al. (41) estimated that of the 1000 Mmol of anthropogenically emitted Hg over the past century, 95% has accumulated in surface soils. The authors predict that even if emissions reductions occurred, leaching of Hg from terrestrial systems would persist. Our data support this hypothesis for nonwetland watersheds in our study, since increased Hg1 concentrations and loadings appeared to be associated with particle-bound Hg. However, in wetland areas, our W E calculations (eq 3 ) suggest shorter residence times and perhaps a more rapid processing of anthropogenically derived Hg. However, it must be noted that in the watersheds studied, the largest percentage of wetland
surface area was about 48%, with the remaining 52% comprisedof upland forest soils. Leaching of upland soils and subsurface flow to wetlands could continually supply Hg for subsequent transport or methylation. The release of Hg from wetland and forest regions is a particularly important considerationfor biotic uptake and bioaccumulation in higher trophic levels-especially fish (42). Our results suggest that Hg is efficiently transferred throughwetland/forestsites in a more reactive form (filtered HgT or MeHg) relative to other land use patterns. Therefore, biotic organisms inhabiting these rivers or downstream impoundmentsand drainage lakes are in contactwith these more labile forms of Hg. Similarly,lakes with hydrologically connected riparian wetland zones may be affected by efficient Hg transfer and methylation. Our results concur with St. Louis et al. (27), suggesting that, in addition to in-lake methylation of Hg, wetland production of MeHg may be an important componentof mass-balancemodels.
Acknowledgments We thank J. Wegner, G. Quinn, M. Walker, and J. Overdier for assistance in field sampling and laboratory analyses. Flow data at a number of sites was obtained by the U.S. Geological Survey, Wisconsin District. We thank three anonymous reviewers for their helpful comments on this manuscript. This research was funded by the Wisconsin Department of Natural Resources, Division of Resource Management and Di-rision of Environmental Quality. Author-SuppliedRegistry Numbers: Hg, 7439-97-6;MeHg, 16056-34-1.
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Received for review December 1, 1994. Revised manuscript received March 20, 1995. Accepted March 29, 1995." E89407352 @Abstractpublished in Advance ACS Abstracts, May 15, 1995.
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