Environ. Sci. Technol. 1998, 32, 456-462
Transport of Mercury in Three Contrasting River Basins STEVEN J. BALOGH,* MICHAEL L. MEYER, AND D. KENT JOHNSON Metropolitan Council Environmental Services, 2400 Childs Road, St. Paul, Minnesota 55106-6724
Total mercury (THg) concentrations and loadings in the Minnesota, Mississippi, and St. Croix Rivers were determined over a 2-year period. These rivers drain watersheds with greatly contrasting land use, soil, and hydrological characteristics. The Minnesota River basin is characterized by a vast artificial drainage network and fertile prairie soils that support the intensive cultivation of row crops. Mercury levels in this river are strongly correlated with total suspended solids concentrations, varying widely in response to precipitation and snowmelt runoff events. The St. Croix River and the headwater Mississippi River (above its confluence with the Minnesota River) drain watersheds characterized by more acidic and sandy soils, more forest and wetland areas, less artificial drainage, and less cultivation as compared to the Minnesota River watershed. Mercury concentrations and loadings in these rivers are much lower than those observed in the Minnesota River, vary over a much narrower range, and reflect a greater dissolved mercury component. These results illustrate the significant influence of watershed characteristics on Hg mobility.
Introduction Anthropogenic mercury (Hg) emissions to the atmosphere are dispersed regionally and globally, leading to the contamination of ambient surface waters (1, 2). Mercury is delivered through atmospheric deposition and hydrologic transport to even remote rivers and lakes. Fish consumption advisories based on Hg contamination of rivers and lakes have become common in parts of North America and Scandinavia where elevated Hg levels in fish have been measured. The transport of Hg from a watershed to the basin outlet reflects the collective influence and interaction of the various geological, climatological, hydrological, soil, and land use/ land cover characteristics of the watershed (3). Hurley (4) investigated the influences of land use/land cover characteristics on total mercury (THg) and methylmercury concentrations and basin yields in 39 rivers in Wisconsin. They concluded that atmospherically deposited Hg undergoes siteand season-specific processing as it is transported from the watershed. In particular, Hg transported from wetland watersheds was thought to be associated primarily with dissolved organic carbon (DOC), while soil erosion and sediment transport were thought to dominate Hg movement in agricultural basins. Agricultural land use practices can have a significant influence on the delivery of sediment and * Corresponding author telephone: (612)602-8367; fax: (612)6028215; e-mail:
[email protected]. 456
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sediment-associated pollutants from watersheds (5-7). Recently, we reported that Hg inputs to the Minnesota River are controlled primarily by suspended sediment input processes (8). Suspended sediment is derived from both upland soil and bank erosion processes. These processes have been magnified by the imposition of artificial drainage networks and extensive cultivation in the basin. This paper expands upon our previous work by comparing 2 years of THg and suspended sediment loading and basin yield data from the primarily agricultural Minnesota River basin to similar data for the Mississippi River above its confluence with the Minnesota River and the St. Croix River. These rivers drain watersheds with contrasting land use, soil, and hydrological characteristics. While diffuse sources dominate THg and suspended sediment loadings in all three rivers (9), delivery processes differ among the watersheds. Contrasting THg and suspended sediment loadings and basin yields in these rivers are interpreted in terms of their different watershed characteristics. Watershed Characteristics. The Minnesota and St. Croix Rivers are the two largest tributaries to the Mississippi River in Minnesota (Figure 1). The Minnesota River drains 44 000 km2 of largely agricultural land in the southwest portion of the state, and the St. Croix River basin comprises 20 000 km2 of mostly forested land in eastern Minnesota and northwestern Wisconsin. The Mississippi River above its confluence with the Minnesota River (hereafter referred to as the headwater Mississippi River) drains 51 500 km2 in the central portion of the state. The climate in the three-basin study area is subhumid continental (10) with dry, cold winters and warm, moist summers. Mean annual precipitation increases from west to east across the study area, from 56 to 81 cm. Mean annual runoff varies from less than 5 cm in the headwaters of the Minnesota River in the west to greater than 36 cm in the upper St. Croix River in the east. Mean annual evaporation varies across the study area from more than 102 cm in the southwest to less than 71 cm in the northeast. Fertile calcareous mollisols are the dominant soil order in the Minnesota River basin (10). These fine-grained soils are highly productive and extensively cultivated. Soils in the St. Croix River basin are primarily alfisols, with regions of spodosols and inceptisols in the upper portion of the watershed. The headwater Mississippi River basin is comprised mostly of alfisols, with mollisols in the southwest and areas of entisols throughout the rest of the basin. With the exception of mollisols in the southwest portion of the headwater Mississippi River basin, soils in the St. Croix and headwater Mississippi River basins are more sandy and less fertile than those in the Minnesota River basin, and cultivation is therefore limited. Agriculture is the dominant land use in the Minnesota River basin (Table 1), where much of the land has been artificially drained to support row-crop farming. Forests and wetlands comprise significant portions of the headwater Mississippi and St. Croix River basins, and natural drainage patterns prevail. The row-cropping agriculture found throughout the Minnesota River basin and parts of the headwater Mississippi River watershed is not widespread in the St. Croix River basin.
Methods Sampling Sites. Samples were collected biweekly over a 2-year period at basin outlet sites (Figure 1). Lock and Dam 1 (LD1; river kilometer UM1364), Fort Snelling (kilometer MI5.6), and Prescott (kilometer SC0.5) are the basin outlets S0013-936X(97)00506-3 CCC: $15.00
1998 American Chemical Society Published on Web 01/15/1998
FIGURE 1. Sampling locations on the Mississippi, Minnesota, and St. Croix Rivers. Sites mentioned in the text are shown with stars (f); basin outlet sites are underlined.
TABLE 1. Land Use Characteristics of Three Watersheds, % of Basin Areaa land use
Minnesota River
headwater Mississippi River
St. Croix River
urban agriculture rangeland forest open water wetlands barren/other total area (km2)
1.8 92 0.9 1.8 2.3 1.3 0.2 44 000
2.6 44 0.0 33 7.7 12 0.7 51 500
0.9 37 0.0 47 3.1 12 0.2 20 000
a Data compiled from U.S. Geological Survey information (11). This information was modified from the USGS Geographic Information Retrieval and Analysis System (GIRAS) database, which was last updated in the mid-1970s. While changes in land use have occurred since these data were collected, we believe the significance of those changes for the analysis presented here to be small.
of the headwater Mississippi, Minnesota, and St. Croix Rivers, respectively. In addition to biweekly sampling, hydrologic event-based sampling at the LD1 and Fort Snelling sites was carried out to characterize the impact of snowmelt and precipitation events on THg loadings in the headwater Mississippi and Minnesota Rivers, respectively. Sampling at LD1 was carried out above the dam, at the influent to the Ford Motor Company hydroelectric building. The Prescott site was sampled at midstream from a bridge, and the Fort Snelling site was sampled from a pier, approximately 15 m from the shore (channel width, 80 m). All sites are considered to be relatively well-mixed and are assumed to provide representative estimates of in-stream loadings. Ongley (12) has shown that single midstream, nearsurface grab samples can be adequately representative of
vertical mean silt+clay concentrations. We expect negligible vertical and cross-sectional variations in silt+clay concentrations. Sampling and Analytical Procedures. Sampling and analytical methods have been described in detail elsewhere (8). All Hg sampling and analysis procedures incorporated appropriately clean techniques. In sampling, an acid-leached 250-mL Teflon bottle attached to a weighted Teflon-coated cable was submerged in the river to a depth of approximately 1 m. A “clean hands/dirty hands” two-person sampling protocol was used (13); sampling staff wore vinyl cleanroom gloves in all sample handling procedures. The sample bottle was transported to and from the sampling site in double zip-close bags. Samples for suspended solids analysis were obtained by submerging a weighted water sampler to a depth VOL. 32, NO. 4, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. River discharge and THg and TSS concentrations at Fort Snelling (km MI5.6), Minnesota River. of approximately 1 m. All samples for Hg analysis were returned to the clean analytical laboratory within 6 h of sample collection. For samples being analyzed for THg, digestion reagents (HNO3, H2SO4, KMnO4, and K2S2O8) were added immediately, directly to the sample bottles in a Class 100 clean hood (8). Samples were then heated at 95 °C for 2 h. Excess permanganate was reduced with hydroxylamine hydrochloride prior to analysis. Selected samples were filtered through 0.4-µm polycarbonate screen membrane filters (Poretics, Livermore, CA) using an apparatus composed entirely of acid-leached Teflon (8). The filtrate was digested as described above and then analyzed for total dissolved Hg. Samples were analyzed for THg using the cold vapor atomic fluorescence technique with single gold-trap amalgamation (14). Analytical QA/QC results were reported in detail in ref 8. Total suspended solids (TSS) concentrations were determined by standard methods (15) using a 1.5-µm glass fiber filter. Discharge Data. River discharge at Fort Snelling was calculated based on stage monitoring data and the U.S. Geological Survey (USGS) rating curve at Jordan (kilometer MI63.4); a 5% increase and 1-day travel lag in discharge between Jordan and Fort Snelling was assumed, according 458
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to the USGS estimation method (16). Discharge on the St. Croix River at Prescott was calculated by adding the discharge of the Apple River to that of the St. Croix River at St. Croix Falls (kilometer SC84.0), multiplying the sum by 1.034 and introducing a 1-day travel lag (16). Discharge data at St. Croix Falls and for the Apple River were obtained from stage monitoring data and the USGS rating curves at these sites. River discharge at LD1 was obtained from U.S. Army Corps of Engineers monitoring records. Continuous discharge and turbidity data from the Jordan site were available via modem, allowing changing conditions on the Minnesota River to be monitored closely. Calculation of Annual Loadings and Basin Yields. Total Hg and TSS loadings at the three basin-outlet sites were calculated from concentration and discharge data. Annual river THg and TSS loadings were estimated by numerical integration of the load data over time, with a time increment of 1 day (8). Daily loads were calculated from measured concentrations and discharge data or estimated by linear interpolation on days when no concentration data were obtained. In calculating river sediment loads from noncontinuous data, the sampling frequency and the frequency and duration of discrete, transient high-loading events must be reconciled to avoid significant error (17, 18). Uncertainties in the load estimates for the Minnesota River were minimized
FIGURE 3. River discharge and THg and TSS concentrations at Lock and Dam 1 (km UM1364), headwater Mississippi River. by intensive sampling during most hydrologic events. By continuously monitoring the turbidity in the Minnesota River at Jordan (kilometer MI63.4, upstream of Fort Snelling), the sampling frequency at Fort Snelling could be increased when river conditions were changing rapidly. On this basis, we believe our load estimates for the Minnesota River to be accurate within (10%. Less frequent sampling on the headwater Mississippi and St. Croix Rivers resulted in higher uncertainties for calculated loads in these rivers. The sampling frequency at LD1 was similar to but slightly less than that at Fort Snelling, and sampling on the St. Croix River was almost exclusively biweekly, resulting in less complete coverage of changing conditions. We estimate our load calculations for the headwater Mississippi and St. Croix Rivers to be accurate within (15% and (20%, respectively.
Results and Discussion Mercury and Suspended Sediment Concentrations. Total Hg and TSS concentrations in the Minnesota (Figure 2), headwater Mississippi (Figure 3), and St. Croix (Figure 4) Rivers varied throughout 1995 and 1996 in response to climatological and hydrological conditions. From December through February of each year, THg and TSS concentrations at all sites were low, reflecting limited runoff from the frozen,
snow-covered watersheds. Total Hg and TSS concentrations increased at all sites in late February or early March as runoff from the spring snowmelt entered the rivers. Precipitation runoff events resulted in fluctuating THg and TSS concentrations at all sites throughout the spring, summer, and fall months. During the open-water season (March-November), THg and TSS concentrations in the Minnesota River (Figure 2) were consistently much higher than those in the headwater Mississippi (Figure 3) and St. Croix (Figure 4) Rivers. Measured THg concentrations in the headwater Mississippi River at LD1 did not exceed 10 ng/L, and those in the St. Croix River at Prescott never exceeded 4 ng/L. In contrast, THg concentrations in the Minnesota River at Fort Snelling routinely exceeded 10 ng/L and approached 80 ng/L during extreme runoff events. Elevated THg and TSS levels were observed in the headwater Mississippi River during the spring flood and after summer precipitation events, but the magnitudes of these fluctuations were substantially less than those observed in the Minnesota River. Total Hg concentrations in the St. Croix River showed spring and fall maxima and varied in response to summer precipitation events. Under baseline wintertime conditions (December-February), THg concentrations in the Minnesota River at Fort Snelling VOL. 32, NO. 4, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. River discharge and THg and TSS concentrations at Prescott (km SC0.5), St. Croix River. averaged 0.96 ng/L, slightly higher than those in the headwater Mississippi River at LD1 (0.63 ng/L) and the St. Croix River at Prescott (0.51 ng/L). In contrast to THg concentrations, dissolved Hg concentrations were highest in the St. Croix River and lowest in the Minnesota River. Dissolved Hg concentrations in the St. Croix River at Prescott and upstream at Stillwater (km SC37.5) averaged 1.9 ng/L (CV ) 0.25; n ) 5), representing over 62% of the THg concentration. Dissolved Hg concentrations in the Minnesota River averaged only 0.41 ng/L (CV ) 0.30; n ) 7) and represented less than 10% of the THg concentration. Dissolved Hg concentrations in the headwater Mississippi River averaged 0.79 ng/L (CV ) 0.36; n ) 8), making up 28% of the THg concentration. Regression analysis showed that THg was not significantly correlated with TSS on the St. Croix River at Prescott (r 2 ) 0.04; n ) 60). This contrasts again with the results from the Minnesota River at Fort Snelling, where a strong correlation was observed (r 2 ) 0.97; n ) 135; slope of the regression line ) 46 ng of Hg/g of SS). Total Hg and TSS were also significantly correlated on the headwater Mississippi River at LD1 (r 2 ) 0.70; n ) 114; slope of the regression line ) 100 ng of Hg/g of SS). These results indicate that the dissolved phase dominates Hg mobility in the St. Croix River. Dissolved organic carbon 460
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(DOC) may play a role in Hg transport. Several studies of Hg transport in runoff from forested catchments have established the association of Hg with DOC (19-21). Total Hg and DOC were strongly correlated in headwater streams on the Canadian shield (22), and Hurley (4) found a strong correlation between dissolved Hg and DOC in Wisconsin rivers during base flow in the fall, but a weaker relationship during high flows in the spring. Further work is necessary to determine whether DOC controls Hg movement in the St. Croix River. In contrast to the St. Croix River, Hg in the Minnesota River is primarily associated with the particulate phase, and THg concentrations appear to be controlled by suspended sediment inputs (8). Between these extremes, the headwater Mississippi River shows significant amounts of both particulate and dissolved Hg. The concentration of Hg on suspended sediments in the headwater Mississippi River is approximately twice that in the Minnesota River (100 ng of Hg/g vs 46 ng of Hg/g, determined from the slopes of the THg:TSS regression lines), but TSS concentrations are much lower, and a significant portion (28%) of the Hg is dissolved. The dissolved Hg component in the headwater Mississippi River has a greater influence on THg concentrations than it does in the Minnesota River but less than in the St. Croix River. The three river basins studied here thus span a range
TABLE 2. Annual Loadings and Basin Yields of THg and Suspended Sediment in Minnesota Rivers Minnesota River headwater Mississippi River St. Croix River
1995 1996 1995 1996 1995 1996
THg load (kg yr-1)
THg yield (g km-2 yr-1)
TSS load (109 kg yr-1)
TSS yield (103 kg km-2 yr-1)
61 42 26 24 8.6 10
1.4 0.97 0.51 0.46 0.43 0.52
1.3 0.90 0.20 0.16 0.027 0.025
29 21 3.8 3.1 1.4 1.2
from predominantly dissolved Hg transport on the St. Croix River to predominantly particulate Hg transport on the Minnesota River. River Loadings and Basin Yields. In both 1995 and 1996, estimated annual THg and TSS loadings and basin yields (loading/basin area) in the Minnesota River were much greater than those in the headwater Mississippi and St. Croix Rivers (Table 2). In 1995, the THg yield from the Minnesota River basin was 1.4 g km-2 yr-1, approximately three times greater than THg yields from the headwater Mississippi (0.51 g km-2 yr-1) and St. Croix (0.43 g km-2 yr-1) watersheds. In 1996, the THg yield from the Minnesota River basin was approximately twice that from each of the other two watersheds [0.97 vs 0.46 g km-2 yr-1 (headwater Mississippi) and 0.52 g km-2 yr-1 (St. Croix)]. Suspended sediment yields showed even greater differences between watersheds. The 1995 TSS yield from the Minnesota River basin was 29 000 kg km-2 yr-1, over 20 times that from the St. Croix River basin (1400 kg km-2 yr-1). A previous survey of Minnesota rivers and streams demonstrated similar differences in TSS concentrations and loadings between these watersheds, attributing the higher TSS yield of the Minnesota River basin to the erosion of fine-grained soils exposed by extensive cultivation (23). Precipitation was 5-15% above normal across the study area in 1995 (24). In 1996, precipitation was approximately 10% below normal in the headwater Mississippi basin and near-normal in the Minnesota watershed, resulting in less runoff and lower THg and TSS loadings and basin yields relative to the previous year (Table 2). Precipitation was approximately 5% above normal in the St. Croix River basin in both 1995 and 1996. Total Hg loading and yield for the St. Croix River basin were 20% higher in 1996 than in 1995, while TSS load and basin yield were slightly (10%) lower. Other studies of THg delivery from watersheds have focused on smaller, more homogeneous catchments, generally reporting higher yields than those observed here. In a study of mercury delivery from 13 small catchments throughout Sweden, THg yields varied from 0.7 to 6 g km-2 yr-1 (19). Total Hg yields from two small (
