Mercury in MN North Shore streams

Statewide, mercury is the major pollutant in about two-thirds of Minnesota 's impaired waters. A number of streams along Minnesota 's North Shore have been listed as impaired by turbidity. These streams are the Brule, Poplar, Beaver, Knife, Lester, and St Louis Rivers (Map of MPCA 2008 draft list of bioaccumulative toxicant impairments PDF).

We don't know as much about mercury in streams as we do about how it behaves in lakes. The USGS has initiated a number of studies aimed at determining how mercury cycles within stream watersheds and food webs. See the USGS for more. Once it is deposited onto a watershed, mercury cycling and bioaccumulation rates within that watershed are controlled by a number of factors including hydrology, water quality, structure of the aquatic food web and temperature. Soils form a major pool for mercury that is closely bound to organic matter in the soils. Although the processes are not yet fully understood scientists think that the release of mercury from this pool is controlled by the decomposition of the soil organic matter and erosion. There is evidence that “old mercury” is processed differently than newly deposited mercury though again, the processes are not fully understood.

Land cover and land use effect the movement of particulate bound mercury in the soils to the sites of methylation which include low oxygenated zones in wetlands and aquatic sediments. Activities that cause erosion such as road building, construction, and some forestry practices can facilitate the movement of this soil bound mercury pool into lakes, wetlands, and streams where the potential for converting this inorganic form into the toxic organic form exists.

A number of studies, some in Minnesota have found that total mercury concentrations in streams are closely related to the concentration of suspended solids. The same is true for streams here in along the MN North Shore. The MPCA ( Anderson 2003) found elevated levels of total mercury in the Poplar River during runoff events when sediment concentrations were high. Samples collected during 2004 through 2006 by WLSSD and NRRI from Kingsbury, Amity, Chester and Tischer Creeks show similar results (Figure 1).

Figure 1. This graph shows the close relationship between total suspended solids
and total mercury concentrations in four Duluth streams.

The MPCA standard for total mercury in the Lake Superior Basin is 1.3 ng/L. They have determined that amounts higher than this will result in the bioaccumulation of mercury in fish tissue that will affect the health of those that consume these fish. Concentrations of total mercury in the four Duluth streams during three different flow regimes are shown in Table 1.

Table 1. Range of total mercury concentrations in the four Duluth streams from 2004 to 2006; n = number of samples (Ruzycki et al 2011)

Flow Regime

Total Mercury (ng/L)
snow melt

2.1 to 15.7

(n = 14)
rain events

1.3 to 28.4

(n = 16)
base flow

1.3 to 3.8

(n = 8)

Methylmercury concentrations in the Duluth streams ranged from 0.06 to 0.30 ng/l.

 

How much Hg gets into the lake?

Analyzing stream water for mercury is expensive and annual flow records are needed to determine how much mercury is carried by a stream annually. However, because THg is so closely related to TSS we are able to estimate mercury concentrations using the Duluth Streams continuous turbidity data. We also have annual flow records for each of the streams. Our estimates show that close to half of the annual mercury load to Lake Superior occurs during snowmelt. Preliminary annual load estimates from 2006 range from 8 to 42 grams of mercury were discharged into Lake Superior from Chester, Tischer, and Kingsbury Creeks. We don't have estimates from Amity Creek due to the loss of the sensor during spring 2006. Close to half of this mercury load is delivered by the spring snow melt.

For more information about mercury in the Western Lake Superior watershed:

References

Ruzycki, E.M., R.P. Axler, J.R. Henneck, N.R. Will, and G.E. Host. 2011. Estimating mercury concentrations and loads from four western Lake Superior watersheds using continuous in-stream turbidity monitoring. Aquatic Ecosystems Health Mgmt Society, in press.