Water samples from the Grand Calumet River/Indiana Harbor Canal and Lake Michigan in Lake County, Indiana, were collected and analyzed for mercury. Sampling was done with ultra-clean protocols, and mercury was analyzed by low-level methods during seasons of contrasting weather and streamflow conditions in August 2001 and May 2002.

Total mercury concentrations in all the Grand Calumet River/Indiana Harbor Canal samples exceeded the 1.3 nanogram per liter Indiana water-quality standard for waters within the Great Lakes system. Total mercury concentrations in the Lake Michigan samples did not exceed the Indiana water-quality standard. Total mercury and methylmercury concentrations were larger in more samples collected during the wet-weather streamflow conditions in May 2002 than in samples collected during the dry-weather streamflow conditions in August 2001. The largest total mercury concentrations were in samples collected from the West Branch Grand Calumet River near wetlands and municipal-effluent outfalls (17.2 nanograms per liter) and in samples collected from the Indiana Harbor Canal near the confluence of the East Branch and West Branch Grand Calumet River (16.0 nanograms per liter).

Particulate total mercury was the predominant form of total mercury detected in samples from the Grand Calumet River/Indiana Harbor Canal. Methylmercury concentrations were no more than 1.5 percent of the total mercury concentrations in August 2001 and no more than 6.2 percent in May 2002. Nearly all methylmercury was particulate and was correlated to concentrations of dissolved solids, total organic carbon, and sulfate. The estimated composition of most of the suspended solids in the water samples from the Grand Calumet River/ Indiana Harbor Canal was sediment larger than medium clay containing minimal organic carbon and plant matter. Total mercury loads in the Indiana Harbor Canal during the time of water sampling were as large as 703 milligrams per hour in August 2001 and 542 milligrams per hour in May 2002. As much as 21 percent of the instantaneous mercury load in some stream reaches could have come from ground-water discharge.

Data from this study have implications for a Total Maximum Daily Load (TMDL) for mercury in the Grand Calumet River/Indiana Harbor Canal. Comparisons of data from this study with historical data do not show substantial changes in the distribution of mercury in the study area from 1994 through 2002. Treated municipal effluent had larger mercury concentrations than industrial effluent and presents a potential for larger mercury loads that could be controlled to achieve a TMDL, based on concentration. Mercury in ground-water discharge may be difficult to control to achieve a TMDL because of its diffuse and widespread distribution.