Predicting the Risk of Mercury Exposure Related to Freshwater Fish Consumption:
A Model for Qualitative Risk Assessment of Environmental Threats to Public Health

Alissa Barron
Bachelor's of Arts in Environmental Studies
May 2001

Mercury is the most toxic heavy metal. It is a serious neurological, developmental, and nephrotoxin at high doses, and exposure to relatively low levels of mercury can cause permanent damage to the kidneys and central nervous system. Once it has found its way into our waters, mercury concentrates through aquatic food chains, most dramatically in fish tissue at levels up to 1,000,000 times the concentrations detected in the water column. The consumption of contaminated freshwater fish represents a significant threat to public health, and fully 68% of fish consumption advisories are due to mercury contamination. The Northeast region of the United States has some of the highest levels of mercury pollution in the country. Rhode Island, however, is the only state in the region without statewide monitoring of fish tissue contaminants. Accordingly, little data exists documenting the levels of mercury deposition in the state's freshwaters, and there is an even greater paucity of data on fish consumption. Consequently, the Rhode Island Department of Health (HEALTH) and Department of Environmental Management (DEM) have identified a need for criteria to help prioritize sites for initial testing of mercury and other contaminant levels in freshwater fish tissue. This study aims to provide HEALTH and DEM with such a set of criteria for determining potentially high risk monitoring sites for mercury. The Pawtuxet watershed was chosen as a pilot site for the study.

Using a ranking system of six different indicators, I have addressed whether it is possible to use available data to predict the risk of mercury exposure related to freshwater fish consumption in Rhode Island. These indicators were defined as

• The distribution and abundance of selected freshwater fish species throughout the state's waters;
  
• The distribution of known fishing activity;
  
• The location of public boat launch ramps, docks, and bridges as access points to potential fishing sites;
  
• Existing fish-tissue data on mercury concentrations in the selected species in select bodies of water;
  
• The location of near-field, traditional sources of atmospheric deposition of mercury; and
  
• The pH of individual waterbodies as an indicator of water quality and the potential for the methylation of mercury.
  

My findings indicate that there is a risk of mercury exposure when eating fish in the Pawtuxet watershed. This risk is not distributed evenly throughout the watershed, however, and my analysis conflicts with the common belief that the rural northwestern lakes and streams should be the target of future surveillance activities. This work indicates that the areas of the south and eastern Pawtuxet may have greater public health significance and should be a primary focus of future surveillance efforts in this watershed. Fish populations, access to fishing spots, and waters we know to be fished are all concentrated along the southeastern edge of the Pawtuxet, which is also the most densely populated area of the watershed. Larger bodies of water also presented a higher risk overall, and lakes and reservoirs a higher risk than streams or most rivers.

Based on the above findings, the study culminates in recommendations for high-risk mercury sites that should be considered priorities for a fish-tissue monitoring program and for data collection on fish consumption patterns and fishing locations. Finally, the study itself represents a model of qualitative risk-based assessment procedures that can be used to maximize public resources when significant empirical data on contaminant risk is unavailable.