Final Report: Social Impact Assessment of Human Exposure to Mercury Related to Land Use and Physicochemical Settings in the Alabama-Mobile River System

EPA Grant Number: R827168
Title: Social Impact Assessment of Human Exposure to Mercury Related to Land Use and Physicochemical Settings in the Alabama-Mobile River System
Investigators: Bonzongo, Jean-Claude J. , Warner, Kimberly A. , Lyons, W. Berry , Bryan, Hobson C. , Roden, Eric E. , Ward, Milton G. , Chaubey, Indrajeet
Institution: The University of Alabama at Birmingham , University of Florida , University of Arkansas - Little Rock , The Ohio State University
Current Institution: The University of Alabama at Birmingham , The Ohio State University , University of Arkansas - Little Rock , University of Florida
EPA Project Officer: Chung, Serena
Project Period: December 21, 1998 through December 20, 2001
Project Amount: $804,534
RFA: Water and Watersheds (1998) RFA Text |  Recipients Lists
Research Category: Water , Watersheds


In the past decade, mercury (Hg) concentrations above levels that could pose human health risks have been measured in predatory fish from many rivers and reservoirs in the southeastern region of the United States. The geologically diverse Mobile Alabama River Basin (MARB), the focus of this study, is the fourth largest in the United States in streamflow, and the largest drainage to the Gulf of Mexico east of the Mississippi. The MARB, particularly its Coastal Plain portion, was hypothesized to be vulnerable to Hg contamination in aquatic food chains, due to the coexistence of both natural and human imposed conditions, which favor the production/accumulation of methyl-Hg (MeHg). Our research addressed the following hypotheses: (1) nutrient loading from certain land-use activities and increased sedimentation above water impoundments create conditions favorable for MeHg production; (2) increased sulfate loading from mining operations results in increased MeHg production; (3) abundant wetlands within the MARB contribute to MeHg loads downstream and in fish; and (4) fish tissue MeHg levels are related to water levels and net rates of MeHg production in sediments.

Based on these hypotheses, the objectives of this research project were to: (1) determine levels and speciation of Hg in different compartments of various aquatic systems in the MARB; (2) investigate the linkage between land use types or the presence of wetlands and microbial processes associated with MeHg production; (3) use geographic information systems (GIS) to represent spatially arranged data and ultimately to predict Hg levels in fish; and (4) use a participatory approach to environmental decisionmaking to ameliorate conflict, and achieve an effective public understanding and support for Hg policy. As part of the latter objective, a social and economic impact-assessment of potential remedial alternatives and policy recommendations was conducted.

Summary/Accomplishments (Outputs/Outcomes):

Overview. The biophysical component of this research project addressed the first three objectives in two phases. Phase 1 involved a wide survey of Hg distributions in water, sediment, largemouth bass, and ancillary parameters at 52 sites within the MARB impacted by one or more of our hypothesized factors. GIS tools were employed to confirm land use and land cover (LULC) in major river subbasins and in smaller tributaries and catchments influenced by hypothesized impact factors and to select sampling sites. Most of the sites were in the Coastal Plain physiographic province, with a few sites in the Appalachian Plateau, where most current and past mining activities are located. Finally, Hg data from this study were used in combination with the National Atmospheric Deposition Program-Mercury Deposition Network data to develop a general mass balance for Hg in the MARB.

In Phase 2, we addressed our hypotheses concerning natural and human-imposed conditions favoring the production/accumulation of MeHg, by conducting both field-intensive investigations and a series of laboratory experiments. To constrain some of the variability associated with Hg species distributions in the entire MARB watershed observed during Phase 1, a field effort was focused on one recreational pool, with four sites representing different impact types. Laboratory experiments examined: (1) the microbial production of MeHg from ambient Hg in native sediments in the presence of water constituents simulating different land use impacts; and (2) the potential for microbial transformation of added Hg and MeHg under a variety of terminal electron accepting processes (TEAPs) relevant in MARB sediment.

Data from both phases were incorporated into a database (Microsoft Access) with query capabilities that could be linked and displayed spatially using ARC_GIS.

The fourth objective was accomplished by incorporating public involvement in all stages of the biophysical research. Representatives of key stakeholder groups were informed and involved throughout the project through early meetings on project design, intentions and hypotheses, dissemination of a newsletter on progress of the research, and meetings and other communication at the end of the project to apprise them of results and invite policy discussion and recommendations. The social assessment/public involvement framework enabled information exchange between key stakeholder groups to minimize polarization over issues, and translated potential policy alternatives into human outcomes that inform the policy decisionmaking process.

Total Hg in Water and Sediment. Total Hg (THg) concentrations in unfiltered water and sediment samples from the Phase 1 survey were relatively low, ranging from 0.2 to 3.8 ng/L (average: 1.1 ng/L, n = 52) and 0.1 to 103 ng/g (average: 32 ng/g, n = 52), respectively. Aqueous THg was positively associated with iron-rich suspended particles, while sediment THg was positively associated with fine iron-rich particles and organic matter, which accumulate in depositional areas. These low THg levels and their quasi-uniform distribution in the MARB suggest that weathering and atmospheric deposition are the primary discrete sources of Hg found in MARB waters.

MeHg in Water and Sediment. Aqueous MeHg concentrations ranged from less than below detection limit (BDL) to 1.5 ng/L; average: 0.3 ng/L (n = 52) with no strong associations with either suspended solids or dissolved organic matter. Sediment MeHg levels (range: < 0.01-3.1; average: 0.6 ng/g; n = 52) were a weak, but significant function of sediment THg, indicating that Hg loading is one important factor governing MeHg production. Sediment organic matter and water column nutrients (N, P, or their ratio), alone or in combination with other variables, explained between 44-89 percent of the variability in sediment MeHg levels in different major river basins or the watershed as a whole.

The highest sediment concentrations of both THg and MeHg were found in tributaries draining mining impacted sites in the Appalachian region, where the highest sulfate concentrations are found, and in certain smaller catchments impacted by agriculture and/or wetlands. The lowest sediment values were associated with open river sites, which are subject to more scouring and sediment transport. In contrast, aqueous MeHg concentrations in some river channels were as elevated as those from smaller catchments impacted by agricultural and wetland LULC, suggesting these areas are sources of MeHg to the main river channel.

Mercury Levels in Largemouth Bass. The concentrations of Hg in largemouth bass tissues spanned more than two orders of magnitude, from 0.02 to 2.8 mg/kg wet weight (average: 0.45 mg/kg; n = 96). The mean largemouth bass concentration (0.45) exactly matched the mean for all national largemouth bass collected during the U.S. Environmental Protection Agency National Mercury Survey. Twelve percent of fish had Hg concentrations qreater than or equal to 1 ppm, the level at which consumption advisories are posted in Alabama, while the average fish Hg concentrations were qreater than or equal to 0.5 ppm at 21 (41 percent) of the sites. Coefficients of variation in fish Hg concentrations from any one site were usually high, averaging 68 percent.

Recreational pool-averaged bass Hg concentrations increased progressively downstream from the Black Warrior to the lower Tombigbee river junction (from 0.1-0.7 ppm), moving from higher elevations in the Appalachian to the Coastal plain, where increased sedimentation, wetland cover, and high sulfate loads may provide a favorable combination of factors enhancing MeHg production and bioaccumulation. Considering that the vast majority of Hg in top predatory fish exists as MeHg, Alabama largemouth bass typically contain more than 1 million times more MeHg than is present in the waters in which they reside. This exemplifies an efficient pathway of MeHg biomagnification in MARB food webs.

Hg Mass Balance Estimate. We estimate the annual watershed Hg input from the atmosphere at 1,250 kg/yr, and the annual flux of Hg to Mobile Bay at only 138 kg/yr. These calculations suggest that roughly 90 percent of Hg entering the MARB from atmospheric sources is retained within the watershed, with most of it is associated with the landscape.

Experimental Findings. Experiments simulating land use impacts showed that organic matter and nutrient additions (N and P) had the most pronounced effect on MeHg production from ambient Hg in MARB sediments, as compared to either sulfate addition or pH change. Experiments examining microbial transformation of added Hg and MeHg in MARB sediments showed that ferric iron-reducing conditions generally inhibited Hg methylation, while methylation rates were similar under sulfate-reducing and methanogenic conditions. In contrast, MeHg degradation was observed under all three TEAPs. Hence, the presence of an iron-oxide-rich surface layer with active MeHg demethylation and suppressed Hg methylation may decrease the flux of MeHg to the overlying water column.

Evaluation of Hypotheses. The collective results of our field and experimental studies provide sound support for most (but not all) of the project hypotheses, as summarized below.

Effect of Nutrients. Nutrient loading to aquatic systems stimulates primary production and the resulting increased organic matter deposition to sediments is likely to provide substrates for methylating microorganisms and to promote the depletion of oxygen in sediments, thereby creating conditions favorable for methylation. The positive relationship between sediment MeHg levels and sediment organic matter and aqueous nutrients support this basic hypothesis. Results from our experimental studies confirmed that the activities of sediment microorganisms responsible for Hg transformations are stimulated by inputs of these nutrients. LULC surrounding streams was found to be an important determinant of nutrient loads, thus providing a link between LULC and MeHg production.

Effect of Dams. Hg levels in largemouth bass sampled above dams appeared to be dependant on geographic position and water level, with shallower dams favoring increased fish Hg burdens. MeHg levels in sediment samples collected behind dams were not as high as in some smaller creeks impacted by agriculture and wetland land cover. However, dams affect many of these creeks by restricting water flow and delivery of fine particles to the main river channel. The retention of fine particles and organic matter in many of these shallow creeks was associated with elevated sediment Hg and MeHg levels.

Effect of Sulfate Loading. Some of the highest sediment Hg and MeHg levels found in the Phase 1 survey were from sites impacted by mines with elevated sulfate levels. However, neither water nor fish sampled at those sites had particularly high MeHg or THg levels, suggesting that in these hydrodynamic systems, MeHg flux from sediments is likely transported downstream. Elevated sulfate levels are suspected of contributing to the higher fish tissue Hg levels observed downstream in one recreational pool (focus of Phase 2 study), where other conditions favorable for MeHg production in the coastal plain region exist. However, our experimental results suggested that: (1) sulfate may not stimulate MeHg production if more energetic electron acceptors, such as iron oxides are present; and (2) sulfate is not required for Hg methylation to proceed.

Effect of Wetlands. Mercury burdens in fish sampled in creeks draining wetlands were dependant on the watershed area of those creeks, suggesting that not all wetlands have equal impact. MeHg bioaccumulation in largemouth bass tissues appeared to be enhanced in certain wetland-impacted creeks where potassium levels were depressed. Low potassium levels may indicate areas of high primary production rates where enhanced MeHg production and bioaccumulation occur. Many wetland-impacted creeks provide the favorable conditions discussed above, which enhance MeHg production and it is suspected that these areas contribute to the occasionally high largemouth bass Hg levels found in many areas of the MARB.

Fish Tissue Levels of Hg Are Related to Water Levels and Net Rates of MeHg Production in Sediments. We found no such relationships in our Phase 1 data set, but some limited support in the smaller Phase 2 field study. We argue that the dynamic nature of the MARB system, which transports large volumes of water and fine sediment during frequent storms and flooding, coupled with differential fish movement and feeding habits, are responsible for the lack of support for this hypothesis at many locations and times of the year.

Management Strategies and Social Impact Assessment. Several watershed management strategies to limit MeHg production and bioaccumulation are suggested by our findings, including: (1) erosion control to decrease input of Hg associated with soil particles to the water; (2) reduction of nutrient loading to limit instream production of organic matter; and (3) restoration of water flow regimes (e.g., through dam removal, or at a minimum more consideration of potential effects of dams on MeHg bioaccumulation in proposals for new dam construction). These potential strategies were put forward to a group of key stakeholders at a final meeting to discuss policy alternatives.

Two basic public policy alternatives were discussed at the policy forum:

1. Remedial (environmental management)

  • Reduction of Hg loading to the MARB.
  • Watershed management strategies to reduce MeHg production and bioaccumulation (above).
2. Nonremedial (minimization of human risk from Hg exposure)
  • Maintain current system of fish consumption advisories.
  • Adjust action level used to issue advisories.
  • Combined research and educational campaign to reduce health risks from Hg exposure.

Results from an informal survey of stakeholders indicated that environmental group representatives preferred taking immediate action to reduce human Hg exposure, while state and industry officials opted for additional research before doing so. Responses to the idea of lowering the Alabama action level were mixed. The greatest consensus centered on the need for watershed management measures to control MeHg production and bioaccumulation, and the need for education. All respondents favored increased public environmental education initiatives and campaigns to identify populations at risk. Thus, education appears to be a key ingredient in obtaining effective support from the general public for any policy alternative.

Incorporation of the project early into the policy arena helped to make preliminary findings known, and to broaden the agenda of the National Mercury Forum (focused mainly on the Gulf of Mexico) to include the team's freshwater findings. Subsequently, some agency officials now support a second-stage effort to determine specific groups "at risk" from high fish consumption and to determine appropriate amelioration policies. Ultimately, the University of Alabama research, in combination with the National Mercury Forum and resulting increased agency focus on Hg in fish have created a sense of urgency to resolve how state officials define risk. Pressure is mounting for the state to lower its current standard from 1.0 to 0.5 ppm in accordance with many other states' standards and the World Health Organization. Depending on how risk is defined, officials may consider a blend of alternatives to address Hg issues, including attention to land-use, nutrient loading, and water flow issues, fish advisories, and accompanying educational campaigns.

The interdisciplinary approach of this project illustrates the utility of integrating biophysical research with social impact assessment (SIA) and public involvement to affect both the conduct of the research and public policy. Disconnections between basic research in the sciences and policy have long been a concern by funding institutions. The project team made this connection effectively. Researchers from different disciplinary backgrounds interacted as a team bound to the common ground of involving policy officials and other stakeholders and translating findings into their social and economic implications. Conflict and stalemate over issues were avoided by the incorporation of a public participation process, and the project team received valuable information from scientists representing relevant industries and government oversight agencies to extend and complement analyses.

Two major conclusions emerge from the social science disciplinary perspective of the project: (1) the applicability of a planning/research management SIA model for linking research to policy was demonstrated; and (2) more generally, the utility of this connection for research to shape policy demonstrates a wider applicability of SIA than has generally been the case. SIA thus represents an instrument for policy, not simply an adjunct to environmental impact assessment.

Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 26 publications 4 publications in selected types All 4 journal articles
Type Citation Project Document Sources
Journal Article Bonzongo JCJ, Lyons WB. Impact of land use and physicochemical settings on aqueous methylmercury levels in the Mobile-Alabama River System. Ambio 2004;33(6):328-333 R827168 (Final)
not available
Journal Article Lyons WB, Bonzongo JC. Mercury in the environment: sources, pathways, and fates. Materials and Geoenvironment 2001;48(1):1. R827168 (Final)
not available
Journal Article Samya M, Snow H, Bryan CH. Integrating social impact assessment with research: the case of methylmercury in fish in the Mobile-Alabama River Basin. Impact Assessment and Project Appraisal 2003;21(2):133-140. R827168 (Final)
not available
Journal Article Warner KA, Roden EE, Bonzongo JC. Microbial mercury transformation in anoxic freshwater sediments under iron-reducing and other electron accepting conditions. Environmental Science and Technology 2003;37(10):2159-2165. R827168 (Final)
not available

Supplemental Keywords:

mercury, methylmercury, land use, biotransformation, bioaccumulation, social impact assessment, watershed management, policy, Alabama, AL, coastal plain, cycling, geographic information system, GIS, environmental policy., RFA, Scientific Discipline, Geographic Area, Water, Water & Watershed, Ecology, Hydrology, Environmental Chemistry, State, Chemistry, Wet Weather Flows, Ecological Risk Assessment, Agronomy, Biology, Watersheds, Mercury, aquatic, fate and transport, aquatic ecosystem, anthropogenic stress, watershed, food chain, agricultural discharges, social impact assessment, runoff, urban runoff, Alabama (AL), citizen perceptions, soils, mercury cycling, aquatic degradation, biogeochemical cycling, methylmercury, watershed influences, methylation, GIS, water quality, marine environment, public policy, bioaccumulation, land management, methylization of mercury

Progress and Final Reports:

Original Abstract
  • 1999 Progress Report
  • 2000