Potential Monomethylmercury Production in Bioreactors and Wetlands Intercepting Elevated Nitrate Loads in Iowa

Date: 
Aug 2016

Issue

Bioreactors and wetlands are edge-of-field practices that have been proven effective at reducing nitrate loads associated with tile drainage. Both practices rely on denitrification under anaerobic conditions, which creates the potential for two undesirable outcomes — increased emissions of greenhouse gases (GHG), and the production of monomethylmercury (MeHg).

Objective

The overall goal of this study is to evaluate the net production of MeHg in woodchip bioreactors and wetlands intercepting tile drainage in Iowa, and to identify system variables that affect MeHg production. This work is intended to determine the risk and conditions of MeHg production in these systems, so guidance can be provided on strategies to optimize nitrate removal while minimizing that risk.

Approach

Research already underway is measuring nitrate removal and GHG emissions in both wetlands and woodchip bioreactors in Iowa. This project will leverage these ongoing research efforts to examine MeHg production in these same systems. This effort will be coordinated with research on the performance of Iowa CREP wetlands and with bioreactor research at the ISU Agricultural Engineering Research Farm, in a system of nine bioreactors constructed with funding from the Iowa Nutrient Research Center in 2013. Samples will be analyzed for MeHg and total mercury through a collaborative agreement with the USGS Mercury Research Lab in Madison, Wisc. Sampling design will encompass seasonality of MeHg production and results will be analyzed in conjunction with measurements of nitrate removal and GHG emissions.

Project Updates

December 2018

Sample collection continued into December for the six wetlands. The experimental bioreactor site experienced a malfunction this year that unfortunately limited sample collection from that site. Researchers are beginning to look at results from the USGS Mercury Lab in Wisconsin. 

September 2018

Sample collection continued into December for the six wetlands. The experimental bioreactor site experienced a malfunction this year that unfortunately limited sample collection from that site. Researchers are beginning to look at results from the USGS Mercury Lab in Wisconsin. 

December 2017

Sample collection continued into December for the six wetlands. The experimental bioreactor site experienced a malfunction that limited sample collection from that site. Sample results from the USGS Mercury Lab in Wisconsin are being analyzed. There is some presence of MeHg and THg in the samples analyzed thus far, however the relative importance of the levels has not yet been assessed.

December 2017

Preliminary review of results indicate there is some presence of methyl mercury (MeHg) and total mercury (THg) in the samples analyzed thus far, however the relative importance of the levels has not yet been assessed. It appears that seasonal variations in hydrologic loading has some effect on the concentrations of MeHg and THg seen both in the wetlands and from the contributing watershed. Further sampling should be conducted to fully understand these preliminary results.

September 2017

Sample collection continued approximately twice each month for the six wetlands. The experimental bioreactor site experienced a malfunction during the summer, but it is hoped the site will be operational before the end of the year. Team members are beginning to look at results from the USGS Mercury Lab in Wisconsin. 

June 2017

Sample collection continued this quarter, with sampling every two to three weeks in six wetlands and three bioreactors. This sampling pattern will continue until November when it is expected the wetlands will freeze and bioreactors be shut down. This sampling period will provide results indicative of the seasonality of MeHg production within these systems. 

March 2017

Sample collection continued, with sampling every two to three weeks in six wetlands and three bioreactors. Researchers will continue to collect samples until November when it is anticipated the wetlands will freeze and bioreactors will be shut down. This sampling period should provide results indicative of the seasonality of MeHg production within these systems. 

 

December 2016

Pre-freeze winter water samples were collected from 12 wetlands and preserved for analysis. Additional mid-winter samples will be collected before shipping samples to the lab for analysis. Samples are shipped in large coolers, so it is more practical to wait until all winter samples have been collected before shipping.

September 2016

This quarter study locations were identified, and a winter sampling plan devised. Sampling protocols were established, and sample collection bottles requested from the lab that will process the samples.

Category: