Browsing by Subject "Porewater"

Now showing 1 - 3 of 3
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    Abiotic transformations of pesticides in prairie potholes
    (2012-08) Zeng, Teng
    The prairie pothole region (PPR) is among the most extensively altered ecosystems on Earth. This region covers approximately 780,000 km2 of central North America, and contains numerous glacially formed wetlands embedded in an agricultural landscape. These wetlands, commonly known as prairie pothole lakes (PPLs), provide essential ecosystem services. Over the last 150 years, agricultural drainage has resulted in severe loss of native prairie wetlands. The remaining PPLs continue to be threatened by nonpoint source pesticide pollution from agriculture. Currently, little is known about the fate and persistence of pesticides in PPLs. In this work, the abiotic transformations of commonly used pesticides in PPL sediment porewaters and surface water were explored. Chloroacetanilide and dinitroaniline pesticides were found to react rapidly with naturally abundant reduced sulfur species (i.e., hydrogen sulfide and polysulfides) in sediment porewaters via nucleophilic substitution and reduction reactions, respectively. Dissolved organic matter (DOM) was also found to play a vital role in the reductive transformation. Next, the photodegradation of a suite of pesticides was investigated in PPL surface water under both simulated and natural sunlight. Enhanced pesticide removal rates pointed to the importance of indirect photolysis pathways involving photochemically produced reactive intermediates such as singlet oxygen and triplet excited-state DOM. Finally, the sedimentary sulfur speciation was examined by sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Sulfur species in PPL sediments were found to consist of organic (di)sulfides, sulfonate, sulfate, and the mineral pyrite. Notably, the fractional abundances of reduced and oxidized sulfur species fluctuate on a seasonal basis.
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    The Effect Of Climate Change On Mercury In Boreal Peatlands
    (2023-09) Pierce, Caroline
    Mercury is a ubiquitous pollutant that accumulates in peatlands, an ecosystem highly sensitive to climate change. Methylmercury (MeHg) is a neurotoxin that is capable of biomagnifying in food webs. We examined the effects of increasing temperature and elevated atmospheric carbon dioxide (CO2) on the concentration of total mercury (THg) and MeHg in peatland soil. This research was performed at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, an ecosystem-scale manipulation in an ombrotrophic bog in northern Minnesota, USA, which includes five temperature levels (ambient plus above- and below-ground warming), with ambient or enhanced CO2 concentration. Increased temperature led to decreased MeHg concentrations in peat and increased THg and MeHg in porewaters. This decrease in peat MeHg, and increases in THg and MeHg in porewater could be caused by more rapid decomposition of the peat leading to mercury mobilization, increased methylation/demethylation rates, or increased gaseous mercury emission. The response to elevated atmospheric CO2 was limited to the surface depths of peat. Total mercury and MeHg decreased in the peat and increased in the porewater. There are no known direct effects of CO2 on mercury cycling so this finding is likely due to changes in other response variables such as the lowering of the water table or changes in the proportion of different plant species. Overall, we observed that temperature and CO2 had significant but subtle effects on THg and MeHg retention in peat. Our findings indicate that mercury concentrations in peat may decrease with climate change which may shift the system from a mercury sink to a mercury source.
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    The effect of high sulfate loading on methylmercury production, partitioning, and transport in mining-imapcted freshwater sediments and lakes in northeastern Minnesota
    (2015-02) Bailey, Logan Timothy
    Methylmercury (MeHg) is a highly toxic form of mercury with the ability to bioaccumulate in food webs. The bioaccumulation of MeHg leads to elevated MeHg levels in fish tissue and poses a threat to public health. Thus MeHg concentrations in surface waters - which may be a result of water column MeHg production, or sediment MeHg production and subsequent flux from sediment porewater - are of particular concern. The production of MeHg from inorganic mercury (iHg) is primarily a result of sulfate-reducing bacteria (SRB) activity in anoxic aquatic environments.Ongoing and historic mining activity on the Mesabi Iron Range (Minnesota, USA) has led to elevated sulfate levels in the downstream waters of the St. Louis River watershed. In an effort to understand the effect of mining-related sulfur-loading on the production and partitioning of MeHg, sediment samples were collected and analyzed from sulfur impacted and non sulfur-impacted lakes and wetlands within the watershed. Additionally, the water column and inlet and outlet streams of a mesotrophic lake (Lake McQuade) were sampled intensively during summer stratified conditions in order to identify the sources and sinks of MeHg to the lake system and determine the potential for MeHg export downstream.Results suggest that dissolved sulfide plays a large role in governing MeHg dynamics in sulfate-impacted freshwater sediment. Consistent with previous research, net MeHg production appeared to be inhibited in sediments with dissolved sulfide >60 uM. However, these high concentrations of dissolved sulfide were accompanied by increased partitioning of MeHg into the porewater phase, potentially increasing the fraction of MeHg available to be transported into surface waters.Sediment at sulfate-impacted sites was generally characterized by high dissolved sulfide and a low potential for long-term net MeHg production. However, the accumulation of dissolved sulfide in sediment porewaters can be limited by the availability of free labile iron (Fe2+) and consequent iron-sulfide precipitation reactions. In the results presented here, high sulfur-loading at two sites appeared to have consumed the available free labile iron and created conditions which allowed for the accumulation of dissolved sulfide and inhibition of MeHg production in the sediment. However, relatively high sulfur-loading(>100 mg/L) to a third site where iron remains in excess of sulfur in sediment may have led to robust net MeHg production, in absence of inhibitory dissolved sulfide concentrations. Accumulation of MeHg in the hypolimnion of Lake McQuade occurred during summer 2012 during a time when bottom water sulfate was being consumed. Though some uncertainty remains as to the ultimate source of the MeHg, estimates of MeHg inputs and outputs to the hypolimnion suggest that water column production was a primary source of MeHg to the hypolimnion during the stratified summer months. Following the wet spring months when inputs were dominated by upstream flows, the flux of MeHg across the limnetic surface was estimated to be the primary source of MeHg to the epilimnion during the stratified summer months. However, most of MeHg input to the epilimnion was apparently degraded prior to being exported to the outlet stream. Thus, despite mid-summer accumulation of MeHg in the hypolimnion, the combination of stratification and substantial degradation in the epilimnion acted to limit export of MeHg out of Lake McQuade.As a whole, Lake McQuade acted as small net source of MeHg to the surrounding water system during the summer months of 2012. Evidence points to a brief rise in MeHg export immediately following lake turnover in Mid-August due to the release of hypolimnetic MeHg to surface waters during lake mixing.