Year Established: 2015 Start Date: 2015-03-01 End Date: 2016-02-28
Total Federal Funds: $29,592 Total Non-Federal Funds: $59,588
Principal Investigators: GeneHua Ng, Amy Myrbo
Abstract: The health of Minnesotas wild rice is a focal point for three major stakeholders: the native Ojibwe and other Minnesotans hold it as a crucial cultural and economic resource, environmentalists want to preserve its pristine aquatic habitat, and mining companies need to know safe sulfate discharge levels into wild rice streams to fuel the northern Minnesota economy while mitigating environmental impact. At the center of these concerns is Minnesotas unique and stringent standard for sulfate concentrations in surface waters, which was motivated to preserve the health of wild rice, but was developed without an understanding of how sulfate appears to impact it. A recent assessment by the Minnesota Pollution Control Agency (MPCA) determined that the toxicity is likely attributed to sulfide, which is produced when sulfate is reduced in sediment porewater, but can then be attenuated by precipitation with ferrous iron (Fe2+). However, an independent review panel found that the MPCAs study lacked adequate explanation for site-specific biogeochemical mechanisms, which hampers a decision for whether or how to revise the current standard. Clarifying the effect of sulfate in streams is particularly urgent from both a scientific and regulatory standpoint, because the MPCAs field campaign found unexplained occurrences of high sulfate but minimally impacted wild rice at many stream sites, which are also direct discharge points of elevated sulfate from economically important mining activity in the state. In the prevailing conceptual model, diffusion from surface water is the source for sulfate that is reduced to toxic sulfide in porewater, and locally available Fe in the sediment is the source of Fe2+ that can precipitate and remove the sulfide. Here, we propose to investigate a new conceptual model in which groundwater also influences porewater sulfide concentrations. Sulfate may be transferred more efficiently to porewater through groundwater transport than through diffusion from surface water, and groundwater may serve to transport available Fe for sulfide immobilization. A masters student will investigate these links through an in-depth study of a pair of stream sampling sites, where she will collect new data on the physical hydrology and water quality in the stream, the underlying porewater, and the connected groundwater system. We will also measure the previously unconstrained redox state of Fe in the sediment, which will provide better understanding of how Fe moves and mitigates sulfide in porewater. Our dataset will build off of experimental mesocosm work and previous field work from the recent MPCA-led assessment. In our proposed approach, a reactive transport model will serve as the platform to integrate our diverse data set and incorporate hydrogeochemical exchange among the connected groundwater, hyporheic, and surface water systems. We will then use the reactive transport model to test conceptual models of groundwater influence on sulfate, sulfide, and Fe. This project provides a new groundwater perspective on a pressing water quality issue for the state, and more broadly lends insight into how groundwater can play a role in surface water quality, a complex environmental and ecosystem concern with nationwide importance.