Nitrate Reduction Potential via Reservoir Water Level Management in Central Iowa

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Nitrate Reduction Potential via Reservoir Water Level Management in Central Iowa: Background:Lake Red Rock reservoir is situated on the Des Moines River and part of the nationwide Sustainable Rivers Program (SRP). Through the SRP, scientific experts and stakeholders have developed water management strategies that provide greater ecosystem benefits for the Des Moines River. Adaptively managing the reservoir to meet environmental targets is now possible due to recent revisions to the Water Control Manual and adoption of a “conservation band”. The introduced flexibility permits water managers and biologists to implement unique strategies that target needs of aquatic dependent species within the reservoir and downstream of the dam, in the river. The Des Moines River drains a watershed of approximately 14,800 mi², flowing from the headwaters in Minnesota through north-central and southeastern Iowa to its confluence with the Mississippi River at Keokuk. In 1969, the Corps completed construction of the Red Rock dam about thirty miles downstream of the city of Des Moines, which was intended to mitigate downstream flooding. The 15,250 acre-Red Rock reservoir stores 189,000 acre-feet of water for a distance of 18 miles upstream from the dam. Land use in the watershed is predominately agricultural, consisting of approximately 67% row crops of corn and soybeans. The Des Moines River exports significant nitrogen loads to the Mississippi River and contributes to Gulf of Mexico hypoxia. Downstream of the reservoir at Ottumwa, Iowa, surface water from the Des Moines River used for drinking water supply is impaired for nitrate-nitrogen (nitrate). Increased sedimentation and nutrient loading in Red Rock reservoir is contributing to water quality degradation within the lake itself, with blue green algal blooms and elevated levels of E.coli bacteria restricting recreational use and harming biodiversity. Recent studies suggest that Saylorville reservoir in Iowa is reducing nitrate concentrations (Schoch et al, 2009) and loads (Stenback et al., 2014) in the river. Hansen et al. (2015) used a time-series approach to quantify the relative importance of various N removal processes and suggested that denitrification was the dominant removal process (60.9%) followed by algal assimilation (37.9%) and sedimentation (1.2%). With denitrification providing a potentially important process for nitrate reductions in the Red Rock reservoir and in the Des Moines River downstream of the dam, it is important to assess the potential for management of Red Rock hydrology to achieve much needed downstream nitrate concentration reductions. Work associated with this funding opportunity is part of a larger effort to connect reservoir operations and science. The overall effort at Lake Red Rock involves consideration of a broad set of natural resources and processes related to pool levels. Knowledge obtained through this work will help inform future management at Lake Red Rock and similar Midwestern reservoirs in the United States. Results are also expected to help quantify environmental responses to a range of operations, which would thereby enhance the Corps ability to communicate environmental benefits associated with operational decisions Brief Description of Anticipated Work:This agreement represents an opportunity to enter into a cooperative agreement for understanding the effects of reservoir water level management on denitrification potential at a Corps reservoir in central Iowa. The overall purpose of this work is to determine whether a net reduction in contributions of nitrate from the outflow of Lake Red Rock can be achieved through reservoir operations optimized for nitrate reduction. Work consists of three independent phases. Phase One uses existing data sets associated with operations at Lake Red Rock to characterize the delta area and collecting samples to assess baseline water and sediment nutrient conditions. Phase Two involves field work for wetland, delta, and reservoir sediment and water biogeochemistry assessment. Phase Three involves analysis of field data, reporting and communications. Phase One:Lake Red Rock delta geomorphology will be characterized using historical aerial photos, pool stage records, LiDAR elevations, and lake survey data. Delta characterization will delineate the size and inundation frequency of wetland area available to support denitrification potential in three areas of the lake. Baseline water quality at random sites to document mixing in the delta and sediment nutrient data at continuous monitoring sites will be used to characterize biogeochemistry in the 10,000 acre delta area and in adjacent lake sediments. A literature review will be done to identify and summarize past works related to denitrification potential in reservoirs. Sources with operational connections will be emphasized. The literature review should clarify the: 1) degree to which different aspects of denitrification are known, 2) reasons why poorly understood aspects remain uncertain, and 3) potential for additional research (ie, Phases Two and Three of this Statement of Interest) to advance scientific knowledge related to reservoir operations and denitrification. The primary product of Phase One is the delta characterization. Delineation of the 10,000 acre delta into areas of common attributes (soils, elevations, land classifications) is envisioned as a shared and critical dataset fundamental to investigation, quantification, and communication of relationships between delta natural resources and pool level management. It is expected to serve as a spatial foundation for study and optimization of operations related to denitrification and sediment deposition dynamics as well as delta utilization by waterbirds, herptiles, emergent wetlands, vegetation, and other natural resources. A short report in the format of an ERDC Technical Note (<10 pages excluding references) documenting literature review, detailed methods descriptions, and delta delineation conclusions will be prepared no later than 4 months from date of CESU award. Phases Two and Three are contingent on both successful completion of the delta delineation and a clear understanding of expected value of additional field work related to denitrification based on the literature review. Phase Two: Phase Two includes two field years of continuous nitrogen biogeochemistry monitoring at three locations in Lake Red Rock. 1. Biogeochemistry meters will be established in a perennial wetland station to monitor wetland nitrate dynamics in the sediment and water column during the growing season. 2. Another station will be set offshore and adjacent to the wetland to evaluate reservoir sediment and water nitrate dynamics. 3. The wetland and reservoir stations will be relocated to delta wetlands when the delta is flooded for waterfowl migration. Nitrate - transects or random samples will be collected in delta to check whether nitrate concentrations in delta waters are consistent (i.e., waters are well-mixed). The Phase Two deliverables are brief annual reports for In-Progress Review reporting and annual and final datasets. Phase Three: Phase Three is for data analysis, reporting. Existing data pertaining to denitrification at reservoirs will be analyzed. Results will be summarized in technical reports and other communication materials. Reports and materials should focus on providing insights that aid development of best reservoir management practices for positive environmental outcomes related to denitrification. Phase Three deliverables are a final report in an ERDC Technical Report format, webinars and conference presentations yet to be determined, and journal publications are encouraged.