Engineering topics

Brooks, Scott C.

Publications and source records attributed to Brooks, Scott C..

7 records

DOE OSTI2024

Self‐Potential Tomography Preconditioned by Particle Swarm Optimization—Application to Monitoring Hyporheic Exchange in a Bedrock River

Abstract A self‐potential (SP) data‐inversion algorithm was developed and tested on an analytical model of electrical‐potential profile data attributed to single and multiple polarized electrical sources. The developed algorithm was then validated by an application to SP‐monitoring field data measured on the floodplain of East Fork Poplar Creek, Oak Ridge, Tennessee, to image electrical sources in areas conducive to preferential flow into the flood plain from the bedrock‐lined riverbed. The algorithm combined stochastic source‐localization by particle‐swarm‐optimization (PSO) of electrical sources characterized by simplified geometries with source tomography by regularized weighted least‐squares minimization of a quadratic objective function. Prior information was incorporated by preconditioning the tomography algorithm by PSO results. Variable percentages of random noise were added to analytical‐model data to evaluate the algorithm performance. Results indicated that true parameters of single‐source models were inverted and approximated with small residual error, whereas inversion of analytical‐model data representing multiple electrical sources accurately approximated the locations of the sources but miscalculated some parameters because of the non‐uniqueness of the inverse‐model solution. Source tomography applied to analytical model data during testing produced a spatially continuous parameter field that identified the locations of point‐scale synthetic dipole sources of electrical current flow with varying degrees of accuracy depending on the prior information incorporated into the tomography. When applied to SP‐monitoring field data, the algorithm imaged electrical sources within a known fault that intersects the bedrock riverbed and flood plain of East Fork Poplar Creek and depicted dynamic electrical conditions attributed to hyporheic exchange.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Rogers Quarry Special Studies Report 2024

Rogers Quarry (RQ) has a history of elevated selenium (Se) concentrations in water and fish caused by inputs from the Y-12 National Security Complex Filled Coal Ash Pond (FCAP). Selenium is acutely toxic at high concentrations, and chronic toxicity occurs at low aqueous concentrations because Se bioaccumulates in fish tissues. The US Environmental Protection Agency (EPA) recently developed water quality criteria for Se that include fish tissue as well as aqueous Se concentrations. The water column concentration criterion is 1.5 µg L -1 for lentic ecosystems and 3.1 µg L -1 for lotic ecosystems. The fish tissue criteria include both an egg/ovary concentration (15.1 µg/g dry weight) and a concentration for whole-body (8.5 µg/g dry weight) or fish muscle (11.3 µg/g dry weight). The egg/ovary criterion supersedes the fillet and aqueous concentrations if measured because Se toxicity manifests via reproductive effects in birds and fish. It is maternally transferred through eggs, leading to teratogenicity (i.e., deformities, developmental effects) in young fish. Historical bioaccumulation data suggest that Se is likely the primary contaminant of concern in RQ, but more recent sampling shows that aqueous As concentrations have been elevated in upper McCoy Branch (MB) leading to RQ. More recently, Se concentrations in fish collected from upper MB were found to be above the tissue criterion for Se for whole-body fish. In FY 2019, actions were taken to deepen the constructed wetland just downstream of the FCAP to increase its ability to remove coal ash particulates from the water column before water enters MB. Sampling was conducted throughout MB in FY 2020-2024 to evaluate the effectiveness of the actions taken in the FCAP wetland and to measure Se concentrations in biota throughout the watershed. The methods and results from a water and fish field collection campaign in April 2024 are presented in this report.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Comparison of machine learning and electrical resistivity arrays to inverse modeling for locating and characterizing subsurface targets

Here, this study evaluates the performance of multiple machine learning (ML) algorithms and electrical resistivity (ER) arrays for inversion with comparison to a conventional Gauss-Newton numerical inversion method. Four different ML models and four arrays were used for the estimation of only six variables for locating and characterizing hypothetical subsurface targets. The combination of dipole-dipole with Multilayer Perceptron Neural Network (MLP-NN) had the highest accuracy. Evaluation showed that both MLP-NN and Gauss-Newton methods performed well for estimating the matrix resistivity while target resistivity accuracy was lower, and MLP-NN produced sharper contrast at target boundaries for the field and hypothetical data. Both methods exhibited comparable target characterization performance, whereas MLP-NN had increased accuracy compared to Gauss-Newton in prediction of target width and height, which was attributed to numerical smoothing present in the Gauss-Newton approach. MLP-NN was also applied to a field dataset acquired at U.S. DOE Hanford site.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Non-mercury methylating microbial taxa are integral to understanding links between mercury methylation and elemental cycles in marine and freshwater sediments

The goal of this study was to explore the role of non-mercury (Hg) methylating taxa in mercury methylation and to identify potential links between elemental cycles and Hg methylation. Statistical approaches were utilized to investigate the microbial community and biochemical functions in relation to methylmercury (MeHg) concentrations in marine and freshwater sediments. Sediments were collected from the methylation zone (top 15 cm) in four Hg-contaminated sites. Both abiotic (e.g., sulfate, sulfide, iron, salinity, total organic matter, etc.) and biotic factors (e.g., hgcA, abundances of methylating and non-methylating taxa) were quantified. Random forest and stepwise regression were performed to assess whether non-methylating taxa were significantly associated with MeHg concentration. Co-occurrence and functional network analyses were constructed to explore associations between taxa by examining microbial community structure, composition, and biochemical functions across sites. Regression analysis showed that approximately 80% of the variability in sediment MeHg concentration was predicted by total mercury concentration, the abundances of Hg methylating taxa, and the abundances of the non-Hg methylating taxa. The co-occurrence networks identified Paludibacteraceae and Syntrophorhabdaceae as keystone non Hg methylating taxa in multiple sites, indicating the potential for syntrophic interactions with Hg methylators. Strong associations were also observed between methanogens and sulfate-reducing bacteria, which were likely symbiotic associations. The functional network results suggested that non-Hg methylating taxa play important roles in sulfur respiration, nitrogen respiration, and the carbon metabolism-related functions methylotrophy, methanotrophy, and chemoheterotrophy. Interestingly, keystone functions varied by site and did not involve carbon- and sulfur-related functions only. In conclusion, our findings highlight associations between methylating and non-methylating taxa and sulfur, carbon, and nitrogen cycles in sediment methylation zones, with implications for predicting and understanding the impact of climate and land/sea use changes on Hg methylation.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2023

Towards Development of a Conceptual Model for Mercury in Bear Creek, Oak Ridge, Tennessee (FY23 Update)

Mercury concentrations in fish in Bear Creek are elevated and comparable to concentrations seen in fish in East Fork Poplar Creek (EFPC) on the Oak Ridge Reservation, even though aqueous inorganic mercury concentrations are orders of magnitude lower in Bear Creek than in EFPC. Acknowledging that the relationship between aqueous and fish tissue mercury concentrations is not linear, and that methylmercury (MeHg) concentrations are likely more related to fish tissue concentrations than aqueous total mercury (Hg T ) concentrations, MeHg production is not easily predicted or controlled. In Bear Creek, where aqueous Hg T concentrations are low, factors other than mercury loading drive the transformation of mercury to MeHg and subsequent trophic transfer to fish. Initial development of a waste disposal facility (Environmental Management Disposal Facility; EMDF) in Bear Creek Valley has begun, and operation of the EMDF has the potential to increase mercury inputs to Bear Creek. Consequently, understanding the factors contributing to elevated MeHg concentrations in water and fish in Bear Creek has increased relevance and importance. This report summarizes data from recent and historical compliance and investigatory studies with an eye toward building a conceptual model to understand the processes affecting mercury transport and transformation in the Bear Creek watershed, as well as to highlight key knowledge gaps in our understanding of these processes that warrant further investigation. The conceptual model will provide a strong technical basis for prioritizing and optimizing potential mitigation actions or best management practices to minimize potential negative effects of the EMDF related to mercury in Bear Creek.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2023

Rogers Quarry Special Studies Report 2023

Rogers Quarry (RQ) has a history of elevated Se concentrations in water and fish caused by inputs from the Y-12 National Security Complex Filled Coal Ash Pond (FCAP). Selenium is acutely toxic at high concentrations, and chronic toxicity occurs at low aqueous concentrations because Se bioaccumulates in fish tissues. The US Environmental Protection Agency (EPA) recently developed water quality criteria for Se that include fish tissue as well as aqueous Se concentrations. The water column concentration criterion is 1.5 µg L -1 for lentic ecosystems and 3.1 µg L -1 for lotic ecosystems. The fish tissue criteria include both an egg/ovary concentration (15.1 μg/g dry weight) and a concentration for whole-body (8.5 μg/g dry weight) or fish muscle (11.3 μg/g dry weight). The egg/ovary criterion supersedes the fillet and aqueous concentrations if measured because Se toxicity manifests via reproductive effects in birds and fish. It is maternally transferred through eggs, leading to teratogenicity (i.e., deformities, developmental effects) in young fish.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2023

Geoelectric Characterization of Hyporheic Exchange Flow in the Bedrock‐Lined Streambed of East Fork Poplar Creek, Oak Ridge, Tennessee

Abstract A multimethod geoelectric survey was implemented between January and March 2022 along a 220‐m long reach of the bedrock‐lined streambed of East Fork Poplar Creek in Oak Ridge, Tennessee to identify locations of surface‐water and groundwater exchange and characterize the subsurface flow paths that convey water between the stream and flood plain. A waterborne self‐potential (WaSP) survey was completed in January 2022 to measure the electric streaming‐potential field in the stream. Electric resistivity tomography (ERT) was performed in March 2022 on the flood plain adjacent to the WaSP survey reach to map the electric resistivity distribution and characterize the hydrogeology and subsurface flow paths that facilitate surface‐water and groundwater exchange in the bedrock‐lined stream. The combination of WaSP and ERT data support the qualitative interpretation that surface‐water and groundwater exchange likely occurs along fractures in outcropping bedrock and along two fault lines that intersect the limestone creek bed.

54 ENVIRONMENTAL SCIENCES↗