Shyam K. Nair

Modeling the potential effects of atrazine on aquatic communities in midwestern streams

The comprehensive aquatic systems model for atrazine (CASM(ATZ)) estimates the potential toxic effects of atrazine on populations of aquatic plants and consumers in a generic lower-order midwestern stream. The CASM(ATZ) simulates the daily production of 20 periphyton and 6 aquatic vascular plant species. The modeled consumer community consists of 17 functionally defined species of zooplankton, benthic invertebrates, bacteria, and fish. Daily values of population biomass (grams of carbon per square meter) are calculated as nonlinear functions of population bioenergetics, physical-chemical environmental parameters, grazing/predator-prey interactions, and population-specific direct and indirect responses to atrazine. The CASM(ATZ) uses Monte Carlo methods to characterize the implications of phenotypic variability, environmental variability, and uncertainty associated with atrazine toxicity data in estimating the potential impacts of time-varying atrazine exposures on population biomass and community structure. Comparisons of modeled biomass values for plants and consumers with published data indicate that the generic reference simulation realistically describes ecological production in lower-order midwestern streams. Probabilistic assessments were conducted using the CASM(ATZ) to evaluate potential modeled changes in plant community structure resulting from measured atrazine exposure profiles in 3 midwestern US streams representing watersheds highly vulnerable to runoff. Deviation in the median values of maximum 30-d average Steinhaus similarity index ranged from 0.09% to 2.52% from the reference simulation. The CASM(ATZ) could therefore be used for the purposes of risk assessment by comparison of site monitoring-based model output to a biologically relevant Steinhaus similarity index level of concern. Used as a generic screening technology or in site-specific applications, the CASM(AT) provides an effective, coherent, and transparent modeling framework for assessing ecological risks posed by pesticides in lower-order streams.

Transport, chemistry, and thermodynamics of uranium hexafluoride in the atmosphere—evaluation of models using field data

Abstract Accidental releases of uranium hexafluoride (UF6) can occur from nuclear fuel cycle facilities. Upon release to the atmosphere, UF6 enters into exothermic chemical reactions with atmospheric water vapor producing hydrogen fluoride (HF) and uranyl fluoride (UO2F2); HF undergoes further polymerization, depolymerization, and hydrolysis. A number of models have been used in the past to simulate the transport of UF6 and its reaction products in the atmosphere. Performances of two models, HGSYSTEM/UF6 and SAIC were assessed by comparing model predictions with field measurements. Field data for UF6 were available from three experimental releases made at Bordeaux in France between 1986 and 1989; an accidental release at Gore, Oklahoma; and an accidental release at the Comurhex Plant in France. The Gore and Comurhex data are of questionable quality since they were from a real accident. The Bordeaux data are of better quality since they were from a research-grade study. Predictions from the HGSYSTEM/UF6 model were within an order of magnitude of the observations, with most within a factor of two of the observations. Most predictions from the SAIC model were within an order of magnitude of the observations, but the model also over-predicted beyond an order of magnitude for a few observations. Detailed sensitivity analyses were also conducted on all modules of the HGSYSTEM/UF6 and SAIC models. At large distances from the source, the output concentration of total uranium is most sensitive to meteorological parameters; and at distances close to the source, it is most sensitive to certain source-specific and meteorological parameters.

A comparative study of the modeled effects of atrazine on aquatic plant communities in midwestern streams.

Potential effects of atrazine on the nontarget aquatic plants characteristic of lower-order streams in the Midwestern United States were previously assessed using the Comprehensive Aquatic System Model (CASMATZ ). Another similar bioenergetics-based, mechanistic model, AQUATOX, was examined in the present study, with 3 objectives: 1) to develop an AQUATOX model simulation similar to the CASMATZ model reference simulation in describing temporal patterns of biomass production by modeled plant populations, 2) to examine the implications of the different approaches used by the models in deriving plant community-based levels of concern (LOCs) for atrazine, and 3) to determine the feasibility of implementing alternative ecological models to assess ecological impacts of atrazine on lower-order Midwestern streams. The results of the present comparative modeling study demonstrated that a similar reference simulation to that from the CASMATZ model could be developed using the AQUATOX model. It was also determined that development of LOCs and identification of streams with exposures in excess of the LOCs were feasible with the AQUATOX model. Compared with the CASMATZ model results, however, the AQUATOX model consistently produced higher estimates of LOCs and generated non-monotonic variations of atrazine effects with increasing exposures. The results of the present study suggest an opportunity for harmonizing the treatments of toxicity and toxicity parameter estimation in the CASMATZ and the AQUATOX models. Both models appear useful in characterizing the potential impacts of atrazine on nontarget aquatic plant populations in lower-order Midwestern streams. The present model comparison also suggests that, with appropriate parameterization, these process-based models can be used to assess the potential effects of other xenobiotics on stream ecosystems.

A spatial model for restoration of the upper Mississippi River ecosystems

A series of locks and dams were constructed and put into operation on the Upper Mississippi River in the 1930s to facilitate commercial navigation. As a result, historical floodplain landscapes were altered. For example, islands characterized by floodplain forests experienced prolonged unfavourable hydrologic conditions and were eliminated from many areas of the river. The distribution and extent of other large river habitat types (e.g., wetlands, secondary channels) were also impacted. In addition, large areas of open water habitat were created through the impoundment of the river. Proposed management plans for the Upper Mississippi River include (1) modernization of the locks and dams to improve navigation efficiency, and (2) ecological restoration to conditions more characteristic of pre-impoundment. The purpose of the work reported here is to describe and apply a spatially explicit comprehensive aquatic systems model (SECASM). The SECASM is offered as one approach for evaluating the anticipated outcomes of alternative management and restoration actions (e.g., island creation, floodplain forest restoration, water level management). The model simulates spatial-temporal changes in the distribution and extent of five land-use types representative of the Upper Mississippi River floodplain: prairie, marsh, upland woody vegetation, surface water, and combined urban/agricultural areas. The SECASM has a spatial resolution defined by 100 x 100-meter grid elements (i.e., 1 ha) and operates using a daily time step for simulated durations up to 100 years. Transitions of habitat types within each grid element are determined by a combination of rule-based algorithms and ecological process equations. The model outputs are amenable to the production of landscape maps and the calculation of landscape metrics (e.g., lacunarity index) that usefully summarize landscape patterns. The ability of the SECASM to realistically describe alterations in Upper Mississippi River floodplain landscapes was evaluated by using Pool 5 land-use patterns reported for 1890 as an initial condition, simulating 100-y of landscape change (including impoundment), and comparing model results with reported conditions for 1989. The SECASM was subsequently used to examine several hypotheses concerning landscape impacts of impoundment, outcomes of alternative restoration actions, and the potential effects of nutrient enrichment.

Modeling the effects of thiamethoxam on Midwestern farm ponds and emergent wetlands

Potential toxic effects of thiamethoxam on nontarget organisms and the community structure of a generic Midwestern farm pond and emergent wetland were assessed using 2 versions of the comprehensive aquatic system model: CASMGFP , a generic farm pond model, and CASMGWL , a generic wetland model. The CASMGFP and CASMGWL are integrated bioenergetics-based and habitat quality models that describe the daily biomass values of selected producer and consumer populations representative of generalized Midwestern farm ponds and emergent wetlands. The CASMGFP demonstrated the ability to reproduce values of population biomass reported for Midwestern (and other) pond ecosystems; the CASMGWL provided a similar modeling capability for Midwestern emergent wetlands. Lethal and sublethal effects of thiamethoxam were modeled as extrapolations of laboratory toxicity assays using the CASMGFP and the CASMGWL . Time series of daily environmental concentrations of thiamethoxam constructed for 6 regional pesticide applications across the United States failed to produce any calculated impacts on modeled population biomass or changes in community structure of modeled trophic guilds in the CASMGFP or the CASMGWL . However, evaluation of systematically increased daily concentrations demonstrated the ability of both models to simulate direct and indirect toxic effects of this pesticide. The present model study suggests that process-based food web/ecosystem models can be used to characterize the potential ecological effects of thiamethoxam on generalized farm pond and emergent wetland ecosystems. Environ Toxicol Chem 2018;37:738-754.