Chapter 8 Contaminant Exposure

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8.1 Overview

The contaminant exposure submodel simulates mercury and methylmercury uptake across the gills for migratory fish moving through spatially and temporally variable contamination fields. Exposure risk is governed by contaminant concentration, suspended particulate matter (SPM), and metabolic rate, with concentrations normalized across the spatial domain and depth-averaged SPM modulating risk to reflect particle-bound transport. Agents accumulate both instantaneous and cumulative exposure, allowing estimation of bioaccumulation risk over the migration season. This formulation reflects established mechanisms of methylmercury (MeHg) uptake in fishes, where exposure during active movement occurs by branchial uptake across the gills (Hall et al. 1997; Evans 1987). The gill epithelium serves as the primary interface for this exchange, with uptake regulated by diffusion, ion transport, and local water chemistry, and increasing with metabolic demand as elevated respiration enhances water flux across the gills; physiological stress can further modify uptake through changes in ion regulation and membrane permeability (Trudel and Rasmussen 2006).

8.2 Purpose

To evaluate mercury and methylmercury exposure and bioaccumulation risk for migratory fishes by integrating patch level contamination, ionregulatory stress, depth averaged suspended particulate matter, and metabolism driven uptake processes in estuarine and coastal systems.

8.3 Entities, State Variables, and Scales

8.3.1 Spatial and Temporal Scales

Spatial Unit: Patch (3 m x 3 m resolution)
Temporal Unit: 5 minute time steps (tick)

8.3.2 Global Variables

Variable Initialized Value Justification
MeHg-threshold 15 ug per kg Screening threshold for methylmercury (National Oceanic and Atmospheric Administration (NOAA) 1990; Gaudet et al. 1995)
Hg-threshold 150 ug per kg NOAA guideline for mercury contamination (National Oceanic and Atmospheric Administration (NOAA) 1990; Gaudet et al. 1995)
min-Hg, max-Hg Calculated from input layers Required for Hg normalization across domain
min-MeHg, max-MeHg Calculated from input layers Required for MeHg normalization across domain

8.3.3 Patch Variables

Variable Name Definition
mercury Mercury concentration at this patch.
methylmercury Methylmercury concentration at this patch.
SPM Depth averaged suspended particulate matter at this patch.
Hg-exp-risk-alewife Patch level mercury exposure risk from alewives.
Hg-exp-risk-stripedbass Patch level mercury exposure risk from striped bass.
MeHg-exp-risk-alewife Patch level methylmercury risk from alewives.
MeHg-exp-risk-stripedbass Patch level methylmercury risk from striped bass.

8.3.4 Agent Variables

Variable Name Definition
metabolism-rate Current metabolic rate used to scale uptake risk.
hg-exposure-duration Number of ticks spent above the mercury threshold.
mehg-exposure-duration Number of ticks spent above the MeHg threshold.
hg-uptake-risk Instantaneous mercury uptake risk per tick.
mehg-uptake-risk Instantaneous methylmercury uptake risk per tick.
hg-total Cumulative mercury uptake.
mehg-total Cumulative methylmercury uptake.
hg-exposure-total Total non normalized Hg exposure.
mehg-exposure-total Total non normalized MeHg exposure.
hg-exposure-total-normalized Sum of normalized Hg exposure values.
mehg-exposure-total-normalized Sum of normalized MeHg exposure values.

8.4 Process Overview and Scheduling

  1. Retrieve methylmercury (ng g⁻¹), and depth-averaged SPM values from the current patch (\(\frac{kg}{m^3}\)).

  2. Normalize patch contaminants between 0 and 1.

  3. Compare patch levels to stress thresholds.

  4. Update exposure durations (ticks).

  5. Compute uptake risk using metabolism ((energy units) tick⁻¹), ion-regulatory stress (dimensionless), patch methylmercury (ng g⁻¹), and SPM (\(\frac{kg}{m^3}\)).

  6. Add uptake risk to cumulative body burden (dimensionless), initialized at zero but configurable based on user-defined inital conditions.

  7. Update patch-level species-specific risk metrics.

8.5 Design Concepts

Basic Principles: Contaminant uptake follows toxico-kinetic principles, where absorption is driven by dissolved methylmercury concentrations and mediated by physiological processes, including metabolic rate and stress, that regulate uptake, retention, and elimination (e.g., Wang and Wong (2003); Trudel and Rasmussen (2006)). SPM represents the hydrodynamic transport medium for sediment-bound contaminants and regulates the availability of dissolved MeHg in the water column, modulating exposure by influencing the concentration of dissolved contaminants available for branchial uptake.

Emergence: Exposure and risk patterns emerge from individual movement across heterogeneous contamination fields, coupled with metabolic demand (energy units per tick) and stress-driven physiology (dimensionless).

Adaptation: Agents do not actively avoid contamination, but their internal physiology modulates uptake in response to metabolic changes.

Objectives: This model does not give agents risk minimization behavior. Instead, risk is assessed to evaluate plausible exposure pathways and cumulative toxic exposure.

Sensing: Agents track patch-level mercury, methylmercury, and SPM, along with internal metabolic state, to quantify contaminant exposure and flux; this is a model construct for accounting purposes and does not represent biological sensing.

Stochasticity: Variation in spatial contaminant fields or individual characteristics and state creates stochastic variation in exposure.

Observation: Outputs include instantaneous exposure risk, cumulative exposure risk, normalized exposure totals, and patch level species specific contributions.

8.6 Initialization

Variable Initialized Value Justification
mercury Input layer Hydrodynamic or field derived contaminant layer.
methylmercury Input layer Same as above.
SPM Depth averaged input Reflects hydrodynamic particle transport.

8.7 Submodels: Mercury Exposure and Uptake

8.7.1 Normalization

\[ MeHg_{normalized} = \frac{MeHg_{patch} - MeHg_{min}}{MeHg_{max} - MeHg_{min}} \hspace{2cm} \text{(8.1)} \]

Where:

  • \(MeHg_{normalized}\) is the normalized methylmercury concentration (dimensionless).

  • \(MeHg_{patch}\) is the patch methylmercury concentration at the agent location (ng g⁻¹).

  • \(MeHg_{min}\) is the minimum methylmercury value for normalization, relative to minimum contaminant concentration observes in system (ng g⁻¹).

  • \(MeHg_{max}\) is the maximum methylmercury value for normalization, relative to maximumcontaminant concentration observes in system (ng g⁻¹).

Clamped to the range 0 to 1.

8.7.2 Exposure Duration

\[ MeHg_{exp} = MeHg_{exp} + 1 \quad \text{if } MeHg_{patch} > MeHg_{threshold} \hspace{2cm} \text{(8.2)} \]

Where:

  • \(MeHg_{exp}\) is the Duration of exposure above threshold (ticks).
  • \(MeHg_{patch}\) is the patch-level methylmercury used in feeding uptake (ng g⁻¹).
  • \(MeHg_{threshold}\) is the threshold concentration for exposure duration tracking, initalized at the Effects Range–Low for MeHg (ERL = 15 ng g⁻¹) (National Oceanic and Atmospheric Administration (NOAA) 1990).

8.7.3 Uptake Risk

SPM acts as a scaling factor on exposure risk, representing increased availability of dissolved MeHg associated with elevated SPM concentrations.

\[ MeHg_{risk} = Met_{tick} \cdot MeHg_{normalized} \cdot (1 + S_{stress}) \cdot (1 + SPM) \hspace{2cm} \text{(8.3)} \]

Where:

  • \(MeHg_{risk}\) is an agent’s instantaneous methylmercury exposure risk (dimensionless).
  • \(Met_{tick}\) is the total metabolic rate for the current simulation tick (energy units).
  • \(MeHg_{normalized}\) is the normalized methylmercury concentration (dimensionless).
  • \(S_{stress}\) is an agent’s physiological salinity stress index (dimensionless).
  • \(SPM\) is the suspended particulate matter (SPM) concentration on the patch (\(\frac{kg}{m^3}\)).

8.7.4 Cumulative Uptake

\[ MeHg_{total} = MeHg_{total} + MeHg_{risk} \hspace{2cm} \text{(8.4)} \]

Where:

  • \(MeHg_{total}\) is an agent’s cumulative methylmercury body burden (dimensionless).
  • \(MeHg_{risk}\) is an agent’s instantaneous methylmercury exposure risk (dimensionless).

Patch-level and individual-level storage based on species for cumulative duration risk experienced:

\[ MeHg_{exp-risk} = MeHg_{exp-risk} + MeHg_{risk} \hspace{2cm} \text{(8.5)} \]

\[ MeHg_{exp-patch} = MeHg_{exp-patch} + MeHg_{risk} \hspace{2cm} \text{(8.6)} \]

Where:

  • \(MeHg_{exp-risk}\) is the cumulative exposure-weighted risk per individual (dimensionless).
  • \(MeHg_{exp-patch}\) is the patch-level cumulative exposure contribution (dimensionless).

8.8 Netlogo Implementation

References

Evans, D H. 1987. “The Fish Gill: Site of Action and Model for Toxic Effects of Environmental Pollutants.” Environmental Health Perspectives 71 (April): 47–58. https://doi.org/10.1289/ehp.877147.
Gaudet, C., S. Lingard, P. Cureton, K. Keenleyside, S. Smith, and G. Raju. 1995. “Canadian Environmental Quality Guidelines for Mercury.” Water, Air, & Soil Pollution 80 (1): 1149–59. https://doi.org/10.1007/BF01189777.
Hall, B. D., R. A. Bodaly, R. J. P. Fudge, J. W. M. Rudd, and D. M. Rosenberg. 1997. “Food as the Dominant Pathway of Methylmercury Uptake by Fish.” Water, Air, and Soil Pollution 100 (1-2): 13–24. https://doi.org/10.1023/A:1018071406537.
National Oceanic and Atmospheric Administration (NOAA). 1990. Sediment Quality Guidelines Developed for the National Status and Trends Program. National Oceanic; Atmospheric Administration. https://rais.ornl.gov/documents/ECO_BENCH_NOAA.pdf.
Trudel, Marc, and Joseph B Rasmussen. 2006. “Bioenergetics and Mercury Dynamics in Fish: A Modelling Perspective.” Canadian Journal of Fisheries and Aquatic Sciences 63 (8): 1890–902. https://doi.org/10.1139/f06-081.
Wang, Wen-Xiong, and Raymond S. K. Wong. 2003. “Bioaccumulation Kinetics and Exposure Pathways of Inorganic Mercury and Methylmercury in a Marine Fish, the Sweetlips Plectorhinchus Gibbosus.” Marine Ecology Progress Series 261: 257–68. https://doi.org/10.3354/meps261257.