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Atmospheric deposition to Utah Lake: science panel subgroup splits between ~32 metric tons TP (majority) and much higher estimates (minority); modeling team to跑
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Summary
A science-panel subgroup presented competing estimates for atmospheric deposition (AD) of nutrients to Utah Lake. The subgroup majority recommended ~32 metric tons of total phosphorus (TP) with a 31–45 range for model inputs; a documented minority argued for much larger TP inputs (recommended ~150 TP, with scenarios up to ~250 TP). The steering committee agreed modelers will run a wider set of scenarios to capture uncertainty.
The Utah Lake Water Quality Study’s atmospheric deposition (AD) subgroup presented analyses and a documented split over how to interpret field AD data and which numerical AD inputs to supply to the nutrient model.
Process and datasets: Samuel Wallace summarized subgroup methods and data sources, including a Williams compilation (2017–2020) and a Wood/Miller dataset. The subgroup met multiple times to scrub units, screen for contamination, and develop an approach to gap‑fill and map shoreline fluxes. The subgroup used a decision tree that favored screened samplers and unscreened samples with metadata indicating no contamination; unscreened samples lacking metadata were excluded from the primary analysis.
Majority recommendation: Mike Brett, representing the majority perspective, said the subgroup’s transparent process identified a small number of extreme samples (notably six Saratoga Springs samples reported to contain sweat bees) that dramatically inflated median fluxes. Brett summarized the majority load recommendation for model inputs as roughly 32 metric tons of TP per year (subgroup range 31–45 metric tons). He defended applying attenuation factors for shoreline samplers based on particle-size principles (larger particles settle faster) and multiple reality checks, including mass-balance comparisons and national deposition-model outputs that aligned with the majority’s order-of-magnitude AD estimates.
Minority perspective: Theron Miller (minority) disagreed, arguing some high values represent real episodic deposition tied to wind-driven dust events and regional transport rather than contamination and that some screening choices and attenuation assumptions undercount legitimate loads. Miller and other minority supporters noted that different reasonable treatments of outliers and attenuation produce much larger TP estimates (memo recommendations in the Williams analysis included point estimates near 150 TP and scenarios to ~250 TP per year). Miller warned that excluding episodic high-wind events risks removing the very events that deliver most AD to the lake.
Insect contamination dispute: The group explicitly diverged on whether insects found in buckets represent contamination or legitimate deposition. Brett and other majority members argued that insect-laden samples had nutrient concentrations orders of magnitude above unscreened medians and therefore likely biased depositional estimates; minority members argued insects may fall to the lake surface and that the sampler design and episodic wind events complicate attribution. “There were six Saratoga samples reported to be contaminated with sweat bees; removing those reduced the estimate by roughly 50%,” Brett said. Thermon Miller countered that high wind events align with many high samples and should not be dismissed without dedicated study.
Steering-committee direction and modeling response: Steering-committee staff (Division of Water Quality) and managers stated the modeling team will run a broader set of AD inputs than originally planned to test sensitivity and incorporate minority recommendations in uncertainty analyses. Scott Daley (Division of Water Quality) and John Mackey explained model runs will include the Wasatch Front Water Quality Council’s higher recommendations alongside the subgroup’s central estimate to see how outcomes vary with AD choices.
What this means: The disagreement does not represent a final regulatory decision but affects model calibration and the range of outcomes the steering committee will evaluate during implementation planning. The subgroup documented both majority and minority positions and provided the modeling team with the subgroup analyses, assumptions and ranges so the modelers can test outcomes across that documented uncertainty.
Ending: Panelists asked for and received commitments that modelers would explore expanded AD ranges and explicit sensitivity testing. The subgroup’s documented disagreement, along with provided minority documentation, will be carried into model runs and uncertainty reporting rather than being resolved in this meeting with a vote.
Note on evidence and availability: Panelists and observers requested additional supporting publications and data; Theron Miller offered to circulate related papers he cited and Janice Braney confirmed paleo and core data are publicly posted.
