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USGS presents salinity forecasts; advisory council eyes single monthly 'upper‑brine' metric for management
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Summary
USGS briefed the council that South Arm shallow salinities measured in late February averaged about 115 g/L and that spring/fall 2025 salinity could range widely by climate scenario. Committee members debated a consistent monthly 'upper‑brine' salinity metric for the Comprehensive Management Plan and whether to publish one or two values when a deep brine layer is present.
Christine, a USGS scientist, told the Utah Great Salt Lake Advisory Council that recent shallow salinity samples taken on Feb. 27 show values between about 108 and 116 grams per liter and that the South Arm's most recent salt‑mass measurement is roughly 894,000,000 tons.
Those measurements underpin USGS forecasts presented to the council: under average climate inputs the forecasted June (spring) salinity is about 108 g/L, a warm‑dry scenario yields about 120 g/L, and a wet‑cool scenario about 92 g/L. For October (fall) 2025 the model's median estimate is roughly 113 g/L with a plausible range from about 92 to 131 g/L.
The forecast uses the February 27 sampling as a starting condition (volume‑weighted salinity ~115 g/L and South Arm volume ~6,280,000 acre‑feet) and incorporates flows through the new breach, evaporation, monthly inflows and a modeled north‑to‑south flux. Christine said the NRCS peak elevation forecast of about 4,193.8 feet would place the lake between the average and wet‑cool scenarios.
Why it matters: the council is preparing a Comprehensive Management Plan (CMP) that must marry elevation targets with salinity‑sensitive ecological and industrial uses. Committee members said a single, repeatable salinity number that reflects conditions where biota live — the upper brine layer — would be easier to communicate to policymakers and the public than a multi‑point sampling record.
How the number would be calculated: Christine described a candidate method that computes a monthly, volume‑weighted salinity for the top 5 meters of the water column and uses samples from four South Arm monitoring sites (2565, 2267, 2767 and 3510). She noted removing or weighting individual samples (for example, the shallow 2267 site influenced by Bear River inflow) can change the monthly estimate; she demonstrated median/mean, maximum and several subset approaches and showed volume‑weighting closely aligns with the distribution of medians in most months.
Debate and caveats: members flagged the deep brine layer, which when present can hold an estimated median ~12% of South Arm salt mass, and asked whether a single published salinity should include or exclude that reservoir. Christine said the upper‑brine volume‑weighted value best reflects near‑surface conditions affecting pelagic ecosystems, while reporting an accompanying deep‑mass number would show how much salt is sequestered below.
Direct quotes: “Our most recent shallow salinities range from about 108 grams per liter to 116 grams per liter,” Christine said; she also presented the mass estimate: “894,000,000 tons is our most recent measurement.” In framing forecast sensitivity she added that the wide range of likely outcomes “is an interesting observation” and urged caution in over‑interpreting month‑to‑month shifts.
What happens next: staff and USGS will refine the recommended monthly upper‑brine metric and work on display and documentation for the CMP and public dashboard. The council discussed publishing either a single volume‑weighted monthly salinity or two numbers (with and without the deep‑brine contribution) plus an upper‑/deep‑mass breakdown to preserve scientific nuance while giving the public a clear, communicable indicator.
The council also asked USGS to continue refining isomass curves and to explore how the metric will be calculated when a deep brine layer is detected.
