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- in food webs,14 with nearly 80% of all river fish consumption Published: November 8, 2024
- anoxic conditions and the prevalence of anaerobic pro- of a large reservoir complex, identified that 29−31% of the
- cesses.22,25 These conditions establish riparian zones as filter-passing MeHg (f.MeHg) and 38−40% of the particulate
- riparian zone processes in the formation of MeHg is basin.48,50,51 Nearly 85% of the irrigated acreage in the entire
- riparian zones that support a diverse range of vegetation.52 Our (0.3 μM glass fiber filters, Advantec, GF7513MM) and stored
- of the diversions and return waters, making it difficult to inorganic sulfide (S(-II), preserved at 50% volume/volume
- Geological Survey (USGS) Equal Width Index method (n = characterization (SUVA254, spectral slope ratio (SR)). The U.S.
- 1−2 per site, n = 18 total).56 River flow and velocity were Geological Survey (USGS) Reston Stable Isotope Laboratory
- the six major tributaries (n = 1−2 per tributary, n = 7 total) MeHg from total Hg concentrations (eq S1). Gravimetric
- and 24 irrigation return drains (n = 1 per drain) that were Hg(II) and MeHg concentrations were calculated by dividing
- pore water SO42−; n = 2 for pore water NO3−) were organic matter (DOM) specific ultraviolet absorbance at 254 nm
- 3.1. Hydrology. Surface water velocities were measured at leaving a minor unaccounted portion of 5.6% in the hydrologic
- ranged from 0.27 to 0.72 m/s and displayed distinct hydrologic hydrology (Supporting Information, Section S3) estimated the
- 579.2) exhibited fluctuating velocity with a peak of 0.72 m/s, accounting (99.7−101%) of the hydrologic budget along the
- and Section 3 (km 571.7−556.5) showed a notable decrease in Snake River. This full accounting of the Snake River hydrologic
- transect. Although the median Cl− concentrations in the main account for 88.0% of the total DOC load through the reach of
- 24.7 mg/L, respectively) were not statistically different (p- contribute 7.3% of the total load based on measured DOC
- were of lower Cl− concentration thus diluting Cl− in the main 4.7% of the DOC load was not attributed to the surface water
- (2.86 mg NO3− (as N)/L) marginally higher, though not main stem surface waters (109 vs 24.6 mg/L, respectively; p-
- significant in either case (Figure S4B). Median concentrations value < 0.001; Figure S9A), which supports the hypothesis that
- mg/L, respectively) were marginally higher, though not and loads.48 POC concentrations followed a similar increasing
- significant, than the main stem Snake River (47.6 mg/L) trend (2.1-fold increase) along the reach of the synoptic survey
- The measured loads of Cl−, SO42−, and NO3− decreased in tributaries and irrigation drains (1.51 vs 1.95 mg/L,
- Similarly, the cumulative instream loads, which only account waters (0.80 mg/L; Figure S9B). Our load analysis of the
- 86.7%, 73.4%, and 67.0% of the Cl−, SO42−, and NO3− across and POC across the study reach. Only 67.3% of the estimated
- inorganic anions from irrigation drains were assessed using an (main stem Snake River and tributaries, 38.7%) and irrigation
- estimate of irrigation drain discharge along this stretch of the drains (28.6%), with 32.7% of the load unaccounted (Figure
- Snake River (13.7 ± 1.4 m3/s)48 and the average measured S10A). For POC, 64.7% could be accounted for (53.5% from
- the total Cl− load, 15.2% of the total SO42− load, and 20.5% of particulate loads are strongly associated with the dynamics of
- agricultural drains, we can account for 94.3%, 88.6%, and Snake River, as fluctuations in measured loads coincided with
- 87.4% of the increases in loads of Cl−, SO42−, and NO3−, changes in river velocity (Figures S2A and S10). In summary,
- riparian pore waters, tributaries, and irrigation drains, and spatially in We observed only 10.8% unaccounted load in the f.Hg(II),
- (B) main stem surface waters (blue), (C) riparian pore waters (solid with 75.6% attributed to the upgradient Snake River and
- red), and (D) riparian sediments (brown) over 164 km of the Snake tributaries and 13.6% to the irrigation drains (Figure S14A).
- The unaccounted portion of f.Hg(II) (10.8%), though
- 3B). Concentrations of p.MeHg increased dramatically from unaccounted load at 28.5%, with 47.5% of the total load
- the initial upgradient to final downgradient sampling locations, attributed to the upgradient Snake River and 28.9% attributed
- to the irrigation drains (Figure S14B). In summary, load surface waters. For the total MeHg load, 57.9% was from the
- comparisons confirm that hydrologic inputs and particle upgradient Snake River and major tributaries and 7.9% was
- behavior govern filter-passing and particulate Hg(II) in the from the irrigation drains, with 34.2% unaccounted for (1.65
- fractions, the analysis of f.MeHg identified 52.4% load for MeHg Formation. The chemistry of the pore waters
- only 3.1% contributions from the irrigation drains, leaving by a degree of suboxic conditions, sediment chemicals
Methods (brief)
- Figure 1. Map of Snake River synoptic survey conducted in July 2022; markers indicate locations of sample collection of main stem surface and
- absorbance at 254 nm (SUVA254).34 DOM plays a crucial role were collected along the reach of the Snake River to assess if
- nutrients and suspended solids.48,50,51 The irrigated landscape metal samples were also collected. Particulate organic carbon
- and main stem of the Snake River are separated by narrow (POC) and particulate nitrogen (PN) samples were filtered
- a comprehensive understanding of the intricate dynamics At each of the 16 main stem sites, pore water was collected
- 2.2. Sample Collection & Processing. Complete details cores (6.25 cm diameter core barrel, 7−20 cm depth) were
- for sample collection and processing are provided in the collected at each of the 16 main stem sampling locations, and
- when Hg(II) methylation is expected to be relatively high surrogate for organic matter content). Biota samples, collected
- reflects similar riverine characteristics to that of the inflowing 2.3. Sample Analyses. Complete details on sample
- summer months has been observed in many other riverine (Supporting Information, Section S1.2). In brief, water samples
- systems.54,55 The main stem of the Snake River was sampled at were analyzed in the Poulin Lab at the University of California,
- main stem sampling sites, surface water was collected from particulate metals (Fe, Mn), inorganic anions (Cl−, NO3−,
- Current Profiler. Average discharge measurements at each collected glass fiber filters. The USGS Mercury Research
- Q-Rev software.57 A multiparameter sonde (Aqua TROLL ments (total Hg and MeHg) on the water samples (filter-
- (DO), and temperature concurrent with discrete sample content. Inorganic divalent Hg (Hg(II)) concentrations, both
- collection. Surface water samples were also collected from filter-passing and particulate, were calculated by subtracting
- actively discharging into the Snake River at the time of sample their volumetric counterpart concentrations by TSS. Biota
- samples were collected in 2 L polyethylene terephthalate glycol (Corvallis, OR). A comparison of field replicates is provided in
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