Research about Cache Creek Settling Basin

USGS2026

Transient methylmercury formation during wetland soil rewetting is governed by organic matter availability rather than coagulant history

Rewetting of previously drained wetland soils can induce rapid redox shifts that influence mercury (Hg) cycling and methylmercury (MeHg) production. We investigated how prior exposure to common metal-salt and biopolymer coagulants influences Hg transformations during wetland soil rewetting and whether organic matter (OM) availability modulates these effects. Wetland soils (∼300 µg kg −1 total Hg) were incubated in triplicate soil-water microcosms for 16 days following rewetting. Treatments included soils previously exposed to ferric sulfate, polyaluminum chloride, or a chitosan-based coagulant, alongside untreated controls. Upon rewetting without additional OM, redox potential declined to ∼250 mV, consistent with manganese-reducing conditions, and MeHg concentrations remained relatively low in water (0.05–0.18 ng L −1 ) and soils (1–2 µg kg −1 ) across all treatments. In contrast, addition of algal-derived OM (Spirulina powder) rapidly decreased redox potential to 0–100 mV and produced a short-lived pulse of MeHg formation in all treatments, with aqueous MeHg peaking at ∼30–40 ng L −1 and soil MeHg at ∼6–8 µg kg −1 on days 2–4 before declining. Coagulant treatments increased transient inorganic Hg concentrations in overlying water (45–75 ng L −1 ) relative to controls (∼30 ng L −1 ) but did not enhance higher MeHg production. Results demonstrate that OM-driven redox dynamics, rather than prior coagulant exposure, control transient MeHg production during wetland soil rewetting, highlighting how episodic carbon inputs can regulate mercury transformation in managed and natural wetlands.

California↗

USGS2017

Geochemistry of mercury and other constituents in subsurface sediment—Analyses from 2011 and 2012 coring campaigns, Cache Creek Settling Basin, Yolo County, California

Cache Creek Settling Basin was constructed in 1937 to trap sediment from Cache Creek before delivery to the Yolo Bypass, a flood conveyance for the Sacramento River system that is tributary to the Sacramento–San Joaquin Delta. Sediment management options being considered by stakeholders in the Cache Creek Settling Basin include sediment excavation; however, that could expose sediments containing elevated mercury concentrations from historical mercury mining in the watershed. In cooperation with the California Department of Water Resources, the U.S. Geological Survey undertook sediment coring campaigns in 2011–12 (1) to describe lateral and vertical distributions of mercury concentrations in deposits of sediment in the Cache Creek Settling Basin and (2) to improve constraint of estimates of the rate of sediment deposition in the basin. Sediment cores were collected in the Cache Creek Settling Basin, Yolo County, California, during October 2011 at 10 locations and during August 2012 at 5 other locations. Total core depths ranged from approximately 4.6 to 13.7 meters (15 to 45 feet), with penetration to about 9.1 meters (30 feet) at most locations. Unsplit cores were logged for two geophysical parameters (gamma bulk density and magnetic susceptibility); then, selected cores were split lengthwise. One half of each core was then photographed and archived, and the other half was subsampled. Initial subsamples from the cores (20-centimeter composite samples from five predetermined depths in each profile) were analyzed for total mercury, methylmercury, total reduced sulfur, iron speciation, organic content (as the percentage of weight loss on ignition), and grain-size distribution. Detailed follow-up subsampling (3-centimeter intervals) was done at six locations along an east-west transect in the southern part of the Cache Creek Settling Basin and at one location in the northern part of the basin for analyses of total mercury; organic content; and cesium-137, which was used for dating. This report documents site characteristics; field and laboratory methods; and results of the analyses of each core section and subsample of these sediment cores, including associated quality-assurance and quality-control data.

California↗