Increases in mercury emissions from desert soils in response to rainfall and irrigation

As part of an international Hg flux intercomparison at the Steamboat Springs, Nevada, geothermal area, several dynamic soil flux chambers and micrometeorological gradient systems were operated over desert soils in early September 1997. A series of unanticipated convective rain cells impacted the site with the first rainfall in {approximately}90 days, and the initial 4-cm rainfall increased soil moisture from {approximately}0.01 to 0.06{percent} (vol/vol). Several chambers were operating prior to the events, and two were deployed over wet soils following rainfall. Rainfall resulted in an immediate and steep rise in ambient air Hg concentrations and soil Hg emissions which persisted for 12{endash}24 hours. Fluxes increased most quickly and to a greater degree over the wettest soils, and the rate of increase was related to chamber design and flushing rate. The flux response was also apparent in the micrometeorological data. In general, soil emissions increased by an order of magnitude following the rain, and reached levels {approximately}6 times above those at the same time the previous day. These fluxes were significantly correlated with temperature, radiation, humidity, wind speed, and soil moisture. After drying for {approximately}40 hours, selected soil plots were manually irrigated with low-Hg-distilled water. Mercury emissions responded similarly across the threemore » treated sites, uniformly increasing from {approximately}60 ng m{sup {minus}2} h{sup {minus}1} pretreatment to {approximately}650 ng m{sup {minus}2} h{sup {minus}1} posttreatment, which was a factor of {approximately}6 higher than adjacent control soils. Possible causes of the increases in flux include soil gas displacement, desorption of Hg{degree} by water molecules, and desorption of Hg(II) and subsequent reduction in solution. The kinetics of the flux response, combined with local soil and climatic conditions, suggest that Hg emissions were responding primarily to soil moisture and solar radiation. These data have interesting implications for the role of changing regional climates on biogeochemical cycling of Hg.« less