Mercury, monomethyl mercury, and dissolved organic carbon concentrations in surface water entering and exiting constructed wetlands treated with metal-based coagulants, Twitchell Island, California

Abstract

Coagulation with metal-based salts is a practice commonly employed by drinking-water utilities to decrease particle and dissolved organic carbon concentrations in water. In addition to decreasing dissolved organic carbon concentrations, the effectiveness of iron- and aluminum-based coagulants for decreasing dissolved concentrations both of inorganic and monomethyl mercury in water was demonstrated in laboratory studies that used agricultural drainage water from the Sacramento–San Joaquin Delta of California. To test the effectiveness of this approach at the field scale, nine 15-by-40 ‑ meter wetland cells were constructed on Twitchell Island that received untreated water from island drainage canals (control) or drainage water treated with polyaluminum chloride or ferric sulfate coagulants. Surface-water samples were collected approximately monthly during November 2012–September 2013 from the inlets and outlets of the wetland cells and then analyzed by the U.S. Geological Survey for total concentrations of mercury and monomethyl mercury in filtered (less than 0.3 micrometers) and suspended-particulate fractions and for concentrations of dissolved organic carbon. In the control wetland cells, total mercury concentrations in filtered water samples ranged from 0.94 to 2.47 nanograms per liter (ng/L) at the control inlets and from 0.84 to 2.63 ng/L at the control outlets, and particulate total mercury concentrations in water ranged from 0.27 to 1.49 ng/L at the control inlets and from 0.17 to 1.11 ng/L at the control outlets. Monomethyl mercury concentrations in filtered water ranged from 0.16 to 0.88 ng/L at the control inlets and from 0.13 to 1.30 ng/L at the control outlets; particulate monomethyl mercury concentrations in water ranged from 0.03 to 0.24 ng/L at the control inlets and from 0.03 to 0.23 ng/L at the control outlets. Dissolved organic carbon concentrations in water ranged from 7.9 to 26.7 milligrams per liter at the control inlets and from 8.5 to 28.0 milligrams per liter at the control outlets. Following coagulation, but prior to passage through the wetland cells, coagulation treatments transferred dissolved mercury and carbon to the particulate fraction relative to untreated source water: at the wetland cell inlets, the coagulation treatments decreased concentrations of filtered total mercury by 59–76 percent, filtered monomethyl mercury by 40–70 percent, and dissolved organic carbon by 65–86 percent. Passage through the wetland cells decreased the particulate fraction of mercury in wetland cells that received coagulant-treated water. Changes in total mercury, monomethyl mercury, and dissolved organic carbon concentrations resulting from wetland passage varied both by treatment and season. Despite increased monomethyl mercury in the filtered fraction during wetland passage between March and August, the coagulation-wetland systems generally decreased total mercury (filtered plus particulate) and monomethyl mercury (filtered plus particulate) concentrations relative to source water. Coagulation—either alone or in association with constructed wetlands—could be an effective way to decrease concentrations of mercury and dissolved organic carbon in surface water as well as the bioavailability of mercury in the Sacramento–San Joaquin Delta.