EVOLVING ARSENIC AND SELENIUM TRENDS BENEATH A DRAINED TERRESTRIAL MARSH

An evolving groundwater flowpath was studied beneath a historic marshland. Located in San Diego Creek Watershed of Orange County California, the historic "Swamp of the Frogs Marsh" was drained in the late 1800s. Today, groundwater beneath the historic marsh is suboxic to oxic. Groundwater was sampled at nine points along the groundwater flowpath. Along the first half of the 1 km flowpath, groundwater moves through the outer fringes of the historic swamp. There, groundwater salinity does not change in concentration and remains slightly saline, whereas arsenic increases from 13 ug/L to 90 ug/L dissolved As. Selenium increases from 50 to 228 ug/L dissolved Se in this upper region. Over 98 percent of the arsenic and selenium along the first half of the flowpath is in the form of arsenate and selenate, the most oxidized forms of these ions. Iron and manganese are not detectable, and almost all of the nitrogen is in the form of nitrate. Along the lower half of the flowpath, groundwater moves into and through the interior of the historic marsh. There, salinity doubles and we observe a correlative increase of chloride, bromide, sulfate, and arsenic; the latter reaching 196 ug/L dissolved As. Selenium decreases substantially to about 60 ug/L Se along the lower flowpath. Over 98 percent of the arsenic is arsenate, but only 85 percent of the selenium is selenate in the lower flowpath region, the rest is present at selenite, an intermediate redox form of Se. Organic forms of arsenic and selenium are not detected along the full flowpath. There is a small amount of manganese (<70 ug/L) and trace amounts of ammonium in the lower flowpath area, but no detectable iron. Oxygen and deuterium isotope values do not change along the full flowpath, eliminating pure evaporation as a possible explanation for salinity and arsenic enrichment in the lower flowpath area. Based on our data, we propose a model of selenium oxidation and arsenic desorption (but not reductive dissolution) from sites on iron and aluminum oxides and clays within the upper part of the flowpath, and leaching of evaporative efflorescent salts (containing arsenate) in the lower flowpath region. Geomorphic and geochemical analysis suggest that the lower flowpath region, in the interior of the historic marsh, was once a phreatic salt playa.