ARSENIC MOBILIZATION AND DISPERSION FROM AN ARSENIC-SULFIDE ENRICHED BIOTITE GNEISS

Arsenic up to 6940 mg/kg in soil in a farm field, and 310 µg/L in residential wells down gradient, were discovered in Warren County, NJ in 2003-2004 and 2008-2009 respectively. The impacted area is in the Highlands Physiographic Province and locally consists of an arsenic sulfide mineralized Precambrian biotite gneiss in the upland and Paleozoic non-carbonate and carbonate rocks in the valley. Interestingly, furnace slag and mine waste occur locally as ore deposits of magnetite were actively mined and smelted. It was therefore uncertain if the As in soil and groundwater is completely natural in origin. In 2008 the NJDEP-New Jersey Geological Survey investigated the sources, mobilization and transport of As there. This involved reviewing existing data and collecting and analyzing water, soil and rock samples. Samples from 3 monitor wells in slag waste had only a maximum of 11 ppb As and therefore was not a major source. Backhoe excavations under the As hot spots in the farm field exposed saprolite with relict gneissic foliation a few feet below the surface. Split spoon sampling revealed a relict pebble in the saprolite at approximately 50 feet below grade containing arsenopyrite (AsFeS) and Lollingite (FeAs2) in a matrix of feldspar, quartz and unweathered and weathered biotite. Saprolite tested via EPA SPLP showed that As is readily mobilized at up to 50 ppb. Electron analytical analysis of the pebble revealed biotite with iron oxide weathering products rimming sheet edges and isolated segregations within the sheets containing up to 4 weight percent As. Soil with similar As enriched biotite was found in yards of residences with well water enriched in As and Sb. It was proposed that the geologic formations underlying the farm field containing As bearing sulfide minerals extend under the area of affected wells. A rock core collected adjacent to those homes by NJGS in 2011 confirmed that and therefore the well boreholes are open in As-sulphide enriched metamorphic rock. It is proposed that when those sulfide minerals oxidize, As is mobilized. Much of that As adsorbs onto the secondary iron oxides associated with biotite and some is transported directly into groundwater. This As enriched soil may have been a secondary source of As to groundwater due to phosphate application in the upgradient farm field.