Influence of groundwater-lake interactions on the subsurface mobility of arsenic at beaches of the Great Lakes

The role of groundwater-lake interactions in the cycling of trace elements is not well understood in the Great Lakes environment. This paper presents field measurements and numerical modeling that provide insight into the geochemical conditions and groundwater flows controlling the accumulation and mobility of arsenic in permeable nearshore aquifers and its potential discharge to the Great Lakes. Field data were collected via shore-normal monitoring transects installed at beach sites on Lake Erie and Lake Ontario. Detailed pore water chemistry analyses revealed elevated arsenic (up to 0.056 mg/L) 1 - 2 m below the shoreline at all beach sites. The groundwater discharge flux was also highest at the shoreline at all sites. The distributions of species in the aqueous and sedimentary phases suggest that arsenic mobility is strongly linked with iron redox cycling. A layer of iron (hydr)oxides was present at the sediment-water interface near the shoreline and this layer may be accumulating arsenic and preventing its release to nearshore waters. Numerical modeling combined with field measurements indicate that wave-induced recirculation across the aquifer-lake interface can be significant and this recirculation may set up the redox gradient that causes iron (hydr)oxides to precipitate below the shoreline. Although widespread across multiple beach sites, the source of arsenic remains unknown. If naturally occurring, however, the accumulation of arsenic in beach aquifers may have important implications for arsenic cycling in the Great Lakes particularly as changing lake hydraulic conditions may lead to the release and subsequent discharge of arsenic to nearshore waters. The interacting nearshore geochemical and hydrological processes examined are pertinent to quantifying the flux of other chemicals (e.g., nitrate, ammonium, phosphorous) to the Great Lakes via the groundwater pathway.