Abstracts

151 - Prevalence of Arsenic Enrichment Near the Sediment-Water
Interface Along Shorelines of the Great Lakes
Sabina Rakhimbekova, Clare Robinson, & Denis O’Carroll
Department of Civil and Environmental Engineering – University of Western Ontario,
London, Ontario, Canada
The mobility of arsenic in groundwater is controlled by interacting biogeochemical and
hydrological processes – these processes are complicated near surface water-groundwater
interfaces due to the mixing of two distinct water entities. The mixing of anoxic groundwater
with toxic lake water recirculating across the sediment-water interface sets up a reaction
zone characterized by sharp pH and redox gradients where a range of chemicals (e.g.,
arsenic) undergo important transformations. While prior research has revealed arsenic
enrichment near the sediment-water interface along the shores of the Great Lakes (up
to 56 µg/L), there is limited understanding of the ultimate source of arsenic, the physical
and geochemical processes governing the arsenic behaviour and if arsenic enrichment in
beach aquifers is a naturally occurring widespread phenomenon. This paper presents pore
water and sediment chemistry data obtained near the sediment-water interface at a wide
range of permeable Great Lake shorelines. Elevated As concentrations at a number of field
sites indicates the process may be naturally occurring and suggests that lake water with
trace concentrations of arsenic may deliver arsenic to the beach aquifer as it recirculates
across the sediment-water interface. Arsenic may then be sequestered by metal oxides that
precipitate below the beach face in distinct redox zones. The accumulation of arsenic in
the beach aquifer creates a potential risk of elevated arsenic concentrations being rapidly
released back to the surface water under favorable hydrologic and geochemical conditions
(e.g., high organic loading from algae washup). The paper also explores potential scenarios
that may cause mobilization of arsenic from the aquifer and subsequent release to adjacent
surface waters.
152 - Quantification of groundwater discharge along shorelines of
the Great Lakes using Radon-222
Tao Ji & Clare Robinson
Department of Civil and Environmental Engineering – Western University, London,
Ontario, Canada
Groundwater discharge can be a significant pathway for transporting pollutants, including
nutrients, organic contaminants and trace metals, into nearshore waters of the Great
Lakes. Groundwater as a contributor of nutrients to the Great Lakes has received increased
attention in recent years, particularly with respect to the potential for onsite sewage
systems to be a source of elevated nutrient levels in nearshore waters. There is, however,
limited understanding and methods available to quantify the groundwater discharge to the
Great Lakes, including regional-scale methods to identify hotspot areas of discharge. Radon
( 222 Rn, t 1/2
= 3.8 days) is a suitable natural tracer for estimating groundwater discharge
because it is a conservative gas and its activity is typically 3-4 orders of magnitude higher
in groundwater than in surface water. Regional-scale 222 Rn surveys were conducted along
160 IAH-CNC 2015 WATERLOO CONFERENCE