PNNL-13501 - Pacific Northwest National Laboratory

Biogeochemical Processes and Microbial Characteristics Across Groundwater Surfaces
Study Control Number: PN00014/1421
Jim Fredrickson, David Geist
The hyporheic zone is defined as the region where groundwater mixes with surface waters resulting in distinct
geochemical gradients that can influence the composition and activity of the associated microbiota. In spite of the
potential importance of this zone on the transport of groundwater contaminants into surface waters and exposure to
sensitive biota, very little scientific information is available regarding the microbiology and geochemistry of this
environment.
Project Description
We evaluated a freeze-core method for collecting samples
suitable for microbiological and geochemical analyses
from the groundwater/surface water mixing zones of
large, cobble-bed rivers. The development of effective
methods for collecting samples from the groundwater/
surface water interaction zone will allow researchers to
investigate 1) how microbial communities within the
mixing zones are affected by hydrologic and geochemical
processes occurring at groundwater/surface water
boundaries, and 2) how microbial communities may affect
contaminant transport, and ultimately fate and effects,
across groundwater/surface water boundaries. As a
consequence of developing this unique capability, future
programs can be undertaken to study microbiological and
geochemical processes within these mixing zones that can
provide additional information about how the
geochemistry, hydrogeology, and microbial communities
interact to affect contaminant transport through this
dynamic interface. This will result in an improved
conceptual model of the structure and function of
groundwater/surface water mixing zones and will reduce
the uncertainties inherent in current numerical models of
contaminant transport from groundwater to surface water.
Resulting scientific information will allow for a more
accurate prediction of contaminant fate and transport and
effects on surface water biota.
Introduction
Chemical, physical, and microbiologic processes within
the groundwater and vadose zones beneath the Hanford
Site are being studied by Laboratory scientists to better
understand the fate and transport of contaminants as they
migrate with groundwater in the unconfined aquifer
toward the Columbia River. Previous and current
research has focused on processes in the saturated zone
and more recently, the vadose zone. Although the
200 FY 2000 Laboratory Directed Research and Development Annual Report
fundamental processes that control contaminant fate and
transport in these regions of the subsurface are still poorly
understood, the microbiological, geochemical, and
physical properties of these regions have been reasonably
well characterized. In contrast, there is little or no
information regarding the microbiological and
geochemical characteristics within groundwater/surface
water interaction zones, especially in large rivers such as
the Columbia. This is largely because of the difficulty in
sampling the subterranean environment beneath large,
cobble-bed rivers.
The main objective of this project is to develop and
evaluate methods for collecting samples from the
groundwater/surface water mixing zones suitable for
microbiological and geochemical characterization and to
obtain preliminary scientific information on this zone in
the Columbia River. Presently, techniques do not exist
that can provide relatively undisturbed samples from the
mixing zones beneath large, cobble-bed rivers such as the
Columbia. The results of this effort will be primarily
proof of principle in nature but are also expected to
provide some of the first scientific insights into the
structure and function of microbial communities that
reside in these zones. The development of these
approaches provide a unique research capability within
the DOE system and will enable the development of
future projects and programs investigating processes in
and the characteristics of this zone.
Approach
A modified freeze-core technique was used to collect
sediment blocks within the hyporheic zone. Freeze-core
techniques are frequently used in lotic environments
(streams and rivers) to obtain in situ samples of benthic
fauna within stream-bed sediments and to characterize
sediment size distribution. In the modified freeze-core
technique, three tubes are driven into the sediments in a