Investigating groundwater-surface water processes in a Chalk catchment in South East England using fluorescence properties of dissolved organic matter

Understanding the hydrochemical functioning of lowland permeable catchments in the
UK is important for effective environmental management of river and wetland ecosystems.
Anthropogenic tracers such as CFCs and SF6 have been useful tools in providing
a framework for understanding groundwater flow and mixing processes within
a part of the Lambourn Chalk catchment at Boxford, South East England. The fluorescence
properties of groundwater and surface water samples from the site were
examined to investigate the use of dissolved organic matter (DOM) as a natural tracer
to better understand groundwater-surface water processes. Fluorescence centres were
observed in fulvic acid-like, aromatic protein and microbial by-product like regions
of the emission-excitation matrix in both groundwater and surface water samples. A
decrease in the fluorescence intensities of the fulvic acid-like material was observed
with depth (down to 25m) in the Chalk interfluve and adjacent to the river highlighting
the role of the soil zone as an important source of DOM. Groundwater from chalk
beneath the gravels show fluorescence signatures similar, although less intense, to that
of the river indicating a degree of mixing between water bodies. Fluorescence centres
from groundwater in the gravels adjacent to the river show less of a river signature and
are therefore perhaps partially isolated with the river system. These findings corroborate
the conceptual model of groundwater movement and demonstrate the potential of
intrinsic fluorescence as a natural tracer for investigating groundwater-surface water
interactions.