Catchment tracers reveal discharge, recharge and sources of groundwater-borne pollutants in a novel lake modelling approach
Authors: Kristensen Emil , Madsen-Østerbye Mikkel , Massicotte Philippe , Pedersen Ole , Markager Stiig , Kragh Theis 
Published: February 2018

Abstract. Groundwater-borne contaminants such as nutrients, dissolved organic carbon
(DOC), coloured dissolved organic matter (CDOM) and pesticides can have an
impact the biological quality of lakes. The sources of pollutants can,
however, be difficult to identify due to high heterogeneity in groundwater
flow patterns. This study presents a novel approach for fast hydrological
surveys of small groundwater-fed lakes using multiple groundwater-borne
tracers. Water samples were collected from the lake and temporary groundwater
wells, installed every 50 m within a distance of 5–45 m to
the shore, were analysed for tracer concentrations of CDOM, DOC, total
dissolved nitrogen (TDN, groundwater only), total nitrogen (TN, lake only),
total dissolved phosphorus (TDP, groundwater only), total phosphorus (TP,
lake only), δ18O ∕ δ16O isotope ratios
and fluorescent dissolved organic matter (FDOM) components derived from
parallel factor analysis (PARAFAC). The isolation of groundwater recharge
areas was based on δ18O measurements and areas with a high
groundwater recharge rate were identified using a microbially influenced FDOM
component. Groundwater discharge sites and the fractions of water delivered
from the individual sites were isolated with the Community Assembly via Trait
Selection model (CATS). The CATS model utilized tracer measurements of TDP,
TDN, DOC and CDOM from the groundwater samples and related these to the
tracer measurements of TN, TP, DOC and CDOM in the lake. A direct comparison
between the lake and the inflowing groundwater was possible as degradation
rates of the tracers in the lake were taken into account and related to
a range of water retention times (WRTs) of the lake (0.25–3.5 years in
0.25-year increments). These estimations showed that WRTs above 2 years
required a higher tracer concentration of inflowing water than found in any
of the groundwater wells around the lake. From the estimations of inflowing
tracer concentration, the CATS model isolated groundwater discharge sites
located mainly in the eastern part of the lake with a single site in the
southern part. Observations from the eastern part of the lake revealed an
impermeable clay layer that promotes discharge during heavy precipitation
events, which would otherwise be difficult to identify using traditional
hydrological methods. In comparison to the lake concentrations, high tracer
concentrations in the southern part showed that only a smaller fraction of
water could originate from this area, thereby confirming the model results.
A Euclidean cluster analysis of δ18O isotopes identified
recharge sites corresponding to areas adjacent to drainage channels, and
a cluster analysis of the microbially influenced FDOM component C4 further
identified five sites that showed a tendency towards high groundwater
recharge rate. In conclusion, it was found that this methodology can be
applied to smaller lakes within a short time frame, providing useful
information regarding the WRT of the lake and more importantly the
groundwater recharge and discharge sites around the lake. Thus, it is a tool
for specific management of the catchment.