4.2. Groundwater – surface water interactions
Excavation of spawning areas to recover and transplant ova formed part
of experimental work in the 1990s to test whether transplanting ova from
areas of high to low spawner density might increase juvenile production
through reductions in density dependent mortality (Youngson and McLaren,
1998). Indeed, some have advocated for similar approaches more recently,
with a focus on moving wild salmon fry rather than ova (Young, 2017).
Surprisingly high ova morality was found at some of the most heavily
used, and apparently suitable, spawning sites on the Girnock (Malcolm et
al., 2005). It was initially hypothesised by fisheries researchers that
fine sediment infiltration was causing anoxic conditions through low
interstitial velocity in redds. However, the “fine” sediment fraction
in the Girnock was found to be comprised mainly of sand (Moir et al.,
2002). Furthermore, interstitial velocities were often high and not
clearly correlated with oxygen concentrations (Malcolm et al., 2011).
Subsequent process-based investigations, including the first use of
continuous optical dissolved oxygen sensors capable of being deployed in
the hyporheic zone (the streambed interface between surface water and
groundwater), showed that some spawning sites were vulnerable to anoxic
conditions where chemically reduced groundwater discharges occurred
through the stream bed (Malcolm et al., 2004a; 2006). These areas of
groundwater discharge coincided with valley constrictions that forced
upwelling groundwater into reaches of the stream where sedimentary
conditions were good for spawning but provided sub-optimum habitat in
terms of sufficient oxygen levels to sustain ova (Malcolm et al., 2005).
Such conditions were shown to be worse in wetter winters, when
groundwater fluxes were higher, and less prevalent in drier winters when
well-oxygenated stream water dominated the hyporheic zone (Soulsby et
al., 2008). So, it seems spawning site selection in the Girnock
represents a trade-off between optimal sedimentary conditions and the
risk of de-oxygenation in some years.
These local investigations into the hydrology of specific spawning
reaches were subsequently placed in a catchment-scale context through
characterisation of deeper groundwater flow paths (Fig. 6) as part of
hillslope hydrology research in the catchment. This has used a
combination of synoptic surveys of environmental tracers in springs and
streams (Soulsby et al., 2007; Blumstock et al., 2015; Scheliga et al.,
2017); sampling deeper and shallower wells (Blumstock et al. 2016;
Scheliga et al., 2018, 2019), geophysical mapping (Soulsby et al.,
2016b) and modelling (Ala aho et al., 2017). Much of this work focused
in the smaller, more accessible 3.2km2 Bruntland Burn
sub-catchment of the Girnock, with broadly similar overall landscape
properties (topography, soils, land use etc.) and hydrology where
logistical challenges to monitoring are less severe (Birkel et al.,
2014).
These studies have shown extensive groundwater storage in glacial
deposits in the Girnock which in valley bottom areas can be
~ 40m deep. Generally, groundwater circulation here is
slow due to the low permeability of these drifts and deeper groundwater
is probably not well-connected to streams. Coarser, shallower drifts on
the hillslopes have higher permeability and more dynamic groundwater
responses. These appear to be fed by fracture networks in the granite
and other rocks exposed on the catchment interfluves which are activated
in wetter periods (Scheliga et al., 2017, 2019). Groundwater chemistry
is highly variable reflecting geology and residence times; but is mostly
strongly alkaline from calcareous rocks in the drifts (Soulsby et al.,
2007). Groundwater in granite is less alkaline (Soulsby et al., 1998).
The spatial and temporal variability of groundwater inputs has a strong
influence on the chemistry of the stream, and the chemistry of the
hyporheic zone (and redd environment) depends on local
hillslope-aquifer-stream connectivity (Malcolm et al., 2005; Soulsby et
al., 2009).