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05 - INVESTIGATING WATER FLOW AND CONTAMINANT PATHWAYS TO RIVERS USING CHEMICAL
HYDROGRAPH SEPARATION
Jenny Deakin1*, Bruce Misstear, B.1, A. Murphy1, M. Dowling1 and Ray Flynn2
1
Department of Civil, Structural and Environmental Engineering, Trinity College Dublin
2
School of Planning, Architecture and Civil Engineering, the Queen’s University of Belfast
ABSTRACT
Diffuse contaminants originating on the land surface can make their way into rivers via a
number of different pathways, including overland flow, interflow, shallow groundwater flow
and deep groundwater flow. Identification of the key pathway(s) delivering contaminants to
a receptor, for example a river, is of considerable importance for implementing successful
water management strategies. Traditionally, physical hydrograph separation methods, such
as master recession curve analysis and digital filtering methods, have been used to
determine the proportion of flow that contributes to streams via the different pathways.
However these methods have significant limitations. Chemical separation of stream event
hydrographs can be used to supplement these more traditional approaches to reduce
uncertainties and provide more robust conceptual models. The underlying premise is that
groundwater with a relatively long residence time has a much higher mineral content (e.g.
silica) than rainfall, while water derived principally from overland or near surface flow may
be higher in other constituents, such as organic carbon, than groundwater. If a distinct,
stable chemical signature can be derived for each of the end members during a rainfall
event, then the proportion of flow from each of the different pathways can be determined.
This technique has been applied only rarely in Ireland thus far, but has been widely used in
the UK and further afield. Various natural chemical tracers such as silica, chloride, dissolved
organic carbon and other major ions, and contaminants (as artificial tracers) such as nitrate
and phosphate, are being trialed in four Irish catchments as part of the ‘Pathways Project’, a
project funded under the EPA STRIVE Programme to develop a national catchment
management tool. The tool will enable practitioners to identify the critical source areas for
diffuse contaminants, and the key pathways by which they travel to receptors.
INTRODUCTION
The flow in a stream or river is derived from rainfall that travels through its catchment via a
number of different pathways, through or over geological materials (bedrock, subsoils and
soils). These pathways typically include overland flow, interflow and baseflow, although the
precise definition of these terms varies across the literature.
Determining the contribution of the different hydrological pathways to the total flow in a
stream is an important factor in understanding the stream’s origin and characteristics, and
hence its water quality. Water moving along the different pathways assimilates different
minerals, organic matter, nutrients and other contaminants, depending on the geological
characteristics of the pathway, the residence time, and the contaminants it encounters
along the way. Different parts of a catchment and different pathways, will contribute
different quantities of flow and contaminants to the stream than others. Understanding
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flow and contaminant transfer along the pathways within a catchment is critical for
designing and implementing cost effective aquatic contaminant management strategies.
The Contaminant Movement along Pathways Project (“Pathways Project”) is an EPA STRIVE
funded project led by the Queen’s University of Belfast (QUB), partnered with Trinity College
Dublin (TCD) and the University College Dublin (QUB). The project will address the challenge
of integrating knowledge of hydrological processes and water-borne contaminant fate and
transport via each of the different pathways, to assess their impact on aquatic ecosystems.
These findings will be used to develop and populate a catchment management tool.
Traditionally, physical hydrograph separation methods have been used to provide insights
into the contribution of different pathways to streams, but as reported at this conference in
2009 by Misstear et al., there are several limitations with these methods (see also Misstear
and Fitzsimons, 2007; and Misstear and Brown, 2008). Increasingly, albeit mainly in
countries outside Ireland, chemical mixing models and hydrograph separation methods
have been used to support traditional hydrograph separation methods using physical data.
This paper looks at these chemical methods and reports on some initial findings from their
application in Ireland. Use of the methods will be further progressed in Irish conditions as
part of the Pathways Project.
DEFINITION OF PATHWAYS
There are a number of different definitions in the literature for the various pathways
contributing flow to a stream. The definitions being used in the Pathways project are shown
in Fig. 1 and are summarised below from Archbold et al (in press).
Fig. 1. Components of flow contributing to a stream in poorly productive (left), and productive
(right) bedrock aquifer settings. Low or high permeability subsoil may overlie either bedrock
aquifer type. This conceptual model was developed by the Working Group on Groundwater
(reported in RPS, 2008).
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Overland flow is generally regarded as sheet flow occurring on the land surface (Shaw,
1994). It is sometimes termed surface runoff or direct runoff and produces a rapid response
in a stream hydrograph. The most common type of overland flow in temperate climates
such as Ireland’s is flow that results only after the soil becomes saturated; this is usually
termed saturation excess overland flow (Nash et al., 2002).
The definition of the term interflow (also commonly referred to as throughflow) is
particularly variable in the literature. Notwithstanding this, interflow can generally be
considered to describe any lateral subsurface flow that occurs between the ground surface
and the water table (Dingman, 2002; Nash et al., 2002; Chin, 2006). As such, interflow can
occur in both the topsoil and subsoil, and may include unsaturated matrix flow, bypass or
macropore flow, saturated flow (from locally perched water tables) and possibly artificial
field drainage.
Overland flow and interflow together are sometimes referred to as quickflow (Chin, 2006),
although some interflow pathways are clearly more rapid than others. In contrast to these
relatively rapid flow components, the term baseflow is used to describe the ‘slow flow’
contribution to the river hydrograph from groundwater discharge. Dingman (2002)
describes baseflow as ‘water that enters [the stream] from persistent, slowly varying
sources and maintains streamflow between water input events’.
The definition of baseflow developed for the Irish context by the Irish Groundwater Working
Group (reported in RPS, 2008) includes shallow groundwater flow, conceptualized as
occurring within the top few metres of weathered and fractured bedrock, and considered to
be the main groundwater pathway in poorly productive aquifers. It also includes the deep
groundwater flow that occurs in the main body of the bedrock aquifer, which is of greater
importance as a flow pathway in productive (especially regionally important) aquifers. Deep
groundwater flow is considered to be equivalent to the long-term sustainable yield of a
groundwater flow system.
Discrete fault or conduit flow was added as a further pathway in the conceptual model in
recognition of the fact that large faults or conduits (for example in karst aquifers) can
transmit larger quantities of groundwater than the surrounding areas of less fractured
bedrock (RPS, 2008). Finally, flow through artificial agricultural drains (drain flow) is also
being included to consider impacts via that pathway on the contaminant water balance.
TRADITIONAL PHYSICAL HYDROGRAPH SEPARATION STUDIES
The early methods of separating out the quickflow and baseflow contributions to the river
hydrograph were based on graphical analysis of the hydrograph. Different methods such as
the Constant Slope method, or Concave method, simply connected a point at the beginning
of the rising limb to the end of the falling limb using different inflection points of the
recession curve, with flow above the line being considered the quickflow component. The
Barnes method (Barnes, 1939) distinguishes three components of flow on a semilogarithmic plot of log Q versus t.
Master recession curve analysis looks at the different points along an average recession
curve where the trend changes and the different flow components cease to contribute.
Master recession curve analysis (using the matching strip and tabulation methods) was
employed in a major recent Irish study concerned with groundwater and surface water
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interactions to distinguish the baseflow (deep groundwater) contributions to river flow (RPS,
2008).
Automated filtering separation techniques, such as the Institute of Hydrology method (IoH,
1980 and 1989) and the Boughton method (Boughton, 1995) are also widely used. These
methods have the advantage that they provide rapid, objective and repeatable means of
estimating the baseflow index, although they are relatively arbitrary and are not based on
hydrological principles (Nathan and McMahon, 1990).
Insights into flow pathway contributions can also be gained from frequency analysis of river
flows, including preparation of flow duration curves which show the percentage of time that
a particular flow is exceeded or equalled. Quantitative indices, such as the Q90/Q50 ratio,
have been developed to indicate the contributions from groundwater storage (Nathan and
McMahon, 1990; Brodie and Hostetlier, 2005). Qualitative indications of the relative
importance of baseflow contributions between different catchments can also be gleaned
from the shape of the flow duration curves: if the section of the flow duration curve below
the median flow (Q50) has a shallow slope, this indicates a continuous baseflow
contribution to the river, whereas a steep curve indicates comparatively smaller
contributions from groundwater storage (Brodie and Hostelier, 2005). More generally, the
Q95 flow has been employed as a rule of thumb to permit comparison of baseflow regimes
from different stream/river discharge monitoring points.
However, all the approaches for physical hydrograph analysis have significant limitations
(Archbold et al, in press), including:
• recession curve analysis assumes a linear response, whereas parts of the flow system
may behave in a non-linear manner;
• the distinction between baseflow and interflow is often arbitrary;
• bank storage effects often are not taken into account;
• analytical equations such as the Boughton 2-parameter algorithm involve subjective
decisions about the choice of values for the empirical constants;
• digital filtering techniques, in the words of Nathan and McMahon (1990) are ‘just as
arbitrary and as physically unrealistic as, say, the separation of baseflow based on a
series of straight lines’; and
• indices based on frequency analysis do not necessarily correlate strongly with flow
processes.
These limitations were discussed at the 2009 National Hydrology Conference by Misstear et
al. (2009), and are also reported in Misstear and Brown (2008) and Misstear and Fitzsimons
(2007).
CHEMOGRAPHS AND HYDROCHEMICAL MIXING MODELS
Analysis of river flow hydrograph data can be enhanced by incorporating an understanding
of the changes in river water chemistry with changes in flow. Changes in isotopes, sediment
and micro-organisms also provide insights but are not further discussed here.
The application of water chemistry and natural chemical tracers for hydrograph separation
is a technique that has been around for some time (e.g. LaSala, 1967; Pinder and Jones,
1969). It is currently widely used in the UK, particularly in Scotland and Wales, but it has not
been applied often in Ireland. The approach depends on the principle that as rainfall, which
is chemically relatively pure, infiltrates through the landscape, the solute concentration
increases with increased residence time and contact with geological materials. The
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contribution from different land uses and artificial human impacts also plays a role. As a
rainfall event progresses in a stream catchment, the water chemistry of the stream changes,
much as the flow changes, with varying contributions from each of the different flow
pathways. Regular or continuous water sampling throughout a rainfall event allows
chemographs to be generated that can then be related to the changes in flow.
Different hydrochemical characteristics can often be associated with flow along each of the
different pathways. For instance, overland flow might be relatively high in organic carbon
which is found in the shallow soil layers, but low in mineral content due to the relatively
rapid travel times to the stream. The chemical makeup of deep groundwater on the other
hand, will more usually reflect the higher degree of mineralisation due to the longer contact
time with the bedrock, with a lower degree of organic content which has usually been
removed by the time the water reaches the stream.
Fig. 2 shows an example of a chemograph from an event in January 2010 in the White River
in Co. Louth, where silica and conductivity concentrations were measured hourly with
continuous water level monitoring. The graph reflects the dilution of the silica and electrical
conductivity in the stream as the water levels rose, and as the proportion of less mineralised
quickflow increased. As water levels decreased there was a subsequent increase in both
chemical tracer concentrations once the quickflow subsided and the more mineralised
baseflow component became dominant again.
Simple hydrochemical mixing models can be generated to help characterise and
conceptualise the components of flow derived from different pathways. Fig. 3 shows a
hydrological mixing diagram generated by Soulsby et al. (2003) for a small agricultural
catchment in northeast Scotland. They used silica and nitrate to distinguish the
contributions of flow from three different pathways: overland flow, shallow subsurface flow
via artificial agricultural drains, and groundwater.
Figure 2: Silica and electrical conductivity changes with water level changes during a rainfall event
in the White River catchment, Co. Louth in January 2010.
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In their catchment, they found that overland flow was dominated by waters that were low
in silica and intermediate in nitrate; groundwater flow was also intermediate in nitrate but
was high in silica; while subsurface drain flow was high in both nitrate and silica. The mixing
model shows the distinctive chemical characteristics of flow from each the three pathways,
sometimes also known as end members. If the characteristics of the end members can be
identified, their proportional contribution to the stream can then be determined from the
hydrograph by carrying out a chemical hydrograph separation, provided that their
compositions remain relatively constant throughout a hydrological event.
Fig. 3. An example of a three component hydrological mixing diagram for stream flow samples in
an agricultural catchment in northeast Scotland using nitrate and silica as tracers. Three
hydrological pathways were identified: overland flow (OF), subsurface flow (SSF) and groundwater
(GW) (from Soulsby et al., 2003).
CHEMICAL HYDROGRAPH SEPARATION
The literature shows that hydrograph separation based on chemical, or other water quality
parameters is predominantly used for separation into two or three components, but
occasionally up to five (Uhlenbrook and Hoeg, 2003). Two-component separations usually
separate hydrographs into event or direct runoff, and pre-event or subsurface waters, whilst
three-component separations introduce a third soil water component that can be
sufficiently chemically distinguished from the other two. This third component is typically
loosely referred to as intermediate flow and can have a variety of definitions, the primary
one being that it is different to overland flow and baseflow (Archbold et al, in press).
The hydrograph separation method uses a simple mass balance equation to quantify the
contributions of different stream flow components to the total flow (Pinder and Jones,
1969). The classic two component mixing equation is as follows:
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QtrCtr = QdrCdr + QgwCgw
where Qtr is the total runoff; Qdr is the direct runoff; and Qgw is the groundwater runoff; and
Ctr is the tracer concentration in total runoff; Cdr is the tracer concentration in direct runoff;
and Cgw is the tracer concentration in groundwater. This equation can be modified to
incorporate additional stream flow components, as described in Ogunkoya and Jenkins
(1993):
QtrCtr = Q1C1 + Q2C2 + ... + Qm Cm
where the subscripts 1, 2, … m, are the various assumed components of the total runoff. The
number of components to be distinguished dictates the number of tracers that need to be
used: n tracers are needed to determine n+1 stream flow components with the equation
being simultaneously solved for each.
Tracers must be conservative, must distinguish the different stream flow components
chemically, and must not be impacted to a significant degree by chemical or biological
processes at the scale or timescale of interest (Hooper and Shoemaker, 1986). The scale and
timescale of interest are important as they allow certain exceptions to be made for the
purposes of particular studies, which would not otherwise hold true. Soulsby et al. (2003)
for instance, used nitrate as a tracer and whilst they admit it is not completely conservative,
they, and other authors (e.g. Durand and Torres, 1996) have found it can be assumed to be
conservative in some catchments during storm episodes where rapidly changing flow paths
are the main control on stream chemistry.
Chemical hydrograph separation studies in Ireland
The authors are only aware of three completed known studies in Ireland to date that have
employed chemical tracer methods for baseflow analysis, although there are a number of
studies currently underway through the Pathways Project and the Agricultural Catchments
Programme being carried out by Teagasc.
The most comprehensive of the studies was a doctoral study carried out at the University of
Ulster by Dellwyn Kane (2009) in the Oona catchment in Co. Tyrone, in which three end
members were distinguished: overland flow, shallow soil water (~10 cm) and deep soil
water (~30-40 cm), using silica and the spectral absorbance coefficient at 254 nm1. Kane
sampled at two catchment scales (~0.1 and 5 km2) during five storm events and looked in
some detail at the statistical uncertainties in the methodology. He then used the results to
develop a conceptual model for the hydrology of drumlin morphology areas such as large
parts of the Oona catchment.
The two smaller studies were carried out by undergraduate civil engineering students at
Trinity College Dublin. The first was conducted in the Loughlinstown River catchment in
south Co. Dublin where Dowling (2009) separated the winter hydrograph into two
components, baseflow and quickflow. In this study, baseflow was considered to be flow via
subsurface pathways having significant interaction with the geological materials, and
quickflow was flow with short residence times, chemically similar to rainfall. Dowling tested
five different tracers: pH, conductivity, chloride, silica and acid neutralising capacity, and
1
The spectral absorbance coefficient is a measure of the amount of UV light absorbed by water samples at a
wavelength of 254 nm. It provides a proxy for the organic content of the water.
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compared the results of each of the hydrograph separations with those derived from the
more traditional physical methods discussed above.
Dowling found that all tracers tested, except pH, gave reasonably consistent estimates of
the winter baseflow, with an average of 64% of streamflow. This estimate was comparable
with those derived from the recession curve and flow duration curve analyses (65%) which
were also based on the data collected during the same winter period. In terms of the
physical methods, he found that the constant slope and Eckhardt (2005) digital filtering
method appeared to provide the closest approximations (Table 1).
Separation technique
% baseflow
Hydrochemical
Acid neutralising capacity
Chloride
Electrical conductivity
Silica
pH
65
55
68
69
94
Physical
Recession curve
Flow duration curve
65
65
Graphical
Straight line
Constant slope
Concave
32
58
50
Digital filtration
Lyne and Hollick filter
Chapman filter
Eckhardt filter
40
33
58
Average all methods
58
Table 1. Estimates of percentage baseflow using different hydrograph separation techniques,
Loughlinstown River, Co. Dublin (after Dowling 2009)
The large range in baseflow estimates derived using the physical methods highlights the
need to employ a number of different analytical techniques in carrying out hydrograph
separations. Additional, detailed hydrological and hydrogeological investigations would be
required to validate the hydrograph separation results.
The second study was carried out by Murphy (2010) in the White River in Co. Louth. Twopart chemical hydrograph separations into baseflow and quickflow, using both silica and
electrical conductivity, were carried out on three events, one more successfully than the
others due to the vagaries of forecasting the weather (Fig. 4).
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Fig. 4. Hydrochemical separation of an event in January 2010 in the White River, Co. Louth into
baseflow and quickflow.
Murphy found that the ‘baseflow’ contribution to the stream during an event in January
2010 was relatively high at 73%. This is because ‘baseflow’ in this case represents all waters
that have been mineralised to some degree due to contact with the geological materials, as
distinct from rainwater and very quick overland flow which have very little interaction with
the soils.
The results from the silica and the chloride separations were within 2% of each other, which
is not surprising as, in the absence of significant contamination, both of these natural
tracers tend to increase in concentration with increased residence time in the geological
materials (refer to Fig. 2), notwithstanding that, for silica particularly, there are upper limits
based on their equilibration with other minerals. However, these baseflow estimates were
higher than those derived using the recession curve (Fig. 5; 63%2) and the constant slope
graphical method (49%), while the concave method matched well at 74%.
To conceptualise these results, 63% of the flow (from the recession curve method) responds
relatively slowly and is reasonably high in mineral content reflecting the slow flow
component. However, based on the hydrochemical estimate of baseflow, at least some of
the flow that responds more quickly also has a higher mineral content than might otherwise
be expected for quickflow. Flow along this pathway has travelled more rapidly to the river
than the slow flow groundwater component, but has still had enough contact time with the
geological materials to raise the mineral content. This means that there is likely to have
been some older pre-event soil waters stored in or close to the stream bank, that was
quickly pushed into the stream before the stream began to rise sufficiently to reverse the
gradients and dilute this groundwater contribution with the less mineralised rainwater.
2
The 63% baseflow is reinterpreted as the sum of Murphy’s (2010) ‘baseflow’ and ‘interflow’ together as the
latter are very similar.
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Fig. 5. Recession curve analysis for White River, Co. Louth. (Based on data from 3 events in winter
2009/2010).
It has been recognised in recent years that in some hydrogeological scenarios, e.g. incised
stream valleys, headwater streams or low permeability environments such as this, rapid
increases in the groundwater runoff to streams can occur with comparatively modest
quantities of infiltrating water (Younger, 2007). The mechanism that drives the rapid
increase is the conversion of the capillary fringe. The capillary fringe is a tension saturated
layer overlying the saturated water table, in which all pores are 100% filled with water but
the water is held in tension with a pressure less than atmospheric pressure. A sudden
change in pore pressure, to that greater than atmospheric pressure, can almost
instantaneously convert the capillary fringe into true groundwater. This results in a rapid
increase in the water table to a height dictated by the prior thickness of the capillary fringe,
which then increases the hydraulic gradient to the stream and leads to the rapid runoff of
groundwater (Fig. 6).
Fig. 6. Conversion of the capillary fringe to groundwater, leading to a rapid rise in the water table,
steeper hydraulic gradients and hence rapid delivery of older, pre-event groundwater to the
stream.
The results in the White catchment corroborate the findings of the early investigators using
hydrochemical methods who showed that, contrary to previous understanding,
groundwater or old/pre-event water, can be a significant contributor to storm flow,
releasing between one quarter and more than three quarters of peak flow events in some
cases (Pinder and Jones 1969; Hooper and Shoemaker, 1986; Wels et al., 1991).These
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results also highlight the importance of developing a sound conceptual model to interpret
what the pathway components actually physically represent in the landscape. Using a
combination of chemical and physical hydrograph analyses is a useful technique that
provides greater insights than using one or other method on its own.
DISCUSSION AND CONCLUSIONS
Knowing the proportion of flow in a stream derived from each of the different pathways is
important for understanding contaminant fate and transport. Firstly, different contaminant
types are often associated with different pathways. Phosphorus, for example, is often
associated with suspended sediment and critical source areas (areas which contribute
disproportionately high levels of a contaminant) for phosphorus will therefore be more
likely to occur in places where overland flow contributes significantly to a stream. Nitrate,
on the other hand, is relatively conservative in aerobic systems where little biological uptake
occurs. Under these circumstances, it has little interaction with the soils and bedrock and it
consequently can be delivered to a stream via the groundwater flow pathway (Archbold et
al, in press). Critical source areas for nitrate may therefore be more remote from a receptor,
may take longer to reach it, and may provide a more sustained nutrient input.
Secondly, using the ‘source-pathway-receptor’ model, the risk of contamination in a stream
(receptor) occurring is greater where there are contaminant sources located along a
particular pathway that is a significant contributor to a stream, than if there is only a weak
pathway link. Having an understanding of the proportional contribution of that pathway to
the total flow in a stream, as well as the concentration of contaminants from a source,
allows the contaminant load from that pathway to be calculated. Contaminant loads are an
important factor in understanding the potential ecological impacts to a stream. Strong
conceptualization of the pathways is therefore essential for targeting and implementing
successful, cost effective, catchment management strategies.
Traditional approaches to understanding pathways, involving physical hydrograph
separation analysis, are limited in their application as they provide a relatively arbitrary and
simplistic interpretation of what is actually happening in the ground. The results from
different analytical methods can also vary widely leading to possible subjective assessments
of results. Chemical hydrograph separation is a useful additional tool for improving our
understanding of flow through the landscape to streams.
The current Pathways Project is aiming to integrate knowledge of hydrological processes
and water-borne contaminant fate and transport, via each of the different pathways, to
assess their impact on aquatic ecosystems. The project will combine traditional hydrograph
separation techniques, hydrochemical methods, site specific investigations into each of the
different pathways, more regional approaches using geological and hydrogeological GIS
information, and finally modelling of flow along each of the pathways. The attenuation of
contaminants and their impacts on stream ecology are also being investigated. These
findings will be used to develop and populate a catchment management tool which can then
be used to target and implement efficient and cost effective catchment management
strategies to meet Water Framework Directive objectives.
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ACKNOWLEDGEMENTS
The Pathways Project refers to the EPA STRIVE research project “2007-W-CD-1-S1: Contaminant
Movement Along Pathways”, which is a collaborative project lead by the Queen’s University of
Belfast (QUB), partnered with Trinity College Dublin (TCD) and University College Dublin
(UCD). The authors would like to thank the EPA and the Steering Committee for the funding and
support for the project. The authors would also like to acknowledge the contribution to the
project by colleagues in TCD, QUB and UCD.
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