3. Method development process
1.0 Aim
This paper provides the justification and basis for the inclusion of a dilution factor into the threshold setting method.
The issue of dilution of a pollutant on its journey to an ultimate receptor is discussed and the aim is to provide one method regardless of the nature of the ultimate body; that is, it is intended that the receptor could be a surface water (such as a river, lake or a dependent ecosystem) or, potentially, a groundwater. However, in our view it will normally be the case that dilution would only be applied for surface waters that are fed partially by groundwater
2.0 Introduction
Groundwater is a precious resource that can often be a source of water for other water features. Certainly, many of the surface water features that we see and use in the
environment are supported by groundwater flow. However, for many water features there may not be a single source of water but two or more, for example a river may be supported by both direct capture of rain water and by discharge of groundwater. One consequence of this is that the concentration of a pollutant in a body that is supplied by groundwater will depend, in part, on the proportion of the flow which is provided by the groundwater.
This situation is most easily visualised in the diagram below where the surface stream is supplied both by rain water running directly off the hills AND by groundwater discharging to the banks or bed of the stream. Groundwater discharging in this way is often termed
“baseflow” since in times of low rainfall or indeed low meltwater supply (that is, normally in the summer months) the groundwater provides the basic level of flow to the stream.
Stream
Groundwater flow Surface run-off
Deliverable 18 47 / 63 2.1 Conceptual models
In applying the dilution Tier for setting thresholds it is essential that a sound conceptual understanding of the system exists. This could include consideration of the physical and hydraulic aspects of flow such as: volumes, distances, dispersion, matrix or fracture flow etc.
Some of this information will be available from the WFD characterisation and from the aquifer typology proposed in BRIDGE (D10). Nevertheless, there will be a strong requirement for body-specific data on flow volumes and rates, which may include seasonal effects.
3.0 Determining the dilution factor
The dilution factor that is suitable for a body may be simply expressed as that fraction of the total flow in the body that is provided by the groundwater for which a threshold is to be set.
In the example below the dilution factor is the fraction of the total stream flow which is coming from the groundwater, that is
Dilution factor (DF) = Qgw/(Qgw + Qsw)
Where
Qgw is the Volume rate of flow of the groundwater;
Qsw is the Volume rate of flow of the surface water.
And hence DF values will lie in the range of 0 to 1.
Note that the surface flow may itself have originally come from groundwater baseflow further up the catchment but beyond the boundaries of the area controlled by the environmental
objectives considered for this groundwater body and surface water body.
For a surface water body or any section of a such a body the dilution factor can be estimated by determining the proportion of streamflow that is derived from baseflow rather than surface water run-off.
Baseflow from groundwater Qgw
Surface flow from beyond the body boundary Qsw *See note
Dilution factor (DF) = Qgw/(Qgw + Qsw)
Deliverable 18 48 / 63 3.1 Methods of determining baseflow
There are several ways of assessing the contribution of the groundwater to the surface water system i.e. baseflow. These can be field measurements, resulting in true figures, or
estimates of the proportion, resulting in a fraction. [Full details available in Chapter 7, D10 ].
Different methods each have different advantages and disadvantages including factors such as costs, repeatability, ability to rework results to allow further understanding of the
catchment or groundwater body etc. The methods include:
• Age analysis, by assessment of hydrochemical mixing
• Tracer analysis, using either totally natural tracers or tracers originating from historical pollution events.
• Temperature or Water Quality surveys
• Low flow assessment
• Hydrograph separation
• Numerical modelling of flows, using deterministic or statistical modelling
Further details on each of the methods are available in the Annexes to Chapter 7, D10. The preferred method, by the BRIDGE project, is hydrograph separation. This must show natural conditions and therefore normalised data-sets must be used so that the effects of permanent discharges, such as industrial or wastewater treatment work discharges, are removed.
Member States can however determine the baseflow, and therefore the dilution factor, by their own method depending on the pressures, data available and the scale.
3.2 Appropriate scale of assessment:
Where possible, it is best to carry out assessment at a groundwater body scale. This does not preclude a more detailed scale study, or modelling study, if it is appropriate. Later, when and if additional detail is required, the method can be adjusted to assess what are the most important mechanisms in the system, for example land drainage.
3.3 Implications for different Body types
Rivers and streams. The methodology produced is particularly suitable and easiest to use when associated with a river or stream since it is more likely that appropriate amounts and quality of data will be available.
Lakes: It is also possible to use this method for lakes although the level of monitoring available may restrict the applicability of the results.
Groundwater Dependent Terrestrial Ecosystems (GWDTE): The scarcity of flow monitoring and water balance assessments associated with GWDTE may also reduce the applicability of the method when assessing the interactions with GWDTE. Sites should be assessed individually to determine applicability. As the groundwater dependence of a GWDTE is partly controlled by an assessment of the ecology (See conclusions of Chapter 8, WP2) then the requirement to carry out an assessment of the association of a GWB with a GWDTE is limited anyway.
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Estuaries and coastal waters: The comparative volume of the sea water and the influence of the tides, is so great that it is very difficult to assess the baseflow input from a GWB. Very site-specific studies are likely to be required to enable an estimation of the input from the GWB to the SWB. It is recommended that the scale of the problem is assessed, by determining how many estuaries and coastal waters fail their EQS, then how many can be explained by the likely input from river drainage. Any situations that are not explained by the interactions of the SWBs themselves may need to be investigated further to see if the GWB is inputting, and having a detrimental impact.
3.4 Seasonality issues
Clearly, the proportion of flow in a receptor that is provided from groundwater may vary during the annual water cycle. In particular, surface waters may experience high surface flow in the winter and spring months from rainfall or snowmelt respectively. Conversely the groundwater baseflow may be a high proportion or indeed all of the flow in the drier summer months. The conceptual model must therefore take account of the impact of these changing proportions and reflect the sensitivity of the receptor through the year for example in taking a view on whether an appropriate EQS should be the maximum permissible or the annual average concentration.
4.0 Using a dilution factor
When calculating a dilution factor it is important that the conceptual model confirms that
• dilution is currently acting to decrease the pollutant concentration at the receptor(s)
• dilution will also act in the future (during the lifetime of the river-basin management plan at least) to decrease the pollutant concentration at the receptor.
The assessment for the future is particularly important in those cases where the existing conceptual model shows that the pollutant has not yet reached the receptor for hydraulic flow reasons. Only once a good case has made been should a dilution factor be included in the determination of the pollutant threshold. Where it is not possible to make a case that this will be protective of the body then the threshold determination should revert to the earlier Tier.
The potential effect of the dilution factor on the ultimate threshold values can be seen in the diagram below. The greater the dilution in the receiving body e.g. river the smaller becomes the Dilution Factor and the larger the eventual threshold value.
Concentration
Dilution factor Reference e.g. Surface water EQS
0 1
Tier 3 value EQS/DF
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The use of dilution and/or attenuation (see next Annex) will require that some existing
monitoring data is available to allow a conceptual understanding of the flow system involved.
Moreover, although the Tiers in this method allow dilution and attenuation to be considered separately it is recognised that in some cases the data will not be sufficiently detailed to do this. Instead, a combined factor may be needed to express both dilution and attenuation.
In the end we must rely on the person making the assessment to have a sound conceptual understanding of the system involved. Where there is insufficient information a conservative assumption should be made that the pollutant is supplied entirely from groundwater and therefore the dilution factor would be 1.
The easiest way to employ the dilution factor in decision making will be as a direct comparison of groundwater data with the environmental standard in the Tier 3.
Ie Dilution factor (DF) = Qgw/(Qgw + Qsw) and then Tier 3 threshold = QS / DF
However, it may be desirable to use existing monitoring on, for example a river reach using groundwater and surface water monitoring data directly, that is, to divide surface water environmental standard by DF to determine concentration that has to be exceeded in
groundwater, in order for groundwater to potentially cause surface water body to fail to meet it’s environmental objectives.
Comparison using both surface water and groundwater monitoring data:
Calculation:
( GWconc. x DF ) + (SWupstream conc.x (1-DF)) = SWdownstream conc.
Where:
GWconc is the concentration of pollutant in the groundwater body
SWupstream conc is the concentration of pollutant in the upstream surface water samples.
SWdownstream conc is the concentration of pollutant in the downstream surface water samples.
DF is the dilution factor.
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Annex V: Pollutant attenuation by naturally occurring physical, chemical and biological
processes (Groundwater Threshold Values at Tier 4).
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Annex V
Pollutant attenuation by naturally occurring physical, chemical and biological processes (Groundwater Threshold Values at Tier 4).
A. Hart, J. Hookey & C. Tomlin
Environment Agency of England and Wales, Solihull, United Kingdom
Contents
1.0 Aim... 53
2.0 Introduction, context and concepts ... 53
2.1 Distance to receptor... 53
2.2 Conceptual model ... 54
3.0 Attenuation processes ... 54
4.0 Demonstrating natural attenuation... 54
4.1 Pollutant is known or predicted to have reached receptor ... 56
4.2 Pollutant is predicted to impact the receptor in the future ... 57
5.0 Monitoring ... 58
6.0 References... 58
7.0 Example ... 58
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Annex V
Pollutant attenuation by naturally occurring physical, chemical and biological processes (Groundwater Threshold Values at Tier 4).
1.0 Aim
This paper is intended to provide the basis for the inclusion of attenuating processes (which may act to decrease the amount, concentration or flux of a pollutant on its travel to a receptor some distance away) into the threshold derivation method.
2.0 Introduction, context and concepts
In the context of the Tiered approach to threshold setting discussed in the main text the inclusion of attenuation factors in threshold setting represents the highest and most sophisticated Tier. As a result this Tier will normally require the greatest amount of information on both the pollutant and the environmental setting and therefore the greatest effort from skilled assessors in order to give adequate assessment of the information. A possible consequence is that it may be difficult to obtain sufficient data to adequately progress this Tier in all cases. Indeed, in many cases the level of information and detail required may mean that the attenuation Tier will be unavailable within the threshold setting process and instead will constitute part of the investigation process for bodies where the TV (set at Tier 3 or below) has been exceeded.
Nevertheless, there are a range of processes in groundwaters and aquifers which, under the right circumstances, may act to attenuate a pollutant as it moves to a receptor. These are discussed briefly below and but more fully in earlier Chapters.
This paper attempts to provide one method regardless of the nature of the ultimate receptor;
that is, it is intended that the receptor could be a surface water (such as a river, lake or a dependent ecosystem) or a groundwater. An important point to remember is that the quality standard which is considered for a particular receptor may therefore be derived from a wide range of different sources (e.g. a surface water EQS value, a groundwater natural
background, local groundwater quality standards such as those in Flanders, or detailed ecological criteria). This in turn will affect how the attenuation is assessed and may require varying emphasis on the measures of mass, flux or concentration of contaminant.
2.1 Distance to receptor
A key consideration is that there must be space and time for any attenuation processes to act. In practice, this will normally mean that there must be sufficient distance between the pollutant (as measured in the groundwater body) and the receptor at risk, plus a sufficiently rapid attenuation rate for appreciable attenuation to take place. Generally though, the attenuation may begin as soon as the groundwater flows because it include processes (e.g.
dispersion, diffusion) that only reduce concentration but not the contaminant mass.
Therefore the attenuation will increase when the flow distance is increased. This is particularly the case if biological degradation is one of the active attenuation processes.
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Such attenuation along a flowpath therefore implies that the receptor assessment point must be spatially separate from the pollutant monitoring observation point(s) in the groundwater body. Clearly there are other factors involved also including access to reactants, access to organisms, time and speed of travel etc. However, this need for distance implies that attenuation should be considered where, for example:
• Pollutant is flowing from a groundwater body to a surface water receptor (for example a chalk aquifer discharging to a stream)
• Pollutant is flowing from a groundwater body to a dependent terrestrial ecosystem (for example groundwater feeding a wetland)
• Pollutant is flowing from a groundwater to second groundwater (for example limestone aquifer flowing into millstone grit)
2.2 Conceptual model
It is essential that a conceptual understanding of the groundwater and receptor system exists. Since attenuation is considered as the highest Tier available it is presumed that a suitable starting point will be the conceptual model developed as part of considering the effect of dilution in Tier 3. This existing model should at least include consideration of the physical and hydraulic aspects of flow such as: volumes, distances, dispersion, matrix or fracture flow etc. In addition to these in order to adequately address attenuation the new conceptual model will need to include consideration of the geochemistry of the
hydrogeological formation(s) along the flow-path and the potential biochemical degradation mechanisms that may occur. Some of this information may be readily available from the aquifer typology proposed in work packages 1 and 2. Nevertheless, there will be a strong requirement for body-specific data on flow and reaction rates.
3.0 Attenuation processes
The physical, chemical and biological processes which occur in aquifers and which may act to naturally attenuate pollutants are well known and widely reviewed for the purposes of treatment of point source pollutants. Several Member States have their own guidance on assessment of natural attenuation (see for example Sinke, 1999 or Environment Agency, 2000). The same processes can also act over much larger scales and would include:
• Diffusion/Dispersion
• Volatilisation
• Sorption (or precipitation)
• Reaction (e.g.chemical oxidation/reduction)
• Biological degradation
Readers are also referred to the previous Annex IV on dilution and previous chapters from work packages 1 and 2 for detailed descriptions of these processes.
4.0 Demonstrating natural attenuation
This section provides a framework for assessing the extent of natural attenuation that may be acting on a pollutant on its journey to a receptor. Assessment must include demonstration that:
• Attenuation is currently acting at a sufficient level to protect the considered receptor:
• Attenuation will act in the future (during the lifetime of the river-basin management plan at least) at a sufficient level to protect the receptor.
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It would also be sensible to consider any potential adverse impacts of attenuation, for example the generation of by-products. The assessment for the future is particularly important in those cases where the existing conceptual model shows that the pollutant has not yet reached the receptor for hydraulic flow reasons. Only once a sound case has been made that attenuation will be protective of the receptor should an attenuation factor be included in the determination of the pollutant threshold.
Is the observed receptor concentration lower than the
monitored groundwater concentration allowing for dilution? ie is there evidence that natural attenuation processes are acting at present ?
Is receptor concentration predicted to exceed the Tier 3 concentration due to hydraulic flow now or in future ? ie is the low concentration observed because pollutant has yet to reach the receptor simply based on flow rate
Natural attenuation is not currently acting. Retain Tier 3 as threshold value
No
Yes
Develop conceptual flow model of hydraulic constraints on pollutant transport through the aquifer – estimate pollutant travel time to receptor
No
Refer to Tier 3
Yes
Develop conceptual model of
attenuation effects on pollutant during transport through the aquifer.
Estimate future pollutant loading to receptor.
If needed develop further numerical model of attenuation effects on pollutant during transport through the aquifer. Estimate future pollutant loading to receptor.
Calculate attenuation factor (AF) for pollutant.
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Note that the flowchart assumes that, where there it is not possible to make a case that attenuation will be protective of the receptor then the threshold determination should revert to the earlier dilution Tier.
4.1 Pollutant is known or predicted to have reached receptor
Where pollutants are known to have reached the receptor at maximum concentration and flux the attenuation factor may be calculated from the ratio of the observed concentration of pollutant in the receptor and the Tier 3 dilution factor (DF).
Note: this surface flow may itself have originally come from groundwater baseflow further up the catchment but beyond the boundaries of the area controlled by the environmental objectives considered for this groundwater body and surface water body.
So the attenuation factor (AF) = Cobs/(Cmon*DF)
Monitored groundwater concentration Cmon
Receptor concentration Cobs
Attenuation factor (AF) = Cobs/(Cmon*DF) Tier 3 Diluted concentration Cdil Baseflow from groundwater Qgw
Surface flow from beyond the body boundary Qsw *See note
Dilution factor (DF) = Qgw/(Qgw + Qsw)
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4.2 Pollutant is predicted to impact the receptor in the future.
Where it is known or predicted from the conceptual model that a pollutant will impact the receptor in future but has not yet had sufficient time to travel from the monitored locations to the receptor a more complex calculation may be needed.
The concentration that is predicted to reach the receptor should be calculated using an appropriate decaying transport model such as the Domenico equation (Domenico P, 1987, Domenico and Schwarz, 1990). The acceptability of different models may vary by Member State and readers should consult local guidance on applicability of natural attenuation models in their Member State.
The attenuation factor is then given by AF = Ccalc/(Cmon*DF) where Ccalc is the predicted attenuated concentration at the receptor boundary.
For clarity these calculations are likely to require significant amounts of data and
interpretation. An example of the data needs of an analytical transport model is given in the box below and although some terms such as flow velocity may be readily available others
interpretation. An example of the data needs of an analytical transport model is given in the box below and although some terms such as flow velocity may be readily available others