Contract n° SSPI-2004-006538
BR B R ID I D G G E E
B B a a c c k k g g r r o o u u n n d d c c R R i i t t e e r r i i a a f f o o r r t t h h e e I I d d e e n n t t i i f f i i c c a a t t i i o o n n o o f f G G ro r o un u n d d wa w at t er e r t t h h re r es sh ho o l l ds d s
Research for Policy Support
D10: Impact of hydrogeological conditions on pollutant behaviour in groundwater and related ecosystems.
Volume 2
Due date of deliverable: March 2006 Actual submission date: May 2006
Start date of the project: 1st January 2005 Duration: 2 years Organisation name of lead contractor for this deliverable: BRGM
Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)
Dissimination level PU Public
PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission
Services)
CO Confidential, only for members of the consortium (including the Commission Services)
3 / 204
Content of Volume 2
Chapter 7 - Groundwater / Surface water interactions... 5
7.1. Introduction ... 7
7.2. Aim... 7
7.2.1. Scale ... 7
7.3. Types of surface water systems ... 8
7.3.1. Streams and Rivers ... 9
7.3.2. Lakes ... 10
7.3.3. Estuaries and other transitional waters... 11
7.3.4. Coastal waters ... 12
7.3.5. Heavily Modified Water Bodies and Artificial Water Bodies... 12
7.4. Chemical/substance concerns... 13
7.4.1. Chemicals considered... 13
7.4.2. Present Pressures ... 14
7.5. Processes and controls at catchment scale ... 16
7.5.1. Proportion of surface water derived from the groundwater pathway compared with water derived from the surface water pathway... 17
7.5.2. Variables influencing input / proportion of water entering the surface water via the groundwater pathway at a catchment scale ... 17
7.5.3. Methods of determining baseflow ... 22
7.5.4. Review of methods ... 27
7.5.5. Available data-sets / knowledge available for use within Member States... 31
7.5.6. Summary / Discussion of “catchment-scale”... 40
7.6. Processes and Controls at a local scale... 41
7.6.1. Groundwater – Surface water interactions at a local scale... 41
7.6.2. Methods of measuring flow in the hyporheic zone and riparian zone .... 44
7.6.3. Attenuation of contaminants in the hyporheic zone and riparian zone... 46
7.6.4. Methods available for in situ monitoring of biogeochemistry. ... 50
7.6.5. Review of data-sets / knowledge available for use within Member States... 51
7.6.6. Conceptual models ... 54
7.6.7. Summary / Discussion of “local scale” ... 55
7.7. Conclusions ... 56
7.8. References ... 56
Chapter 8 - Groundwater / Dependent terrestrial ecosystem Interactions... 179
8.1. Introduction ... 181
8.1.2. Groundwater dependency... 181
8.2. Aim... 182
8.3. Types of water dependent terrestrial ecosystems ... 182
8.3.1. Division between groundwater and surface water fed ecosystems ... 182
8.3.2. Landscape location ... 183
8.4. Linking landscape location and water transfer mechanisms ... 184
8.4.1. Flat area wetlands... 185
8.4.2. Slope wetlands... 186
8.4.3. Depression wetlands... 186
8.4.4. Valley bottom wetlands ... 187
8.5. Processes and controls on the groundwater system and the GWDTE... 189
8.5.1. Catchment scale controls... 189
8.5.2. Local scale controls ... 191
8.6. Chemical /substance concerns... 193
8.6.1. Chemicals considered... 193
8.6.2. Present Pressures ... 193
8.6.3. Attenuation in GWDTEs... 194
8.7. Review of methods. ... 194
8.7.1. Data sets required... 195
8.7.2. Data sets available for use... 196
8.8. Discussion ... 199
8.9. Conclusions ... 200
8.10. References ... 200
5 / 204
Chapter 7
Groundwater / Surface water interactions
Jan Hookey (EA) Arno Aschauer (UBA-A) Rossitza B. Gorova (EEA)
G. Fritsche (HLUG) Mette Dahl (GEUS)
Enn Loigu (UT) Hélène Pauwels (BRGM)
Sandra Bertin (AETS) Zoltàn Simonffy (BME)
Manuela Ruisi, Paolo Traversa (Abteverre) Stanislaw Witczak, Jaroslaw Kania (DHWP/AGH)
Teresa Melo (UNI-AVEIRO) With the contribution from
David Allen, Marianne Stuart (British Geological Survey)
7.1. Introduction
Many of the surface water features that we see and use in the environment are supported by groundwater flow. The Water Framework Directive [WFD] recognises this and, in its consideration of Status objectives for groundwater bodies, the WFD requires that the Status of the groundwater and surface water bodies be linked. Thus, good groundwater chemical status requires the chemical composition of the groundwater body to:
- not exhibit the effects of saline, or other, intrusion;
- not exceed the quality of standards applicable under other Community legislation, as set out in Article 17;
- neither result in failure to achieve the environmental objectives for surface waters or any significant deterioration of the ecological or chemical quality of such bodies, nor to result in any significant damage to terrestrial ecosystems that depend directly on the groundwater body.
It is this last point, the dependence of groundwater body status on its effects on surface water systems and terrestrial ecosystems, which requires the nature of groundwater-surface water interactions to be considered.
7.2. Aim
The aim of this chapter is to review the interactions between groundwater and surface water that may affect the flow and quality of water between the two bodies, and so affect Status determinations made as part of the implementation of the Directive.
We shall consider the amount of water that passes from groundwater into the surface water and the processes that may occur at the interface between the groundwater and surface water. The groundwater – surface water interface is a region that is often marked by strong physical and chemical gradients and active biological populations.
7.2.1. Scale
When considering the interactions it is also important to recognise the influences at different scales. The variations across a large area, or region, influence the distribution of groundwater and surface water within an entire system. This may be across a groundwater body, within a river basin or across a catchment. As Member States have defined groundwater bodies in different ways the terminology for this larger scale consideration will vary. For the purposes of this chapter the larger, wide scale controls and processes have been referred to as “catchment scale”.
At a smaller, more localised scale, there is the term “reach”. A hydrological definition of a river reach is “a straight, continuous, or extended part of a river, stream or restricted waterway” (Parker, 1997). This can be quite a short length. Despite this, the term may also be used to describe quite a long length of river, often between sampling points. Within a river reach or at the margins of a lake, the groundwater – surface water interactions can also be considered at much smaller, more localised scale, across the actual interface of the groundwater-surface water. For the purposes of this chapter the small scale controls, and processes, have been referred to as “local scale”.
7.3. Types of surface water systems
Surface water systems considered include those classified as Surface Water Bodies in the WFD, such as streams and rivers, lakes, estuaries and coastal waters. Some surface water systems may also be included in the WFD classification of Heavily Modified Water Bodies / Artificial Water Bodies, such as canals, drainage ditches, or reservoirs.
Surface water systems all form component parts of the hydrological cycle, enabling the transportation of water from precipitation, via streams, rivers and lakes, drainage ditches and canals, ultimately to the estuaries and coastal waters and back to the oceans, from where it evaporates to form precipitation again (figure 7.1 and photograph 7.1). Surface water systems can be found in a variety of different geomorphological settings and geological settings that influence the flow characteristics and response time in individual drainage basins.
Figure 7.1 - The Water Cycle.
[Environment Agency. Water Framework Directive, Guiding principles on the technical requirements. June 2002].
Photograph 7.1 - Transport of water from upland stream to river, lake and sea, Cumbria, England (J. Hookey).
7.3.1. Streams and Rivers
Streams may develop, eventually forming rivers, in a variety of areas. Their formation will be influenced by on the geological strata, with less permeable geological formations encouraging stream development. The more permeable geological formations generally lead to the promotion of groundwater flow and little stream flow, although streams will form where the watertable outcrops. The geology (for example the hydraulic properties, stratigraphy and structural setting) combined with the geomorphology will influence the response time of the stream or river. The types of geomorphological situations include high upland or mountainous locations, narrow river valleys or gorges, and lowland river valleys or outwash plains (photographs 7.2, 7.3 and 7.4).
Photograph 7.2 - Upland stream, Austria (J. Hookey).
Photograph 7.3 - Lowland river, Yorkshire, England (J. Hookey).
Photograph 7.4 - River meandering along valley bottom, Scotland (J. Hookey).
7.3.2. Lakes
Lakes may form either on-line with a stream or river, or off-line from a stream or river.
On-line lakes have a stream or river input to them and an outflow to a stream or river. The influence of the lake will be to slow the water flow. This type of lake may have a component of groundwater flow as its input but will definitely have a surface water component of flow.
They are normally located in river valleys, and often in the more lowland reaches of a river system (photograph 7.5).
Off-line lakes may have small streams feeding surface water “run-off” to them but do not have an outlet to a stream or river. They may also have a component, or rely almost totally on, input from groundwater flow. Off-line lakes exist in depressions such as high mountain cols (photograph 7.6). Particular types of off-line lakes, often associated with specific processes may form. Examples include volcanic lakes which form in the intra-calderic or marginal depressions of volcanic structures, or lakes which form in kettle holes, the depressions in glacial drift deposits formed by isolated blocks of melting ice.
Photograph 7.5 - On-line lake, with catchment behind, Cumbria, England (J. Hookey).
Photograph 7.6 - Upland, off-line lake, Cumbria, England (J. Hookey).
7.3.3. Estuaries and other transitional waters
Estuaries mark a transition between the fresh water environment and the coastal environment. The groundwater component of water input is likely to be limited in comparison to the fresh surface water input and the seawater input because of the volume of surface water that drains to the estuary and low gradients, etc.
The contribution of groundwater to flow within an estuary will be partly controlled by the permeability of the underlying strata. As an estuary often marks a zone of deposition of sediments the thick layer of sediments that builds up could reduce the interaction with the groundwater body (photograph 7.7). Groundwater bodies close to estuaries can show a measureable water level change, and quality impacts, associated with tidal influences. Other transitional waters include areas of brackish water or lagoons.
Photograph 7.7 - Ravenglass Estuary, Cumbria, England – interaction of river water and seawater (J. Hookey).
7.3.4. Coastal waters
Coastal waters may receive water directly from groundwater bodies. The contribution may be minimal in comparison to the volume of the sea, especially if the geology is relatively impermeable. There are, however, documented cases of fresh water upwelling at significant distances off-shore from springs. This usually occurs in locations where the geology comprises fractured aquifers or extremely karstic environments. Groundwater bodies close to the coast can show a measureable water level change, and quality impacts, associated with tidal influences.
7.3.5. Heavily Modified Water Bodies and Artificial Water Bodies
Both Heavily Modified Water Bodies and Artificial Water Bodies may need to be considered depending on individual circumstances, mainly based on how the interaction with the groundwater body has been impacted (figure 7.2).
7.3.5.1. Heavily Modified Water Bodies
Heavily Modified Water Bodies include those that have undergone substantial physical alteration to permit navigation, water storage, flood defence and land drainage. Rivers may have been enlarged, straightened and deepened to achieve these alterations. Rivers may even have been re-routed or covered to accommodate urban development and transport links. They may have been diverted to provide power for mills or dammed to provide hydropower, or for public water supply.
x x x x
Engineering limits the interaction of groundwater body with the surface water system.
Impermeable canal lining.
Groundwater interaction flow limited.
Engineering does not prevent interaction of groundwater body with surface water system.
Gabion wall style flood defences, not keyed in to underlying clay.
Groundwater free to flow beneath
Impermeable flood defences, keyed in to underlying clay.
. Groundwater flow interrupted, and limited.
Semi- permeable or permeable canal lining.
Groundwater flow still possible although.
Figure 7.2 - Examples of how engineering may, or may not, impact on groundwater flow in similar circumstances.
Photograph 7.8 - Engineered banks of river, Strasbourg, France (J. Hookey).
Estuaries and coastal waters may have been physically modified by flood defence works to provide improved harbour and port facilities and for residential development.
Heavily Modified Water Bodies are considered separately in the WFD but they can still be impacted by adjacent groundwater bodies and so need to be considered.
7.3.5.2. Artificial Water Bodies
Artificial Water Bodies are man-made surface water bodies, which have been designed to serve a particular purpose but which can also support important aquatic ecosystems. These include canals, some docks and man-made reservoirs.
7.4. Chemical/substance concerns 7.4.1. Chemicals considered
The Water Framework Directive highlights a range of substances that may cause a water body to fail its environmental objectives. These include:
- Priority hazardous substances and priority substances discharged to surface water defined under Article 16 (3) of the WFD;
- Emerging contaminants (mainly carcinogens and endocrine disruptors);
- Substances for which guideline thresholds need to be set under Articles 3 & 4 of the old GWD (defined in Annexes I and III).
The full list is available (in Annex 1) but these substances have been divided into a number of property-related classes in order to discuss their behaviour in groundwater/surface water (or groundwater dependant terrestrial ecosystems) interactions.
The property related classes considered include:
- Heavy metals;
- Organo metals;
- Nitrogen compounds;
- Anions;
- Pesticides;
- Hydrocarbons;
- Chlorinated hydrocarbons;
- Alkyl phenols and related compounds;
- Endocrine disruptors and others.
7.4.2. Present Pressures
While there is a large number and range of substances that may be relevant to consider, not all of these groups of substances will be of equal concern in all waters, or indeed in all Member States. Table 7.1 shows a summary of principal areas of concern for member states, taken from one or more of:
- the preliminary WP2 questionnaire;
- the more-detailed WP2.2 questionnaire response;
- the formal WFD Article 5 characterisation submissions from the Member States to the Commission.
Table 7.1 - Principal substances of concern for groundwater quality for Member States in order of stated importance.
1 2 3 4 5 6
Austria Nitrate Pesticides (Atrazine, Bentazone)
Pesticide metabolites (Desethyl atrazine) Belgium*
Bulgaria Nitrate Sulphate Iron and manganese
Abandoned mine areas Heavy metals (Cr, Zn, Cu) Denmark Nitrate Phosphate Pesticides
England and Wales
Nitrate Urbanisation quality
problems
Pesticides and sheep dip
Phosphate Mining:Heavy metals,
chloride and sulphate.
Chlorinated solvents
Estonia Nitrate
France Nitrates Pesticides and local point source pollutions
Hydrocarbons High natural background level for elements such as As, Se, Pb
Emerging pollutants
Germany* Nitrate Pesticides
Italy Nitrate
Hungary Nitrate from Abandoned Local point
agriculture, farms and settlements
mines (only 1 ground- water body)
sources of pollution by different substances Lithuania*
Netherlands*
Poland Iron and
manganese Nitrate and
nitrite Pesticides Heavy metals BTEX Chlorinated solvents
Portugal Nitrate Pesticides Mining and
heavy metals Hydrocarbons Solvents Scotland Nitrate Phosphate Pesticides
and sheep dip Spain
Northern Ireland
Diffuse pollution from agriculture and forestry
* Information from initial WP2 questionnaire.
Information input to table directly at Orleans workshop.
While there are some concerns identified that only affect a few partners, such as hydrocarbons and chlorinated solvents, several Member States share the same concerns.
The key pressures appear to be diffuse pollution caused by - nutrients such as nitrates and phosphates;
- pesticides and their breakdown products;
- and heavy metals (photograph 7.9), particularly from former mining activity.
Photograph 7.9 - Former Lead Mine, Yorkshire, England, discharging water rich in heavy metals (J. Hookey).
Returning to the property related classes, the following numbers of partners reported a concern with respect to groundwater bodies:
- Heavy metals. Five partners (out of 12) reported a concern due to heavy metals, mainly associated with former mining activity. They were Bulgaria, England & Wales, Hungary, Poland and Portugal.
- Organo metals. No partners reported a concern, but organo metals may not have been considered during WFD classification as there may have been insufficient data, or routine testing may not be in place for organo metals in groundwater bodies.
- Nitrogen compounds. All twelve partners that responded reported concerns relating to high nitrate readings. These appear to be associated with either agriculture or with urbanised areas.
- Anions. Two partners (out of 12) reported a concern due to anions. They were Bulgaria and England & Wales. The high readings appear to be associated with elevated Chloride or Sulphate levels associated with former mining activity. It is possible that the number of countries reporting this should have been higher but as the main problem associated with mining discharge is the heavy metals it may have been overlooked.
- Pesticides. Six partners (out of 12) reported a concern over elevated levels of pesticides, or their derivatives, in the groundwater. They include Austria, Denmark, England & Wales, Germany, Poland and Portugal.
- Hydrocarbons. Two partners (out of 12) reported a concern relating to the presence of hydrocarbons. They include England & Wales and Poland.
- Chlorinated hydrocarbons. Three partners (out of 12) reported a concern relating to the presence of chlorinated hydrocarbons or solvents. They were England & Wales, Poland and Portugal.
- Alkyl phenols and related compounds. No partners reported a concern, but alkyl phenols may not have been considered during WFD classification as there may have been insufficient data, or routine testing may not be in place for alkyl phenols and related compounds in groundwater bodies.
- Endocrine disruptors and others. No partners reported a concern, but endocrine disruptors and other substances may not have been considered during WFD classification as there may have been insufficient data, or routine testing may not be in place for endocrine disruptors and other substances in groundwater bodies.
7.5. Processes and controls at catchment scale
If a high concentration of a substance is detected in a surface water system it could be derived from contributions either purely from surface water, purely from groundwater, or from a mixture of both surface water and groundwater. The high concentration may be derived from the natural background quality of the contributions from the surface or groundwater systems or it could be elevated due to anthropogenic influence. Then, if the elevated concentration is causing a reduction in Status of the surface water body, or causing a significant trend, it will be necessary to investigate. The investigation will include an assessment of whether the groundwater body is linked to, and contributing to, the impact on the surface water system as this may influence the Status of the groundwater body too.
In order to establish the influence of the groundwater body on the surface water system it is important to determine the contribution of water from the groundwater body. This must include both the volume of flow and the proportion / concentration of substance derived from
the groundwater body. This section concentrates on the controls on the volume of flow within a catchment; and the subsequent section focuses on the interaction of the groundwater with the surface water.
7.5.1. Proportion of surface water derived from the groundwater pathway compared with water derived from the surface water pathway
As precipitation hits the ground it will either form surface run-off, potentially accumulating to form streams and rivers, or may infiltrate in to the ground to form a component of the groundwater environment. [Geological controls may allow groundwater to reappear at the surface, via springs, feeding in to streams and rivers and, likewise, surface water to enter the groundwater system too.] By assessing the reaction of a surface water system to precipitation events it is possible to assess the proportion of water that will take the direct surface (overland) flow route, compared with the proportion that infiltrates into the ground and takes the groundwater flow route to the stream or river. The proportion that takes the groundwater flow route to the stream, or river, is normally referred to as “baseflow”. There are a number of variables that influence the route precipitation takes to a surface water system and several ways of assessing the volume of baseflow contribution, via groundwater, to a surface water system.
7.5.2. Variables influencing input / proportion of water entering the surface water via the groundwater pathway at a catchment scale
Variables that will influence the proportion of water that takes each route and, thus, the magnitude, speed and duration of the response of the river hydrograph, include:
- geology;
- drift (and soil) cover;
- land use;
- land drainage;
- seasonal variation;
- geomorphology;
- climatic variation.
Each variable is considered at a catchment / groundwater body scale and is described in greater detail in the following sections:
7.5.2.1. Geology
The character of the geological strata (sub-surface material and the structural setting) in a catchment is of critical importance to the relationship between surface run-off and the water infiltrating the ground to form sub-surface flow. As a result it is necessary to consider the geological setting of a river basin to determine whether it is predominantly a surface water fed (hydrographic) or a groundwater fed (hydrogeological) catchment (river basin) and consequently whether the contribution to the water course from groundwater is major or minor.
Photograph 7.10 - Eire (J. Hookey).
If the geological strata is effectively impermeable, for example a clay, then very little infiltration to the groundwater will occur, with the majority of water forming surface water run- off direct to the stream system. This will produce a rapid response of a large magnitude and relatively short duration on a stream flow hydrograph.
Where the geological strata has sufficient porosity and permeability to act as an aquifer, then the precipitation will infiltrate to form groundwater. In these catchments a groundwater contribution to an effluent (gaining) stream will be expected, with the nature of the contribution depending on the hydraulic properties of the aquifer. For example, a porous, permeable, homogeneous, isotropic sandstone would be expected to allow rapid infiltration of precipitation and will be likely to provide a high relative contribution of groundwater to the streamflow. The resulting hydrograph will show a more delayed response, will be of lesser magnitude and of greater duration than that for a clay catchment as the response would be partially dependent on the slower groundwater flow.
The geological strata considered in this chapter includes:
- Limestone (karstic and non-karstic);
- Chalk;
- Crystalline basement;
- Shale (and Schist);
- Sand and gravel;
- Sandstone;
- Volcanic rocks;
- Evaporites;
- Clays and marls.
Photograph 7.11 - Clay with Drift cover, Wales (J. Hookey).
Photograph 7.12 - Sandstone, Cumbria, England (J. Hookey).
Photograph 7.13 - Limestone outcrop, Dorset, England (J. Hookey).
Photograph 7.14 - Flow from discrete fractures, Switzerland (J. Hookey).
7.5.2.2. Drift and soil cover
The nature of material covering the bedrock may be very important in the relationship between the groundwater and surface water, as it will influence the amount of infiltration
which occurs, which in turn will affect the amount of water entering the groundwater system.
For example, a thick clayey soil or drift layer will inhibit the vertical flow component of infiltration and will promote surface runoff so that even where the underlying material is permeable the groundwater component of discharge to the surface water body will be minimised. The resulting hydrograph will be of a more rapid, flashy (greater magnitude and limited duration) nature compared with that from a thinner or more permeable soil. If, conversely, the drift is sandy then recharge to an underlying aquifer will be promoted; and if the bedrock has low permeability then the drift itself may act as the source of groundwater flow. This would result in a slower, delayed, response on the hydrograph, with a smaller peak flow but a longer duration.
7.5.2.3. Land use
Human activity affecting land, such as urbanisation, forestry or agriculture may affect important hydrological variables such as soil water storage or infiltration and thus influence the proportion of water that becomes surface run-off compared with the water infiltrating the ground to form sub-surface flow.
Urbanisation, for example, is likely to result in the creation of surfaces which are less permeable than those of the natural materials which they replace. This will result in an increase in the surface water component of flow to a stream, producing a flashier hydrograph, with peaks which are higher and earlier than in the natural system. The difference between the natural hydrograph and that resulting from urbanisation will be greatest where the permeability contrast between the original ground surface and the new urbanised surface is greatest. Leakage from water supply and waste water disposal networks can also be a factor in urban water budgets.
Afforestation is likely to reduce peak flows, while deforestation may intensify surface flooding by reducing water storage and infiltration.
The nature of crop types (including afforestation) will influence the rate of actual evapotranspiration and therefore the availability of water. Evapotranspiration can vary greatly with crop type, soil moisture deficit and temperature.
7.5.2.4. Land drainage
Agricultural drainage alters near-surface water levels in order to make land more accessible, or productive, and therefore inevitably effects the proportions of water flowing by different routes to a surface water body, and thus the resulting hydrograph. The effect of drainage varies with soil type; for example drainage of heavy clay soils, which otherwise are susceptible to surface saturation, tends to reduce large and medium flood peaks. For more permeable soils, drainage tends to increase flood flows (Ward and Robinson, 2000).
7.5.2.5. Seasonal variation
Both the amount of the groundwater contribution to streams and its relative contribution to surface flow will vary seasonally as a result of variations in recharge amount and intensity.
The amount of groundwater contribution will tend to vary with the hydraulic gradient to the stream and will therefore tend to be higher when groundwater levels are high as a result of antecedent recharge - generally during the late winter. However the proportion of groundwater in streamflow will be highest when the surface runoff contribution is least, during the summer. Where streams drain catchments that consist entirely of permeable rocks, natural summer streamflow will be almost entirely derived from groundwater.
The colder, winter season may influence the timings and relative contribution of inputs to groundwater or surface water due to precipitation falling as snow or the ground being frozen, and subsequently due to snow melt.
The seasonal variation in groundwater level may also affect the length of the stream in permeable catchments. During the summer the stream will rise at its perennial head; as groundwater levels rise during the winter the length of flowing stream will increase to reach a maximum as groundwater levels peak. The length of the resulting section of intermittent stream depends broadly on the gradient of the stream bed and the specific yield of the aquifer.
7.5.2.6. Geomorphology
Basin factors such as catchment size and shape and variations in slope, aspect and altitude will alter the timing and shape of the flood hydrograph in the surface water system. Thus for example the combination of basin shape and drainage network pattern influences the size and shape of flood peaks at the basin outlet (Ward and Robinson, 2000).
The shape of the river basin will influence the response time, with long thin basins taking a long time to achieve through flow of water after a storm, compared with more circular shaped basins.
The size of a river basin will influence response time, with large river basins, often taking a long time to respond, compared to smaller basins.
Relief can influence location of likely precipitation with maximum amounts of rainfall often being associated with upland / mountainous areas and rain- shadow effects often occurring in the lea of mountainous areas. Relief can influence response times with the steep slopes in some basins accelerating run-off, as in upland areas and high mountains, compared with the slower response of low gradients such as lowland river valleys and outwash plains.
Factors associated with altitude, such as temperature, may also influence response times due to the conditions at certain times of year. In winter conditions in the high mountainous areas the precipitation may fall as snow, or be held up on glaciers or snow fields, and so not be initially available as surface water run-off (photograph 7.15). Later in the season, it may contribute additional run-off in the form of meltwater.
Photograph 7.15 - Snow during winter conditions, at altitude in the Pyrenees, France.
7.5.2.7. Climatic variation
The climate, the long-term manifestations of weather, will vary on a wide-scale with latitude (distance from equator or poles) combined with the flow path of the wind current (whether it crosses a continent or maritime environment). A country near the equator will generally experience hotter, drier conditions. A country that is nearer the pole will generally experience the coldest weather conditions. Countries influenced by a wind current from a continental source will experience greater extremes of climate than those influenced by wind current from a maritime source.
7.5.3. Methods of determining baseflow
There are several ways of assessing the contribution of baseflow (broadly equated to groundwater) to a surface water system. These can broadly be grouped into physical and chemical approaches and can result in true figures, or estimates of baseflow proportion.
Different methods have different advantages and disadvantages including factors such as accuracy, cost, repeatability etc.
7.5.3.1. Physical methods
7.5.3.1.1. Hydrograph analysis
The streamflow response (expressed as a hydrograph) to a rainfall event has commonly been described in terms of a rapid response (quickflow) followed by a delayed flow response:
this latter phenomenon often being referred to as baseflow. In process terms this classic hydrograph response has often been envisaged to consist of a number of components which occur broadly sequentially (although with overlap) after a rainfall event. These are: channel precipitation, overland flow, throughflow and true groundwater flow. Water from melting snow will also follow one or more of these flow paths.
A common approach to analysing hydrographs is to separate the quickflow component from the delayed, baseflow component. It is then not uncommon to assume that the baseflow component is broadly equivalent to the groundwater contribution to streamflow. In reality however, given the somewhat arbitrary nature of the separation between the quickflow and the delayed flow, and the likely temporal overlap between the flow processes (e.g. slow throughflow and the true groundwater flow) the relationship between baseflow and groundwater flow is not simple. On the other hand, groundwater flow tends to be a major long-term component of total runoff, and becomes particularly important during long dry periods when surface runoff does not occur.
Despite the lack of precise correlation between baseflow and groundwater flow, baseflow separation from flow hydrographs has long proven to be a valuable approach and hydrograph analysis is an important and widely used research and management tool.
Baseflow separation from streamflow data may be undertaken for individual events (e.g.
storms) in a variety of ways, commonly graphical approaches based on aspects of the hydrograph shape sometimes combined with inferences from catchment characteristics. It is however important to recognise that while the separation procedures are usually based on physical reasoning, the quantitative approaches are somewhat arbitrary and often based on subjective judgements.
During dry periods, the hydrograph recession curve (commonly expressed as an exponential recession) can be used to provide baseflow estimates, or measured low flow data during these periods may be taken as equating to baseflow values.
In order to undertake baseflow separation on a continuous basis, empirical separation algorithms are often used. Once the baseflow component has been separated a baseflow index (BFI) may be calculated. This is the ratio of baseflow to total flow in the stream and commonly is broadly equated to the relative contribution of groundwater flow to the total stream discharge.
A description of various approaches used to identify the baseflow component of streamflow is given in Annex 2.
Strengths
• The physical hydrograph separation approach is based on historical river gauging so the estimate of groundwater contribution can take both seasonal and annual changes into account.
• The collection of data is relatively inexpensive.
Limitations
• This method only really identifies quickflow and delayed flow (baseflow) components of streamflow, but much of the baseflow within an event, and immediately after the event may not be true groundwater and may be associated with slow drainage from superficial deposits and runoff from more distant parts of the catchment.
• The methods of separation are often somewhat arbitrary
• The result does not explain why the surface water / groundwater split is as it is. The result is an integration over the entire catchment to the point measured i.e. gives no spatial information on which parts of the catchment contribute the most.
7.5.3.1.2. Modelling
There are two main types of modelling – deterministic and statistical.
(i) Deterministic modelling can be further subdivided, as the modelling technique involved may be lumped, semi-lumped or fully distributed. Deterministic modelling simulates the key physical processes of the flow system - rainfall, evapotranspiration, surface runoff and groundwater recharge (which then becomes the groundwater component of river flow). The model is based on real parameter values with the use of real input data and its outputs are compared against real field data e.g. flows.
(ii) Statistical modelling is similar to the deterministic model but it uses effective parameter values to deal with multiple processes or parametric uncertainty. The values are optimised to replicate observed response.
Strengths
- Modelling methods test the conceptual understanding of how a catchment is working. A model can be used in an iterative way, developing as the conceptual model changes.
Modelling gives a spatial and temporal understanding of a catchment and allows predictions to be run to see change in baseflow with change in landuse, etc. The model can also be used to give flow statistics e.g. flow duration, etc. Statistical modelling has the added advantage that it starts to look at the confidence in model predictions.
Limitations
- Any model will only provide one, of possibly many, explanation of the understandings of catchment characteristics and groundwater / surface water interaction.
7.5.3.2. Hydrochemical methods
There is a spectrum of hydrochemical methods available that can be applied to estimating the contribution and quality impact of groundwater baseflow to surface water. These range from the simple mixing model through to sophisticate and data intensive chemical hydrograph separation. Approaches are listed below in order of increasing complexity with details provided in Annex 2.
Main strengths
- Hydrochemical methods can be good at distinguishing different types of water.
Main weaknesses
- It can be difficult to get a truly representative sample from surface water.
- Many methods need relatively expensive specialised sampling or analyses.
- The data can be difficult to interpret.
7.5.3.2.1. Simple mixing model
The basis of the technique is to estimate the percentage of the river flow that has entered the river via the groundwater pathway versus the faster overland/surface runoff pathway using major and minor ion chemistry. This requires the determination of the quality of the groundwater, the surface runoff and the mixed water in the river.
Strengths
- This is a relatively quick and inexpensive technique.
- It can be used where there is little or no historical flow or rainfall data.
Limitations
- It gives a “snap-shot” of relative contributions.
- It does not give any idea of the volumes involved.
7.5.3.2.2. Temperature / water quality surveys
A temperature survey is based on identifying temperature inputs indicative of groundwater flow along a river reach. In summer conditions, when the surface water is warm, the groundwater inputs can be identified by lower temperature inputs. Conversely, in winter conditions the groundwater inputs to the river are comparatively warm. A water quality survey of a river can also be used to identify inputs typical of groundwater.
Strengths
- This method gives a detailed picture of where the interaction is important along the river reach.
Limitations
- It does not give overall catchment information at a groundwater body scale.
7.5.3.2.3. Tracers
Tracers are substances which move as part of the water flow and conventionally are unreactive substances which can be considered to be conservative. Different tracers can be applied to different time scales due to their different transport mechanisms especially in the unsaturated zone. While solute tracers are moved advectively with the seepage water, gas tracers can pass the unsaturated zone diffusively through the air phase. For application to the estimation of baseflow there are a number of categories which could be used to distinguish between ground and surface water.
Natural substances only present/or different in the groundwater component of flow due to its longer residence time or different recharge history e.g. strontium isotopes, stable isotopes of water, 85Kr, and 39Ar.
Substances present in modern surface water and/or rainfall from current pollution but absent in older groundwater e.g. CFCs, SF6.
Substances present in the groundwater component of flow from historic pollution events as they have long since disappeared from the surface water flow path e.g. tritium.
Strontium isotopes
The principle of the approach is that Sr isotopic variations observed in water can be explained by mixing of Sr of different isotopic signatures coming from different sources. Thus in the simple case where two Sr sources with known Sr signature are identified, the 87Sr/86Sr ratio of the mixing can be determined.
Strengths.
- Strontium is a natural tracer of water-rock interactions.
- The method is suitable for work on a relatively small scale.
Limitations
- This method can be affected by atmospheric inputs, but this contribution can be subtracted.
- Anthropogenic influences (fertilisers, de-icing salts) can also perturb the natural signal, but as for the atmospheric inputs, this contribution can be corrected for.
Stable isotopes of water (δ18O, δ2H)
Stable isotopes of the water molecule (δ18O, δ2H) can also be used for tracing the groundwater contribution to the river and the river contribution to the groundwater. Most of the applications of stable isotopes of hydrogen and oxygen in hydrology make use of the variations in isotopic ratios in atmospheric precipitation, i.e., in the input to a hydrological system under study.
Strengths
- The method uses water molecules as natural internal tracers.
- The isotopic fraction factors are well known.
- A relatively quick and inexpensive technique which can be used where there is little or no historical flow or rainfall data.
Limitations.
- A good conceptual understanding of the catchment is required as some results vary with temperature, and therefore with altitude and latitude.
Chlorofluorocarbons and sulphur hexafluoride
The chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) can be used to trace relatively modern water in the flow system. Unlike tritium, these gases are well-mixed in the atmosphere and their input functions are better characterised. For example a bulk age for groundwater recharged from the present back to the 1970s can be calculated. Water older than this would not be expected to contain CFCs. This can be compared with surface water which would be expected to have a modern signature.
Strengths
- The cost of analyses is moderate.
- The sampling procedure is relatively simple.
- The method is sensitive because it has a low detection limit.
Limitations
- CFCs can be affected by local anthropogenic inputs, such as landfills.
Tritium (3H)
Tritium is the best-known dating agent for recent groundwater. There was a major input to the water cycle fron aerial thermonuclear testing which peaked in the early 1960s and the presence of 3H in groundwater indicates that at least a proportion of the water was recharged within the last 40-50 years. The majority of the 3H released has now decayed to 3He and its usefulness as a tracer has been much reduced.
Strengths
- There are no interferences.
Limitations
- The environmental concentrations of tritium now very low.
- Tritium is not well mixed in the atmosphere so quantifying the source term can be difficult.
- The method is not good for very slow moving groundwater systems.
7.5.3.2.4. Hydrograph separation using water quality
Inorganic water quality data can be combined with information from tracers and flow measurements in ground and surface water to obtain a detailed understanding of the contribution of groundwater to surface water. The principle is to solve a chemical- mass balance equation of dissolved constituents in stream flow at a particular location in the stream at a specified time to obtain the discharge of groundwater into the stream.
Strengths
- It can give a longer term view of relative contributions.
- It contributes to understanding of the differences between the groundwater and surface water.
- It gives an idea of resource available because includes flow or rainfall information.
Limitations
- The concentrations measured in the groundwater and run off may not actually represent the baseflow contribution to surface water.
7.5.4. Review of methods
7.5.4.1. Methods used for determining baseflow from groundwater in Europe From the WP2.2 questionnaire response the following countries have a national methodology available for determining baseflow:
Is there is a method for deriving Base Flow available?
Is this type of data available on a national or local scale?
Austria Uncertain No
Belgium*
Bulgaria Hydrograph Separation Local scale
Low flow assessment National, a few tens of km between measuring points
Denmark
Deterministic modelling (DK model) National, 1x1 km grid Base Flow Index from Hydrograph separation
and modelling widely used. National scale England and
Wales
Tracers Local scale
Estonia Hydrograph separation Local scale
France Hydrograph separation Recession curve analysis or Hydrological lump models
Local scale and National data bank
Germany* Base Flow Index from hydrograph separation National scale and Federal State scale Base Flow Index from hydrograph separation National scale
Hungary
Typology Italy Base Flow from hydrograph separation Local scale
Lithuania*
Netherlands*
Poland* Base Flow from Hydrograph separation National scale Portugal Base Flow from Hydrograph separation Local scale Scotland* Base Flow Index from Hydrograph separation
used National
Spain
* Information from original WP2 questionnaire
• Information input direct to table at Orleans workshop
All the methods put forward are suitable for assessing baseflow contribution, and there are advantages and disadvantages to each method. It can be seen that a variety of methods are in use but that hydrograph separation is the method most commonly used by partners to determine baseflow [10 out of 12 responses] at a national scale. Out of the ten partners that use hydrograph separation as their main method of establishing baseflow there are three that appear to use the resulting information for further interpretation and modelling at a national scale. It appears that methods such as groundwater quality monitoring methods, and the use
of tracers, are widely used although usually only to determine groundwater – surface water interactions at a local scale.
More detailed information was requested on hydrograph separation and tracer studies in order to assess the extent of data available and the format in which it was kept.
• Hydrograph Separation
The following table summarises the responses to specific questions about the methods of groundwater flow contribution to surface water.
Is the baseflow figure a true figure of groundwater contribution or an indicator?
Does method allow for variation with geology, land
use etc
How is information
stored?
Who carried out assessments and
when?
Austria True figures (mm/a; % precipitation/region)
Allows variation with geology;
soil type; land use
Data sheets and GIS
DANUBS (2003); local scale
Belgium*
Bulgaria Natural groundwater resources in different geological formations and aquifers (m3/s) and rates of groundwater flow(l/s/km2)
Method applied for different geology in varied terrain
Paper map, text
National Institute of Hydrology and Meteorology – river flows
Denmark True figure Yes Digital Assessment: Danish
Land Development Service (DDH). From about 1920’s – present.
DK model: GEUS (1995- 2005) England and
Wales
Indicator, shown as a baseflow index fraction
Hydrology Of Soil Types (HOST) allows for soil and geology variations
Electronic databases.
Map?
Institute of Hydrology (now Centre for Ecology and Hydrology, CEH)
Estonia - Yes, seasonal
variation reflects conditions in river basin
Electronic - Excel worksheets
Dr Arvo Jarvet, University of Tartu
France True figure specially where piezometric data cannot be used ( Karsts) and where
runoff/drainage ratio is in favour of drainage ( percolation)
Allow variation with land use as well as with rainfall or geology
National data bank “Hydro”
Research Institutes, States agencies, consultant agencies
Germany* - Yes Digital models Example: F.Z.Julich,
2003-2005 + for federal states (a manifold of several case studies) Hungary Indicator, Base Flow Index Variation in
types of surface water body and aquifers
Digital map BME (University) 2004
Italy True figures (mm/a
%precipitation/region) Variation with geology, soil type and land use
GIS University and
National Environment Agency
Lithuania*
Netherlands*
Poland* Indicator Paper maps
and partly electronic
Institute of Meteorology and Water Management and Polish Geological Institute.
Portugal True figure Allows for
geological variation
GIS Civil Engineering
National Laboratories (LNEC) 2003
Scotland* - - - -
Spain
* Information from initial WP2 questionnaire
• Age of water
Given that the proportion of water derived from deep in the aquifer compared with water from shallow depths is likely to vary with age, the questionnaire requested information on water determining the age of water. Responses are summarised in the following table.
Is there any routine testing or local scale testing carried out to determine the age of the water?
Austria Local case studies only. Routine testing to be developed soon.
Belgium*
Bulgaria Local scale – isotopes.
Denmark
3H used in National monitoring programme (1989-1997). CFCs used after 1997. New approach uses 3-D integrated groundwater-surface water model and simulated transport of environmental tracers (e.g. CFC’s, 3H/3He, 85Kr and SF6).
England and Wales No routine testing. A few specific studies e.g. tritium and more recently CFCs, SF6
Estonia -
France* 14C, 3H, CFC but only at study sites
Germany* Local scale testing for local problems only. No systematic testing.
Italy Local scale testing.
Hungary Some use of 3H and 14C Lithuania*
Netherlands*
Poland* Local scale testing (3H, SF6 and 14C)
Portugal Local case studies only, using 3H and 14C. Some artificial tracers.
Scotland*
Spain
* Information from WP2 Questionnaire
This shows that the age of water is frequently determined by the use of tracers, either natural or introduced. These methods predominantly appear to be used for detailed, local scale testing, rather than at a regional or national scale.
Hydrograph separation techniques are widely used but usually for larger scale systems with good flow gauging data. Details from the questionnaires show that the hydrograph separation methods used by various partners use the same concept, but the methods are slightly different so the results would not be totally comparable.
The data produced from the hydrograph separation techniques is often held on paper maps, GIS and electronic databases. This would allow the information to be easily handled.
In some countries the method of determining baseflow data has been enhanced by modelling techniques. For example, modelling of water quality in Poland. Denmark, England and Wales also use modelling, on a national scale linked to other catchment variables (land-use, drainage, etc.) to assess groundwater – surface water interactions.
The modelling method used in Denmark is far more detailed and concentrates far more on land-drainage. Due to geological circumstances in Denmark the type of model is perfectly suited to their needs, but it appears to be less easily adapted to the more wide-scale view of the groundwater bodies required for the different geological and geomorphological situations in all Member State countries.
Also of interest, regarding suitable scale and information requirements, were the comments from Denmark regarding the observed relationship of groundwater bodies to the surface water system (or Groundwater Dependent Terrestrial Ecosystem [GWDTE]). “Interactions between different types of groundwater bodies (local versus regional size and direct versus indirect contact) and GWDTE and surface water bodies have been incorporated in the River Valley Types of the GSI typology. These characteristics of the groundwater body influence the amount and stability of groundwater discharge to the GWDTE and surface water bodies (Dahl et al., 2004).”
7.5.4.2. Applicability / Future use of methods
It is important that the method chosen for use by a Member State is consistent with the data available and the pressures. The scale of the method should be considered. Ideally a hydrograph separation method should be used at a large scale but tracer or water quality / temperature methods can be used at a more localised smaller scale.
The method should be adjusted to account for the most important mechanisms. This is particularly appropriate for modelling specific important mechanisms, for example land drainage. Where possible, it is best for the scale to allow initial assessment at a groundwater body scale. This will not preclude a more detailed scale study if it is available.
As Member States have access to varied amounts of data and may be considering different pressures it seems appropriate to suggest a tiered approach to determine which model type is appropriate for use.
Tiered approach to determining suitable method for use
Baseflow estimation methods.
Tier 1
- Little data
- Age/ Tracer/ Low Flow Survey
- Provides relative importance of groundwater input
Tier 2
- More data
- Hydrograph separation with regional modelling
- Gives better understanding of relative importance of groundwater input and spatial understanding within catchment
Tier 3
- Large data requirement - Deterministic modelling
- Gives predictive understanding of groundwater input under different conditions within catchment
- Allows assessments of future changes to be made
Increasing effort, cost, complexity and reliability of results.
If adequate resources and data are available the ideal method to use for considering the contribution of groundwater to a surface water body at a catchment, or groundwater body scale, is hydrograph separation combined with deterministic modelling. As outlined in section 7.5.2, there are many variables / complex factors governing the proportion of surface water to groundwater flow in a surface water body at any particular given point and time. By combining an assessment of the response times of a river to precipitation events and seasonal changes, by assessing the hydrographs, with basic knowledge of the catchment soil type, geology and landuse, etc., a model of the catchment can be derived. Ultimately modelling can allow the base flow data to be used as a predictive tool by linking the results with data on land drainage, land use, geology, etc., and to give flow statistics. By linking base flow data to all the variables in a catchment, or groundwater body, and having all the variables available in electronic (or GIS) format it becomes a very powerful tool. The data produced can be transferred to provide predictions, and assess the impact of changes, for ungauged catchments of similar geological / soil type / land-use nature.
As the questionnaire responses show there are several Member States that collect the data required to carry out wide-scale base flow assessment by hydrograph separation techniques and carry out predictive work by modelling the possibilities of doing this have been further researched in this paper.
7.5.5. Available data-sets / knowledge available for use within Member States 7.5.5.1. Baseflow from hydrographs
Data input requirements for baseflow estimation by hydrograph separation:
The required data inputs to estimate baseflow by hydrograph separation including river water level or flow gauging from a representative network with regular (preferably daily) readings.
River flow gauging at national scale
River flow gauging at local scale
Number of monitoring locations
Frequency of measurements
Austria Yes Yes 800 (600) Daily
Belgium*
Bulgaria Yes Yes ? Daily and monthly
Denmark Yes Yes A few hundred Daily
England and Wales Yes Yes 1,349 Daily and sub-daily
Estonia Yes Yes 30 rivers daily
France Yes Yes 2,400 Variable
Germany* Yes Yes Several thousand Daily
Hungary Yes Yes 330 Daily and sub-daily
Italy Yes Yes - Every 10 minutes
Lithuania*
Netherlands*
Poland* Yes Yes Approx 1,000 Daily
Portugal Yes Yes Daily
Scotland* Yes Yes
Spain
“Sub-daily” includes several times per day (i.e. 2, 4, 12), hourly, every 15 minutes and continuous, etc.
* Information from initial WP2 questionnaire
It is recognised that baseflow assessments from hydrograph separation can provide an indication of groundwater flow to surface water systems. It may not give an exact figure, due to several influences, but it gives a reasonable estimate at a large scale.
From the information in the table detailing input requirements it appears that most Member States (11/12) have daily gauging of water levels and flow in a good network of streams and rivers. Some Member States have separate water level monitoring of lakes. From this initial information it would, therefore, appear that most Member States have the ability to carry out baseflow estimation by hydrograph separation from data they already collect. The following table shows that the data is frequently stored in electronic format that suggests that it could potentially be used in assessments at a large scale, appropriate to use at groundwater body scale.
This initial idea should be viewed with some caution, however, as a further, more detailed, assessment of the information shows that there may be some localised problems with this idea. Potential problems include data storage, inadequate monitoring, and other strong influences on flow regimes that mask information on baseflow (including land drainage, releases from hydroelectric power systems and snowmelt, etc.). The data collation and storage issue is discussed further after the appropriate tables. The flow regimes that can mask the information on baseflow from groundwater are discussed further here.
In Denmark the input to surface water systems is strongly influenced by land drainage. The conceptual models, the specific monitoring method used and the models used in Denmark allow for this to be taken in to account. The models used are specifically developed for the circumstances, and will give a far more accurate result than trying to use a more generic model.
One partner, Austria, has raised specific concerns regarding the use of flow gauging data for the determination of baseflow from groundwater. The concerns raised may apply in limited areas of several other Member States too.
- The generation of hydroelectric power in many rivers in Austria leads to the storage and controlled release of water. This would result in hydrographs that partly reflected the flow regime associated with the release of water for power generation and would make it hard to interpret the information. It is possible to subtract the influence of this flow to enable correct interpretation but it would require additional information to be collected and additional data interpretation. A more detailed model would potentially be required where hydroelectric power stations are situated.
- The impact of snowmelt on the flow and level of streams and rivers in Austria is ascertained by assessing water level data and temperature data from gauging stations.
This effect will partially mask the baseflow data. As the impact of snowmelt has been studied historically it should be possible to use the information to subtract the influence of the snowmelt flow from the hydrograph. It should, therefore, be possible to ascertain enough information to enable an interpretation of a hydrograph to determine baseflow. A more detailed model would potentially be required in areas where there is significant influence of surface water flow by snowmelt.
While it is possible to allow for the impact of both hydroelectric power station discharges and for snowmelt it is likely that the resulting calculation of the proportion of baseflow is likely to be less accurate than on a less influenced system. As additional data collection is required and more detailed modelling is expected there is likely to be an impact on time and costs in the Member States affected.
Estonia also reported poor correlation of data during periods of “ice jam” (when the streams and rivers are frozen.
