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Research for Policy Support
D10: Impact of hydrogeological conditions on pollutant behaviour in groundwater and related ecosystems.
Volume 3
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)
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Content of Volume 3
Chapter 9 Impact of quantitative alteration on groundwater quality ... 5
9.1 What means change of quantitative status?... 7
9.1.1 Synthesis ... 7
9.1.2 Direct groundwater abstraction ... 7
9.1.3 Man induced groundwater injection/infiltration... 15
9.1.4 Indirect impact of human activities on recharge/discharge ... 20
9.2 Quantity related aquifer responses and triggered processes with impact on groundwater quality ... 25
9.2.1 Seawater intrusion in coastal aquifers and fresh water intrusion in marine aquifers ... 26
9.2.2 Leakage/draining between aquifers ... 36
9.2.3 Lowering/fluctuations of the groundwater table in aquifers due to pumping or natural variations... 40
9.2.4 Quantitative status of associated surface waters... 45
9.2.5 Direct injection and irrigation return flow... 45
9.3 Matrix Actions/quantity impact/triggered processes/quality parameter influenced ... 53
9.4 Assessment of quantitative impacts on quality ... 55
9.4.1 Time scales of quantitative and qualitative stresses on groundwater systems... 55
9.4.2 General procedure for practical assessment approach ... 61
9.4.3 Quantifying hydrodynamical changes ... 62
9.4.4 Quantifying hydrochemical impact (I): natural hydrochemistry ... 66
9.4.5 Quantifying hydrochemical impact (II): changes of the natural hydrochemical system ... 68
9.4.6 Quantifying hydrochemical impact (III): cross-correlation of hydrodynamical changes and hydrochemical impacts... 68
Chapter 9
Impact of quantitative alteration on groundwater quality
Wolfram Kloppmann (BRGM)
Carlos Martínez, José Antonio de la Orden, Juan Grima (IGME) Marc Van Camp, Marleen Coetsiers, Kristine Walraevens (LAGH-UGent)
Andres Marandi, Enn Karro (UT)
Jaroslaw Kania, Stanislaw Witczak, Andrzej Zuber (DHWP/AGH) Teresa Melo (Univ. Aveiro)
Jan Hoockey (EA)
The overall information provided in this chapter is structured as follows:
- listing of typical situations where change of quantitative status of a groundwater body may impact qualitative status (both human induced changes and natural/hydroclimatic changes);
- relation between:
. (1) human action on quantity, . (2) aquifer response,
. (3) triggered physical and chemical processes and . (4) quality changes.
. These relations are described in detail in chapter 9.2 and are synthesised in chapter 9.3 in form of a matrix.
- methods for detecting relation between quantitative and qualitative status changes (chapter 9.4);
- illustration through some case studies (in chapter 9.2 and case study files in Annex 1);
- synthesis: Implication for pollutants behaviour according to properties described within WP1- Implications for defining thresholds.
9.1 What means change of quantitative status?
9.1.1 Synthesis
Wolfram Kloppmann, BRGM
This chapter focuses on the types of human action and natural variations that have an influence on groundwater quantity. The different categories of human actions are briefly synthesised. The focus lies both on the question “why”, i.e. the human needs and uses leading to actions of groundwater abstraction/injection/indirect modification (e.g. agriculture, industry, etc.) and on the question “how”: which technical devices are used and which is the impact of their use on groundwater quantity. Special focus lies on the quantity impact of each action that can be expected as a function of:
- (1) quantity abstracted or injected (directly or indirectly, by modification of natural recharge);
- (2) “geometry” of the human action (e.g. depth and repartition of wells, surface area of landuse changes…);
- (3) aquifer parameters (storage coefficient, transmissivity, specific yield…), types of aquifers (karstic, fissured, porous);
- (4) geometry of aquifers/hydraulic behaviour (mono-, multi-aquifer systems, confined-, semi-confined, unconfined).
9.1.2 Direct groundwater abstraction Wolfram Kloppmann, BRGM
9.1.2.1
9.1.2.2 Introduction
It seems obvious that direct groundwater abstraction figures among the quantitatively most important human interventions with impact on groundwater quantity. Information on 25 EU countries obtained from expert questioning (EEA, 1999) indicates that the most common and important human interventions are related to groundwater abstraction. Globally, about one fifth of total water abstractions in the EU and the USA is groundwater. The total percentage of groundwater abstraction (21% for the USA in 2000, USGS 2000, 18% for the EU
according to OCDE, 12% according to EEA, EEA 1999) gives only a rough indication of the relative importance of groundwater in the overall water supply. The share of groundwater varies considerably for different categories of use and for different countries:
- Categories: For some categories, groundwater makes up more than 50 % of water abstractions (domestic use with 98% of groundwater, lifestock with 57% in the USA) or more than 1/3rd (public supply with 37%, irrigation with 42%) according to USGS (2000).
- Countries: The relative availability of groundwater resources compared to surface water resources induce huge regional differences in the relative share of groundwater abstraction ranging between 91% for Iceland and 9% for Belgium and Spain (EEA, 1999).
Total amounts of extracted water and groundwater in the US (Figure 0-1) followed roughly the increase of population till the eighties, where a peak was reached due to large industrial, irrigation, and thermoelectric-power withdrawals, and is more or less constant since. The total amount of groundwater extraction in the US in 2000 was 0.32 billion m3.
Figure 0-1: Trends in population and freshwater withdrawals by source in USA, 1950–2000 (USGS, 2000)
These total volumes of groundwater abstraction have to be compared to the long-term recharge if the long-term sustainability of groundwater resources exploitation is to be estimated. Excess abstraction with respect to recharge is generally referred to as over- exploitation. Difficulties of this concept lie in the fact that
- the long-term recharge is often difficult to estimate
- adverse effects (physical, chemical, economic, ecological or social) can arise locally or regionally at different time scales even if the strict criterion of “over-exploitation” is not matched (see discussion in EEA, 1999)
In the context of this report, we will focus on the effects of groundwater abstraction on groundwater quality. Such effects can occur even if groundwater abstraction is quantitatively sustainable only by changes in the groundwater flow patterns. These effects can occur locally or globally, whereas “over-exploitation” is a term that refers to the groundwater resource (groundwater body) as a whole. We will therefore not use this term in the following discussion.
This Chapter focuses on the following main points:
- In which ways will groundwater abstraction change groundwater quality?
- What is the spatial and temporal scale of this influence?
9.1.2.3 Impact of pumping on groundwater quality
The following table resumes the pressures leading to direct groundwater extraction, the concrete action on groundwater quantity, the resulting aquifer responses triggering physical and chemical processes. The final quality changes are resumed in chapter 9.2
Human pressure Human action on quantity
Aquifer response
Triggered
physical and chemical
processes
Quality changes
Public water supply
Domestic supply Agriculture (lifestock, irrigation) Mining Industry
Cooling-electricity
Drilling of wells
Pumping from wells and boreholes,
Water evacuation from deep mines and open pits, Groundwater
drainage from drainage
channels
Drawdown of the unconfined
groundwater table or the pressure level (confined) leading to a cone of depression Horizontal
deviation of the natural flow pattern
Vertical deviation for partially penetrating wells and multi-layer aquifers
Gradient changes with respect to surface water bodies (sea, rivers, lakes, wetlands)
Mixing of waters from different aquifers through vertical or lateral inflow (different water qualities, pollution status, redox state, salinity)
Mixing with seawater
Changes in the contribution of surface water to the groundwater Oxygenation of anoxic
groundwater Salinisation Dissolution/
precipitation
processes in particular in the vicinity of the well Pollution by substances used for drilling or well design
resumed in chapter 9.2
9.1.2.4 Spatial and temporal scale of the influence of groundwater extraction
In a large majority of cases, groundwater is extracted from wells or boreholes. They represent punctual “singularities” in the horizontal groundwater flow and will deviate this horizontal flow over a defined surface of the aquifer. Their range of impact will depend on the - vertical extension well/borehole and well design (multi-level well, connecting several
aquifers, fully or partly penetrating well):
- the rate of discharge
- the aquifer parameters (transmissivity, porosity, natural gradients)
- the connection to surface waters (in particular for wells in alluvial aquifers with a significant contribution of bank filtrate)
- in general: the spatial setting of boundary conditions (recharge boundaries like surface waters feeding the aquifer, discharge boundaries like impermeable boundaries or surface water receiving groundwater)
- the type and substances used for drilling (direct impact of the well drilling on groundwater quality)
• Area of impact of a pumping well on groundwater flow:
Pumping in a groundwater withdrawal well will induce a drawdown of the wellhead, the cone of depression (either of the free groundwater table in the case of an unconfined aquifer, or the potentiometric surface for confined aquifers). The area where such a drawdown is measurable is called the zone of influence (see Figure 0-2). When pumping starts, the depression cone will grow (transient state) till steady state conditions are reached. The final extension of the zone of influence will depend on aquifer properties (transmissivity…), on well characteristics, on boundary conditions and on the discharge rate. All flow lines passing through the zone of influence are horizontally deviated with respect to natural flow directions.
The three-dimensional volume within an aquifer that contributes groundwater to a pumping well is called zone of contribution (see Figure 0-2). This zone is limited by groundwater divides or other physical boundaries. It can be divided into zones of transport defined by the volume within an aquifer that contributes water to the withdrawal well (or any discharge feature) within a specific travel time. In principle, outside the zone of influence, natural flow conditions prevail and no impact of pumping in terms of quantity or quality is expected. On the other hand, any quantity or quality change within the zone of contribution will have an impact on the quantity and quality within the zone of contribution and can also potentially change the shape of the zone of contribution.
Zones of influence of different discharge features may superpose so that pumping leads to a large scale drawdown within aquifer systems even if individual wells are expected to have only a limited impact.
Figure 0-2: Zone of contribution, influence and transport under sloping water-table conditions (Renken et al., 2001)
• Impact on vertical flow, mixing between aquifers, pollution by the well:
Any drilling or digging of wells disturbs the natural quantity and quality conditions in a groundwater body. This is particularly relevant when a scientifically sound characterisation of groundwater quality is searched for (Kloppmann et al., 2001). The main disturbances (Figure 0-3) are related to:
- The construction of the well (drilling, digging): If drilling additives other than water are used, these additives will give rise to a local pollution around the borehole. Once pumping has started additives from the borehole, the annular space and the aquifer in the vicinity of the borehole will progressively be eliminated. The time lag of complete elimination will depend on the extent of penetration of the pollutants into the groundwater body, of the nature of pollutants (adsorption on aquifer/borehole material), the pumping rate and other factors related to the well construction. The annular space around the tubing represents a favourite pathway of gases (O2, CO2), surface waters, and surface pollutions into the groundwater body. A badly conceived annular space can lead to local oxidation processes,
especially when it brings into contact dissolved oxygen and anoxic groundwaters. This can lead to scaling (metal hydroxide precipitation) that can make wells unusable.
- The material of the tubing: Metal tubings can induce a local pollution with heavy metals.
This is relevant for monitoring wells where such pollutions can mimic a general pollution of the groundwater body.
annularspace
+O2,CO2, surface water, pollutions, drilling additives tubing metals...
Zone of impact of the well construction
annular space +O2,CO2,...
Zone of impact of the well construction
(a) No pumping (b) Pumping
Figure 0-3: Impact of well construction on groundwater quality due to the use of drilling additives - The conception of the well: Multi-layer aquifers where different permeable horizons are separated by more or less impermeable layers frequently show distinct and highly contrasted groundwater qualities for the different aquifers (in particular deep, saline, anoxic layers and shallow, fresh, oxygenated, often polluted layers). Even if vertical hydraulic gradients exist between the different aquifers, mixing is prevented by the impermeable layers (Figure 0-4 a).
Drilling a multi-level well, screened in the different permeable horizons into such a system leads to hydraulic short circuits between the aquifers and to mixing, following the vertical gradients (Figure 0-4 b). Salinisation of shallow freshwaters, penetration of anoxic groundwaters into oxidised layers and, in general, all types of chemical re-equilibrations (dissolution/precipitation etc.) are the consequence. Even simple temperature contrasts (thermal water inflow) can lead to chemical re-equilibration and therefore to quantity changes. In most cases such thermal waters will also be highly mineralised. Such hydraulic short circuits also frequently occur through a badly conceived or constructed annular space even for wells that are screened only in one permeable layer.
Aquifer 1
Aquifer2
wellhead aq 1
wellhead aq 2
Aquifer1
Aquifer2
wellhead aq 1
wellhead aq 2 espace annulaire
Quality ? Hydraulic short circuit by a multi-level borehole or a badly conceived annular space
Figure 0-4: Vertical mixing of groundwaters in multi-layer aquifers (a) due to hydraulic short circuits by multi-level wells (several screened intervals in the same tubing) or by badly constructed annular space
(b)
The different local quality changes around a borehole will superpose so that it can be extremely difficult to asses the non-impacted groundwater quality on a larger than local scale (Figure 0-4 a). A solution that is adopted for high quality monitoring of multi-layer aquifers or for sustainable exploitation of a multi-layer groundwater resource are multi-tube boreholes with annular spaces and screenings that isolate each of the permeable layers (Figure 0-4 b)
Aquifer 1
Aquifer 2 Wellhead aq 1
Wellhead aq 2
Aquifer 2
Quality????
Aquifer 1 global
pumping
Quality1 Quality2
(a) Superposition of local quality impacts of pumping well
(b) Technically sound solution of multitube boreholes with distinct screened intervals
Figure 0-5: (a) Superposition of local quality impacts around a pumping well (hydraulic short circuits, contamination by drilling material and well material) (b) Technically sound solution of independent
tubings specifically screened in separate aquifers and isolating annular space.
Vertical mixing can also occur within a single confined or unconfined aquifer when pumping is performed in partly penetrating wells. Such wells will lead to a vertical flow component in the vicinity of the bottom of the screened interval.
More details and case studies are given in chapter 9.2.2.
• Connection of aquifers to surface water (sea, rivers…) and impact of pumping:
Surface waters that are hydraulically connected to groundwaters constitute boundary conditions to the aquifer, either recharging boundaries if the surface waters feed the groundwater body (groundwater surface above the potentimetric surface of the groundwater) or discharging boundaries if the surface water is alimented by the groundwater (e.g. base flow of rivers). The gradient between surface and groundwater can vary along the year depending on the surface water levels. Pumping will decrease the potentiometric surface of the groundwater within the zone of influence (depression cone). This can lead to steeper gradients and to increased flow of surface water to the aquifer in cases where surface waters feed the groundwater. In cases, where groundwater flows into the surface water (river baseflow, coastlines), the natural gradient groundwater-surface water can be inversed leading to surface water inflow into an aquifer that wound naturally aliment the surface water body.
A specific case is seawater intrusion along a coastline (Figure 0-6). The shape of the natural interface between fresh and saline water is determined by the density difference between saltwater and fresh water. Under natural conditions, this interface will intersect the earth surface at the shoreline and show a slope towards the inland so that continental freshwaters are underlain by salt water near the cost. Wells near the coastline can lead to
- upwelling of saltwater through introduction of a vertical flow component in areas, where freshwater is underlain by saltwater.
- horizontal displacement of the salt-fresh interface through horizontal flow in inland direction where natural flow is towards the ocean.
Several case studies of seawater intrusion into coastal aquifers are mentioned in chapter 9.2 and in annex 1.
Water table Water table
Water table Water table
Freshwater, density ρ Saltwater, density ρsinterface
Freshwater, density ρ interface
Saltwater, density ρs
Ocean Ocean
Figure 0-6: Position of the saltwater-freshwater interface in a coastal aquifer, natural conditions and pumping conditions.
• Time scale:
Pumping consists in a hydraulic disequilibration of the groundwater body. A new equilibrium will be reached after a given time lag leading to a modified but stable flow pattern in the groundwater body. Depending on factors related to pumping (geometry of discharge device and discharge rate) and to the aquifer (permeabilities), the time to reach this new equilibrium will be more or less long. As pointed out by EEA (1999), a continuous decrease of groundwater levels is not synonymous to overexploitation of the aquifer. It only means that the groundwater body is still in a “transient state” and that the new equilibrium is not yet reached. Transient state may last for hundreds or thousands of years in cases of aquifers
with relatively low permeability. This also implies that the zone of influence will continuously grow larger till steady state conditions are reached.
9.1.3 Man induced groundwater injection/infiltration Wolfram Kloppmann, BRGM
On a global scale, direct, man induced groundwater recharge is negligible compared to natural recharge and even to direct groundwater abstraction. Nevertheless, on aquifer scale, direct injection/infiltration can induce significant modifications of the quantitative and qualitative status of the reservoir. Motifs for direct injection/infiltration can be classified as follows:
- to use aquifers as a reservoir for temporary storage of water for later use, this implies an at least temporary amelioration of the quantitative status of the aquifer;
- to use the aquifer as a reservoir for fluids that cannot be used or are undesirable at the surface. These can be fluids originating from the aquifer itself (oilfield brines, geothermal fluids), from other aquifers or from the surface (liquid waste, industrial brines, etc.).
Aquifers are also increasingly used for gas storage (CO2 sequestration);
- to voluntarily change flow patterns in the aquifer by injection in the aim to deviate or block contaminant plumes or to build up hydraulic screens against saline intrusion.
Human induced recharge of aquifers can be active infiltration/injection or passive (e.g. losses from water distribution networks or agricultural backflow). Common to all injection systems is the fact that the waters that are injected into a natural groundwater system are
- foreign to this natural system (e.g. treated waste water);
- or originate from the natural system but have been transformed before reinjection (e.g.
cooling or degassing of geothermal fluids).
This induces mixing of fluids with different chemical and/or physical characteristics within the aquifer leading to:
- direct contamination by undesirable or toxic substances;
- and/or shifts of the chemical equilibria in the groundwater mass leading to “secondary”
quality changes (e.g. mixing of oxygen-rich fluids with reducing groundwater leading to mobilisation/fixation of trace element, mineral precipitation/dissolution, etc.).
These mechanisms are common to other triggering actions that induce mixing of different fluids and are discussed in chapter 9.2. In the following, we will have a closer look on several types of human actions that induce active or passive recharge of aquifer systems, artificial recharge, geothermal and oilfield extraction-injection loops, liquid waste disposal, agricultural return flow, reflooding of mines.
9.1.3.1 Artificial recharge (Management Aquifer Recharge MAR)
Management Aquifer Recharge MAR (http://www.iah.org/recharge/), commonly termed
“artificial recharge”, is a groundwater management option that allows:
- restoration of the groundwater levels indicating amelioration of the quantitative status;
- amelioration of the infiltrating water quality (SAT, soil aquifer treatment) and of the groundwater quality (e.g. when groundwater is naturally saline);
- storage of temporarily more abundant surface waters for periods of lower water availability;
- beneficial changes of the flow patterns (hydraulic barrier against saline intrusion…), and - the use of the aquifer as water distribution system. Water types used are more or less
treated wastewater, stormwater, surface water, bank filtrate of rivers, desalinated brackish or seawater.
The main injection/infiltration systems are surface reservoirs, injection wells, trenches, basins. Enhanced bank filtration into aquifers hydraulically connected to rivers or lakes, artificial flooding of flood plains and surplus irrigation are other techniques of artificial recharge. The two main configurations are ASR (Aquifer Storage and Recovery) and ASTR (Aquifer Storage, Transfer and Recovery) (Rinck-Pfeiffer, 2004). In the first case groundwater is injected in the aquifer and recovered at the same point at a later time (e.g.
injection well = pumping well). In the second case, injection and abstraction are spatially separated. Spatial and timescale of the two configurations are different. Both the zone of influence and the residence time of injected water are potentially lower for ASR than for ASTR (Figure 0-7).
a b Zone of influence
Residence time
ASR ASTR
Figure 0-7: Differences between ASR (aquifer storage and recovery) and ASTR (aquifer storage, transfer and recovery). a) spatial configuration (Rinck-Pfeiffer, 2004), b) spatial influence on the
natural aquifer system and residence time of injected water.
Impact on resource quality and the related risks are main concerns about MAR and are the subject of numerous ongoing projects and case studies (see chapter 9.2.6.), e.g. the 6th FP STREP ReclaimWater (contract 018309).
9.1.3.2 Geothermal facilities
Reinjection of generally highly saline fluids produced in geothermal fields into the geothermal reservoir fulfils two conditions of optimal development and management of a geothermal resource (Stefánsson, 1997):
- geothermal brines have to be considered as waste and regulations in most countries do not allow surface disposal of these brines. Reinjection avoids environmentally more harmful solutions as brine disposal in the sea;
- reinjection is a powerful method for increasing the longevity of the geothermal resource and the amount of energy that can be extracted.
It is evident that the impact on groundwater resources can be expected to be limited as:
- geothermal resources are mostly situated in depths that are excluded for “normal”
groundwater uses;
- fluids encountered in these depths are usually too saline to be used for any other purpose than heat production;
- in order to be efficient, re-injection has to be performed within the exploited reservoir and any loss of fluids is avoided in an optimised system. This limits the potential contamination of other (shallower) reservoirs by the geothermal loops.
Nevertheless, reinjection of the cooled fluids, oversaturated with respect to mineral phases like silica, will induce important changes of the reservoir geochemistry that can lead, via mineral precipitation to a decrease of porosity and permeability of the reservoir and a decline of the yield of the geothermal resource. We have here a kind of feedback loop starting with a quantity impact of a human action on the reservoir (pumping/injection of geothermal fluids) triggering a quality response of the system (scaling) that has itself an impact on the potential quantity of fluids/heat that can be recovered from the system. Supplemental water from additional sources, such as surface waters, storm waters, ground water, and wastewater treatment effluent, is sometimes injected in addition to geothermal fluids to replace mass lost through condensate evaporation (Figure 0-8).
Figure 0-8: Example for the extent of groundwater extraction-reinjection volumes in the southern production area of the Los Azufres geothermal field (Mexico) (Arrelano & al., 2005).
9.1.3.3 Thermal storage, low enthalpy geothermal systems
Ground source heat pumps (GCHP) and underground (aquifer) thermal energy storage (UTES or ATES) are now in a phase of rapid market penetration in European countries (Germany, Switzerland, Austria, Sweden, Denmark, Norway, France) and in the USA (Sanner & al., 2003). The principle is to combine a heat pump with an underground heat exchanger. The system can be used for heating (heat extraction from the ground) or for thermal energy storage (for cooling purposes, storage of solar and waste heat). Figure 0-9 shows the increasing number of installed heat pumps in several European countries. The highest share of ground coupled heat pumps in total residential heating demand reaches 1%
in Sweden and Switzerland (Sanner & al., 2003). The heat exchange system can be open loops (pumping-reinjection of groundwater) or closed (borehole heat exchange BHE). With
respect to groundwater bodies, in terms of quantity-quality considerations, only an open system is relevant. Because water is not consumed by GCHP systems, in practice, the injected flow will be the same as the pumped flow, so that the overall quantity impact will be negligible. The quality of GCHP injectate also usually reflects the characteristics of the source ground water. As for high enthalpy geothermal power plants, the main quality impact will be due to cooling or heating leading to modification of geochemical equilibria within the aquifer. Main processes are dissolution/precipitation (scaling). Injectate can also contain:
metals leached from the pipes and pumps; bacteria (where oxygen, nutrients, and a source of bacteria are present); precipitated ferric iron solids (where dissolved iron is present in source water, and the GCHP system introduces oxygen); and chemical additives sometimes used for disinfection or corrosion prevention (EPA, 1999). Some examples (Griffoen and Appelo, 1993) will be discussed in sub-chapter 9.2.6.
Figure 0-9: Number of installed heat pump units in some European countries (Sanner & al., 1999;
Donnerbauer, 2003). Dark-coloured columns: 1998, GSHP only (Sanner, 1999); Light-colour columns:
2001, all HP (Donnerbauer, 2003). Cited from Sanner & al. (2003).
9.1.3.4 Injection wells as part of groundwater remediation strategies
Current technology for treatment of groundwater contaminated with chlorinated hydrocarbons as well as with metals and/or radionuclides is “pump and treat”, followed by disposal or reinjection of treated water (Figure 0-10). This process can be costly and inefficient due to the difficulty of completely removing the contaminated groundwater and sorption of contaminants on mineral surfaces. The quantity/quality impact of re-injection can be considered as negligible with respect to the initial contamination.
Figure 0-10: Example for the extent of groundwater extraction-reinjection as part of a decontamination strategy of a contaminated aquifer (Christ & al., 1999).
Salt water intrusion barrier wells are used to inject water into a fresh water aquifer to prevent the intrusion of salt water. Control of salt water intrusion through the use of these wells may be achieved by creating and maintaining a "fresh water ridge" (EPA, 1999). Alternatively, injection-extraction systems are used that inject fresh water inland, while intruded salt water is being extracted along the coast.
Injected waters include untreated surface water, treated drinking water, and mixtures of treated municipal wastewater and ground or surface water. Because protection of drinking water supplies is the major goal of a salt water intrusion barrier well and the injectate typically meets drinking water standards, salt water intrusion barrier wells are unlikely to receive spills or illicit discharges of potentially harmful substances. Ground water monitoring and toxicological, chemical, and epidemiological studies have found no measurable adverse effects on either ground water quality or the health of the population ingesting the water, when the injectate was treated wastewater effluent (EPA, 1999).
9.1.3.5 Reclamation of mines
Mined out portions of underground mines are frequently filled with mixtures of water and sand that may contain a large variety of other materials (ashes, coal cleaning waste, mill tailings, acid mine drainage treatment sludges, flue gas desulfurisation sludges…) in the main goal of subsidence control. EPA (1999) reports 5,000 documented and around 8,000 estimated mine backfill wells in the US. Given the complex contamination situation of groundwater in most mining areas, it seems difficult to distinguish the impact of backfill injections from the high background contamination. The probability that backfill injection will contribute to groundwater contamination depends on the nature of the inectate as well as on a variety of site specific characteristics (mineralogy, hydrogeology, geochemistry…). No incidents of contamination of drinking water resources could be directly linked to mine backfill in the US (EPA, 1999).
9.1.3.6 Brine injection from desalination/oil-gas exploration
Petroleum bearing formations usually contain highly saline fluids. These are sedimentary brines, either fossil, evaporated and geochemically altered seawater or secondary brines from solid salt dissolution. These brines are produced together with oil and gas and traditionally dumped into pits and gullies, nowadays mostly reinjected into their original or another deep aquifer formation.
The potential impact of these practices can be considered much higher than in the case of high enthalpy geothermal production-injection loops where usually both production and
injection wells are carefully engineered because power production and the longevity of the reservoir depends on the well quality. In the Texan Gulf Coast Aquifer, thousands of injection wells operated in the past and present so that injection conditions are much less constrained than in the case of geothermal brines and environmental impacts have been observed, in particular groundwater salinisation = (Hudak and Wachal, 2001). Figure 0-11 shows gives an order of magnitude of injected volumes in the case of large oilfields (Birkle & al., 2005).
Figure 0-11: Example for the extent of surface water injection/groundwater-reinjection into the Samaria-Sitio Grande oil field, Mexico (Birkle & al., 2005).
9.1.4 Indirect impact of human activities on recharge/discharge Carlos Martínez, José Antonio De la Orden, Juan Grima, IGME
Human activities may affect groundwater and associated ecosystems in a number of ways. A distinction must be made between direct and indirect effect of such activities. Sometimes, there is a direct relationship between an action and its consequences on the environment. It is the case of groundwater exploitation and the change in natural groundwater regime.
In the second case, the impact is indirect, and sometimes can be observed a long time after human disturbances on the subsurface hydraulic environment, due to the inertia of aquifers in its response to external influences. These are much more difficult to evaluate, because of the complicated interactions between land, soil and vegetation.
For a given pressure, and looking from the point of view of recharge and discharge, groundwater can be altered in its quantity and in its quality. In this chapter, a general overview of how anthropogenic stresses can affect groundwater in an indirect way will be provided from the point of view of recharge and discharge, in terms of both quality and quantity.
9.1.4.1 Human activities to take into account
Activities that can have an indirect impact on groundwater resources can be categorized in a number of groups:
• Industrial and transport activities
During the last century, the Earth's average surface temperature rose by around 0.6°C.
Evidence is getting stronger that most of the global warming that has occurred since the beginning of the industrial revolution, over the last 50 years, is attributable to human activities. In its Third Assessment Report, published in 2001, the Intergovernmental Panel on Climate Change projects that global average surface temperatures will rise by a further 1.4 to 5.8°C by the end of this century. This global temperature increase is likely to trigger serious consequences for humanity and other life forms alike, including a rise in sea levels of an estimated 9 to 88cm by the end of this century, which will endanger coastal areas and small islands, and a greater frequency and severity of extreme weather events.
Human activities that contribute to climate change include in particular the burning of fossil fuels, which cause emissions of carbon dioxide (CO2), the main gas responsible for climate change, as well as other 'greenhouse' gases. In order to bring climate change to a halt, global greenhouse gas emissions must be reduced significantly.
Climate models are predicting serious problems in the future. Dramatic changes in groundwater recharge and discharge regimes and interactions between groundwater and surface water systems are expected, as aquifers are replenished by effective rainfall.
Changes in the volume and distribution of recharge will determine the effect of climate change in groundwater resources.
From the point of view of wetlands, directly or strongly dependent on groundwater, the increase in temperature, sea level rise and changes in precipitation will affect them in a significant way. In a case where the water table lowers due to a diminishing recharge caused by human actions there may be severe negative effects. It must be kept in mind that in many wetlands, the water table is very shallow. When groundwater level falls, the water level can even disappear, drying the wetland. Very often, groundwater lowering processes occur together with a general worsening of groundwater quality. Sea level rise, on the other hand, could produce coastal erosion and sea water intrusion in coastal wetlands and deltas.
Nevertheless, the incomplete state of knowledge does not permit a fully integrated assessment of climate change to make a distinction between natural and anthropogenic causes.
• Deforestation
Brought about by the conversion of forests and woodlands to agricultural land, commercial logging, fires and other factors, deforestation is originating the loss of the protective cover of vegetation.
In this context, soil erosion is an ongoing process. As deforestation increases, a soil moisture deficit is originated. It implies that groundwater recharge diminishes, because part of the surface water goes directly into streams and is never available as groundwater recharge.
Nevertheless, sometimes groundwater recharge may be incremented as a consequence of vegetation loss, especially in arid and semiarid areas, due to the fact that evapotranspiration decreases, and more water is available to recharge the aquifer.
Land conversion from grasslands and forests to agriculture have been major contributors to climate change until the industrial revolution. Nowadays, industry and transportation can be
considered as main contributors, but still an estimated 20% of the total amount of emissions from anthropogenic sources comes from deforestation. Special attention must be paid in this context to the burning of trees, which releases huge amounts of carbon dioxide into the atmosphere.
• Urbanization
It can affect infiltration, as soils that have been covered with impervious surfaces can not absorb precipitation and therefore there is a decrease in the recharge. Consequently, there is a fall in natural discharge to rivers, springs or directly into the sea in coastal aquifers. It is not only the effect of housing to account for, but streets, paved roads, parking lots and even cemeteries are factors to consider when making calculations about recharge diversion.
A second impact to account for is the rising of groundwater levels beneath the cities. As cities developed in the last century, water extraction evolved over time, from intensive groundwater exploitation to the abandonment of water wells in the cities, due to pollution of water resources, migration of industry to outer areas or some other reason.
Groundwater quality and quantity can be modified too as an indirect result of water losses in urban supply nets and sewage pipelines. The first case produces an increase of groundwater recharge. A good example is the Alicante village of Vergel, where more than 45% of water circulating by the urban supply net is lost, due to the bad conservation of the infrastructure.
This water reaches the saturated zone, becoming recharge. The second case, that is, losses in sewage nets origin groundwater pollution. The effects of this kind of pollution are not yet well known. In Spain, the Spanish Geological Survey (Instituto Geológico y Minero de España) has made a study to assess the influence of both the accidental spills in sewage nets and the spills coming from industries located inside the municipalities on groundwater quality. The effect of oil and other hazardous pipeline releases is magnified in recharge areas, due to the incorporation of pollutants into the aquifer.
A special type of urbanization is happening in coastal areas, for example in the Mediterranean coast, with recreation facilities. Golf courses may increase recharge, as recycled waste water is used to irrigate them and they are able to retain and permeate. On the other hand, if groundwater is used for irrigation of the course, the total amount of permeation could diminish instead.
• Waste disposal
Waste disposal sites are a well known source of groundwater contamination. Sewage lagoons, landfills and waste disposal facilities in general can affect recharge or discharge depending on their location. Storage and management infrastructure for waste in the vicinity of such facilities are factors to be considered when estimating indirect impacts.
The location of individual sewage treatment systems (septic systems) for homes in rural areas or buildings not connected to city sewer system is another factor to consider when evaluating risk of indirect impact to groundwater resources. High densities of such systems near recharge areas must be controlled for assessing potential impacts.
• Resource extraction
Underground or open cast mines, quarries and mineral extraction activities are likely to produce changes in quantity and quality of groundwater. In terms of quantity, if a deep mine
intersects an aquifer, variations in natural flow regime will be produced, and changes in recharge or discharge could happen. In terms of quality, a spillage or discharge of pollutants could reach the water table and affect the quality of groundwater in recharge areas.
Preparatory activities are to be considered when examining the mining sector, as large withdrawals of groundwater take place when dewatering a mine in preparation for mining.
Recharge may be affected by these activities and, if we take a closer look at the oil industry, also by the heavy use of groundwater in oilfield injection activities for enhanced oil recovery.
• Agriculture
Land use planning has often been based in requirements of demand. Changing land uses like putting irrigation into previous dry farming lands, can originate an increase of groundwater recharge by backflows. After, groundwater level rises and, sometimes, as happened in India, a negative effect is produced. This is the flooding of irrigation farmlands, which can even cause their abandonment. In the last years new irrigation areas have been created and dryland development has been made. They have contributed to an increment of groundwater recharge. Clearing of native vegetation in recharge areas, like upper parts of a catchment (which usually correspond to lowest agricultural productivity areas) could increase the amount of recharge as well. A side effect of such an increase in recharge could be that salts contained in the unsaturated zone are flushed into underlying aquifers, causing land salinity in some cases.
The opposite phenomenon can occur too, that is, if irrigation water is pumped from aquifers, groundwater levels can diminish and cause a reduction of natural discharges. If discharges are linked to wetlands, the negative effects can even cause the drying of wetlands. Other negative effects are those related to groundwater quality. It is very frequent that water quality worsens when intensive exploitation occurs. In general, land use change is the human activity that produces the most negative indirect impacts on groundwater.
Diffuse pollution due to the use of fertilisers in agriculture, mainly in irrigated areas, is the main cause of high nitrate concentrations in groundwater and, depending on the cultivated area, recharge could be affected.
• Infrastructure works
The construction of a dam or a reservoir can modify the relationship between surface water and groundwater, increasing the charge of aquifers and therefore, originating changes in recharge and discharge processes.
River regulation using reservoirs, channel impermeabilization of natural streams, canal and irrigation channels, etc. are some other factors to account for.
Construction of roads can affect both recharge and discharge. If the road is in a recharge area, pollutants may be released accidentally during construction or operation phases, or in the case of northern countries, salt added to melt snow in winter could get into groundwater.
In this case, pollutants are added to the aquifer, while if the road is in a discharge area, pollutants are washed into surface water. Movement of equipment and supplies could have an impact as well.
9.1.4.2 Examples
In Spain, there are several examples of indirect impact of human activity on groundwater recharge and discharge. The most important cases are related to land uses changes. A very well known example is the Mancha Occidental groundwater body. During the last century, in the fifties, a very dramatic change in land use occurred: more than 200,000 hectares of previously dry farmlands had irrigation added. Groundwater was used for irrigation. The pumped volumes have increased with time and total irrigated hectares. Now, the difference between natural recharge and pumped volume is about 250Mm3/year, which has caused the decrease of groundwater level of up to 30 meters in some areas. As a consequence, many surface streams have disappeared or changed into ephemeral, and so has happened with the natural discharge to the Tablas de Daimiel wetland, currently a National Park. Nowadays, this wetland only gets water from transfers coming from the Tajo-Segura aqueduct. This water has a worse quality. May be this is the best example of how wetlands can be destroyed by human activities, like the change of land uses.
This Spanish National Park (RAMSAR zone) has suffered along the last 55 years a very important reduction of its area. Now, the current extension is seven times less than the original one. Moreover, groundwater discharges through several springs, known as “Ojos del Guadiana” have disappeared since 25 years. The same has happened to the indirect discharges to the Cigüela (formerly among 0 y 324Mm3/year) and Azuer (formerly among 0 and 61Mm3/year) rivers. These discharges are almost zero since the beginning of the eighties of the last century. This quantitative data is very significant, but there is too a negative qualitative effect. What happens is that water coming to the wetland has not any groundwater component. So, the water quality is much more worse because groundwater has a conductivity of 73µS/cm, while surface water exceeds 3,000µS/cm. Salinity in the wetland has increased as an indirect consequence of human actions. Likewise, studies made have detected a diminishing of the number of plant species in the wetland, from 22 in 1976 to only 5 in 1995 (Alvarez-Cobelas & al., 2001).
The reason for this dramatic change is the drying of the wetland. It began in 1955 because of the malaria. There was an urgent need for Spain to put into exploitation more farmlands to alleviate the famine that Spain suffered after the civil war. This phenomenon reduced the former 150km2 of the wetland by a 130km2. Putting into exploitation new irrigation areas implied the need to irrigate an area 10 times higher than the previous one. The consequence is the intensive exploitation of groundwater.
Another example in Spain is the groundwater body of Mancha Oriental. Here a similar process occurred, a change in land use to put into exploitation big irrigation areas resulted in an intensive exploitation of groundwater during the last 30 years. Groundwater table has fallen up to 60 meters in some places. The hydraulic regime has changed dramatically. The aquifer drained more than 300Mm3/year to the Jucar River, and now this amount is less than a third (MOPTMA, 1993). Currently, some river stretches are even dry. A progressive salination of groundwater for irrigation has occurred, which implies a salination of the soil and the impoverishment of its agrarian characteristics.
Another similar example is located in the Duero river basin, the groundwater body known as
“Los Arenales”. The problem is very similar. Several natural discharges to lagoons and small wetlands have disappeared, as a consequence of the change of land use. The water level has diminished up to 30 meters in the last 20 years (Castaño, 1999).
It is remarkable too that in Spain, in the Jucar river basin located inside the Valencia Autonomous Region, most of the new dams and reservoirs have among their objectives to
increase the recharge of aquifers. That is what is happening with the Algar dam, on the Palancia River, in Valencia Province. One of the objectives of this dam is to alleviate the current groundwater resources imbalance, which has been assessed in 7Mm3/year, due to irrigation use of groundwater. This imbalance has caused a progressive movement of the saline edge inland, and a salination of many exploitation wells. The dam is located on a permeable site; so it is thought that recharge to aquifers will increase up to 13Mm3/year. This new situation would end the current imbalance, and restore the equilibrium. It is remarkable that this fact of the high permeability of the dam site forced the abandonment of the construction project in the past. Nowadays, this characteristic is useful because the need for the aquifer’s replenishment is very high. This example shows how a planned human activity tries to modify the recharge regimen of aquifers, which are in very bad conditions caused by previous unplanned activities.
The abolished Spanish Hydrological Plan included a water transfer between the Ebro river basin and the southernmost part of Spain, which is very dry. One of the objectives of the transfer was to artificially recharge many overexploited aquifers in the Alicante province.
9.1.4.3 Recommendations for further research and international cooperation
- Evaluating indirect impact of human activity on groundwater is not easy, because it depends on a number of factors, so good knowledge of the effects of human activities on groundwater recharge is needed.
- Indeed, the first action to be taken to protect water resources from human interference is a better understanding of the water cycle, by means of further research on quantification of recharge and development of recharge monitoring techniques.
- Waste disposal facilities on groundwater recharge areas should be avoided. On the other hand, facilities located on discharge areas should be controlled, because although the fate of pollutants is surface water, well pumping near surface water bodies could induce recharge.
- Land use planning must take into consideration environmental, social and economic constraints in the long term to keep a balance of the indirect impact of human activity on recharge and discharge areas
- Related to agricultural practices, codes of good agricultural practices are to be implemented to reduce pollution induced by intensive agriculture in recharge areas.
- Modelling of unsaturated zone is needed for assessing groundwater recharge. In this way, different scenarios could be analysed to predict effects of climate change in recharge.
9.2 Quantity related aquifer responses and triggered processes with impact on groundwater quality
This part of the report deals with the two questions that are the main point of chapter 9:
- what is the response of aquifers to human actions on groundwater quantity?
- for each category of response: what physical or chemical processes are triggered by human actions on groundwater quantity and how do these processes act on groundwater quality (release of contaminants, etc.)?
Each of the following subchapters contains the following information:
- “what is…” introduction with a short glossary of main terms or explications of the subject from reference textbooks;
- what human action with impact on quantity?
- what is the aquifer response? (sums up what was said in chapter 9.1 but here in relation to the processes;
- what physical/chemical processes are triggered? (e.g. mixing with other groundwater bodies, with seawater, with surface water, chemical reactions with aquifer material, direct pollution accompanying injection, release or fixation of contaminants…);
- what is the quality impact of the processes?
- case studies for this process.
The following subchapters are organised by categories of aquifer responses.
9.2.1 Seawater intrusion in coastal aquifers and fresh water intrusion in marine aquifers
Marc Van Camp and Kristine Walraevens, Laboratory for Applied Geology and Hydrogeology, Ghent University
9.2.1.1 Introduction
Groundwater quality in coastal aquifers is largely influenced by the interaction between the sea and the bordering aquifer systems. This interaction can result in freshening of saline aquifers or salinization of fresh water bodies. In complex cases even both situations can be found in the same aquifer system. While the main mechanism for salinization or freshening is hydrodynamically driven (groundwater flow), also physical and chemical processes within the aquifer will alter groundwater composition. Cation exchange is in many cases an important process to consider; it results in a hydrochemical spectrum of groundwater types reflecting both the hydrodynamical and hydrochemical characteristics of the aquifer.
In order to understand the natural groundwater quality and the controlling processes in a coastal aquifer, it is crucial to known both the hydrodynamical and hydrochemical behaviour and the way these are linked together, because usually only the interaction between them can explain observed quality distributions. Where mixing of fresh and salt water occurs, density-driven flow may become important and change quality distribution, while the groundwater composition itself influences hydrodynamics. An integrated approach of both aspects is indispensable.
Coastal aquifers are mostly characterized by the confrontation between marine and continental conditions. This may result in the salinization of fresh aquifers, or conversely, the freshening of saline aquifers.
Salinization can be induced by natural events or by anthropogenic causes (table 9.1). Each of these mechanisms has its own time scale (ranging from years to hundred of thousand of years), and the present fresh/salt water distribution in a coastal aquifer is very often determined by the long-term hydrogeological and physiographical history of the region.
mechanism
causes freshening salinization
natural - development of dune belt
- marine regression - sea transgression anthropogenic - artificial recharge
- river/canal outflow
- marine intrusion by overexploitation - upconing by overexploitation
Table 0.1 - Causes for freshening and salinization processes.
9.2.1.2 Processes and mechanisms affecting groundwater quality in fresh/salt water aquifers
When analyzing natural groundwater quality controls in coastal aquifers, one has to consider the specific role of the factors determining groundwater composition under salinizing or freshening conditions. These factors are:
- the composition and mixing ratio of end members;
- hydrodynamics: mass transport processes:
. advection due to groundwater flow, . hydrodynamic dispersion (mixing);
- chemical reactions:
. within the water phase,
. with the gas phase in the unsaturated zone, . with the aquifer matrix.
• Composition of end members
The end members to be considered in coastal aquifers are seawater and fresh recharge water, the latter generally conceived as infiltrating rain having dissolved calcite. The main ions in this recharge water result from calcite dissolution (Appelo & Postma, 1993), and the watertype is F-CaHCO3O according to the classification of Stuyfzand (1986). The seawater type is S-NaClO. Besides the major ions, the classification name expresses the chloride content (fresh F: <150mg/l; fresh-brackish Fb: 150-300mg/l; brackish B: 300-1,000mg/l;
brackish-salt Bs: 1,000-10,000mg/l; salt S: 10,000-20,000 mg/l; hyperhaline H: >20,000mg/l) and the cation exchange code. The latter indicates either a surplus (+) of marine cations ((Na++K++Mg2+)corrected > √(½Cl-)), or a cation exchange equilibrium (0) (-√(½Cl-) ≤ (Na++K++Mg2+)corrected ≤ √(½Cl-)), or a deficit (-) of marine cations ((Na++K++Mg2+)corrected < -
√(½Cl-)). All concentrations are expressed in meq/l. The parameter (Na++K++Mg2+)corrected represents the marine cations in the sample that are not due to admixture of the seawater end member, and is calculated as the sum of measured marine cations, from which 1.061Cl- is subtracted, the latter representing the marine cations in the end member fraction ((Na++K++Mg2+)/Cl- = 1.061 for mean ocean water). A surplus of marine cations (+) points to cation exchange resulting from freshening, whereas a deficit (-) indicates cation exchange caused by salinization. The end members by definition show cation exchange equilibrium (0).
• Density-dependent flow
As seawater has a higher density than fresh water (about 2.5% more), groundwater flow will be partially driven by density differences. In this case the groundwater flow equation will contain density dependent terms. Where spatial variations in fluid density are present, such as in coastal aquifers, the density variations can substantially affect rates and patterns of fluid flow. In many of these hydrogeologic settings, an accurate representation of variable density groundwater flow is necessary to characterize and quantify groundwater flow rates, travel paths, and residence times. Density dependent flow can be described using the concept of “fresh water heads”, in which the real pressure in a point in the aquifer, which depends on the density variation of the water in the overlying part of the aquifer, is replaced by a fresh water column (with density one) corresponding with the same pressure. Mapping
and modelling of hydraulic heads in coastal regions should always consider this variable density effects.
• Advection, dispersion, mixing and diffusion
In most of the groundwater bodies the groundwater itself is not static but is flowing, due to variations in hydrostatic pressure or hydraulic head. Flow velocities of groundwater are usually in the range of meters per year, or higher in the proximity of pumping wells. The displacement of the groundwater or advection is usually the main driving force by which groundwater can be replaced by other groundwater with another composition. Advection is described by the law of Darcy and flow velocities can be computed for a given hydraulic gradient, using local values for hydraulic conductivity and effective porosity:
velocity gradient k
= ne × with: velocity = groundwater flow velocity (L/T),
gradient = hydraulic gradient (-), k = hydraulic conductivity (L/T), ne = effective porosity.
Individual water particles follow pathlines or streamlines, but due to different pathways within the porous system, and to local variations in hydraulic conductivity occurring in the aquifer sediments, some water particles will move faster or slower, generating a mixing zone where there is a gradual change in water composition if a front is advancing in the aquifer system.
This process is called mechanical dispersion and is dependent on the flow velocity and the dispersivity, a quantification of the effect of sediment heterogeneity. Dispersivity is not an isotropic parameter, but can change over orders of magnitude relative to the direction. It is highest in the flow direction. This property has to be represented by a three dimensional tensor.
When groundwater is not flowing but is stagnant, mass transfer by concentration gradients can occur by the process of diffusion. The effect of diffusion is described by the laws of Fick (Fetter, 1995). Fick’s first law says thet the mass of fluid diffusing is proportional to the concentration gradient:
F D dC
d dx
= −
with: F = mass flux of solute per unit area of unit time, Dd = diffusion coefficient (L2/T),
C = solute concentration (ML-3).
Systems where the concentrations are changing with time, are described by Fick’s second law:
dC
dt D d C
d dx
= 22
Diffusion coefficients are well known, but in most aquifer systems the process is of minor importance unless evolution during long time periods is considered. Diffusion may be significant near the contact with under- or overlying aquitard layers, in which more saline waters are still present. The values for Dd usually range between 1 en 2 10-9m2/sec (Fetter, 1995).
Usually mechanical dispersion and diffusion are added in a single term and called hydrodynamic dispersion. Combining dispersion and advection the advection-dispersion equation can be derived, describing the change in concentration of a solute species in function of time for the one dimensional case:
∂
∂
∂
∂
∂
∂ C
t D C
x vx C
= 2 − x 2
with: vx = groundwater flow velocity (x component) (L/T), D = dispersion (L2/T).
• Speciation
The chemical form in which an element occurs in water is called its speciation. Species of an element can have a different stoichiometric or structural form. Besides aqueous species, also gaseous and solid species can exist. The concept of speciation is central to equilibrium and kinetic aspects of aquatic chemistry, and therefore groundwater composition. Elements can exist in a variety of oxidation states, protonated and deprotonated forms and free or complexed ion forms. In natural groundwater many tens of different species can be recognized. For given (measured) element concentrations, the speciation can be calculated using thermodynamical equilibrium data. Practically this is done using a computer program (e.g. PHREEQC, Parkhurst & Appelo, 1999). Changes in the speciation of an element can be triggered by altered redox conditions in the groundwater. Transition of oxidation states can influence the mobility of elements in water and transformations of one species into another can induce changes in groundwater quality.
• Cation exchange
Cations may be attracted to the region close to a negatively charged clay mineral surface and held by electrostatic forces. The amount of cations that can be exchanged is determined by the cation exchange capacity (CEC), quantifying the number of moles of adsorbed ion charge that can be desorbed per unit mass of sediment (e.g. expressed as meq/100g sediment). CEC values for some clay minerals are listed in table 9.2 (Stumm and Morgan, 1995). The CEC values of an aquifer will depend on the clay content of the sediments and reliable values can only be found by measurements.
Clay Exchange capacity (meq/100g) Kaolinite 2.3
Illite 16.2
Montmorillonite 81.0
Table 0.2 - CEC values for selected clay minerals.
Because the clay minerals show a selective preference for different ions, a general order of affinity can be given (Hoffmeister series):
Ca > Mg > K > Na.
• Dissolution and precipitation of mineral phases
Because groundwater is always in contact with the aquifer material, rock-water interaction is important and the dissolution of minerals can increase the concentration of related elements.
Mineral phases can be formed by precipitation if concentrations in the groundwater are higher than the solubility of the mineral.
Essential for the dissolution/precipitation processes is the concept of saturation:
- the saturation ratio is defined:
SR IAP Ks
= with: SR = saturation ratio,
IAP = Ion activity product, Ks = solubility;
- the saturation index is defined as:
SI = log10(SR) with: SR = saturation ratio,
SI = saturation index.
The relations between saturation state, SR, SI and impact on processes and concentrations are given in table 9.3. After the SR and SI values are calculated for a mineral phase, based on the groundwater composition, the possible dissolution or precipitation is known. Notice however that some dissolution and precipitation processes are very slow and that these reactions are kinetically controlled.
saturation state SR SI process concentrations
oversaturated > 1 > 0.00 precipitation decreasing
saturated 1 0.00 equilibrium constant
undersaturated < 1 < 0.00 dissolution increasing Table 0.3: Relation between saturation state and the precipitation and dissolution processes.
Because many of the salinizing/freshening aquifers have a marine origin, calcium carbonate in the form of shells is usually present in the sediments, and the groundwater will become (more or less) equilibrated with calcite. While the solubility of calcite in pure water is rather small (log K = -8.48, only 0.12mmol/l will dissolve), it can be better dissolved when some amount of CO2 is present in the water:
CaCO3(s) + CO2(g) + H2O = Ca2+ + 2HCO3-. A distinction must be made between open and closed system dissolution.
Open system dissolution occurs when the CO2(g) concentration stays constant. This can only happen where the groundwater has direct contact with the atmosphere: at and above the water table. Partial pressure of the CO2 in the atmosphere is around 10-3.5, but due to microbiological activity in the soil, elevated values may occur, often 10-2 and even up to 10-0.5. This kind of dissolution usually appears in the unsaturated zone, if the soils are not (yet) decalcified.
Closed system dissolution occurs when the groundwater has no direct contact with the atmosphere and only dissolved CO2 can be consumed for the dissolution of calcite. This happens below the water table.
As can be seen in table 9.4 open system dissolution will dissolve more calcite, what will be reflected in higher calcium and bicarbonate concentrations.
