3 Challenges of mapping buried valleys

Transcription

1 3 Challenges of mapping buried valleys 3 Challenges of mapping buried valleys 3.1 The EU and the groundwater The overall objective of the EU Water Framework Directive is about achieving good quality for all waters, including groundwater, which should not be polluted at all. With regard to groundwater, the European Commission in its introduction to the Directive in essence states that it combines prohibition of discharging polluting agents directly to the groundwater with monitoring of the groundwater quality. It requires the reversal of any humanly induced upwards pollution trend and limits the extraction of groundwater to a sustainable level. It follows that fulfilment of these requirements require knowledge of the location, size, and quality of the groundwater resources among other things. Indeed, the Directive contains instructions for a 2-stage characterisation of all groundwater resources, including location, boundaries, geology, pollution threats et cetera. The EU is taking the issue of groundwater protection even further. A proposed daughter directive based on Article 17 of the Water Framework Directive is currently being discussed. This is the new Groundwater Directive whose purpose is establishing specific measures to prevent and control groundwater pollution. If this is to be fulfilled, data on the groundwater resources and the subsurface geological strata housing them the aquifers become very important. 3.2 The importance of buried valleys When mapping the subsurface geology for the purpose of showing the presence of groundwater resources and establish their whereabouts, size and natural protection against pollution, a number of challenges have to be dealt with. Some of these apply to all types of aquifers, whereas others have special relevance when considering buried valleys. Mapping buried valleys for groundwater raises technical as well as social and planning issues. The first question is why, apart from purely scientific reasons, should society be concerned about buried valleys at all? In light of the EU Water framework Directive, the answer to this is twofold: Firstly, aquifers in buried valleys in northwestern Europe themselves contain important groundwater resources. Secondly, buried valleys can effect their surroundings in the sense that they can constitute a pathway, though which water and pollutants may flow from the surface to otherwise well-protected aquifers outside the buried valley itself. Apart from this, if potable groundwater is a limited and valuable resource, as much as possible should be known about buried valleys and their vulnerability to pollution, so they can be taken into account by water planning authorities. A number of topics with regard to mapping of buried valleys are discussed below. 3.3 Mapping The methods used for mapping buried valleys are mostly the same as those used for subsurface geological mapping in general. However, the particular geological characteristics of buried valleys suggest that meticulous planning of the mapping process and a thorough assessment of the mapping techniques useful in each case of mapping a buried valley are especially important. Buried valleys are generally oblong, narrow structures eroded up to several hundred metres down into the surroundings, which are often much older than the buried valley itself. The buried valleys can have complex internal structures due to their complex mode of formation, very often with several episodes of erosion and deposition of sediments having occurred within the same valley (Jørgensen & Sandersen 2006). The valleys often consist of several channels eroded into each other, where each channel has been filled with sediments between each erosive incident. Some channels are filled with meltwater sand and gravel, whereas others contain silt or clay of various 21

2 PETER ERFURT types. This means that the nature of the valley infill material can change over very short distances, both vertically and horizontally, and that no systematic distribution of the various types of sand and clay valley infill appears to exist, not even within the same valley. In addition, glacially induced tectonic movements and re-mobilisation of older sediments during the Pleistocene valley formation can further complicate the picture (Jørgensen & Sandersen 2004). The very changeable geological make-up of buried valleys make many of them difficult to map out in detail. It does not, however, exclude the existence of mappable aquifers. Individual or sometimes several successive erosion and deposition episodes can produce large coherent bodies comprised of sediments of a similar nature, if not necessarily of the same age. These may fill a substantial part or sometimes even dominate the framework of a particular buried valley. Such a large, homogenous sediment body may for instance consist of coarse material such as sand and gravel, and then constitutes an aquifer that may be mapped. The occurrence of buried valleys in an area is often indicated by descriptions of samples from existing boreholes, where a thick succession of Pleistocene sediments in one or more boreholes in an area where older rocks generally lie close to the surface may signify a buried valley (Jørgensen & Sandersen 2004). Other signs are sudden changes in water chemistry, sometimes accompanied by a deepening in the level of the redox front, formed when oxidizing groundwater penetrates into the reduced sediments of the subsurface. This happens because a buried valley can constitute a recharge area with a high rate of groundwater flow compared with its surroundings, drawing down oxygenated water at a relatively high rate where confining clay layers are missing in the valley sediments because of erosion or non-deposition. Since no two valleys are identical, the methods used in mapping them must be considered in each case, even if buried valleys in general may be divided into specific groups (see Chap. 2 and Chap. 6). An example of problems encountered when mapping buried valley geology is given below: There may be a problem of separating the buried valley and its surroundings. For example, a sand-dominated buried valley eroded into sandy Neogene deposits is not easy to map out by electromagnetic or electric geophysical methods because of the lack of resistivity contrast between the younger and older sediments, see Chapter 4 of this handbook. A buried valley can cut through older deposits containing well-protected aquifers and cause short-circuit between these and younger water of possible lower quality infiltrating downwards through the valley sediments (Fig. 3.1). Construction of a good groundwater potentiometric map and the setting up of a groundwater model may help to delineate this threat. The age of water from different wells in an area can show if younger water is infiltrating quickly and may influence the quality of the older, deep aquifers. Fig. 3.1: A Pleistocene buried valley filled with outwash sand (yellow) and scattered bodies of clay (brown), cuts through a series of continuous Neogene deposits of sand (light blue) and clay (dark blue). The arrows show possible exchange of groundwater between the buried valley and the surrounding, older and otherwise well-protected aquifers. The buried valley may thus constitute a threat to the groundwater resources in the area, ranging beyond the immediate borders of the valley itself. 22

3 3 Challenges of mapping buried valleys A seismic survey may delineate the valley profile and show in one or more seismic profiles across the valley where its borders toward the surroundings are located, but will not disclose much about the nature of the valley sediments. Drilling a well will do this (and simultaneously provide information on water chemistry), but only at a point in the valley, and drilling is expensive. Combining lithological and logging information from wells with an electromagnetic survey over part of the valley can expand the model of the internal valley lithology. It is necessary to obtain information on any clay layers protecting the valley aquifers from polluting agents from the surface by slowing down infiltrating groundwater and accelerating decomposition of unwanted substances, and this may make it necessary to use still other methods of mapping. The effectiveness of the aquifers natural protection is mainly a function of aquifer recharge rate, the thickness of the oxidised zone, the distribution, thickness and type of covering clay layers, and the ability of the valley sediments to chemically reduce substances such as nitrate. Sediment and water analyses can show whether there are substances present, capable of protecting the aquifers by chemical reaction with the infiltrating water. For example, the content of the mineral pyrite in a sediment strongly influences its nitrate reduction capacity. A high content of finely disseminated pyrite may be capable of virtually removing dissolved nitrate in infiltrating groundwater for hundreds of years, while slowly advancing the border between oxygenated and reduced conditions (the redox-front ) towards the aquifer. While the removal of nitrate from the infiltrating groundwater is desirable, the thickening of the oxidized part of the subsurface may eventually become a problem, and at the redox-front itself the pyrite-nitrate reaction may liberate problematic concentrations of heavy metals such as nickel, contained in the crystal lattices of the pyrite grains, to the groundwater. On the other hand, petroleum hydrocarbons such as diesel oil as well as certain pesticides are bacterially digested in the oxidized zone, where nitrate is stable. The above is an example of the possible challenges in mapping a buried valley. When starting up such a project, it is important to form an idea a preliminary model of the general type of valley to be mapped. Furthermore, the example makes it evident that, with the exception of the most simple valleys, only a programme using more than one mapping technique is going to result in a useful understanding of buried valleys, their groundwater resources and their impact on the surroundings. The integration of different mapping methods is crucial. Chapter 4 of this handbook explains the different mapping methods applicable to buried valleys, and discusses their strengths and weaknesses. Chapter 6 suggests a course of action for mapping the different types of buried valley. 3.4 Quality and quantity Groundwater quality, as determined by chemical and bacteriological analyses of a numerous set of substances, is one of the possible ways to characterize the state of the water environment, and is a measure of the suitability of the water for drinking-water extraction. The quality is also an indication of the extent to which the groundwater is affected by human activity. Article 1 of the Water Framework Directive lists as one of its purposes a contribution to the provision of the sufficient supply of good quality surface water and groundwater as needed for sustainable, balanced and equitable water use, as well as a significant reduction in pollution of groundwater. To this end, the Directive among others defines the terms Groundwater status the general expression of the status of a body of groundwater, determined by the poorer of its quantitative status and its chemical status and Good groundwater status the status achieved by a groundwater body when both its quantitative status and its chemical status are at least good. 23

4 PETER ERFURT Fig. 3.2: The EU Water framework Directive definition of good groundwater chemical status. Good groundwater chemical status is the chemical status of a body of groundwater, which meets all the conditions set out in Table of Annex V to the Directive, see Figure 3.2. Thus, in the light of the Directive, mapping buried valleys is not only the geological mapping of aquifers, but also an assessment of the quality in the context of whether it is adversely affected by natural conditions or by human activity of the groundwater bodies within these aquifers and the temporal development of this quality. Water analyses may show if the water quality is already affected by problems and needs remediation, if there are changes in the water quality over time, and whether the aquifer needed for drinking-water supply may be adversely affected by neighbouring aquifers of inferior quality. The occurrence of contaminants of natural origin, such as salt water infiltration, upconing of water coloured by dissolved organic matter ( brown water ) or a rise in the concentration of arsenic and nickel may indicate that a detailed strategy is needed for utilisation of the aquifer, and that the aquifer is used to the limit of sustainability, or is indeed overdrafted in regard to being a renewable resource. Contaminants of direct anthropogenic origin such as nitrate, pesticides, chlorinated solvents and petroleum hydrocarbons means an aquifer under pressure from human activities, and indicates the need for investigations and protective action. The Directive specifies quantitative status as an expression of the degree to which a body of groundwater is affected by direct and indirect abstractions. Good quantitative status is the status defined in Table of Annex V in the Water Framework Directive, see Figure 3.3. To determine the robustness of the aquifer to climate changes and water extraction, a combination of tools are at hand. Mapping the physical size of the resource, conducting pumping tests on wells and carrying out a programme of sounding the depths to the water table in the available wells in the given area, in order to finetune the groundwater potentiometric map, will provide data on the groundwater flow and hydraulic properties of the aquifer. Understanding the behaviour of complicated flow systems, and predicting how they will behave in the future, relies upon modelling the real hydrogeological system (Fetter 2001). 24

5 3 Challenges of mapping buried valleys Fig. 3.3: The EU Water framework Directive definition of good groundwater quantitative status. Showing how a groundwater system such as a buried valley will react to various stresses and interact with its surroundings, involves using the acquired geological data to set up a conceptual geological model. Based upon this and the available hydrological information, a dynamic groundwater model can be devised, through which various possible scenarios may be run. Modelling thus plays an important part in assessing the level of exploitation, which is sustainable and will not damage the resource or permanently alter its properties. 3.5 Protection Protection is really two topics, firstly the natural protection of the aquifers referred to in Section 3.3, and secondly the protection of the groundwater quality that can be applied by human groundwater stakeholders, which in broadest sense are everyone dependent on groundwater for drinking-water. Protection of the water quality in an aquifer is one of the most important aspects of groundwater management, considering the number of sources of potential groundwater contamination (Sandersen & Jørgensen 2003). On one level this is achieved by implementing proper well field strategies concerning physical quality of the water wells, well spacing and pumping rates in well fields where naturally occurring phenomena such as salt or brown water may present a problem. On another, there may be serious problems with controlling contamination caused by human activity. A most important aspect of preventing groundwater pollution is the identification of the aquifer recharge areas in the mapping process, and the subsequent protection of the aquifer in such areas, in which potential contaminant sources should be closely controlled (Fetter 2001), by the removal of hot-spots and regulation of contaminant discharge to the environment. Sand-dominated buried valleys, essentially constituting one large aquifer, may comprise very large recharge areas for both the valley aquifer itself and the surrounding aquifers outside the buried valley structure. 25

6 PETER ERFURT Clean-up of contaminant hot-spots such as chlorinated solvents, heavy metals or petroleum hydrocarbons, whose sources may be landfills, former industrial sites or spills from underground storage tanks, involves physical removal or the application of various techniques of in-situ remediation. This will reduce single point-source threats to the aquifer. Areally distributed sources or nonpoint sources such as nitrates and pesticides derived from agricultural activities are more complicated to deal with. It is mainly done by legislation designed to limit their introduction into the groundwater. This ranges from banning certain types of pesticides and limiting the use of synthetic and natural fertilizers in vulnerable areas, to setting up financial incentives for local participation in projects such as re-forestation and wetland restoration projects that apart from other issues also benefits the groundwater. An important aspect of protecting the groundwater is the realisation by the individual citizen that she or he can make a contribution. It is important that landowners, in town or country, realize that they live in an area where it is possible to affect the groundwater resource that may later be used for drinking water. The citizens should be informed about the correct use of fertilizers and pesticides and encouraged to minimize their use to the lowest possible level. 3.6 Planning and social issues The mapping of buried valleys and their groundwater resources provides a large amount of information relevant to water professionals, public policymakers and the general public. In order for this information to be used in a manner beneficial to society, a number of issues must be considered. Action plans Public dialogue Finance The technical and scientific data from the mapping process form the basis for preparing an action plan, which is a strategy for exploiting and protecting the groundwater in an area, designed to ensure balanced, sustainable and equitable use. The action plan shows the current state of the groundwater and presents the initiatives that are necessary for keeping the groundwater in a good state or improve it if necessary. An action plan is basically an agreement between the different stakeholders, stating what and when to do, as well as by whom the necessary action is to be taken. The action plan itself is usually the result of a combined effort by public authorities and stakeholders such as waterworks and landowners organisations. It is most important to create a sense of ownership for the plan, common to all the parties involved. The planning staff will have a number of requirements to be fulfilled and questions to be answered in order to create an effective action plan. They should be presented to the mapping staff at an early stage in the mapping process. The mapping staff must have time consider how to answer these questions, and for developing possible alternative mapping strategies if such are necessary to fulfil planning staff needs. Thus, a close dialogue between the mapping and planning staff, from early on in the process, is very important for producing a functioning action plan. From the Water Framework Directives opening reflections it may be inferred that involving the stakeholders and general public living in the area affected by the action plan is important. This is a necessary pre-requisite for public understanding and acceptance of the proposed action towards groundwater protection. Involvement and information of the affected parties should start already at the beginning of the mapping process, with presenting to the public what is done and why through media announcements as well as presentations by the mapping staff to the various parties making up the action plan staff. Public meetings should be arranged during all stages of the mapping and planning work when the need for citizen involvement is felt, but are especially important when the action plan is ready for publication. During the public hearing-phase, the action plan and the result of the mapping lying underneath it, should be presented and discussed at a meeting to which all citizens in the general area are invited. 26

7 3 Challenges of mapping buried valleys Most of the groundwater mapping and preparation of action plans is financed by public money. Water-consuming industries such as large dairy companies may finance their own mapping projects in cooperation with the water authorities, but the cost of hydrogeological work is mainly paid over the taxes. In Denmark, there currently is a duty on groundwater extraction by waterworks, agriculture and industry, the amount being dependent of the size of the extraction permission. A similar duty, depending on the annual amount of extracted groundwater, is raised in Schleswig-Holstein and a few other german federal states. The yield of this duty is earmarked for groundwater mapping and preparation of action-plans, but cannot be used for implementation of these plans. Protective action such as re-forestation and other nature restoration projects as well as compensation to farmers for limiting the use of fertilizers and pesticides may be financed in part by the national governments and in part by the EU. 3.7 References EU Water Framework Directive (2000): Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for the Community action in the field of water policy", October 23, Fetter CW (2001): Applied Hydrogeology, 4th edition, Prentice-Hall, NJ. Jørgensen F and Sandersen P (2004): Kortlægning af begravede dale i Jylland og på Fyn. Opdatering De jysk-fynske amters grundvandssamarbejde (in Danish). Jørgensen F and Sandersen P (2006): Buried and open tunnel valleys in Denmark erosion beneath multiple ice sheets. Quaternary Science Reviews 25: Sandersen P and Jørgensen F (2003): Buried Quaternary valleys in Western Denmark occurrence and inferred implications for groundwater resources and vulnerability. Journal of Applied Geophysics 53:

8 28 PETER ERFURT