Groundwater protection plan for the Managua aquifer - development of a planning tool

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1 Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHS Publ. no. 222, Groundwater protection plan for the Managua aquifer - development of a planning tool CECILIA SCHARP Division of Land and Water Resources, Royal Institute of Technology, S Stockholm, Sweden Abstract In the southern drainage basin to Lake Managua in Nicaragua, groundwater is the main source of water supply to both urban and rural populations as well as to industry and agriculture. Managua, the capital, is located on top of the aquifer. The aquifer is of complicated volcanic origin and the potential for contamination seems to be very high. Contamination threats have been identified as originating from both point and non-point sources. The study presents a strategy for developing a protection plan for the Managua aquifer based on the assessment of risk for contamination of the aquifer and the relative value of the water to the society. Specific criteria for selecting methods to be implemented have been identified suited to the prevailing conditions within the country. Inter-institutional cooperation has proved to be of great importance for the success of introducing groundwater protection measures. WATER SUPPLY SITUATION Groundwater is of utmost importance for the water supply to households, industry and agriculture in Managua with surroundings. Well over one million people live in the city itself which is almost one third of the country's total population. The migration rate from the rural areas into the capital is very high and the population increase ratio is estimated to be 7% annually. Rural-urban migration gives a rise to spontaneous settlements which increase the demand for basic services such as water supply. The water supply system is constructed to serve around 0.5 million people. The volcanic aquifer is located south of Lake Managua and partly directly under Managua city (Fig. 1). Water is extracted from deep drilled municipal wells scattered over the area and from private wells mainly serving industries. The main well field is located just south to the international airport. Annual withdrawal is approximated to 40 million m 3. The yearly possible groundwater withdrawal has been estimated to 108 million m 3 (Instituto Nicaraguense de Acueductos y Alcanterillados (INAA), 1992). Perennial surface waters are limited with the exception of Lake Managua, Lake Masaya and some small crater lakes which are recharged primarily by groundwater and act as a huge wells. Lake Managua has a naturally poor quality and in addition it is suffering from contamination from untreated sewage water and effluent from industry released without treatment into the lake. This implies that the lake cannot be used for water supply in any sector of the society. Lake Masaya is also suffering from pollution from the surrounding agricultural areas and from the city of Masaya. A Japanese project together with the water authority (INAA) are currently making estimations on groundwater availability for future water supply.

2 444 Cecilia Scharp Fig. 1 Study area, Lake Managua with its groundwater recharge area covering an area of 880 km 2. THE STUDY AREA The study area is defined as the groundwater recharge area to the Managua aquifer covering an area of about 880 km 2 reaching from the hilly volcanic region in the south, with an altitude of 900 m a.s.l., to the gently sloping plane towards Lake Managua, some 35 m a.s.l. to the north. Mean annual precipitation in the area amounts to 1250 mm distributed during the wet season May through October. The potential évapotranspiration is estimated to be 2300 mm. November to March is referred to as the dry season. The geology is of volcanic origin and characterized by a complex stratification originating from different geologic era and with varying degree of consolidation. From a hydrogeologic point of view the most important formations are: - basaltic-andesitic compact agglomerates, tuff and pyroclastic flows of Plio- Pleistocene age (Middle las Sierra Group); - lava flows and pyroclastic flows and fall out deposits of Pleistocene age (Masaya Group Volcanic); - alluvial deposits of Holocene age composed of mainly sandy and clayey sediments with pyroclastic material. Modern Holocene deposits are also common in the area but they are generally located above the groundwater level. The Holocene lava deposits have an important role

3 Groundwater protection plan for the Managua aquifer 445 for the groundwater recharge regime, the infiltration is almost direct as there is no soil or vegetation cover. The aquifer is regarded as open. The soils are light sandy, and silty soils, highly porous and easily eroded. The plane closer to lake Managua consists of alluvial soils with a relatively higher clay content. The groundwater recharge area is subdivided into three sub areas: the (a) Eastern sub-area, (b) the Central area and (c) Western sub-area, according to the identified groundwater divides. Work has been initiated in Eastern sub-area which covers an area of about 450 km 2 (Fig. 1). The water quality in the aquifer is naturally good and well suited for water supply in most locations. The water is mainly of C0 3 + HC0 3 type with a ph varying between However, in some wells water is influenced by volcanic activity and less suited for domestic use. PROBLEM IDENTIFICATION Within the study area both quantity and quality aspects is important to address when planning for future water supply. The city is in a position where it need to increase the water supply for the growing population and thereby the withdrawal from the aquifer. There is an urgent need to find new possible locations for well fields. Overdraft with lowering of the groundwater level has been recorded in several of the water supply wells in the central area close to the lake (Instituto Nicaraguense de Acueductos y Alcantarillados, 1992). As a result of declining groundwater level it has been recognized as an obvious risk that polluted water from Lake Managua may intrude into the aquifer and deteriorate the quality. Furthermore, a full spectrum of threats to the quality of the water can be identified in the area with close links to the actual land use. Three major groups of contaminant sources can be identified: - effluent from industries, - leakage from spontaneous settlements and waste disposals, - spreading of agrochemicals. A primary problem, with regard to pollution transport, is the location of the city on top of the aquifer. About 80% of the country's industry is located in the urban area and its activity constitutes a great risk of contamination of the aquifer. Often the industries use old technology and treatment of sewage water is non-existent. Chemicals used in the processes are often stored directly on the ground and local waste disposals are placed on the industrial plot. Waste handling in general is under-dimensioned for the city's need and only one official waste disposal site exists. Spontaneous settlements can be found scattered over the city wherever space is available. Most of the settlements does not have proper waste and water handling. Pesticides and fertilizers have been used in excess in the agricultural areas of the drainage basin during several years. Non-degradable pesticides like toxaphene have been produced within the industrial area, and the use have been highly subsidised by the government for cotton production. Today the main use has stopped due to low prices on cotton and withdrawn subsidises. Still there is no information available about the risk of pesticides stored in the unsaturated zone and slowly moving downwards, into the aquifer. Traces of pesticides have though been found in excavated wells within the Sabana Grande-Masaya area close to a property for storage of agrochemicals (INAA, unpublished report).

4 446 Cecilia Scharp Institutional aspects Experience of groundwater protection activities is very limited in Nicaragua today. Preventive measures consist mainly of direct well-head protection and basic monitoring of water quality in production wells. There is a general lack of physical planning which means there is no proper land-use planning. Limited background information is available for groundwater managers and decision makers when handling for example issues on location of environment endangering activities with regard to groundwater contamination. Further more, there is an unclear division of responsibilities between authorities working with groundwater management, and an insufficient legislation dealing with water issues. The legislation mainly deals with exploitation issues and not management and protection of the resources. AIM The overall aim of this work is to develop and test a strategy for groundwater protection which is suitable for Nicaragua and other countries with similar conditions. The work includes support to responsible institutions to help them develop an understanding of the need for groundwater protection and to find methods to develop a plan to protect the groundwater from anthropogenic contamination, adapted for local conditions. GROUNDWATER PROTECTION STRATEGY A protection plan is a basic groundwater protection tool. The plan will not substitute the necessity of performing detailed studies in every specific case or location, but aims at serving as a screening tool for groundwater managers and decision makers. It will identify areas that need specific attention and provide guidelines on how to improve land-use. Further, it shows which activities can be allowed within an identified protection zone in order to minimize the risk of harmful fluxes of contaminants to groundwater. The plan does not include implementation of remedial actions, but considers and gives recommendations on how this should be done and shall be complemented by an action plan (Nordvâstra Skânes Kommunfôrbund, 1992). The aims of the protection plan are: - to draw guidelines for general groundwater protection against different types of threats within a groundwater recharge area; - to identify areas that has a special protection value or protection need. The working steps for the development of the protection plan have been identified to be the following: - problem identification, - identification of role of institutions, - defining of conceptual model, - defining of strategy for the protection plan, - carrying out of analyses, - implementation of plan, - preventive and remedial actions,

5 Groundwater protection plan for the Managua aquifer 447 Basic concepts Landscapes differ in vulnerability to impacts from various types of threats imposed on the land surface. They are assessed in different ways according to their importance to the society or as an ecological element. The methodology identified for the development of the protection plan is based on two basic concepts firstly assessment of the risk of contamination of the aquifer and secondly the relative value it represents to the society (Fig. 2). A similar approach is used by Nordvâstra Skânes Kommunfôrbund (1992). Guiding criteria for selection of methods to test have been that the method must be: general, covering extensive areas, systematic, easy to implement, based on existing data and information, standardized to give comparable results, transferable to other areas with similar conditions, using an adequate scale. The choice of methods and scale must reflect the purpose. The issue of data availability and the use of proper scale is discussed by Foster (1990). The contamination risk to groundwater can be defined as the interaction of a contamination source and the vulnerability of the groundwater, as proposed by Foster (1987) and Holmberg et al. (1990) and others. The intrinsic characteristics of the unsaturated and saturated zone interact with the contamination load (amount, flux, inventory of quantity and quality assessment of vulnerability assessme ntof demands inventory and characterisation of contaminantes assessment of relative value as sessment of risk for contamination Fig. 2 Identified strategy for the development of a protection plan for the Managua aquifer.

6 448 Cecilia Scharp mobility, persistence) to form the contamination risk. Vulnerability of groundwater is often defined as an intrinsic property of soil and groundwater and can be referred to as natural vulnerability. However, some concepts does include concentrations, fluxes and characteristics of the contaminants. Vulnerability is either determined only for the vertical flow of water and contaminants, or include also the horizontal flow within the saturated zone. In the first case vulnerability generally express the possibility of introduction of a contaminant into the saturated groundwater zone. In the latter vulnerability is related to water quality in an abstraction point such as a well. Methods for vulnerability assessment are for practical and economic reasons often limited to evaluating only static parameters. In this study an American method called "DRASTIC, a standardized system for evaluating groundwater pollution potential using hydrogeological settings" by Aller et al. (1987) has been selected for implementation. The system uses seven hydrogeological parameters for evaluating the vulnerability: depth to groundwater, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity of the aquifer. The vulnerability is estimated in relative, qualitative terms. This vulnerability assessment will later be compared with an even simpler method based on lithographic zoning. To be able to evaluate the risk for groundwater contamination, as defined above, contamination sources have to be identified and then characterized according to properties that have a decisive influence on the transportation of contaminants. Foster & Hirata (1990) have developed a guide for characterization of contaminants, especially for Latin American conditions, that partly is used in this study. Areas where high vulnerability coincides with a contamination source with mobile, non-degradable contaminants will be identified as high risk areas where protection measures have to be initiated. The assessment of the relative value of the aquifer consists of two separate surveys first an assessment of the available quantities and quality in different parts of the aquifer. This study is followed by a water demand survey which tries to establish the present need and the future demands for the society together with an identification of areas that are of special importance from an ecological point of view. The results from these two assessments are combined to produce a final score, in the same way as for the assessment of risk, which shows in relative, qualitative terms areas of the aquifer which are especially valuable. Results are presented as thematic overlay maps each accompanied with a comprehensive manual which describes the used methods, limitations of method, how data were selected and the conclusions made during the work. These maps are not to be used separately but form the basis of the protection plan. By presenting results on overlays the information is prepared for storage in a database and as input data for the introduction of a Geographical Information System (GIS). GIS is considered to be an excellent tool for planning activities of this type. DISCUSSION AND CONCLUSIONS As the study is under implementation no final conclusions can be drawn at this stage but some general reflections are made. For the vulnerability assessment the DRASTIC system has shown to be a possible alternative. The aim and the scale of the method basically coincide with the defined

7 Groundwater protection plan for the Managua aquifer 449 criteria in the study. It has been possible to make evaluations with DRASTIC as a guide also in volcanic heterogeneous media. One issue is though how to handle lakes of importance to groundwater recharge in an area. Furthermore, there have been no major obstacles with gathering and evaluating available information and data for the hydrogeological parameters. But as the availability of necessary data, of good quality, is limited informal contacts between experienced professionals have been of utmost importance for the evaluation of the information. There have been very limited possibilities for complementary hydrogeological investigations due to the high costs. An issue under consideration has been the validation of the results of the assessment. It has been considered important to validate the findings with the best local hydrogeological knowledge there is. The results were discussed in a reference group. It is also important not to regard the results as static, they have to be updated when new information is obtained. Further, it can always be discussed if a simpler method, with less parameters, would have beenmore appropriate to use and also given the same results. Groundwater protection is very seldom the responsibility of only one institution but is more commonly shared between several. An important achievement is the increased hydrological knowledge base at institutions in Nicaragua is enlarged. Considerable efforts are made to form a hydrogeological conceptual model common for all institutions as basis for the vulnerability assessment and groundwater protection strategy. To succeed with groundwater protection strategy it is necessary to promote an interinstitutional cooperation where the institutions together define common goals. These institutions are the players who have an interest in groundwater management and who can become the future users of a protection plan, (Fig. 3). It is important that they are involved in the methodology development at an early stage. In this project, interinstitutional cooperation has ensured that the consumers' needs are met. Good working Knowledge about the resources and responsibility for development and protection Groundwater managers and consumers,.' *' ' < Protection plan - I * h!<.v- J Education and development Fig. 3 The importance of involving parties concerned have been identified at an early stage in the project.

8 450 Cecilia Scharp relationships have improved the flow and exchange of data and information between institutions. Further, the activities have lead to skilful persons with local knowledge coming together and mobilizing an interest and a foundation for groundwater protection in Nicaragua. Acknowledgement This research is carried out within the framework of the Sustainable Use of Water Resources Project in Nicaragua. It is financed by the Swedish International Development Authority (SIDA) and implemented by Centre for International Technical and Educational Cooperation (CITEC) at the Royal Institute of Technology (KTH). I would like to thank my colleagues and friends in the SUWaR-Nicaragua project who are actually performing the field work and CITEC for supporting me and giving me the opportunity to participate in the project and to perform my research. REFERENCES Aller, L., Bennett, T., Lehr, J. H. & Petty, R.J. (1987) DRASTIC: A Standardised System or Evaluating Groundwater Pollution Potential Using Hydrogeological Settings. National Water Well Association, Dublin, Ohio, USA. Foster, S. S. D. (1987) Fundamental concepts in aquifer vulnerability, pollution risk and protection strategy. In: Vulnerability of Soils and Groundwater to Pollution (ed. by van Duijvenbooden& van Waegeningh). Proceedings and Information, TNO Committee on Hydrogeological Research, The Hague, no. 38. Foster, S. S. D. &Hirata, R. (1990) Groundwater Pollution Risk Assessment: A Methodology Using Available Data. WHO- PAHO/HPE-CEPIS technical manual, Lima, Peru. Holmberg, M., Johnston, J. & Maxe, L. (1990) Mapping groundwater sensitivity to acidification in Europe. In: Impact Models to Assess Regional Acidification(ti. by J. Kàmari), International Institute for Applied System Analysis, Austria. Instituto Nicaraguense de Acueductos y Alcantarillados and Japan International Cooperation Agency (1992) The Study on Water Supply Project in Managua, Progress Report (2). Nordvàstra Skânes Kommunfôrbund (1992) Ôversiktlig Skyddsplan for Grundvatten (General protection plan for groundwater). Consultant Report by VBB/VIAK, Malmb.