METHODS FOR THE IMPROVEMENT OF THE WATER SUPPLY
IN THE CHACO OF PARAGUAY

Michael von Hoyer

Introduction

The Chaco (240 000 km² ) occupies approximately two third of the territory of the Republic of Paraguay, but only two percent of the population live in this region (Fig. 1). The extreme subtropical climate, the lack of a modem infrastructure and above all the scarcity of potable water has so far prevented a large scale colonisation of the area. At present the country experiences a high population growth which the traditional settlement areas in the east of the Rio Paraguay cannot support. The development and colonisation of the Chaco is therefore the declared objective of the Government of Paraguay. With the fragility of the ecosystem of the Chaco much effort is given to achieve a sustainable development of the area.

The limited availability of drinking water being a principal constraint for the establishment of settlements and for the cattle ranging industry, the Governments of Germany and Paraguay have, under a bilateral agreement of cooperation, initiated a project in 1992 with the objective to improve the accessibility to good quality drinking water in urban and rural areas of the Chaco.The project is planned to carry on until the year 2000. To achieve its goals, the project has pursued the philosophy of working at local level with local institutions and direct contact with the water consumer. The positive results and the cooperative response from the population have shown that the right approach has been chosen.

The project is being carried out by the Federal Institute for Geosciences and Natural Resources (BGR) as the German partner and the Direcci6n de Recursos Hidricos (DRH) residing in Filadelfia in the Central Chaco, as the Paraguayan institution being in charge of the water resources development in the Chaco. The project aims at three target groups with differing objectives: (a) Dirección de Recursos Hídricos : training of technical and scientific personnel, introduction of modem methods for the exploration and development of groundwater resources combined with the installation of the neccessary technical equipment; (b) population of the urban centres : development of methods for an improved underground storage of fresh groundwater, protection of fresh groundwater resources against contamination; (c) population of the rural areas i.e. campesinos, indigenous Indians: construction of water supply systems with participation of the village communities.

Strengthening of the local water development institution

Project activities center on the Direcci6n de Recursos Hídricos (DRH). Due to the lack of larger surface water bodies, groundwater is of great importance for the water supply. Much investigative work had been done by the DRH in the nineteen seventies and eighties with the support of the UNDP but knowledge about the hydrogeological conditions was still fragmentary. The project therefore combines the investigation of the groundwater resources with the training of the personnel. Starting with university leavers, who had majored in geology, the German experts introduce the young geologists to the practical hydrogeological work, whilst the theoretical base knowledge of hydrogeology is acquired in university master study programmes after completion of all field work. Important for the long term working capability of the DRH is the fact that all trained personnel retains permanent posts with the DRH, thus the acquired knowledge remains and can be productive within the region of the Chaco.

In order to enable the DRH to perform standard hydrogeological investigations in the Chaco, the project purchased drilling equipment and hydrogeological field instruments, equipped a water quality laboratory and installed computer hard and software. With the drilling rigs the DRH does not only carry out exploration work but constructs water wells for farms and settlements, thus generating an income that is reinvested in the maintenance of equipment and offices and offers a certain degree of independence from the budget of the central government. The water quality laboratory, the only one in western Paraguay, is frequently consulted by farmers and town engineers. The project has succeeded in establishing the DRH as an institution that offers services to the region. In turn it receives the political support of the local and provincial authorities which is indispensable for its future existence.

Of course the influence of the DRH which is an organisation under the Ministry of Agriculture and Lifestock Farming does not remain restricted to the Chaco region, in so far as the professional expertise of the hydrogeologists flows into countrywide development planning, e.g. through major contributions with the drafting of a water legislation.

Investigation of the groundwater resources

The main objective of the hydrogeological investigations carried out by the project was to improve the knowledge of the groundwater conditions in the central part of the Chaco where three Mennonite colonies have led to a population concentration (Fig. 1). A brief overview shall explain the hydrological conditions of the area which govern the availability of drinking water.

Since the nineteen eighties the annual mean precipitación is between 800 and 900 mm. In individual years up to 1,600 mm have been recorded. The larger part of these rains occur during the months November to March at the time of maximum temperature and the highest rate of plant growth and hence highest evapotranspiration so that a major portion of the rainwater is directly evaporated. To develop storage of rainwater in order to bridge the annual long dry period is the essential task if an uninterrupted water supply is to be guaranteed. Three methods are in use and their technology has been further developed by the project.

(a) Rainwater harvesting: Rainwater is being collected from house roofs and drained into underground cisterns from where it is pumped into a raised water tank. Water then is piped into the house. A cistern capacity of 25,000 litres per person is recommended. Provided that rain pipes and cisterns are kept clean, a good quality water supply is obtained. Of course this water collection system requires a fairly large and rigid roof area plus electricity for the water lifting pump. Therefore it can only be operated with town or farm houses. The poor population in the rural settlements living in grass or wood shingle covered huts cannot utilize this technology.

(b) Water wells: In the area between the towns of Mariscal Estigarribia in the west and Loma Plata in the east groundwater occurs at shallow depths ( 3 to 13 in below ground surface) in fine grained sands. The groundwater is generally saline with the exception of a few specific localities where fresh groundwater lenses are encountered which are recharged by the annual rains. The project has developed methods for the detection of such fresh water lenses (field survey of soils, vegetation, drainage combined with the interpretation of air photos and satellite images plus handdrilled investigation holes). The lenses are generally small and thus sufficient only for the supply of homesteads and small settlements. Overpumping leads to the intrusion of the surrounding saline water.

The natural process of rainwater infiltration in a morphological depression underlain by sands with the subsequent formation of a fresh water lense, floating on the surrounding saline groundwater, can be reproduced artificially. In a morphological depression which receives the surface runoff of a larger catchment area an open air reservoir, a socalled tajamar, is excavated by earth moving equipment. The dimensions of the reservoir are determined by the lateral extent of the sandy formation, the depth to the groundwater table and the slope stability of the reservoir walls. Storage volumes normally are in the range of 10,000 to 20,000 m³. In urban areas the runoff from the network of street ditches may be channeled into tajamares. In the towns of Filadelfia, Loma Plata and Neu-Halbstadt a system of several infiltration tajamares with production wells supply the commercial centre, small industrial plants, schools, hospitals and hotels. The project has studied the artificial recharge process in model cases and defined the parameters which determine the size of the fresh water body underneath a tajamar as well as the quantity of extractable fresh water.

(c) Surface water storage: Extensive areas of the Central Chaco are covered by loams and sandy clays and the technique of artificial infiltration of rainwater cannot be practised. These are as well the areas where only saline groundwater is encountered. Here tajamares are excavated in the low permeable formation and surface water is stored during the dry months. This method of water supply is the most unfavourable; firstly because of the high water loss due to evaporation (1,500 mm per year) and secondly because of the poor water quality. The tajamar water carries a high content of clay and silt in suspension together with a multitude of microorganisms many harmful to man. Intestinal infectious diseases are abundant. This observation given in respect with the tajamar water is also valid for the water found in lagoons and water holes. Most of the indigenous Indian population and the poor campesinos living in remote areas rely however exclusively on such unhygienic water resources.

Direct measures to provide drinking water for the poor rural population in the Chaco

In view of the inadequate water supply of the rural population in the Chaco, the project embarked on the construction of water supply systems in small settlements in 1994. Depending on the local water resources conditions one or a combination of the above described water supply techniques was implemented.

From the onset the project pursued the aim that the water supply systems to be constructed should be operated and maintained by the village communities. Consequently, the technology used had to be simple and adapted to the local supply conditions and the communities had to be involved from the planning right through to the construction stage. The baseline adopted for the supply standards is: a guaranteed year-round water supply if possible from groundwater, water points in form of cisterns with handpumps at a maximum distance of 400 in from each household.

The project reacts on applications from village communities, whereby the application has to state the input of the community. In the majority of cases the contribution from the village population is labour. It follows the planning phase whereby the village community has to form a water committee which identifies the water needs and at the same time names the persons responsible for the organisation and execution of work. Project staff and water committee plan the supply network and water points. Often this is a repeated process until the final layout is agreed upon. During the construction phase the project supplies the building material and necessary tools free of cost. The building material is delivered not in bulk but as work progresses, in order to avoid waste and to call in the work contribution from the villagers. The project staff teaches the workers in the various mechanical skills needed and makes sure that the work is carried out as planned. Running the water project in this manner means of course that the community dictates the work speed, it guarantees however that the village people carry the project all the way, which makes it, " their projecf ". This is essential for the operation and maintenance of the installations in the future.

Technically modified windpumps are used to lift water from wells and tajamares and pump the water into the mains under low pressure. Flexible PE-water pipe of 37 mm diameter carries the water to underground cisterns, where the flow is regulated by float and valve. From here the water is lifted by hand pumps. The cisterns have a storage capacity between 5,000 and 8,000 litres and serve as reservoirs when there is no wind. Where muddy tajamar water is the only supply, slow-flow-gravity sandfilters, built next to the cisterns, achieve a part reduction of the silt load and germs and improve the taste of the water.

From 1994 until 1997 the project has constructed water supply systems in 22 villages with a total of approx. 6,000 inhabitants. Applications from additional 10 villages will be processed during 1998.

Technical staff of the DRH has been trained to carry out maintenance services that require the facilities of a mechanical work shop. Thus the village people can call on these services when major repairs become necessary. To secure the financial means for the maintenance work, agreements have been signed by the provincial administrations in the Chaco and the DRH according to which the provinces will remunerate the DRH for work performed for the communities.