As a project aim is to obtain a sustainable water supply throughout the year, shallow dug wells (up to some 10 m) could not be envisaged. Alternatively, deeper drillings in the crystalline rock basement seemed to be appropriate. In view of the experiences gathered with the many actual effective deep drillings that followed (number including project phase III: about 2000), the areas encountered within this project may be characterized by three statements, which initially had to be considered as mere guesses.

• From a hydrogeological point of view, this region must be considered as being difficult as larger aquifers are exceptions. Ground water reserves which are usable throughout the year can be recognized only in some single cases, but not in general, through terrain characteristics and an analysis of data related to the surface cover.

• Locally significant water reserves cannot be expected anywhere in the undisturbed crystalline rock basement formation (deeper than 20–30 m).

• In crystalline zones, successful drillings must reach fractures associated with water-bearing strata; as the fracture intensity may be locally very variable, the choice of the drilling point, even on a small scale, will generally determine the success or failure of the drilling.

Water Supply for Vavuniya Town

A daily consumption of 50–75 liters per capita was calculated for the approximately 20,000 inhabitants of Vavuniya town which is spread over more than 10 sq. km. The existing water supply system was absolutely insufficient and could not be practically incorporated into the current water demand planning scheme. For this reason, it was necessary to build a certain number of wells with a permanent yield totaling 20 l/s.

In view of the previously mentioned geological features, it was not possible to obtain this water quantity with one or a few wells. It seemed better to look for several particularly appropriate fracture formations at depths of more than 30 m, in order to be able to drill a series of independent deep wells. The following discussions focus on the use of a combined method which—contrary to general expectations—turned out to allow a remarkably reliable detection of relatively narrow fracture zones.

Recognizable topographic and morphologic features were exploited to preselect an area in the immediate proximity of the town, which was tentatively considered as being appropriate; this area was then analyzed in detail by means of two different techniques. In the selected area, the project manager first determined the suspected exact locations of promising fracture zones by means of the dowsing technique. Then, geo-electrical techniques were used in the predefined area, which extended over a couple of kilometers [1,7], in order to investigate general information on the underground structure and the possible existence of larger fracture zones up to a depth of some 50 m. The result showed that up to a depth of 25 m the soil resistivity was very high and exceeded 500 Ohm meter; only underneath, could numerous inhomogeneities be noted along with values of the order of 100 Ohm meter in some areas, which indicated the possible availability of larger aquifers.

To get additional and more reliable information about the area, Schröter determined 13 spots for systematic test drillings in the spatially confined area. For that purpose, he used mainly the dowsing technique, which allowed him also to roughly predict the respective expected yields; the latter information was due to his ability to evaluate and differentiate his subjective dowsing reactions, a particular aspect of dowsing, which will be treated in part 3.2. After this, drillings were carried out up to a depth of 50 m. In most cases usable water strata were struck, roughly as predicted. Schröter claimed to have used the information on the actually encountered yields to improve the future interpretation of his subjective dowsing perceptions in the actual working area.

The interpretation of the geo-electrical measurements and the results of the consecutive test drillings revealed that abundant aquifers could not be found at arbitrary locations in the crystalline bedrock, and that yields exceeding 1 l/s per borehole might only be obtained in exceptional cases. All the test drillings reached fracture zones, but their yields were different from each other; eight of these drillings were later equipped with hand pumps. The geo-electrical technique in particular seemed not to be able to locate the best drilling sites with sufficient accuracy and to differentiate excellent from less productive spots.

Subsequently, Schröter determined by means of his dowsing technique seven other spots where fractures were particularly extended. He managed to locate these points in the proximity of the town to avoid long water pipelines. At each position three to five closely spaced drillings were carried out to a depth of approximately 50 m in order to enlarge the respective well surfaces. As the detected fractures were apparently very narrow and without good connections to each other, small blastings at the bottom of the boreholes helped to increase the yield from typically 0.3 l/s to 1.6 l/s through an enlargement of the well surface, whereby the draw-down remained moderate. Ultimately, for all the 21 drillings relatively large water quantities could be obtained, in one case over 7 l/s for a group of three boreholes, and always of excellent drinking water quality. Out of the 34 initially drilled holes (including all test drillings) 29 were converted into individual wells or so-called star-wells and were integrated into the water supply distribution network.

Overall, the well system provided a maximum of 25 l/s for permanent use and was therefore sufficient for the water supply of the whole town. Utilizing many pump tests and the water table decline observed as a function of the drained yields, sufficient data could be collected to establish a hydrogeologic model for the investigated area [7]. Accordingly, the precipitation quantity, as well as the permeability of the partly fractured underground up to a depth of 50 m should be sufficient to ensure on a long term basis a sustainable potable bulk water supply from all the new wells.

Experimental results and theoretical calculations revealed that deep borehole drilling yields, exceeding 1 l/sec, can only be obtained in particularly extended fracture zones. But those are rather rare and do not show up clearly enough at the surface to be found easily or even by high chance. With respect to this conclusion, and in view of the evaluation of underlying problems, all the specialists involved in the project have come to essentially the same opinion and agree. It is worth mentioning that the necessary expenditures for the water program designed and set up by means of the described combined procedure—with inclusion of the installation—only amounted to one tenth of the initially estimated input. The former plan was to treat the water of the Malvatu Oya River and to pump water over 32 km distance through pipelines to the town. The actually adopted solution, however, allowed saving of much time, avoided expenditure on treatment of shallow lying water and main tenance of a long water pipe with pump stations, and led to a long term sustainable water supply actually exceeding the initially planned quantities.

Following the experiences and successes gathered during the first part of the project, the following facts may be mentioned:

Allowing for generally acceptable efforts and expenditures, the classically available procedures were appropriate for the usual and very rough approximate drilling spot location; more exact siting within, say, a meter was not possible, but would have been necessary in the case of very narrow fractures. The complementary use of the dowsing technique filled this prospecting gap and enabled higher rates of success in locating fractures and related drilling spots which ultimately led to an optimum borehole yield.

At this stage, the described results do not yet provide proof of the relevance of the dowsing technique, but the quoted findings could also be corroborated in various subsequently executed GTZ projects. Accordingly, the conjecture finds increasing support that the use of dowsers in hydrogeological exploration may substantially increase the success rate of a water development project.

Water Supply in Rural Zones and Shore Lines

Except for the immediate coastal region, one found in the districts of Vavuniya and Mullaitivu the same geological features as in the already described environment of Vavuniya Town. But the exigencies of the water supply differed in an essential point from the conditions relevant for the urban zone of Vavuniya Town.

The project area covered 4160 sq. km of the thinly and least populated zones of Sri Lanka (44 inhabitants/sq. km) and 350 of the most problematic villages have been selected whose 50,000 inhabitants should have an all-year-round water supply. As the village populations amounted to a few hundred inhabitants respectively, spread over a large area, more than one well per village had to be built. For an estimated consumption of 15 - 35 l per person and day, a relatively low mean well yield of 5–10 l/min. could be considered as adequate. Consequently, it was possible to equip the wells with handy, economical and low-maintenance hand pumps.

The advantage compared with the prospecting in Vavuniya Town consisted of the fact that maximum yields were not at all necessary; it appeared sufficient to locate much smaller fracture zones. Because of the desire to secure the water supply throughout the year, these sites had to be found in the crystalline rock basement. Unlike high-yielding springs, a more frequent presence of such smaller aquifers could be expected. The possibility of a greater choice of well locations meant that features such as the desires of the local beneficiaries, the hygienic aspects, access or improvement of access and a central situation in the village could be incorporated. The disadvantage was that less productive aquifers, which may exhibit partly only a few centimeters wide fractures, represent in general smaller perturbations in the basement and are more difficult to locate with classical methods; in parts, detection is virtually impossible with the available techniques. Likewise, similar difficulties showed up in many projects carried out by other institutions, where very small fractures and weathered zones had to be located exclusively by conventional procedures (see below).

The location of the first hundred well points followed the previously mentioned combined procedure [5, 6]. Whenever possible, the formerly mentioned classical indications were used to recognize basement structures and eventually promising zones which had to be roughly preselected in the area chosen by local responsible authorities. One has to take into account the fact that this procedure was difficult: wells had to be located in populated areas according to the desires of the village people, i.e. in civilized and topographically modified areas, where the use of classical procedures was problematic or impossible. In view of this situation, Schröter employed his dowsing technique and located such places which should be situated, in his opinion and according to his experience, quite precisely on appropriate, weakly developed fracture zones. The next step was, if technically possible, the checking of such places by means of methods such as the geo-electrical technique (Schlumberger method). In almost all the areas checked by this method, it was revealed that the fracture sites first determined by Schröter could be confirmed by anomalies of the soil resistivity. More precisely: on the one hand, these measurements by themselves could, of course, not be uniquely interpreted in terms of precise and safe drilling points; on the other hand, the places could not be clearly excluded on account of a complete lack of soil resistivity gradients.

Fig. 2. Yield distribution according to Table 1. (F: dry drillings, i.e. below 5 l/min.). For conventional well prospecting with relatively low production, one would expect, contrary to the observed distribution, continuously decreasing parts. The clearly visible deviation from this points to a different mechanism.

The combined procedure appeared to be economically optimal as enough water could be found in apparently small shallow fractures at depths below 30 m. Not a single drilling hole had been deepened when the expected average yield could be proven after a six hours pump test. That meant, though, that the respective maximum and sustainable permanent yields, as well as the corresponding water drawdown were generally unknown; otherwise, one should have drilled deeper as envisaged and the pump tests including well construction should have been done with much more effort. With regard to the objectives of the project, both procedures were not necessary, nor could they be carried out within the given time scale and the financial cost constraints.

Results from the first 100 drillings in Vavuniya District, obtained under the above-mentioned restrictive conditions, are compiled in Table 1, ordered in nine yield categories. It is revealed that the effectively encountered water quantities were very variable, but mostly exceeded the necessary minimum yield. Nevertheless, it is noteworthy that the wells delivered, on the average, relatively low yields according to the requirements stated beforehand, contrary to the situation in Vavuniya Town where much higher yields were demanded and obtained. Furthermore, it is interesting to point out that the yield distribution displayed in Table 1 differs from the one which would be expected when only conventional prospecting methods are used: due to the spatially extended distribution of the productivity of fractured rock domains, the number of smaller yields should first dominate and then decrease with increasing yield of the wells. But as the actually resulting distribution partly reveals a contrary behavior, it seems fair to conclude that a different, goal-oriented mechanism may lie behind the applied dowsing procedure.

It should be kept in mind that only drinking water counted as success. A too high percentage of salt, iron, nitrate, nitrogen and fluorides would have led to the abandonment of the borehole. Thanks to the ability to interpret his subjective dowsing reaction, Schröter was able to give at least indirect hints in this respect before drillings took place; in this way the number of wells which presented a bad water quality were insignificant. This part of the dowsing spectrum turned out to be significant especially for finding sites on shore lines, but its real significance is not yet well proven and should be checked in further tests, along with consideration of future new theories.