S1004 The Groundwater Survey by using Resistivity Method and its Application in the Republic of Rwanda

Transcription

1 S1004 The Groundwater Survey by using Resistivity Method and its Application in the Republic of Rwanda K. Nakamura* (NTC International Co., Ltd.), T. Aizawa (Suncoh Consultants Co., Ltd.), S. Matsuda (NTC International Co., Ltd.) Summary This paper presents result of the groundwater survey by using resistivity method and its application especially in the developing countries based on the lessons learnt through the feasibility study of irrigation facility rehabilitation project in Republic of Rwanda in eastern Africa. As a result of the study, the water catchment structure was examined by resistivity survey, and availability of water resource was also confirmed. In addition, it was confirmed that the interpretation of the resistivity survey results for the water catchment structure in the laterite soil of this country is appropriate. This suggests that there is sufficient possibility of project cost reduction due to early implementation of resistivity survey. In the future, it is considered that similar interpretations can be made in the case of constructing an alternative water facility in the vicinity of a place already having a spring as in this time April 2018, Yogyakarta, Indonesia

2 1. Introduction In developing countries such as Africa, water demand has been steadily increasing year by year with economic growth and population increase, and water shortage is a serious problem. In order to improve capacity of reservoir, raising dam height is commonly applied; however it often requires drilling of alternative wells for the ones to be submerged under the water. Moreover, a project proponent estimates more budgets considering failure of constructing alternative wells. However, the location that is easy to construct is choose sometimes and success probability of alternative wells is not necessarily high. Based on this background, groundwater survey was conducted in the feasibility study stage by using resistivity method on the assumption that construction cost can be judged accurately by examining candidate sites of alternative wells. This paper presents result of the groundwater survey by using resistivity method and its application especially in the developing countries based on the lessons learnt through the feasibility study of irrigation facility rehabilitation project in Republic of Rwanda (Rwanda) in eastern Africa by Japan International Cooperation Agency (JICA). 2. Groundwater Survey 2.1. Outline of the Survey Rwanda located in Eastern Africa (Fig.1) is known for its civil war in 1990 s. Currently the country s economy, tourist number, human development index have grown rapidly so called as The Miracle of Africa. However, rainfed agriculture which is easily influenced by climate alternation is dominant in the country yet. Hence, further introduction of irrigation agriculture and rehabilitation of the aging irrigation facilities are prioritized as one of the key national development issues in the country. In response to this, JICA decided to implement a feasibility study for rehabilitation of irrigation facilities by grant aid cooperation scheme in future. The feasibility study unveiled that existing water facility used by local community are likely to be submerged due to rise in high water level of dam by the project. Hence, Republic of Rwanda provision of alternative water facilities to the community was planned by the project as a mitigation measure against the anticipated adverse impact by the project. In this context, in order to identify appropriate drilling location(s) for the alternative wells, underground water survey by the 2D-Resistivity survey method was planned. Fig.1 Location map of Rwanda 2.2. Hydrogeological Characteristics Rwanda is a landlocked country located between the eastern and western rift valleys in the eastern Africa and its topography is converted into a semi-plain and forms a country of thousand hills. One of major rivers in the country, Akagera River threads among the hills can be said as one of the headstreams of River Nile because it flows into the Lake Victoria in Uganda. In terms of geology, basement rock is composed of metamorphic and granite rocks of Pre-Cambrian period and outcrop of these rocks are often found on the road slope. Surface layer is these high weathered rocks, or reconsolidated sediment and it forms very hard and red-colored surface layer in the bare land. Annual rainfall amount is 1,000mm which almost equivalent to two-third of the one fall in Japan, but intermittent and showery rain is recorded only in the rainy season. Existence of the water facilities that use natural spring and being used by local community throughout the year indicates the possibility of existence of rich precipitation and aquifer underground Survey Site

3 Structure of the affected facility is very simple and it collects water through the steel pipe horizontally inserted to the spring point and its outlet area is protected by concrete (Fig.2). The affected facility is located between valley plain and hills behind. Hence, it is expected that rainwater that infiltrated underground can flow down slowly through the weathered layer and appear on the ground which is affected by erosion. In addition, several springs were observed around the affected facility. This meant aquifer and underground water in slope is non-uniformly distributed, and a hydrogeological structure that accumulates underground water is formed around the affected water facility. Therefore, survey line for the 2D-Resustivity survey was placed to the perpendicular to the slope direction order to clarify underground water structure in horizontal direction of the slope. Fig.3 shows existing water facility to be affected and slope behind Result and Discussion (1) 2D-Resistivity Survey 200m length of 2D-resistivity survey was conducted on the slope which located behind of the existing water facilities and the elevation which will not be affected by rise in full water level. Electrode arrangement is Pole-Pole method with electrode interval of 2.0m and maximum survey depth of 20 meter. Conditions, equipment and items for the Survey are shown in Table 1 below. In consideration of rise in full water level by the project, elevation of the alternative water facility should be 3m higher than the existing one and relative height of the survey points was set as 12m accordingly. Fig.2 Existing Water Facility (close-up view) Fig.3 Existing Water water facility Facility to be and affected hills behind Table.1 Conditions, equipment and items for the Survey Experiment conditions -Survey length: 200m -Survey interval: 2.0m -Survey depth: 20m -Electrode arrangement: Pole-Pole method Equipment and items used -Measuring Unit :M C OHM -Electrode: Earth rod φ19mm L=50cm 200pcs -Electro cable: Single 0.5sq 2,000m Survey result is shown in Fig.4 below. Electrical resistivity with a range of 20 to 2000Ω-m was confirmed and shown in the analyzed cross section. Among this, high resistivity area in the surface layer around 3 to 5 m depth from the ground level corresponds to dried topsoil, colluviums, and highly weathered rocks. Low resistivity area with tens Ω-m was estimated as clayish weathered rocks based on the denuded geological characteristics observed in surrounding area. Furthermore, layers of around 10m depth is classified into three according to the distance from the beginning point: low resistivity (distance between 0 to 60m with resistivity of about 80Ω-m), mid-resistivity (distance between 60 to 120m with resistivity of about 200Ω-m), and high resistivity (distance between 120 to 200m with resistivity of around 800Ω-m).

4 Fig.4 Result of the 2-D Resistivity in Cross Section Considering (i) the different resistivity found in the aforementioned deepest layer is caused by rock types or grain size that composes the rocks, (ii) existing facility is located at almost centre of the survey line, and (iii) its spring water depth is around 12m, distribution of highly weathered rocks and groundwater recharge in this layer was presumed. (2) Test Boring Survey The test boring survey was conducted to target the deepest distribution points of the low resistivity area in the second layer. Because, it was presumed that low resistivity layer contains many clay and it constitutes impermeable layer which prevent outflow of groundwater. Advantages to implement the test boring survey at feasibility study stage is assuredness and economic efficiency. The test boring cost is cheaper than the one for actual boring at construction stage with 50% success probability. In fact, the test boring survey and pumping test are implemented at survey stage in Japan if certain conditions are satisfied. Moreover, even if groundwater is not confirmed in the test boring survey, it will enable to make a more reliable plan. Elevation of the test boring survey was set as 12m higher than the current water level at the pipe of existing water facility. As a result of the survey, clayey weathered rocks from 0 to 7.6m depth, repeated washout of core from 9 to 18m depth, and effuse of muddy water and blowout of freshwater from 18 to 19.5m depth were confirmed. Photo of boring core and comparison of resistivity survey and test boring survey are shown in the Fig.5 and Fig.6 respectively. At the end of the boring survey, pumping test was conducted by temporary hand-pump supplementary. It took about 2 to 3 hours to get clean water from the boring hole with pumping discharge of 10 litters per minute. Decrease of water level in the boring hole was not confirmed during the pumping test. Although the pumping test by hand-pump might be insufficient to verify the water yield, adequate water yield of 150 litters per minute is expected because diameter of boring hole in construction stage will become bigger and inside of well hole will be cleaned. Based on the test boring survey and pumping test, distribution of the targeted aquifer is clarified physically Fig.5 Boring Core Fig.6 Contradistinction of test boring and resistivity

5 and drilling point of the alternative facility is set at 7.0m higher than the existing facility. 2.5 Challenges of resistivity survey in developing countries Not limited to the resistivity survey, but it is often uncertain that availability of equipment and items needed for the survey in the developing countries. Hence, the availability must be checked through the local staffs or partnering company prior to the flight. In this survey, a measuring instalment and electro cables were procured in Japan and brought to Rwanda while battery and charger for measuring instalment and measurement tape were purchased in Rwanda because its availability was confirmed in advance. In consideration of freight and rough road conditions in Rwanda, a heavy-duty measuring instalment M C OHM was selected for the survey. However, it doesn t have a function to display waveform, though it should have considered. Importance of prior preparation of equipment and items for the resistivity survey in developing country was re-realized. 3. Discussions and Conclusion In this survey, the water catchment structure was examined by resistivity survey, and availability of water resource was also confirmed physically by trial digging in addition to the selecting the location. As a result, it was confirmed that the interpretation of the resistivity survey results for the water catchment structure in the laterite soil of this country is appropriate. This suggests that there is sufficient possibility of project cost reduction due to early implementation of resistivity survey. In the future, it is considered that similar interpretations can be made in the case of constructing an alternative water facility in the vicinity of a place already having a spring as in this time. Reference European Association of Geoscientist and Engineers (2014):Application Manual of Method to Engineering and Environmental Problems, Chapter 6. The Geoelectrical Method.