Geophysical Investigations of a Rural Water Point Installation Program in Nampula Province, Mozambique

There are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the provinces of Nampula and Cabo Delgado. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the uitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50 %. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate. In total 11 sites were investigated, including seven boreholes with sufficient yield and five boreholes with insufficient yield. A perpendicular cross with two 400m survey lines were made over 7 boreholes and single 400m survey lines were made over 5 boreholes. Due to lateral variation the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate

A hydrogeological study of the Nhandugue River, Mozambique – A major groundwater recharge zone

The Nhandugue River flows over the western margin of the Urema Rift, the southernmost extension of the East African Rift System, and marks the north-western border of Gorongosa National Park, Mozambique. It constitutes one of the major indispensable water resources for the ecosystem that the park protects. Our study focused on the hydrogeological conditions at the western rift margin by resistivity measurements, soil sampling and discharge measurements. The resistivity results suggest that the area is heavily faulted and constitutes a major groundwater recharge zone. East of the rift margin the resistivity indicate that solid gneiss is fractured and weathered, and is overlain by sandstone and alluvial sediments. The top 10-15 m of the alluvial sequence is interpreted as sand. The sand layer extends back to the rift margin thus also covering the gneiss. The sandstone outcrops a few kilometers from the rift margin and dips towards east/south-east. Further into the rift valley, the sand is underlain by lenses of silt and clay on top of sand mixed with finer matter. In the lower end of the investigated area the lenses of silt and clay appears as a more or less continuous layer between the two sand units. The topmost alluvial sand constitutes an unconfined aquifer under which the solid gneiss forms a hydraulic boundary and the fractured gneiss an unconfined aquifer. The sandstone is an unconfined aquifer in the west, becoming semi-confined down dip. The lenses of silt and clay forms an aquitard and the underlying sand mixed with finer matter a semi-confined aquifer. The surface runoff decreases downstream and it is therefore concluded that surface water infiltrates as recharge to the aquifers and moves as groundwater in an east/south-eastward direction

Combined electrical resistivity tomography and magnetic resonance sounding investigation of the surface-water/groundwater interaction in the Urema Graben, Mozambique

This study focusses on the hydrogeology of Urema Graben, especially possible interactions between surface water and groundwater around Lake Urema, in Gorongosa National Park (GNP). Lake Urema is the only permanent water source for wildlife inside GNP, and there are concerns that it will disappear due to interferences in surface-water/groundwater interactions as a result of changes in the hydraulic environment. As the lake is the only permanent water source, thiswould be a disaster for the ecosystem of the park. The subsurface geology in Urema Graben was investigated by 20 km of electrical resistivity tomography (ERT) and three magnetic resonance sounding (MRS) surveys. The average depth penetration was 60 and 100 m, respectively. The location of the ERT lines was decided based on general rift morphology and therefore orientated perpendicular to Urema Graben, from the transitional areas of the margins of the Barue platform in the west to the Cheringoma plateau escarpments in the east. ERT andMRS both indicate a second aquifer, where Urema Lake is a window of the first upper semi-confined aquifer, while the lower aquifer is confined by a clay layer 30–40 m thick. The location and depth of this aquifer suggest that it is probably linked to the Pungwe River which could be a main source of recharge during the dry season. If a dam or any other infrastructure is constructed in Pungwe River upstream ofGNP, the groundwater level will decrease which could lead to drying out of Urema Lake

Geophysical Investigations of a Rural Water Point Installation Program in Nampula Province, Mozambique

There are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the provinces of Nampula and Cabo Delgado. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the uitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50 %. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate. In total 11 sites were investigated, including seven boreholes with sufficient yield and five boreholes with insufficient yield. A perpendicular cross with two 400m survey lines were made over 7 boreholes and single 400m survey lines were made over 5 boreholes. Due to lateral variation the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate

Borehole Logging and Slug Tests for Evaluating the Applicability of Electrical Resistivity Tomography for Groundwater Exploration in Nampula Complex, Mozambique

In Nampula province, Mozambique, there is a high number of water wells considered as having failed for having too low a pumping yield. Two Electrical Resistivity Tomography (ERT) measurement campaigns were conducted in the area for evaluating the reasons of failures. However, in some cases it was difficult to verify and interpret the ERT results by only using the inadequate lithological description presented in drilling reports. In this paper the integration of borehole logging and slug testing is presented as a solution to add more information and to enhance the interpretation of ERT models. The borehole logging tool measured resistivity, magnetic susceptibility and natural gamma. The logging results proved that the ERT models are accurate in estimating the resistivity for basement (>1400 Ωm), fractured layer (220–1400 Ωm), semi-weathered layer with clay accumulation (10–220 Ωm), and weathered and leached layer (220–2700 Ωm). The slug testing gave results of high hydraulic conductivity (K) values where the ERT indicates well-developed weathered and fractured layers, and low K values where these are less developed. The borehole interpretation can be extrapolated using the ERT model to give a geometric characterization of the aquifer. Therefore, the implementation of the ERT method in groundwater exploration is encouraged

On the applicability of geophysics for groundwater investigations in areas with lack of hydrogeological information. Examples from Mozambique

The work presented in this thesis has aimed to improve the knowledge about the groundwater occurrence in different geological conditions in Mozambique. The main focus in this study has been on the understanding of hydrogeological conditions in area with little background information by use of geophysical investigation. The lack of background information is a problem in many countries because the existing information is not well organized, old reports are missing in the archives and data is not published in scientific journals. In such conditions, the approach of combining different geophysical methods measuring different physical properties of geological material can improve the interpretation.Four geophysical methods have been assessed regarding their applicability in solving real hydrogeological problems in Sofala and Nampula provinces in Mozambique. The implemented methods are electrical resistivity tomography (ERT), magnetic resonance sounding (MRS), induced polarization (IP) and geophysical borehole logging (BL). Also other methods such slug tests, dilution tests and grain size analyses were included in this study.The ERT method was used throughout the investigation however the interpretation of the results were often not conclusive due to lack of reference data. The Interpretation of ERT results compared to groundwater extraction rates from new boreholes (in Nampula provincne) showed that areas with a well-developed weathered layer have better potential for groundwater extraction. The MRS method work well in unconsolidated sediments in an attempt to understand the interaction between surface water and groundwater in the Urema floodplain, Sofala province. However, when MRS was used to identify the main reason for low possible groundwater extraction rates in some of the wells, MRS did not work well due to the low signal caused by low water content in combination with different types of noise, both natural and anthropogenic. IP results, presented as normalized chargeability, give a good contrast between clay and sand layers and between clay layers and unweathered rock in Mongicual district, Nampula province. The high value of normalized chargeability found in sand layers in this study are attributed to a content of heavy metal minerals. The electrical conductivity of groundwater increases when these minerals are dissolved. Borehole loggings together with ERT results have shown that the lithological descriptions in the drilling reports are sometimes inconsistent. The inconsistence may be caused by the mix of site information or copy and paste of the description from different reports. As a consequence of this inconsistency, it is expected that even some VES presented in the borehole drilling report are presumably wrong.Combining hydrogeophysical methods and other non-geophysical methods has proven to be useful when no other information is available. This effort has increased the knowledge of hydrogeological situation in different parts of Mozambique. Therefore, it is recommended to continue this but with more control (to reduce the uncertainties) on how they can be interpreted together and thereby overcome the lack of background data

Borehole Logging and Slug Tests for Evaluating the Applicability of Electrical Resistivity Tomography for Groundwater Exploration in Nampula Complex, Mozambique

In Nampula province, Mozambique, there is a high number of water wells considered as having failed for having too low a pumping yield. Two Electrical Resistivity Tomography (ERT) measurement campaigns were conducted in the area for evaluating the reasons of failures. However, in some cases it was difficult to verify and interpret the ERT results by only using the inadequate lithological description presented in drilling reports. In this paper the integration of borehole logging and slug testing is presented as a solution to add more information and to enhance the interpretation of ERT models. The borehole logging tool measured resistivity, magnetic susceptibility and natural gamma. The logging results proved that the ERT models are accurate in estimating the resistivity for basement (>1400 Ωm), fractured layer (220–1400 Ωm), semi-weathered layer with clay accumulation (10–220 Ωm), and weathered and leached layer (220–2700 Ωm). The slug testing gave results of high hydraulic conductivity (K) values where the ERT indicates well-developed weathered and fractured layers, and low K values where these are less developed. The borehole interpretation can be extrapolated using the ERT model to give a geometric characterization of the aquifer. Therefore, the implementation of the ERT method in groundwater exploration is encouraged

Geophysical Investigation of a Rural Water Point Installation Program in Nampula Province, Mozambique

There are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the province of Nampula and Cabo Delgado in crystalline rock zone. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the suitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50%. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate of boreholes. In total, nine boreholes with sufficient yield, and five boreholes with insufficient yield were investigated. In both VES and ERT, the resistivity values indicate 3 different layers. One surface layer with resistivity between 220-5000+ Ωm, a second layer with lower resistivity value, varying from 10-220 Ωm, less than 10 Ωm in some places, and a third layer with high resistivity values, 220-5000+ Ωm, increasing with Depth. Due to lateral variation, the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate

A hydrogeological study of the Nhandugue River, Mozambique - A major groundwater recharge zone

The Nhandugue River flows over the western margin of the Urema Rift, the southernmost extension of the East African Rift System, and marks the north-western border of Gorongosa National Park, Mozambique. It constitutes one of the major indispensable water resources for the ecosystem that the park protects. Our study focused on the hydrogeological conditions at the western rift margin by resistivity measurements, soil sampling and discharge measurements. The resistivity results suggest that the area is heavily faulted and constitutes a major groundwater recharge zone. East of the rift margin the resistivity indicate that solid gneiss is fractured and weathered, and is overlain by sandstone and alluvial sediments. The top 10-15 m of the alluvial sequence is interpreted as sand. The sand layer extends back to the rift margin thus also covering the gneiss. The sandstone outcrops a few kilometers from the rift margin and dips towards east/south-east. Further into the rift valley, the sand is underlain by lenses of silt and clay on top of sand mixed with finer matter. In the lower end of the investigated area the lenses of silt and clay appears as a more or less continuous layer between the two sand units. The topmost alluvial sand constitutes an unconfined aquifer under which the solid gneiss forms a hydraulic boundary and the fractured gneiss an unconfined aquifer. The sandstone is an unconfined aquifer in the west, becoming semi-confined down dip. The lenses of silt and clay forms an aquitard and the underlying sand mixed with finer matter a semi-confined aquifer. The surface runoff decreases downstream and it is therefore concluded that surface water infiltrates as recharge to the aquifers and moves as groundwater in an east/south-eastward direction. (C) 2011 Elsevier Ltd. All rights reserved