Salt distribution in the Senegal middle valley : analysis of a saline structure on planned irrigation schemes from N'Galenka creek

In the middle Senegal valley, the saline soil distribution is not related to the present faint topography. The absence of a relationship is one of the major constraints in establishing new irrigation schemes. The salt distribution was studied to understand its variability, and to describe its structure and spatial arrangement. Saline areas were delineated by measuring the electromagnetic soil conductivity (ECm), a rapid technique with a portable instrument (EM38). The results indicate that the saline soils are distributed as strips. A detailed examination revealed that the major strip is actually composed of two parallel minor strips, and a comparison with aerial photographs showed that one lies in a former creek bed, and the other fringes it on the southern bank. The strip is intersected by an actual creek bed, indicating that the salt distribution is ancient, related to previous geo-morphology, and does not result from a recent remobilisation of the marine salt deposits incorporated in the soil. The identification of this relationship between the present saline soil distribution and previous geo-morphology allowed us to survey the whole N'Gualenka region (about 6000 ha) using ECm measurements on selected transects. (Résumé d'auteur

Actual and potential salt-related soil degradation in an irrigated rice scheme

Abstract Salt-related soil degradation due to irrigation activities is considered a major threat to the sustainability of rice cropping under semi-arid conditions in West Africa. Rice productivity problems related to soil salinity, alkalinity and topographic position were observed in an irrigated rice scheme in southern central Mauritania. Detailed study of soils in a toposequence revealed that highest topsoil salinity and alkalinity were found at the shallow soils (<1.2 m) of the middle and upper slopes. Here, soils have formed in situ from the schist parent rock, which releases carbonate rich salts upon weathering. Within these soils large differences in salinity and alkalinity level occur at short distances, indicating minimal groundwater flow and a strong variation in the geochemical composition of the vertically positioned bedrock. Further downslope, soils have a (partly) colluvio-alluvial origin. Here, sedimentation during annual floods increased soil depth (>2.5 m) and salinity levels remained low due to leaching. Foum Gleita's irrigation water used is amongst the most alkaline in the Sahel. However, no clear indications of secondary salinization or alkalinization due to irrigation activities were observed. A comparison of historical data revealed no significant changes of topsoil salinity and pH over the last 30 years. The PHREEQC 2.0 model was used to study actual and potential development of soil salinity and alkalinity problems, by simulating excessive concentration of the irrigation water through evaporation. The evolution into a strongly sodic-alkaline solution due to precipitation of Mg-calcite and -silicate minerals did not fit with current composition of ground and surface water, which showed geochemical control of alkalinity at high concentrations. Incorporation of cation exchange processes, using a small (1.0 mmol c per 100 g dry soil) but calcium saturated CEC, resulted in a better fit with field data. Results indicate that the soil's buffer capacity to counteract alkalinization processes is large. However, the soil water and salt balance needs to be quantified in order to determine development rate and equilibrium levels of soil salinity and alkalinity for different soil type  water management combinations. This study does neither reject the hypothesis that salt-related soil degradation jeopardizes the sustainability of rice cropping in the Sahel, nor does it provide evidence for its verification. However, our results are in line with other studies in west Africa, in that current salt-related production problems are inherited, rather than being induced by irrigated rice cropping.

Actual and potential salt-related soil degradation in an irrigated rice scheme

Abstract Salt-related soil degradation due to irrigation activities is considered a major threat to the sustainability of rice cropping under semi-arid conditions in West Africa. Rice productivity problems related to soil salinity, alkalinity and topographic position were observed in an irrigated rice scheme in southern central Mauritania. Detailed study of soils in a toposequence revealed that highest topsoil salinity and alkalinity were found at the shallow soils (<1.2 m) of the middle and upper slopes. Here, soils have formed in situ from the schist parent rock, which releases carbonate rich salts upon weathering. Within these soils large differences in salinity and alkalinity level occur at short distances, indicating minimal groundwater flow and a strong variation in the geochemical composition of the vertically positioned bedrock. Further downslope, soils have a (partly) colluvio-alluvial origin. Here, sedimentation during annual floods increased soil depth (>2.5 m) and salinity levels remained low due to leaching. Foum Gleita's irrigation water used is amongst the most alkaline in the Sahel. However, no clear indications of secondary salinization or alkalinization due to irrigation activities were observed. A comparison of historical data revealed no significant changes of topsoil salinity and pH over the last 30 years. The PHREEQC 2.0 model was used to study actual and potential development of soil salinity and alkalinity problems, by simulating excessive concentration of the irrigation water through evaporation. The evolution into a strongly sodic-alkaline solution due to precipitation of Mg-calcite and -silicate minerals did not fit with current composition of ground and surface water, which showed geochemical control of alkalinity at high concentrations. Incorporation of cation exchange processes, using a small (1.0 mmol c per 100 g dry soil) but calcium saturated CEC, resulted in a better fit with field data. Results indicate that the soil's buffer capacity to counteract alkalinization processes is large. However, the soil water and salt balance needs to be quantified in order to determine development rate and equilibrium levels of soil salinity and alkalinity for different soil type  water management combinations. This study does neither reject the hypothesis that salt-related soil degradation jeopardizes the sustainability of rice cropping in the Sahel, nor does it provide evidence for its verification. However, our results are in line with other studies in west Africa, in that current salt-related production problems are inherited, rather than being induced by irrigated rice cropping.

Comparison of electrical resistivity by geophysical method and neutron probe logging for soil moisture monitoring in a forested watershed

Geophysical methods are becoming more popular nowadays in the field of hydrology due to their time and space efficiency. So an attempt has been made here to relate electrical resistivity with soil moisture content in the field. The experiments were carried out in an experimental watershed 'Mulehole' in southern India, which is a forested watershed with approximately 80% red soil. Five auger holes were drilled to perform the soil moisture and electrical resistivity measurements in a toposequence having red and black soils, with sandy weathered soil at the bottom. Soil moisture was measured using neutron probe and electrical resistivity was measured using electrical logging tool. The results indicate that electrical resistivity measurements can be used to measure soil moisture content for red soils only

A study on the status of fluoride ion in groundwater of coastal hard rock aquifers of south India

India has an increasing incidence of fluorosis, dental and skeletal, with nearly about 62 million people at risk. High fluoride groundwaters are present especially in the hard rock areas of the country. This paper analyzes the most extensive database on fluoride and other chemical constituent distribution in the coastal hard rock aquifers of Thoothukudi district. A total of 135 samples were collected and analyzed for major cations and anions to assess the geochemical process. The fluoride concentration in drinking waters varied from BDL to 3.2 mg l-1 in the study area. Majority of the samples do not comply with WHO standards for most of the water quality parameters. The saturation index of fluorite saturation index was used to correlate with F- to identify their relationship to increase of fluoride levels. The correlation between the F- concentration and the water type was also attempted. Spatial distribution of fluoride in groundwater was studied to understand the influencing factors. The relationship of F- with HCO- 3, Na+ and pH concentrations were studied and found that HCO- 3, has good correlation with F- than the other parameters