ICDP workshop on the Lake Tanganyika Scientific Drilling Project: a late Miocene–present record of climate, rifting, and ecosystem evolution from the world's oldest tropical lake

The Neogene and Quaternary are characterized by enormous changes in global climate and environments, including global cooling and the establishment of northern high-latitude glaciers. These changes reshaped global ecosystems, including the emergence of tropical dry forests and savannahs that are found in Africa today, which in turn may have influenced the evolution of humans and their ancestors. However, despite decades of research we lack long, continuous, well-resolved records of tropical climate, ecosystem changes, and surface processes necessary to understand their interactions and influences on evolutionary processes. Lake Tanganyika, Africa, contains the most continuous, long continental climate record from the mid-Miocene (∼10 Ma) to the present anywhere in the tropics and has long been recognized as a top-priority site for scientific drilling. The lake is surrounded by the Miombo woodlands, part of the largest dry tropical biome on Earth. Lake Tanganyika also harbors incredibly diverse endemic biota and an entirely unexplored deep microbial biosphere, and it provides textbook examples of rift segmentation, fault behavior, and associated surface processes. To evaluate the interdisciplinary scientific opportunities that an ICDP drilling program at Lake Tanganyika could offer, more than 70 scientists representing 12 countries and a variety of scientific disciplines met in Dar es Salaam, Tanzania, in June 2019. The team developed key research objectives in basin evolution, source-to-sink sedimentology, organismal evolution, geomicrobiology, paleoclimatology, paleolimnology, terrestrial paleoecology, paleoanthropology, and geochronology to be addressed through scientific drilling on Lake Tanganyika. They also identified drilling targets and strategies, logistical challenges, and education and capacity building programs to be carried out through the project. Participants concluded that a drilling program at Lake Tanganyika would produce the first continuous Miocene–present record from the tropics, transforming our understanding of global environmental change, the environmental context of human origins in Africa, and providing a detailed window into the dynamics, tempo and mode of biological diversification and adaptive radiations.© Author(s) 2020. This open access article is distributed under the Creative Commons Attribution 4.0 License

Estimation of groundwater recharge in Bugesera region (Burundi) using soil moisture budget approach

Groundwater recharge for Bugesera region, a potable water-scarce area in northeastern Burundi, is computed using the soil moisture budget technique. Five evapotranspiration methods including Hamon, Hargreaves, Thornthwaite and two modifications of the original Thornthwaite method are evaluated in comparison to the reference evapotranspiration method, i.e. the FAO Penman-Monteith equation for years where complete climatic data is available. The evapotranspiration calculated by the aforementioned methods along with rainfall data are used to compute the soil moisture budget. The latter is calculated using the methodology devised by Thornthwaite & Mather (1955). Recharge calculation is performed using both the Thornthwaite Monthly Water-Balance Model (henceforth TMWB model) and excel sheets wherein each term of the soil moisture budget is computed separately. The results of evapotranspiration calculations show that, while the other evaporation methods slightly to moderately underestimate or overestimate the pote tial evapotranspiration in comparison to the FAO Penman-Monteith method, Hargreaves equation aberrantly overestimates this parameter. Likewise, groundwater recharge estimated using Hargreaves' evapotranspiration is dramatically reduced in comparison to the other evapotranspiration methods. Moreover, this study clearly shows that the time discretisation used in recharge calculations has important consequences, the use of smaller time steps leading to enhanced recharge. This better corresponds to reality. Compared to the recharge values obtained on a daily basis with Penman-Monteith evapotranspiration, the TMWB model which is on a monthly basis, using Hamon's evapotranspiration, gives the best approximations of reality, with the advantage of needing much less data. The distribution pattern of monthly recharge features a bimodal pattern somewhat similar to that of the monthly rainfall with an important peak in April

Degradation of groundwater quality in coastal aquifer of Sabratah area, NW Libya

Overextraction of groundwater is widely occurring along the coast where good quality groundwater is at risk, due to urbanization, tourist development and intensive agriculture. The Sabratah area at the northern central part of Jifarah Plain, Northwest Libya, is a typical area where the contamination of the aquifer in the form of saltwater intrusion, gypsum/anhydrite dissolution and high nitrate concentrations is very developed. Fifty groundwater samples were collected from the study area and analysed for certain parameters that indicate salinization and pollution of the aquifer. The results demonstrate high values of the parameters electrical conductivity, sodium, potassium, magnesium, chloride and sulphate which can be attributed to seawater intrusion. The intensive extraction of groundwater from the aquifer reduces freshwater outflow to the sea, creates drawdown cones and lowering of the water table to as much as 30 m below mean sea level. Irrigation with nitrogen fertilizers and domestic sewage and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for the high nitrate concentration towards the south of the region. Seawater intrusion and deep salt water upconing result in general high SO42- concentrations in groundwater near the shoreline, where localized SO42- anomalies are also due to the dissolution of sebkha deposits for few wells in the nearby sebkhas. Upstream, the increase in SO42- concentrations in the south is ascribed to the dissolution of gypsum at depth in the upper aquifer

Occurrences of evaporitic salts in Bugesera region (Burundi) and relation to hydrogeochemical evolution of groundwater

Several studies conducted on occurrences of evaporitic salts in various parts of the world often interpret the formation of such salts as the result of the evaporation of subsurface brines, saline lake waters and sea water-contaminated groundwater, on one hand, and the deposition of sea sprays and wind-transported salts, on the other hand. This study seeks to understand processes governing the formation of evaporitic salts in a humid tropical context and to establish a link between the occurrence of these salts and the different processes controlling the hydrogeochemical evolution of groundwater in Bugesera region, in north-eastern Burundi. Unlike previous studies, salt occurrences in the study area occur in a continental and humid tropical environment where chemical analyses of water samples from several small lakes reveal rather fresh water. While salt incrustations reported in the western Burundi occur in a sedimentary basin which is made up of a mixture of lacustrine and fluviatile sediments, the evaporitic salts in the depression of Bugesera appear in the upper layers of the weathered overburden developed on Precambrian basement rocks. These facts suggest that the lithological nature and setting of the area may not be the key factor controlling the formation of these salt deposits. X-ray diffraction (XRD) analyses performed on saline soils from two quarries in Bugesera region confirm the presence of salt minerals including particularly gypsum (CaSO4 center dot 2H(2)O), halite (NaCl) and thenardite (Na2SO4). Other minerals identified such as goethite, kaolinite, muscovite and quartz may be part of the soil matrix. Chemical analyses on leachates of soil samples from the 2 quarries in Bugesera region confirm their saline nature as evidenced by high TDS (1826-17,679 mg/l) and pH values (8.3-8.5). The predominance of Na+, Mg2+, Ca2+, K+, SO42-, Cl- in the chemical composition of the dissolved salts clearly reflects their mineralogical composition. This study clearly demonstrates that these evaporitic salts originate from the combination of three processes, namely the dissolution of aluminosilicate minerals which enrich groundwater in different salt-forming ions, the capillary rise which lifts the shallow groundwater and the dissolved ions up to the reach of evaporation and ultimately the evapoconcentration which precipitates the salts following well-established pathways. These findings are well supported by the results of the simulation of the evapoconcentration process with the PHREEQC hydrochemical model

Estimation of depth to fresh-salt water interface and its implications for sustainable groundwater resource management: a case study of the Coastal strip of Dar es Salaam, Tanzania

Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting over-exploitation is entailing the risk of seawater intrusion. This problem has been systematically investigated for the first time, and appears to be wide-spread. Determining the depth of the fresh-salt water interface was deemed crucial in the view of sustainable exploitation of the Dar es Salaam quaternary coastal aquifer system. Vertical electrical soundings (VES) and horizontal resistivity profiles have shown a decrease of resistivity with depth and toward the coastline. Cross sections have shown the presence of salt water in the aquifer near the coastline, and the boundary between fresh and saline waters is mapped. The surface prospection resistivity data showed comparable results as reflected in hydrochemical/lithological cross sections and resistivity well logs. Salt water was found mostly in the area within 2 km of the coastline, and the depth to the interface in this zone is ranging from 1.3 to 20 m. Away from the coastline, the interface (if present) is at greater depth. VES executed at Kurasini (4 km from the sea) showed the fresh-salt water interface at a greater depth (43 m)

Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl- near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na-Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca-HCO3 and Ca-Mg-HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na-HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system