Olivier Ribolzi

A short review of fecal indicator bacteria in tropical aquatic ecosystems: knowledge gaps and future directions

Given the high numbers of deaths and the debilitating nature of diseases caused by the use of unclean water it is imperative that we have an understanding of the factors that control the dispersion of water borne pathogens and their respective indicators. This is all the more important in developing countries where significant proportions of the population often have little or no access to clean drinking water supplies. Moreover, and notwithstanding the importance of these bacteria in terms of public health, at present little work exists on the persistence, transfer and proliferation of these pathogens and their respective indicator organisms, e.g., fecal indicator bacteria (FIB) such as Escherichia coli and fecal coliforms in humid tropical systems, such as are found in South East Asia or in the tropical regions of Africa. Both FIB and the waterborne pathogens they are supposed to indicate are particularly susceptible to shifts in water flow and quality and the predicted increases in rainfall and floods due to climate change will only exacerbate the problems of contamination. This will be furthermore compounded by the increasing urbanization and agricultural intensification that developing regions are experiencing. Therefore, recognizing and understanding the link between human activities, natural process and microbial functioning and their ultimate impacts on human health are prerequisites for reducing the risks to the exposed populations. Most of the existing work in tropical systems has been based on the application of temperate indicator organisms, models and mechanisms regardless of their applicability or appropriateness for tropical environments. Here, we present a short review on the factors that control FIB dynamics in temperate systems and discuss their applicability to tropical environments. We then highlight some of the knowledge gaps in order to stimulate future research in this field in the tropics.

Land Use and Water Quality Along a Mekong Tributary in Northern Lao P.D.R.

Improving access to clean water has the potential to make a major contribution toward poverty reduction in rural communities of Lao P.D.R. This study focuses on stream water quality along a Mekong basin tributary, the Houay Xon that flows within a mountainous, mosaic land-use catchment of northern Lao P.D.R. To compare direct water quality measurements to the perception of water quality within the riparian population, our survey included interviews of villagers. Water quality was found to vary greatly depending on the location along the stream. Overall, it reflected the balance between the stream self-cleaning potential and human pressure on the riparian zone: (i) high bacteria and suspended load levels occurred where livestock are left to free-range within the riparian zone; (ii) very low oxygen content and high bacteriological contamination prevailed downstream from villages; (iii) high concentrations of bacteria were consistently observed along urbanized banks; (iv) low oxygen content were associated with the discharge of organic-rich wastewater from a small industrial plant; (v) very high suspended load and bacteria levels occurred during flood events due to soil erosion from steep cultivated hill slopes. Besides these human induced pollutions we also noted spontaneous enrichments in metals in wetland areas fed by dysoxic groundwater. These biophysical measurements were in agreement with the opinions expressed by the majority of the interviewees who reported poor and decreasing water quality in the Houay Xon catchment. Based on our survey, we propose recommendations to improve or maintain stream water quality in the uplands of northern Lao P.D.R.

Soils and Brine Geochemistry and Mineralogy of Hyperarid Desert Playa, Ouargla Basin, Algerian Sahara

The Ouargla basin is located in hyperarid north Africa and characterized by an endoreic landscape (playa) with Solonchak (accumulated soluble salts) and Gypsisol (accumulated gypsum) soils, and subsurface groundwater. The chemical and mineralogical specificity of this hyperarid ecosystem has been compared to other areas under arid environment (Tunisian sub-Sahara and Algerian steep-lands). Chemical data on the major and minor elements, such as Sr2+, were obtained, and geochemical studies of groundwater, soil solution samples and brines have been performed. In total, 84 samples (42 groundwater, 39 soil solutions, 3 brines) were collected. X-ray diffraction and micromorphological observations (SEM with microprobe) were also performed in order to analyze soils and efflorescence. The concentrations of chemical components showed a strong variability (standard deviation of 1.15 for Cl− and 1.08 for Na+) which was reduced by an increasing ratio between concentrated and diluted samples. The Principal Component Analyses showed that an increase in the concentration of soluble ions was responsible for 56% of the total variance. When the soluble salt concentration increased, the chemical composition changed from Ca-sulfate dominance to Na-chloride dominance. Thermodynamic equilibrium data indicated that these salt solutions were controlled by the following precipitation sequence: calcite > gypsum > mirabilite > halite. This sequence was confirmed by X-ray diffraction and micromorphology. Genesis of dehydrated forms of CaSO4 · 2H2O were recognized by: (1) partial dehydration of gypsum by sun radiation on the soil surface, and (2) a decrease in water activity (<1) relating to an increase in ionic strength (>3 molc kg−1 of H2O) of concentrated solutions. The presence of mixed sulfates (glauberite, bloedite, and others) appears to be a distinctive feature of the hyperdesert soil. In this environment they are more stable than simple sulfates. A discrete mineral involving a minor element was observed, and celestite (SrSO4) was thermodynamically predicted. This mineral results from the coprecipitation of Sr2+ with gypsum. Micromorphologically, celestite crystals occur as inclusions and within the periphery of gypsum crystals. The seasonal and daily thermal amplitudes in the desert environment determine the precipitation pattern into the Na–Ca–SO4–H2O system (mirabilite-thenardite-halite paragenesis), controlling the process of dissolution/ precipitation of gypsum. The hyperarid environment has a remarkable chemical and mineralogical reactivity because of the high solubility and hydration status of minerals, and its sensitive reactivity to climate variation. Based on these results, dehydrated CaSO4 is proposed as a diagnostic pedological criterion for delineating hyperarid and arid ecosystems.

Trapping efficiencies of cultivated and natural riparian vegetation of northern Laos

In northern Laos, intensification of cultivation on sloping land leads to accelerated erosion processes. Management of riparian land may counteract the negative impacts of higher sediment delivery rates on water quality. This study assessed water and sediment concentration trapping efficiencies of riparian vegetation in northern Laos and the effect of cultivation of riparian land on water quality. Runoff flowing in and out of selected riparian sites was monitored by means of open troughs. In 2005, two native grass, two bamboo, and two banana sites were monitored. In 2006, adjacent to steep banana, bamboo, and native grass sites, three upland rice sites were established and monitored. Water trapping efficiency (WTE) and sediment concentration trapping efficiency (SCTE) were calculated on an event basis; means and 95% confidence intervals (CIs) were estimated with a bootstrapping approach. Confidence intervals were large and overlapping among sites. Seepage conditions severely limited trapping efficiency. Native grass resulted in the highest WTE (95% CI, -0.10 to 0.23), which was not significantly different from zero. Banana resulted in the highest SCTE (95% CI, 0.06-0.40). Bamboo had negative WTE and SCTE. Median outflow runoff from rice sites was nine times the inflow. Median outflow sediment concentration from rice sites was two to five times that of their adjacent sites and two to five times the inflow sediment concentration. Although low-tillage banana plantation may reduce sediment concentration of runoff, cultivation of annual crops in riparian land leads to delivery of turbid runoff into the stream, thus severely affecting stream water quality.

Study of infiltration in a Sahelian gully erosion area using time-lapse resistivity mapping

Abstract Observing that concentrated runoff destroys indurate and impermeable surface horizons to form gullies on Sahelian slopes, we investigated whether these gullies are preferential places for deep infiltration and groundwater recharge processes. The primary aim of this study is to determine if resistivity mapping is an appropriate method to use for locating recharge zones from the surface. The study area, in northern Burkina Faso, is a typical (1 ha) gully erosion area located at the outlet of an 82-ha catchment with solonetz soils and a crystalline basement. Taking advantage of a long dry season followed by a short rainy season, we made use of a time-lapse approach to carry out electrical resistivity mapping and monitor apparent resistivity variations that occurred in the soils during the rainy season, between June and September. We made nine apparent resistivity maps in the year 2000 and two in January and March 2001. To monitor expected infiltration and percolation to depths of 5 m or more, we laid out Wenner array profiles with an inter-electrode spacing of 5 m. The time-lapse mapping was also controlled with: (i) neutron probe measurements; (ii) resistivity measurements on outcrops during infiltration tests; (iii) electrical resistivity logging in auger holes. Geophysical results showed that the apparent resistivity parameter can either decrease (typical case) or increase (unexpected case) after a rain. Neutron probe measurements indicated that infiltration varies within a few decimeters even at the centre of the main gully. Using one dimensional (1D) modelling based on resistivity variations monitored during infiltration tests, we concluded that apparent resistivity variations are linked to the presence of carbonate in the soils. When soluble carbonates are present, the resistivity of the infiltrated layer varies from 220 Ω m (dry state) to less than 5 Ω m (wet state), bringing about a decrease in apparent resistivity value for the 5m spacing. In the absence of carbonate, resistivity varies from 1500 to 180 Ω m, but produces an increase of the apparent resistivity value for the same spacing. Consequently, we found time-lapse apparent resistivity mapping to be an efficient way to delineate certain soil properties. It also provided additional information about punctual observations. However, our results have led us to conclude that the 5-m inter-electrode spacing is too large to monitor this type of shallow infiltration phenomenon and that the effect of temperature on resistivity should be considered when comparing maps over the period of a few months. Furthermore, this type of survey should be controlled using electrical loggings in auger holes, or electrical soundings in order to get a better understanding of in-depth resistivity variations. Finally, this survey indicated that deep infiltration processes are not occurring below the gully situated on the slope. Further studies are required downstream to identify the location of groundwater recharge in Sahelian crystalline contexts.

Evaluation of Molecular Methods To Improve the Detection of Burkholderia pseudomallei in Soil and Water Samples from Laos

ABSTRACT Burkholderia pseudomallei is the cause of melioidosis, a severe and potentially fatal disease of humans and animals. It is endemic in northern Australia and Southeast Asia and is found in soil and surface water. The environmental distribution of B. pseudomallei worldwide and within countries where it is endemic, such as the Lao Peoples Democratic Republic (Laos), remains unclear. However, this knowledge is important to our understanding of the ecology and epidemiology of B. pseudomallei and to facilitate public health interventions. Sensitive and specific methods to detect B. pseudomallei in environmental samples are therefore needed. The aim of this study was to compare molecular and culture-based methods for the detection of B. pseudomallei in soil and surface water in order to identify the optimal approach for future environmental studies in Laos. Molecular detection by quantitative real-time PCR (qPCR) was attempted after DNA extraction directly from soil or water samples or after an overnight enrichment step. The positivity rates obtained by qPCR were compared to those obtained by different culture techniques. The rate of detection from soil samples by qPCR following culture enrichment was significantly higher (84/100) than that by individual culture methods and all culture methods combined (44/100; P < 0.001). Similarly, qPCR following enrichment was the most sensitive method for filtered river water compared with the sensitivity of the individual methods and all individual methods combined. In conclusion, molecular detection following an enrichment step has proven to be a sensitive and reliable approach for B. pseudomallei detection in Lao environmental samples and is recommended as the preferred method for future surveys.

Use of fallout radionuclides ((7)Be, (210)Pb) to estimate resuspension of Escherichia coli from streambed sediments during floods in a tropical montane catchment.

Consumption of water polluted by faecal contaminants is responsible for 2 million deaths annually, most of which occur in developing countries without adequate sanitation. In tropical aquatic systems, streambeds can be reservoirs of persistent pathogenic bacteria and high rainfall can lead to contaminated soils entering streams and to the resuspension of sediment-bound microbes in the streambed. Here, we present a novel method using fallout radionuclides (7Be and 210Pbxs) to estimate the proportions of Escherichia coli, an indicator of faecal contamination, associated with recently eroded soil particles and with the resuspension of streambed sediments. We show that using these radionuclides and hydrograph separations we are able to characterize the proportion of particles originating from highly contaminated soils and that from the resuspension of particle-attached bacteria within the streambed. We also found that although overland flow represented just over one tenth of the total flood volume, it was responsible for more than two thirds of the downstream transfer of E. coli. We propose that data obtained using this method can be used to understand the dynamics of faecal indicator bacteria (FIB) in streams thereby providing information for adapted management plans that reduce the health risks to local populations.Graphical AbstractGraphical abstract showing (1) the main water flow processes (i.e. overland flow, groundwater return flow, blue arrows) and sediment flow components (i.e. resuspension and soil erosion, black arrows) during floods in the Houay Pano catchment; (2) the general principle of the method using fallout radionuclide markers (i.e. 7Be and 210Pbxs) to estimate E. coli load from the two main sources (i.e. streambed resuspension vs soil surface washoff); and 3) the main results obtained during the 15 May 2012 storm event (i.e. relative percentage contribution of each process to the total streamflow, values in parentheses)

Effect of land use and hydrological processes on Escherichia coli concentrations in streams of tropical, humid headwater catchments

Lack of access to clean water and adequate sanitation continues to be a major brake on development. Here we present the results of a 12-month investigation into the dynamics of Escherichia coli, a commonly used indicator of faecal contamination in water supplies, in three small, rural catchments in Laos, Thailand and Vietnam. We show that land use and hydrology are major controlling factors of E. coli concentrations in streamwater and that the relative importance of these two factors varies between the dry and wet seasons. In all three catchments, the highest concentrations were observed during the wet season when storm events and overland flow were highest. However, smaller peaks of E. coli concentration were also observed during the dry season. These latter correspond to periods of intense farming activities and small, episodic rain events. Furthermore, vegetation type, through land use and soil surface crusting, combined with mammalian presence play an important role in determining E. coli loads in the streams. Finally, sampling during stormflow revealed the importance of having appropriate sampling protocols if information on maximum contamination levels is required as grab sampling at a fixed time step may miss important peaks in E. coli numbers.

Hydrological Regime and Water Shortage as Drivers of the Seasonal Incidence of Diarrheal Diseases in a Tropical Montane Environment

Background The global burden of diarrhea is a leading cause of morbidity and mortality worldwide. In montane areas of South-East Asia such as northern Laos, recent changes in land use have induced increased runoff, soil erosion and in-stream suspended sediment loads, and potential pathogen dissemination. To our knowledge, few studies have related diarrhea incidences to catchment scale hydrological factors such as river discharge, and loads of suspended sediment and of Fecal Indicator Bacteria (FIB) such as Escherichia coli, together with sociological factors such as hygiene practices. We hypothesized that climate factors combined with human behavior control diarrhea incidence, either because higher rainfall, leading to higher stream discharges, suspended sediment loads and FIB counts, are associated with higher numbers of reported diarrhea cases during the rainy season, or because water shortage leads to the use of less safe water sources during the dry season. Using E. coli as a FIB, the objectives of this study were thus (1) to characterize the epidemiological dynamics of diarrhea in Northern Laos, and (2) to identify which hydro-meteorological and sociological risk factors were associated with diarrhea epidemics. Methods Considering two unconnected river catchments of 22 and 7,448 km2, respectively, we conducted a retrospective time series analysis of meteorological variables (rainfall, air temperature), hydrological variables (discharge, suspended sediments, FIB counts, water temperature), and the number of diarrheal disease cases reported at 6 health centers located in the 5 southern districts of the Luang Prabang Province, Lao PDR. We also examined the socio-demographic factors potentially affecting vulnerability to the effect of the climate factors, such as drinking water sources, hygiene habits, and recreational water exposure. Results Using thus a mixed methods approach, we found E. coli to be present all year long (100–1,000 Most Probable Number or MPN 100 mL-1) indicating that fecal contamination is ubiquitous and constant. We found that populations switch their water supply from wells to surface water during drought periods, the latter of which appear to be at higher risk of bacterial contamination than municipal water fountains. We thus found that water shortage in the Luang Prabang area triggers diarrhea peaks during the dry and hot season and that rainfall and aquifer refill ends the epidemic during the wet season. The temporal trends of reported daily diarrhea cases were generally bimodal with hospital admissions peaking in February-March and later in May-July. Annual incidence rates were higher in more densely populated areas and mostly concerned the 0–4 age group and male patients. Conclusions We found that anthropogenic drivers, such as hygiene practices, were at least as important as environmental drivers in determining the seasonal pattern of a diarrhea epidemic. For diarrheal disease risk monitoring, discharge or groundwater level can be considered as relevant proxies. These variables should be monitored in the framework of an early warning system provided that a tradeoff is found between the size of the monitored catchment and the frequency of the measurement.

From shifting cultivation to teak plantation: effect on overland flow and sediment yield in a montane tropical catchment

Soil erosion supplies large quantities of sediments to rivers of Southeastern Asia. It reduces soil fertility of agro-ecosystems located on hillslopes, and it degrades, downstream, water resource quality and leads to the siltation of reservoirs. An increase in the surface area covered with commercial perennial monocultures such as teak plantations is currently observed at the expanse of traditional slash-and-burn cultivation systems in steep montane environments of these regions. The impacts of land-use change on the hydrological response and sediment yields have been investigated in a representative catchment of Laos monitored for 13 years. After the gradual conversion of rice-based shifting cultivation to teak plantation-based systems, overland flow contribution to stream flow increased from 16 to 31% and sediment yield raised from 98 to 609 Mg km−2. This result is explained by the higher kinetic energy of raindrops falling from the canopy, the virtual absence of understorey vegetation cover to dissipate drop energy and the formation of an impermeable surface crust accelerating the formation and concentration of overland flow. The 25-to-50% lower 137Cs activities measured in soils collected under mature teak plantations compared to soils under other land uses illustrate the severity of soil erosion processes occurring in teak plantations.