Diatom transfer-function of total phosphorus in an urban water supply reservoir in Brazil: inference of baseline conditions for tropical reservoirs

Although reservoirs are similar to natural lakes in many respects, such driving forces as water retention time and watershed features can play important roles in the limnology of manmade lakes. With the goal of investigating how these factors influence the limnology of tropical reservoirs, physical and chemical variables were measured at four sampling sites in two reservoirs in southern Brazil, from June 2002 to June 2003. Funil Reservoir is located in one of the most‐populated areas in the country, in the Paraíba do Sul river basin, which drains and drastically influences the water quality of the reservoir. In contrast, Lajes Reservoir is located in a well‐preserved area, with its water retention time varying from six to 30 times longer than for Funil Reservoir. Funil Reservoir is a turbid (median euphotic zone = 4.3 m), eutrophic reservoir (median total phosphorus (TP) = 3.1 µm), with a high phytoplankton biomass (median chlorophyll‐a concentration = 10.0 µg L−1). In contrast, Lajes Reservoir is a transparent (median euphotic zone = 9.2 m), mesotrophic water system (median TP = 1.0 µm), with a low phytoplankton biomass (median chlorophyll‐a = 1.9 µg L−1). Both reservoirs were stratified during the summer months, but isothermy was only observed in Funil Reservoir. Because of its short water retention time, Funil Reservoir is a much more dynamic system than Lajes Reservoir, with a pronounced temporal pattern related to changes in its water column and its phytoplankton biomass. Spatial heterogeneity is more evident in Lajes Reservoir, mainly as a consequence of its location in a preserved area, long water retention time and the presence of net cages for fish culture in the waterbody. The typical spatial zonation found in reservoirs, related to nutrient sedimentation and light availability, however, is more evident in Funil Reservoir than in Lajes Reservoir. Despite the similarities between these two water systems, which are in the same geographical region with similar climate, and are comparable in size, the distinct watershed features and water retention time are responsible for marked differences between these reservoirs.

The Dr. João Penido Reservoir is an 80-year-old tropical impoundment that serves as the main drinking water supply for the city of Juiz de Fora, Brazil. The reservoir is an important environment for the region, serving as a study site for the scientific community. The main objective of this research was to calculate quality indices for water and the trophic state of the reservoir. We performed physical–chemical and microbiological analyses on water samples collected at 5 points in the reservoir between July 2011 and March 2013, a period that characterizes both dry and rainy periods. The parameters analyzed were water temperature, dissolved oxygen, pH, turbidity, total solids, biochemical oxygen demand, nitrate, total phosphorus, chlorophyll a, thermotolerant coliforms and total coliforms. Indices modified for tropical waters, the Water Quality Index (WQIIgam) and the Trophic State Index (TSILamp, a variation of the Carlson Index typically used for temperate zones) were calculated for each sample. The results showed the prevalence of a “mesotrophic” state and a “medium” level of water quality in the Brazilian reservoir. It is likely that the age, forms of use, and occupation of the watershed have produced negative impacts on the aquatic ecosystem over the last 80 years. Water quality clearly demonstrates the safety and environmental health of a watershed. This study presents data on the water quality and trophic state of the Dr. João Penido Reservoir and provides guidelines for developing and implementing an integrated management plan for the preservation of its watershed.

A process-based model was used to simulate a hindcast based on the worst historical water quality condition of a tropical urban reservoir. Paranoá Lake is located in Brasília-DF, Brazil, and went through intense eutrophication in the 70s and 90s, with an important cyanobacterial bloom event in 1978. The parameters of phytoplankton were calibrated, focusing on the group of Chlorophyta (green algae) and Cyanobacteria (blue-green algae) at four depths (1, 10, 15 and 20m). The results indicated that the model was able to reproduce the Cyanobacteria biomass in comparison with the observations (RMSE=22-29.10-3 mgC L-1). On the other hand, the simulated Chlorophyta biomass showed good agreement with the observed data only in the bottom layer (RMSE=29. 10-3 mgC L-1 at 20m). In the hindcast simulation, the model was able to predict a significant increase in cyanobacterial biomass facing a water quality deterioration. In the meantime, the simulated Chlorophyta biomass decreased, which may indicate the phytoplankton group succession in response to the environmental conditions.

Low salinity waterflooding and CO2 injection are enhanced oil recovery (EOR) methods that are currently growing at a substantial rate worldwide. Linking these two EOR methods appears to be a promising approach in mature fields and for the exploration of post- and pre-salt basins in Brazil. Moreover, the latter reservoirs already have high CO2 content in the gas phase. Interfacial phenomena between fluids and rock in low salinity brine/CO2 environment still remain unclear, particularly the wettability behavior induced by the pH of the medium. In this study, coreflooding experiments, zeta potential, contact angle, interfacial tension (IFT), and pH measurements at ambient and reservoir conditions were performed to investigate the influence of the rock composition and brine/CO2 mixtures at different pH values for low salinity water-CO2 EOR (LSW-CO2 EOR) applications in Brazilian reservoirs. Brazilian light crude oil, pure CO2, and different brine solutions were used to represent the fluids in actual oil reservoirs. The experiments were carried out on Botucatu sandstone samples, with mineralogy determined by energy dispersive X-ray analysis. Coreflooding experiments were conducted by injection of 10 pore volumes of high salinity water followed by low salinity water. Contact angles, IFT and pH measurements at atmospheric and elevated pressures were performed in a high-pressure view cell (Pmax = 10,000 psi, Tmax = 180 °C) by different methods. The contact angle results were compared to those of earlier publications for other rock types. Increased oil recovery was observed in the coreflooding experiments during LSW injection. In addition, the effluent pH during LSW injection increased 0.7-4.3 points more than initial pH in high salinity water injection. Zeta potential measurements confirmed expansion in the water film on Botucatu sandstone surface at low salt concentrations. These observations indicate that during LSW injection solely, an increase in pH would increase water wettability of Botucatu sandstone, as all edges and faces of its surface become negatively charged and may repel polar compounds in crude oil. On the other hand, contact angle experiments reveal that water wettability is further enhanced in LSW when CO2 is dissolved in the water, and the system changes to acidic conditions. It seems that a change in the medium pH enhances interactions with water molecules for which the respective interfacial energy decreases, and hence the contact angle as well. Therefore, low salinity brine/CO2 mixtures may synergistically lead to increased oil recovery by decreasing the contact angle. This study advances the understanding of interfacial properties and wettability behavior in low salinity brine/CO2 environment, facilitating the design of LSW-CO2 EOR applications in Brazilian fields. Moreover, the study provides useful information for oil companies that have acquired mature wells and exploration blocks in Brazil, supporting them in operational and economic decisions.

Role of the sediments of two tropical dam reservoirs in the flux of metallic elements to the water column

Where are the South American freshwater turtle blood flukes (Trematoda: Spirorchiidae)? The first morphological and molecular analysis of spirorchiid cercariae from freshwater snails in Brazil

The income of small-scale fisheries of two urban reservoirs in Brazil: Represa Billings (127 km2) located in the metropolitan region of São Paulo, the capital city of the state of São Paulo and Lago Paranoá (38 km2) located in Brasília (DF), the capital city of Brazil were calculated and compared. Both fisheries are mainly based on the alien Nile tilapia Oreochromis niloticus (more than 80% of the total catch). Although these reservoirs are nearly 900 km apart, their native fish fauna belong to the Upper Paraná Province. The Represa Billings fishers have, on average, a daily profit of 15.8 R dollars (8.4 U dollars mostly employing gill nets) and Lago Paranoá fishers 46.6 R dollars (24.9 U dollars, mostly employing cast nets), which is a statistically significant difference (P < 0.001). The profit of the fisheries is explained by the factor "reservoir" and covariate "days of fishing". Due to the increasing violence in the metropolitan region of São Paulo, the Represa Billings fishery is vanishing.

Characterization of tilapia farming in net cages at a tropical reservoir in Brazil

While the presence of microcystin-LR (MC-LR) in raw water from eutrophic reservoirs poses human health concerns, the risks associated with the ingestion of MC-LR in drinking water are not fully elucidated. We used a time series of MC-LR in raw water from tropical urban reservoirs in Brazil to estimate the hazard quotients (HQs) for non-carcinogenic health effects and the potential ingestion of MC-LR through drinking water. We considered scenarios of MC-LR removal in the drinking water treatment plants (DWTPs) of two supply systems (Cascata and Guarapiranga). The former uses coagulation/flocculation/sedimentation/filtration/disinfection, while the latter has an additional step of membrane ultrafiltration, with contrasting expected MC-LR removal efficiencies. We considered reference values for infants (0.30μg L-1), children/adults (1.60μg L-1), or the population in general (1.0μg L-1). For most scenarios for Cascata, the 95% upper confidence level of the HQ indicated high risks of exposure for the population (HQ > 1), particularly for infants (HQ = 30.910). The water treatment in Cascata was associated to the potential exposure to MC-LR due to its limited removal capacity, with up to 263days/year with MC-LR above threshold values. The Guarapiranga system had the lowest MC-LR in the raw water as well as higher expected removal efficiencies in the DWTP, resulting in negligible risks. We reinforce the importance of integrating raw water quality characteristics and treatment technologies to reduce the risks of exposure to MC-LR, especially for vulnerable population groups. Our results can serve as a starting point for risk management strategies to minimize cases of MC-LR intoxication in Brazil and other developing countries.

The study goal was to determine spatiotemporal variations in chlorophyll-a (Chl-a) concentration using models that combine hydroclimatic and nutrient variables in 150 tropical reservoirs in Brazil. The investigation of seasonal variability indicated that Chl-a varied in response to changes in total nitrogen (TN), total phosphorus (TP), volume (V), and daily precipitation (P). Therefore, an empirical model for Chl-a prediction based on the product of TN, TP, and normalized functions of V and P was proposed, but their individual exponents as well as a general multiplicative factor were adjusted by linear regression for each reservoir. The fitted relationships were capable of representing algal temporal dynamics and blooms, with an average coefficient of determination of R2 = 0.70. The results revealed that nutrients yielded better predictability of Chl-a than hydroclimatic variables. Chl-a blooms presented seasonal and interannual variability, being more frequent in periods of high precipitation and low volume. The equations demonstrate different Chl-a responses to the parameters. In general, Chl-a was positively related to TN and/or TP. However, in some cases (22%), high nutrient concentrations reduced Chl-a, which was attributed to limited phytoplankton growth driven by light deficiency due to increased turbidity. In 49% of the models, precipitation intensified Chl-a levels, which was related to increases in the nutrient concentration from external sources in rural watersheds. Contrastingly, 51% of the reservoirs faced a decrease in Chl-a with precipitation, which can be explained by the opposite effect of dilution of nutrient concentration at the reservoir inlet in urban watersheds. In terms of volume, in 67% of the reservoirs, water level reduction promoted an increase in Chl-a as a response to higher nutrient concentration. In the other cases, Chl-a decreased with lower water levels due to wind-induced destratification of the water column, which potentially decreased the internal nutrient release from bottom sediment. Finally, applying the model to the two largest studied reservoirs showed greater sensitivity of Chl-a to changes in water use classes regarding variations in TN, followed by TP, V, and P.

The Carlson’s (1977) Trophic State Index (TSI) is a widely employed tool to estimate the degree of eutrophication in a reservoir. In Brazil, the need of a classification system that would take into account regional characteristics employed adjusted indexes generated by data from reservoirs in the southeastern region of the country. Current research compares responses for Carlson’s TSI (1977) and its derivations for Brazilian reservoirs from data collected in the Pereira de Miranda Reservoir (State of Ceara, Brazil), and analyzes the influence of local conditions on results and their applicability to reservoirs in the semiarid region. TSIs were calculated by data on total phosphorus, chlorophyll a and water transparency. The reservoir was estimated as mesotrophic based on the chlorophyll a variable, and between eutrophic and hyper-eutrophic when based on total phosphorus data and water transparency. Results showed the need to consider intrinsic factors in the discussion on the applicability of TSIs to reservoirs in the semiarid region since the peculiar hydro-climatic conditions and morphometric characteristics make them even more vulnerable to disturbance agents, such as winds which have a significant influence on processes that determine the trophic state.

Guarapiranga Reservoir is the second most important public water supply in Sao Paulo, Brazil and has been eutrophic for several decades. We inferred the major ecological shifts for the period 1919–2010 related to multiple stressors (forest flooding, hydrological change, use of algicide and eutrophication), using geochemistry (TOC, TN, TP, C/N, δ15N, δ13C) and diatom assemblages in a short (75-cm) sediment core. Thirty-two diatom species were abundant in the core and stratigraphically constrained incremental sum of squares analysis enabled identification of three diatom zones and four subzones, i.e. depths at which marked changes in species composition occurred. Early diatom assemblages were dominated by benthic, oligotrophic taxa, mainly Eunotia, influenced by flooded vegetation after dam construction. A shift to dominance by a planktonic species (Eunotia tukanorum) occurred ca. 1932, during the period of initial physical disturbance and early use of the water body as a public water supply. Diatoms and geochemical variables show that the reservoir was oligotrophic from ~1919 to 1947. Eutrophication began ~1975 and by the early 1980s the reservoir had become eutrophic, in response to an explosive increase in human population in the watershed. Severe cultural eutrophication has persisted since ~1990. Higher concentrations of copper in the sediments, beginning in 1991, reflect the increased use of copper sulfate to control cyanobacteria blooms and provide a chronological marker. Higher δ15N values in recent sediments indicate greater sewage inputs and low C/N values reflect the predominant contribution of algae to sediment organic matter. Eutrophic taxa Cyclotella meneghiniana and Nitzschia sp. dominate recent diatom assemblages, along with Aulacoseira granulata, a species that is tolerant of copper sulfate. Diatom assemblages reflect multiple stressors, however, geochemical information provides a better understanding of the early phase of the reservoir. Paleolimnologically documented trophic state changes in this important drinking water supply are largely attributable to increased urbanization of the drainage basin and inputs of sewage. Management efforts should focus on mitigating this nutrient source.

It is reported for the first time in Brazil and South America the presence of cylindrospermopsin (CYN) in water supply reservoirs. CYN is a powerful hepatotoxic alkaloid implicated in outbreaks of human sicknesses. We detected CYN in different sources of water in Northeastern Brazil using molecular and immunological techniques. The highest concentrations of toxin occurred in the Jucazinho reservoir with the phytoplankton containing the potentially CYN-producing C. raciborskii and Sphaerospermopsis aphanizomenoides (previously known as Aphanizomenon aphanizomenoides). The polyketide synthase (PKS) and peptide synthetase (PS), which are directly related to the ability to produce CYN, were found in all the analyzed samples. The result of the present study emphasizes the need to improve monitoring of CYN in water bodies used for drinking and recreation, in order to avoid exposure of human populations to this toxin.

SummaryLow-salinity waterflooding and carbon dioxide (CO2) injection are enhanced oil recovery (EOR) methods that are currently increasing in use worldwide. Linking these two EOR methods is a promising approach in the exploration of mature fields and for post- and presalt basins in Brazil. Moreover, the latter reservoirs already exhibit a high CO2 content by nature. Interfacial phenomena between fluids and rock in a low-salinity water-CO2 (LSW-CO2) environment remain unclear, particularly the wettability behavior that is related to the pH of the medium, among others. This study investigates the influence of rock composition and pH of the brine on reservoir wettability through coreflooding and zeta potential experiments in LSW and determination of contact angles and interfacial tension (IFT) in the crude oil-LSW-CO2 system at reservoir conditions. Brazilian light crude oil, pure CO2, and brine solutions of different concentrations and compositions were used to represent the fluids in actual oil reservoirs. The experiments were carried out on Botucatu sandstone, Indiana limestone, and calcite crystal samples, with mineralogy determined by energy dispersive X-ray (EDX) analysis. Coreflooding experiments were conducted by the injection of 10 pore volumes (PVs) of fourfold diluted synthetic reservoir brine (SRB), followed by 10 PVs of 40-fold diluted SRB to evaluate the low-salinity effects. Interfacial properties, such as contact angle and IFT, as well as density and pH, were determined at elevated pressures to evaluate the synergistic effects between CO2 and salt content. In addition, geochemical modeling using PH REdox EQuilibrium (in C language) (PHREEQC) was performed to predict the in-situ pH and match with the experimental data. An increase in oil recovery and pH of the effluent was observed in the coreflooding experiments during diluted SRB injection. The ionic concentrations of the effluent samples also indicated illite dissolution. Furthermore, zeta potential measurements confirmed the expansion of the water film and shift from positive to negative surface charge of Botucatu sandstone for salt concentrations less than 80,000 mg/L at pH &amp;gt; 7, whereas in Indiana limestone, negative surface charge was only observed in deionized water at pH &amp;gt; 9. These observations indicate that during LSW injection alone, an increase in pH will favor a thicker water layer on the Botucatu sandstone surface that in turn increases water wettability and results in increased oil recovery. Conversely, the presence of CO2 in LSW causes a decrease in the pH of the medium, which is related to further enhancing water wettability when linking pH with contact angle measurements. It seems that a change in the pH of the brine induced by CO2 solubility in LSW enhanced interactions between the rock surface and water molecules. The respective interfacial energy then decreased, resulting in a decreasing water contact angle. It was also noticed that seawater-CO2 systems caused salt precipitation and mineralogical changes in carbonate and sandstone rock induced by calcite and kaolinite dissolution, respectively. This study contributes substantially to the understanding of interfacial properties and wettability behavior in LSW-CO2 systems, facilitating the design of LSW-CO2 EOR applications in Brazilian fields or even CO2 storage. Moreover, the study provides useful data for oil companies that have acquired mature wells and exploration blocks in Brazil, supporting them in operational and investment decisions.

Calcium carbonate scale formation is a significant challenge in oil production systems, particularly in intelligent completion valves, where complex flow and thermodynamic conditions can favor crystallization. Understanding and mitigating scale deposition in these components is crucial to ensuring long-term system integrity and efficiency. This study aims to map the risk of calcium carbonate scaling in intelligent completion valves by performing a sensitivity analysis of operational conditions, using a comprehensive multiphysics modeling framework. A numerical model integrating flow dynamics, thermodynamic equilibrium, and crystallization kinetics was developed. Flow characteristics within the valve geometry were evaluated using a dimensionless formulation to determine pressure, velocity, and volumetric flow distribution. A population balance model was employed to predict the mass of calcium carbonate deposition based on the fluid composition and operational conditions. Simulations considered a range of volumetric flow rates (2500 to 7500 barrels per day), temperatures (60 to 85 °C), and pressures (400 to 580 bar), using representative oil and water compositions from Brazilian reservoirs. The results revealed that higher flow rates, elevated temperatures, and lower pressures significantly increase the risk of carbonate scaling calcium. These conditions enhance supersaturation, thereby promoting nucleation and crystal growth. Deposition was predominantly observed in regions with high velocity and surface area, such as constrictions and trim locations within the valve. This study introduced a novel numerical approach that can guide the optimization and future control of operational parameters to mitigate scale formation in intelligent completion valves. A risk map was developed to evaluate scaling potential under various reservoir conditions, supporting more informed decision-making in flow control design and operation.