High Recharge Rates Research Articles

The impact of managed aquifer recharge on the fate and transport of pesticides in agricultural soils

Groundwater aquifers worldwide experience unsustainable depletion, compounded by population growth, economic development, and climate forcing. Managed aquifer recharge provides one tool to alleviate flood risk and replenish groundwater. However, concerns grow that intentional flooding of farmland for groundwater recharge, a practice known as Ag-MAR, may increase the leaching of pesticides and other chemicals into groundwater. This study employs a physically based unsaturated flow model to determine the fate and transport of residues of four pesticide in three vadose zone profiles characterized by differing fractions of sand (41 %, 61 %, and 84 %) in California's Central Valley. Here, we show that the complex heterogeneity of alternating coarse and fine-grain hydrogeologic units controls the transit times of pesticides and their adsorption and degradation rates. Unsaturated zones that contain a higher fraction of sand are more prone to support preferential flow, higher recharge rates (+8 %), and faster (42 %) water flow and pesticide transport, more flooding-induced pesticide leaching (about 22 %), as well as more salt leaching correlating with increased risks of groundwater contamination. Interestingly, considering preferential flow predicted higher degradation and retention rates despite shorter travel times, attributed to the trapping of pesticides in immobile zones where they degrade more effectively. The findings underscore the importance of considering soil texture and structure in Ag-MAR practices to minimize environmental risks while enhancing groundwater recharge. The study also highlights that selecting less mobile pesticides can reduce leaching risks in sandy areas.

Recharge Assessment in Greek Karst Systems: Methodological Considerations and Implications

The recharge of karst aquifers is significant for the effective management of groundwater resources, and its estimation should be tailored to accommodate their specific hydrogeological characteristics. This study provides a two-step methodological approach for the determination of karst aquifer recharge. Initially, mean annual recharge rates were estimated in the karst system of Ziria (Southern Greece) utilizing the APLIS and modified APLIS methods in order to decipher which was the most suitable version for recharge assessments. The results indicated similar mean recharge rate values at 42.7% and 41.4%, respectively, but significant differences in the spatial distribution. The modified methodology emerged as a more accurate and realistic approach, mainly due to the incorporation of permeability assessments. The final phase of the methodological approach involved the application of modified APLIS in two additional karst hydrosystems, Planitero and Xiromero, while a quantitative cross-comparison of the recharge rates was obtained for a deeper understanding of the factors controlling the groundwater recharge process. In Ziria, recharge rates exhibit a relatively uniform distribution throughout the area, with a median value of 46.7%. Conversely, in Planitero, High recharge rates (60–80%) occupy 56.8% of the surface, while in Xiromero, Moderate recharge rates (40–60%) dominate, representing 53.4% of the land coverage. These variations underscore the spatial heterogeneity of recharge within the karst systems, highlighting the importance of considering local geological and hydrological conditions in its assessments. The methodological approach of this study is flexible and can be adapted to different karst sites for the determination of recharge regimes, contributing to the alleviation of the groundwater depletion issue.

Estimating vadose zone water fluxes from soil water monitoring data: a comprehensive field study in Austria

Abstract. Groundwater recharge is a key component of the hydrological cycle, yet its direct measurement is complex and often difficult to achieve. An alternative is its inverse estimation through a combination of numerical models and transient observations from distributed soil water monitoring stations. However, an often neglected aspect of this approach is the effect of model predictive uncertainty on simulated water fluxes. In this study, we made use of long-term soil water content measurements at 14 locations from the Austrian soil water monitoring program to quantify and compare local potential groundwater recharge rates and their temporal variability. Observations were coupled with a Bayesian probabilistic framework to calibrate the HYDRUS-1D model and assess the effect of model predictive uncertainty on long-term simulated recharge fluxes. Estimated annual potential recharge rates ranged from 44 to 1319 mm a−1 with a relative uncertainty (95 % interquantile range/median) in the estimation of between 1 % and 39 %. Recharge rates decreased longitudinally, with high rates and lower seasonality at western sites and low rates with high seasonality and extended periods without recharge at the southeastern and eastern Austrian sites. Higher recharge rates and lower actual evapotranspiration were related to sandy soils; however, climatic factors had a stronger influence on estimated potential groundwater recharge than soil properties, underscoring the vulnerability of groundwater recharge to the effects of climate change.

Supercapacitor Properties of rGO-TiO2 Nanocomposite in Two-component Acidic Electrolyte

The electrochemical properties of the highly porous reduced graphene oxide/titanium dioxide (rGO/TiO2) nanocomposite were studied to estimate the possibility of using it as a supercapacitor electrode. Granular aerogel rGO/TiO2 was used as an initial material for the first time of manufacturing the electrode. For the aerogel synthesis, industrial TiO2 Hombikat UV100 with a high specific surface area and anatase structure was used, and the aerogel was carried out with hydrazine vapor. Porous structure and hydrophilic-hydrophobic properties of the nanocomposite were studied with a method of standard contact porosimetry. This is important for a supercapacitor containing an aqueous electrolyte. It was found that the hydrophilic specific surface area of the nanocomposite was approximately half of the total surface area. As a result of electrochemical hydrogenation in the region of zero potential according to the scale of a standard hydrogen electrode, a reversible Faraday reaction with high recharge rate (exchange currents) was observed. The characteristic charging time of the indicated Faraday reaction does not exceed several tens of seconds, which makes it possible to consider the use of this pseudocapacitance in the systems of fast energy storage such as hybrid supercapacitors. Sufficiently high limiting pseudo-capacitance (about 1200 C/g TiO2) of the reaction was obtained.

Hypogene speleogenesis related to porphyry magmatic intrusions and its influence on subsequent karst evolution in the Peruvian high Andes

Carbonate rocks of Cretaceous and Jurassic age are widespread in the Peruvian Andes where they host numerous magmatic intrusions and related ore deposits. Most of Peru's metal ore deposits are located at high elevations within a narrow, tectonized carbonate-rock belt extending over 2000 km in length. Many of the limestone formations are karstified and characterized by high recharge and percolation rates, well-developed subsurface drainage and complex flow patterns. Knowledge of karst in the Andes is generally limited, although karst is recognized to cause extreme hydrogeologic complexity and great challenges in development of mining facilities located in carbonate settings. Aspects of the origin of void/conduit systems (i.e. speleogenesis) and karst evolution are rarely addressed in environmental impact and risk assessment studies.This study in the Antamina area in the east side of the Cordillera Occidental, home to the world largest known skarn copper‑zinc deposit, demonstrates that the current karst system has formed as the result of polygenetic and multi-phase development. Structure-controlled hypogene cave systems are well-preserved and widespread in the area and present a foremost example of endogenous hypogene speleogenesis related to magmatic intrusions. Hypogene karst structures have strongly influenced subsequent epigene karstification, which commenced at the Pliocene-Pleistocene boundary. Changes in the base-level position due to the landscape evolution and Pleistocene glaciations were major additional factors that influenced epigene karst development. The findings of this study are potentially relevant to assessing karst origin and hydrogeology in many carbonate-hosted ore deposits in the Peruvian Andes and other regions. This study shows that the speleogenetic and evolutionary approach is indispensable in the development of sound conceptual models of groundwater flow in karst terrains and the assessment of karst-related hazards and risks.

The Cauê Aquifer on the eastern limb of the Moeda Syncline: characterization, impacts, and flow in the western Iron Quadrangle (Quadrilátero Ferrífero), MG, Brazil

The Cauê Aquifer is an important water supplier to the third largest urban agglomeration in Brazil. The Metropolitan Region of Belo Horizonte is in the Iron Quadrangle, site of intense iron ore mining, requiring large volumes of water and mine dewatering. In this context, we evaluate the regional impact on groundwater level between 2000 and 2019 based on a conceptual hydrogeological flow model on the eastern limb of the Moeda Syncline. This study includes the definition of boundary conditions and aquifer units, the calculation of hydrodynamic parameters, water balance, renewable reserve, and flow analysis. The Cauê, Gandarela, and Moeda aquifers are considered aquifer units, among which the Cauê Aquifer showed the highest hydraulic conductivity (9.45×10-7 m/s on average) as well as the highest recharge rate, with 38.67% rainfall. The average volume pumped was estimated at 41 million m³/year, equivalent to about 150% of the renewable reserve of the Cauê Aquifer, calculated at 27 million m³/year. Underground flow in the area tends southward, and the Cauê Aquifer acts as a water divide, which is less evident in the center of the area, where the greatest impact on groundwater level lowering occurs.

Biological and Physical Clogging in Infiltration Wells: Effects of Well Diameter and Gravel Pack.

Gravity-driven infiltration into the shallow subsurface via small-diameter wells (SDWs), i.e., wells with an inner diameter smaller than 7.5cm (3inches) and no gravel pack) has proven to be a cost-efficient and flexible tool for managed aquifer recharge (MAR), as it provides relatively high recharge rates with minimal construction effort. SDWs have a significantly smaller open filter area than larger diameter wells with gravel pack, making the infiltration of low-quality waters through these wells more at risk clogging. To investigate their susceptibility for biological and physical clogging, 24 physical models with different well setups were evaluated by infiltrating either nutrient-poor but turbid water or nutrient-rich but clear water. The experiments showed that smaller diameters and the lack of a gravel pack increase the well's susceptibility to both kinds of clogging. However, this effect was observed to be much more pronounced for physical than for biological clogging. Our conclusion is that SDWs show severe disadvantages with respect to the infiltration of highly turbid waters in comparison to large diameter wells with a gravel pack. Nevertheless, this disadvantage is much less severe when it comes to the infiltration of clear but nutrient-rich waters (e.g., treated wastewater). Depending on the economic and geological circumstances of a MAR-project, this disadvantage could be outweighed by the significantly lower construction costs of SDWs.

Stormwater harvesting in ephemeral streams: how to bypass clogging and unsaturated layers

To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.

Groundwater vulnerability mapping using the Susceptibility Index (SI) method: Case study of Takelsa aquifer, Northeastern Tunisia

The Takelsa phreatic aquifer currently shows stress signs and the contamination risk of groundwater by different chemical elements and pesticides is a likely concern. Here we assessed the aquifer vulnerability using the Susceptibility Index (SI) method, considered as a specific vulnerability method that takes into consideration the impact of land uses chemical fertilizers, anthropogenic activities and natural factors as a source of contaminants in groundwater. WetSpass and MODFLOW models were applied to quantify groundwater recharge and water table depth, the two of the most important parameters in groundwater vulnerability mapping.The obtained recharge map for 2015 showed values ranging between 0 and 342 mm/year with an average value of 111 mm/yr, corresponding to about 16% of the annual average rainfall. Regarding the groundwater flow model and for a steady and transient state, a reasonable correlation between the observed and simulated values was obtained confirming that results can be used to establish the vulnerability map.The Susceptibility Index (SI) results showed that the groundwater vulnerability of the studied aquifer ranges from low to very high. Most of the area (90%) has moderate to very high pollution susceptibility mainly due to low slope, agricultural land, high recharge rate, sandy soil dominance and low water table depth. The validation of the obtained map was undertaken through a comparison of areas of high salinity hazard with their relative vulnerability index. For most wells, the different salinity hazard parameters matched with areas of moderate to high vulnerability.The integration between the SI method and WetSpass and MODFLOW models for the study area is a valuable and applicable approach for the assessment of the groundwater vulnerability.

Peak grain forecasts for the US High Plains amid withering waters

Irrigated agriculture contributes 40% of total global food production. In the US High Plains, which produces more than 50 million tons per year of grain, as much as 90% of irrigation originates from groundwater resources, including the Ogallala aquifer. In parts of the High Plains, groundwater resources are being depleted so rapidly that they are considered nonrenewable, compromising food security. When groundwater becomes scarce, groundwater withdrawals peak, causing a subsequent peak in crop production. Previous descriptions of finite natural resource depletion have utilized the Hubbert curve. By coupling the dynamics of groundwater pumping, recharge, and crop production, Hubbert-like curves emerge, responding to the linked variations in groundwater pumping and grain production. On a state level, this approach predicted when groundwater withdrawal and grain production peaked and the lag between them. The lags increased with the adoption of efficient irrigation practices and higher recharge rates. Results indicate that, in Texas, withdrawals peaked in 1966, followed by a peak in grain production 9 y later. After better irrigation technologies were adopted, the lag increased to 15 y from 1997 to 2012. In Kansas, where these technologies were employed concurrently with the rise of irrigated grain production, this lag was predicted to be 24 y starting in 1994. In Nebraska, grain production is projected to continue rising through 2050 because of high recharge rates. While Texas and Nebraska had equal irrigated output in 1975, by 2050, it is projected that Nebraska will have almost 10 times the groundwater-based production of Texas.

An approach to map karst groundwater potentiality in an urban area, Sete Lagoas, Brazil

ABSTRACT Due to complex hydrogeological contexts, mapping groundwater potential zones in karst regions is a difficult task. This study was conducted on a karst terrain in Sete Lagoas, Brazil, where rapid urban growth without adequate water resources management resulted in the drilling of unproductive wells and possible aquifer over-exploitation due to the concentration of wells. The objective was to develop a method for mapping groundwater potentiality for karst aquifers, serving as water resources management tool, based on 3D geological modelling, field work and pumping test data, remote sensing, geophysics analysis and production of thematic maps, resulting in the hydrogeological conceptual model. The groundwater potentiality map showed that most potential zones are on the central area, limited by grabens, where limestones are in contact with unconsolidated sediments, on areas with higher density of morphostructural and geophysical lineaments, mainly in east–west and north–east directions, near karst surface features, on thicker karstified zones, with higher recharge rates.

Regional Groundwater Modeling of the Guarani Aquifer System

The Guarani Aquifer System (GAS) is a strategic transboundary aquifer system shared by Brazil, Argentina, Paraguay and Uruguay. This article presents a groundwater flow model to assess the GAS system in terms of regional flow patterns, water balance and overall recharge. Despite the continental dimension of GAS, groundwater recharge is restricted to narrow outcrop zones. An important part is discharged into local watersheds, whereas a minor amount reaches the confined part. A three-dimensional finite element groundwater-flow model of the entire GAS system was constructed to obtain a better understanding of the prevailing flow dynamics and more reliable estimates of groundwater recharge. Our results show that recharge rates effectively contributing to the regional GAS water balance are only approximately 0.6 km3/year (about 4.9 mm/year). These rates are much smaller than previous estimates, including of deep recharge approximations commonly used for water resources management. Higher recharge rates were also not compatible with known 81Kr groundwater age estimates, as well as with calculated residence times using a particle tracking algorithm.

Numerical investigation of streamtube approach to model flow in heterogeneous unsaturated sandy soils

The streamtube approximation has been commonly used for upscaling of flow and transport in spatially heterogeneous unsaturated soils. This study investigates the prediction ability of streamtube approach to model flow in heterogeneous unsaturated sandy soils under a rainfall event at a plot scale. Towards this aim, a numerical model for solving 3-D Richards’s equation was developed and verified for 1-D and 3-D cases. Turning band method was used to generate the random field of soil hydraulic parameters. The 3-D soil field heterogeneity was represented by considering variation in both van Genuchten parameter (α) and Ks. For streamtube models, two cases of heterogeneity were investigated: i) variability in Ks only, and ii) variability in both Ks and α. Both the parameters were assumed to follow a log-normal distribution. For different boundary conditions at the soil surface and varying soil heterogeneity, the average Darcy's flux and mean field-scale soil moisture obtained using streamtube approach were compared with 3-D numerical solutions. The results identify that for high recharge rate at the soil surface, streamtube model is a good approximation of the 3-D heterogeneous soil field, and the average difference in mean soil moisture obtained from 3-D model and streamtube based 1-D model was below 10% when variability in both Ks and α was considered for all timesteps. The inclusion of variability in α leads to improvement in prediction capability of streamtube model. Further, regression equations were developed that can be utilized to predict the mean field soil moisture for heterogeneous 3-D sandy soil fields given the mean field soil moisture obtained from streamtube approach for both the cases. The regressions equations follow exponential model for case-1 and linear model for case-2 with R2 values 0.80 and 0.99, respectively. The developed equations can be reliably used to predict mean field soil moisture for heterogeneous sandy soil fields.

Estimation of groundwater recharge in savannah aquifers along a precipitation gradient using chloride mass balance method and environmental isotopes, Namibia

The quantification of groundwater resources is essential especially in water scarce countries like Namibia. The chloride mass balance (CMB) method and isotopic composition were used in determining groundwater recharge along a precipitation gradient at three sites, namely: Tsumeb (600 mm/a precipitation); Waterberg (450 mm/a precipitation) and Kuzikus/Ebenhaezer (240 mm/a precipitation). Groundwater and rainwater were collected from year 2016–2017. Rainwater was collected monthly while groundwater was collected before, during and after rainy seasons. Rainwater isotopic values for δ18O and δ2H range from −10.70 to 6.10‰ and from −72.7 to 42.1‰ respectively. Groundwater isotopic values for δ18O range from −9.84 to −5.35‰ for Tsumeb; from −10.85 to −8.60‰ for Waterberg and from −8.24 to −1.56‰ for Kuzikus/Ebenhaezer, while that for δ2H range from −65.6 to −46.7‰ for Tsumeb; −69.4 to −61.2‰ for Waterberg and −54.2 to −22.7‰ for Kuzikus/Ebenhaezer. Rainwater scatters along the GMWL. Rainwater collected in January, February and March are more depleted in heavy isotopes than those in November, December, April and May. Waterberg groundwater plots on the GMWL which indicates absence of evaporation. Tsumeb groundwater plots on/close to the GMWL with an exception of groundwater from the karst Lake Otjikoto which is showing evaporation. Groundwater from Kuzikus/Ebenhaezer shows an evaporation effect, probably evaporation occurs during infiltration since it is observed in all sampling seasons. All groundwater from three sites plot in the same area with rainwater depleted in stable isotopic values, which could indicates that recharge only take place during January, February and March. CMB method revealed that Waterberg has the highest recharge rate ranging between 39.1 mm/a and 51.1 mm/a (8.7% – 11.4% of annual precipitation), Tsumeb with rates ranging from 21.1 mm/a to 48.5 mm/a (3.5% – 8.1% of annual precipitation), and lastly Kuzikus/Ebenhaezer from 3.2 mm/a to 17.5 mm/a (1.4% – 7.3% of annual precipitation). High recharge rates in Waterberg could be related to fast infiltration and absence of evaporation as indicated by the isotopic ratios. Differences in recharge rates cannot only be attributed to the precipitation gradient but also to the evaporation rates and the presence of preferential flow paths. Recharge rates estimated for these three sites can be used in managing the savannah aquifers especially at Kuzikus/Ebenhaezer where evaporation effect is observed that one can consider rain harvesting.

Understanding groundwater salinization mechanisms to secure freshwater resources in the water-scarce city of Maputo, Mozambique

In this study hydrochemical, isotopic and multivariate statistical tools are combined with a recharge analysis and existing geophysical data to improve understanding of major factors controlling freshwater occurrence and the origins of high salinities in the multi-layered coastal aquifer system of the Great Maputo area in Mozambique. Access to freshwater in this semi-arid area is limited by an inefficient public supply network, scarce surface waters, long droughts and an increasing population growth. Groundwater has a large potential to enhance water security, but its exploitation is threatened by both coastal and inland salinization mechanisms that are poorly understood. A GIS approach is utilized to classify potential recharge zones based on hydrogeological properties and land use/cover, whereas potential recharge rates are estimated through a root zone water balance method. In combination with water stable isotope data results reveal that extreme rainfall events provide the most relevant contributions to recharge, and interception and evaporation play an important role in the low recharge areas. Hierarchical clustering of hydrochemical and isotopic data allows the classification of six water groups, varying from fresh to brackish/salt waters. Corresponding scatter plots and PHREEQC modelling show evaporation and mixing with seawater (up to 5%) as major processes affecting salinity in the area. The co-occurrence of high alkalinity and Cl concentrations, in combination with piezometric and geo-electrical data, suggests that: 1) inland brackish/salt groundwater is caused by mixing with seawater trapped within clay layers; and 2) brackish/salt surface waters result from seepage of brackish groundwater into rivers and wetlands, followed by evaporation, hence increasing salinity and δ18O values. Mixing with small fractions of trapped seawater as main salinity source, rather than halite dissolution, is further corroborated by Br/Cl ratios of brackish/salt water samples near the ocean ratio. Cation exchange upon salinization is mainly observed in the semi-confined aquifer, while freshening takes place in the phreatic aquifer, particularly in areas presenting high recharge rates.

Understanding recharge of soil water in a sand dune at the Nuoertu of Badain Jaran Desert using isotopes of H and O

A typical megadune in the hinterland of Badain Jaran Desert has been monitored from 2013 to 2014. Six soil profiles have been analyzed for the recharge source and movement mechanism combined with Cl− concentration and stable isotopes. The analysis results show that local precipitation is the main recharge source of the soil water while shallow groundwater only influences the soil water which interfaces groundwater table. The recharge rates are calculated by chloride mass balance method for one hydrological year. The high recharge rates also reflect the significance of local precipitation for recharging soil water at the Nuoertu area in this desert. The variation characteristics of oxygen and hydrogen isotopes of soil profiles suggest that the piston-type flow should be the primary mode in the Nuoertu area. This approach is applicable to analyzing soil water characteristics of unsaturated zone in similar arid desert areas around the world.

Prediction of Groundwater Quality Trends Resulting from Anthropogenic Changes in Southeast Florida.

The effects of surface water flow system changes caused by constructing water-conservation areas and canals in southeast Florida on groundwater quality under the Atlantic Coastal Ridge was investigated with numerical modeling. Water quality data were used to delineate a zone of groundwater with low total dissolved solids (TDS) within the Biscayne aquifer under the ridge. The delineated zone has the following characteristics. Its location generally coincides with an area where the Biscayne aquifer has high transmissivities, corresponds to a high recharge area of the ridge, and underlies a part of the groundwater mound formed under the ridge prior to completion of the canals. This low TDS groundwater appears to be the result of pre-development conditions rather than seepage from the canals constructed after the 1950s. Numerical simulation results indicate that the time for low TDS groundwater under the ridge to reach equilibrium with high TDS surface water in the water-conservation areas and Everglades National Park are approximately 70 and 60 years, respectively. The high TDS groundwater would be restricted to the water-conservation areas and the park due to its slow eastward movement caused by small hydraulic gradients in Rocky Glades and its mixing with the low TDS groundwater under the high-recharge area of the ridge. The flow or physical boundary conditions such as high recharge rates or low hydraulic conductivity layers may affect how the spatial distribution of groundwater quality in an aquifer will change when a groundwater flow system reaches equilibrium with an associated surface water flow system.

GIS-based FRASTIC model for pollution vulnerability assessment of fractured-rock aquifer systems

All groundwater is vulnerable to pollution, but the level of susceptibility depends on the spatial peculiarities of a region. Among the numerous vulnerability assessment methodologies, the geographical information system-based DRASTIC model is the most sophisticated and achieves reliable results even in complex areas. Fractured-rock aquifers are not only complex, but they are also potential sources of radioactive and other types of waste and are liable to high recharge rates. Hence, they can serve as a conduit for fluid (and potentially) contaminants. Therefore, in this study, a new parameter, ‘fracture media’ (F), is fashioned to replace the weakest parameter (i.e. depth to water, D) in the standardized DRASTIC model to create a new model (FRASTIC) for the complex fractured aquifer system. The proposed model was tested on a case study area (Kano, Nigeria), and the FRASTIC minimum and maximum indices obtained were in the range of 63–170. The region was characterized as having very low (covering 0.16% of the total study area), low (55.52%), medium (42.53%), and high (1.79%) vulnerability based on the standardized classification system. In addition, the modified FRASTIC model was further developed using the sensitivity analysis (SA) and recorded the highest vulnerable area coverage (32.86%) within Kano. Thus, the modified FRASTIC model is appropriate for the complex nature of the study area because it contributes appreciably to defining the vulnerable zones. The single-parameter SA was employed to obtain effective weights for two modified models, which were then implemented to improve efficacy. Conventional nitrate validation indicated that there is a strong correlation between nitrate and vulnerability for the modified models. In addition, a new approach for identifying the relationship between the water quality and vulnerability indices was developed by this study and was proven to be an effective validation tool.

Assessment of groundwater vulnerability in the Daule aquifer, Ecuador, using the susceptibility index method

The Guayas region in Ecuador is economically very important, producing 68% of the national crops. The main agricultural activities threaten the groundwater therein with nitrate contamination given the large fertiliser and water needs. The present work tests the applicability of the susceptibility index assessment method in evaluating the impact of agricultural activities on groundwater quality, using as a case study an aquifer of the Guayas river basin in Ecuador. The index adapts four parameters of the DRASTIC method and incorporated a new land use parameter. Results show that the areas highly vulnerable to contamination are located in irrigation perimeters of dominant paddy fields associated with the high recharge rates in the alluvial deposits. Respectively, moderately vulnerable and low-vulnerability areas correspond to aquatic environments and forests, semi-natural zones and water bodies. One of the main contributions of the Daule aquifer vulnerability is likely its wide, flat topography. A great part of the aquifer is at high risk of contamination by nitrates if a code of good agricultural practices is not applied. Therefore the implementation of a monitoring network to control the nitrates concentrations is the first step to assure groundwater quality for drinking purposes.

Estimativa de recarga subterrânea a partir da Separação de escoamento de base na bacia hidrográfica do Rio Ibicuí (América do Sul)

Este estudo se propõe a estimar as taxas de recarga subterrânea na Bacia Hidrográfica do Rio Ibicuí, localizada no Estado do Rio Grande do Sul, a partir da utilização do método de separação do escoamento de base. O principal aquífero da Bacia é o Sistema Aquífero Guarani (SAG), nas suas áreas de confinamento (Oeste) e afloramento (Central). A separação do escoamento de base foi realizada por meio do filtro numérico de Eckhardt (2005) com dados de séries históricas de vazão disponibilizados pela Agência Nacional das Águas. As estimativas de recarga foram comparadas com as características geológicas, hidrogeológicas e pedológicas encontradas na área de estudo. As taxas de recarga variaram de 88 mm/ano a 378,7 mm/ano (13% a 47%, respectivamente da precipitação efetiva nas sub-bacias, e 6% a 21% da precipitação total). Os resultados mostraram que as maiores taxas de recarga<strong> </strong>ocorreram nas unidades geológicas com transmissividades mais elevadas e solos mais permeáveis, e também com precipitações médias mais elevadas. Por outro lado, as sub-bacias com baixas taxas de recarga apresentaram unidades geológicas permeáveis e unidades cristalinas impermeáveis, solos com altos teores de argila e precipitações médias reduzidas.