The spatial and temporal variability of the hydro-climate as well as land use and land cover (LULC) changes are among the most challenging problems facing water resources management. Understanding the interaction between climate variability, land use and land cover changes and their links to hydrology, river morphology and ecohydrology in the Dinder and Rahad basins in Sudan is confronted by the lack of climatic, hydrological and ecological data. This book investigated the impacts of land degradation on the Dinder and Rahad hydrology and morphology, and interlinkage to the ecohydrological system of the Dinder National Park (DNP) in Sudan. It used an ensemble of techniques to improve our understanding of the hydrological processes and LULC changes in these basins. This included long-term trend analysis of hydroclimatic variables, LULC changes analysis, field measurements, rainfall-runoff modelling, hydrodynamic and morphological modelling of the Dinder river and its floodplain, with special focus on the Mayas wetlands. Moreover, this research is the first study to investigate the eco-hydrology of the DNP. It is expected that the results of the study will be beneficial to all stakeholders concerned and support decision-making processes for better management of water resources and ecosystem conservation in the area and possibly beyond.@en

Understanding the spatiotemporal dynamics of surface water in varied, remote and inaccessible isolated floodplain lakes is difficult. Seasonal inundation patterns of these isolated lakes can be misestimated in a hydrodynamic model due to the short time of connectivity. The seasonal and annual variability of the Dinder River flow has great impact on what is so called Mayas wetlands, and hence, on the habitats and the ecological status of the Dinder National Park. This variability produces large morphological changes due to sediment transported within the river or from the upper catchment, which affects inflows to Mayas wetlands and floodplain inundation in general. In this paper, we investigated the morphological dimension using a quasi-3D modelling approach to support the management of the valuable Mayas wetlands ecosystems, and in particular, assessment of hydrological and morphological regime of the Dinder River as well as the Musa Maya. Six scenarios were developed and tested. The first three scenarios consider three different hydrologic conditions of average, wet and dry years under the existing system with the constructed connection canal. While the other three scenarios consider the same hydrologic conditions but under the natural system without an artificial connection canal. The modelling helps to understand the effect of human intervention (connection canal) on the Musa Maya. The comparison between the simulated scenarios concludes that the hydrodynamics and sedimentology of the Maya are driven by the two main factors: a) the hydrological variability of Dinder River; and b) deposited sediment plugs in the connection canal.@en

The implementation of afforestation programs in arid environments in northern China had modified the natural vegetation patterns. This increases the evaporation flux; however, the influence of these new covers on the soil water conditions is poorly understood. This work aims to describe the effect of Willow bushes (Salix psammophila C. Wang and Chang Y. Yang) and Willow trees (Salix matsudana Koidz.) on the soil water conditions after the summer. Two experimental plots located in the Hailiutu catchment (Shaanxi province, northwest China), and covered with plants of each species, were monitored during Autumn in 2010. The monitoring included the soil moisture, fine root distribution and transpiration fluxes that provided information about water availability, access and use by the plants. Meanwhile, the monitoring of stable water isotopes collected from precipitation, soil water, groundwater and xylem water linked the water paths. The presence of Willow trees andWillow bushes reduce the effect of soil evaporation after summer, increasing the soil moisture respect to bare soil conditions. Also, the presence of soil water with stable water isotope signatures close to groundwater reflect the hydraulic lift process. This is an indication of soil water redistribution carried out by both plant species.

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The Nile Delta Aquifer (NDA) is threatened by salt water intrusion (SWI). This article demonstrates an approach for identifying critical salinity concentration zones using a three-dimensional (3D) variable-density groundwater flow model in the NDA. An innovative procedure is presented for the delineation of salinity concentration in 2010 by testing different simulation periods. The results confirm the presence of saline groundwater caused by SWI in the north of the NDA. In addition, certain regions in the east and southwest of the NDA show increased salinity concentration levels, possibly due to excessive groundwater extraction and dissolution of marine fractured limestone and shale that form the bedrock underlying the aquifer. The research shows that the NDA is still not in a state of dynamic equilibrium. The modeling instrument can be used for simulating future scenarios of SWI to provide a sustainable adaptation plan for groundwater resource.@en

This paper examines the long-term trends of streamflow, rainfall and temperature over the Dinder and Rahad River basins. Streamflow of the Rahad River showed significant increasing trends in both the annual and seasonal flows. There was no detectable change in the mean annual and seasonal flow patterns of the Dinder. However, the analysis of seasonal maxima suggested a shift towards decreased flows during the high flow period (August) and increased flows during the low flow period (November). The Dinder maxima of August decreased from 517 m³/s over the early part of the record (1972–1991) to 396 m³/s over the latest years (1992–2011). The mean annual temperature showed significant increasing trends at the rate of 0.24 and 0.30°C/decade in the examined stations. Rainfall showed no significant change. The result of this study suggests other factors than climate variability (e.g., land use and land cover changes) to be responsible for streamflow alterations.

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In 2015, UN Member States adopted the 2030 Agenda for Sustainable Development, including Sustainable Development Goal 6 (SDG 6): "Ensure availability and sustainable management of water and sanitation for all". Commonly known as the 'water goal', SDG 6 went well beyond the limited focus on water supply and sanitation in the Millennium Development Goals (MDGs) and recognized the importance of all aspects of the water cycle in development and that water was embedded directly and indirectly in all 17 SDGs. In 2018, the UN published a report: "Sustainable Development Goal 6 Synthesis Report on Water and Sanitation 2018" (referred to in this paper as 'the report') that reviewed progress with SDG 6 at global and regional levels. Overall, the report concluded there was progress, but it was too slow, and the world was not on track to achieve SDG 6 by 2030 without a significant change of gear. The report was written primarily for those working in sustainable development to guide finance and resource allocation, but there was much embedded in the report that was of value to those engaged in research and in developing the much-needed capacity to plan and manage water resources, particularly in developing countries. This paper attempts to distill these issues and to ask how those involved in education, training, and research could contribute to enabling and accelerating progress towards achieving SDG 6. Three key areas of engagement were identified: the urgent need for more data and improved monitoring to assess SDG 6 progress and to enhance decision-making, the need to address the serious lack of human and institutional capacity that was constraining progress, and the challenge of taking research into policy and practice. Note: This paper is a review of selected aspects of the report (in which production the authors were chiefly involved as coordinators and editors), and as such most of the facts, figures, and discussion in this review are taken from the report. For this reason, we have not continually attributed them to the report to avoid repetition. However, in some cases, we have attributed report material to the primary sources where we considered it important to do so. We have also attributed material we have included, and which is not cited in the report. A review inevitably depends, to some extent, on the views of the reviewers and as such we have tried to make it clear where we are expressing our personal views rather than those expressed in the report. The report contains full references to all the primary sources.

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Phosphorus is a nutrient necessary for the development of crops and is thus commonly applied as fertilizer to sustain agricultural production. It occurs naturally, in indefinite quantities of uncertain quality in phosphate rock formations, but also accumulates in urban and livestock wastewater wherefrom it is often lost as a pollutant. Recovering phosphorus from wastewater, however, is feasible through struvite crystallization technologies and has the potential to reduce phosphorus pollution of the environment as well as lower the agricultural demand for artificial P fertilizers. In this study, we developed a model to assess the global potential of P fertilizer recovery from wastewater and to visualize its trade at sub-national resolution. Results show that humans discharge a maximum of 3.7 Mt P into wastewater, thereby potentially satisfying 20 % of the global fertilizer demand. Provided 2015 market dynamics, however, the model determines that only 4 % of this discharge is technologically and economically recoverable in a market that offers cheap rock phosphate products also. The results of this study demonstrate that in the current economic context, phosphorus recovery from wastewater offers only a small contribution to resolving global phosphorus issues. Nevertheless, this recovery offers many wastewater treatment facilities the opportunity to contribute to creating sustainable communities and protecting the environment locally, while reducing their own operational costs.

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As Egypt's population increases, the demand for fresh groundwater extraction will intensify. Consequently, the groundwater quality will deteriorate, including an increase in salinization. On the other hand, salinization caused by saltwater intrusion in the coastal Nile Delta Aquifer (NDA) is also threatening the groundwater resources. The aim of this article is to assess the situation in 2010 (since this is when most data is sufficiently available) regarding the available fresh groundwater resources and to evaluate future salinization in the NDA using a 3D variable-density groundwater flow model coupled with salt transport that was developed with SEAWAT. This is achieved by examining six future scenarios that combine two driving forces: increased extraction and sea level rise (SLR). Given the prognosis of the intergovernmental panel on climate change (IPCC), the scenarios are used to assess the impact of groundwater extraction versus SLR on the seawater intrusion in the Delta and evaluate their contributions to increased groundwater salinization. The results show that groundwater extraction has a greater impact on salinization of the NDA than SLR, while the two factors combined cause the largest reduction of available fresh groundwater resources. The significant findings of this research are the determination of the groundwater volumes of fresh water, brackish, light brackish and saline water in the NDA as a whole and in each governorate and the identification of the governorates that are most vulnerable to salinization. It is highly recommended that the results of this analysis are considered in future mitigation and/or adaptation plans.

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The Eastern Nile Basin is facing a number of transboundary issues, including water resources development, and the associated impacts. The Nile Basin, particularly the Eastern Nile Sub-basin, is considered as one of a few international river systems of potential conflicts between riparian countries. The Eastern Nile is characterized by the high dependency of downstream countries on river water generated in upstream countries, with limited or no contribution to the runoff itself. The aim of this paper is to analyze optimal scenarios for water resources management in the Eastern Nile with regard to hydropower generation and irrigation development. A hydro-economic optimization model based on Genetic Algorithm has been used to determine the maximum benefits for two scenarios: (i) non-cooperative management of hydraulic infrastructure by the riparian countries (status quo), and (ii) cooperative water resources management among the riparian countries. The hydro-economic model is developed using a Genetic Algorithm and deterministic optimization approach covering all hydraulic infrastructures in the Eastern Nile, existing and planned, including the Grand Ethiopian Renaissance Dam (GERD). The results show that cooperative management yields an increase in hydro-energy returns for all countries compared to the status quo, with a very high increase in Ethiopian's returns, as expected. Non-cooperative system management would negatively impact the hydro-energy of Egypt compared to the cooperative management (reduced by 11%), without a significant increase of hydro-energy for Ethiopia. For Sudan, the results show that hydropower generation benefits from the presence of GERD, in both management scenarios. Non-cooperative management of the system, along with the internal trade-off between irrigation and hydropower facilities, would negatively impact irrigation supply in Sudan. The findings support the argument of positive impact of GERD development on the three Eastern Nile riparian countries, Ethiopia, Sudan and Egypt, provided that the three countries agree to manage the system cooperatively.@en

The variability of rainfall and climate, combined with land use and land cover changes, and variation in geology and soils makes it a difficult task to accurately describe the key hydrological processes in a catchment. With the aim to better understand the key hydrological processes and runoff generation mechanisms in the semi-arid meso-scale Kaap catchment in South Africa, a hydrological model was developed using the open source STREAM model. Dominant runoff processes were mapped using a simplified Height Above the Nearest Drainage approach combined with geology. The Prediction in Ungauged Basins (PUB) framework of runoff signatures was used to analyse the model results. Results show that in the headwater sub-catchments of Noordkaap and Suidkaap, plateaus dominate, associated with slow flow processes. Therefore, these catchments have high baseflow components and are likely the main recharge zone for regional groundwater in the Kaap. In the Queens sub-catchment, hillslopes associated with intermediate and fast flow processes dominate. However, this catchment still has a strong baseflow component, but it seems to be more impacted by evaporation depletion, due to different soils and geology, especially in drier years. At the Kaap outlet, the model indicates that hillslopes are important, with intermediate and fast flow processes dominating and most runoff being generated through direct runoff and shallow groundwater components, particularly in wetter months and years. There is a high impact of water abstractions and evaporation during the dry season, affecting low flows in the catchment. Results also indicate that the root zone storage and the parameters of effective rainfall separation (between unsaturated and saturated zone), quickflow coefficient and capillary rise, were very sensitive in the model. The inclusion of capillary rise (feedback from the saturated to unsaturated zone) greatly improved the simulation results.

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The Eastern Nile riparian countries are currently developing several reservoir projects to contribute to the needs for energy and food production in the region. In the absence of formal mechanisms for collaboration, the transboundary nature of the Eastern Nile basin makes water resources development challenging. The large seasonal and interannual variability of the river flow increases those challenges. This paper assesses the implications of water resources development in the Eastern Nile basin on water availability for hydropower generation and irrigation demands at country and regional levels, using simulation and scenario analysis methods. Twelve scenarios are used to test developments of several dams and irrigation demands, Grand Ethiopian Renaissance Dam (GERD) operation options, and unilateral (status quo) versus integrated transboundary management of dams. A RIBASIM model that included 20 dams and 21 irrigation schemes was built, using a complete data set of 103 years at a monthly time step. Four indicators have been used for evaluating the performance of the system: hydroenergy generation (MWh=year), reliability of irrigation supply (%), reservoir net evaporation (10⁶ m³/year), and flow regimes of rivers (m³/s). The results show that in case of managing the system in an integrated transboundary manner and without new irrigation development projects, GERD would increase the hydroenergy generation in Ethiopia by + 1,500% and in Sudan by + 17%, with a slight reduction in Egypt of -1%. Supply reliability of existing and planned irrigation schemes in Sudan would not be practically influenced by the GERD, but the reliability will be reduced by about 8% when upstream development and new irrigation expansion materialize. Full development of the Eastern Nile basin would reduce the irrigation supply reliability in Egypt to 92% compared to the base scenario (100%). Compared to integrated management, unilateral management would increase the hydroenergy generation in Ethiopia (+16%), increase the rate of evaporation losses in the basin (+15%), and reduce the irrigation supply reliability in Sudan after full development of dams and irrigation projects (-10%). Water resources development would have considerable but varying effects on the countries.

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Chemical hydrograph separation using electrical conductivity and digital filters is applied to quantify runoff components in the 1,640 km² semi-arid Kaap River catchment and its subcatchments in South Africa. A rich data set of weekly to monthly water quality data ranging from 1978 to 2012 (450 to 940 samples per site) was analysed at 4 sampling locations in the catchment. The data were routinely collected by South Africa's national Department of Water and Sanitation, using standard sampling procedures. Chemical hydrograph separation using electrical conductivity (EC) as a tracer was used as reference and a recursive digital filter was then calibrated for the catchment. Results of the two-component hydrograph separation indicate the dominance of baseflow in the low flow regime, with a contribution of about 90% of total flow; however, during the wet season, baseflow accounts for 50% of total flow. The digital filter parameters were very sensitive and required calibration, using chemical hydrograph separation as a reference. Calibrated baseflow estimates ranged from 40% of total flow at the catchment outlet to 70% in the tributaries. The study demonstrates that routinely monitored water quality data, especially EC, can be used as a meaningful tracer, which could also aid in the calibration of a digital filter method and reduce uncertainty of estimated flow components. This information enhances our understanding of how baseflow is generated and contributed to streamflow throughout the year, which can aid in quantification of environmental flows, as well as to better parameterize hydrological models used for water resources planning and management. Baseflow estimates can also be useful for groundwater and water quality management.

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Increasing water demand coupled with limited water resources has given rise to the need for sustainable water resources development in the Nile River Basin. Managing the basin’s water resources is complex because of its transboundary nature, exacerbated by environmental degradation and strong climate variability. The Nile basin water resources have been extensively studied during the last 125 years for planning and management purposes, in particular with regard to the use of blue water in the downstream part of the basin, though recently some studies have also focused on the upper parts. These studies show that there is no convergence of development plans emerging among the Nile riparian countries. This paper reviews river basin water resource models as applied in the Nile River Basin, distinguishing between simulation, optimization and combined simulation and optimization models. The paper concludes by identifying knowledge gaps to guide future research on water resources planning and management in the Nile.

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Understanding the land use and land cover changes (LULCCs) and their implication on surface hydrology of the Dinder and Rahad basins (D&R, approximately 77 504 km2) is vital for the management and utilization of water resources in the basins. Although there are many studies on LULCC in the Blue Nile Basin, specific studies on LULCC in the D&R are still missing. Hence, its impact on streamflow is unknown. The objective of this paper is to understand the LULCC in the Dinder and Rahad and its implications on streamflow response using satellite data and hydrological modelling. The hydrological model has been derived by different sets of land use and land cover maps from 1972, 1986, 1998 and 2011. Catchment topography, land cover and soil maps are derived from satellite images and serve to estimate model parameters. Results of LULCC detection between 1972 and 2011 indicate a significant decrease in woodland and an increase in cropland. Woodland decreased from 42 to 14 % and from 35 to 14 % for Dinder and Rahad, respectively. Cropland increased from 14 to 47 % and from 18 to 68 % in Dinder and Rahad, respectively. The model results indicate that streamflow is affected by LULCC in both the Dinder and the Rahad rivers. The effect of LULCC on streamflow is significant during 1986 and 2011. This could be attributed to the severe drought during the mid-1980s and the recent large expansion in cropland.

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The interactions between groundwater and surface water have been significantly affected by human activities in the semi-arid Hailiutu catchment, northwest China. Several methods were used to investigate the spatial and temporal interactions between groundwater and surface water. Isotopic and chemical analyses of water samples determined that groundwater discharges to the Hailiutu River, and mass balance equations were employed to estimate groundwater seepage rates along the river using chemical profiles. The hydrograph separation method was used to estimate temporal variations of groundwater discharges to the river. A numerical groundwater model was constructed to simulate groundwater discharges along the river and to analyze effects of water use in the catchment. The simulated seepage rates along the river compare reasonably well with the seepage estimates derived from a chemical profile in 2012. The impacts of human activities (river-water diversion and groundwater abstraction) on the river discharge were analyzed by calculating the differences between the simulated natural groundwater discharge and the measured river discharge. Water use associated with the Hailiutu River increased from 1986 to 1991, reached its highest level from 1992 to 2000, and decreased from 2001 onwards. The reduction of river discharge might have negative impacts on the riparian ecosystem and the water availability for downstream users. The interactions between groundwater and surface water as well as the consequences of human activities should be taken into account when implementing sustainable water resources management in the Hailiutu catchment.

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The water balance dynamics and runoff components of a tropical forested catchment (46 km2) on the southwestern Pacific coast of Nicaragua were studied combining hydrometry, geological characterization and hydrochemical and isotopic tracers (three-component hydrograph separation). The climatic water balance was estimated for 2010/11, 2011/12 and 2012/13 with net values of 811 mm year-1, 782 mm year-1 and –447 mm year-1, respectively. Runoff components were studied at different spatial and temporal scales, demonstrating that different sources and temporal contributions are controlled by dominant landscape elements and antecedent rainfall. In forested sub-catchments, permeable soils, stratigraphy and steep slopes favour subsurface stormflow generation contributing 50% and 53% to total discharge. At catchment scale, landscape elements such as smooth slopes, wide valleys, deeper soils and water table allow groundwater recharge during rainfall events. Groundwater dominates the hydrograph (50% of total discharge) under dry prior conditions. However, low soil infiltration capacity generates a larger surface runoff component (42%) under wet prior conditions which dominates total discharge. Our results show that forested areas are important to reduce surface runoff and thus soil degradation, which is relevant for the design of water management plans. Editor D. Koutsoyiannis Associate editor D. Gerten@en

The Upper Blue Nile is home to a large human and livestock population that live in diverse biophysical and socio-economic environment. The basin is increasingly experiencing multi-dimensional pressures including population growth, climate change and variability, deforestation, land/soil degradation, as well as increasing upstream-downstream tension on water use rights. Understanding the dynamic interactions of land use and water resources in the basin comes at the forefront of any effort to improving the livelihood and sustainability in the basin. As part of a study to develop a decision support system for an integrated natural resources management for the Upper Blue Nile basin, methods and techniques are developed to identify land use suitably based on various biophysical and socio-economic factors, and water resources availability in the basin. Possible biophysical and socio-economic land use change drivers on a mesoscale catchment, Jedeb, were identified using historical remote sensing data as well as primary data sources such as field observations, questionnaires and interviewing key stakeholder informants. Then, major land use change drivers were ranked using regression analysis (PCA). A hydrological model was setup using the Soil and Water Assessment Tool (SWAT) for the Jedeb catchment. Based on the identified high impact land use change drivers and additional factors such as agro-ecological zones and water resources availability (output from the hydrological model), a land use suitability analysis for the catchment was developed using the SITE (SImulation of Terrestrial Environments) generic land use change modeling framework. The land use and hydrological models exchange yearly simulation results to determine land use suitability analysis and impacts of land use change on components of the basin's hydrology, and vice-versa. Land use scenario was analyzed by assuming a 20% increase in population in the catchment. Preliminary results indicate a clear shift mainly from grassland land use type to cultivation land use type. It is concluded that the techniques and methodologies used in this study especially for integrating the two models can be used for a more realistic and thorough analysis of land use suitability and water resources dynamics and assessment study. Outputs of this study are used as inputs to develop a spatial decision support system for an integrated assessment and management of land use and water resources in the basin.@en

Tracer testing is an efficient and versatile method to characterize hydrological processes both in surface waters and in the subsurface, and with synthetic DNA (deoxyribonucleic acid) experiments can be performed with a theoretically unlimited number of tracers. We explored the use of six synthetic DNA tracers in injection experiments in the Strijbeekse Beek and the Merkske, two small brooks in the Netherlands. We wanted to test the applicability of synthetic DNA markers in combination with the quantitative polymerase chain reaction (qPCR), an analytical technique used to determine DNA concentrations. The most important results from the two experiments were that DNA concentrations during breakthrough ranged from 0·1 to 100 fM (femtomolar = 10−15 mol/l), and that DNA mass transport in the stream was not retarded compared to transport of an injected NaCl mass. However, DNA mass was substantially reduced, most likely as a result of adsorption, attachment, decay and/or biological uptake processes in the stream itself. Although more insight in the various mass balance components of DNA mass in the stream is required, we concluded that the use of synthetic DNA markers in combination with qPCR has potential for spatially distributed surface water-groundwater interaction tracer experiments.@en

In health impact assessments, the sticking efficiency of a bacteria or virus population largely determines the transported distance of that biocolloid population, and hence, the potential health impact. However, at the same time, one of the most difficult parameters to estimate is the lower value of the sticking efficiency that should be used in calculating the health impact. In this paper, we introduce the concept of the minimum sticking efficiency (αi) value of a bacteria population, including a method to determine the minimum sticking efficiency. Thereto, sticking efficiency distributions of 6 environmentally isolated Escherichia coli strains were determined by carrying out laboratory column experiments over a transport distance of about 5 m. Experiments were conducted in de-mineralized (DI) water and in artificial groundwater (AGW). Sticking efficiencies were calculated for column segments (at varying distances from top of column) and fractions of total bacteria mass input in each segment were estimated by mass balance. The sticking efficiencies were highest close to the top of the column, near the point of bacteria mass input (0.103-0.352 in DI, and 1.034-9.470 for AGW) and reduced with distance with the lowest αi values (10- 5-0.06 in DI and 0.006-0.283 in AGW) determined at the two most distant column segments (between 2.33 and 4.83 m from the top of the column). Power-law distributions best described the relationship between fraction of cells retained, Fi, and αi. The minimum sticking efficiency was defined as the sticking efficiency belonging to a retained bacteria fraction of 0.001% of the original bacteria mass (total number of cells) flowing into the column (F = 10- 5), and coinciding with a 99.999% reduction of the original bacteria mass, and minimum sticking efficiencies were extrapolated from the fitted power-law distributions. In the DI experiments, minimum sticking efficiency values ranged from as low as 10- 9 (for E. coli strain UCFL-94) to 10- 2 (for E. coli strain UCFL-348); in the AGW experiments, minimum sticking efficiency values ranged from 10- 6 (for strain UCFL-94) to ≥ 1(for strain UCFL-348). We concluded that in quantifying health impacts of biocolloids traveling in aquifers, the concept of the minimum sticking efficiencies, and the percentage of individual biocolloids of a total population having such low sticking efficiency, together with an inactivation rate coefficient, can serve as a useful tool to determine the maximum transported distance as a worst case scenario, and, hence, the potential health impact.@en

Recharge of waste water in an unconsolidated poorly sorted alluvial aquifer is a complex process, both physically and hydrochemically. The aim of this paper is to analyse and conceptualise vertical transport mechanisms taking place in an urban area of extensive wastewater infiltration by analysing and combining the water balance, the microbial (Escherichia coli) mass balance, and the mass balance for dissolved solutes. For this, data on sediment characteristics (grain size, organic carbon, reactive iron, and calcite), groundwater levels, and concentrations of E. coli in groundwater and waste water were collected. In the laboratory, data on E. coli decay rate coefficients, and on bacteria retention characteristics of the sediment were collected via column experiments. The results indicated that shallow groundwater, at depths of 50 m below the surface, was contaminated with E. coli concentrations as high as 10⁶ CFU/100 mL. In general, E. coli concentrations decreased only 3 log units from the point of infiltration to shallow groundwater. Concentrations were lower at greater depths in the aquifer. In laboratory columns of disturbed sediments, bacteria removal was 2-5 log units/0.5 cm column sediment. Because of the relatively high E. coli concentrations in the shallow aquifer, transport had likely taken place via a connected network of pores with a diameter large enough to allow bacterial transport instead of via the sediment matrix, which was inaccessible for bacteria, as was clear from the column experiments. The decay rate coefficient was determined from laboratory microcosms to be 0.15 d^{- 1}. Assuming that decay in the aquifer was similar to decay in the laboratory, then the pore water flow velocity between the point of infiltration and shallow groundwater, coinciding with a concentration decrease of 3 log units, was 0.38 m/d, and therefore, transport in this connected network of pores was fast. According to the water balance of the alluvial aquifer, determined from transient groundwater modelling, groundwater flow in the aquifer was mainly in vertical downward direction, and therefore, the mass balance for dissolved solutes was simulated using a 1D transport model of a 200 m column of the Quaternary Alluvium aquifer. The model, constructed with PHREEQC, included dual porosity, and was able to adequately simulate removal of E. coli, cation-exchange, and nitrification. The added value of the use of E. coli in this study was the recognition of relatively fast transport velocities occurring in the aquifer, and the necessity to use the dual porosity concept to investigate vertical transport mechanisms. Therefore, in general and if possible, microbial mass balances should be considered more systematically as an integral part of transport studies.

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