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

Africa, though reported to be the least urbanized continent, is recognized as one where the rate of urbanization is highest. The development and expansion of informal settlements in the suburbs of the cities is widespread, while they harbor the majority of the urban population. Slums are characterized by, among other things, poorly constructed houses, poor water supply and sanitary conditions, and lack or inadequate support services. Besides the spreading of diseases related to surface water (e. g., malaria), one of the main problems associated with sanitation and water in slum areas is related to the pollutant load entering and leaving the slum catchment, either as surface water or groundwater. This is polluting drinking water or causing eutrophication of surface water, due to the extremely high nitrogen and phosphorus fluxes discharging those slum catchments. The SCUSA research project aims at identifying the most sustainable sanitation solutions in urban slums, including the most important parameters determining and guaranteeing sustainability. Our approach is multidisciplinary, and should therefore yield answers with a financial, social, technical, and environmental view on the extremely complex field of sanitation in urban slums.

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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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Though coliform bacteria are used worldwide to indicate faecal pollution of groundwater, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown. To investigate the effect of dissolved organic carbon (DOC) on the attachment of E. coli to saturated goethite-coated sand, we carried out column experiments with E. coli with and without humic acid (HA) in monovalent and divalent salt solutions. To characterize sorption of DOC and attachment of E. coli, we measured the pH of the influent and effluent, the cation concentrations and the zeta potential of particles. Depending on the chemistry of the E. coli suspension, the normalized breakthrough concentrations were over 80 times higher in columns treated with HA compared with columns not treated with HA. However, this difference was not constant: there were time-dependent variations in attachment of E. coli to the collector surface, and in the chemical composition of the bacterial suspension. Reduction in removal occurred because HA altered the surface charge of the collector and also sterically hindered E. coli. In addition, reduction of removal in a CaCl2 bacterial suspension was probably caused by site-blocking mechanisms between HA and Ca2+ ions. Our results indicate that in the presence of DOC, the concept of geochemical heterogeneity in explaining attachment of biocolloids has limited relevance.@en

Though coliform bacteria are used worldwide to indicate fecal pollution of groundwater, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown. We evaluated the occurrence of both straining and attachment of E. coli ATCC25922 in columns of ultra-pure, angular, saturated quartz sand. The column experiments were conducted over a wide range of porous medium sizes, column heights, input concentrations, and pore water flow velocities. Straining and attachment were examined by modelling the breakthrough curves (with HYDRUS 1D). In addition, model output was compared with measured strained and attached bacteria via column extrusion experiments (in which sand was extruded from the column and placed in excess water) and flow reversal experiments (in which the pore water flow direction was reversed, thereby dislodging strained bacteria). Our model consisted of an attachment rate coefficient and a straining rate coefficient; both of these decreased with transport distance. The straining rate coefficient also decreased in a Langmuirian way, in response to the filling of available pore space, which in turn depended on influent bacteria concentration, quartz grain diameter, and transport distance. The maximum strained fraction was 25-30% of total bacteria mass applied to the column; the maximum attached fraction was 30-35%. The fit between modelled and measured (strained and attached) bacteria masses was acceptable, as was the sensitivity of the model output to fitted parameter values. Our results lead to a new description for the time-dependent mass balance of strained bacteria, which entails using three fitting parameters. The results also imply that column experiments in combination with retention profiles (or various column lengths) are not enough to explain the retention processes in a column. Column extrusion and flow reversal experiments provide vital additional information on the occurrence and magnitude of straining. Our straining model could be of assistance in evaluating the importance of straining and in incorporating the straining process in bacteria transport modelling.

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Because of heterogeneity among members of a bacteria population, deposition rates of bacteria may decrease upon the distance bacteria are transported in an aquifer. Such deposition rate decreases may result in retained bacteria concentrations, which decrease hyper-exponentially as a function of transport distance, and may therefore significantly affect the transport of colloids in aquifers. We investigated the occurrence of hyper-exponential deposition of Escherichia coli, an important indicator for fecal contamination, and the causes for such behavior. In a series of column experiments with glass beads of various sizes, we found that attachment of E. coli decreased hyper-exponentially, or, on logarithmic scale in a bimodal way, as a function of the transported distance from the column inlet. From data fitting of the retained bacteria concentration profiles, the sticking efficiency of 40% of the E. coli population was high (α=1), while the sticking efficiency of 60% was low (α=0.01). From the E. coli total population, an E. coli subpopulation consisting of slow attachers could be isolated by means of column passage. In subsequent column experiments this subpopulation attached less than the E. coli total population, consisting of both slow and fast attachers. We concluded that the main driver for the observed dual mode deposition was heterogeneity among members of the bacteria population. Intra-population may result in some microbes traveling surprisingly high distances in the subsurface. Extending the colloid filtration theory with intra-population variability may provide a valuable framework for assessing the transport of bacteria in aquifers.

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The transport of bacteriophage PRD1, a model virus, was studied in columns containing sediment mixtures of quartz sand with goethite-coated sand and using various solutions consisting of monovalent and divalent salts and humic acid (HA). Without HA and in the absence of sand, the inactivation rate of PRD1 was found to be as low as 0.014 day- 1 (at 5 ± 3 °C), but in the presence of HA it was much lower (0.0009 day- 1), indicating that HA helps PRD1 to survive. When the fraction of goethite in the sediment was increased, the removal of PRD1 also increased. However, in the presence of HA, C/C0 values of PRD1 increased by as much as 5 log units, thereby almost completely eliminating the effect of addition of goethite. The sticking efficiency was not linearly dependent on the amount of goethite added to the quartz sand; this is apparently due to surface charge heterogeneity of PRD1. Our results imply that, in the presence of dissolved organic matter (DOM), viruses can be transported for long distances thanks to two effects: attachment is poor because DOM has occupied favourable sites for attachment and inactivation of virus may have decreased. This conclusion justifies making conservative assumptions about the attachment of viruses when calculating protection zones for groundwater wells.@en

Escherichia coli and thermotolerant coliforms are of major importance as indicators of fecal contamination of water. Due to its negative surface charge and relatively low die-off or inactivation rate coefficient, E. coli is able to travel long distances underground and is therefore also a useful indicator of fecal contamination of groundwater. In this review, the major processes known to determine the underground transport of E. coli (attachment, straining and inactivation) are evaluated. The single collector contact efficiency (SCCE), η0, one of two parameters commonly used to assess the importance of attachment, can be quantified for E. coli using classical colloid filtration theory. The sticking efficiency, α, the second parameter frequently used in determining attachment, varies widely (from 0.003 to almost 1) and mainly depends on charge differences between the surface of the collector and E. coli. Straining can be quantified from geometrical considerations; it is proposed to employ a so-called straining correction parameter, αstr. Sticking efficiencies determined from field experiments were lower than those determined under laboratory conditions. We hypothesize that this is due to preferential flow mechanisms, E. coli population heterogeneity, and/or the presence of organic and inorganic compounds in wastewater possibly affecting bacterial attachment characteristics. Of equal importance is the inactivation or die-off of E. coli that is affected by factors like type of bacterial strain, temperature, predation, antagonism, light, soil type, pH, toxic substances, and dissolved oxygen. Modeling transport of E. coli can be separated into three steps: (1) attachment rate coefficients and straining rate coefficients can be calculated from Darcy flow velocity fields or pore water flow velocity fields, calculated SCCE fields, realistic sticking efficiency values and straining correction parameters, (2) together with the inactivation rate coefficient, total rate coefficient fields can be generated, and (3) used as input for modeling the transport of E. coli in existing contaminant transport codes. Areas of future research are manifold and include the effects of typical wastewater characteristics, including high concentrations of organic compounds, on the transport of E. coli and thermotolerant coliforms, and the upscaling of experiments to represent typical field conditions, possibly including preferential flow mechanisms and the aspect of population heterogeneity of E. coli.

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Though coliform bacteria are used world wide as an indication of faecal pollution, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown, especially for the period after the clean bed collision phase brought about by prolonged infiltration of waste water. In this research, the breakthrough curves of E. coli after total flushing of 50-200 pore volumes were studied for various influent concentrations in various sediments at different pore water flow velocities. The results indicated that straining in Dead End Pores (DEPs) was an important process that dominated bacteria breakthrough in fine-grained sediment (0.06-0.2 mm). The filling of the DEP space with bacteria took 5-65 pore volumes and was dependent on concentration. Column breakthrough curves were modelled and from this the DEP volumes were determined. These volumes (0.21-0.35% of total column volume) corresponded well with values calculated with a formula based on purely geometrical considerations and also with values calculated with a pore size density function. For this function the so-called Van Genuchten parameters of the sediments used in the experiments were determined. The results indicate that straining might be a dominant process affecting colloid transport in the natural environment and therefore it is concluded that proper knowledge of the pore size distribution is crucial to an understanding of the retention of bacteria.

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Lack of water management in the Sana'a Basin in Yemen has led to mining of groundwater and massive groundwater quality deterioration. In the last four years, management of waste water has changed dramatically and entire city neighborhoods have been connected to a conventional sewer system. In this paper, the effects of this measure on long-term groundwater quality development of the aquifers underlying Sana'a and, more specifically, on water quality of the public supply peri-urban wellfields are analyzed. The results, obtained with a transient groundwater model, indicated that by 2020 the construction of the sewerage will have considerably reduced the area polluted by groundwater, but the process is slow. Furthermore, construction of the sewerage will hardly affect the groundwater quality of the wellfields, since flow is not directed towards most of the production wells. The Yemeni authorities should realize that less expensive sanitation alternatives are available, but they need user participation, which, in turn, would raise public awareness that water supplies and sanitation are not to be seen as solely a government responsibility.@en

To elucidate the parameters determining the transport of Escherichia coli in aquifers, the attachment of E. coli in low concentrations to column sediments was investigated. The sediments comprised 0.18-0.50 mm quartz sand, grains coated with goethite, calcite grains or grains of activated carbon (AC), in varying fractions (λ=0, 0.05, 0.1, 0.2, 0.4, 0.7, 1.0) and all of similar diameter to the quartz sand. The weighted sum of favourable and unfavourable sticking efficiencies (αtotal) showed that upon increasing the fraction of favourable mineral grains (λ) there was an initial rapid increase, which then slowed down. This was most pronounced in the AC experiments, followed by the calcite experiments and then the goethite experiments. We ascribe this non-linear relation to surface charge and hydrophobic heterogeneity of the E. coli population.@en

In Sana'a, the capital of the Yemen Arab Republic, a major well inventory was compiled in 1995 during which samples were analysed for major cations and anions. Five years later the opportunity was taken to repeat the exercise on a sub-set of the original wells. The results showed that groundwater in the urban area was characterised by high concentrations of almost all major cations and anions due to the continuous infiltration of wastewater into the aquifers via cesspits. The dominant watertype appeared to be CaCl₂. The Cl^--concentration ranged from 3 to 10 mmol/l and NO₃^--concentration ranged from 1 to 3 mmol/l while NH₄⁺ was absent in all samples. It is concluded that cation exchange has taken place. Ca²⁺ in groundwater has been enriched, while Na⁺, K⁺ and NH₄⁺ have been depleted. Groundwater affected by wastewater had pH values of 0.5-1 unit lower than groundwater not affected by wastewater, indicating that acidification has taken place. Over the period between the two surveys, concentrations of almost all major anions and cations increased, while pH decreased, both owing to the continuous infiltration of wastewater. An exploratory one-dimensional transport model of a 200 m column of the aquifer underlying Sana'a showed that, over a 15-year period of continuous wastewater infiltration, a quarter of the NH₄^- present in raw sewage would oxidise to NO₃^- thereby producing acidity and some 60% would be adsorbed. The model indicates that after 50 years of wastewater infiltration, exchange of NH₄⁺ has become limited due to the limited cation exchange capacity (CEC) of the soil. Therefore more NH₄⁺ will be oxidised to NO₃^- and [NO₃^-] in groundwater will rise. At the same time, groundwater in the zone of NH₄^- oxidation will become very acid due to a lack of buffering minerals. The modelling studies, together with the results from the surveys, tend to indicate that up to 12% of the current population of the city could be dependent on contaminated groundwater for their drinking water supply.

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The geohydrological and hydrogeochemical factors that determine the contribution of PO4 from exfiltrating groundwater from a semiconfined aquifer to the PO4 balance of surface water, especially in ditches, were studied in a Dutch polder. Numerical flow modelling based on field data indicates that 30-90% of the upward seepage from the underlying aquifer is discharged via drains and the rest via the ditch bottom. Water level measurements and profiles of pore water quality in the ditch bottom show that groundwater flows to the ditch and to underground drains in winter, whereas in summer the water infiltrates the ditch bottom probably because of the high evapotranspiration rates. Phosphate stored in Fe oxihydroxides in the ditch bottom cannot be made available as a nutrient to the surface water during the growing season.@en