Influence of Extreme Hydrologic Events on the Occurrence of Microbial Species in Regional Groundwater Systems
The mobility of microbial species originating from surface sources such as animal manure is influenced both by the intrinsic permeability of the subsurface sediment and by the nature of the hydrologic driving factors associated with climatic variability. Extreme hydrologic events such as exceptionally heavy periods of precipitation or snowmelt represent conditions that may produce rapid infiltration and potential redistribution of microbial loads through surface runoff. In this study, the occurrence of various bacterial indicator species (total coliforms (TC), aerobic endospores and E. coli) was tracked in a network of over 20 monitoring wells situated in a vulnerable wellhead protection area of a municipal well field located near Woodstock, Ontario during snow melt periods. Water quality, hydraulic head and groundwater temperature were also monitored during the course of the investigation. The wells were situated along a groundwater flow path at various depths below the water table and sampled frequently throughout an annual cycle. Samples were also obtained from surface water and tile drains during winter and spring melt and from the municipal wells. The data indicate that the elevated microbial concentrations in both the tile drains and shallow aquifer system correlate to ephemeral hydrologic events that develop during intense melt periods. Surface runoff mobilizes microbial species from significant distances away from the municipal wells to within the immediate vicinity of the wells, significantly increasing the potential for capture in the municipal supply subsequently increasing the risk water quality. Higher concentrations in the shallow aquifer systems were observed in response to ephemeral hydrologic events, yet occurrence in the municipal wells was very infrequent even under these conditions of high vulnerability. The occurrence patterns in the municipal wells, however, appear to correlate to the snowmelt periods, although after a significant time lag related to the distance between the infiltration area and the well positions. Numerical simulation is used to quantify the potential migration patterns and associated travel times for microbial species entering the subsurface near the municipal well field under these extreme hydrologic conditions.
Mechanisms of Pathogen and Surrogate Transport in Porous Media: Concurrent Effects of Grain Characteristics, NOM and Ionic Strength
It is widely accepted that riverbank filtration (RBF) can provide substantial reductions in the concentrations of both microbial and chemical contaminants while providing more consistent water quality to subsequent treatment processes. Factors such as experimental scale, subsurface heterogeneity, and variable flow paths and fluxes have made it difficult to relate laboratory outcomes to field performance. Field studies have been plagued with inadequate consideration of ground water flow, reliance on unproven "surrogate" parameters, non-detects at the extraction well, and limited sampling. As a result, a treatment-technique type of approach has been utilized to describe subsurface and operational conditions that result in effective RBF. While it is generally understood that parameters such as ionic strength, the presence of natural organic matter (NOM), and media size and shape characteristics affect pathogen transport in porous media, one major limiting factor in the development of regulatory credits and predictive models is the lack of understanding of the concurrent effects of such parameters. To provide guidance for assessing the efficacy of RBF processes, the present investigation is focused on evaluating the concurrent effects of these parameters on pathogen transport in RBF environments. This work details the complete outcomes of a factorial experimental investigation of the concurrent impacts of the four parameters: ionic strength, NOM concentration, grain size, and uniformity coefficient on pathogen and surrogate transport in porous media. Duplicate column studies have been conducted to evaluate the transport of Cryptosporidium oocysts and Salmonella typhimurium bacteria in saturated sandy environments; PR772 bacteriophage, and 4.5 μm and 1.5 μm microspheres are also being used as surrogates for pathogen transport. The strain of Salmonella was selected due to the direct link to human illness in the Grand River watershed. Preliminary results indicate that ionic strength effects are greater than NOM effects. Grain size also significantly impacts pathogen transport. Experiments conducted at rates relevant to both GWUDI and conventional filtration systems have indicated that removals of 4.5 μm microspheres appear to be somewhat reliable surrogate indicators of oocyst removal. No apparent correlation between oocyst removals and removals of bacteria (Bacillus spores) was observed, underscoring that relying on assumed correlation between removals of surrogate parameters target organisms may not be appropriate during RBF investigations.
Demonstrating Effective In Situ Filtration of Pathogens in Saturated Environments: Laboratory and Field Considerations
As both water treatment costs and demands for potable water increase, many municipalities around the world are considering low-cost, effective treatment technologies such as riverbank filtration (RBF), which involves situating municipal ground water wells in close proximity to surface water bodies such as rivers to induce conditions of downward infiltration from the river across part or all of the riverbed in the vicinity of the well. A variety of frameworks in various jurisdictions have been proposed for assessing groundwater under the direct influence of surface water (GWUDI/GUDI) for the ultimate purposes of determining appropriate levels of subsequent treatment or source water protection. While it is relatively easy to observe deterioration of production well water quality and to require more extensive treatment, relying on the subsurface for effective in situ filtration of compounds of public health significance (e.g., pathogens) necessarily requires more extensive demonstration. This work details and summarizes unpublished work from four separate studies focused on improving the water industry's capacity to assess the pathogen removal efficacy of in situ filtration processes such as RBF. Both laboratory and field studies investigating the transport of Cryptosporidium oocysts, Bacillus subtilis spores, and Salmonella typhimurium bacteria in saturated sandy environments have indicated that experimental conditions (e.g., microorganism densities present during the studies) can impact transport outcomes by over 3-log. Laboratory experiments have also indicated that experimental configuration (e.g., column orientation) can significantly impact pathogen transport. Accordingly, laboratory and field data must be carefully examined when outcomes are extrapolated for the purpose of policy development.
Regional-scale pathogen transport in a fractured rock aquifer located in an agricultural watershed
A regional-scale groundwater monitoring study has been intiated in an agricultural watershed, near Perth, Ontario, Canada. The study area is characterized by sparsely-fractured Precambrian syenite-migmatite overlain by 0-3m of Paleozoic Nepean Sandstone. Rock outcrops are common in this terrain, but overburden thickness is greater than 4m in some locations. Twenty-two bedrock wells were drilled between 2004 and 2008 to depths from 30-45m below ground surface (bgs) in a 100 km2 area. Hydraulic testing to identify horizontal fracture features was completed in each well and most were instrumented with multilevel piezometers. Results of this study indicate that areas of minimal overburden must create direct transport pathways for pathogens, such as E. coli. Little overburden coupled with recharge events creates an optimal environment for the introduction of pathogens to fractured rock aquifers. Bacteria occur most often in shallow piezometer sections indicating direct connection to the surface. However, bacteria were also found in deep piezometers (~30m bgs) suggesting that vertical fractures encourage transport to deeper horizontal fractures. Maximum E. coli counts were greater than 400 cts/100ml in a shallow piezometer on a rock outcrop and 80 cts/100ml in a deep piezometer after a prolonged rain event in the summer of 2008. These new results indicate that positive bacterial counts do not directly relate to sources close to obvious shallow outcrop areas.
Numerical modeling of multiple nitrate sources affecting the groundwater quality of private wells
The use of hydraulic data alone has proven to be insufficient to constrain transient simulations of mass transport. Recent developments in analytical methods, especially in the measurement of stable isotopes in water, have opened new possibilities to interpret transient groundwater flow and mass transport mechanisms. In that perspective, a numerical model was developed to represent the transient transport of nitrates in the Wilmot River watershed in Prince Edwards Island. This area is characterized by intensive agricultural land use, especially potato crops using large quantities of chemical fertilizers. The groundwater quality in many wells in the watershed has been deteriorating over the years, with the average nitrate concentration now reaching 7 mg/L, while some individual wells are above the maximum concentration limit of 10 mg/L. To evaluate the contribution of different nitrate sources to groundwater, seasonal concentrations of nitrate ion isotopes were measured in groundwater (N-15 and O-18). The dual isotope analysis allows the quantification of the proportions of nitrate species in groundwater, providing a geochemical mixing model of the different nitrate sources. The isotopic results obtained from the domestic wells within the watershed were used to develop and constrain a 3D groundwater flow and transport regional numerical model. Conceptually, the model reproduces the flow and transport conditions of the fractured upper 20 m of the aquifer. Since this part of the aquifer contains most the water available for domestic use, simulation results demonstrate that this groundwater is highly vulnerable to surface contamination and responds rapidly to changes in contaminant input.
Low-Temperature Geothermal Systems and Groundwater Protection
The development of low temperature geothermal systems for space heating or cooling residential, institutional, commercial and industrial buildings has been steadily increasing in Canada and nations world-wide, primarily due to the associated environmental and cost benefits. Although geothermal systems are generally regarded as a 'green' and 'sustainable' energy source, recent studies have highlighted that the migration of thermal anomalies associated with these subsurface systems can result in adverse impacts to environmental receptors and these systems should be a consideration in groundwater protection. This is particularly true where groundwater is important in the regulation of surface water temperatures and where temperature dependent reactions might affect groundwater quality. In this study, environmental implications of geothermal systems will be evaluated by monitoring groundwater temperatures in wells surrounding existing geothermal developments in different geophysical areas of Nova Scotia. The collected data, in conjunction with existing data, will be used to create numerical models capable of predicting the nature and extent of thermal changes caused by the long-term operation of a geothermal system. Observations and model results will be used to assist in the creation of guidelines to develop geothermal resources in an optimal and sustainable manner for both the thermal application and the environment.
Evaluation of areas of contribution and water quality at receptors related to TCE plumes in a valley fill aquifer system
The Val-Belair sector is located within Quebec City, about 20 km from downtown. Potential source zones and TCE plumes in groundwater are found at the western limit of the sector. At the center of the sector, four municipal water supply wells pump groundwater from an aquifer in surficial sediments where dissolved TCE is found. Private residential wells are also found in the sector. The Nelson River and its tributaries drain the sector and flows from west to east. New characterization results and available data were used to develop a numerical model of groundwater flow and mass transport to 1) define geological and hydrogeological contexts, 2) delineate the distribution of TCE and identify its migration paths and 3) evaluate the effect of TCE on the water quality of receptors (Nelson River, municipal and residential wells). In the sector, 30 to 40 m of sediments filling a buried valley form two aquifers separated by an aquitard: an unconfined deltaic aquifer at surface, an underlying silty prodeltaic aquitard and a semi-confined aquifer of deltaic sands and diamictons. Groundwater exchanges between the aquifers are generally downward through the aquitard, but near the Nelson River there is upward flow. Monitoring has led to sparse TCE detections in the Nelson River, regular detections at a mean value of 0.62 μg/L at one municipal well, occasional detections at another well and no detection at the other two wells. No TCE was detected in private wells, which are located outside the migration paths of TCE plumes. The context and numerical modeling with particle tracking and mass transport show the relationships between the two source zones, three TCE plumes and three receptors. Municipal wells pump in the semi-confined aquifer at a level appearing sustainable, but use most of the recharge in the sub-watershed. Areas of contribution to the wells thus cover almost all the study area with a complex pattern. These wells compete with the effect of the Nelson River to drain groundwater flow. Mass transport shows that most of the TCE mass flux from the TCE plumes ends up in the Nelson River, but at low concentrations, thus restricting TCE concentrations in the municipal wells at levels much lower than the maximum concentration limit.
Monitoring subsurface microbial and nutrient transport to assess treatment capability of at- grade septic system designs
By design, septic systems release pathogenic microbes, nutrients and other chemical contaminants into the subsurface and have the potential to adversely impact groundwater quality. Newer at-grade septic system designs discharge wastewater effluent on the soil surface, however, relatively little research has been conducted on transport processes and treatment efficacy for these systems. The objective of this study was to investigate physical, biological, and chemical processes beneath two at-grade wastewater treatment systems. Secondary treated effluent from the Fish Creek wastewater treatment plant in Calgary is being applied to soil through the two at-grade systems in volumes equivalent to a three-bedroom household. A dye tracer was also introduced with the effluent to aid in the evaluation of subsurface flow patterns and the identification of soil sampling locations. An extensive vadose zone monitoring system, consisting of suction lysimeters, tensiometers, time domain reflectometry probes, thermistors, and soil vapour probes, was installed to track the effluent through the soil profile. Fecal coliform, total coliform, and E. Coli, as well as other physical and chemical parameters, are being monitored in-situ. Soil samples for microbial and chemical analysis have also been obtained by excavating portions of the infiltration area beneath the two systems. Chemical and dye tracers showed relatively rapid migration of effluent to depths of up to 1.5 m below surface. Preliminary pathogen results indicate an approximately four log reduction in E. Coli concentrations at 10 cm depth and six log reduction at 60 cm depth. Continued monitoring of these pilot systems will provide valuable data on subsurface pathogen migration and the suitability of at-grade systems for treating wastewater and reducing the risk of groundwater contamination.
Development and field testing of an alternative latrine design utilizing basic oxygen furnace slag as a treatment media for pathogen removal
In densely-populated communities in developing countries, appropriate setback distances for pit latrines often cannot be met. An alternative latrine was designed that incorporates two permeable reactive media to treat pathogens and nitrate from effluent. Basic oxygen furnace (BOF) slag in contact with wastewater effluent elevates pH to levels (> 11) that inactivate pathogens. Saturated woodchip creates reducing conditions that encourage the growth of denitrifying bacteria which remove NO3-. The field application was constructed in Santo Antônio, a peri-urban community located 25 km south of the city of São Paulo, Brazil. A 2-m diameter pit was excavated to a depth of 4 m into the sandy-clay unsaturated zone. A geotextile liner was emplaced to create saturated conditions in the 0.5-m thick woodchip barrier. Above the woodchip barrier, a 1-m thick layer of BOF slag mixed with pea gravel and sand was emplaced. A series of filter layers, grading upward from coarse sand to fine gravel, where placed above the BOF layer, and gravel was also infilled around the outer perimeter of the excavation, to ensure O2 diffusion into the design, the formation of biofilm, and degradation of organic material. A control latrine, constructed with similar hydraulic characteristics and nonreactive materials, was constructed at a locality 100 m away, in the same geological materials. Total coliform, thermotolerant coliform, and E. coli are removed by approximately 4-5 log concentration units in less than one meter of vertical transport through the BOF slag media. In the control latrine, comparable reductions in these pathogenic indicators are observed over three meters of vertical transport. Removal of sulphur-reducing Clostridia, Clostridium perfrigens and somatic coliphage are also achieved in the alternative design, but initial concentrations in effluent are low. Some measurable concentrations of pathogen indicators are measured in lysimeters below the BOF layer, but are associated with low-TDS, neutral water that is infiltrating in from the sidewall of the excavation. Oxygen concentration is augmented (5 mg L-1) in the alternative latrine compared to the control design (1-2 mg L-1), suggesting that conditions for biofilm development are improved. The decline in pH between sampling events after 42 and 82 days of wastewater application suggest that the potential for base release is decreased over time. Somatic coliphage concentrations are 1-2 log concentration units lower in stainless steel lysimeters compared to concentrations measured in adjacent pan lysimeters, suggesting that the filtration of coliphage by the porous cup may negatively bias sampling.
The Use of Reactive Materials in Septic Systems for Pathogens and Nitrate Removal
The developing countries have an urgent need for cheap and efficient techniques for the improvement of sanitary conditions in areas without public water supply and sewerage system, especially in suburban regions or irregular occupation areas, where there is a great lack of social assistance. In this type of situations, the inhabitants use dug wells for water use and cesspits for disposal of sewage, which usually contaminates the groundwater with nitrate and microorganisms. As part of a study aiming to develop new sewage treatment systems in an irregular occupation area located at the District of Barragem, south region of the municipality of São Paulo (Brazil), a conventional cesspit (named as "Control") and an alternative septic system were constructed and monitored for a year. The design of the alternative septic system included a 1m thickness reactive barrier constituted by BOF (Budget Oxygen Furnace - a byproduct of the steel-making industry) for pathogens removal, then 1m sand package where the wastewater is oxidized and at the bottom the wastewater is in contact with a 0,5m thickness reactive barrier constituted by sawdust (carbon source), where redox conditions are very reducing and denitrification and even methanogenesis can take place. The chemical and biological data collected in the alternative septic system showed complete removal of the pathogens in the BOF barrier, then nitrification occurred between the BOF and the bottom of sand package. However denitrification in the sawdust barrier was incomplete because of the high pH caused by the BOF materials, which can reduced the number of denitrifiers bacteria present in the sawdust barrier. Isotope analyses that are been carried out in the residual nitrate will provided more information about the extent of the denitrification reaction in the alternative septic system. In case of the control cesspit, it was observed the occurrence of high concentration of ammonium, dissolved organic carbon, CO2, CH4 and low dissolved oxygen, which means the whole cesspit works like a septic tank, and probably nitrification will occur below the cesspit. It is possible that saturated conditions were achieved within the control cesspit, which reduced the input of oxygen in the wastewater. This study have shown for a better treatment of wastewater, the design should be a sand bed below septic tank for nitrification occurrence, followed by sawdust barrier for denitrification reaction, and then BOF barrier, whose high pH produced will be responsible for microorganisms removal.