Grantee Research Project Results
2012 Progress Report: Evaluation of Sanitary Sewers as a Source of Pathogen Contamination of Municipal Water Supply Wells
EPA Grant Number: R834869Title: Evaluation of Sanitary Sewers as a Source of Pathogen Contamination of Municipal Water Supply Wells
Investigators: Bradbury, Kenneth R. , Borchardt, Mark , Gotkowitz, Madeline B
Institution: University of Wisconsin - Madison , Marshfield Clinic Research Foundation , Wisconsin Geological and Natural History Survey
EPA Project Officer: Page, Angela
Project Period: June 1, 2011 through May 31, 2013 (Extended to December 31, 2013)
Project Period Covered by this Report: August 1, 2012 through August 1,2013
Project Amount: $598,580
RFA: Advancing Public Health Protection through Water Infrastructure Sustainability (2009) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
Objective:
This project investigates the sources and transport pathways of human viruses from a presumed source in near-surface sanitary sewers to deeply cased municipal water-supply wells. The research objectives are 1) to quantify the presence of pathogenic viruses in groundwater near urban sewer systems in hydrogeologic settings appropriate for loading groundwater; 2) to establish correlations between virus presence and sewer characteristics such as age, construction, materials, depth, and overall condition; 3) to evaluate the transport pathways of pathogenic viruses to deep supply wells; and 4) to use numerical modeling to develop estimates of the amount of sewer exfiltrant reaching groundwater and the probability of contamination of nearby water supply wells, and to extrapolate this information to other areas. Work on this project began in the fall of 2011 after notification of the award was received in mid-July, 2011. The first year of the project included site selection, site characterization, monitoring well installation, and two months of groundwater sampling.
Progress Summary:
Year 2 Work Effort
The second year of the project, August 2012 – August 2013, focused on groundwater sample collection and data base development. Samples were collected twice per month from sixteen monitoring wells and from the sanitary sewer system. Six municipal supply wells were sampled once per month. Surprisingly, there were relatively few delays due to winter weather or equipment failures. All wells were sampled four times for major ions and three times for environmental isotopes. Monitoring at each site included recording groundwater levels at 20–minute intervals with pressure transducers. The recording interval was shortened to once per minute for several weeks of the project period. Sample collection was completed in June, 2013.
Work effort during July and August 2013 shifted to sample analysis for viruses and total coliform at the USDA-Agricultural Research Service Laboratory (ARS) in Marshfield, Wisconsin. Data compilation included development of a comprehensive sanitary sewer database, including pipe material, length, depth, and age of mains and laterals in the two municipalities within the study region. Precipitation records, as well as sanitary and storm sewer flows were compiled for the period of interest.
Preliminary results
Preliminary laboratory analyses indicate 17 of 455 groundwater samples, 3.7%, were positive for human enteric viruses. Of the 24 wells sampled during the project period, nine were virus-positive once and four were positive twice. Eleven of the 24 wells were not virus-positive during the study period. This differs markedly from previous studies of this groundwater system, in which 47% (70 of 148) samples collected from six municipal supply wells were virus positive (Bradbury et al., 2013. Source and Transport of Human Enteric Viruses in Deep Municipal Water Supply Wells. Environmental Science & Technology, 47[9]).
Figure 1. Daily precipitation, depth to water table and dates of virus-positive samples.
Discussion
The relatively low detection rate from this study may be related to precipitation extremes during the study period. As shown in Figure 1, the study area experienced drought conditions during the summer of 2012 and high precipitation totals in the spring of 2013. The virus-positive rate increased from less than 1 percent (1 out of 130 samples) from June through September to 7% (16 of 202 samples) from September through February. Surprisingly, there were no virus positive samples among the 105 samples collected in March through mid-May, a time with precipitation and high water table conditions.
These results suggest that more than one temporally variable physical process affects sewer exfiltration. Figure 2 illustrates the potential role of water table elevation relative to the sewer main. Under condition A, if the water table exceeds the elevation of a gravity-fed sewer, infiltration from groundwater to sewers would occur, rather than exfiltration. Such conditions may have occurred over a large portion of the study region in the Spring of 2013, under very high water table conditions. However, force mains, several of which are present in the study area, are a potential source of exfiltration under high or low water table conditions (C and D). This hypothesis is currently under evaluations using groundwater elevation data collected for this project and the GIS of sewer elevations from the study area.
Figure 2. Cross sections showing possible locations of a sewer relative to the water table. A: gravity drain sewer below water table; B: gravity-drain sewer above water table; C: force main above water table; D: force main below water table. H1 and H2 represent hydraulic head inside and outside the sewer. Arrows show directions of potential sewer leakage.
Although speculative in nature, the low virus detection rate in groundwater and conditions of very high and very low water table elevations warrant additional analysis of conditions shown in Figure 2A and 2B. Virus transport may be affected by a flushing mechanism within the vadose zone that is sensitive to infiltration and depth to water. As illustrated in Figure 3A, drought conditions with little to no infiltration may result in leaked sewage remaining in the vadose zone. Infiltration may initiate transport of raw sewage to the water table, but transport through the vadose zone limits transport to groundwater (Figure 3B). High recharge and water table rise can result in sewer leaking directly to the groundwater system (Figure 3C). However, the notable lack of virus detections during high water table conditions during spring 2013 (see Figure 1) suggests the conceptual model shown in Figure 3D, where water table conditions can be so high that sewer exfiltration ceases.
Figure 3. Conceptual models of recharge, sewage exfiltration, and groundwater infiltration. A: drought conditions with sewer leakage remaining in the vadose zone; B: Infiltration flushes some leakage to the water table; C: Recharge raises water table and sewage leaks directly to saturated zone; D: Water table elevation exceeds that of the sewer, exfiltration ceases and groundwater infiltrates to sewer.
Future Activities:
Work on the project continues this fall with data compilation, statistical analyses, and numerical simulations of groundwater flow and transport at the field sites. The spatial and temporal relationships between virus positive samples, precipitation, groundwater recharge, and the elevation of sanitary sewers and the water table are of particular interest.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 6 publications | 3 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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Gellasch CA, Bradbury KR, Hart DJ, Bahr JM. Characterization of fracture connectivity in a siliciclastic bedrock aquifer near a public supply well (Wisconsin, USA). Hydrogeology Journal 2013;21(2):383-399. |
R834869 (2011) R834869 (2012) |
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Gellasch CA, Wang HF, Bradbury KR, Bahr JM, Lande LL. Reverse water-level fluctuations associated with fracture connectivity. Groundwater 2014;52(1):105-117. |
R834869 (2011) R834869 (2012) |
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Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.