SANITARY SEWER
EXFILTRATION &
INFILTRATION RISK
ASSESSMENT
Meredith S. Moore
Penn State MGIS Program
Advisor: Dr. Barry Evans
GEOG 596A, Fall 2014

 Background – explanation of the problem
 Previous Study Methodology
 Objectives
 Approach – identification and incorporation of relevant datasets
 Proposed Methodology
 Goals & Deliverables
 Timeline
 References

Water Quality Perspective: Localized study from
2000 implicated sewage as a pollution threat to groundwater.
Broken sewer pipes allow for exfiltration of sewage into the surrounding soil.
http://www.krwa.org/water-wastewater/

Sewer Utility Perspective: Infiltration of groundwater causes infrastructure capacity problems.
Groundwater infiltration into sewer line documented during CCTV investigation

Elevation Comparisons: Example
Comparison of sewer line and groundwater elevations can be straightforward with survey-grade stream cross section elevation data.

Determining Groundwater Elevation

Scoring Exfiltration Risk

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Determining Groundwater Elevation
Existing regional water table maps for the NC Piedmont are not considered useful due to variable local conditions.
 Available Datasets:
 Flood study: surveyed stream cross section elevations
 Bare Earth LiDAR-derived DEMs for streambed and ground surface elevations
 Well depth and static water level data
 Depth to Water raster developed by USGS in 2001
 Soil Surveys
 National Wetlands Inventory
 CCTV infiltration study data

Survey-grade grade elevation points were collected along cross sections on FEMAregulated streams for a
Mecklenburg County flood study.

DEMs were developed from bare earth LiDAR data that was flown in
2012. Not all parts of the county have been processed. The FEMA stream centerline layers were “burned in” using this surface.

Despite good coverage of wells with static water level data; it is unknown if the water level reflects the level of the surficial aquifer, or the pressure of groundwater within bedrock fractures.
Relevant wells can possibly be extracted using bedrock depth data from soil surveys.

Static Water Levels
 The static water level of wells drilled into water-bearing fractures in bedrock is related to the pressure within the aquifer, not the elevation of the surficial aquifer.
http://www.maine.gov/dacf/mgs/explore/water/facts/aq-01.gif

An existing groundwater elevation surface developed by USGS in
2001 is not intended for site-specific analysis. It is a modeled surface and uses low resolution elevation and slope data.
Redeveloping this surface with recent, high-resolution LiDAR is a potential approach.

The 1980 Mecklenburg
County NRCS soil survey includes areas with shallow bedrock and a shallow water table.

The National Wetlands
Inventory map service provides a reference for locations that can be assumed to have a water table near the ground surface.

Charlotte-Mecklenburg
Utilities has noted the locations of groundwater infiltration during CCTV investigations. These locations can be used to
“test” different groundwater elevation surfaces for accurate predictions.

Incorporating Sewer Infrastructure
Information: Scoring
This table shows the failure potential scores assigned to different pipe materials during a previous study. Higher scores indicate a higher likelihood of pipe failure. If a sewer line had been recently rehabbed, a score of 1 was automatically assigned.
This table shows the scores assigned to different flow volume ranges during a previous study.
Higher scores represent more potential damage that can result from infiltration of groundwater or exfiltration of sewage.

 Develop interpolated groundwater elevation raster and raster using
USGS methods with higher-quality data. Test these two rasters and the existing USGS raster for accuracy with CCTV infiltration data.
 Assign scores to pipes for material type, flow volume, and rehab status.
Consider utilizing other available datasets.
 Select pipes at risk for infiltration due to position below the water table. Develop infiltration risk ranking using pipe material and rehab status scores.
 Score pipes above the water table for exfiltration risk based on their distance above the water table (closer = higher risk score). Develop exfiltration risk ranking using these scores; as well the pipe material, flow volume, and rehab status scores.

 Deliver a prioritized dataset to the engineers that plan pipe rehabilitation and repair scheduling.
CMUD will be concerned with both exfiltration and infiltration risk assessment.
 Provide an exfiltration risk dataset to Water
Quality to support targeted water quality monitoring.

 Continue with analysis through winter and early spring
 Late Spring/Early Summer Conference such as
Esri’s Southeast User Conference

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Bannister, Roger A. (2013). Incorporating Professional Judgment into Groundwater Contouring Tools within GIS.
Provided by Roger Bannister.
Department of Agriculture, Conservation and Forestry. (2005). Sand and Gravel Aquifers. Retrieved from http://www.maine.gov/dacf/mgs/explore/water/facts/aquifer.htm
[Accessed 09/18/2014].
Fels, John E., & Matson, Kris C. (1996). Approaches to Automated Water Table Mapping. Retrieved from http://www.ncgia.ucsb.edu/conf/SANTA_FE_CD-ROM/sf_papers/matson_kris/santa-fe.2.html
[Accessed
09/03/2014].
Mecklenburg County Geospatial Services (2014). Mecklenburg County Well & Flood Study layers [Data files].
Provided by Jamie Metz of Mecklenburg County Geospatial Services.
USDA, NRCC. (2013). Soil Survey Geographic (SSURGO) database for Mecklenburg County, North Carolina [Data file]. Available from http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx
[Accessed 09/03/2014].
USFWS. Map Service for National Wetlands Inventory. Available from http://107.20.228.18/arcgis/rest/services/Wetlands/MapServer
USGS. (2001). Estimated Depth to Water, North Carolina [Data file]. Provided by Silvia Terziotti of USGS.
USGS. (2012). Groundwater Frequently Asked Questions. Retrieved from http://nc.water.usgs.gov/about/faq_ground.html
[Accessed 08/22/2014].