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Pollution Risk Assessment Methods

SubLoad Model

 

This is a simple Excel-based model that predicts nutrient load and runoff for development projects.
MANAGE Method (Method for Assessment, Nutrient-loading, And Geographic Evaluation) This is a Geographic Information System (GIS)-based tool that informs decisions about risks associated with land use. This method utilizes ArcView.


 

SubLoad Model

What is it?
The Cooperative Extension SubLoad model is a simple spreadsheet model used to calculate hydrologic and nutrient loading (Nitrogen only) expected to groundwater based on the developement of small mixed use or residential developments. It is not appropriate for use on major industrial or commercial developements as the loadings associated with these uses are not represented in the model.

Download the SubLoad Model (SubLoadmodel (xls))

For more information please contact Marie Esten, RI NEMO, 401-874-5687 or mesten@uri.edu

 

MANAGE Method

What Is It? a basic description of the method
How Does It Work? an overview of how to complete the method
Pollution Risk Rating Indicators a summary of indicators used in the method
Interpreting the Results an explanation of how the results can help you
Limitations things to keep in mind
Case Studies actual applications of the method
Technical Documentation and Join Tables  

 

What is it?
The Cooperative Extension MANAGE (Method for Assessment, Nutrient Loading And Geographic Evaluation) method is used for risk assessment. It is a Geographic Information System (GIS)-based tool that informs decisions about risks associated with land use. The method uses a series of environmental indicators to rank risks to surface- and groundwater resources. The indicators can be varied based upon the objectives of the user and the characteristics of the site. The results of the model are presented in a variety of formats, including maps and data tables. The focus of the method is on identifying high risk situations that can lead to impaired water quality, allowing for the identification of appropriate management options.

 

How Does It Work?
Readily-available Geographic Information System (GIS) data including streams and ponds, pollution point sources, and soils, are processed using an ArcView tool. The output of this step are summary data about the study area. Summary data might include percentages of land use types on specific soils and percentage of land use types in riparian areas.

This summary data is then input into an Excel spreadsheet model which calculates values for risk indicators such as stormwater runoff, nutrient loading estimates, and impervious surface.

There are additional risk indicators used in the MANAGE method that are not generated by the Excel spreadsheet model. Rather, they are determined by mapping the data and visually interpreting the data. For example, mapping locations of leaking underground storage tanks (LUSTs) and determining which are in areas likely to contaminate the water resource of interest.

Yet another risk indicator option used in the MANAGE method are monitoring data associated with the study area. For example, water quality data for wells, available from RI HEALTH, can support the overall risk assessment.

 

Pollution Risk Rating Indicators
Watershed land use and other natural features used as “indicators” of watershed health were chosen based on their documented relationship to water quality conditions. The MANAGE method always utilizes more than two indicators to avoid over-reliance on one or two factors. This is especially important where inputs are not well documented adding additional uncertainty to the analysis results.

A sub-set of the following indicators are often used:

Land use – Watershed wide
High intensity land use
Impervious surface area
Forest and Wetland
Septic Systems per acre
Riparian (Shoreline) Land use
Riparian High intensity land use
Riparian Impervious surface area
Riparian Forest and Wetland
Disturbed Riparian Area (inverse of Riparian Forest and Wetland)
Existing or potential pollution sources
Mapped pollution sources within study area, within 200 ft buffer to surface waters and tributaries, or within public well inner protected radius (200 ft bedrock well; 400 ft gravel well)
Soils – Risk to groundwater
Very sandy, rapidly permeable soils
Soils – Risk to surface water and/or shallow groundwater
Presence of restrictive layers
High Water Table
Erosion potential
Combined land use/natural features
High intensity land use on highly permeable soils
High intensity land use within shoreline zone
Erodible soils in vacant, unprotected areas
Hydrologic budget and nutrient loading estimates
Phosphorus to surface runoff (lbs/acre/year)
Nitrogen loading to groundwater recharge (lbs/acre/year)
Nitrate-N concentration to groundwater recharge (mg/L)
Other Pollution Sources and Hydrologic Modifications
Point sources, discharges to surface or groundwater ie: salt storage, underground storage tanks, livestock, kennels, etc.
Receiving Water Characteristics
Existing condition of surface water body - Nutrient enrichment level, visual and physical condition, compliance with water quality goal, etc.
Sensitivity to impact of surface water, flushing time, depth, shoreline configuration, etc.
Outflow water quality or well water quality, history of contaminant detects within 5 years
Sensitivity to impact of aquifer – bedrock (low risk) or sand and gravel well (high risk), USGS vulnerability rating, etc.

 

Interpreting the Results
For each indicator a rating of risk is needed. The risk level categories (i.e. low is less than 10%) should be defined at the outset of the project. The outputs from the Excel spreadsheet model, any outside data sources, and visual interpretation of maps are categorized in a way that determines the risk level (low, medium, high, or extreme). Each risk level has an associated numeric value, called a scoring value. The risk level and scoring value are determined for each indicator. For example, if the model indicates that high intensity land use comprises 12% of the study area, then the risk level is medium and the scoring value is 5. The overall risk rating can be determined by taking a sum of the scoring value for each indicator.

The following table highlights the risk rankings used for the indicators discussed above. The values in the table are conservative, in order to highlight water quality threats at a stage where management actions can resolve problems.

 
Pollution Risk Rating and Associated Scoring Value
 
Low
Medium
High
Extreme
 
0
5
10
25
Land use – Watershed wide
High intensity land use
< 10%
10 – 14%
15 – 25%
> 25%
Impervious surface area
< 10%
10 – 14%
15 – 25%
> 25%
Forest and Wetland
> 80%
50 – 80%
20 - 49%
< 20%
Septic Systems per acre
< 0.10
0.10 – 0.23
0.24 – 0.49
0.50 - 1.15
Riparian (Shoreline) Land use
Riparian High intensity land use
< 5%
5 - 9%
10 - 15%
> 15%
Riparian Impervious surface area
< 5%
5 - 9%
10 - 15%
> 15%
Riparian Forest and Wetland
> 95%
80 - 95%
60 - 79%
< 60%
Disturbed Riparian Area (inverse of Riparian Forest and Wetland)
< 5%
5 - 19%
20 - 40%
> 40%
Existing or potential pollution sources
Mapped pollution sources within study area, within 200 ft buffer to surface waters and tributaries, or within public well inner protected radius (200 ft bedrock well; 400 ft gravel well)
Mapped – No specific ranking
Soils – Risk to groundwater
very sandy, rapidly permeable soils < 10% 10 - 60% > 60%
Soils – Risk to surface water and/or shallow groundwater
Presence of restrictive layers
< 2%
2 - 10%
> 10%
High Water Table
< 5%
2 - 20%
> 20%
Erosion potential
< 5%
2 - 20%
> 20%
Combined land use/natural features
High intensity land use on highly permeable soils
< 5%
5 - 15%
5 - 15%
> 30%
High intensity land use within shoreline zone
none
1 - 5%
5 - 15%
>15%
Erodible soils in vacant, unprotected areas
Mapped – No specific ranking
Hydrologic budget and nutrient loading estimates
Phosphorus to surface runoff (lbs/acre/year)
< 0.46
0.47 - 0.68
0.69 - 0.93
>0.93
Nitrogen loading to groundwater recharge (lbs/acre/year)
< 5.4
5.4 - 8.0
8.1 - 16
>16
Nitrate-N concentration to groundwater recharge (mg/L)
< 2
2 - 4.9
5 - 7.9
8-10
Other Pollution Sources and Hydrologic Modifications
Point sources, discharges to surface or groundwater ie: salt storage, underground storage tanks, livestock, kennels, etc.
Not rated, may be mapped
Receiving Water Characteristics
Existing condition of surface water body - Nutrient enrichment level, visual and physical condition, compliance with water quality goal, etc.
Not rated
Sensitivity to impact of surface water, flushing time, depth, shoreline configuration, etc.
Not rated
Outflow water quality or well water quality, history of contaminant detects within 5 years
Trace
< 1/2 MCL1
> 1/2 MCL1
Violation
Sensitivity to impact of aquifer – bedrock (low risk) or sand and gravel well (high risk), USGS vulnerability rating, etc.
Not rated

 

Limitations of the Method
As with any model/method there are limitations that should be kept in mind when applying the model/method. The following are limitations of the MANAGE method:

1.) The Excel spreadsheet model should not be used in predominantly urban areas because surface water in these environments is often controlled by drainage structures, so the runoff component of the model is generally inaccurate in urban areas. In addition, soils are often so disturbed that they do not replicate natural flow patterns.

2.) The scale of the GIS data used in the method needs to be appropriate. If the study area is too small, the data may imply greater accuracy than really exists. For example, a scale of 1:24,000 means that 1 inch on the map equals 24,000 inches or 2,000 feet on the ground. When we zoom in to a larger scale, the data become less accurate and obvious errors begin to appear.

3.) The ArcView tool used to generate data for input into the Excel spreadsheet model is coded specifically for Rhode Island soils. Therefore, it is not appropriate for use in any other state.

4.) As in any model, the quality of the output is only as good as the quality of the input. If your data is old or inaccurate, then it will affect the output products. Although values are reported by the model, it is important to think of them not as absolute values but as estimates.

 

Case Studies

Wickford Harbor: Coastal Watershed Example (PDF, 3098 KB) This example utilized MANAGE to create a management plan for control of pollutant impacts to Wickford Harbor in Southern Rhode Island. Subwatersheds of Wickford Harbor were compared to prioritize mitigation efforts. Indicators such as percentage of impervious surface in the subwatersheds were used to rank the potential to contribute pollutants to the Harbor.
Woonsocket: Urban Buildout Example for a Surface Water Watershed (PDF, 1892 KB) A buildout analysis of a Woonsocket, Rhode Island surface water supply watershed was undertaken. The method for conducting a buildout analysis is highlighted in this example.
  Manville: Suburban Wellhead Protection Area (Coming Soon!) MANAGE was used to update a Source Water Assessment for a Wellhead Protection Area in Northern Rhode Island. This included the use of indicators to rank potential risks to the wellhead and associated groundwater. Indicators included highly permeable soils, the number of potential pollutant sources, and well water contaminant detections.

 

Technical Documentation and Join Tables

Technical documentation of the MANAGE method GIS components as well as join tables and legends generally used in MANAGE are provided on the following page: Mapping Links and Resources.

Background information on the development and use of the MANAGE method are located in the National Decentralized Water Resources Capacity Development Project: Wastewater Planning Hnadbook, Mapping Onsite Treatment Needs, Pollution Risks and Management Options Using GIS, Feb. 2004.       

Copyright 2006 URI Water Quality Program 
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