|
|
Pollution Risk Assessment
Methods
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?
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.
|
