Ground Water Susceptibility Analysis

Ground Water Potential Susceptibility Analysis Risk Ranking Matrix

Potential Susceptibility = Sensitivity + Vulnerability

Sensitivity is based on water well, aquifer, and recharge factors.

Vulnerability is based on the types and numbers of significant potential sources of contamination and their distance from the well.

Sensitivity Factors and Weighting

Sensitivity factors include well depth, age of well, average ground water velocity in the aquifer, and soil recharge potential.  Each factor is weighted by assigning a percentage of the overall sensitivity value. The Sensitivity Ranking Range is from 1 to 10.  If data for any sensitivity component is unavailable the default will be 10.   

1. Well Depth (50%)

Shallow wells are more vulnerable to contamination than deeper wells that are naturally protected by confining clay layers.  Rankings were assigned to depth ranges.  Wells in the shallowest depth range are assigned a ranking of 10 while wells in the deepest depth range are assigned a ranking of 1, using the ranking system described in Chapter 6. 

Ranking:

18 - 30

Feet

= 10

31 - 51

Feet

= 9

52 - 86

Feet

= 8

87 - 145

Feet

= 7

146 - 245

Feet

= 6

246 - 414

Feet

= 5

415 - 699

Feet

= 4

700 - 1179

Feet

= 3

1180 - 1989

Feet

= 2

1990 - 3354

Feet

= 1


2. Age of Well (10%)

The older the well, the more sensitive.  As wells age, the grouting and casing can deteriorate. Also, the Louisiana Water Well Construction Standards were promulgated in November of 1985.  Wells constructed prior to this date may not be properly completed and may therefore be more vulnerable to contamination.  Deteriorated or improperly constructed wells can provide conduits for contaminants to enter the aquifer from the surface or shallow subsurface.  Age ranges are ranked from 1 to 10 using the ranking system described in Chapter 6.

Ranking:

     1 - 2

years

= 1

   >2 - 3

years

= 2

   >3 - 4

years

= 3

    >4  6

years

= 4

    >6 - 9

years

= 5

  >9 - 15

years

= 6

>15 - 23

years

= 7

>23 - 36

years

= 8

>36 - 56

years

= 9

        >56

years

= 10

 

3. Aquifer Average Ground Water Velocity (25%)

Contaminants will move through aquifers having higher ground water velocities faster than through those with lower velocities.  Aquifers were ranked according to their average ground water velocity using the ranking system described in Chapter 6. 

Ranking:

  • Terrace = 10

  • Jasper (Miocene)*= 8

  • Evangeline = 6

  • Red River Alluvial = 5

  • Mississippi River Alluvial  = 4

  • Chicot = 4

  • Sparta = 4

  • Southern Hills = 4

  • Cockfield = 3

  • Catahoula (Miocene)*  = 2

  • Carrizo-Wilcox = 1

* Shown as Miocene of Central LA on Recharge Potential Map.

4. Soil Recharge Potential (15%)

Soil recharge was based on the Louisiana Recharge Potential Map developed by the Louisiana Geological Survey for DEQ.  The recharge potential for each soil association within the geologic recharge areas is based on such soil characteristics as parent material; subsoil texture, permeability, and drainage; surface slope; and surface runoff.  These characteristics affect the movement of water from the surface, through the soil horizons, and into the underlying geohydrologic systems.  The interpretations are based on the soil characteristics up to six feet below the surface.  Soils overlying the alluvial aquifers have no recharge potential designation on the map but will be assigned a moderate ranking based on soil associations having areas of both high (point bar and natural levee deposits) and low (backswamp areas) recharge characteristics.  Rankings were assigned on a scale of 1 to 10: High recharge potential was assigned a ranking of 10; Medium recharge potential was assigned a ranking of 5; low recharge potential was assigned a ranking of 1.

Ranking:

High - well to excessively drained sands and gravel with rapid rates of infiltration and low runoff potential = 10. 

Moderate - moderate to well drained soils having medium textures and moderate rates of water transmission.  Some areas have both high and low recharge = 5.

Low - poorly drained silts and clays with slow infiltration rates and high runoff potential = 1.

Sensitivity Ranking Examples:

Optimum Water Well (lowest sensitivity)

Depth  = 2000 feet : 1 (ranking) X .50 (weighting) = .50 

Age = 1 : 1 (ranking) X .10 (weighting) = .10 

Aquifer = Wilcox : 1 (ranking) X .25 (weighting) = .25 

Soil Recharge = low : 1 (ranking) X .15 (weighting) = .15 

Total = 1

Worst Case Scenario (highest sensitivity)

Depth  = 25 feet : 10 (ranking) X .50 (weighting factor) = 5 

Age = 62 : 10 (ranking) X .10 (weighting) = 1 

Aquifer = Terrace : 10 (ranking) X .25 (weighting) = 2.5 

Soil Recharge = high : 10 (ranking) X .15 (weighting) = 1.5 

Total = 10

Vulnerability Factors and Weighting

Vulnerability factors include the types and number of potential sources of contamination and their distance from the well.  The ranking scheme for vulnerability was based on sources or causes of contamination of ground water impacting public water supplies in the past, review of ranking schemes in the literature, and the experience of the DEQ Wellhead Protection Program staff.  Potential sources of contamination were ranked as high, medium, or low risk.  The potential sources will then be weighted according to their proximity to the well.

SIGNIFICANT POTENTIAL SOURCES OF CONTAMINATION AFFECTING GROUNDWATER

Higher Risk

  • Abandoned Water Well

  • Above Ground Storage Tank

  • Agriculture Chemical- Formulation/Distribution (pesticide/insecticide)

  • Animal Feed Lots/Dairies (including impoundments)

  • Battery Recyclers

  • Body Shop/Paint Shop

  • CERCLA Site

  • Chemical Plant

  • Class V Injection Wells

    • Motor Vehicle Waste Disposal Wells

    • Industrial Waste Disposal Wells

    • Large Capacity Cesspools

  • Dry Cleaner/Laundromat

  • Inactive/Abandoned Site

  • Military Facility

  • Petroleum (includes bulk plants)

  • Septic System

  • Truck terminal

  • Underground Storage Tank

  • Wood Preserving Plant

Medium Risk

  • Airport

  • Auto/Boat/Tractor/Small Engine Shop

  • Class I Injection Well (Industrial & Hazardous)

  • Class II Injection Well (Oil & Gas)

  • Class III Injection Well (Mining Salt or Sulphur)

  • Furniture Stripping

  • Inactive Water Well

  • Oil/Gas Well and Associated Drilling Activities (including impoundments)

  • Oxidation Pond

  • Promiscuous (unpermitted) Dump

  • Railroad Yard - Switching

  • Railroad Yard- Loading and Offloading

  • Railroad Yard- Maintenance

  • Sand/Gravel Pit

  • Sanitary Landfill (active or inactive)

  • Sewer Treatment Plant (including impoundments)

  • Surface Impoundments (industrial)

Lower Risk

  • Asphalt Plant

  • Car Wash

  • Cemetery
  • Funeral Home

  • Golf Course

  • Hospital

  • Irrigation Well

  • Lumber Mill

  • Metal Plating/Metal Working

  • Nuclear Plant

  • Paper Mill

  • Pipeline Compressor Stations

  • Plant Nursery

  • Port Facilities

  • Power Plant

  • Printing Shops

  • Salvage Yard

  • Sewer Lift Station

Line Potential Sources of Contamination

Railroads, Pipelines and Sewer Lines, Roads, and Hazardous Waste Transportation Routes are Line Potential Sources of Contamination subject to spills and leaks.  They will be rated based on a pertinent number per square mile in the delineated area.  (Also septic systems).


Weighting of Potential Sources of Contamination

A six-tier approach will be used to rate potential sources of contamination according to their distance from the well.  Potential sources of contamination within 1000 feet of the well are considered most critical, and five tiers will be utilized inside of 1000 feet as follows:

Distance from Well Potential Source of Contamination

  High Medium Low
0' - 200' 25 12.5 2.5
>200' - 400' 20 10.0 2.0
>400' - 600' 15 7.5 1.5
>600' - 800' 10 5.0 1.0
>800' - 1000'  5 2.5 0.5
> 1000' 1 0.5 0.1

A graphic example of the tiered weighting system is attached to the end of this matrix.

In the cases where there are two or more wells clustered together, an inventory and a potential vulnerability number will be assigned to each well separately. The potential vulnerability score for each well will be divided by the unit area of the specific delineated area for that well. This will allow for comparison of the relative potential susceptibility totals of all wells in a system and statewide.

A 1000' radius translates to 0.1127 square miles.

A 2640' radius (1/2 mile) translates to 0.785 square miles.

A one (1) mile radius translates to 3.142 square miles.

Large systems will automatically have large totals because of the number of wells.  The potential vulnerability number divided by the unit area eliminates the bias introduced by large systems with a large number of wells.

Vulnerability Ranking Examples:

Potential Sources of Contamination >1000 feet from well:

6 USTs = 1.0 (weighting) X 6 = 6.0

1 Car Wash = 0.1 (weighting) X 1 = 0.1

1 Dry Cleaner = 1.0 (weighting) X 1 = 1.0

1 Boat Repair Shop = 0.5 (weighting) X 1 = 0.5

Total = 7.6

Potential Sources of Contamination within 1000 feet of well:

3 USTs 300' from well = 20 (weighting) X 3 = 60

2 USTs 500' from well = 15 (weighting) X 2 = 30

1 UST 120' from well = 25 (weighting) X 1 = 25

1 Car Wash 700' from well = 1 (weighting) X 1 = 1

1 Dry Cleaner 800' from well = 10 (weighting) X 1 = 10

1 Boat Repair Shop 600' from well = 5 (weighting) X 1 = 5

Total = 131

Potential Vulnerability = Vulnerability Score / unit area of SWPA

Assuming both are a 1-mile radius SWPA:

7.6 / 3.142 = 2.4

131 / 3.142 = 41.7

Initial Potential Susceptibility Analysis for Each Public Water Supply System

An initial potential susceptibility analysis, or Phase I analysis will be conducted for each water system when the assessment is complete.  As previously mentioned the comparative, or Phase II, analysis will be conducted when all assessments are complete.  The Initial, or Phase I, analysis will consist of:

1. A sensitivity analysis will be conducted for all PWS wells based on the Sensitivity Scale of    1 - 10 developed for the Matrix:

 

 

Sensitivity Ranking

Range

Low

1 - 3

Medium

4 - 7

High

8 - 10

2. The relative vulnerability of the system to potential sources of contamination will be given as the number of high, medium and low risk potential sources of contamination identified within 1000 feet of the well and greater than 1000 feet from the well within the source water protection area.  Potential sources located within 1000 feet of the well are assumed to pose a higher risk due to their proximity to the well.

Example of How the Final Potential Susceptibility Number is Determined for a Well

POTENTIAL SENSITIVITY RANKING NUMBER  8
POTENTIAL VULNERABILITY RANKING NUMBER  + 6
TOTAL 14

For a one well water supply system, this is the potential susceptibility number for that ground water system.

For a two well system, if the second well has a potential susceptibility number of 16, the water system potential susceptibility number is 14 + 16  = 30/2=15. 

The logarithmic formula in Section 6.3.2 of the document is then applied to the range of ground water public supply system potential susceptibility numbers statewide for a final comparative ranking between 1 and 10.  

Sample Ground Water System Source Water Protection Area (1000' Radius)


ADDENDUM 1

Revision to SWAP GW Susceptibility Analysis

January 14, 2000

The SWAP GW Susceptibility Analysis did not include provisions for line features.  We identified roads, railroads, and pipelines as line features from databases that need to be included in the analysis.  In addition, we will be including a septic tank density in the area representing a two-year time of travel based on average aquifer system velocities. Below is a revised scheme including these features. 

Currently for ground water, sensitivity and vulnerability are weighted equally.  This will not change.  What will change is that the vulnerability number will be derived from ground-truthed features (GTF), line features from databases (LFD) and a septic tank density number (STDN).  Each of the three line features named above will account for 4% of the total of the vulnerability number (2% each of the susceptibility number).  Septic tank density will account for 4% of the total of the vulnerability number (2% of the susceptibility number). GTFs will account 84% of the total of the vulnerability number (42% of the susceptibility number). 

The calculations will be as follows: 

The LFDs will each be represented as a density per square mile of each delineated SWPA as calculated using the most current US Census Tiger file.  For example, if there are 10 miles of railroads in a 4 square mile SWPA, the density would be 2.5 miles of railroads per square mile.  The same SWPA could have 20 miles of highways resulting in a density of 5 miles of highways per square mile.  There could be 4 miles of pipelines in this SWPA resulting in a density of 1 mile of pipeline per square mile.

Finally, the septic tank density inside the two-year time of travel could be 16 per square mile. 

After all assessments are completed, the above densities for EACH line feature and the Septic Tank Density would be ranked from 1 - 10 using the logarithmic formula.  For example, the above railroad (RR) density might rank as 5.  The highway (HW) density might rank as 5 and the pipeline (PL) density might rank as 7.  The septic tank density might rank as an 8.

We might assume using the example from Appendix R of the SWAP Document that the GTF number is 643.3 pts per square mile (resulting in a ranking of 6 using the logarithmic formula) and that the Sensitivity Number for the well is 8.  The calculation for the final susceptibility ranking would be performed as follows: 

Sensitivity Ranking Number is:                    8 and has 50% weight

Vulnerability is derived as follows: 

            GTF Ranking number is  6 and has 42% weight

            LFD (RR) Ranking Number is  5 and has 2% weight

            LFD (HW) Ranking Number is  5 and has 2% weight

            LFD (PL) Ranking Number is  7 and has 2% weight

            Septic Tank Density Ranking  Number is 8 and has 2% weight

Therefore: 

Sensitivity                                                         8 * .50             =            4.0

Vulnerability               

             GTF                                         6 * .42            =            2.52

LFD (RR)                             5 * .02            =            .10

             LFD (HW)                           5 * .02            =            .10

             LFD (PL)                             7 * .02            =            .14

            Septic Tank Density                8 * .02            =            .16

             Total Vulnerability                             =            3.02

TOTAL SUSCEPTIBILITY NUMBER is 4.0 + 3.02 =  7.02

The logarithmic formula would be used to rank this total susceptibility number from 1 - 10 after ALL Total Susceptibility Numbers are determined for ALL GW SWPAs.

Minor Aquifers 

There are 4 minor aquifers in the State of Louisiana sourced by public supply wells where no hydrogeologic data are available.  Based on their lithologies and consultation with U.S.G.S., default values from similar aquifers of the same age will be used.  One of the aquifers is of Miocene age and the other three are of Eocene age.  The Eocene Age aquifers are most commonly recognized as confining units.  These ?aquifers are actually sand stringers present within the clay of the confining unit.  Due to their limited lateral extent and thickness the well yields are low; all factors indicative of low ground water velocity.  The following are the default values: 

Minor Aquifer

Default Major Aquifer

2-yr TOT (ft)

Rank

Castor Creek (Miocene)

Jasper

968.8

8

Cook Mountain (Eocene)

Wilcox

62

1

Cane River (Eocene)

Wilcox

62

1

Naborton (Eocene)

Wilcox

62

1

The entries in the data dictionary will be as follows; 

Castor Creek Aquifer

            Aquifer Name:              Castor Creek

            Aquifer System Name:            Miocene Series

Cook Mountain, Cane River, or Naborton Aquifer

            Aquifer Name:              Cook Mountain, Cane River, or Naborton

            Aquifer System Name:            Eocene Series

The Miocene Series default numbers are 968.8 ft. TOT and Rank = 8

The Eocene Series default numbers are 62 ft. TOT and Rank = 1

ADDENDUM 2

 Revision to SWAP GW Susceptibility Analysis

July 24, 2000

The SWAP GW Susceptibility Analysis and its addendum dated January 14, 2000 did not include provisions for oil and gas wells.  In the previous addendum, we identified roads, railroads, and pipelines as line features from databases that needed to be included in the analysis.  In the January 14, 2000 addendum, we also included a septic tank density in the area representing a two-year time of travel based on average aquifer system velocities. Below is a revised scheme including oil and gas well density as determined by the Louisiana Department of Natural Resources SONRIS database.  

As in the previous addendum, the ground water sensitivity and vulnerability will be weighted equally.  What will change is that a new category, oil / gas well density will be added to the vulnerability calculations. The vulnerability number will be derived from ground-truthed features (GTF), line features from databases (LFD), a septic tank density number, and the new category: oil / gas well density number.  Each of the three line features from databases, septic tank density number, and the oil / gas density number named above will account for 4% of the total of the vulnerability number (2% each of the susceptibility number). GTFs will account for 80% of the total of the vulnerability number (40% of the susceptibility number). 

The calculations will be as follows: 

The LFDs will each be represented as a density per square mile of each delineated SWPA as calculated using the most current US Census Tiger file.  For example, if there are 10 miles of railroads in a 4 square mile SWPA, the density would be 2.5 miles of railroads per square mile.  The same SWPA could have 20 miles of highways resulting in a density of 5 miles of highways per square mile.  There could be 4 miles of pipelines in this SWPA resulting in a density of 1 mile of pipeline per square mile. The septic tank density inside the two-year time of travel could be 16 per square mile.  Finally, the SWPA could have 20 oil / gas wells resulting in a density of 5 per square mile. 

After all assessments are completed, the above densities for EACH line feature, the septic tank density and the oil / gas well density would be ranked from 1 - 10 using the logarithmic formula.  For example, the above railroad (RR) density might rank as 5.  The highway (HW) density might rank as 5, the pipeline (PL) density might rank as 7.  The septic tank density might rank as an 8.  The oil / gas well density might rank as a nine.

We might again assume, using the example from Appendix R of the SWAP Document,  that the GTF number is 643.3 pts per square mile (resulting in a ranking of 6 using the logarithmic formula) and that the Sensitivity Number for the well is 8.  The calculation for the final susceptibility ranking would be performed as follows: 

Sensitivity Ranking Number is:                    8 and has 50% weight 

Vulnerability is derived as follows: 

            GTF Ranking number is                      6 and has 40% weight

            LFD (RR) Ranking Number is            5 and has 2% weight

            LFD (HW) Ranking Number is            5 and has 2% weight

            LFD (PL) Ranking Number is            7 and has 2% weight

            Septic Tank Density Ranking  Number is                    8 and has 2% weight

            Oil / Gas Well Density Ranking  Number is                    9 and has 2% weight

Therefore:

Sensitivity                                             8 * .50             =            4.0

Vulnerability               

            GTF                                         6 * .40            =            2.40

LFD (RR)                             5 * .02            =            .10

            LFD (HW)                           5 * .02            =            .10

            LFD (PL)                             7 * .02            =            .14

            Septic Tank Density                8 * .02            =            .16

            Oil / Gas Well Density 9 * .02   =            .18

             Total Vulnerability                             =            3.08

TOTAL SUSCEPTIBILITY NUMBER is 4.0 + 3.08 = 7.08

The logarithmic formula would be used to rank this total susceptibility number from 1 - 10 after ALL Total Susceptibility Numbers are determined for ALL GW SWPAs.  

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