A NUMERICAL CLASSIFICATION SYSTEM TO DETERMINE
	OVERALL SITE SUITABILITY FOR SUBSURFACE WASTEWATER DISPOSAL
October, 1987

Donald C. Hoxie, PE*			Russell C. Martin, PE*		David P. Rocque, SS*

The State of Maine abandoned the traditional percolation test in 1974 and replaced it with a system of site evaluation to determine suitability for subsurface wastewater disposal.  These evaluations are performed by individuals licensed by the Department of Human Services.  In 1986 there were 10,037 permits issued for subsurface wastewater disposal systems including new and replacement systems.

THE MAINE SUBSURFACE WASTEWATER CODE

The Maine system is based on identification and description of the soils using the United States Depart of Agriculture classification terminology.  Parent material and texture determine the wastewater application rate and the depth to seasonal high water table (SHWT), impervious layer or bedrock determines the elevation of the bottom of the disposal field and the suitability or unsuitability of the property.  Undeveloped properties with less than fifteen inches of original soil above the limiting factor are considered unsuitable for any form of subsurface wastewater disposal.

The U.S. Soil Conservation Service rates 93% of Maine soils as having “severe” limitations for subsurface disposal with less than 2% rated as having “slight” limitations.  Table 1 illustrates the soil types by parent material and the percentage of systems installed on these soil groups in 1986.

Table  1.    	                  Percentage of State and Systems by Soil Parent Material

Parent Material		Percentage of State		          Percentage of Systems
	
Glacial Tills			77.0					63.5	
Lacustrine & Marine		  9.5					  9.2	
Stratified Drift			  8.5					22.1	
Organic			  3.4					  0.0	
Alluvial			  1.3					  0.1	
Mixed Origin			  0.5					  5.1	

D.C. Hoxie, Director; R.C. Martin, Program Manager: D.P. Rocque, State Site Evaluator; Maine Department of Human Services.

Table 2.	Percentages of State and Systems by Soil Limiting Factor
Limiting Factor	Percentage of State	             1986
Percentage of Systems	
Shallow to Bedrock (<48*)	           19.4	          13.0	
Well Drained (SHWT>48*)	           30.5	          22.0	
Mod. Well Drained (SHWT48-15*)	           22.9	          57.0	
Somewhat Poorly Drained (SHWT15-6*)	           22.1	            7.0	
Poorly  & Very Poorly Drained (SHWT<6*)	             5.0	            1.0 	

THE NEW SYSTEM VARIANCE PROCEDURE

Because of the diminishing number of sites with “suitable soils” and the belief that other site characteristics should be taken into consideration the Department developed the New System Variance procedure.  This procedure assigns points to various site and system design characteristics and sets a minimum passing score of 50 points, with 65 points required for properties in Shoreland Zoning areas, and 75 points for lots in proposed subdivisions.  Properties not meeting the requirement of original soil over limiting factor are judged by this system.  Table 3 includes the current point assignments.

Table  3.	Current New System Variance Criteria
	
Soil Profile              2, 3, 7                      1, 8, 9            4                 5,6,11                10
Soils Points                 15                            10              7                     3                     - -	
Depth to SHWT(cm)  35      33     30     28     25           23-15         Less than 15
Drainage Points          20      15       9       6       3               0                 -   -	
Size of Hectares     +4      4. 0. 2. 4    2.0     1.6   1.2    0.4    0.2        <0.2
Property Points       20             15        10       8       6      4        2        -10             --	
Terrain Position             Knoll            Sideslope             Lowland              Depression
           Points                     5                      3                         -5                      - -	
Waterbody Distance(s)            +76               75-46           45-30                  Less than 30
Setback      Points                       5                    3                  0                          - -    	
Water Supply                  Type            Public                Drilled       Other    Resource Protection
Zone                                Points              5                           3             0             - -  	
Type of        Type   Comm<378       Comm  378-1135     SFD   Comm 1139-2838  Comm>2838
Development Points           3                               3               0                          -5                    -10 	
Design           Volume                           Minimum+66%             Minimum+33%            Minimum
Flow               Points                                         10                                     5                               0 	
Separation      Depth                             Minimum+100%            Minimum+50%            Minimum
Distance         Points                                          10                                    5                               0	
Additional Technique      Sand Filter      Peat Filter     Curtain Drain    Loam Liner  Press.
Treatment  Points                    5                     5                         5                     3                 3	

ll new system variances are reviewed by the Department.  Since the program’s inception in 1981 almost 2,000 variances have been considered with 81% approved and 19% denied.  The failure rate for these systems has been no higher than for all new systems; approximately 1 in 1000.  The number of variance applications has increased dramatically in the last year, up 67% over 1986.  Applications for the first quarter of the current year suggest that new system variances will be up by 44% in 1988.

Table 4.    Number of New Systems Variances by Year

Fiscal year
(Jul 1 – June 30	
Total	
Approved	
Denied

1982	

274	

227	

47	
1983	277	216	61	
1984	208	155	53	
1985	282	248	34	
1986
1987

Totals

1st Qtr.  1988
	313
324

1878

  196

	254
424

1524

  167	59
100

354

  29

	

OVERALL POINT CLASSIFICATION SYSTEM

In response to the increase in the number of new system variances and our good experiences with these systems, we are proposing a point classification system to be used for all properties.  This system would base the suitability for building on an assessment of the overall site instead of a single parameter, depth of original soil to limiting factor.  As envisioned, this system would incorporate all the current approved new system variances as systems in full compliance with the rules.  Minimum point totals would be established for individual lots (65 pts.), subdivision (70 pts.), shoreland lots (75 pts.), and sand and gravel aquifers (80 pts.).

Limiting Factors in the Soil

The important limiting factors are seasonal water table, restrictive layer, and bedrock.  The first two elements of the classification system assign points based on the depth of original soil present above the limiting factor(s).  More points are assigned for deeper soil strata assuming a greater degree of treatment before the effluent reaches the groundwater table.  Figure 1 is a graphical representation of the points assigned in Tables 5.1 and 5.2.

Table  5.1  Limiting Factor – Seasonal High Water Table or Restrictive Layer

(cm)
Depth To 122+ 122-110  109-99  98-86  85-76  75-66  65-56  55-46  45-33  32-25  24-15

Points          20          18           16         14       12      10          8         6         4          2         0

Table 5.2  Limiting Factor – Bedrock

(cm)
Dept 122+ 122-114  113-104  103-97  96-89  88-79  78-71  70-64  63-56  55-46  45-38

Points     20         18            16          14       12        10        8          6         4         2         0

Property Size

Decreased overall development density is achieved by assigning higher point values for larger lots.  No attempt is made to determine lot size by soil type due to the extreme variability of soils within the boundaries of typical house lots.  These points are assigned in Table 5.3.

Table 5.3    Size of Property
(he)		4.0-	3.6-	3.2-	2.8-	2.4-	2.0-	1.6-	1.2-	0.8-	0.4-
Average +4.0 	3.6	3.2	2.8	2.4	2.0	1.6	1.2	0.8	0.4	0.2
______________________________________________________________________________

Points       20	18	16	14	12	10	    8	   6	    4	    2	   0

Position in the Landscape 

Table 5.4 attempts to encourage development on sloping sites which usually have better drainage.  This is illustrated by Figure 2.

Table 5.4    Terrain

Position	Knoll			Upland		Sideslope	Lowland	Depression

Points		   6			    4		       2		       0		         -

Setback from Waterbodies

The minimum setback by rule is 30 m (100 ft.).  Table  5.5 awards more points at this horizontal setback distance is increased.

Table  5.5  Waterbody Setback
 (m)
Distance  To		+70		70-56		55-41		40-30		Less Than  30

Points			  10		    7		    3	                0

Property  Line  Setback

This criteria was included to encourage greater separation between individual disposal fields and to reduce the potential interference with abutting properties during construction.  Table 5.6 assigns these points.

Table 5.6      Property Line Setback

Distance To	
+30	
30-26	
25-21	
20-11	
10-06	5-3	Less than 3
5-3	Less than 3

Points
	  
   5	    
  4	 
   3	   
  2	  
   1	  
  0      -	

Total  Wastewater  Flow

This section provides a slight penalty for large cluster systems.  With a greater wastewater flow concentrated in a smaller area they present a greater potential for outbreak and groundwater contamination than individual systems.  Table 5.7 assigns these points.

Table  5.7    Total Wastewater Flow
						
(I/d)
Volume	
Less Than 1100	
1101-2200	
2201-2300	
3301-4400	
4401-.5500     5501-7500

Points	
          15
	
       12	
       9	
       6	
        3                     0	

Table  5.8                         Design Wastewater  Flow

(I/d)
Volume	
Min+70%	
Min+55%	
Min+40%	
Min+25%	
Min+10%     Min
	
Points	      10
	        8	        6	        4
	      2                0	

Separation Distance from Limiting Factor

Table 5.9 assigns points for an increased vertical separation distance between the bottom of the disposal field and the limiting factor.  This will provide for more treatment of the wastewater prior to contact with the groundwater table and will in part make up for the lack of depth of original soil in the less well drained conditions.

Table 5.9               Separation Distance from Limiting Factor

(cm)
Depth	
Min+40	
Min+32	
Min+24	
Min+16	
Min+8      Min
	
Points	  10	   8	  6	   4	2	0

Type of Water Supply

Table 5.10 recognizes the lesser potential for water supply contamination when an off-site source is utilized, and the greater security of a drilled well over that of a dug well.

Table 5.10            Type of Water Supply

Type	Off-site Water Supply	On-site Drilled Well	On-site Dug Well
	
Points
	                5	              3	               0	

Width of Disposal Field

This section is designed to encourage narrower disposal fields to provide a) an increased sidewall to bottom area ratio to minimize the problem of compaction, smearing, and dirty stone;  b) a greater soil profile area downslope to intersperse and treat the wastewater: and c)  generally less fill and reduced opportunity for differential settling.

Table 5.11	Disposal Field Design

(m)
Width	
Less Than 1.5	
1.5-2.5	
2.6-4.0	
4.1-5.0	
Greater Than 5.0

Points	
       10	
   8	
   6	
    4	
          2

Additional Treatment

The last portion of the classification system provides additional points for site modification on properties with perched seasonal water tables and the use of pretreatment technologies to provide a higher quality effluent reaching the disposal field.
        
Table 5.12                   Additional Treatment

Technique	Sand/Peat Filters
Profiles 4,5, 6 & 8	Curtain Drains
Profiles 4,5,6 & 11	Nitrate Removal

Points	
             5	
             5	
         10

The following table is an illustration of the application of the point classification system described in this paper to a sample five lot subdivision.  The system clearly shows which lots are well suited for on-site wastewater disposal in their native state and which lots will need to use additional design factors to achieve acceptable scores.  It is anticipated that this process will be of great use to local planning boards in evaluating subdivisions.  Instead of simply knowing that  all proposed lots “pass” they will be able to identify the marginal lots and either eliminate them or require additional on-site engineering in the system design.

Table 6.           Application of Table 5 to a Five Lot Subdivision

Point
Group	
Lot #1	
Lot #2	
Lot #3	
Lot  #4	
Lot  #5
	
Limiting
Factor	SHWT @ 30cm
(2  pts)	SHWT @40cm
( 4     pts)	SHWT @ 50cm
( 6   pts)	SHWT @ 80
(12  pts)	SHWT>122cm
(20  pts)
	
Dept to
Bedrock	BDRK @ 60cm
(4  pts)	BDRK @75 cm
(8  pts)	BDRK @ 78cm
(8  pts)	BDRK @115cm
(18  pts)	BDRK> 122cm
(2o  pts)
	
Size of
Property 	0.8 HE
(3  pts)	1.0	HE
(4  pts)	0.9 HE
(4 pts)	0.5 HE
(2  pts)	1.1	HE
(4  pts)
	
Position  in
Landscape	Upland
(4  pts)	Upland
(4  pts)	Sideslope
(2  pts)	Sideslope
(2  pts)	Sideslope
(2	pts)
	
Water Body
Setback	> 70m
(10  pts)	>70m
(10  pts)	>70m
(10  pts)	60m
(7  pts)	60m
(3	pts)
(4	
	
Property
Line	20m
(2  pts)	10m
(1 pt)	5m
(0  pts)	6m
(1  pts)	25m
(3  pts)
	
Total
Waste Flow	2000 LPD
(12  pts)	2000 LPD
((12  pts)	2000 LPD
(12  pts)	2000 LPD
(12  pts)	2000 LPD
(12  pts)
	
Site  Points	37	43	42	54	64
	
Design
Flow	MIN + 70%
(10   pts)	MIN + 55%
(8  pts)	MIN + 40%
(6  pts)	MIN +25%
(4  pts)	MIN + 10%
(2  pts)
	
Separation
Distance	MIN + 40cm
(10  pts)	MIN + 32cm
(8  pts)	MIN +24cm
(6  pts)	MIN + 16cm
(4  pts)	MIN + 8cm
(2  pts)
	
Water
Supply	Drilled
(3   pts)	Drilled
(3  pts)	Drilled
(3  pts)	Drilled
(3  pts)	Drilled
(3  pts)
	
Design
Width	1.Om
(10  pts)	1.Om
(10  pts)	1.	Om
(l0 pts)	1.	Om
(10  pts)	2.	Om
(10  pts)
	
Additional
Treatment	NONE
(o   pts)	NONE
(0   pts)	NONE
(0   pts)	NONE
(0   pts)	NONE
(0   pts)
	
Engineering
Points	
33	
29	
25	
18	
16
	
Total
Points	
70	
72	
67	
72	
80	

In summary we believe our point classification system for all lots utilizing subsurface wastewater disposal is a more thorough, rational, and logical method for determining site suitability than our present method of looking only at the depth of original soil over the limiting factor.  This classification system will provide for better overall land utilization, protection of the natural environment, and a careful integration of the disposal field with the unique characteristics of each site.