We bought a house built in 1996 with a sand filter septic system. We got the following diagram of the system:

enter image description here

but it might as well be ancient runes to us.

Specific questions:

  1. What does "D.F.S.B.L." mean?
  2. What are the circled discontinuities? Are they just copy artifacts, or do they mean something?
  3. What do the hexagons denote?
  4. I see a label for "Abandoned and capped laterals", but it's not clear to me if there are any. What is the effect if there are?
  5. "Wet" area: I would expect the area where the drainage pipes are to be "wet", but for some reason, the "wet area to be avoided" is completely disjoint from the pipes. Why? What activities am I supposed to be "avoiding" in this area?
  6. What activities should I be avoiding over the area with the septic lines?
  • 1
    The circled discontinuity at the top is a copy artifact. evidenced by nearly all the lines including non septic lines having it. Commented Mar 28, 2022 at 16:46
  • Regarding the wet area, someone already mentioned to put nothing heavy there, but also do not plant trees, etc. They weigh much and their ever-growing roots may damage the field.
    – kackle123
    Commented Mar 28, 2022 at 20:57
  • 1
    @kackle123 I don't see where anyone else said that. In fact, isherwood said that the wet area is not related to the septic at all, which I assume means "no restrictions" (other than "don't put a septic system there").
    – crockeea
    Commented Mar 29, 2022 at 1:16
  • @crockeea isherwood's selected answer (right now) says "Keep concentrated loads off the area. This would include vehicles and heavy equipment, building foundations, etc."
    – kackle123
    Commented Mar 29, 2022 at 13:24
  • That's over the septic lines, as opposed to over the area labeled as "wet".
    – crockeea
    Commented Mar 29, 2022 at 16:20

3 Answers 3


What does "D.F.S.B.L." mean?
Based mostly on context, I'd guess Drain Field Setback Boundary Line (or ...Set Back Line). It would be the practical extremity of the septic system area.

What are the circled discontinuities?
Possibly variances resulting from surveys taken from different points or sources. There would be some error and subjective judgement variation expected.

What is the effect of abandoned or capped laterals?
If anything, collapse due to age or load could result in minor sinking of grade. I won't guess whether it's a concern with respect to septic system operation. You'd want a local expert to assess that.

What are "wet areas to be avoided"?
My speculation is that these are areas with high water table, where septic system function would be impaired or would violate wetland protections.

What activities should I be avoiding over the area with the septic lines?
Keep concentrated loads off the area. This would include vehicles and heavy equipment, building foundations, etc.

Protect the area from frost. If the septic system is at or above frost depth for your area, use heavy mulch or other means to insulate the area, capturing ground heat to prevent freezing.


Adding to @isherwood's answer, I believe that half of the drainfield that the drawing lays out is not actually installed, but is a representation of how the laterals can be arranged in the reserve drainfield in case the original drainfield fails. The section to the left in the drawing appears to be labeled "100% Reserve Area", and the laterals in this section are drawn in dashed lines, as opposed to the laterals in the primary area which are drawn in continuous lines. The word reserve is hard to see, but it's slightly above and to the right of the cedar tree above the "wet area". The two horizontal lines running across the drawing in that area make me wonder if the drawing was printed on two pages, then taped together before photocopying, leading to the apparent disconnects in many of the lines as they cross the horizontal lines.

There appears to be no connection between the primary drainfield and the reserve area, which is why I suspect it's not installed. Normally it would just be shown as a rectangle the same size as the primary field, but in this case it's not obvious that a drain field can be fit into that area with all the obstructions so the designer lid out the plan.

  • Designing a reserve field at the time the original field is designed is quite normal as a design requirement. It is normally only installed when and if needed. The area is reserved (you can't build something else on it.)
    – Ecnerwal
    Commented Mar 29, 2022 at 1:04

I see circles denoting trees. Apparently this extensive system was routed around some large trees. Field lines are not wet if the system is working well. I once had a septic system that was not built as well as yours; one branch went through a low spot , ensuring that it was always damp( I planted asparagus on it so I did not need to ride the mower over it). I can't read the drawing but you may have an aerobic tanks ( air pump) before the sand filter. As large and complex as the field system is ,it was likely in conformance with a relatively comprehensive local ordinance. Septic can be fine ; I have family in western Nebraska sandy land, they do not know where their 40 + year old field tiles are because they never needed any service. I had a large vegetable garden the was partly over one leg and never saw a difference compared to other areas.

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