Sewer smell in basement

Am hoping I come to this board having already done most of the "common sense" homework...here's the story:

Moved into a 1957 built ranch with a full, unfinished basement a few years ago. The previous owners had a perimeter drain installed around the perimeter of the basement as they had intentions of finishing the basement.

The perimeter drain empties into (2) sump pits - I'm told (2) pits were necessary because of the length of the drain. These pits pump out into an easement that is over 70' away from the house. One of these pits is connected to the radon fan system we had to have installed after the home inspection during the home purchase process (not sure if that is relevent or not).

I have (2) outbound sewer lines that exit the house through the basement wall about 48" above the basement floor. These lines are both 4" and are the original cast iron drain lines. They empty into our septic tank that is 20-25" away from the house.

The (2) sump pits are in close proximity to where the (2) sewer lines exit the house - they are on the same wall and are all within 15' of each other. So the sewer lines do exit through the ground directly above where the sump pits are gathering the most direct water that comes into the drain system if that make sense.

ONLY when we have very intense rains - a sewer smell invades the basement. The source is these 2 sump pits - so the smell is being brought into the home by the ground water that is rushing into the pits via the perimeter drain.

The furnace/air conditioner grabs the scent and pushes it upstairs unless we turn it off when the scent is in the basement. We currently have a fan set up near one of our basement windows that helps to vent the basement, but it takes some time before the scent is completely gone.

I have made sure that all gutters are draining far away from the house. There was one downspout that emptied right on top of where one of the sewer lines runs from the house to the septic tank, but I have have attached corrigated to that downspout and have it emptying 30' away from the area now.

The slope of our yard over the sewer lines and septic tank does run away from the house. There are a few areas where there are some depressions that could gather water and I could stand to level things out a bit, but I definitely don't have water running towards the house/foundation.

So I've read that there could potentially be crack in my sewer line(s) outside the home and the soil has become contaminated - the rain water could be carrying this down into the perimeter drain and into the home. This is one
explanation that I have not yet been able to look into.

Is there a "test" for this possible cause, or do I just have to replace the outbound lines to see if that fixes it? When doing this repair, do I also need to replace as much of the soil as possible that may have become contaminated?

Any other possible causes I'm overlooking?

Any other information I can share to help diagnose the problem?

As I browsed through the forums (again), I ran across another plausible explanation. In this thread, Mike shares:

This sounds to me to be a classic case of an old septic system drainfield sewer water backflowing into another drainfield line (from the sump pump) whenever the ground is saturated from heavy rainfall. The problem is your soil type. Clay-based soils are notorious for this.
I know of entire old rural subdivisions in this area built with all septic systems in clay soils, and every time that there is a ground-saturating storm (especially hurricanes) everyone's septic system backs up into their home. There is no way that these subdivisions would ever get approval today.
Back to your problem...the best solution in my opinion is to replace (move if possible) or add to your septic system drainfield with new additional lines, preferably with a built-up (18" of sand, for instance) bed above the drainfield for run-off and absorption. Your Health Department septic system technicians should be trained to custom design such systems for that soil type.
A more immediate solution would be to install a check valve on the discharge side of your sump drain line to prevent the sewer water from backflowing into it and getting back into your home.
It sounds like you may have to do a major drainfield replacement or improvement project.
Good Luck!
Mike

Our soil has some pretty decent amounts of clay...

In the above situation, it was proposed that the water is is backing up through the system into the sump pit somehow. In my case, somehow the water is making it's way out of the system into the ground and then into my perimeter drain.

If the drain field is overwhelmed with water, is it possible for that water to be making it's way through the soil all the way into my perimeter drain - from nearly 35-40' away?

If this were the case, I'm wondering if an external french drain installed along the outside of the house between the septic system and the foundation would solve the problem. That way I would be catching the contaminated water before it made it's way into the interior perimeter drain...thoughts?

This is a tricky situation. Before you spend all that time, money and effort to dig up around your home, seal the tops/openings of your sump pits and any drain that runs to them that does not have a p-trap on them. You may find that the odour could be coming from somewhere else ie: the pipes exiting the house in the immediatly location of your sump pits.

I'd try a dye test to see if it is indeed the septic leaking. This test may not be 100% conclusive as dye does not always make it through the soil but if it does show up in the sump pits then you do have your answer...

http://www.prestodye.com/water-trace.html

I would suspect the line to the tank is leaking or, the connection at the tank.
A camera inspection may reveal the problem.

Thanks for the input guys.

Krow - I'm 99% sure that the source of the smell is the perimeter drain pits - and only during extremely heavy rain. But I'm open to considering I could be wrong...are you thinking that the drain lines leaving the house might be cracked inside the basement and allowing the scent in? If that were the case, wouldn't they leak as well? Or am I not completely following?

Redwood - the home inspector did a dye test when we purchased the home, but I only recall him checking to see if it ended up popping up in the yard where the drainfield is located - I don't recall if he double checked the sump pits for the perimeter drain...I'll definitely run a dye test the next time we are getting a heavy rain and see what happens...

Trust me - I don't want to do more digging than necessary, but my wife and I are REALLY ready to solve this thing.

As an aside, one of the first projects we intend to do in the basement (after we get this figured out) is install a full bathroom down there. This will require that we tie into one of the drain lines and also reroute a bit of the drainline plumbing that is coming down from the first floor full bath to clean up the ceiling space a bit. Since all the drainlines are original cast iron, I'm likely going to replace all the interior drain line plumbing to PVC - so I'm not overly opposed to continuing that all the way to the septic tank...

Any other thoughts?

When you do the dye test put something white in the sump so you can shine a light through the water to the white object. It will make detection of the dye easier...

I don't see on that website where you can actually order the dye - am I missing it or is this stuff available at the big box DIY stores?

Any big difference on the colors available?

It is available on line and at some supply houses.
Google the name
Get whatever color you think will show up best.
Or, whatever color you can find

Redwood - follow up question for you. If the line to the septic tank isn't leaking into the ground (this is what we are confirming or not with the dye test), then what else could be causing the scent?

Just continuing to think about the situation while I wait for the next strong rain to blow through and I can run the dye test...

I'm not sure but lets run with the most probable possibility for now...

Gotcha - like I said, I'm open to other possibilities, but I've been down in the basement once a heavy rain has been going for awhile and the scent definitely begins in the pits...I've put my head right down by them and can start to faintly smell it at first and then eventually it gets much stronger.

When I have time I'll draw up some quick layouts of the backyard and also the basement plumbing so everyone has visuals....

Update - we opened up the cleanout and took a peek down the main drain line as it runs out the foundation and towards the tank. Right on the other side of the foundation it appears a joint has been disrupted and is allowing effluent to leak into the ground right next to the house. This is likely getting washed down into the perimeter drain when we get heavy rains...so it appears that some repair work is in order.

Luckily the lines aren't that deep, so I'm going to uncover them and replace with PVC all the way to the tank. Not that much different than what is going on in this thread actually...

While I'm at it, I'm going to replace some lines inside the house as well - questions related to that are over here in this thread.