Please describe your procedure for tying monitoring wells. eg/ do you use GPS, total station, or levels?
> Please describe your procedure for tying monitoring wells. eg/ do you use GPS, total station, or levels?
Personally I have always used a conventional instrument and a level. Higher precision than GPS.
B-)
Typically the horizontal location is less relevant than the vertical to the environmental engineering firm. So we locate the lid either with GPS or a total station and then run levels through the top of casing.
Total station with double and triple checks sometimes. Different setups to the same pairs. In one case we were working around a fairly deep rock quarry with wells both at ground level and on the floor of the quarry. Total station was much more convenient than forty-leven turns with a level.
I have used a combination of all three where there are several and some of them aren't GPSable.
Set a couple of control points and run the receivers static while doing total station ties.
I have run levels from bench marks or used GPS derived elevations depending on the situation.
I might bring in my control via GPS, but all my locations are done with a Robot.
If they are on federal or state property we are required to report elevations to the 0.01’ (we use a digital level for this task) and horizontal to the nearest 1.0’ (We use GPS for this)
Project specifications dictate the approach. I've done some MW jobs where the wells are miles apart and the specs are 3' H and 0.5' V, so I used RTK with 3 separate observations.
Some small-site MW jobs I do call for 1' or 0.5' H with the V expressed to (but not necessarily accurate to) 0.01'. Some of these call for H and/or V relative to the site rather than to a specific datum. I use a total station for these, measuring H to the center of the cover and V to a punch mark on the rim, followed by a measure-down to the casing using a tape and a torpedo level. If they want datum ties, I'll usually do that with GPS and/or a passive bench mark, as appropriate.
California's GeoTracker program has detailed specs that require a datum tie for H and optionally V. That seems to be what I get mostly these days, though I only do a handful a year anymore. I'll use GPS to control H and maybe the V, and a total station for the on-site ties.
There was a time early in my proprietorship when MW work accounted for a big share of my business, but that hasn't been the case for 15 years or more.
> Typically the horizontal location is less relevant than the vertical to the environmental engineering firm. So we locate the lid either with GPS or a total station and then run levels through the top of casing.
This is our proceedure as well. We also locate borings, which are sometimes referred to as "wells", and these we will do with GPS, since it is essentially a ground shot, with no casing.
:good:
I use GPS for project control. Total station for well location/elevation. Elevation requirement is 0.01'. My level always goes along for the ride, especially if wells have been reset. Typically though most project work is for new or additional wells which have to located anyway. Since most times I am alone the total station is key, D&R to monitoring well top of casing in to mark on rim. If it is level work I go to the job when someone is there to sample the wells.
Paul in PA
depends, what they are, there are monitoring wells and there are monitoring wells, some are for a gas station leak and cover about a city block and there are others that are spread over a 100000ac area. One gets total station and levels the others are totally done with GPS (we used to do them with a t2 and Topcon). The funny thing is when they ask for the elevations to 0.01' lol.
You just have to educate the client about that.
We do a few of these
We place a permanent mark, usually a ramset nail, in the concrete surrounding the well head
We gps both the top of the well tube and the nail
We run precise levels through through both the nail and the well tube.
We have an adapter that fits in the top of the well tube that centers the gps pole and provides a good surface for levelling
I always GPS the horizontal location of the well and level the elevations. The 0.01' vertical requirement precludes the use of GPS for elevations as far as I am concerned. If there are existing wells on site I usually try to obtain the old certifications and level between those to verify the vertical datum and then use leveling to transfer those elevations to the new wells. The only time I use the total station is if the horizontal locations are not possible with GPS (wells inside of a building or in dense woods).
Elevations n these to the 0.01' are ridiculous - probably twice the order order of magnitude more precise than the sensors used to detect contaminants in the boreholes. But, the Gubmint form says 0.01'...
We do a lot of piezometers at dams, which are basically monitoring wells where they want to check the elevation of the water surface.
Almost all of them are wide open (i.e. GPS could be used), but it is just as easy to use a total station. BTW, a D/R shot with a total station can easily be as accurate as a level if shots are short. The piezometers are usually at the top, on the face, and at the bottom of the dams, so there are large differences in elevation. Very difficult to run levels to. Total station is the way to go.
I've shot a lot of them over the years, mostly at a landfill. I usually use GPS for them, but if there is some kind of special spec, I will use a level. The truth is,
GPS is almost always adequate for the work, no matter what the spec says.
I've gotten that expunged from projects that I've worked on. I'm sitting in a meeting with a couple of "senior" geologist when the requirement came up. They both burst out laughing, the one says; "I don't know how that crap filtered into these specs, but it's silly, nearest .1' is what I want. Like we can measure it that good anyway".
There usually is a magic marker mark on the rim of a PVC pipe, either inside a larger metal pipe or under a plastic cap and that is the measure point and what is located. The plastic pipe is usually cut crudely and is jagged and the mark is 3/8" wide and there is more than .01' slope across it.
They drop a device into the pipe and if they are within .01' each time I would be shocked, that assumes that the drill hole is plumb and they actually use the measure point and not somewhere else on the rim.
Anyway, unless you are in a tight area and can digital level through the wells then the .01' is a pipe dream.
Speaking of magic marker on the top of the pipe.
The first time I ever encountered a monitoring well was back in the 70s at a manufacturing plant that was closed down due to ground contamination. They had drilled a bunch of wells to get a handle on the solvents that had been dumped in the soil. We had to locate the wells and get grades.
I was making an ink mark on the jagged PVC top so they could refer to our grade point and dropped the cap from my magic marker down the new well. The soil tech saw it and had a fit. So much for that well....
Ken
Same Thing in Bathymetry Survey
A client wanted that our Bench Marks used for RTK observation of sounding data be within 1st order accuracy - ~0.004/km loop. The loop from the primary BM was around 1 km. I remember that our error was around 0.006 m. Client said we need to do a rerun.
My boss countered that the 2 mm was meaningless because the echo sounder has a minimum reading of 0.10 m. Not to mention the error due to sediment deposits.
Sometimes these bookish clients don't really understand the purpose of the errors encountered in different types of work.