Groundwater Sampling Protocol

[cwarf]

I have been charged with reviewing our company SOP for groundwater sampling of monitoring wells.

Our current SOPs provide guidelines for groundwater sampling by bailing and low flow. The low flow SOP follows EPA guidance, so there isn't much to modify. However, the SOP for bailing discusses collecting stabilization parameters between each well volume until the well has stabilized to within 10%. Based on the brief internet research I've done - I found that the purpose of the specified number of well volumes (based on the state the sampling is being conducted in) is to remove the stagnant water within the well formation so that you are sampling "fresh" water. Do any other companies require stabilization parameters to be within 10% for bailing?

Have any companies minimized the practice of groundwater sampling by bailing because of the potential of bad sampling habits by field personnel?

 

[soiledup]

It is standard practise within our company to purge 2.5 times, the volume of the the well to ensure that you are recovering a 'fresh' groundwater sample. However, you honestly do not want to get me started on the bad sampling habits of field personnel because it's : a) not good for my blood pressure & b) would take far too long to explain.

regards

Soiledup

 

[greenone]

Yes, if we are bailing, we require the successive grab samples to be within 10% of each other.
If the well is producing so little that collecting one sample takes all day because it won't recover quickly after purging, we note that in the report, and consider relocaitng the well.

 

[fattdad]

When purging a rapidly recovering well, 2 to 3 times the well volume is a good practice as cited above. Here is the BIG question in my mind: What is a well volume? If you install a 2-in diameter well into an 8-in diameter boring, the well screen is 5 ft long and the gravel pack is two feet above the screen, the well volume should (my opinion) consider the effective radius and length of the gravel pack. The effective radius being the product of the gravel pack porosity and the difference between the well-screen diameter and the hole diameter.

In slowly recovering wells, we would just fully evacuate the well and then consider the inflowing water as formation water.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[cwarf]

Is sampling by bailing a thing of the past? I've been reading websites that really discourage the use of bailing and promote the use of low-flow sampling methods. Some even recommend sampling using passive diffusion (no purge).

We sample for all types of analytes (GRO, DRO, VOCs, Metals, Herbicides, etc). I know that bailing can lead to volatilization of the water, but what about other pros and cons are there?

I feel that low-flow sampling is a great method to use (when executed properly), but it can add significant cost to a sampling event. Do the pros justify the increased cost?

 

[greenone]

It's not a matter of justifying the increased cost. It's a matter of complying with regulatory requirements. We have agencies in CA that require low flow sampling instead of bailing.

 

[mcb02]

The Site I am presently on just completed a study on the cost-effectiveness and accuracy between the standard low-flow sampling and Passive Diffusion Bags. Depending on the number of samples needed to collect per year, the Passive Diffusion Bags can save a bit of money, and have proven (at least in my circumstance) to be as accurate, if not more accurate than low-flow sampling techniques. It is accepted by most governmental and state agencies because you are sampling a discrete interval, especially if the well in question must be sampled at two separate depths.

 

[cfj104]

Just my 2 cents, but we never use bailers for sampling from wells. We use either a bladder pump or grundfos for low-flow sampling.

For low-flow sampling we have guidelines that the measured parameters (DO, ORP, Temp, pH, turbidity...) must be within before we can sample once the readings have stabilized. We use a horiba to collect the water parameters.

Also, being someone who occasionally has to do the field sampling, I highly prefer the passive diffusion bags. They are easy to set-up and retrieve. You are also able to sample at a more specific point (Such as a 1 foot area of a 10 foot screen).

If you do use bailers, be sure to replace the bailer and string after each well to avoid contaminating any future samples.

 

[Thunderthief]

Speaking for personal experience, most of the states where I work (New England) are moving towards requiring low-flow-- nobody's banned bailing yet, at least that I know of, but that may be on the horizon. Things have already got to the point where the MADEP and CTDEP basically put a big fat mental asterisk after results from bailed sampling. I haven't used passive bags yet, but they do sound interesting.

I still use bailers when installing wells-- mostly as a surge block, to shake things loose so I can evacuate as much of the fines as I can. The issue I have with pumps are, well, first it's hard to fit dedicated submersible units into a budget for clients with limited resources (i.e. homeowners) and most of the nondedicated units, such as peristaltic or bladder pumps, have a max depth-- my Cole-Parmer peri pump works like a champ for shallower wells, but the bottom drops out once you get down past about 25 feet. They also have their limitations-- the CTDEP, for example, doesn't like people to use peri pumps for sampling volatiles, since they believe the peristaltic action aerosolizes some of it and skews the results. As far as materials costs, anything you save on bailers you wind up spending on tubing.

I typically try to remove as much water as I can-- personally I agree with the above post by fattdad, and think that 2.5 is an ok rule-of-thumb minimum given that your well is of a certain depth and with a certain type of sandpack, but purging more is always better than not purging enough. I usually try for a minimum of 5, or until the parameters stabilize. If it's a well with a high recharge in a high-sensitivity area, where I need to be certain of what's in the groundwater, I'll leave a purge pump on the thing for a couple hours and purge ridiculous amounts of water- one of the Connecticut regs I really dislike is they don't allow filtering of metals samples, so it's best to get as clean a sample as possible.

SW846 does, however, consider a well to have been sufficiently developed if you pump or bail it dry. Whether the recharge would meet your parameters and data quality objectives is a whole different ball of fish.