Patrick Sutton
    Low-Flow Purge and Sample - Turbidity
    Topic posted April 26, 2011 by Patrick SuttonSuper Contributor , last edited January 19, 2012
    1134 Views, 13 Comments
    Low-Flow Purge and Sample - Turbidity

    When collecting groundwater samples using low-flow, how would you recommend collecting turbidity measurements?  I've seen a T-valve used in the tubing prior to the flow-through cell and I've also seen multi-meters equipped with turbidity probes inside the flow-through cell. 

    Thanks, Patrick   



    • Tom Speight

      We use the flow-through cell.

    • Grant Austin

      It's a pain, but I use a seperate nephlelometer. The water quality meter that I often use does not have turbidity measurement capability.

    • schelius

      I've always used a YSI-8260 or 8260XL, but there are other models and brands of WQ meters with flow-through cells which also have a built-in turbidity probe. The flow-through cell is nice, but go with whatever you feel more comfortable with, or even try both and compare.

    • tinaparsh

      The built-in turbidity meters on a YSI are the most convenient, but the downside is they are not as accurate as using a separate WQ meter.  I've often seen the YSIs malfunction or give negative turbidity readings (I think they require frequent cleaning and calibration).  For a separate turbidity meter I'd recommend a LaMotte.

      • Tom Speight

        I had the same problem with the Horiba units.  The equipment vendor we rent WQMs from says those things are a pain to keep on-spec.

    • Patrick Sutton

      Thanks, that's good to know.  I like the idea of using the flow-through cell for convenience.  Maybe we'll rent a multi-meter several times with the turbidity probe to get a better sense of how well it operates.

      Overall, I personally like the ASTM standard for low-flow sampling a lot more than the EPA guidance.  The ASTM doesn't specify how turbidity should be measured (in versus out of the flow-through cell).  However, I noticed that EPA Region 1 and 9 have revised the 1996 EPA guidance for low-flow sampling to specifiy collecting the turbidity measurements before entering the flow-through cell to avoid biases that might be caused by sediment build up in the cell.  I'd rather say that turbidity can be measured in the cell (because it's convenient), but if sediments start to build up in the cell then 1) the pump rate should be lowered, if possible, to reduce turbidity and 2) the cell should be disconnected from the purge tubing and rinsed out while the pump continues to run.  

    • John Dougherty

      we use a turbidity meter outside of the flow cell.

    • Dan Fluman

      Nephelometers are cheap - even a small bench unit that plugs into a 12V adaptor in your vehicle can be used.  Turbidity should be taken prior to and after purging begins.  Once the pH, SC, ORP, DO and Temp have stabilized, the second grab should be taken.  My wells (150+) are set up with TFE tubing perforated 3-5' above the bottom of the well which allows "hookup and go".  The downside is that you tend to see false degradation of WQ over time in slow moving groundwater situations because you are no longer pulling out 3 volumes of water from the casing which helps keep tehe casing clear in most instances (except for Fe/Mn/Mg clogging due to high metal and O2 content).  With this in mind, wells need to be redeveloped every few years. 

    • kevin

      If your a hydrogeologist, remember why you are measuring turbitity in the first place. 

    • Emily B

      I've used both a separate unit (Hach) and a combined flow cell (Horiba).  For the most accurate "snapshot" reading of turbidity with a separate meter, you'll need to grab some water from the tubing before it hits the flow cell.  It can be inconvenient, but it can also help move things along if you've had a particularly turbid stretch at the beginning.  The downside I see to using an in-cell turbidity measurment is that once you have silt (or other "chunks") inside the flow cell, you have to wait for it to work its way out before you get an accurate reading of the newly entering water.

    • Tom Speight

      Has anyone done an a/b test to see what results you get from the different approaches?  E.g. put two tubes down one well, one going to an all-in-one with a flow cell and the other having a separate unit in-line?

      If the separate units give better readings, I should probably switch. 

      Anecdotally-- the most tedious day of field work I ever had was sampling using a WQM with a bladder pump.  They take forever even to fill up the flow-through cell.  TWO HOURS for each well.

    • Jack Siegel, PG

      Horiba U-10 or 20, with a flow-through cell and set your tubing intake depth to the upper 6-inches of the water column. pumping rate should be such that you dont get air bubbles; which means your withdrawl is at or below the rate of recharge. have fun with that.

      • RobE

        Actually, you should set the intake within the upper portion of the screened interval, otherwise you are purging stagnant water above the well screen.  Low flow purging and sampling is meant to avoid that.