Wednesday, April 14, 2010

Groundwater Monitoring

I need help with the science of groundwater monitoring. Our existing observation well network consists of 287 (or so) wells that are spacially located across the district via a 6.25 mile hexagonal grid.  Statistical work said this spacing and configuration would give us an acceptable accuracy to describe the water table attitude with the least number of wells to be measured. 

Subsequent statistical analysis confirms that the current network is in fact satisfactory enough to determine, over longer time frames, the areas of the district where water level changes are relatively worse than, or better than, average.  However, it is NOT sufficient enough to monitor the water levels in smaller areas, or for shorter  time frames - which is what we need now in the designated high priority areas.

How then is the best way to improve the accuracy of these smaller observation well networks?  Is it simply a matter of density?  The pundits tell us that all else being equal and with our variability in our aquifer bedrock, it takes approximately 4 times the well density to double our statistical accuracy.  Do we use data loggers or continue with annual tape measurements?

Would an index well (or two or three) be a better approach?  Theoretically, once the index well is data logged, corrected for barometric influences and has a recovery curve applied so that its full recovery level can be ascertained, just a few of these could more accurately reflect what is actually going on in these smaller areas from year to year - and maybe even one if we choose wisely.  But then we have the issue in the short term of comparing new, corrected and massaged data with the existing unmassaged data.  And the water users are not keen on one index well replacing the 9 obs we currently have in this specific area.

I'd be interested in hearing from the science community on any other approaches that would be available for consideration - preferably other approaches that have worked in other places.


  1. Wayne,

    My expertise is in monitoring surface water but I'll give it a shot.

    You have thought out the first step in determining where and how might be the best approach. Your thoughts of the different technologies might give the best accuracy for your time and efforts.

    1st question. What amount of accuracy do you want in the reading? +/- 0.01 ft, 0.05 ft, 0.10 ft? How much range do you want from the instrument (let's say a submersible pressure probe)? Common ranges are 5.0 psi, 10 psi and 15 psi. The 5 psi probe has a usable range of 11.5 ft and has the best resolution. The 10 psi probe has twice the range but the resolution will be less. The 15 psi probe has even less. The trade off is range vs resolution is directly related to my initial question about the accuracy you want.

    2nd question. You mentioned about data logging. What did you have in mind? Data logging can be done in the probe or externally by a dedicated data logger. My question is how much you want to do in the long run? The in situ data logger is inside the probe is the most inexpensive option but very limited as compared to an external data logging solution (that can be used on other projects). Also, how do you want to monitor and maintain them?

    One last thought about equipment. Your district and the farmers it supports will want the best bang for the buck. Factor the O&M costs for Operational and maintenance support. A higher quality of equipment costs more initially, but will usually be more accurate and be more consistent in the long term. Considering that the bulk of your program costs will be in the personnel supporting it, resist the inexpensive stuff.

    Accuracy of your end product (the collected data) rests on a couple of different factors. I've briefly covered the equipment aspect. Accuracy also rests in the knowledge of the people in the process and the consistency of their field discipline (note keeping & methods).

    You will find that with all things, accuracy happens on several levels in data collection.

    It is a collection of knowledge, using good equipment and having field discipline.

    Wait a second? What was the question?


  2. Delbert: Nice to hear from you again. And with so much good information, too. What a bargain! Question 1: We normally get our tape measurements to .01 feet, so I'm assuming we'd need the same accuracy from any logger. When I mentioned logger, I was referring to a slim-line pressure transducer that takes periodic (hourly, daily, etc.) readings (corrected for barometric pressure). These pressure heads are then translated into feet of water above the sensor - hence water level elevation. We at this time have 5 of these surrounding a dedicated index well trying to see how indicative the index well really is, and what kind of an area it can represent.

    While I think the index well will work, I'm not sure how wide of an area it is going to be representative of. If we need 4 index wells to represent a township area, we may as well increase our tape measurement points and continue to kriege or TIN a water level elevation - it'll be less expensive.

    What I don't really know is the statistical accuracy of our current obs well network, or what level of accuracy we need to describe the water table changes in our smaller, high priority areas (tending to be 2-5 townships in size (46,000 ac - 115,000 ac)) in 2-year time frames. I think the current obs well network is +/- 10 feet, so I'm assuming we'd need somewhere in the order of +/- 2.5 feet accuracy to adequately capture changes in the water levels in these smaller areas from year to year. However, I have not yet found a statistician that wants to convince me that this is right or not.

    I will take your final comments to heart as I see that you are exactly right. We are entering new ground in reducing water use in these areas to achieve a specified goal - we had better be able to measure well enough to show what we've really done - not always an easy task.

    The question was....

  3. You can find out a bit of info on how Groundwater Monitor Well Installation works.