Methods detail - sampling
1. Sampling from monitor wells
We use Masterflex battery-powered peristaltic pumps to draw water from wells. We use hoses with 0.25 inch inside diameter and thick polyethylene walls so that suction does not collapse the hose. The hose is polyethylene to ensure that there is minimal sorption of any dissolved chemicals inside the hose during sampling.
A few field measurements are done while sampling from a monitor well.
We insert the hose into the well to near the bottom. The tip is blunted so it does not catch on the inside well joints.
We begin pumping to purge the well to make sure that we withdraw groundwater just entering the well being sampled, not water that has been sitting in the well for a while. (Water in the well is much more open to the atmosphere than water in the aquifer or soil, thus the chemistry may change, and the inside surface of the well is different from what water encounters in gravel, sand or soil thus the microbial community may differ.) We pump the well while measuring temperature, specific conductance, and pH in an overflowing cup at the pump outlet hose. Stability of these parameters is a clue about fresh groundwater entry.
It is important to clean hoses between samples so that there is no carryover from the prior sampling point to the current sampling point. We clean the inside of the hose by pumping through a half gallon of deionized water, and we wipe down the outside of the hose with a paper towel moistened with deionized water. We store the hoses in bags when not attached to the pump, and we change bags between trips. As another way of ensuring that the current sample is not influenced by prior uses of the hose or the cleaning water, the purging process for the current well pumps through plenty of current well water.
While most of the wells sampled in this way were installed by Cornell, in a few cases we use this same equipment and approach with a well owned by the cooperator that does not have its own pump.
There is a specific HowTo page for monitor well sampling.
2. Sampling from owner wells
We draw samples from taps in most cases. We allow the owner’s pump to run for a few minutes to make sure that we are getting fresh groundwater rather than water sitting in plumbing for days or longer. An owner well that has been used actively in recent days does not need much of this kind of purging, one that has been idle needs longer.
The usual field measurements are done while sampling from an owner well, except that depth to groundwater can usually not be measured and we don’t measure temperature et al while letting the pump run before sampling.
There is a specific HowTo page for sampling wells that have their own pumps.
3. Sampling from groundwater-fed ponds
A pond may be sampled by dipping in a container or using a peristaltic pump and hose. When a hose is used, it must be cleaned inside and out with deionized water after each sample, as for groundwater well sampling. Purging is not relevant. We rinse the container with sample water before filling it.
The usual field measurements are done while sampling from a pond. Depth to groundwater is of course zero.
There is a specific HowTo page for technical details of pond sampling.
4. Sampling from lakes
Lake sampling is done with a Kemmerer sampler that allows sampling from a specific depth range. This device can also be used for sampling deep inlets. Inlets can also be sampled with a peristaltic pump and hose, as for ponds.
Volunteers have portable meters for measurements in field. Cornell samplers use the usual Hanna meters.
There is a specific HowTo page for details applicable to Cornell SWL personnel’s sampling. (CSLAP sampling reference).
5. Sample preservation until analysis
The chemical content of a sample can change between when the sample is collected and when it is analyzed, thus we must preserve samples following lab guidance and professional standards. The NYSDEC pesticide lab has always recommended freezing and has confirmed the longevity of known pesticide concentrations for various durations of frozen storage. Until freezing a day or two after the samples are collected, the samples are held on ice in insulated containers or refrigerators. The lake volunteers use their home freezers, and Cornell SWL has commercial refrigerator and freezers.
Aliquots (small sub-samples) for analytical work must be made before first freezing, or frozen samples must be thawed to make them. We strive to thaw a minimum number of times and only for as long as needed to make the aliquots, making all aliquots except the cations one in one session. We make aliquots for cations before first freezing, since they can be stored in a refrigerator instead of frozen. Since the samples arrive a few at a time and some analyses are months away, we defer making the rest of the aliquots (for alkalinity, anions, and pesticides) until the first of these analytical procedures, or shipment to the DEC lab, is imminent. There are economies of labor in working with larger batches of samples for making aliquots.
The detailed cation aliquot acidification protocol is in a HowTo page, and sample storage is in another HowTo page.
Last updated 2023-08-17. sp17 AT cornell.edu