Methods detail - analytical

This page is about how we measure the physical or chemical state of water, while at a sampling site or in a Cornell or NYSDEC laboratory. This provides a second level of detail for certain facets of water measurement.

1. Field measurements: water level

We use Solinst “sounders” to measure where the water table is below the rim of the Cornell monitor well, and when possible an owner well.

Model 102	Model 102 mini

These meters work by making a sound and flashing a LED when a weighted probe tip on their cable touches water. The user raises and lowers the probe in the well to zoom in on where the sound first begins. The cable is graduated in millimeters above the probe tip, and holding it against the rim of the well establishes how far down the probe tip is below the rim.

We call the well rim (without well cap) elevation the datum, a reference point. The distance of the datum above or below the land surface can be measured, allowing converting the depth-below-rim value to depth-below land surface.

A monitor wells sampling HowTo page covers operation of these devices.

2. Field measurements: chemical and physical

We measure pH, temperature, and specific electrical conductance (EC) using handheld meters by Hanna Instruments.

Low range (0-3999 μS/cm)	High range (>4 mS/cm)
Model 98129	Model 98130

These meters are identical except for the 98129 reporting in microsiemens (μS/cm), topping out at 3999 μS, and the 98130 reports in millisiemens (mS/cm). We have one site whose EC is typically above the maximum 3999 value of the 98129, sometimes above 5 mS/cm on the 98130. These meters report specific conductance which is expressed as if the water temperature were 25 degrees C. Conductance increases with temperature.

We immerse the meter in a cup containing water drawn from the well, pond, or lake. While purging a well, we let water flow through the cup with the meter immersed and observe changes in the quality of water being brought up. When the measurements stabilize, particularly temperature and specific conductance, that is an indicator that the pumping is drawing in fresh groundwater instead of water that has been sitting in the casing for a while.

A detailed HowTo page covers operation.

3. Cornell lab measurements: anions, alkalinity, cations

These secondary analyses all provide context for the pesticide measurements.

3.1 Anions

We measure four anions: nitrate NO₃^-, sulfate SO₄^-2, phosphate PO₄^-2, and chloride CL^- using an ion chromatograph (IC). Cornell SWL staff operate the IC as guests at a USDA lab facility on the Cornell Ithaca campus.

This is covered in a HowTo.

3.2 Alkalinity

Combined with pH, alkalinity tells us carbonate (CO₃^-2) and bicarbonate (HCO3^-) anion concentration.

We measure alkalinity with a handheld Hanna HI775 freshwater alkalinity meter, using a ~12 mL aliquot. We measure lab pH in the aliquot before transferring the liquid into a vial for the alkalinity meter. Hanna provides a calibration checking liquid. The full procedure is in a HowTo.

3.3 Cations

An analytical chemist colleague at the same USDA lab on the Cornell campus where we do anion testing analyzes samples for a range of cations such as sodium and calcium. The procedure is described in a HowTo.

4. NYSDEC lab: pesticides and breakdown products

(need methods narrative and perhaps a picture from NYSDEC lab)

The NYSDEC lab routinely analyzes many samples for the State’s pesticide regulatory and enforcement program, for many different pesticides. The lab is very well equipped and staff highly experienced, and the data quality are the most reliable of any measurements in the project.

Many of the lab’s tests on water samples are performed with a liquid chromatograph coupled with a tandem mass spectrometer (Deeper). Analytes are tested in groups having common preparation steps. The machine can typically determine concentrations down to 0.1 or 0.01 micrograms per liter (μg/L) reliably in water samples, the level depending on the analyte.

Last updated: 2023-08-17, sp17 AT cornell.edu.