Shallow groundwater wells are installed at most aquatic field sites to measure high temporal resolution groundwater elevation, temperature, and specific conductance. Sensor-specific details maybe found in the data product documentation on the NEON data portal.
Spatial Layout of Groundwater Wells
Up to eight wells are installed at each field site. The spatial orientation of groundwater wells at each site is informed by topographical and logistical considerations and is meant to capture hyporheic and shallow, unconfined aquifer hydrology and chemistry.
Some wells are placed near the stream, river or lake feature for each site to capture hyporheic flow. These groundwater wells are also placed in close proximity to sensor stations in the water body to enable direct comparisons to surface water temperature and chemistry data. Other wells are located further from water bodies to allow for characterization of the magnitude and direction of groundwater flow, and depending on local conditions, enable sampling of less directly connected groundwater flow paths.
Each well is equipped with a data logger to capture changes in groundwater elevation, temperature, and fluid electrical conductivity every 5 minutes. Data loggers are located 0.50 m from the bottom of the screened interval for wells greater than 3 m total depth, while shallow wells less than 3 m total depth have the data logger located 0.2 m from the bottom of the well screen.
Note the placement of groundwater wells in the schematics below:
One exception to the method described above is NEON permafrost sites (OKSR, TOOK, and CARI) where dynamic water table boundaries present significant challenges in measuring absolute groundwater elevation. This is due to surface elevation fluctuations from frost heave and seasonal fluctuations in the underlying active layer, which is the ground layer above the permafrost that seasonally freezes and thaws.
In permafrost ecosystems, the active layer is a primary ecohydrologic driver, influencing nutrient cycles, biological processes, and downstream water quality. For these NEON sites, key measurements of the active layer including the depth to liquid water from the ground surface, if present, and depth to the active layer thaw are collected weekly. Similar to groundwater elevation in stationary ecosystems, these data are useful for addressing water table related questions of groundwater availability and seasonal variability. As with all NEON sites, data loggers capture temperature and fluid electrical conductivity.
Using Co-located Sampling Methods
In addition to groundwater sensor data collection, groundwater samples are collected twice per year for the same suite of water chemistry parameters collected from surface water samples. Water chemistry samples are collected from a subset of four wells at each NEON site with wells.
For rivers and wadeable streams, the four sampling wells are selected in attempt to cover all of the following categories: upstream, downstream, right bank, and left bank. Preference is also given to wells that are closer to the surface water chemistry sampling locations. For lakes, the four sampling wells are selected with two on the inlet side and two on the outlet side of the lake. Consistent sampling of the same set of four wells at each site is necessary for evaluation of seasonal responses in groundwater constituent concentrations.
Additional Instrument Measurements
Groundwater Related Data Products
- Temperature of groundwater
- Elevation of groundwater
- Specific conductivity in groundwater
- Groundwater and active layer measurements at permafrost sites
Why These Measurements
There are important linkages and feedbacks between groundwater and streams, rivers, and lakes. These groundwater-surface water interactions can critically influence nutrient cycling, temperature regimes, and instream habitats. Collecting data related to these linkages is a integral aspect of better understanding how aquatic ecosystems change over time. For example, seasonal groundwater withdrawals for human uses in agriculturally dominated areas may drive groundwater height and flow direction, which will likely impact surface water level, temperature, and chemistry.