Near-Surface Monitoring tools enable both direct and indirect detection of CO2 above underground storage reservoirs. These tools include geochemical monitoring in the soil and vadose zone, geochemical monitoring of shallow groundwater or surface waters, surface displacement monitoring, and ecosystem stress monitoring. The purpose of these monitoring approaches is to detect near-surface manifestations of CO2that may be leaking from geologic storage.
Geochemical monitoring in the soil and vadose zone involves direct sampling of CO2 and its reaction products, as well as sampling for tracers that were injected into underground storage along with the CO2. Geochemical monitoring of groundwater involves installation of shallow monitoring wells for detecting any changes in groundwater chemistry related to CO2 injection. Such geochemical sampling techniques provide valuable direct measurements of CO2 and associated indicators, but characterizing a large area requires many individual data collection points.
Surface displacement measurements are designed to detect uplift of the land surface that may have been caused by CO2 injection within the storage formation. Ecosystem stress monitoring is aimed at mapping vegetative stress that may have resulted from elevated CO2 levels near the soil/atmosphere interface. Remote sensing data can provide highly precise surface displacement measurements and indications of vegetative stress over a large area. However, the data can be a challenge to interpret where site conditions are complex.
Research Agenda and Challenges
Research is needed to develop near-surface monitoring tools for detecting possible releases of CO2 in the vadose zone and in shallow groundwater and surface waters. Its important to note that near-surface measurements complement atmospheric measurements, because natural variations in CO2 levels in the near-surface ecosystem are minimal. Shallow groundwater monitoring is obviously important for protection of underground sources of drinking water. Techniques are needed for monitoring large areas associated with CO2 storage projects.
Pathways for near-surface monitoring research include:
NETL-Supported Near-Surface Monitoring Research
NETL supports projects that are addressing research challenges associated with Near-Surface Monitoring. Examples of projects supporting this key technology include: (1) development of low-cost, autonomous hardware which will detect CO2 in groundwater and automatically process data; (2) development of a network of shallow subsurface sensors to monitor statistical fluctuations of CO2concentrations to detect any excess emissions indicating leakage from a storage site; and (3) development of a procedure for 14C tagging CO2 streams prior to injection into a geologic formation as a method for leakage detection.
The MVA webpage offers links to detailed information on projects performing research in this area.