Coastal ecosystems play an important role in interfacing between marine and terrestrial environments. These systems host complex dynamic processes associated with strong physical gradients between freshwater and saltwater, ground water and surface water, and between high and low permeability soils. Therefore, the structure and stability of coastal environments have a critical impact on environmental resources, ecosystems, and global climate change.

Coastal ecosystems are ecologically fragile but provide important ecosystem services and are often understudied. Physical sampling methods such as coring and well installation provide direct information but are limited in spatial and temporal resolution. Moreover, the disturbance caused by drilling or coring may damage fragile sediment structures causing ecological harm. In addition, these conventional sampling methods are time and cost consuming.

As part of a DOE-funded Phase 2 SBIR proposal, Vista Clara and University of California Santa Cruz (UCSC) conducted in-situ measurements of soil hydrogeology in a salt marsh system (Figure 1) using Nuclear Magnetic Resonance technology.

Figure 1. NMR Investigation at a salt marsh system during high tide. The wells used for NMR investigations are depicted with red arrows. Location A is closer to the water while location B is further from the water.

The Dart 1.75-inch tool was used to measure high-resolution hydrogeologic properties at several monitoring wells installed along the marsh. These NMR measurements are non-destructive, non-radiogenic, and provide high-resolution evaluation and visualization of crucial hydrogeological properties such as water content associated with different porosities, porosity, hydraulic conductivity, and transmissivity. These key properties are measured in real time, thus providing immediate assessment of the studied environment.  Figure 2 shows the results of NMR logging conducted at two different wells. The water content, porosity, and hydraulic conductivity is significantly different between the two locations. Here, location A is closer to the open water and location B is closer to the upland.  We collected variable water filled pore size distributions at both locations. In addition, the total water content at location A is significantly higher from that measured at location B, as expected.

Figure 2. Dart NMR logging results at two locations along the salt marsh. These logs reveal a distinct difference in total water content and variable porosity distributions between the two locations.

Non-invasive investigations are crucial for accurate characterization of coastal ecosystems that are ecologically and structurally fragile. In addition to well logging, we conducted surface NMR measurements to non-invasively study shallow subsurface changes associated with different time points along the tidal cycle. These measurements were conducted using the GMR Flex instrument connected with pre-polarization and small figure eight coils, with sensitivity of up to 2m in depth (Figure 3). These investigations were conducted near the wells at location B depicted in Figure 1.

Figure 3. Surface NMR setup used to collect time-sensitive data.

Figure 4 shows the results of surface NMR experiments conducted at high tide vs. low tide. We were able to detect a significant change in signal decay distribution (Figure 4A) and mobile water content associated with low tide as compared to high tide (dash rectangle, Figure 4B).

Figure 4. Results of surface NMR experiments conducted at two different time points associated with high and low tide at the salt marsh.

The advantage of surface NMR over other tools is that this measurement is done completely non-invasively, it does not require the installation of wells that may destruct fragile coastal ecosystem, thus allowing measurement to be conducted at any chosen location.

This work was supported by US Department of Energy Grant number DE-SC0021480. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the US Department of Energy.