What goes on in the subsurface may not just stay in the subsurface

Of all of the themes covered in this year’s Van Tuyl lecture series, one of the more pervasive and intriguing concerns the intersection between two or more disciplines. The multidisciplinary intersection was on the display of hydrogeology and geophysics as presented by Dr. Kamini Singha of Pennsylvania State University. Singha and her team have used innovative approaches to analyze an area of the hydrologic system termed the hyporheic zone.

The hyporheic zone is the intermediate zone between the groundwater aquifers and the actual stream or river and, for the purposes of the presentation, serves as a transient storage for water and particulate material. Studying the hyporheic zone is convenient as it exists as part of the near surface stream system. “It’s fun to work in stream systems,” said Singha, “you can actually see whats happening.” The main goal for Singha and her team has been to determine the approximate storage capacity for these zones in terms of both volume and holding time along with how water actually moves through and interacts with the subsurface environment.

In order to accurately gauge the effects of these environments, models need to be made that mirror what may actually happen in the subsurface. Because these areas are relatively unstudied, for Singha, it meant going out to several stream environments and using geophysical techniques to monitor the movement of water within the subsurface. “We are going to try to make our model fit the field data,” stated Singha, “that way, when we look at our model, it should be somewhat close to what is really happening.” In order to construct these models, Singha and her team utilized tracers in the flow in the form of salts along with electrodes that could detect the higher conductivity allotted by the salinity from the tracer.

The pathways and movement of the water can be seen and, because of this, the holding time of material in the hyporheic zone can be determined. The main problem with the system comes through resolution of data. “When we are near the electrodes we can see clear[ly],” motioned Singha. It is when the data collected is far from the electrodes that the resolution drops off significantly. Though there is less data away from the electrodes, the models afforded the scientists a smoother look at the data or as Singha termed it, “the beer goggles version of the subsurface.”

For the field work, Singha and her team used several different sites. Most notable was the H. J. Andrews Experimental Forest in central Oregon. This forest was perfect for testing, as it has extremely wet winters and very dry summers that allowed for tests at multiple flow levels to be conducted. The first experiment carried out was useful for determining that the flow direction in the subsurface changes, albeit by small amounts, over the seasons. This change allows for water to be kept in the environment for when it needs it and was useful as a first set of data. A much more detailed experiment was carried out at a different watershed within the forest and proved to be invaluable for the data it provided.

When the tracer was added during high flow rates, the salts spread out more quickly through the basin and began to highlight some of the more active area of the hyporheic zone. When the tracer flow was stopped, the salts were flushed out of the system quickly, leaving just a rim of salt. When the tracer was applied to a slower flow rate, it did not spread out nearly as much, but the salt remained in the zone for a much longer period of time. The stream also reacted fairly quickly to the addition of the tracer and the salt was seen immediately in most parts of the system, showing that it did not take much effort for material to jump from the stream into the storage space. “Even through the beer goggles, we can say something about the extent and pathways within the zone,” quipped Singha.

The key lessons learned from this experiment are not surprising, but may have profound ramifications for future hydrologic knowledge. First off, streams dynamically respond to groundwater conditions, it is not just a simple in-out reaction. Second, exchange between these systems depends upon space and time. Different stream systems will have different interactions and how long a material is in the system will dictate how much it infiltrates into both systems. Of the more interesting side lessons learned was that the system does not entirely reset. Over the course of the testing, adding salt to the stream caused the stream’s salinity to increase over the long term. One look at the data that was collected shows the rings of salt still present after long periods of time. This reflection raises questions over the long term impact of pollutants even after they are thought to be removed entirely from a stream system.

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