By Thomas Guengerich, New Mexico Tech
New research by New Mexico Tech professor Dr. John Wilson and doctoral student Katrina Koski sheds further light on the transfer of groundwater between very fast flow paths within karst rock formations and the much larger volume of karst rock with slow flow, with implications for water chemistry and contaminant behavior.
Using funding from the New Mexico Water Resource Research Institute, Wilson and Koski conducted two-dimensional computer modeling to predict how water is transported through subterranean systems and what factors influence the rate and direction of flow.
This project, titled “Computational Fluid Dynamics Modeling of Karst Conduit-Matrix Exchanges with Relevance in Contaminant Transport, and Chemical Reactions,” has laid the groundwork for future field studies, which recently received funding from the National Science Foundation (NSF) and the U.S. Environmental Protection Agency (EPA).
Karst is a geological term that refers to underground formations that resemble Swiss cheese. On a large scale, we refer to karst features as caves or caverns. Generally speaking, karst refers to regions where rock has begun to dissolve chemically or “undergone chemical dissolution.” Karst aquifers are important water sources; they supply water to 25 percent of the United States, and some regions rely almost entirely on such formations; Florida, as an example, gets 90 percent of its water from karst. Also, much of the southern New Mexico Pecos Watershed is karst.
Wilson’s work examines the “hyporheic zone,” which is the area where water flows back and forth from the cave – or conduit – to the karstic rock formations – or matrix. Hyporheic zones and hyporheic flow was first characterized by biologists studying streams where surface water descends underground and later reappears in the stream.
Chemists soon started examining surface hyporheic zones to explain what happens to dissolved solid organics (and other chemicals) as they move through hyporheic zones, as well as looking at how aquifer water mixes with surface water. Now, hydrologists like Wilson and Koski are breaking ground on subterranean hyporheic zones. In fact, Wilson was the first hydrologist to suggest that hyporheic zones could exist at the margin of a karst conduit.
“If water stays only a short time, it’s hyporheic flow,” Wilson said. “Through modeling, we are showing the propensity for deep flow. One interesting finding is how the variation in the karst wall topography impacts hyporheic flow.”
Scallops – or patterned undulations in the cave walls – create eddies in the flow, and hyporheic flows. Many karst conduits are air-filled with a riverlike flow at the bottom. This project specifically examines conduits that are completely filled with water. In some cases, given the right pressure, water can even flow upward through the cave roof into the matrix.
Karst conduits typically have porous and permeable walls. Conduits range in size from building-sized to conduits too small for a diver. Water follows flowlines from high pressure to low pressure. For instance, a flooding event creates high-pressure in the conduit, forcing flow into the matrix. Water tends to flow quickly through the karst conduit and slowly through the matrix – or the aquifer. Conduits respond quickly to precipitation, while aquifers respond – or recharge – very slowly in response to rainfall.
In their modeling, Wilson and Koski accounted for the various characteristics of the karst conduit and the rock matrix, varying water pressures, conduit geometries and flow rates. They also applied standard physics models that describe flow rate – like Darcy’s Law and the Navier Stokes Equations. 3 January 2013 The resulting models show water flow paths in conduit and matrix, as well as the travel time through the hyporheic zone.
As the project’s title suggests, Wilson’s NM WRRI research examines water chemistry and the transport of contaminants. They considered how groundwater in the karst aquifers changes chemically as it is transported through conduits and matrix.
Groundwater contains varying levels of organic and inorganic chemicals – both natural and anthropogenic. As water moves through a hyporheic zone, dissolved chemicals undergo reduction-oxidation – or redox reactions. Water will enter the matrix in one chemical state, travel through the hyporheic zone and return to the conduit in a different state. Wilson and Koski’s models show that water entering the matrix at Point A may stay sequestered longer than water entering at Point B. The longer water is sequestered, the more redox reactions take place.
“We know the chemical reactions taking place,” Wilson said. “There’s a cascade of reactions as the water moves through the matrix. We are looking at how water gets sequestered, and how it is transformed into something less mobile and less toxic.”
In the absence of sunlight and biological factors found on the surface, water becomes anaerobic and then begins to lose other elements. At some locations where long-sequestered water re-enters the conduit, researchers are even finding mineral deposits.
Wilson and Koski are examining these chemical changes that occur in the water.
This simulation depicts a cross-section of flow in a karst conduit and induced hyporheic flow in the surrounding rock matrix that is located both above and below the conduit. The ceiling above the conduit has two large cupolas while the floor is lined with regularly-spaced features called scallops. The upper left (a) depicts relative flow speed (red = fast, blue = slow) with different color scales in conduit and matrix. The upper right depicts the distribution of fluid pressure (color, red = high, blue = low), fluid velocity (arrows), and flow paths in the matrix. The ceiling morphology drives the hyporheic flow deeper into the matrix ceiling above the conduit than the smaller scallops drive flow into the matrix below the floor. With a porous and morphologically complex ceiling and floor, there is an interaction between the floor and ceiling morphology that creates nested hyporheic flow paths in the matrix on the other side of the conduit. The relative age of hyporheic flow is shown in the lower left (color: red = old, blue = new) while in the lower right is the highly variable spatial pattern of relative residence time for the returning hyporheic floor to the conduit from the ceiling (top) and floor (bottom). The illustrated domain is 2m wide and 2.5m tall.
Additionally, the researchers also look at speleogenesis – or cave formation. They are looking at the chemical reactions that cause the karst rock, which can be almost spongy, to disintegrate over time.
“Rock will basically dissolve from the inside-out,” Wilson said. “It’s rotting at depth. We can see the reactions and the enlargement of karst features.”
The next step in the research of karst hyporheic flow and water chemistry will be field studies. Wilson and Koski are working on a field study in Wakulla, Florida, near Tallahassee. Using a $387,000 grant from the NSF, they will scuba dive into the karst terrain. (Actually, they will hire NSF-approved divers because the target location is both deep and difficult to dive.) They will take core samples and install sondes that will measure and transmit data on water pressure, temperature and chemistry. The array of instruments will be the first such observatory dedicated to the study of hyporheic flow and chemistry. Koski, who earned her master’s at New Mexico Tech in 2000, will use that field work for her doctoral dissertation.
“We were selecting an important scientific question for the dissertation, one that had not been answered before,” Wilson said. “Katrina wanted to do her Ph.D. in flowing caves. I’d been doing research in air-filled caves and how gasses exchange. I thought her proposal was interesting.”
Koski also landed an EPA Star Fellowship to support her work. The Fellowship, which includes a stipend for Koski, allowed Koski’s salary from the NSF grant to fund another graduate student, Kenneth Salaz, who originally earned his bachelor’s at New Mexico Tech in 1998 and was Tech’s top undergraduate student that year, earning him the coveted Brown Medal.