The lowering of the earth's surface (land subsidence) has many causes. Two of the most common causes are: (1) removing geologic material by mining, tunneling, and solution cavities; and (2) producing fluids such as groundwater, oil, gas, steam, and geothermal brine. Removal of solid material from depth dominates the first, along with the subsequent bridging of overlying strata that weaken and eventually collapse into the excavation. Gradual compression of small pore spaces over a large region in response to interstitial fluid removal dominates the second cause, along with rearrangement of the particulate structure of sedimentary material that slowly reaches a sturdier, more compact configuration. The second process has plagued Las Vegas Valley (as well as other regions and cities worldwide) for decades. Long Beach, Mexico City, and parts of the San Joaquin Valley have subsided as much as 9 meters due to fluid withdrawal. Land subsidence is the surface expression of cumulative deformation at depth.
NBMG has focused on two aspects of subsidence due to fluid withdrawal. NBMG has been monitoring the subsidence of Las Vegas Valley and analyzing the data for several years. It has become increasingly involved in investigating the complex physical processes that cause subsidence and the related fissures that slowly crack the land surface. NBMG is now recognized as a world leader in understanding these processes and in advancing the state of the science in predicting subsidence and fissuring due to fluid withdrawal.
Recently, a symposium on land subsidence was held during the 1995 Annual Meeting of the Association of Engineering Geologists (AEG). Not only did John Bell and Don Helm of NBMG present papers at the symposium but seven NBMG-supported students and former students at the University of Nevada also attended, and four of them presented papers. All six papers pertained to subsidence. As the symposium convener quipped, "AEG needs a separate chapter in the Proceedings for NBMG contributions."
NBMG contributions to the symposium did not cease there. Don Helm was invited to chair the session on "Fissuring and Modeling." A greater honor was his receiving the award for outstanding paper of the year in the field of environmental and engineering geology, which was presented at the annual banquet of the AEG. The award-winning paper was "Hydraulic forces that play a role in generating fissures at depth" and appeared in the Bulletin of the Association of Engineering Geologists, vol. 31, no. 3, p. 293-302 (1994).
The Fifth International Symposium on Land Subsidence (FISOLS), sponsored by UNESCO and the International Association of Hydrological Sciences, was held in The Hague, the Netherlands, in October 1995. Three oral papers and three poster sessions were presented by NBMG faculty and students. Don Helm chaired the session on "Theory and Modelling" and was invited to give the summarizing (or "crystal ball gazing") address.
A number of interesting new research topics are being addressed by the NBMG subsidence team in addition to monitoring and analyzing data. One new topic is the recently observed fissuring in Stead (north of Reno) and the possibility of its being caused by slow tectonic creep. This represents a newly postulated mechanism by John Bell.
Another new topic is the occurrence over time of a series of parallel fissures on the Yucca dry lake bed on the Nevada Test Site. Each fissure is nearly 2 kilometers long, is narrow and deep, and is separated from its nearest neighbor by about 200 meters. Field tests by the U.S. Geological Survey indicate that the fissures probably reach the top of the regional aquifer at a depth of nearly 500 meters. The earliest fissure was observed more than four decades ago, before both aquifer pumping and weapons testing. Ponded water tends to enter these fissures during rainstorms. These deep vertical fissures greatly enhance the potential hydraulic connection between the regional aquifer and any contaminant that may inadvertently accumulate at the land surface that drains toward the Yucca dry lake bed. The physical processes that cause these fissures are poorly known. When, where, and why they occur are questions being addressed by NBMG. Until these questions are answered, it is impossible to predict if the existing fissures will grow and if new ones will appear. Any temporary storage of radioactive material on the land surface on or near the Yucca dry lake bed should be done with utmost care.
A third new topic is the occurrence of fissures and subsidence near Honey Lake in response to geothermal development. Don Helm has been asked to become involved in a fourth problem that is related to the possible sinking or breaching of barrier islands southeast of Melbourne, Australia, in response to onshore and offshore fluid withdrawal.
Finally, a scientist from Vietnam (Mr. Q.T. Nguyen) has joined the NBMG subsidence research team as a UNR graduate student. He is interested in studying ongoing subsidence of Hanoi due to groundwater pumping. Virtually all freshwater in Hanoi is supplied from subsurface aquifers. Unfortunately, only about 60 percent of the freshwater requirements are currently being met. More groundwater development in expected. However, floods and other serious hazards are already occurring and may reach disaster proportions before subsidence is controlled.
NBMG is also conducting theoretical work on subsidence under Don Helm's guidance. A groundwater hydrologist from the USGS Division of Water Resources (Tom Burbey) developed a three-dimensional subsidence computer code as his doctoral dissertation. The development of this code represents a significant step forward. Additional work regarding, for example, (1) the upward propagation of fissures from depth and (2) treating the deformation of sedimentary material more nearly as the deformation of particulate material than as the deformation of massive rock is getting under way. The predicted results that are based on particulate "fluid" analogy match observed behavior of saturated soils in the laboratory much more closely than was previously possible when an elastic "solid" analogy was used.
Practical hydro-geo-environmental problems are being addressed by NBMG. In the process, a new discipline is being forged. Other disciplines such as groundwater hydraulics, soil mechanics, rock mechanics, geology, seismology, and earthquake engineering have looked along the edges of aquifer mechanics but have largely ignored the fundamentals. NBMG is recognized internationally as being in the forefront of developing the discipline of aquifer mechanics, a new branch of scientific inquiry within the earth sciences.
---Donald Helm, Research Hydrogeologist