-Bernardo Beate, Ecuador
-Zinzuni Jurado-Chichay, United States
-Craig Forster, United States
-Dina Lopez, Canada
-Luz Luna, El Salvador
-Guillermo Moran, El Salvador
-Alfredo Rene Roldan Manzo, Guatemala
-Carlos Pulinger, El Salvador
-Sharon Templeton, United States
The volcano-groundwater interaction study group was charged with the task of identifying hydrogeology-related projects that would complement volcanological research at the Santa María Decade Volcano. Although four project ideas are proposed, none stands alone as a hydrogeology-specific study. Each project, however, supports one or more aspects of the volcanological studies.
Evaluating Explosive Activity at Santiaguito Dome
The overall goal of this project is to evaluate the mechanics of explosive activity at Santiaguito Dome and to assess the probability for explosions to trigger dome collapse. A more detailed description of the overall project objectives are provided by the "Dome Growth" study group.
Objectives of Complementary Hydrologic Studies
The objectives of the proposed hydrologic studies include:
1) estimating phreatic levels in the vicinity of the dome,
2) assessing changes in chemical and thermal conditions within the dome as indicated by changes in groundwater chemistry, temperature, and isotopic signatures
3) estimating the volume of subsurface water passing through the dome,
4) evaluating possible relationships between explosive activity and both short and long-term variations in rainfall.
Relatively routine hydrology studies (spring mapping, groundwater sampling, and rainfall monitoring) and subsequent analyses can provide the information needed to attain the objectives outlined above. Data collection must be closely coordinated with the concurrent volcanological observations (e.g., soil gas and fumarole surveying and sampling, video recording of explosions and eruptions, characterizing eruptive products, and monitoring both seismic activity and dome growth) outlined by the "Dome Growth" study group.
The hydrology studies should emphasize characterizing the input, output, and throughput of heat, fluid, and solutes through the dome area. Precipitation falling within the 1902 crater and adjacent slopes of Santa María volcano likely forms the primary source of water moving through the dome. Estimates of total annualized rainfall and seasonal variations in rainfall can be obtained by monitoring rainfall near the dome (perhaps by locating a rainfall gage near the Hotel de Magermann). An automatic monitoring station that requires little attention throughout the year would provide the detailed record needed to evaluate the way that temporal variations in rainfall might influence variations in explosive activity. Finances and manpower permitting, it would be preferable to have a complete meteorological station established near the dome and a least four automatic rainfall gages deployed on the slopes of the Santa María volcano. Adopting this strategy would provide a better description of rainfall distribution near the dome and the complementary meteorological data (air temperature, wind speed, etc.) that should be collected as base line information for use by those involved in the Santa María Decade Volcano studies.
Total fluid output, and temporal variations in the volume of fluid output, can be estimated by measuring rates of fluid discharge from springs located downslope from the dome and by measuring discharge rates of streams found in the headwaters of the Río Nimá I & II and the Río El Tambor. Difficulties associated with maintaining automatic monitoring equipment suggests that these observations are best made manually during frequent and regular visits by field personnel. Spring and streamflow gaging data would be augmented by estimates of fluid losses occurring through evaporation and explosive activity at the dome.
Rates and patterns of groundwater flow are controlled in part by the phreatic level within both the Santa María volcano and the Santiaguito dome. Locating the elevations of both cold and hot springs near the dome and on the adjacent slopes of Santa María will provide a basis for estimating minimum elevations of water levels in the immediate vicinity of the dome. This information will aid in assessing the depth of eruptive phreatic activity and provide insight into the hydraulic gradients that drive fluid flow through the dome. Obtaining reasonable estimates of fluid fluxes through the dome will assist in computing a thermal energy balance because a significant percentage of heat entering the dome is likely dissipated by advective transport in the groundwater flow system.
Monitoring temperature, chemistry, and isotopic characteristics of several thermal springs located immediately downslope of the dome may provide direct indications of changes in chemical and thermal regimes within the dome that, in turn, may presage important changes in explosive activity. Because anecdotal information suggests that measurable changes in spring temperature and discharge rate will be found on a seasonal basis, it will be important to make frequent observations (at least 20 times per year) to separate dome-related variations from those caused by the background seasonal variations.
Data collected in the proposed study should be used to construct plausible conceptual (and possibly numerical) models of fluid flow and heat transfer at the site of the active volcanic dome. Modeling studies that incorporate a simplified representation of the physical processes should yield insight into the way that different hydrothermal regimes might lead to ultimate dome collapse.
1) The proposed hydrologic studies should be designed and implemented in concert with associated volcanological studies of dome growth at Santiaguito Dome.
2) The monitoring activities should be carried out over at least a two year period in order to provide a reasonable basis for assessing the possible magnitude of seasonally-derived variations in our observations.
3) Regular monitoring should be carried out by INSIVUMEH staff with seasonal assistance from university students and foreign scientists.
4) A minimum effort should include at least twice-monthly visits to manually collect rainfall data, groundwater samples, and measure spring and stream flows.
5) The field-based hydrologic studies should be funded as an integral part of the dome growth studies, or at least as a clearly connected companion study.
6) Follow-up modeling studies might best be carried out by INSIVUMEH staff or university students within training programs supervised by foreign scientists.
Application of Study Results
This hydrologic study does not directly produce an assessment of the probability that explosive activity at Santiaguito dome will trigger a dangerous dome collapse, rather supporting information needed to make such an assessment is obtained.
Baseline Hydrogeology Studies
Baseline studies of the regional and local groundwater flow systems should be implemented to better asses how groundwater flow systems might modify thermal regimes, influence local hydrothermal systems, and influence deep-seated volcanic processes in the vicinity of the Santa María volcano. These studies should form an integral component of a full-fledged sequence of volcanological studies carried out as part of the Santa María Decade Volcano studies.
Relatively routine hydrology studies (spring mapping, groundwater sampling, and rainfall monitoring) and subsequent analyses can provide the information needed to attain the objectives outlined above. Data collection must be designed to maximize the use of existing information available from geothermal exploration activities at Zuñil, Meteorological observations at Quezaltenango, and any groundwater resource studies. A 20 by 20 km region is suggested; extending from Quezaltenango on the east and centered, in a north-south orientation, on the Santa María volcano. Specific field-based tasks should focus on surveying the location, elevation, discharge rate, chemistry, temperature, and isotopic signature of all wells and springs found within the study area. Subsequent analysis would use the data to constrain numerical models of non-isothermal groundwater flow that, in turn, would aid in assessing the way that advective transport of heat by groundwater flow influences both regional and local thermal regimes. In addition to providing baseline information for the Decade Volcano studies, results of this project would assist geothermal exploration activities at the Zuñil geothermal well-field.
The hydrologic studies should be coordinated with other activities that could be included within the hydrology project or carried out within other volcanological studies. For example, detailed geological mapping, fumarole surveying and sampling, soil gas surveys, and shallow heat flow mapping would assist in constraining models of the hydrothermal system.
1) The proposed hydrologic studies should be designed and implemented once it is clear that a full-fledged sequence of vulcanologic studies will be carried out for the Santa María Decade Volcano.
2) Although monitoring is not suggested at first, once the baseline survey is complete and other volcanological data are obtained, it may be appropriate to monitor conditions at one or more locations within the proposed study area.
3) The baseline hydrologic studies could be carried out by INSIVUMEH staff with assistance from INDE staff, university students, and foreign scientists.
4) The hydrologic studies should be funded as an integral and fundamental element of the Decade Volcano program.
Application of Study Results
Although this hydrological study does not directly assist in minimizing volcanic hazards, the resulting data provide information needed to better understand the hydrothermal setting of Santa María volcano; one of 13 volcanoes with sufficient risk of major eruptions to warrant designation as a Decade Volcano.
Slope-Stability Assessments in Areas of Extensive Hydrothermal Alteration
Altered fault rocks similar to those found at the 1991 Zuñil landslide are likely to be found along the fault-bounded sides of steep-walled valley found in the volcanic terrain of Central America. Earthquake activity associated with active volcanism in the region may trigger destructive landslides. Strategies must be developed to map the distribution of areas of extensive hydrothermal alteration that can lead to landslide activity similar to that experienced at the Zuñil geothermal well-field in 1991. Mapping the distribution of altered fault rocks is best accomplished by developing an improved understanding of how current and former hydrothermal systems operate in faulted volcanic terrain.
Strategies for mapping the distribution of hydrothermally altered fault rocks can be developed by defining the landslide hazards along a 10 km portion of the Samalá Valley centered on the Zuñil geothermal well-field where the 1991 landslide occurred. Tasks should include:
-detailed mapping of faults and volcanic features along the Samalá Valley
-detailed mapping of zones of hydrothermal alteration
-mapping of previous landslide scarps
-collecting and analyzing samples of both altered and unaltered fault rocks
-searching for hydrothermal explosion craters
-using topographic maps and aerial photographs to identify areas of steep slopes
-mapping the location, chemistry, temperature, and isotopic character of fumaroles and thermal springs to assess their impact on the development of weak rocks
Developing a useful strategy for defining the hazards map requires integrating an understanding of the patterns and rates of fluid flow in fault-controlled hydrothermal systems. An excellent example of such a system is exposed in cross section at the landslid scarp found at the Zuñil geothermal well-field. Detailed observations of the chemical, thermal, and isotopic character of fluids, fumarolic gases, soil gases, and altered rocks found within the landslide scarp will assist in identifying the geometry and size of regions of altered rock most likely to fail. The fact that fumaroles exposed by the landslide were not identified prior to slope failure, suggests that hidden fumarolic activity may continue to weaken slopes located in similar environments.
1) The proposed study should be implemented immediately in order to take advantage of the fresh exposures exhumed by the 1991 landslide.
2) Mapping and sampling activities should be carried out by INSIVUMEH and INDE staff with seasonal assistance from university students and foreign scientists.
Application of Study Results
This study integrates hydrological, geochemical, and geological mapping activities and analyses within a project designed to help assess and minimize the risk of landslides similar to the one experienced at the Zuñil geothermal well-field in 1991.
The workshop teams had many specific recommendations in the individual summaries, but there are some repeated themes, many of which are also noted in the unanimous recommendations:
*The need for continuous collection of important data is reiterated by many teams, and is vital for anticipating volcanic events.
*The collaboration of visiting specialists with Guatemalans in teams is essential, and such collaborations should recognize and reinforce the fact that effective hazard mitigation will require a leadership role by Guatemalans.
*Effective communication of the significance of scientific results concerning volcanic hazard mitigation to the general public should be part of the mission of interdisciplinary teams, because this is a vital element for overall success of this enterprise.
*The Guatemalan agencies and institutions interested in volcanic hazard mitigation should be strengthened and encouraged to pool their resources and to coordinate their efforts.