PROCEEDINGS, TOUGH Symposium 2009 Lawrence Berkeley National Laboratory, Berkeley, California, September 14-16, 2009

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MODELING OF HYDROLOGIC AND MAGMATIC INTERACTION AT MASAYA VOLCANO, NICARAGUA

S.C.P. Pearson (1), C.B. Connor (1), W.E. Sanford (2), K. Kiyosugi (1) and H. Lehto (1)

(1) University of South Florida, Dept. of Geology, 4202 East Fowler Avenue,

Tampa, Florida, 33620 USA (2) U.S. Geological Survey, Mail Stop 431, Reston, Virginia 20192, USA

e-mail: spearson@mail.usf.edu

ABSTRACT

The interaction between magma and groundwater can play a pivotal role in volcanic eruptions. The location and style of this interaction is strongly affected by both the local and regional geological structures. By creating TOUGH2 models based on interpretations of magnetic data, and comparing model outputs with self-potential and CO2 profiles, we were able to identify small-scale sealing faults dipping at 60° that redirect upward fluid flow. On a more regional scale, convection within the saturated zone along a 3– 4 km fault on the flank of the Masaya Volcano can explain the diffuse degassing patterns seen at the surface. TOUGH2 models therefore allow us to improve understanding of the volcano-hydrologic system, its surface expressions, and its dominant controls, an understanding vital for improved eruption forecasting.

INTRODUCTION

Interaction between magma and groundwater is an important process in active volcanoes (Hurwitz et al., 2003). If magma heats groundwater directly, an initial phreatic eruption may occur before magma reaches the surface (e.g., Connor et al., 1996). In contrast, boiling of groundwater may create a hydrothermal system that can be in nonexplosive equilibrium with quiescently degassing magma for extremely long periods of time (Ingebritsen et al., 2006). Alternatively, direct interaction between groundwater and magma may result in violent phreatomagmatic eruptions (Morrissey et al., 2000). The Masaya Volcano in Nicaragua provides a natural laboratory for studies of long-term magma-groundwater interaction, because it is characterized by persistent, open degassing and a relatively shallow water table. Synthesis of different datasets is necessary to provide a complete view of a shallow hydrothermal system, even on a local scale. Here, we create TOUGH2 models (Pruess, 1991) of the subsurface in an actively degassing area on the flank of Masaya Volcano, from interpretation of transient electromagnetic soundings (MacNeil et al., 2007) and magnetic profiles. We compare surficial model

outputs with CO2 fluxes and self-potential (SP) measurements to deduce localized structures at Masaya Volcano. SP is a useful, if complex, indicator of fluid flux, because a negative SP anomaly is created by recharge of meteoric water (Sasai et al., 1997), and conversely, the interaction between moving pore fluid and the electric double layer at the pore surface generates an electric potential that causes a positive SP anomaly (Overbeek, 1952). CO2 flux is a direct measure of flow of one component of fluid and may therefore be used as a proxy for the energy of the system (Chiodini et al., 2005). In low-temperature systems, this fluid generally results from boiling of the hydrothermal aquifer, although the porous medium through which the gases travel significantly affects the surface outflux (Chiodini et al., 1998; Evans et al., 2001; Lewicki et al., 2004). We also created TOUGH2 models to attempt to understand the volcano-hydrologic system on a more regional scale. These models allow us to identify possible sources for diffuse degassing (fumarole) patterns observed within a 3–4 km fracture zone on the flank of Masaya Volcano. We can therefore determine configurations of the heat source at depth, valuable in understanding how the groundwater and magmatic systems are interacting and where. This, in turn, can greatly help in forecasting future volcanic eruptions, their magnitude, and their location.

MASAYA VOLCANO

Masaya Volcano is one of the most persistently active volcanoes in Central America (McBirney, 1956; Stoiber et al., 1986; Figure 1). It is located within 20 km of Managua, the capital city of Nicaragua, an area with over 2 million inhabitants. Large, explosive Plinian eruptions within the last 6,000 years (Williams, 1983; Wehrmann et al., 2006) have shown that it has the potential to impact Managua residents, their property, and their water resources (Johansson et al., 1998; MacNeil et al., 2007). The currently active (Santiago) crater has been the site of constant degassing since 1993 (Duffell et al., 2003; Stix, 2007) and has the largest reported noneruptive gas flux at any volcano (Stoiber et al., 1986; Horrocks et al., 1999; Burton et al., 2000;

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Duffell et al., 2003). Gas flux and composition from the vent are very consistent (Horrocks et al., 1999) and imply a magma body of approximately 10 km3 (Walker et al., 1993). The subsurface is highly fractured (Walker et al., 1993; Rymer et al., 1998; Williams-Jones et al., 2003).

Figure 1. Aerial photograph showing the location of

the study area on the flank of Masaya volcano. The dashed line represents the inferred fracture zone (Lewicki et al., 2003). Fumarole zones are highlighted by black circles. The inset is a map of the location within Nicaragua.

There is a subtle NE-trending fracture system extending 3–4 km from the summit crater to Comalito cinder cone, and beyond (Figure 1; Lewicki et al., 2003; Pearson et al., 2008). Elevated SP, CO2 flux, and temperature suggest that this fracture zone is approximately 100 m wide. At least three distinct low-temperature (~65°C) fumarole zones occur along the fracture. The fumarole next to the Comalito cinder cone has some of the highest known carbon dioxide fluxes from low-temperature fumaroles, and the gases retain a magmatic component (Lewicki et al., 2003; St-Amand, 1998). This fumarole responds to changes in volcanic activity at the summit vents (Pearson et al., 2008). These observations suggest that the hydrothermal system at Masaya Volcano is isolated within the Masaya caldera complex and is in equilibrium with the magmatic system. Vaporization of groundwater is on the order of 400 kg s-1 (Burton et al., 2000) and creates a hydrologic gradient about Santiago crater that causes flow of groundwater toward the volcano. The depth to the groundwater table is almost 250 m at Santiago crater and shallows to 50 m at Comalito cinder cone as a subdued reflection of topography (MacNeil et al., 2007). For the fumaroles along the fracture zone to respond to volcanic activity, this hydrologic system must be perturbed.

MODELING

The grid We used the Petrasim interface to TOUGH2 to create models that simulate the interaction of a groundwater-air mixture with magmatic heat. We applied EOS3, using air as a simplified approximation of a magmatic gas heat source. The models comprised 900 cells over a regular 30×30 grid. As the data we were attempting to replicate were collected along profiles, we used only two dimensions, a reasonable assumption for the fault system. To represent fluid flow through a vertical fracture, we took advantage of the TOUGH2 code's ability to include the effect of gravity, and the negative z-direction corresponded to depth. The y direction was one unit cell width. Other parameters (Table 1) were inferred from previous studies by Chiodini et al. (2005), MacNeil et al. (2007), or are typical values for fractured basalt. The models were run for an infinite number of time steps over 3 x 109 s, or approximately 250 years, the last time that the fracture zone saw surface eruptive volcanic activity, and therefore a major change in its configuration.

Table 1. Parameters used in TOUGH2 models.

Parameter Value Units

Density 2000 kg/m3 Porosity 0.3 Wet heat conductivity 1.49 W/mCSpecific heat 840 J/kgC Permeability of fumarole rock 1 x 10-13 m2 Permeability of fault 1 x 10-300 m2 Permeability of fractured rock 1 x 10-10 m2

LOCALIZED MODEL OF FUMAROLE ZONE

Magnetic profiles collected over a 150 m×100 m area at the middle fumarole zone on the flank of Masaya Volcano reveal a positive magnetic anomaly to the NW of the study area and a negative one to the SE (Figure 2a). Given that these are normally magnetized basalts of moderate magnetization, modeling shows that this could be explained by at least one dipping fault within the fumarole zone (Pearson et al., 2009 submitted; Figure 2a,b). SP and CO2 profiles show an increase moving towards the SE, and then a rapid decrease (Figure 2b,c). One fault dipping at 60° would result in just one very high, single peak in both datasets, in contrast to the more gradual, and more numerous, peaks observed in the data. Therefore, TOUGH2 was used to create a more detailed model, where the effects of both groundwater and volcanism could be included to attempt to better replicate the data.

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Figure 2. Geophysical data used to develop boundary conditions for Tough2 models. a) Observed magnetic anomaly (red dots) and calculated magnetic anomaly (solid line) used to infer geologic structure shown in (b). b) Light grey is a shallow, more permeable scoria layer. Arrows suggest the direction of fault slip in the layer below. c) SP profiles collected in 2004. d) SP (blue) and CO2 (green) profiles measured nearby in 2006. e) Surface gas flux output from Tough2 model.

Method To replicate the inferred faults within the fumarole zone, we included a second rock type that was relatively impermeable and created a barrier at approximately 60° to the surface. Although faults are generally seen as conduits, they can also serve as barriers to flow (Caine et al., 1996; Fairley ey al., 2003; Marler and Ge, 2003). We propose that mineral precipitation, clay zones, and gouge along the faults have caused them to inhibit flow. Since all of the data collected were a reflection of the shallow system, only the vadose zone down to 250 m was considered. This was replicated by an equal mixture of air and water, with a single layer of air at the top to represent the surface. Different mixtures of air and water, and stratified variations in the proportions of each, were also modeled. Five hundred meters was encompassed in the models to cover the 100 m wide fault zone and the surrounding rock. The other boundary and initial conditions can be seen in Table 2.

Table 2. Conditions used in TOUGH2 models.

Parameter Fumarole zone Fracture zone Length 500 m 3400 m Depth 250 m 3400 m Bottom BC 100°C

1.037 x 105 Pa 50% air

100°C 3.29 x 107 Pa 100% water

Top BC (fixed) 20°C 1.013 x 105Pa

100% air

20°C 1.013 x 105Pa 100% water

Vertical walls No flow No flow Initial conditions 20°C

Atmospheric pressure 50% air

20°C Hydrostatic

pressure 100% water

The heat source below the fumarole zone is unconstrained. Other than the fact that magmatic gases are detected, nothing is known about the heat source and whether it is (a) magma directly below the fracture zone; (b) hot gases flowing from the crater; or (c) heat being transferred from the crater through the saturated zone. Therefore, we modeled three different bottom boundary conditions to represent these, all at 100°C: (a) heat injection; (b) air injection with an enthalpy of 1.509×105 J/kg; (c) water injection with an enthalpy of 2.676×106 J/kg. Rates of injection were determined experimentally by comparison with measured surface temperatures and gas fluxes.

Results All of the models result in gases rising toward the surface and water sinking to the bottom (Figure 3). Since the temperatures are fixed at the top and the injection cells are at 100°C, the temperature profiles all look very similar. Fluid flux increases moving through the footwall toward the fault zone, becomes negligible in the fault zone, and increases again moving toward the next fault. Across the second fault, there is no fluid input at depth in the model, and a convection zone is developed in the hanging wall, which has much lower flux rates than those associated with rising hot fluids. When heat is injected into the system at 1 J/s (Figure 3a) the maximum temperature is 90°C, and there is some circulation of water toward the faults. Air injection at 1×10-5 kg/s results in a maximum temperature of 42°C, but all of the water is driven out of the base of the model. Water injection at 5×10-7 kg/s results in a maximum, basal temperature of 100°C (Figure 3b). Both water and air rise towards the surface, but their flow is redirected along the faults. For both types of mass injection, higher rates result in unstable models.

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Figure 3. TOUGH2 model outputs showing injection of energy into an air-water mixture, with 2 relatively impermeable faults dipping SE at 60°. Black arrows represent gas flux, and gray arrows liquid flux. (a) heat injected at 1 J/s; (b) water injected at 5 x 10-7 kg/s. Gas flux is scaled by a factor of 200 and 500 respectively for comparison.

A thin, more permeable gravel layer, as suggested by magnetic modeling, inhibits and complicates flow very shallowly. With varying proportions of air and water, the circulation of water varies, but the gas flux at the surface remains essentially unchanged. When a stratified mixture of air and water is used to replicate the vadose zone, with all water at the bottom decreasing gradually to no water at the surface, flow at the surface is inhibited, and small pockets of air become trapped within convection cells of water at depth.

Discussion The TOUGH2 models show an increase in gas flux moving towards the faults, with a decrease over them (Figure 3). This is in excellent agreement with the SP and CO2 profiles (Figure 2). Therefore, low-

permeability fault models of the area appear to be supported, with more than one relatively impermeable fault dipping at 60°. These seal upward flow and redirect it along the underside of the fault. The TOUGH2 models provide some constraints on the temperature source at depth. If air or another hot gas is injected, water is driven out of the base of the model, essentially drying out the system, which we do not observe. Injecting heat into the base of the model does result in feasible temperatures and the correct geometry of gas flux. This could correspond to very shallow magma, something that is unlikely in this system, or conduction of heat from the saturated zone. When hot water is injected into the shallow system, representing transfer of heat through the saturated zone, the surface gas flux again has the correct geometry, and the flux is twice that of injecting heat. This therefore seems more likely. To confirm the heat source and rates, drilling would be necessary, because the maximum temperature and water circulation at depth vary between the two models. The magnetic model suggests a shallow scoria layer with a higher permeability. Our model shows that this does not produce the observed surface gas flux. Therefore, a distinct shallow layer, if present, must have a much more gradual permeability boundary with the host rock below. Varying the proportions of air and water within the system does not make a significant difference to surface gas flux; thus, we cannot use soil-moisture-content data to further refine the models. However, the TOUGH2 models show that air content is not systematically increasing approaching the surface, which is not likely anyway because of inhomogeneities within the subsurface.

REGIONAL MODEL OF FRACTURE ZONE

The distance between the Santiago Crater and the Comalito Cinder Cone along the fracture zone is approximately 3,400 m (Figure 1), and TEM soundings have detected the water table at between 51 and 59 m depth (MacNeil, 2007). The three fumarole areas are spaced irregularly along the fracture zone. There are two primary ways to focus these gases along the fracture: (1) localized heat sources at depth; or (2) convection along the fracture. Field observations provide some clues as to which is the more likely mechanism, but TOUGH2 models help to refine these theories.

Method We created models using the parameters in Table 1 and conditions in Table 2, with variable injection along the bottom boundary. Since the depth to the heat source is unknown, a square grid of 3,400 m was

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used for simplicity for models of the saturated zone. All models were homogeneous. Two different models were tried to replicate localized heat sources at depth. The first included only the vadose zone, with depth down to 55 m and length of 3,400 m. This was similar to the fumarole models, with an equal mixture of air and water, but a layer of air at the surface. Water was injected at 100°C at three points along the bottom. The other model injected heat at 3 distinct points at the bottom of the saturated zone. To see if convection occurs and could produce the configuration of heat and gas flux observed along the fracture, we created a uniform heat source along the bottom boundary. Heat was injected at different rates along the bottom of the saturated zone with a cell temperature of 100°C.

Results When hot water is injected into the bottom of the vadose zone at 1 kg/s, hot plumes rise directly to the surface (Figure 4). However, this only occurs when the other cells of the bottom boundary condition are fixed at 20°C. When they are allowed to vary naturally, heat dissipates. When heat is injected at three distinct points into the base of the saturated zone at 20 000 J/s, three plumes develop with a maximum temperature of 74.5°C. Higher rates result in hotter plumes, and lower rates result in plumes that are too cool or do not even reach the surface.

Figure 4. TOUGH2 model output showing hot water injected into a homogeneous air-water mixture to try to recreate the three fumarole zones observed from vadose zone circulation. Water (gray arrows) and gas (black arrows) rise along plumes and sink between them.

Injecting heat at 1667 J/s uniformly along the base of the model results in convection within the system. This creates three distinct zones of elevated

temperature and fluid flux (Figure 5). The maximum temperature is 65°C. Variations in injection rate result in differences in maximum temperature and in the number of plumes created.

Figure 5. TOUGH2 model output showing heat injected uniformly along the base of the saturated zone. Three plumes still arise.

Discussion

Causes of fumarole zones TOUGH2 models reveal that the most likely source for the three distinct fumarole zones is convection within the saturated zone. If the base of the vadose zone is kept cold in all but three zones, the elevated surface fluid flux and temperature could be a simple reflection of variations in fluid flux at depth. However, even given this unlikely scenario, CO2 surveys at the fumarole zones on the flank of Masaya Volcano show that all three zones have fluxes of between 2,000 and 2,255 g m-2 day-1 that are remarkably consistent over extended periods of time. It is unlikely that all three zones would have entirely separate sources but the same surface gas fluxes. When the saturated zone is included, a comparison with the convective model shows that although three distinct sources create three plumes of heat and gas flux, the more simple uniform heat source also does. Therefore, it is more likely that there is one source, and that convection within the saturated zone is causing the fumarole zones observed at the surface. With a constant heat source at depth, convection can develop in groundwater or in gases above the water table. The development of convection cells depends on the Rayleigh number, in porous media given by [Zhao et al., 2003]:

( )0

02

0

µλβρ HTkgc

Ra p ∆= [1]

Substituting values appropriate for Masaya Volcano, found in Table 3 (Chiodini et al., 2005, MacNeil et al., 2007, and Zhao et al., 2003):

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43.1102.1551018010781.920006.0

5

1042

∗×∗×∗∗×∗∗∗

= −

−−

Ra

The Rayleigh number is 0.13 for gases within the vadose zone. The critical Rayleigh number is given by [Zhao et al., 2003]:

( )[ ]( )223

222232 1

HH

HHRa D

critical

π+=

( ) ( )[ ]( ) ( )223

213

22223

2133 1

HHHH

HHHHRa D

critical +++

=π

where H1 is the length of the fracture, H2 is its thickness, and H3 its height. In this case the fracture is 100 m long, 3,400 m wide (as we are studying circulation dynamics along the fracture and not across it) and the depth to the water table (taken as the limit of the circulation regime) is 55 m. The critical Rayleigh number is 3.8×104 for the 2D case and 55 for the 3D case. Therefore, gases circulating within the vadose zone in the fracture will not advect. Even including a 50% mixture of water, the Rayleigh number is 8.8×103, and so heat will be transferred by conduction rather than advection. If the saturated zone is included, the Rayleigh number is 1.6×105. However, the critical Rayleigh number drops to 40 for the 2D case and 1.2×104 for the 3D case. Therefore, convection will readily occur within a saturated zone 3,400 m×3,400 m with a temperature difference of 80ºC across it. Table 3. Properties used in Equation (1). Where there are two values listed, the first is for water and the second for steam.

Parameter Value water / steam

Units

λ0 Thermal conductivity

1.43 W/m°C

H Depth to Water Table

55

m

k0 Intrinsic Permeability

1x10-10 m2

cp Specific heat 4185 / 2000 J/kgK

ρ0 Fluid density 1000 / 0.6 kg/m3

β Thermal expansion coefficient

2x10-4 / 7x10-4

K-1

µ Dynamic viscosity

1x10-3 / 1.2x10-5 Ns/m2

The effect of permeability Permeability is an extremely important factor in controlling fluid flow in faulted volcanic terrains (Todesco, 1995; Caine et al., 1996; Evans et al., 2001; Manzocchi et al., 2008). In our models, a homogeneous fracture is assumed, with a permeability of 1×10-10 m2. If the lower permeability of the fumarole zone is used, it inhibits flow and prevents convection. However, it is likely that the bulk permeability is higher in this fracture than it is within the fumarole zone, where surrounding, unfractured rock is also included in the models. Caine et al. (1996) show that the response of fluid flux also depends on geometry of a fault or fracture zone. Where the damage width of a fault is negligible compared to its total width, strain is localized and flow is inhibited. In contrast, a damage width close to the total width of the fault causes strain to be evenly distributed and the fault to act as a conduit. It is therefore entirely possible that (in addition to permeability variations) the geometry of the fault and fracture zones is causing the difference in fluid flow. There is some evidence, both conceptual and observational, for lateral heterogeneities along and within the fracture. Magnetic and GPR measurements suggest that there are faults within the 100 m wide fracture zone at various points along its length, which may seal or channel flow, and that the fracture dimensions vary along its length. Previous work by Méheust and Schmittbuhl (2001) and Neuville et al. (2006) have shown that fracture roughness can enhance or inhibit flow, but have not shown it to cause the strength of fluid flux variations that we observe at Masaya. As seen in the fumarole modeling, faults can focus flow into an area, but do not prevent flow on the scale seen at Masaya. Permeability can vary by several orders of magnitude within a fault (Manzocchi et al., 2008), and can form low-permeability zones with channels of high permeability (Marler and Ge, 2003; Fairley et al., 2003), but to create three distinct and comparatively strong zones of elevated flux at the surface, very specific heterogeneities would be required. The fault and fracture systems at Masaya Volcano are more extensive and less focused, suggesting that although heterogeneities are playing a role in channeling groundwater and gas flow, they are not the dominant factor.

CONCLUSION

TOUGH2 models are in excellent agreement with geophysical observations suggesting that within the central fumarole zone at Masaya Volcano, there are multiple shallow faults dipping at 60°. These faults channel flow through the hanging wall and inhibit flow across the faults to the footwall. TOUGH2

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models show that flow of hot gases does not cause the temperature or fluid flux distribution observed, and therefore flow of hot water or heat from the saturated zone is the most likely cause of our surface observations. Magnetic models suggest a distinct, shallow, more permeable layer but TOUGH2 models show that this would inhibit gas flow and not result in the surface fluid flux distributions we observe. Therefore, a much more gradual change in permeability must occur across the different layers. Within the fracture zone linking the active crater with the fumarole zones, TOUGH2 models show that convection within the saturated zone can create distinct fumarole zones, even with a constant, uniform heat source at depth. The three fumarole zones observed at Masaya Volcano may therefore be the result of injection of heat at 1667 J/s into the base of a 3400 m deep saturated zone, causing convection within the saturated zone. TOUGH2 models are found to be a powerful tool to improve understanding of the hydrothermal system at both the regional and the local scales, particularly when combined with geophysical measurements. Knowledge of the geological controls on fluid flow help us to understand any changes in fumarole activity in relation to changes within the volcanic and hydrologic systems as a whole.

ACKNOWLEDGMENTS

We are grateful to staff at Instituto Nicaraguense de Estudios Territoriales. Field assistance from students from the University of South Florida Volcanology field class and from the Centro de Investigaciones Geocientíficas at the National Autonomous University of Nicaragua are also gratefully acknowledged.

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