FIRST NATIONAL BANK OF WHITE SULPHUR SPRINGS, MONTANA

THERMAL WATER WELL GEOLOGIC REPORT

SUMMARY AND ANALYSIS

By Darrel Dunn - Hydrogeologist

1978

INTRODUCTION

The purpose of this report is to present the geologic and hydrologic data obtained from the First National Bank of White Sulphur Springs thermal water well, hereafter called the Well. The Well is located near the southeast corner of the First National Bank property which is north of Main Street in the northeast of the northeast of the northeast quarter of section 13, T9N, R6E, Meagher County. This report is based on the information obtained from the Well and a brief inspection of some pertinent published geologic reports. I will briefly describe the information obtained from the Well and present an analysis of that information.

LITHOLOGY

No samples were caught for the portion of the hole from the ground surface to 35 feet. Consequently, no description is available for this interval. From 35 feet to the total depth of the hole, 875 feet, the subsurface material was predominately gray mudstone with varying amounts of pyrite. Some of this mudstone was soft, but most was very hard because it was very well indurated. The most indurated mudstone could have been called argillite. In the interval between 168 feet and 265 feet, the subsurface material was approximately 50 percent silica-cemented quartz sandstone. A few thin beds of sandstone are present above this section and also below it to about 400 feet depth. Yellow, orange and reddish brown clay was reported in the vicinity of 500 feet depth. Considerable pyrite was found associated with all of the subsurface materials; it occurs as aggregates disseminated throughout the materials and as vein filling. Some of the pyrite showed well developed crystal faces suggesting growth in open fractures. None of the subsurface material was found to be porous; consequently, all of the void space in the subsurface must be open fractures. None of the subsurface materials reacted with dilute hydrochloric acid indicating that it is not calcareous. A few calcareous chips were found in the samples, but these were thought to be derived from surficial material that caved into the mud pits and was circulated into the hole. All of the subsurface materials found are consistent with the lithology of the Greyson Shale.

FRACTURES

My best estimate of the location of the greatest concentrations of open fractures is shown on the accompanying stratigraphic column by slash symbols. The location of these intervals is based upon (1) the amount of loose pyrite seen in the samples and the presence of well developed crystal faces on the pyrite, (2) rapid drilling penetration rates that do not seem to be related to soft mudstone, (3) deflections on the temperature log curves, and (4) the depths at which drilling fluid was lost by seeping or flowing into the sides of the hole.

I think that the greatest concentration of open fractures and the major source of the hot water in the hole is in the interval from 150 feet to 250 feet, possibly extending to 320 feet. Caved intervals in the hole are probably the best indication of the presence of highly fractured material, and a caliper log of the hole was reported to have indicated a caved interval existed at 215 to 220 feet and another caved interval was indicated at 170 feet. An additional caved interval was reported at 85 feet, but interpretation of the temperature logs indicates that this interval may either contain slightly cooler water than the lower intervals or it may not be very permeable. The temperature logs indicate that hot water enters the hole from 150 to 320 feet when it is pumped, and the hottest inflow may be in the interval from 150 feet to 190 feet (in the vicinity of the caved interval at 170 feet). In addition, an anomalous penetration rate between 240 feet and 245 feet suggests the presence of open fractures there. Some drilling fluid was lost from the hole when it reached a depth of 255 feet, and circulation was lost when the depth of the hole reached 276 feet. At his latter depth the well was pumped at a rate of approximately 45 gallons per minute (gpm), and very little drawdown was observed. Consequently, the rocks above 276 feet must contain highly fractured zones which are very permeable.

It is noteworthy that this major fracturing is associated with the interval that contains the silica-cemented quartz sandstone layers. The top of the sandstone-bearing interval is at 168 feet and the base is at 265 feet, which is the interval that includes the depths that are thought to contain the greatest concentration of fracture-permeability. This observation is in correspondence with information from the temperature logs and indicates that the sandstones tend to contain more open fractures than the mudstones in the subsurface.

PUMP TEST

The Well was pump tested on August 12, 1978. The interval tested was from the bottom of the surface casing at 100 feet to the top of the cement plug at approximately 330 feet. The Well was first pumped at about 43 gallons per minute (gpm) for a period of about 10 minutes. Then the pump was stopped because I was having difficulty measuring the water level in the Well. The Well was allowed to recover for about 25 minutes. This initial pumping was probably fortunate, because it served to remove relatively cool water from the Well and thereby removed the effect of replacing cold water by hot water on the subsequent water level measurements. The actual pump test began at 10:31 A.M., and the Well was pumped at 42.8 gpm for 605 minutes (about 10 hours). Then the pumping rate was increased to 79.5 gpm for an additional 400 minutes (6.67 hours). After the pump was stopped, the recovery of the water level was measured for 69 minutes. The results of the pump test are best illustrated by the time-drawdown graphs which are presented near the end of this report.

Transmissivity, which is a measure of the ability of the aquifer to transmit water, was estimated from the time-drawdown graphs. Three different values for transmissivity were obtained: one from the initial pumping rate, a second from the stepped-up pumping rate in the later part of the pump test, and a third from the water level recovery measurements that were made after the pump was stopped. These values were 182,000, 103,000, and 262,000 gpd/ft respectively. It is my opinion that 103,000 is the best estimate of transmissivity because it is based on the graph with the least amount of scatter. The value for well loss coefficient calculated from the pump test is 0.00022, which is a fairly high value. It suggests that the lost circulation material used during the drilling of the hole may be partially plugging fractures and causing a high well loss.

Inspection of the time drawdown curves shows that drawdown ceased after 29 to 35 minutes in the first pumping step and after 41 to 51 minutes in the second pumping step. One possible explanation for this stabilization of drawdown is that a "recharge" boundary exists in the vicinity of the Well. This apparent boundary may be a more permeable part of the aquifer; indeed, it may be an indication that the major "conduit" which serves to bring the hot water up from depth is nearby. Another possibility is that this effect is caused by the presence of the lost circulation material in the aquifer; however, I think that the stabilization of water level would have occurred sooner if lost circulation material were responsible. Whatever the cause of the stabilization, the pump test results indicate that the water level in the Well is likely to decline very little after the first hour of pumping at low pumping rates. With regard to the ability of the well to supply water at 50 gpm for heating the bank, calculations using the aforementioned values for transmissivity and well loss indicate that the drawdown in the Well would be only about 1.22 feet. However, since this pump test put very little stress on the aquifer, I think the results should be used cautiously; and I recommend that a pump be set at least 15 feet below ground level. Furthermore, since there will be some heat loss near ground level, you might consider setting the pump near the bottom of the surface casing and even introducing a seal above the pump to reduce the cooling effect of near surface heat exchange.

Before the pump test started, I measured (1) the water level in the pit that serves the Spa Motel, (2) the water level in the ditch north of the Well which carries hot water to the north fork of the Smith River, and (3) the water level in the concrete pipe that carries water away from the fill area south of Main Street. I found that the water level in the Spa Motel pit declined 0.045 feet during the pumping period, the water level in the ditch north of the Well did not decline during the pumping period, and the water level in the concrete pipe declined 0.11 feet. These observations indicate that pumping the Well at low rates will not affect the flow in the ditch north of the Well. The decline in water level in the Spa Motel pit was likely to have been caused by the pumping of water from the pit itself which was occurring during the pump test period. Whatever the cause, the water level in the pit did not decline much; and pumping from the Well at low rates probably will have no significant effect upon the productivity of the pit. The decline in flow rate from the fill area is puzzling; but since the fill area is farther from the Well than the Spa Motel pit, it seems unlikely that the decline was caused by pumping from the Well. However, I need more information on the usual flow regimen of the ditch from the fill area before I could make a reasonably good estimate of the effect of the Well on that ditch.

The temperature of the water was measured during the pump test. The measurements were taken at the discharge end of a hose that carried the water to the Main Street gutter. The temperature of the water near the beginning of the test period was 119^o F. After 136 minutes pumping, the temperature was 117^o F. The change from 119^o F to 117^o F is so small that I doubt if it should be considered significant. Consequently, although the temperature measurements declined during the test, the decline does not seem to exceed that which could be produced by measurement error and variations in heat loss from the discharge hose.

FLOW SYSTEM

The well has provided some information on the nature of the thermal water system in the area. An important consideration is whether the relatively low temperatures measured near the bottom of the hole reflect natural low temperature of the rock and water at that depth or whether they are a result of the invasion of cool bore hole fluid into the fractures at that depth. I do not thank that the cool temperatures at the bottom of the hole were a result of settling of cool water from the top of the hole to the bottom, because this water would have had to pass through the high temperature zone indicated on the static temperature logs between 100 and 200 feet. Hypothetical conditions which may be considered for the bottom portion of the hole are as follows: (1) the rock in the bottom portion of the hole is permeable and contains hot water, (2) the rock in the bottom of the hole is permeable and contains cold water, (3) the rock in the bottom of the hole is low permeable material and contains hot water, and (4) the rock in the bottom of the hole is low permeable material and contains relatively cool water. I think that the first hypothesis (that the rock in the bottom part of the hole is permeable and contains hot water) may be rejected, because if this condition existed, the bottom of the hole would have responded like the top of the hole when the temperature logs were run, and high temperatures would have been measured at the bottom of the hole. I think the second hypothesis may be rejected because relatively cool water coming from permeable material in the bottom of the hole would have produced cooler water near the top of the hole during the time the Well was simultaneously being pumped and temperature logged. The temperature logs show that the (non-pumping) temperature in the upper part of the hole is very close to the pumping temperatures. I think that the third hypothesis (that the rock in the lower part of the hole has a low permeability but contains hot water) is not consistent with heat flow considerations. Low permeability prevents any rapid resupply of heat to the rock by water flowing through in a natural flow system. Consequently, I think the rock may be maintained at a relatively cool temperature by conduction of heat away from that part of the system. This heat flow is the result of natural thermal gradient between rock and water in the warmer part of the ground water system and the surrounding cooler part of the system. Even if it is hypothesized that the thermal water flow in the area is vertical and the hot water in the shallow permeable beds has arrived by being transmitted upward through less permeable beds below it, the observations are not consistent with the hypothesis; because this would imply a relatively high hydraulic head gradient through the low permeable material which in turn would produce flow between the bottom of the hole and the top of the hole while the hole was not being pumped. Such flow would tend to cause any cool water introduced from the hole into the low permeable rocks to move out of the low permeable rocks during the non-pumping period and non-pumping temperature measured near the bottom of the hole would not be low. The fourth hypothesis (that the rocks in the lower part of the hole have a low permeability and contain relatively cool water) seems to be consistent with the temperature logs and other information obtained from the Well and with heat-flow considerations. Low permeable portions of thermal ground water systems should tend to be cooler than associated high permeable portions of the system because they can not conduct a high volumetric flow rate of hot water. Therefore, the heat supply is less and the low permeable portion of the system will tend to remain cooler because of the conduction of heat caused by the temperature gradient between this part of the system and nearby cool parts of the system.

I think the correspondence between permeability and the quartz sandstone beds is not a coincidence. The probability that this well could accidentally be drilled at a location where the boundaries of the sandstone interval and the boundaries of an inclined sheer zone would coincide is too low. I think the fractured sandstone simply tends to be more permeable than the associated fractured mudstone. If this is the case, and the sandstone layer is nearly horizontal, than any horizontal component in the hydraulic head gradient in the system will tend to produce a large horizontal movement of water along the bed. Consequently, the bed would tend to cause hot water to move horizontally away from the hottest part of the system before it continues its upward movement toward discharge area at the ground surface. Therefore, hot water is probably flowing horizontally through this sandstone interval away from the central part of the thermal ground water system where the water is moving upward from the heat source at depth. I would expect the water in the sandstone to become hotter as this source is approached, and water in the mudstone above the sandstone interval should also be come hotter toward the source. Consequently, the source is probably south, southeast, or east of the Well, because water that has come to the surface in the hot springs area southeast of the Well has been reported to be hotter tan any water found in the Well. Weed (1986) visited the area near the end of the last century and reported that the water issued from nine large springs and several seepages whose combined flow was estimated at 13,000 gallons per hour (217 gpm) and he said that water used to supply public baths had a temperature of 123.5^o F (51^o C). Since the hottest temperature measured at the Well was 119^o F, the water must become hotter as the old thermal spring area is approached,

CONCLUSIONS

Information obtained from this thermal water well indicates that 50 gallons per minute may be obtained from the Well without producing adverse effects on the supply of hot water to nearby springs; however, the decline in flow from the fill area south of Main Street during the pump test remains unexplained. Since the Well was pumped over ten hours and the temperature of the water declined only slightly or not at all, and since the Well is probably drawing in water from hotter more permeable parts of the thermal ground water system, it seems fairly unlikely that the temperature of the water from the Well will decline when it is pumped for long periods of time to heat the First National Bank building. However, a temperature decline can not be completely ruled out.

Since sandstone layers located at depths between 150 and 265 feet at the Well site may be conducting hot water away from the source area, it seems likely that these same sandstone layers may be tapped for hot water elsewhere in the vicinity; and the closer the well is to the source area the hotter the water will be.

If further exploration in the area is desired, one approach would be to drill shallow wells to this sandstone interval and measure the temperature of the water encountered in the wells and the hydraulic head of the system at the well sites. Both temperature and head should increase as the source is approached; of course the temperature of the water must be taken into consideration when the head is measured. Having found the location of the hottest water and highest head in the sandstone aquifer, further exploration could be pursued by drilling one or more deep tests. If the hot water is rising essentially vertically from a deep seated heat source, then a deep test in the maximal area indicated by shallow test wells should be successful. However, if the direction of the rise is affected by an inclined fault or sheer zone, then more than one well might be required to explore the deep subsurface. Such exploration would be expensive. However, the deeper water is likely to be much hotter than the water that arrives at the surface because of heat loss due to heat transfer near the surface and because of mixing with cooler surface waters near the surface.

REFERENCES CITED

Weed. W. H. (1986): Geology of the Castle Mountain mining district; U.S. Geological Survey Bulletin 139.

APPENDIX

PUMP TEST

White Sulphur Springs First National Bank, Well #1

August 12, 1978

Measurements by Darrel E. Dunn.

Measuring point is top of fiberglass casing, which is 0.86 ft. above flange. Flange was measuring point when drilling well. Measured by electric sounder unless otherwise noted.