Ground Water Case History 1
GEOPHYSICS AS A TOOL FOR FINDING EARTH MATERIALS THAT CAN TRANSMIT WATER TO A WELL
INTRODUCTION
The National Park Service (NPS) Yosemite Park officials were told in 1983 that they must develop a new water supply sources or construct water treatment plants to eliminate a problem with the bacteria Guardia, now found in the surface water supply systems being used on the valley floor and at the Wawona complex. The order came from the California Department of Public Health (CDPH). CDPH , also ordered the NPS to abandon all existing water supply springs and to develop groundwater abstraction wells to supply these facilities and all other forest service facilities on Park Service land. This included terminating all water usage of the Merced River and all tributaries, as a source of water withdrawn for use on the Yosemite valley floor and the Wawona complex.
The U.S.G.S. Sacramento Office was given the responsibility of developing a single supply supplemental supply water well with a minimum capacity of 800 gal/min. The U.S.G.S. was unsuccessful in their location of a well to produce the required quantity of water and Alvin L. Franks, Ph.D., DBA Consultant in Engineering/Environmental Geology, was engaged by NPS, by way of two separate contracts to locate , drill, test -prove the yield of the wells drilled. The first contract in 1983 was for the locations outside of the valley floor and the second in 1985 was for the well on the valley floor.
The Franks engagement for the location and drilling of the valley floor well began in the fall of 1985, just as a new computerized 24-channel seismic refraction geophysical exploration seismograph had just been released for sale and for rental application. This was one of the first uses of this modern geophysical technology, other than for the exploration for oil and gas. It was the first known use of this instrumentation for the exploration for water. It has progressed since its inception and is now the way to the future of Groundwater Geology.
THE YOSEMITE VALLEY FLOOR
Here, the public health threat was due to the arrival of a new bacterium traced to South America. NPS took heed and declared that the Sierra Nevada river systems and the Yosemite National Park Service surface water diversions, in particular, had to be eliminated, or a very costly treatment plant must be built and put into operation to sterilize the public water supply. The NPS hired the U.S.G.S. to locate and develop a new source from groundwater for use on the valley floor complex. They were not successful and the well they drilled and cased did not meet their requirements of at least 800 gal/min production rate.
At that time Dr. Franks was just completing studies outside of the valley floor and was given a contract to find a location on the valley floor and observe the drilling of the pilot well where a well producing.at least 800 gal/min could be drilled and put into production.
It was very fortunate that Dr. Franks had previously subcontracted a regional geophysical service company to do the hammer seismic work on the locations for the wells not located on the valley floor. This company, Gasch and Associates, had just acquired the latest 24-channel seismic refraction system that could save and combine refraction traces as a means of building a three-dimensional refraction image capable of imaging to tens of thousands of feet below the surface. The field information was collected by way of a computer tape and could only be translated by specialized computer software, then located in Denver Colorado, Houston Texas and another in Alaska.
Based on common Geologic rational and the data from past wells both good and indifferent, it was obvious that the only locations that would prove to yield sufficient water-bearing materials would be in a geologic environment with the absence of the clay soils that had defeated the U.S..S. effort. It would not be located in the deep lake bed deposits that they drilled and tested.
The target zonal aquifer lay beneath the surface on a subsurface ridge, nestled between the 2,000 feet deep upper basin and the almost equally deep lower basin below the main Valley Floor. Both ridge-side flanking basins had been in-filled with fine glacial-lacustrin sediments. Other wells drilled and put into service on this ridge between the basins, showed that there was the possibility of finding an aquifer to supply the required 800 gal/min below the lake bed sediments found at the surface and up to 400 feet below the surface. Translated geomorphologicallly, this would have to be a buried river channel somewhere on this subsurface geological high and below the lake bed deposits that prior wells found extending from 250 to 400 feet below the surface. It would have to be a series of buried river deposited sands and gravels that represented a system that remained close to the same drainage pattern to provide the required depth of course water bearing materials to provide the 800 gal/min yield.
Gasch and Associates, of Sacramento, was engaged to perform the tree-dimensional subsurface survey along this buried subsurface ridge. He, Gasch came through came through with a vibration wave generated by use of a large tank full of water with a gas powered engine providing the vibrating shock wave. The return waves were picked up the on geophones on lines extending from canyon wall north and south and east and west over most of the subsurface geologic high ridge. It took three days and numerous movement of the vibration energy source, to cover the area that had the potential for finding the required buried river deposited gravels that could yield the required 800 gal/min. Their was a minor problem doing the survey under the Merced River which was solved by extending the distance between pick up geophones and entering the data for the computer system to evaluate the change.
The geologically-targeted multi-channel vibration seismic refraction survey worked like a charm and produced a signature image of the entire geologic system. The most promising location showed 375 feet of lake deposited clays, then 25 feet of sands with some gravel then 385 feet of clean gravels with course sands, then 15 feet of gravely sand then soft deeply weathered bedrock at 400 feet depth.
Based on this data another firm was hired to do the drilling and completion of the well. Dr. Franks was paid to act as an observer of this pilot hole drilling task. The small diameter pilot hole was drilled and the cuttings returns and the e-log confirmed the geophysical data without exception. This well was drilled to a total depth of 850 feet and found only weathered bedrock below the 800 feet depth. It had and artesian head of 10 feet above ground surface when the aquifer was penetrated, that had to be controlled by mud weight during drilling, the e-log and other testing items. Flow testing and a short-duration pump tests confirmed that the materials below 400 feet were clean and absent of clay and silt. The well was then drilled out to 24-inch production diameter and cased to 400 feet of surface casing and then with perforated casing and gravel pack to match the materials penetrated to 890 feet with the bottom 10 feet without perforations. The well was tested by others and a pump installed with pipe lines extending up to the hill to the east to the NPS storage tank. The well produced 1,200 gal/min and still had an artesian head as measured at the well head. Dr. Franks was told that the 1,200 gal/min was the total capacity of the pump and restrictions in the pipe line and that with a larger pump, etc. the rate pumping rate could be increased in the future, if required.
GROUNDWATER SUPPLY FOR LOCATIONS NOT ON THE VALLEY FLOOR
Crane Flat Area
This area is at about 200 feet in elevation above the valley floor and close to the north entrance to the park well within the granitic batholith. Glaciation had removed virtually all bodies of ablation till or other course unconsolidated aquifer materials, leaving the sole possibility for developable groundwater being within the anomalous rock-mass fracture zones in the granitic materials. This was a clear case for necessary use of aerial photograph, field geologic mapping and use of available economic geophysical techniques.
Dr. Franks surface geologic mapping of the area indicated only two areas of potentially fractured granitic bedrock. It was proposed to use a one line seismic refraction system to determine the extent and depth of fracturing that could provide the information on fracture zones that would have sufficient storage and pathway for development of a water supply well. As planning for the field geophysical survey was underway, NPS sent a notice prohibiting the use of explosives and a “Hammer” system had to be used to generate the refraction shock waves. The surveys were done and completed in the late summer of 1983 at all four areas, Crane Flat, Foresta, Wawona, and El Portal , prior to the drilling of the first holes at this location.
The system was used at the two potentially fractured bedrock zones. Six lines were run at each location to find the areas with the slowest velocity indicating fracturing of the granitic material. The lines showing the most fracturing were selected for drilling of the test holes. The N.P.S. sent an order to drill all holes to a depth of 400 feet and set only a six feet surface casing with cap after completion of the drilling and long term pump testing was completed at all locations.
Two test were drilled at this location at the two most favorable locations, both to the required 400 feet depth. An air vibration hammering rotating drill was used at all locations. This type of drill rig is the most economic way to drill into granite rock masses. An 8 inch bit was used and the cutting and change in fluid content of the returns were observed and recorded. No cores were take of any of the rocks penetrated anywhere in the park system.
The first well drilled to the required 400 feet showed and inflow of water starting at about 90 feet below the surface at a measured rate of about 35 – 40 gal/min with little or no increase to completion at 400 feet. The weather was changing and snow began to fall so the 6 feet of surface casing was cemented in and the well capped.
The second well again was drilled to the required 400 feet and showed and inflow of water starting at 70 feet increasing to about 20 gal/min at completion of drilling. At that time, October 1983 there was two feet of snow on the ground and the well was capped and operation shut down for the winter.
In the early spring, pump tests were run and showed a long term rate of production for well No. 1 of 30 gal/min and 15 gal/min for well No.2
Foresta Area
This is an area of extensive development of small vacation houses that were given a 50 year? lease for construction and living on N.P.S. land. Most if not all had shallow wells, rainfall collection tanks, etc. The N.P.S. paid for the drilling of the wells as a possible source of water for these vacation homes.
Here the surface geologic mapping, air photos and the seismic refraction data indicted three areas of anomalously-fractured granitic materials. Test well drilled to the required depth of 400 feet had fractured zones for most of the depth of the boring, but all were filled with clay gouge and the total inflow was negligible with the return air. The driller estimated the inflow at one gal/min. The hole was backfilled and abandoned.
Well No, 2 drilled again to the required 400 feet had fractured zones at a depth of 58 feet and had a pumping rate of 15 gal/min determined by the required long term pump testing.
Well No. 3 again drilled to the required 400 feet had fractured zones to a depth of 300 feet and had a pumping rate of 60 gal/min determined by the required long term pump testing.
Wells 2 and 3 were fitted with required 6 feet of surface casing and capped.
Wawona Residential and Commercial Development Area
The Wawona area is located on the high plateau to the south of the south-rim of the main canyon. It is a very extensive development including a golf course, hotels, restaurants, motels, many private homes and N.P.S. facilities. Most if not all of the existing wells do not meet the existing requirement of 50 feet of casing cemented in with all production 50 feet below ground surface. Also the surface waters in the area are not suitable for a public water supply without major treatment as described earlier.
After mapping the geology using all available tools including aerial photos, a few fractured zones in the granite bedrock were found and seismic surveys were performed at the two most favorable locations to determine the breadth and depth of the fractured zones that may provide the pathway of potential water-yielding materials. Test wells were drilled at both locations to a depth of 400 feet. It was found that the majority of the rock-mass fractures were unfilled with fine-grained materials. Long term air lift pump tests confirmed that the materials at both wells were too fine and compacted to yield an aquifer-mass response of more than five gal/min. Both wells were backfilled and abandoned.
Further studies by the U.S.G.S. over a seven-year period with some wells as deep as 2,000 feet did not find water of a quality that could be used and also they produced less than 4 gal/min.
El Portal
This valley-floor area of development is located near the entrance to the park and along the banks of the Merced River. All of the existing water wells did not meet public health standards and were being ordered for closure and abandonment if a 50 feet of surface casing was not cemented in place.
Seismic refraction lines were run at the five locations showing promising by field geologic mapping and engineering indicating that a body of valley floor gravel alluvium could be expected to be over 50 feet thick. This groundwater prospecting work was completed in the late summer of 1984.
All five areas were found to have up to 30 feet or more of gravel below the 50 foot requirement for surface casing ( total aquifer depths of more than 80 feet). Actual proof drilling of the wells was done by others, and apparently the aquifer source bodies proved adequate.