The hydrostratigraphic model developed from this investigation provides for:

1. Identification of three perched aquifer systems and an explanation for the perched aquifer discharge patterns and spatial distribution on the hillsides.
2. Location of the main recharge area for the middle perched system based on geologic, hydrologic and geophysical data.
3. Association between the failure planes for the 1991 and ca. 1979 landslides and the carbonaceous paper shale package.
4. Potential of each aquifer system for slope stability problems.
5. Explanation of well hydrographs responding to a recharge pressure wave.

Three perched aquifer systems (upper, middle and lower) occur within the plateau and have different implications for slope failures. Paleo-stream channels control ground water flow of the upper system that causes point discharge locations on the hillsides. They have very low discharge rates typical of seeps. Any slope failure associated with these individual channels will likely be small relative to the magnitude of failures associated with the middle and lower systems.

The middle system has the greatest discharge rates and lateral extent. Recharge to the middle basalt system is primarily from the irrigation system with unknown quantities recharging from field application of water. Recharge to the middle system occurs in the Fossil Gulch Pond area based on the hydrostratigraphic model. The main component of ground water flow is likely following the southerly dip of the basalt flow. Hydrographs for wells NPS-5, NPS-4 and NPS-3 and water chemistry illustrate a recharge pressure wave propagating through the basalt aquifer. It takes approximately four months for the pressure wave to travel from the recharge area in mid-April to the discharge zones reaching NPS-5 in June, NPS-4 in July and NPS-3 (near the discharge area) in August. The volume of water discharging from the basalt flow saturates sediments in the discharge zones and erodes the slope face causing over-steepened slopes.

The third and lowest perched aquifer system is controlled by paleo-stream channels. This system is particularly susceptible to slope failures because of the presence of a finely laminated paper shale package with occasional deposits of diatomite and volcanic ash. The existing 1991 and ca. 1979 slope failure planes are about 40 feet below the Shoestring Basalt flow which corresponds to the elevation of carbonaceous paper shale package of the lower system. Sparse data exists for this system but recharge likely occurs from leakage of the middle basalt aquifer system and water draining from upper systems into lower systems from monitor wells constructed with long effective screen intervals.

Recommendations for further study are presented below.

1. Continue ground water monitoring on a monthly basis.
2. Continue water quality data collection for all wells and specific perched aquifer discharge locations.
3. Perform ground water tracer tests to define ground water parameters and identify flow paths with discharge locations that will aid mitigative efforts.
4. Based on field data, canal lining efforts need to be focused near the Fossil Gulch Pond area to impact the middle system.
5. Drill a series of drain wells upgradient from the perched aquifer discharge zones down to the regional aquifer. Construct the wells to capture water flowing toward the hillside effectively diverting it down to the regional aquifer. A problem with drain wells will be water quality implications from draining perched aquifer water into lower aquifers (aquifer interconnection). The advantages to this option are:

• Provides a permanent long term solution with little or no maintenance compared to relatively short life spans (30 yrs.) for canal/pond synthetic liners.
• Eliminates potential problems with future irrigation dredging activities to remove sediments in the canal/pond and synthetic liners.
• Lower costs than canal/pond lining. The least expensive option of lining the canals/ponds using HDPE flexible membrane is calculated to be $3.6 million.
• No de-watering pumps which require capitol, maintenance and electrical costs.
• Drain wells can be constructed on federal lands, compared to canal/pond lining that will occur on private land.

Table of Contents
Chapter 1 | 2 | 3 | 4 | 5
Appendix A | B | C | D

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