Description of the Problem

A six-layer hydrostratigraphic model is proposed for the study site based on the geologic and hydrologic models. The six layers are composed of three aquifers and three aquitards and are listed below.

Layer #1 - Upper Perched Aquifer (Tuana Gravel Formation).

Layer #2 - Aquitard (Glenns Ferry Formation).

Layer #3 - Middle Perched Aquifer (Shoestring Basalt flow).

Layer #4 - Aquitard (Baked zone at base of Shoestring Basalt flow and underlying Glenns Ferry Formation).

Layer #5 - Lower Perched Aquifer (Glenns Ferry Formation stream facie).

Layer #6 - Aquitard (Carbonaceous Paper Shales).

Figure 18 illustrates a detailed and accurate vertical scale fence diagram of the hydrostratigraphic model showing the six main geologic layers, monitor well designs and water levels. Figure 19 illustrates a generalized block diagram of the hydrostratigraphic model with three perched aquifers identified as the upper, middle and lower systems. These aquifers flow through layers #1, #3 and #5. Paleo-stream channels control ground water flow in the upper system while the middle system is primarily controlled by the Shoestring Basalt flow. The lower perched system is controlled by a fine-grained stream facie resting on a package of carbonaceous clays. Figures 20, 21 and 22 illustrate photographs of three aquifer discharge areas compared to the model, which explains the discharge spatial distribution on the hillside.

Upper Perched Aquifer System

The upper system is characterized by paleo-stream channels composed of primarily fine sand (Saddler, 1997) along localized, sinuous and meandering paleo-stream channels. Figures 20 and 21 exhibit these localized point discharge paleo-stream channels noted by heavy isolated vegetation growth surrounded by sagebrush. Most of the ground water filled channels are located at the contact (about 3,300 feet above sea level) between the coarser grained Tuana Gravel Formation and underlying fine-grained Glenns Ferry Formation. This system has the shallowest depth below ground surface and shortest ground water flow paths from recharge to discharge. Based on field observations these channels exhibit unconfined flow conditions with discharge rates typical of seeps.

Figure 18 Fence Diagram (omitted from on-line edition)

Figure 19 Block diagram of hydrostrat model (omitted from on-line edition)

Middle Perched Aquifer System

The middle system is primarily constrained to the Shoestring Basalt flow at an elevation of about 3,200 feet in the discharge area and transmits ground water through fractures and joints. General ground water flow likely follows the geologic dip of the flow that was determined from the seismic study to be southerly. The basalt flow is not areally extensive and where the flow pinches out fine-grained sediments control ground water flow.

The geologic and hydrologic models indicate the area of recharge to the basalt aquifer system is primarily in the vicinity of the Fossil Gulch Irrigation Pond. Basalt is observed in direct contact with irrigation water at an elevation of about 3,300 feet in this area. Hydrographs for NPS-5, NPS-4 and NPS-3 illustrate how a recharge pressure wave propagates through the basalt system from recharge areas to the discharge. Geophysical evidence of recharge for the pond area is based on the Misa-a-la-masse and Schlumberger studies performed by Vector Engineering (1994). The high response of Misa-a-la-masse and Schlumberger data near the Fossil Gulch Pond area correlates with the geology and the hydrographs from monitor wells.

This system has intermediate length ground water flow paths from recharge to discharge zones. The basalt flow has the greatest hydraulic conductivity of the three perched systems and it discharges the greatest volume of ground water. Figure 22 illustrates how the greatest vegetation growth correlates to discharge from this aquifer system. The basalt has a basal baked zone that is well lithified but fractured. This baked zone and underlying silty clays are acting as an aquitard to the basalt aquifer. Saturated thickness of the flow is a few feet near the discharge areas but at monitor well N.P.S.-5 it is has a thickness of 60 feet.

Lower Perched Aquifer System

The lower perched aquifer system is characterized by a fine-grained paleo-stream facie resting on a 20-foot thick package of carbonaceous paper shales. In some areas the stream facie exhibits preferential flow paths through paleo-channels as shown on Figures 20 and 22. The lower perched system is the least defined due to a lack of data. However, this system correlates with the elevation of the rotational failure planes for the 1991 and ca. 1979 slope failures (Figure 22).

Ground water recharge to the lower system likely encompasses a larger area and flow paths are the longest of the three perching systems. Additional water may be draining from monitor wells into the lower system due to long effective screen intervals (Appendix C). The discharge volume of the lower system is significantly less than the basalt system and at most locations evapotranspiration is greater than discharge. There is evidence for the expansion of the lower system in recent years. The green vegetation located at a paleo-stream channel in the lower perched system on Figure 20 did not exist a few years ago based on historical photos. Also, water levels in monitor well 9CCC1, which is completed in the lower system, continue to increase (Appendix C).

Figure 20 photo of seeps (omitted from on-line edition)

Figure 21 photo of seeps (omitted from on-line edition)

Figure 22 photo of basalt seep (omitted from on-line edition)

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

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