Hagerman Fossil Beds NM - Landslide (Appendix C)

APPENDIX C
DISCUSSION OF GROUND WATER

Introduction

Ground water data has been collected since 1986 when six monitor wells were installed and 1994 with additional 6 wells. Interpretation of the hydrographs needs to take into account the well construction because of multiple perching ground water conditions. There are two general trends illustrated in the hydrographs; one type is cyclic and the other is nearly level. Hydrograph characteristics are mainly a function of recharge events, aquifer parameters, lengths of flow paths and monitor well design. Figure 28 illustrates the hydrographs for each monitor well discussed in the following paragraphs. The identification number applied to the wells is the standard legal description used by the U.S.G.S. of range, township, section and 1/4,1/4,1/4 referenced to the Boise base line and meridian. All of the wells and springs have the same state, county, range (R13East) and township (T7South) so just the section and 1/4’s will be referenced. The six wells constructed in 1994 are noted with the acronym NPS-# for clarification.

Monitor Well 17ABB1

The hydrograph for 17ABB1 illustrates a cyclic water level response from March 1986 through October 1988. Water levels generally rise from 145 feet below ground surface (B.G.S.) in March 1986 up to 139 feet B.G.S. in 1988 where they appear to stabilize and then generally start dropping down to 148 feet B.G.S. in January 1992. The water level shift in January 1992 is attributed to the change from B.L.M. to U.S.G.S. monitoring. Starting in January 1992 the cyclic pattern is no longer observed and water levels drop only slightly from 146 feet B.G.S. to 148 feet B.G.S. in April of 1996.

Monitor Well 17AAB1

Water levels remain stable at about 111 feet B.G.S. from March 1986 through April 1987 but start to increase reaching a peak of 107 feet B.G.S. on January 1988 and a cyclic pattern starts to develop with water levels dropping again to 110 feet B.G.S. in August of 1988. Levels drop from 110 feet in 1990 down to 115 feet B.G.S. in January 1992 where they level off with only a slight cyclic pattern until August 1993 when the well was pumped by the U.S.G.S. The water level shift in January 1992 is attributed to the change from B.L.M. to U.S.G.S. monitoring. Water levels never recovered from pumping the well and levels remained at about 121 feet B.G.S. through April 1996.

grnd-wtr.gif (10074 bytes)

Figure 28. Static water level hydrographs for monitor wells in Fossil Gulch area (data from U.S.G.S. and N.P.S.).

Monitor Well 9CCC1

Water levels decrease significantly from 161 feet B.G.S. in March 1986 to 202 feet B.G.S. in September 1986. The levels rebound in a broad arch reaching a peak of 191 feet B.G.S. in January 1990 and then decrease again to 197 feet B.G.S. in October 1991. The water levels then rise again to 189 feet B.G.S. in January 1992 and then steadily increase through April 1996 by three feet to 186 feet B.G.S.. The water level shift in January 1992 is attributed to the change from B.L.M. monitoring to U.S.G.S. monitoring.

Monitor Well 9CDC1

Water levels from March 1986 through October 1988 exhibit a cyclic pattern and generally increase from 109 feet B.G.S. in March 1986 to 103 feet B.G.S. in September 1987. Two peaks occur in November 1986 and September 1987 and then a steady decline through October 1988 when monitoring stopped until 1990. The 1990 measurement indicated a dry well and it has been dry since. Water levels appear to have dropped in response to the concrete lining of the Fossil Gulch Canal after the 1987 irrigation season.

Monitor Well 9CBB1

Water levels exhibit cyclical patterns and generally increase from 95 feet B.G.S. in March 1986 to 82 feet B.G.S. in April 1988. Levels then generally decrease from 90 feet B.G.S. in May 1993 to 93 feet B.G.S. in March 1996. The general decrease in water levels from 1988 through 1996 may be due to the concrete canal lining constructed after the 1987 irrigation season. This well may have a leaky surface seal based on inconsistent water levels and a high concentration of nitrates in the water.

Monitor Well 9DCC1

This well was a dry hole and is known to have remained dry until 1994. In 1994 the N.P.S. allowed the Boise State University seismic study team to discharge an explosive in the bottom of the well for the study. The well has remained in this condition.

Monitor Well NPS-1 (9DCB1)

Only a few water levels have been collected for NPS-1. Based on very sparse data, water levels have remained very constant at 233 feet B.G.S. from 1994 to present. The constant water levels may be explained by water filling the PVC end cap on the casing, then overflowing through the screen and moving downward out of the bottom of the well.

Monitor Well NPS-2 (9CDD1)

The static water levels exhibit a low amplitude wave characteristic of the cyclic pattern observed more prominently in other well hydrographs. Water levels have risen from about 220 feet B.G.S. in June 1994 to 218 feet B.G.S. in December 1994 and then back down to 221 feet B.G.S. in July 1995.

Monitor Well NPS-3 (9DCC2)

The hydrograph for NPS-3 exhibits a very low amplitude cycle ranging from a low of 221 feet B.G.S. up to 219 feet B.G.S.. The cycle peaks in December and reaches its low during August. The water level pattern is nearly identical to the hydrograph for monitor well NPS-2 in both amplitude and time of cycle highs and lows. There are no significant long-term changes in water levels.

Monitor Well NPS-4 (5DDD1)

The hydrograph for NPS-4 exhibits a very smooth cyclic pattern with greater amplitude than either NPS-3 or NPS-2. The cycle peaks at 174 feet B.G.S. in November and reaches its low of 179 feet during July where NPS-2 and NPS-3 hydrographs show a water level peak during December and a low during August. The amplitude difference for NPS-4 is about five feet where NPS-2 and NPS-3 is about two feet. There are no significant long-term changes in water levels from 1994 through 1996.

Monitor Well NPS-5 (7AAA1)

The hydrograph for NPS-5 exhibits a very smooth cyclic pattern with greater amplitude than NPS-4, NPS-3 or NPS-2. The cycle peaks at 176 feet B.G.S. in October and reaches its low of 187 feet during mid-May. The amplitude difference for NPS-5 is about 10 feet where NPS-4 is about five feet and NPS-2 and NPS-3 have about two feet of amplitude change. There are no significant long-term changes in water levels from 1994 through 1996.

Monitor Well NPS-6 (9CDD2)

The hydrograph for NPS-6 is very similar to NPS-2 and NPS-3 characterized by a smooth low amplitude cyclic water level pattern observed more prominently in other well hydrographs such as NPS-4 and NPS-5. The cycle peaks at 218 feet B.G.S. in December and reaches its low of 221 feet during August. The amplitude difference for NPS-6 is about three feet, which is similar to NPS-2 and NPS-3.

Summary of Fossil Gulch Area Monitor Wells

Hydrographs for the monitor wells generally show two trends. One type is a cyclic rise and fall attributed to seasonal recharge events to the aquifers. The other is a generally flat hydrograph with some exhibiting a slight decrease in water levels and some a slight increase. The cyclic hydrographs suggest a better hydrologic connection to the recharge source with shorter ground water flow paths. The greater the amplitude of the cycle the better the hydraulic connection to the recharge source. The longer the flow paths are the greater the energy loss in the system which result in lower amplitude cycles, or even flat hydrographs. The basalt flow aquifer usually has cyclic hydrographs associated with it which are also a function of basalt aquifer properties of low storage and high transmissivity.

Flat hydrographs are usually exhibited in wells that encountered only fine-grained sediments. Sediment properties have higher storage and lower transmissivity than basalt and so the recharge pressure wave looses energy at a greater rate resulting in nearly flat hydrographs. Very low amplitude cycles can still be observed in some of these flat hydrographs. The challenge in hydrograph interpretation is from long effective screen intervals in the monitor wells, which can interconnect aquifers.