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TROY EXECUTIVE SUMMARY

REPORT OF GEOLOGIC/HYDROGEOLOGIC SERVICES

CITY OF TROY

LATAH COUNTY, IDAHO

INTRODUCTION

The city of Troy (Troy) is located on the Clearwater Plateau, in central Latah County, Idaho. Troy relies on both surface and ground water resources for their water supply source. The community has a surface water collection facility on Moscow Mountain to the north as the primary source and two wells (Big Meadow well and the Duthie Park well) that supplement during low runoff times. Troy supplies water to approximately 800 people and has requested assistance in evaluating the water supply wells in relation to long-term use.

GEOLOGY

Troy, Idaho is located along the northwestern margin of the Clearwater Embayment of the Columbia River Basalt province. The community is near the contact of the Miocene aged basalt flows and prebasalt crystalline and metamorphic basement rock. Previous researchers produced a geologic map of the Troy 7 ½ minute quadrangle and subdivided the basalts into three units. These units are named the N1 member of the Grande Ronde Formation, the Priest Rapids Member of the Wanapum Formation, and the Onaway Member of the Saddle Mountains Formation. The Onaway, considered the youngest of these three units, was interpreted to rest unconformably in channels eroded into the other basalt units. Regional mapping done by the Idaho Geological Survey for a compilation of the Potlatch 1:100k quadrangle and additional detailed mapping and analytical data determined that the basalt of Onaway is older and that it underlies the Columbia River Basalt Group throughout parts of Latah County. Outcrops near Troy were reexamined and it was determined that the Onaway underlies much of the community and plays a major role in the ground water regime (Figure 1).

HYDROGEOLOGY

The primary water source for Troy is a surface water collection facility that is treated with a slow sand filter. Troy experienced reduced production from the sand filter and subsequently increased production from one of the two water supply wells. The well with increased flow rates began to fail due to over pumping resulting in little to no available drawdown. Reviewing Troy’s production records of the sand filter, and two water supply wells indicated that there was most likely a decreased production from the sand filter due to clogging. Maintenance of the sand filter produced significantly better flow and resulted in lower necessary pump rates from the water supply wells. It is expected that Troy can meet demands in the near future by regular maintenance of the sand filter and intermittent usage of ground water allowing time for wells to recover.

Development of ground water resources to completely meet the demands of Troy has not been successful. Typically, wells north of the community penetrate thin Miocene basalt flows and encounter prebasalt crystalline rocks that do not produce enough water for a municipal supply. The Big Meadow well, located in northeastern Troy, is interpreted to be located in weathered crystalline rocks and/or a sand unit of the Latah Formation. Geologic maps suggest that deep wells in the Grande Ronde Formation would have good potential for major producing aquifers. However, most of the deep wells (500 to 800 feet) in Troy have not been successful in terms of major production. It is now understood that this lack of success is because these wells are completed in the older, weathered basalt of Onaway, not in basalt flows of the Grande Ronde Formation. The Grande Ronde flows occur as thin units in channels eroded into Onaway flows and thick sequences of Grande Ronde do not underlie the community. The Duthie Park well is located in a thick sequence of the basalt of Onaway.

The Columbia River Basalt Group generally consists of basalt flows that make good aquifers, because of thick well-developed flow tops, generally fresh unweathered basalt, and closely spaced fractures. The basalt of Onaway, where exposed in Latah County is often weathered to clay, has thin flow tops, and widely spaced fractures. In Troy, the Onaway is highly weathered and has been mined in the past for the production of clay products. A video log of a city well (Reservoir well) drilled in 1995 shows weathered basalt to approximately 400 feet and the drilling log indicates weathered basalt alternating with unweathered basalt to a depth of over 700 feet. The well produced a maximum of 50 gallons per minute before it partially caved in.

The fact that the Onaway underlies Troy explains some of the communities past difficulties concerning the development of a dependable ground water supply from deep wells. Drilling logs suggest that the Onaway unit extends beneath the subsurface to the west, south, and east of Troy. Those same logs suggest water production zones are not very laterally extensive. For example, an abandoned well (City Hall well) in downtown Troy has a lower water level than expected when compared to the Duthie Park well. In addition, there is no documentation of production from similar elevations between the two wells which are only ~2000 feet apart.

CONCLUSIONS

At the present there are no identifiable targets for a large production well in Troy. A deep well in the Onaway would be expected to produce no more than 50 to 100 gpm and would be only capable of supplementing the current water supply. The well should be planned in advance so that it is properly constructed to maximize the production capabilities. The well location should be as far from the Duthie Park well as possible, possibly on the east end of town. We would recommend that Troy continue to monitor and record water level and pumpage information in the Big Meadow well and Duthie Park wells. The water level and pumpage data should be compiled and reviewed regularly.