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The purpose of the Latah County Hydrologic Characterization Project was to evaluate the potential for shallow groundwater recharge to the upper aquifer in the Moscow area (the “Wanapum aquifer”) through sediments along the granite/basalt contact, in the vicinity of Moscow Mountain. 47 boreholes were advanced into nearsurface sediments and 6 boreholes were cored into the top 20 feet of Priest Rapids basalt in the South Fork of the Palouse River and Paradise Creek drainages. The Wanapum aquifer is generally understood as being hosted within the Miocene Priest Rapids Member of the Wanapum Formation, includes the underlying sediments and sedimentary interbeds, and is believed to be distinct from the deeper Grande Ronde aquifer. Investigation showed a consistent pattern of sediment deposition, in which granite and/or metamorphic rocks were overlain by heterogeneous (intermixed coarse and fine) sediments of moderate to high permeability, ranging in thickness from 14 to 54 feet. Basalt flows in the area of this investigation were ubiquitously overlain by a substantial thickness (20 – 25 feet) of low permeability fine sediments (poorly sorted silts, sands, and clays, and peat); these low-permeability sediments were overlain by heterogeneous sediments of relatively high permeability, similar in character to those overlying granitic bedrock. Many of the boreholes in basalt terrain encountered elevated moisture contents at the contact between the low permeability sediments and the overlying sediments of higher permeability. In addition, hydrologic testing and examination of core recovered from the boreholes that penetrated the Priest Rapids basalt suggests limited hydrologic connectivity with the overlying sediments, even though the basalt interior probably has substantial bulk (fracture) permeability. On the basis of data gathered in this study, we conclude that recharge to the shallow aquifer along the eastern margin of the Palouse Basin is probably limited by thick, low permeability sediments and poor connectivity between the Priest Rapids basalt and the overlying sediments. It appears likely that the majority of infiltration entering the nearsurface sediments flows laterally through discontinuous, high permeability sediments until it discharges into local streams (i.e., the South Fork of the Palouse River, Paradise Creek, or their tributaries) or intersects the land surface as spring discharge. There is some circumstantial evidence for this hypothesis from the presence of numerous springs in the area, and anecdotal observations of water discharging to Paradise Creek from sand lenses exposed by excavations near Joseph Street and Mountain View Road in Moscow, Idaho [personal comm., J. Bush]. Bush also notes that the rapid response of first and second order streams in Latah County, consistent with limited recharge and rapid return flow. Some lateral flow may eventually infiltrate the shallow basalt aquifer, either through spatially-discrete areas of high permeability (e.g., paleo-stream channels), or by flowing westward beyond the area of low permeability sediments; however, evaluating the evidence for either of these possibilities was beyond the scope of the present study.

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Title:
Latah County hydrologic characterization project. Final report
Authors:
Fairley, Jerry P.; Solomon, Mark D.; Hinds, Jennifer J.; Grader, George W.; Bush, John H.; Rand, Amber L.
Date Created (ISO Standard):
2006-09-30
Description:
The purpose of the Latah County Hydrologic Characterization Project was to evaluate the potential for shallow groundwater recharge to the upper aquifer in the Moscow area (the “Wanapum aquifer”) through sediments along the granite/basalt contact, in the vicinity of Moscow Mountain. 47 boreholes were advanced into nearsurface sediments and 6 boreholes were cored into the top 20 feet of Priest Rapids basalt in the South Fork of the Palouse River and Paradise Creek drainages. The Wanapum aquifer is generally understood as being hosted within the Miocene Priest Rapids Member of the Wanapum Formation, includes the underlying sediments and sedimentary interbeds, and is believed to be distinct from the deeper Grande Ronde aquifer. Investigation showed a consistent pattern of sediment deposition, in which granite and/or metamorphic rocks were overlain by heterogeneous (intermixed coarse and fine) sediments of moderate to high permeability, ranging in thickness from 14 to 54 feet. Basalt flows in the area of this investigation were ubiquitously overlain by a substantial thickness (20 – 25 feet) of low permeability fine sediments (poorly sorted silts, sands, and clays, and peat); these low-permeability sediments were overlain by heterogeneous sediments of relatively high permeability, similar in character to those overlying granitic bedrock. Many of the boreholes in basalt terrain encountered elevated moisture contents at the contact between the low permeability sediments and the overlying sediments of higher permeability. In addition, hydrologic testing and examination of core recovered from the boreholes that penetrated the Priest Rapids basalt suggests limited hydrologic connectivity with the overlying sediments, even though the basalt interior probably has substantial bulk (fracture) permeability. On the basis of data gathered in this study, we conclude that recharge to the shallow aquifer along the eastern margin of the Palouse Basin is probably limited by thick, low permeability sediments and poor connectivity between the Priest Rapids basalt and the overlying sediments. It appears likely that the majority of infiltration entering the nearsurface sediments flows laterally through discontinuous, high permeability sediments until it discharges into local streams (i.e., the South Fork of the Palouse River, Paradise Creek, or their tributaries) or intersects the land surface as spring discharge. There is some circumstantial evidence for this hypothesis from the presence of numerous springs in the area, and anecdotal observations of water discharging to Paradise Creek from sand lenses exposed by excavations near Joseph Street and Mountain View Road in Moscow, Idaho [personal comm., J. Bush]. Bush also notes that the rapid response of first and second order streams in Latah County, consistent with limited recharge and rapid return flow. Some lateral flow may eventually infiltrate the shallow basalt aquifer, either through spatially-discrete areas of high permeability (e.g., paleo-stream channels), or by flowing westward beyond the area of low permeability sediments; however, evaluating the evidence for either of these possibilities was beyond the scope of the present study.
Subjects:
hydrologic systems hydrologic basins groundwater recharge sediments hydrologic systems
Location:
Latah County
Collection:
Palouse Basin
Rights:
Rights to the digital resource are held by the University of Idaho. http://www.uidaho.edu/
Publisher:
University of Idaho
Contributing Institution:
University of Idaho
Type:
Text
Format:
application/pdf
Cataloger:
jh
Date Digitized:
2006

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Source
Preferred Citation:
"Latah County hydrologic characterization project. Final report", Idaho Waters Digital Library, University of Idaho Library Digital Collections, https://www.lib.uidaho.edu/digital/iwdl/items/iwdl-2006_fairley_et.html
Rights
Rights:
Rights to the digital resource are held by the University of Idaho. http://www.uidaho.edu/
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