Wednesday, September 17, 2014

Report shows groundwater declines here

Data will be used for flow model in valley


By GREG MOORE
Express Staff Writer

The U.S. Geological Survey is trying to determine whether the groundwater levels in an aquifer important to agriculture in the south valley are in decline. Express file photo

    A report released early this month by the U.S. Geological Survey indicates that the water level in a confined aquifer in the southern Wood River Valley continues to decline, though causes of the decline have yet to be determined.
    Two measurements taken at each of 10 wells in the aquifer, one in 2006 and the other in 2012, showed declines of between one-tenth of a foot and more than 20 feet; average decline was 6.8 feet.
    According to the report, titled “Stream Seepage and Groundwater Levels, Wood River Valley, South-Central Idaho, 2012–13,” the Wood River Valley aquifer system is composed of a single unconfined aquifer that underlies the entire valley and a lower, separated and confined aquifer in the southern Bellevue fan, generally to the south of Baseline Road.
    Average decline of 77 wells measured in the unconfined aquifer was 2.9 feet, though some had declined as much as 14 feet.
    “In the unconfined aquifer, we didn’t see a lot of change,” said the report’s author, Jim Bartolino, Idaho groundwater specialist for the Geological Survey. “But in the confined aquifer, we saw some pretty big declines.”
    Bartolino cautioned that measurements were taken only twice, six years apart, and no one knows what went on in the meantime.
    “I wouldn’t say that’s a trend quite yet,” he said. “If we measure these wells again in five or six years and we still see lower numbers, then I would say that, yes, we’ve got a serious decline going on.”
    A groundwater flow model of the Big Wood River system, scheduled for completion at the end of 2015, is expected to provide hydrologists with the tools to answer that and related questions. The recently compiled data will be included in the model.
    Bartolino said farmers in the southern Bellevue Triangle are “fairly dependent” on the confined aquifer. He said there appears to be some recharge of the confined aquifer from the larger, unconfined aquifer, though hydrologists do not yet have detailed knowledge of how they are connected.
    He said longtime residents in southern Blaine County have reported that there used to be a lot of water-logged land north of U.S. Highway 20, but those areas have been gradually contracting southward.
    Bartolino said flows in the Big Wood River have also been declining, and that seems to tie in to a lower groundwater level.
    “Some of the declines are due to pumping [of groundwater] and some are due to drier conditions,” he said. “But I haven’t been able to tease out how much of each is going on.”
    The groundwater flow model, a collaborative effort between the U.S. Geological Survey and the Idaho Department of Water Resources, will be used for combined administration of groundwater and surface water rights.
Greg Moore: gmoore@mtexpress.com




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