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IGWS - West Nishnabotna Aquifer

West Nishnabotna Aquifer

Regional bedrock aquifers are absent in most of southwest Iowa, which forces water users in this area to rely heavily on shallow alluvial aquifers. Alluvial aquifers are sand and gravel deposits along and beneath with major rivers. The West Nishnabotna aquifer consists of alluvial deposits and some terrace deposits associated with the West Nishnabotna River. The West Nishnabotna aquifer is an alluvial aquifer in southwest Iowa that covers parts of Carroll, Fremont, Mills, Pottawattamie, and Shelby counties (Figure 1). The aquifer varies from 0.25 miles wide north of Manning in Carroll County, to over 3 miles wide in Mills and Fremont counties and ranges in thickness from 3 to over 40 feet.


Figure 1. Extent of the West Nishnabotna aquifer.

The West Nishnabotna aquifer is generally underlain by glacial till. The exception is an area approximately one-mile south of Carson to just south of Macedonia (Pottawattamie County) where alluvial deposits are in direct contact with Pennsylvanian shale and limestone units. The shallow Pennsylvanian bedrock reduces the width of the aquifer in this area to less than one mile.

The West Nishnabotna aquifer supplied urban and rural water users with between 4.2 million gallons of water per day (mgd) to 5.7 mgd from 2004 through 2009. Groundwater withdrawals from four regions, the City of Manning/Ag Processing/West Central Iowa Rural Water Association (MAPWC), City of Harlan, Regional Water, and City of Oakland, comprise approximately 80% of the total withdrawals from the main branch of the West Nishnabotna aquifer (Figure 2).


Figure 2. Percentages of groundwater removed from the West Nishnabotna aquifer by the City of Manning/Ag Processing/West Central Iowa Rural Water Association (MAPWC), City of Harlan, Regional Water, City of Oakland, and others.


Regional groundwater flow is directed toward the West Nishnabotna River in a general southerly direction. The hydraulic gradient is similar to the land surface topography in most locations. During most of the year, the West Nishnabotna River is a gaining stream. Exceptions to this occur during high river stage when temporary bank storage may cause a transient reversal in flow direction, and near major wellfields where pumping stress reverses the groundwater flow direction and creates induced recharge from the river into the aquifer. Groundwater recharge sources are precipitation and infiltration, induced recharge from surface water, and minor discharge from glacial drift and terraces along the valley wall.


West Nishnabotna Aquifer

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