High-density subdivisions in the North Hills north of Helena and declining groundwater levels have caused concern about how much groundwater development can occur, how densely groundwater wells can be spaced, and if restrictions on water usage are needed. Elevated nitrate concentrations detected in some wells created concerns about the use of individual septic systems in places where thin soils overlie fractured bedrock aquifers. These groundwater availability and quality concerns led to the designation of a Temporary Controlled Groundwater Area (CGWA) by the Montana Department of Natural Resources and Conservation (DNRC) in 2002 and again in 2008.
The purpose of the North Hills groundwater investigation was to evaluate the hydrogeology of the North Hills study area in sufficient detail to evaluate the aquifer’s response to current and potential groundwater pumping. The investigation also evaluated the extent and magnitude of water-quality issues.
Water levels in a network of 74 wells and stages and flows at 17 surface-water sites were measured from January 2010 through July 2011. Synoptic water-quality sampling events were conducted in April, August, and October, 2010. Endocrine-disrupting chemicals were sampled for April 2011. Groundwater monitoring wells were drilled and seven aquifer tests were performed to determine aquifer properties. A water budget and numerical groundwater models were constructed to model scenarios evaluating the effects of current and future residential development on groundwater.
The study results include refined groundwater surface maps, aquifer test results, water-quality data, water budget estimates, effect of bedrock faults on the groundwater flow system, groundwater–surface-water interactions, and calibrated groundwater models available for use.
The modeled pumping increases from residential development can cause groundwater level declines and diminishing groundwater outflow to drains and Lake Helena. The impact of a potential 160-acre subdivision was evaluated using various housing densities. Small lots and potentially dense housing leads to more severe groundwater level declines than larger lots with fewer houses. The effect of pumping from 470 households compared to 47 households shows that groundwater drawdown by the denser housing is about 10 times that of the less dense housing. A scenario removing the Helena Valley Irrigation Ditch and its associated irrigation suggests water levels would decline 15 to over 35 feet within a few miles of the canal.
One groundwater sample exceeded the drinking water standard for nitrate (10 mg/L), and 30% of the groundwater samples were above the typical background value of 2 mg/L. The elevated nitrate concentrations were likely from septic effluent.
The DNRC used the study results to allow the CGWA to expire in January 2010.
Download report (12.17 MB)
Download zipped North Hills Area Model Transient files (330.30 MB)
Download zipped North Hills Area Model Steady State files (8.5 MB)
Download zipped Map Files (151.2 MB)
Download zipped Pediment Focus Model Steady State files (8.7 MB)
Download zipped Pediment Focus Model Transient files (147.6 MB)
|Kirk Waren||Team Leader, Senior Research Hydrogeologist, Groundwater Modeler (Retired)|
|Andy Bobst||Senior Research Hydrogeologist|
|Julie Butler||Hydrogeologist (currently with the Massachusetts Dept. of Environmental Protection)|
|Jane Madison||Hydrogeologist (currently with Montana Department of Environmental Quality)|
|James Swierc||Hydrogeologist (Lewis and Clark Water Quality District, LWQPD)|
Data collected for this project can be accessed through the Ground Water Information Center (GWIC) database:
Model scenarios that tested aquifer response from pumping various subdivision densities cause groundwater level declines and diminishing groundwater outflow to drains and Lake Helena.
Content last updated: 1/18/2018 8:14:36 AM