HomeMy WebLinkAbout03 - Groundwater Memo WET - 01-12-21
Butte | Anaconda | Great Falls | Bozeman | Kalispell · www.waterenvtech.com 1
MEMORANDUM
To: Morrison-Maierle, Inc.
From: Michael E. Nicklin, PhD, PE
Date: January 12, 2021
Re: Supplement Explaining Potential or Lack Thereof of Adverse Impacts
Significance of Drawdown
In addition to drawdown that will occur within the permit boundary, some
groundwater level drawdown is projected to occur outside the project boundary. No
adverse impacts associated with drawdown outside the boundary are projected for
three reasons:
a. High groundwater levels tend to be a liability for housing structures (e.g.,
basements and crawl spaces) and other infrastructure. Hence, a slight
drawdown of from 0.5 to 1.5 feet should, if anything, should be considered
an asset.
b. A water level change of 0.5 to 1.5 foot change, over even higher, will not
cause any adverse effect to any properly constructed supply well outside
the project boundary.
c. The actual extent of drawdown is likely much more limited outside the
project boundary than what is presented in Figure 8 of the model report.
This is due to unmapped underground drainage features (e.g., agricultural
drain tile), underground infrastructure such as water and sewer
conveyance features, etc. which act as local limiting boundaries to
groundwater level changes.
Need or lack thereof obtaining surveyed elevations of the existing drain tile
Although surveyed elevation data of the drain tile could be obtained it is deemed
that it would not lead to a substantive change in model calibration and application
efforts. The current assumption of depth to the drain of three feet (based upon a site
measurement near the aquifer test site) coupled with the field test
hydraulic/transmissivity led to an excellent model calibration or fit to the water level data
collected at the site monitoring wells. Hence, it is deemed there would be no benefit or
MEMORANDUM · Northwest Crossing Groundwater Model
Butte | Anaconda | Great Falls | Bozeman | Kalispell · www.waterenvtech.com 2
substantive model improvement if more surveyed data for the existing drain tile were
collected.
Potential for Surface Water/Groundwater Quality Degradation
No degradation of surface water or groundwater quality will be caused by
implementing the drain system. The water quality of the groundwater beneath and
adjoining the property will be unaffected by the drain system. Note that the streams in
the vicinity of the project typically interact with the groundwater system already. Such
interaction will also be the case after the system is installed. Given that groundwater
quality will not be affected by the drain system then likewise the surface water quality
will not be impacted.