10.a2 2022-03-10 Tiller Response to Comments on AOP
Minnesota District
7200 Hemlock Lane, Suite 200, PO Box 1480, Maple Grove, Minnesota 55311
t. (763) 425-4191 f. (763) 425-7173
www.martinmarietta.com
March 10, 2022
Ken Cammilleri, City Administrator
City of Scandia
14727 209th St. N
Scandia, MN 55073
Dear Mr. Cammilleri:
This letter is written in response to an email dated March 7, 2022 from Mr. John Lindell, to the City of
Scandia and received by Tiller Corporation, a Martin Marietta Materials, Inc. company (herein referred to
as Tiller) on March 7, 2022 regarding Tiller’s 2022 AOP Application for our Scandia mine (Site). The email
from Mr. Lindell expressed three separate matters, which include: utilizing current aerial imagery for use
in the AOP application, changes to the surface expression for nearby wet features and mining into the
groundwater. Tiller takes comments from its neighbors seriously. The email received from Mr. Lindell
prompted an evaluation of the matters raised and Tiller offers the following responses.
1. Matters Pertaining to Use of Aerial Imagery in the AOP
a. Mr. Lindell recommends that Tiller’s AOP approval be delayed and Tiller provides an accurate, up
to date aerial photo of the mining property and adjacent properties.
The City of Scandia requires aerial photos to be included in the AOP for the purpose of showing
the active mining area and location of the active stockpiling area.
Figure 3: Aerial Photo includes an aerial photo captured by an unmanned aerial vehicle (UAV) for
areas within the Site, dated September 13, 2021. The aerial photo shows the active mining area
and location of the active stockpiling area, which complies with the City’s required use of aerial
imagery in the AOP. Use of the UAV is limited to Tiller property.
For areas outside of the Tiller property, the most current aerial photo available through
Washington County is used, which is dated 2020.
2. Matters Pertaining to the Surface Expression of Nearby Features
a. Mr. Lindell shared observations that “ponds” on and near his property have exhibited a drier
surface expression more recently in comparison to a few years ago when a wetter surface
expression was present.
These observations are not surprising when considering the precipitation and estimated relative
soil moisture conditions during this timeframe within our local region. Review of the
precipitation trends is important because surface water in the form of precipitation is largely
March 10, 2022
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what influences the characteristics of the shallow depressions associated with the moraine
southwest of the Site. The shallow wet depressions and wetland features located in the area
have been described in the past as being perched, which means the feature is underlain by a low
permeability layer that allows water to perch. These features have water levels that are perched
above the regional water table, thereby relying more on precipitation to maintain wet
characteristics.
With this in-mind, we could expect to see changes in the surface expression of these features in
response to precipitation and climatic trends. The precipitation graph shown below as Figure 1:
Annual Precipitation (2016-2021) illustrates annual precipitation from 2016 through 2021 for
the Lower St. Croix River area. From 2016 through 2018, there was a decline in annual
precipitation followed by a substantial peak in annual precipitation in 2019 and a subsequent
decline through 2021. The peak in annual precipitation for the years mentioned above occurred
in 2019 and correlates with observations that wetter conditions occurred a few years ago.
Figure 1: Annual Precipitation (2016-2021)
We can also look at the Palmer Drought Severity Index (PDSI) to inform on the surface expression
of the wetlands in the subject area. The PDSI is a standardized index based on a simplified soil
water balance and estimates relative soil moisture conditions. The magnitude of PDSI indicates
the severity of the departure from normal conditions. A PDSI value >4 represents very wet
conditions, while a PDSI <-4 represents an extreme drought. Figure 2: PDSI (2016-2021) illustrates
the PDSI in the month of July for the Lower St. Croix River area from 2016 through 2021. As shown
in the figure, from 2016 through 2018 there was a decline in the PDSI or estimate of the relative
soil moisture conditions. Then, in 2019 and 2020, the PDSI indicates very wet conditions followed
by significant drought conditions in 2021.
March 10, 2022
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With the exception of a spike in precipitation in 2019, the annual precipitation had been declining
over the last six years in the Lower St. Croix River area. The PDSI also indicates a downward trend
in estimated relative soil moisture conditions for the last six years during the month of July, with
the exception of the very wet conditions in July of 2019 and July 2020. The drought conditions in
2021 represent a more significant departure toward dry conditions experienced more recently.
The variations in precipitation and estimated relative soil moisture help demonstrate the range in
seasonal variations that can occur each year. For features in the landscape that are influenced by
and respond to precipitation and soil moisture, it can be expected that observable changes will
occur in response to these conditions as they change over time.
A review of a few aerial photos, both historical and recent helps illustrate the range in surface
expressions for the shallow depressions in the area. To provide an area of reference, a yellow
outline is made around the wet perimeter of a feature located in the area subject to Mr. Lindell’s
comment. The yellow outline is based on the wet perimeter as indicated on a 2020 Washington
County aerial photo. Aerial images included are 1970, 2009 and 2020.
3. Matters Pertaining to Mining into the Groundwater
a. Mr. Lindell describes that when Scandia became a City and assumed regulatory authority over
mining, Tiller was authorized to excavate into the groundwater and goes on to suggest that
Washington County prohibits excavation into groundwater.
Washington County does not prohibit excavation into the groundwater. The Washington County
Development Code, Chapter Seven Mining Regulations, Section 5.5 Protection of Water Tables
states that, “Excavation into unconfined (emphasis added) aquifers must be closely monitored
and conducted according to the conditions of the permit.” The County and the City both allow
mining into unconfined aquifers. In addition, they both prohibit excavation into confined
Figure 2: PDSI (2016-2021)
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aquifers. Excavation into the groundwater at the Scandia Site is taking place in an unconfined
aquifer.
Tiller operates its’ mining sites with a great deal of responsibility. As a leader in the aggregate mining
industry in Minnesota, Tiller recognizes the importance of protecting our water resources. Tiller has and
will continue to operate the Site in accordance with local, state and federal rules and regulations.
Please let me know if you have any questions or require additional information on this matter. I may be
reached at (763) 425-4191.
Sincerely,
Christina Morrison, P.G.
Environmental Engineer II
Enc
Tiller Property
Tiller Property
218th Street
¯0 200 400100Feet
1970 Aerial Photo - Scandia
Aerial Photo Source: HIG
Tiller Property
Tiller Property
218th Street
¯0 200 400100Feet
2009 Aerial Photo - Scandia
Aerial Photo Source: NAIP
Tiller Property
Tiller Property
218th Street
¯0 200 400100Feet
2020 Aerial Photo - Scandia
Aerial Photo Source: Washington County