11 Habitat for Humanity Critical Aquifer Memo
Memo
To: Sara Williams, Assistant Planner, City of Yelm
From: Mallory Dobbs, Civil Engineer, LDC, Inc.
Date: July 28, 2022
Re: Habitat for Humanity Yelm Plat Critical Area Report
Introduction
According to the City of Yelm Municipal Code (YMC) 18.21.010.G, critical aquifer recharge areas are
considered critical areas. Per YMC 18.21.070.B, the entire city of Yelm and its urban growth area is
identified as a highly susceptible critical aquifer recharge area. The purpose of this Critical Area Report
is to use scientifically valid methods and studies and field reconnaissance to evaluate the project
proposal and all probable impacts to critical areas.
The proposed Habitat for Humanity Yelm PRD project is located at 407 Longmire Street SE in Yelm,
WA (Thurston County Tax Parcel No. 22719230700). The site is approximately 2.30 acres (100,188
square feet) and is zoned Moderate Density Residential (R-6). The project will include subdividing the
project site into 22 lots and on 16 of the new lots a townhome unity will be constructed on 6 of the
lots a detached single-family house will be constructed. The project will also include the construction
of a new internal private road, frontage improvements along Longmire St SE, utility extensions,
stormwater facilities, and open space.
Per YMC 13.16.020, the city of Yelm utilizes the manual adopted by reference and prepared by the
Department of Ecology that contains BMPs to prevent or reduce pollution (or a technically equivalent
manual approved by the Department of Ecology). Therefore, the proposed project will utilize the 2019
Department of Ecology Stormwater Management Manual for Western Washington (SWMMWW) for the
proposed stormwater improvements thus protecting the critical aquifer recharge area. No other critical
areas are located on the proposed project site.
Stormwater Design
Per Thurston County GIS, the entire parcel is located within the Category 1 Critical Aquifer Recharge
Area (CARA). Per Volume I Section 4.10 of the SWMMWW, at a minimum, basic treatment to remove
solids prior to discharge to an infiltration facility is required. Additionally, it is important to note that
according to the Washington State Department of Health Well Construction Map, there are no wells
within 100’ of the proposed stormwater facilities. The proposed project will create over 5,000 s.f. of
new impervious surfaces and therefore will trigger Minimum Requirements #1-9 for the new and
replaced hard surface and the land disturbed. Source control BMPs will be implemented to prevent any
potential pollution to the aquifer, however, the use of the site is residential and therefore is not
anticipated to have pollution generating activities on-site. The stormwater system for the proposed
project will consist of catch basins located through the private roadway to collect the stormwater runoff
from the majority of the project parcel. The stormwater runoff will then be conveyed to a bioretention
pond that will provide the stormwater runoff with enhanced treatment prior to infiltrating into the
ground. Per Minimum Requirement #5, the proposed roof areas will utilize individual drywells located
on each lot. The roof areas will not be metal or be considered a pollution generating impervious surface
and therefore is safe to infiltrate within the CARA. The proposed utility plan and critical areas map has
been provided as Attachment 1 of this document. The preliminary drainage report prepared by LDC,
Inc. dated July 2022 provides additional information regarding the proposed project improvements.
Habitat for Humanity Yelm PRD – Critical Areas Report
July 27, 2022
Page 2 of 2
LDC, Inc. •Surveying • Engineering •Planning
Conclusion
This report assumes that all proposed improvements will be built per the construction drawings.
The proposed stormwater improvements meet the requirements of the Department of Ecology and
therefore adverse affects to the critical aquifer recharge area are not anticipated with the construction
of this project.