Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
2.6 Environmental Assessment
Wetlands and Riparian Areas
Wetland and riparian areas along the Sanderson Gulch and North Sanderson Gulch corridors are currently
very limited. Figure E-1 in Appendix E delineates significant potential wetland and riparian areas based on
the National Wetlands Inventory, 2009 aerial photography and field observations. Two recent projects
within Sanderson Gulch Park between W. Florida Avenue and S. Zuni Street have re-established a natural
riparian channel edge for approximately 1,600 feet of the channel. In those reaches of Sanderson Gulch
contained within linear parks and open space, significant additional potential exists to reestablish wetlands
and riparian areas. The 2009 Denver Parks and Recreation Gulch Master Plan (Matrix, 2009) recognized
this potential and proposes establishing a sizeable riparian buffer along the channel edge throughout the
Denver reach.
Threatened and Endangered Species
The U.S. Fish and Wildlife Service (USFWS) currently lists the following threatened and endangered
species for Denver and Jefferson Counties (http://www.fws.gov/endangered/):
Denver County Status Jefferson County Status
Piping plover Threatened Mexican spotted owl Threatened
Least tern Endangered Piping plover Threatened
Ute ladies’-tresses orchid Threatened Least tern Endangered
Preble’s jumping mouse Threatened Colorado Butterfly plant Threatened
Pawnee montane skipper Threatened
Canada Lynx
Threatened
Preble’s jumping mouse Threatened
The entirety of the Sanderson Gulch watershed is within the Denver metropolitan Block Clearance Zone for
Ute ladies’-tresses orchid, Preble’s meadow jumping mouse and the Colorado butterfly plant. These
species should not be a concern for projects within the watershed.
USFWS on-line maps currently indicate that the Canada lynx, Mexican spotted owl and the Pawnee
montane skipper are not known or believed to occur within the Denver portion of the watershed east of
Sheridan Avenue. Bird habitat resources for Piping plover, Least tern and the Mexican spotted owl within
the Sanderson Gulch watershed and along the Sanderson and North Sanderson Gulch waterways are
currently limited because the watershed is fully urbanized and there is little riparian corridor adjacent to the
channels. Future design projects will require more detailed assessments of any potential threatened and
endangered species habitat resources within the project limits.
Landfills and Contamination
There are many landfills in the lower Sanderson Gulch area. Figure E-2 in Appendix E presents known
historical landfill areas within the Sanderson Gulch watershed. The source of this information is the City
and County of Denver GIS. A review of environmental records and consultation with Denver Environmental
Health staff indicates that likelihood of encountering severe environmental hazards is relatively low.
As part of the Final DRAFT Mississippi Gulch Outfall Alternatives Analysis and Scoping Study, December
2010, an initial file review was conducted of the regulatory-listed environmental issues potentially affecting
Sanderson Gulch between Lipan Street and the South Platte River in Denver. A letter report dated October
15, 2009 by Matrix Environmental Services, LLC., (MES) to the City and County of Denver presented a
summary of the sites identified during a regulatory search that are adjacent to or up-gradient of the outfall
and may have impacted the drainageway. A First Search regulatory review of sites within a 1-mile radius of
the outfall was conducted. The only sites identified that may have a current impact on the outfall area are
the potential landfills located along lower Sanderson Gulch. Based on the potential presence of this landfill
and because of the highly industrialized areas surrounding the Study Area, a materials management plan
should be implemented for Sanderson Gulch during any intrusive development activities.
Denver Radium Site
UDFCD has been made aware of the potential for radium near the Sanderson Gulch outfall. However, the
threat has been cleared according to research as summarized in the October 15, 2009 report from Matrix
Environmental Services:
“The Denver Radium Site consists of more than 65 properties along the South Platte River Valley in Denver,
Colorado. In the early 1900s, radium was used for commercial purposes including cancer therapy and
research. Several ore processing facilities located along the South Platte River provided a domestic source
to meet the radium market demands. In the early 1920s, radium, vanadium, and uranium ores were
discovered in Africa, and the Denver radium industry dried up. Radioactive soils and debris were
abandoned at the processing facilities and the locations of the sites and contaminated debris piles were
forgotten. In 1979, the EPA re-discovered the Denver Radium Site during a review of old documents and
the State spearheaded several studies to locate the Denver Radium Site properties. The Denver Radium
Site was added to the National Priority List (NPL) of Superfund Sites in September 2003 and was divided
into 11 Operable Units (OUs).’
“Contaminants at the site included radium, thorium, arsenic, lead, and radon gas, and contaminated media
included air, soils, and groundwater. Clean-up was conducted at all 11 OUs and included the removal of
contaminated soil to a permanent offsite disposal site, installation of radon gas ventilation systems, and
contaminant stabilization. In September 2006, the Final Closeout Report for the Denver Radium Site was
completed and the properties were deemed suitable for redevelopment.’
“The Sanderson Gulch outfall is down-gradient of the Denver Radium Site Operable Unit 3 (OU3), bordering
OU3 on the south. Remedial actions at OU3 began in August 1989 and were originally completed in
September 1991. A phased approach to the cleanup allowed onsite businesses to maintain operations
during the excavation and shipment of 63,672 tons of contaminated materials from OU3. Some areas of
OU3 were not excavated and approximately 5,868 cubic yards of contaminated soils were originally left
onsite beneath the groundwater table, near water lines, under South Jason Street, around the Packaging
Corporation of America building, and along South Platte River Drive.’
“In 2001, groundwater was collected from a monitoring well located within the OU3 boundary. The well had
elevated levels of gross alpha and gross beta contamination. In 2003, an additional five monitoring wells
were installed around OU3 and sampled for radionuclide contamination. Of the five new wells, only two
wells exhibited radionuclide contamination. These wells were located within or immediately down-gradient of
the contaminated soils that were left onsite. A final round of groundwater monitoring was completed in
February 2004 and a final report was submitted in May 2004. In the 2008 five-year review of the Denver
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