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Title: Field Evaluation of the Fate of Wastewater Components from Septic Systems”
Principal Investigator's Name(s): Marjorie E. Bedessem, Assistant Prof, and Thomas V. Edgar, Associate Prof, Civil and Architectural Engr., UW.
Objective of the Project: Septic systems have been listed
as one of the top ten sources of ground water pollution by 31 of the 52
reporting states, tribes and territories in the USEPA’s 2000 national
water quality inventory. In particular, septic system leachfields have
been known to release dissolved nitrogen in the form of nitrate into groundwater,
presenting a significant source of pollution. Low cost, passive modifications,
which increase nitrogen removal in traditional leachfields, could substantially
reduce the overall impact on groundwater resources.
In recent years, septic system modifications have been proposed to include
organic-rich anaerobic zones that promote bacterial denitrification. Bacterial
denitrification results in the conversion of nitrate to nitrogen gas and
thereby minimizes the potential for nitrate contamination of ground water.
A modified leachfield design was developed based on the results of column
studies that were conducted through a concurrent federal 319 grant. The
modified design utilized local soils and incorporated an organic-rich
layer of sawdust mixed with native soil. Column studies showed an increase
in total N removal from 31 to 67 % when the denitrification layer was
employed.
The objective of the field study was to furnish an evaluation of the effectiveness
of the modified leachfield design in controlling bacterial and nutrient
contamination (nitrogen compounds) at an actual home installation. The
field evaluation was primarily supported through federal 319 funds, and
this project provided supplemental funding to address development of one
of the monitored field sites. Homeowners in areas considered to be at
risk and representative of existing rural ranchette development in Wyoming
were identified. Homeowner incentives for participation in the program
were provided and installation agreements were successfully signed with
two Wyoming households.
The modified leachfield design was installed at a new homesite in Cheyenne,
WY along with an approved standard leachfield design. An additional system,
consisting of one experimental leachfield trench and a companion standard
leachfield trench, was installed as a replacement for an existing failed
system at a residence in Laramie, WY. Twelve boreholes with nested ceramic
cup lysimeters at varying depths were installed within or adjacent to
the leachfields at each Cheyenne and Laramie location. A pressure vacuum
lysimeter sampling system was constructed at each site to permit vadose
zone as well as saturated zone sampling and allow comparison of the treatment
efficiency of side by side systems. Between the two sites, there are 78
lysimeter sampling ports. Samples for BOD and nitrogen species are taken
from the septic tank and lysimeter ports on a quarterly basis when possible
in order to evaluate the long term treatment effectiveness of the modified
systems. Sample collection and analysis are on-going with two year agreements
from each of the households and options for continued monitoring thereafter.
Publications (and other project products):
Results of the column study phase of the companion project are reported
in “Nitrogen removal in laboratory model leachfields with organic-rich
layers” by Marjorie E. Bedessem, Thomas V. Edgar, and Robert Roll
scheduled for publication in the May/June 2005 edition of Journal of Environmental
Quality.
Student Support
Two undergraduate students in the Civil Engineering Department received
training on this project, Chris Knodel and Ted Cramer. These students
helped to install monitoring systems during leachfield construction, developed
and field tested the sampling apparatus, collected samples and assisted
with the laboratory analyses.
Presentations: Project overview and status was presented at the WWDC-USGS Water Research Program Priority and Selection Committee Summer Meeting, July 23rd 2003. Preliminary results of the field efforts were also presented at the Wyoming Water Forum on May 5, 2004.
Follow-up Funding: The column study and the bulk of the field site installation and sampling effort were funded through the Wyoming Department of Environmental Quality, Water Quality Division, by a Section 319 Non-Point Source Grant from the United States Environmental Protection Agency. Additional funding to maintain student support for continued sampling of the field sites is being requested through an NSF EpScor undergraduate research proposal submitted by Katharine Trowbridge, an undergraduate civil engineering student, for Summer 2005.