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Evaluation of Deep Installations of Septic System Drainfields in Utah: Performance and Treatment Effectiveness

Common practice in the field of on-site wastewater treatment is that drain fields should be
located at shallow depths in order to maximize aerobic decomposition of organic wastewater
contaminants, and enhance evapotranspiration. This results in less transport of contaminants to ground water, and increased removal of nutrients (nitrogen and phosphorus) through vegetative uptake. However, in Utah, drain fields have often been, and continue to be, installed at depths of 8 to 10 feet or even deeper. This practice started many years ago in order to install the systems in soil materials that are more permeable than surface layers. The concern then was to dispose of the wastewater, with little attention given to the ability of the soil system to accomplish degradation of the wastewater contaminants. The practice of deep installation continues today – in some cases disposal is still emphasized instead of treatment-and also because the design process for deep systems results in much smaller systems.

The concern is that treatment is not effective at the depths that septic systems are installed in
Utah. Data will be developed to evaluate this issue.

Benefits to State:

Utah’s local health departments are presently permitting deep on-site systems without clear indication if these systems are adequately treating wastewater contaminants. Results of this research will provide information to the health departments on whether they can either continue permitting these types of systems with confidence or eliminate the use of the systems.

Geographic Areas:

Study Areas: Cache County.

Areas Benefited: All areas of Utah where deep systems are commonly used for on-site
wastewater treatment.

Accomplishments

Findings: With cooperation of the Bear River Health Department eight sites in Cache Valley were identified and sampling equipment installed in drain fields. Four of the sites utilize deep trenches for treatment of wastewater while four utilize shallow trenches. Sampling of the sites has begun and will continue through December of 2010. We have also worked with the owners of the sites to educate them on proper septic system use practices.

Results: All analytical and sampling techniques have been developed and tested. Leachate
samples from the drain fields are in the process of being analyzed for nitrate nitrogen,
phosphorus, coliform bacteria, total suspended solids, and chemical oxygen demand to
determine treatment effectiveness in both the shallow and deep trench systems.


Site locations in Cache Valley
Site locations in Cache Valley.

Deep trench system installation, Wellsville, UT, November 2008

Deep trench system installation, Wellsville, UT, November 2008.

Deep trench installation, Clarkston, UT, November 2008

Deep trench installation, Clarkston, UT, November 2008.

Installation of lysimeters using silica and bentonite.  Access tubes extend to surface and are covered with a grate

Installation of lysimeters using silica and bentonite. Access tubes extend to surface and are covered with a grate.


October 11, 2010