City of Renton
Project: Septic Effluent Loading Impacts
Location: King County, Washington
Effluent from malfunctioning septic systems can introduce nitrates, chlorides,
phosphorus, and other chemicals into aquifers that supply drinking water.
Once these chemicals reach the groundwater, they can move and spread,
creating a potential health hazard. However, effluent contributes to the
recharge that an aquifer receives. As part of an environmental impact
study for a sewer treatment plant, the City of Renton wanted to assess
the impacts of septic effluent on the lower Cedar Valley aquifer.
The challenge for this job was to design an approach that addressed the
environmental concern in a way that was scientifically defensible, yet
We predicted how water quality in the aquifer would change under several
sewage alternatives using typical effluent discharge rates, natural recharge
rates, and data from water quality studies on septic effluent. Our analyses
also considered how much aquifer recharge would be reduced if septic infiltration
were eliminated. To estimate this reduction in recharge, we first needed
to estimate precipitation-related recharge by analyzing rainfall and evapotranspiration
data. We also correlated short-term rainfall recorded by the City of Renton
to the long-term records at SeaTac airport; this analysis allowed us to
establish a reliable statistical basis for long-term precipitation averages
in the project area.
Our work contributed to a successful project permitting effort, ultimately
resulting in construction of the sewer treatment plant.