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Onsite
wastewater systems (OWS), or septic systems, account for roughly 25% of
domestic waste disposal in the United States. OWS discharge is an
important component of TMDL calculation. Most
watershed models require the external estimations of OWS loads based on
GIS data. This approach makes it difficult to develop a TMDL
implementation plan involving OWS conversion or consider projected
trade-offs between OWS vs. centralized sewers.
Systech
modified WARMF, a GIS-based watershed model, to
simulate OWS loading. Algorithms were based on OWS research and
scientific principles. The model now calculates the "edge-of-drainfield"
pollution loads rather than requiring them as input. The modified WARMF was applied
to the 840 km2 (325 mile2) Dillon Reservoir
Watershed in Colorado where over 1000 onsite wastewater systems (OWS) can
be found along the Blue River. WARMF
was set up with digital elevation model (DEM), meteorology, point source,
and land use data. Additional data collected in the watershed for this
project included surface water quality, soil properties, well data, and
the spatial distribution of OWS in the watershed. A biozone module was
developed to represent the treatment processes taking place at the
biologically active soil layer that develops in a soil receiving septic
tank effluent. The build-up of
bacteria increases field capacity, and decreases porosity and the
infiltration rate. The biozone
module was tested with data collected in the laboratory column experiments
using an accelerated dosing rate. The
tested module was then incorporated into WARMF to process the septic
effluent before releasing it to a soil layer.
Hydrology and water quality simulations were run and the model was
calibrated to available observed data.
After establishing a base case, various management scenarios
related to OWS were tested. These
scenarios included the conversion of existing OWS to centralized sewers.
The scenario runs provide information to evaluate the trade-offs between
OWS and centralized sewer systems as well as the general impact of OWS on
surface water quality. A report published by the National Decentralized
Water Resources Capacity Development Project is available for download (Siegrist
et. al 2005). Check our journal articles,
conference publications, and reports
sections for more information.
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Schematic diagram of OWS in WARMF
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