『Abstract
Water in the urban front-range corridor of Colorado has become
an increasingly critical resource as the state faces both supply
issues as well as anthropogenic degradation of water quality in
several aquifers used for drinking water. A proposed development
(up to 1100 homes over two quarter-quarter sections) at Todd Creek,
Colorado, a suburb of Westminster located about 20 miles northeast
of Denver, is considering use of onsite wastewater system (OWS)
to treat and remove domestic wastewater. Local health and environmental
agencies have concerns for potential impacts to local water quality.
Nitrogen treatment in the vadose zone and subsequent transport
to ground water at a development scale is the focus of this investigation.
The numerical model HYDRUS 1D was used, with input based on site-specific
data and several transport parameters estimated from statistical
distribution, to simulate nitrate concentrations reaching ground
water. The model predictions were highly sensitive to mass-loading
of nitrogen from OWS and the denitrification rate coefficient.
The mass loading is relatively certain for the large number of
proposed OWS. However, reasonable values for the denitrification
rate coefficients vary over three orders of magnitude. Using the
median value from a cumulative frequency distribution function,
based on rates obtained from the literature, resulted in simulated
output nitrate concentrations that were less than 1％ of regulatory
maximum concentrations. Reasonable rates at the lower end of the
reported range, corresponding to lower 95％ confidence interval
estimates, result in simulated nitrate concentrations reaching
groundwater above regulatory limits.
Keywords: Urban; Septic systems; HYDRUS; Modeling; Nitrates; Denitrification』
1. Introduction
2. Site description
3. Methodology
3.1. Model selection: HYDRUS 1D
3.2. Model domain, boundary and initial conditions
3.3. Model input parameters
3.3.1. Soil physical parameters
3.3.2. Effluent loading rate
3.3.3. Nitrogen effluent concentration
3.3.4. Nitrification rate and denitrification rate coefficient
parameters
3.4. Model simulations
3.5. Sensitivity of model results to uncertain input parameters
4. Conclusions
Acknowledgements
References