Assessing the risk of violating stream water quality standards

For many years, managing agencies have enacted and enforced water quality standards based on a deterministic evaluation of the stream environments under their control. Given the random nature of the processes occurring within a stream system, the deterministic approach to water quality regulation is subject to obvious shortcomings. In an attempt to improve water quality regulation, a method is presented for quantifying the joint risk associated with dissolved oxygen deficits exceeding a specified standard and the length of such violations within a stream environment. Techniques are employed utilizing the Streeter-Phelps equation in conjunction with Monte Carlo simulation. In addition, flexibility is provided in the formulation by allowing several probability distributions to be assigned to each parameter in the model. A sensitivity analysis is also performed on the joint risk for the various probability distributions and statistical properties assumed for each parameter. Implied in the methods and results presented is the development of improved water quality standards incorporating the inherent stochastic nature of stream environments. -Authors