HR: 15:05h
AN: H53I-06    [Abstracts]
TI: Propagating Water Quality Analysis Uncertainty Into Resource Management Decisions Through Probabilistic Modeling
AU: * Gronewold, A D
EM: adg12@duke.edu
AF: Nicholas School of the Environment and Earth Sciences - Duke University, Box 90328, Durham, NC 27708, United States
AU: Wolpert, R L
EM: wolpert@stat.duke.edu
AF: Nicholas School of the Environment and Earth Sciences - Duke University, Box 90328, Durham, NC 27708, United States
AU: Wolpert, R L
EM: wolpert@stat.duke.edu
AF: Department of Statistical Science - Duke University, Box 90251, Durham, NC 27708, United States
AU: Reckhow, K H
EM: reckhow@duke.edu
AF: Nicholas School of the Environment and Earth Sciences - Duke University, Box 90328, Durham, NC 27708, United States
AB: Most probable number (MPN) and colony-forming-unit (CFU) are two estimates of fecal coliform bacteria concentration commonly used as measures of water quality in United States shellfish harvesting waters. The MPN is the maximum likelihood estimate (or MLE) of the true fecal coliform concentration based on counts of non-sterile tubes in serial dilution of a sample aliquot, indicating bacterial metabolic activity. The CFU is the MLE of the true fecal coliform concentration based on the number of bacteria colonies emerging on a growth plate after inoculation from a sample aliquot. Each estimating procedure has intrinsic variability and is subject to additional uncertainty arising from minor variations in experimental protocol. Several versions of each procedure (using different sized aliquots or different numbers of tubes, for example) are in common use, each with its own levels of probabilistic and experimental error and uncertainty. It has been observed empirically that the MPN procedure is more variable than the CFU procedure, and that MPN estimates are somewhat higher on average than CFU estimates, on split samples from the same water bodies. We construct a probabilistic model that provides a clear theoretical explanation for the observed variability in, and discrepancy between, MPN and CFU measurements. We then explore how this variability and uncertainty might propagate into shellfish harvesting area management decisions through a two-phased modeling strategy. First, we apply our probabilistic model in a simulation-based analysis of future water quality standard violation frequencies under alternative land use scenarios, such as those evaluated under guidelines of the total maximum daily load (TMDL) program. Second, we apply our model to water quality data from shellfish harvesting areas which at present are closed (either conditionally or permanently) to shellfishing, to determine if alternative laboratory analysis procedures might have led to different management decisions. Our research results indicate that the (often large) observed differences between MPN and CFU values for the same water body are well within the ranges predicted by our probabilistic model. Our research also indicates that the probability of violating current water quality guidelines at specified true fecal coliform concentrations depends on the laboratory procedure used. As a result, quality-based management decisions, such as opening or closing a shellfishing area, may also depend on the laboratory procedure used.
DE: 1847 Modeling
DE: 1871 Surface water quality
DE: 1894 Instruments and techniques: modeling
DE: 3245 Probabilistic forecasting (3238)
DE: 3275 Uncertainty quantification (1873)
SC: Hydrology [H]
MN: 2007 Fall Meeting