Miguel A. Medina

Monte Carlo analysis and Bayesian decision theory for assessing the effects of waste sites on groundwater. I: Theory

Abstract A sequential Bayesian risk methodology is outlined for addressing the problem of permitting waste sites under conditions of imperfect information. Topics covered include use of preposterior analysis to evaluate candidate sampling plans and prior/posterior analysis of risk. The algorithm is sufficiently general that it can be adapted to accommodate any specific groundwater contaminant transport model or site configuration. In the following paper an example is provided to show the details of application to a numerical groundwater transport model. The methodology described here is being incorporated into an advisory computer system for North Carolina groundwater quality modeling and management needs.

Incorporating transient storage in conjunctive stream–aquifer modeling

Abstract There has been growing interest in incorporating the transient storage effect into modeling solute transport in streams. In particular, for a smaller mountain stream where flow is fast and the flow field is irregular (a favorable environment to induce dead zones along the stream), long tails are normally observed in the stream tracer data, and adding transient storage terms in the advection–dispersion transport equation can result in more accurate simulation. While previous studies on transient storage modeling account for temporary, localized exchange between the stream and the shallow groundwater in the hyporheic zone, larger-scale exchange with the groundwater in the underlying aquifer has rarely been included or properly coupled to surface water modeling. In this paper, we complement previous modeling efforts by incorporating the transient storage concept in a conjunctive stream–aquifer model. Three well-documented and widely used USGS models have been coupled to form the core of this conjunctive model: MODFLOW handles the groundwater flow in the aquifer; DAFLOW accurately computes unsteady streamflow by means of the diffusive wave routing technique, as well as stream–aquifer exchange simulated as streambed leakage; and MOC3D computes solute transport in the groundwater zone. In addition, an explicit finite difference package was developed to incorporate the one-dimensional transient storage equations for solute transport in streams. The quadratic upstream interpolation (QUICK) algorithm is employed to improve the accuracy of spatial differencing. An adaptive stepsize control algorithm for the Runge–Kutta method is incorporated to increase overall model efficiency. Results show that the conjunctive stream–aquifer model with transient storage can handle well the bank storage effect under a flooding event. When it is applied over a stream network, the results also show that the stream–aquifer interaction acts as a strong source or sink along the stream and is too significant to be ignored. The adaptive stepsize control for stream solute transport improves overall model performance.

Monte Carlo analysis and Bayesian decision theory for assessing the effects of waste sites on groundwater, II: Applications

Abstract In the preceding paper a sequential Bayesian risk methodology is presented for the assessment of the effects of waste sites on groundwater. Specific details of the Monte Carlo implementation depend on the contaminant transport model employed. In the present paper the modifications used to adapt a deterministic numerical transport model for Monte Carlo analysis in the sequential algorithm are presented, followed by a sensitivity analysis of the resulting stochastic transport model. The sensitivity analysis assesses the impacts of the various sources of uncertainty incorporated, and emphasizes the need to explicitly consider such uncertainty in the process of risk assessment for regulatory decision. Similar modifications to a wide range of contaminant transport models have been incorporated into an advisory computer system for groundwater quality modeling and management.

Flood Nonstationarity in the Southeast and Mid-Atlantic Regions of the United States

AbstractA case study examining aspects of nonstationarity in the estimation of flood magnitudes in the Southeast and mid-Atlantic United States is presented here. The focus is on challenges posed by changes in the number and location of stream gauges over time to assess climate variability and land-use land-cover (LULC) change impacts. The study shows that statistical outliers are spatially aligned with LULC change from urbanization independently of time period and record length across the region with substantial increases (>30%) in the magnitude of the 25-year and 100-year events since 1950. Large amplification (>10) of the ratio of flood magnitude to the mean annual flood was detected in expanding urban areas, but only systematically for the 100-year event, suggesting that existing infrastructure constrains the sensitivity of flood response for events below applied engineering design criteria. Overall, annual streamflows do not exhibit statistically significant trends (98% cfl), except for positive tren...

Global Water Crisis and Climate Change

Exceptional, and possibly historic, drought conditions in North Carolina and the rest of the Southeast were endured through part of 2007 and 2008 U.S. Drought Monitor 2007, 2008 . To gain perspective, a statistical analysis of the forty-five-year record of measured flows at a gauging station on the Eno River near Durham, N.C., reveals that the daily river flow averages 3.56 m3 /s 125.75 cfs , compared to 0.29 m3 /s 10.35 cfs from May 30, 2007, to January 30, 2008. During this drought, the minimum average flows were little more than 0.028 to 0.085 m3 /s 1 to 3 cfs at the same location. Although surface storage was replenished through the early months of 2009, groundwater levels remain below average. By contrast, during Hurricane Fran September 1996 the flow averaged 416.26 m3 /s 14,700 cfs , equivalent to at least a twenty-year flood. California has endured three years of drought since the fall of 2006 DWR 2009 and this past summer most of south and central Texas has been suffering from extreme to exceptional drought conditions U.S. Drought Monitor 2009 . An El Nino weather pattern may bring relief through February 2010 Reuters 2009 . Yet, as an annual average, only about 10 million people are exposed to droughts in the United States with zero annual deaths versus over 25 million exposed in China with well over 100 annual deaths as shown in Fig. 1 EM-DAT 2009 . Severe drought has recently prevailed in Northern and Western China 2008–2009 , in major winter wheat producing areas provinces of Henan, Anhai, Shandong, Hebei, Shanxi, Shaanxi, and Gansu with precipitation levels at 70–90% below normal Branigan 2009 . It is considered the worst drought in 60 years in China, with about 9.5 million hectares of winter wheat 44% of total planted damaged. This extreme event was foreseen years earlier: Domroes and Schaefer 2003 studied the temperature and rainfall records of 165 World Meteorological Organization

Coupling High-Resolution Acoustic Sensor Measurements with Analytical and Numerical Porous Media Solute Transport Modeling

This paper presents the development of a novel acoustic sensor that enhances Doppler approaches to monitor groundwater vector flow. The technique is based on a high-resolution acoustic phase measurement: flowing water introduces a shift in the acoustic velocity component between the transmitter and receiver. By switching the transmitter and receiver modes at periodic intervals, the resulting magnitude of the acoustic phase shift yields the speed of water flow. The high-resolution of the measurement comes from the feedback loop architecture that forces an output toneburst to the transmitter to remain in quadrature with the signal recorded at the receiver. Flow velocity measurements in a column filled with saturated sand (subjected to salt tracer injections), were validated in an ASTM standard constant-head hydraulic test column. Several analytical solution solute transport models and a finite-element numerical model were applied to the test column and compared with measured data. Model predictions obtained...