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Lincoln, NE, United States

Ahlfeld D.P.,University of Massachusetts Amherst | Schneider J.C.,Olsson Associates | Spalding C.P.,McDonald Morrissey Associates Inc.
Hydrogeology Journal

Anomalies found when apportioning responsibility for streamflow depletion are examined. The anomalies arise when responsibility is assigned to the two states that contribute to depletion of Beaver Creek in the Republican River Basin in the United States. The apportioning procedure for this basin presumes that the sum of streamflow depletions, computed by comparing simulation model runs with and without groundwater pumping from individual states, approximates the streamflow depletion when both states are pumping. In the case study presented here, this presumed superposition fails dramatically. The stream drying and aquifer-storage depletion, as represented in the simulation model used for allocation, are examined in detail to understand the hydrologic and numerical basis for the severe nonlinear response. Users of apportioning procedures that rely on superposition should be aware of the presence and likely magnitude of nonlinear responses in modeling tools. © 2016 Springer-Verlag Berlin Heidelberg Source

Reichard J.S.,Georgia Southern University | Nelson B.R.,Olsson Associates | Meyer B.K.,Georgia State University | Vance R.K.,Georgia Southern University
Southeastern Geology

St. Catherines Island is a 10 mile (16 km) long barrier island located on the Georgia coast between the Savannah and Altamaha Rivers. Periodic measurements of hydraulic head and water chemistry in the Upper Floridan aquifer were obtained over 21 months from a 5 mile (8 km), north-south transect of four water-supply wells on the island. Head data show that the artesian groundwater follows a general south to north groundwater flowpath into the cone of depression centered near Savannah, Georgia. Previous studies have shown that the loss of artesian head within this system has led to both lateral and vertical saltwater intrusion along the Georgia coast. Saltwater intrusion is of considerable interest because the Upper Floridan aquifer serves as the principal water supply for the southeastern coastal region. Water chemistry data from the Upper Floridan aquifer in this study reveal a consistent mixing pattern along the south to north groundwater flowpath. Total dissolved solids average 358 mg/L in the most up-gradient well and progressively decline to 309 mg/L in the down-gradient well. Similarly, average chloride concentrations along the flowpath decline from 13.4 to 9.1 mg/L and sulfate from 113 to 73 mg/L. The artesian groundwater also has low dissolved oxygen content (4 to 8%) and is under reducing conditions (-0.30 to -0.34 relative V) and slightly alkaline (pH 7.6 to 7.8). Most significant is the decrease in sulfate and chloride concentrations. The decline in sulfate, which represents 82% of the observed decline in total dissolved solids, can be explained by biotic sulfate reduc- tion. However, because chloride is a conservative tracer, its decreasing concentration within the Upper Floridan is due to abiotic mixing and dilution with either surficial freshwater moving downward along the flowpath, or by saline water entering at a discrete point up-gradient of the transect.Samples collected from shallow wells on the island show average chloride concentrations in the surficial aquifer to be 2 to 4 times greater than what is found in the Upper Floridan, thereby eliminating dilution by surficial freshwater as the cause of the decreasing chloride trend in the artesian aquifer. In regards to the intrusion of more saline water, modern seawater can be ruled out because it does not fall along the observed Upper Floridan mixing line as plotted on a trilinear diagram. However, chemical data from nearby Lower Floridan wells do plot up-gradient of the mixing line. Based on these data, it is concluded that saltwater intrusion is taking place within the Upper Floridan aquifer on St. Catherines Island by the discrete upward movement of more saline water from the Lower Floridan along near-vertical joints, faults, and or solution collapse features. These pathways are believed to ultimately be related to recurrent movement of known basement structures located beneath the sedimentary sequence of the Coastal Plain. Source

Taylor C.W.,Olsson Associates | Weldon B.D.,New Mexico State University | Jauregui D.V.,New Mexico State University | Newtson C.M.,New Mexico State University
Practice Periodical on Structural Design and Construction

Ultrahigh-performance concrete (UHPC) develops very high compressive strengths and exhibits improved tensile strength and durability properties that make UHPC a promising material for bridge applications. Through case studies on typical prestressed concrete girder bridges (simple and continuous), the potential impact of implementing UHPC in New Mexico was investigated. Two existing bridges with high-performance concrete girders were redesigned using UHPC with a compressive strength of 155.1 MPa (22,500 psi) and a modulus of rupture of 8.0 MPa (1,160 psi). The redesign used a modified load factor design procedure for the Service III flexure limit state and a modified load and resistance factor design procedure for the ultimate shear limit state that considered the compressive strength, modulus of rupture, and modulus of elasticity of UHPC. Additionally, 15- and 18-mm-diameter (0.6- and 0.7-in.) prestressing strands were investigated. The use of UHPC and 18-mm-diameter (0.7-in.) prestressing strands reduced the required volume of girder concrete by up to 40%. Additionally, the contribution of the steel fibers in the UHPC significantly reduced the required shear reinforcement. © 2013 American Society of Civil Engineers. Source

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