The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en eche... more The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modifi ed by karst processes infl uenced by structural and stratigraphic controls. While the modern confi ned Edwards Aquifer fl ows through cavernous voids at the base of the escarpment, air-fi lled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed.
Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a bor... more Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a borehole in the unsaturated zone of a partially-welded, fractured tuff at Apache Leap, Arizona. One-meter intervals were later isolated at multiple depths and pumped to purge drilling air from each interval. The volume of air purged (at 1 atm, 20 C), ranged from a low of 0.3 m 3 in an unfractured interval, to a high of 252 m 3 in a highly fractured interval. The concentration of SF 6 remained high throughout the purge in all intervals and often increased over time. Measurements of d 13 C, 14 C and CO 2 concentration indicated that atmospheric air was eventually drawn into several of the intervals in spite of the fact that SF 6 concentrations remained high. Possible explanations include mixing of atmospheric air drawn through fractures with partially-purged matrix air, and delayed removal of SF 6 relative to atmospheric gases due to adsorption of SF 6 within the tuff matrix, dissolution into pore water, or diffusion from dead-end pores with restricted openings. In this system, following a long delay between drilling and purging, it was found that the risk of contamination from surface air by over-purging was substantially greater than the risk of contamination from residual drilling air by under-purging.
The B isotopic ratio of a fly ash leachate can be very different from the B isotopic ratio of a n... more The B isotopic ratio of a fly ash leachate can be very different from the B isotopic ratio of a natural groundwater. Mixtures of leachate and groundwater typically result in nonlinear B isotope mixing curves that enable identification and quantification of leachate contamination in a groundwater at much lower levels than possible using concentration analyses alone. Limits on B isotope use for contaminant quantification will exist for some environments such as landfills with multiple ash types, but B isotopic analysis may often remain the preferred method for qualitative identification.
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at t... more This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction this manuscript in whole or in part may be granted by the head of the major department of the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: ^ APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below.
The edge of regional aquifers can be complex hydrodynamic systems with unique flow dynamics, wate... more The edge of regional aquifers can be complex hydrodynamic systems with unique flow dynamics, water quality, and continuity relationships with the main aquifer system. A site near the southwestern margin of the High Plains Aquifer (USA) was investigated to characterize the local hydrogeology and its relationship with the regional aquifer system. Measurements of tritium, ion concentrations, oxygen and hydrogen isotopes, and hydraulic heads documented (1) a discontinuous saturated zone, (2) no inflow to the study area from the regional aquifer, (3) focused recharge beneath playas with limited local mixing between pockets of saturation, (4) outflow orthogonal to the regional aquifer flow direction, (5) localized multi-year reversals in flow direction following high precipitation events, and (6) a magnified influence of the paleo-erosional surface of the basement rock (Dockum Group) on groundwater isolation and flow direction. In isolated areas, groundwater can be trapped on decadal time scales by depressions in the Dockum, or by recharge events that periodically reverse groundwater gradients.
Long-term metal and arsenic mobility between wetlands and lakes: variable histories within the sa... more Long-term metal and arsenic mobility between wetlands and lakes: variable histories within the same floodplain
The theoretical minimum difference between the S 13C of soil COz (a concentration) and soilrespir... more The theoretical minimum difference between the S 13C of soil COz (a concentration) and soilrespired CO* (a flux) was previously thought to be 4.4%0. It is shown here that the difference is a function of the 6 13C of soil-respired COZ, with possible minimum values less than 4.4%0. A simple equation is given which allows the 613C of soil CO2 or soil-respired CO2 to be calculated from four measurable variables. Sampling methodology is investigated demonstrating the need for correcting measured soil-respired CO2 samples for CO, contamination in base reagents, for contributions from atmospheric CO*, and for isotopic fractionation at the base-solution/air interface.
Bald cypress (Taxodium distichum) is known to respond to increases in precipitation with increase... more Bald cypress (Taxodium distichum) is known to respond to increases in precipitation with increased radial growth even when rooted in continuously saturated sediments where water is not a growth-limiting factor. Measurements of ␦ 18 O, Cl Ϫ , 3 H and hydraulic head in surface water and shallow groundwater in an oxbow lake-wetland in northern Mississippi show that rapid downward flow of surface water into the root zone is initiated only after precipitation-induced increases in surface water depth exceed a threshold value. Rapid flow of surface water through the root zone has the potential to introduce oxygen to sediments that would otherwise be anoxic, facilitating nutrient uptake and growth. Climatic reconstruction using tree rings from bald cypress in this environment appears possible because increases in precipitation generally correlate well with increases in water level, which in turn enhances the delivery of oxygenated water to the roots.
Picric acid is an explosive historically produced and disposed at the Louisiana Army Ammunition P... more Picric acid is an explosive historically produced and disposed at the Louisiana Army Ammunition Plant (LAAP) in northern Louisiana. The potential for natural degradation of picric acid was investigated by creating picric-acid slurries with four LAAP sediments of variable composition and monitoring for up to 98 days. The concentrations of picric acid decreased rapidly in all slurries during the first day, attributed to adsorption, followed by slower decreases in some samples due to degradation. Degradation in unsterilized slurries was nearly complete within 80 days for two of the four sediments. Increases in nitrite and nitrate concentration over time were proportional to the loss of picric acid and indicate that at least two of the three nitrite groups were removed from the picric acid molecule. The absence of significant concentrations of compounds with a mass greater than 100 amu in the final solutions suggests that all three nitrite groups were removed. No correlation was found between the degree of degradation and grain size, clay content, organic content, carbonate content, or a suite of element concentrations in the sediment. Degradation in sterilized samples was minimal for all sediment slurries, indicating microbial activity as the primary mechanism of degradation.
ABSTRACT Shortly after peak stage of the 2011 Mississippi River flood, water samples were collect... more ABSTRACT Shortly after peak stage of the 2011 Mississippi River flood, water samples were collected from the river, from sand boils near the toe of the levee, and from actively flowing relief wells over a 55 km stretch north of Vicksburg, MS. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Sand boil discharge was similar to water from relief wells only at a location where the levee sits on ancient channel fill deposits. Seepage at this site is forced along a deeper pathway beneath the fine-grained channel fill. Where the levee sits on sandy point bar deposits, shallow flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but appears to reflect unique weathering or redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.
The radiocarbon activity of carbon collected by vacuum distillation from a single partially satur... more The radiocarbon activity of carbon collected by vacuum distillation from a single partially saturated tuff began to decline after approximately 60% of the water and carbon had been extracted. Disproportionate changes in 14C activity and 613C during distillation rule out simple isotopic fractionation as a causative explanation. Additional phenomena such as matrix diffusion and ion exclusion in micropores may play a role in altering the isotopic value of extracted carbon, but neither can fully account for the observed changes. The most plausible explanation is that distillation recovers carbon from an adsorbed phase that is depleted in 14C relative to DIC in the bulk pore water.
ABSTRACT Identifying and quantifying sources and sinks of CO2 is integral to developing global ca... more ABSTRACT Identifying and quantifying sources and sinks of CO2 is integral to developing global carbon budgets and effectively modeling climate change. Adsorption of CO2 onto mineral and soil surfaces has generally been regarded as an insignificant sink, though few studies have investigated adsorption on natural materials at temperatures and CO2 concentrations relevant to atmospheric or soil zone conditions. In this study, annual adsorption at the scale of North America was modeled for the upper 3 m of the Earth’s surface (the root zone) based on our own and published adsorption data, and results compared with reported estimates for the North American terrestrial carbon sink during 2000–2005. Our results suggest that adsorption can account for 1–3% of the average annual sink during these years. At smaller regional scales where more adsorptive deposits are present, such as volcanic ash or high-organic soils, the sink may be significantly larger.
AbstraetmTbe occurrence and significance of aqueous flow through fractures in unsaturated tuff wa... more AbstraetmTbe occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the Apache Leap Research Site near Superior, Arizona. Water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel, from the saturated zone in a vertical borehole (USW UZP-4), and from both the unsaturated and saturated zones in 14 an angled borehole (DSB). The geochemistry and C activity of water samples from the DSB suggest that most of the recharge to the saturated zone has occurred through fractures, especially beneath the ephemeral streams. Evidence of substantial recent recharge through fractures was found in saturated-zone samples from the mine haulage tunnel using 3H, 634S and SO42-/CI-analyses. Evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the DSB using geophysical measurements from the borehole, water-content analyses from core samples, and ~4C and 3H analyses from pore water extracted from preserved core samples. Post-bomb 14C activity was measured in pore water near fractures just above the saturated zone.
Seepage beneath artificial levees is a common concern during flooding events. Risk of levee failu... more Seepage beneath artificial levees is a common concern during flooding events. Risk of levee failure is elevated when piping erodes channels beneath the levee, evidenced by the formation of sand boils where transported sediments discharge. Along the lower Mississippi River, pathways of floodwater beneath the levee vary with surface geology, following deeper paths where the levee overlies fine-grained channel-fill deposits, and shallower (higher risk) paths where it overlies sand-bar deposits. Shallow, organic-rich alluvial aquifers are often geochemically stratified into upper oxic and lower anoxic zones, raising the possibility of using the geochemical signatures of discharging water from sand boils to differentiate flow pathways. A preliminary investigation north of Vicksburg, MS, (USA) during the 2011 Mississippi River flood demonstrated the potential of using geochemistry to identify deep and shallow pathways, though the study was limited to cation and trace element analyses. Sampling during the 2015 and 2016 floods for temperature, conductivity, redox potential (Eh), dissolved oxygen (DO), major ions, trace elements, tritium, and stable isotope ratios of oxygen, hydrogen, and strontium, facilitated a greater understanding of the nature of flow and geochemical evolution of groundwater in this environment. Characteristics of deeper flow pathways (relative to shallow) included (1) lower Eh and higher Fe and As, reflecting anoxic conditions and high-Fe sediments, (2) proportional increases in Fe and HCO 3 , indicating reductive dissolution of Fe-oxyhydroxides, and (3) higher ratios of Ba/Ca and Sr/Ca, reflecting differences in the elemental composition of minerals with depth. Tritium results indicate that subsurface flow pathways are dynamic, shifting spatially with the rapid changes in hydraulic gradients during and between flooding events. Estimated residence times of groundwater discharging from sand boils and relief wells ranged from essentially zero (discharge of concurrent floodwater) up to a quarter century. Lower strontium isotope ratios (87 Sr/ 86 Sr) were observed in the aquifer relative to river water, though with no clear variation with flow depth. Oxygen and hydrogen isotope ratios (δ 18 O and δ 2 H) show evidence of partial evaporation prior to recharge, also with no apparent variation with subsequent flow depth.
The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en eche... more The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modifi ed by karst processes infl uenced by structural and stratigraphic controls. While the modern confi ned Edwards Aquifer fl ows through cavernous voids at the base of the escarpment, air-fi lled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed.
Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a bor... more Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a borehole in the unsaturated zone of a partially-welded, fractured tuff at Apache Leap, Arizona. One-meter intervals were later isolated at multiple depths and pumped to purge drilling air from each interval. The volume of air purged (at 1 atm, 20 C), ranged from a low of 0.3 m 3 in an unfractured interval, to a high of 252 m 3 in a highly fractured interval. The concentration of SF 6 remained high throughout the purge in all intervals and often increased over time. Measurements of d 13 C, 14 C and CO 2 concentration indicated that atmospheric air was eventually drawn into several of the intervals in spite of the fact that SF 6 concentrations remained high. Possible explanations include mixing of atmospheric air drawn through fractures with partially-purged matrix air, and delayed removal of SF 6 relative to atmospheric gases due to adsorption of SF 6 within the tuff matrix, dissolution into pore water, or diffusion from dead-end pores with restricted openings. In this system, following a long delay between drilling and purging, it was found that the risk of contamination from surface air by over-purging was substantially greater than the risk of contamination from residual drilling air by under-purging.
The B isotopic ratio of a fly ash leachate can be very different from the B isotopic ratio of a n... more The B isotopic ratio of a fly ash leachate can be very different from the B isotopic ratio of a natural groundwater. Mixtures of leachate and groundwater typically result in nonlinear B isotope mixing curves that enable identification and quantification of leachate contamination in a groundwater at much lower levels than possible using concentration analyses alone. Limits on B isotope use for contaminant quantification will exist for some environments such as landfills with multiple ash types, but B isotopic analysis may often remain the preferred method for qualitative identification.
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at t... more This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction this manuscript in whole or in part may be granted by the head of the major department of the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: ^ APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below.
The edge of regional aquifers can be complex hydrodynamic systems with unique flow dynamics, wate... more The edge of regional aquifers can be complex hydrodynamic systems with unique flow dynamics, water quality, and continuity relationships with the main aquifer system. A site near the southwestern margin of the High Plains Aquifer (USA) was investigated to characterize the local hydrogeology and its relationship with the regional aquifer system. Measurements of tritium, ion concentrations, oxygen and hydrogen isotopes, and hydraulic heads documented (1) a discontinuous saturated zone, (2) no inflow to the study area from the regional aquifer, (3) focused recharge beneath playas with limited local mixing between pockets of saturation, (4) outflow orthogonal to the regional aquifer flow direction, (5) localized multi-year reversals in flow direction following high precipitation events, and (6) a magnified influence of the paleo-erosional surface of the basement rock (Dockum Group) on groundwater isolation and flow direction. In isolated areas, groundwater can be trapped on decadal time scales by depressions in the Dockum, or by recharge events that periodically reverse groundwater gradients.
Long-term metal and arsenic mobility between wetlands and lakes: variable histories within the sa... more Long-term metal and arsenic mobility between wetlands and lakes: variable histories within the same floodplain
The theoretical minimum difference between the S 13C of soil COz (a concentration) and soilrespir... more The theoretical minimum difference between the S 13C of soil COz (a concentration) and soilrespired CO* (a flux) was previously thought to be 4.4%0. It is shown here that the difference is a function of the 6 13C of soil-respired COZ, with possible minimum values less than 4.4%0. A simple equation is given which allows the 613C of soil CO2 or soil-respired CO2 to be calculated from four measurable variables. Sampling methodology is investigated demonstrating the need for correcting measured soil-respired CO2 samples for CO, contamination in base reagents, for contributions from atmospheric CO*, and for isotopic fractionation at the base-solution/air interface.
Bald cypress (Taxodium distichum) is known to respond to increases in precipitation with increase... more Bald cypress (Taxodium distichum) is known to respond to increases in precipitation with increased radial growth even when rooted in continuously saturated sediments where water is not a growth-limiting factor. Measurements of ␦ 18 O, Cl Ϫ , 3 H and hydraulic head in surface water and shallow groundwater in an oxbow lake-wetland in northern Mississippi show that rapid downward flow of surface water into the root zone is initiated only after precipitation-induced increases in surface water depth exceed a threshold value. Rapid flow of surface water through the root zone has the potential to introduce oxygen to sediments that would otherwise be anoxic, facilitating nutrient uptake and growth. Climatic reconstruction using tree rings from bald cypress in this environment appears possible because increases in precipitation generally correlate well with increases in water level, which in turn enhances the delivery of oxygenated water to the roots.
Picric acid is an explosive historically produced and disposed at the Louisiana Army Ammunition P... more Picric acid is an explosive historically produced and disposed at the Louisiana Army Ammunition Plant (LAAP) in northern Louisiana. The potential for natural degradation of picric acid was investigated by creating picric-acid slurries with four LAAP sediments of variable composition and monitoring for up to 98 days. The concentrations of picric acid decreased rapidly in all slurries during the first day, attributed to adsorption, followed by slower decreases in some samples due to degradation. Degradation in unsterilized slurries was nearly complete within 80 days for two of the four sediments. Increases in nitrite and nitrate concentration over time were proportional to the loss of picric acid and indicate that at least two of the three nitrite groups were removed from the picric acid molecule. The absence of significant concentrations of compounds with a mass greater than 100 amu in the final solutions suggests that all three nitrite groups were removed. No correlation was found between the degree of degradation and grain size, clay content, organic content, carbonate content, or a suite of element concentrations in the sediment. Degradation in sterilized samples was minimal for all sediment slurries, indicating microbial activity as the primary mechanism of degradation.
ABSTRACT Shortly after peak stage of the 2011 Mississippi River flood, water samples were collect... more ABSTRACT Shortly after peak stage of the 2011 Mississippi River flood, water samples were collected from the river, from sand boils near the toe of the levee, and from actively flowing relief wells over a 55 km stretch north of Vicksburg, MS. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Sand boil discharge was similar to water from relief wells only at a location where the levee sits on ancient channel fill deposits. Seepage at this site is forced along a deeper pathway beneath the fine-grained channel fill. Where the levee sits on sandy point bar deposits, shallow flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but appears to reflect unique weathering or redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.
The radiocarbon activity of carbon collected by vacuum distillation from a single partially satur... more The radiocarbon activity of carbon collected by vacuum distillation from a single partially saturated tuff began to decline after approximately 60% of the water and carbon had been extracted. Disproportionate changes in 14C activity and 613C during distillation rule out simple isotopic fractionation as a causative explanation. Additional phenomena such as matrix diffusion and ion exclusion in micropores may play a role in altering the isotopic value of extracted carbon, but neither can fully account for the observed changes. The most plausible explanation is that distillation recovers carbon from an adsorbed phase that is depleted in 14C relative to DIC in the bulk pore water.
ABSTRACT Identifying and quantifying sources and sinks of CO2 is integral to developing global ca... more ABSTRACT Identifying and quantifying sources and sinks of CO2 is integral to developing global carbon budgets and effectively modeling climate change. Adsorption of CO2 onto mineral and soil surfaces has generally been regarded as an insignificant sink, though few studies have investigated adsorption on natural materials at temperatures and CO2 concentrations relevant to atmospheric or soil zone conditions. In this study, annual adsorption at the scale of North America was modeled for the upper 3 m of the Earth’s surface (the root zone) based on our own and published adsorption data, and results compared with reported estimates for the North American terrestrial carbon sink during 2000–2005. Our results suggest that adsorption can account for 1–3% of the average annual sink during these years. At smaller regional scales where more adsorptive deposits are present, such as volcanic ash or high-organic soils, the sink may be significantly larger.
AbstraetmTbe occurrence and significance of aqueous flow through fractures in unsaturated tuff wa... more AbstraetmTbe occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the Apache Leap Research Site near Superior, Arizona. Water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel, from the saturated zone in a vertical borehole (USW UZP-4), and from both the unsaturated and saturated zones in 14 an angled borehole (DSB). The geochemistry and C activity of water samples from the DSB suggest that most of the recharge to the saturated zone has occurred through fractures, especially beneath the ephemeral streams. Evidence of substantial recent recharge through fractures was found in saturated-zone samples from the mine haulage tunnel using 3H, 634S and SO42-/CI-analyses. Evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the DSB using geophysical measurements from the borehole, water-content analyses from core samples, and ~4C and 3H analyses from pore water extracted from preserved core samples. Post-bomb 14C activity was measured in pore water near fractures just above the saturated zone.
Seepage beneath artificial levees is a common concern during flooding events. Risk of levee failu... more Seepage beneath artificial levees is a common concern during flooding events. Risk of levee failure is elevated when piping erodes channels beneath the levee, evidenced by the formation of sand boils where transported sediments discharge. Along the lower Mississippi River, pathways of floodwater beneath the levee vary with surface geology, following deeper paths where the levee overlies fine-grained channel-fill deposits, and shallower (higher risk) paths where it overlies sand-bar deposits. Shallow, organic-rich alluvial aquifers are often geochemically stratified into upper oxic and lower anoxic zones, raising the possibility of using the geochemical signatures of discharging water from sand boils to differentiate flow pathways. A preliminary investigation north of Vicksburg, MS, (USA) during the 2011 Mississippi River flood demonstrated the potential of using geochemistry to identify deep and shallow pathways, though the study was limited to cation and trace element analyses. Sampling during the 2015 and 2016 floods for temperature, conductivity, redox potential (Eh), dissolved oxygen (DO), major ions, trace elements, tritium, and stable isotope ratios of oxygen, hydrogen, and strontium, facilitated a greater understanding of the nature of flow and geochemical evolution of groundwater in this environment. Characteristics of deeper flow pathways (relative to shallow) included (1) lower Eh and higher Fe and As, reflecting anoxic conditions and high-Fe sediments, (2) proportional increases in Fe and HCO 3 , indicating reductive dissolution of Fe-oxyhydroxides, and (3) higher ratios of Ba/Ca and Sr/Ca, reflecting differences in the elemental composition of minerals with depth. Tritium results indicate that subsurface flow pathways are dynamic, shifting spatially with the rapid changes in hydraulic gradients during and between flooding events. Estimated residence times of groundwater discharging from sand boils and relief wells ranged from essentially zero (discharge of concurrent floodwater) up to a quarter century. Lower strontium isotope ratios (87 Sr/ 86 Sr) were observed in the aquifer relative to river water, though with no clear variation with flow depth. Oxygen and hydrogen isotope ratios (δ 18 O and δ 2 H) show evidence of partial evaporation prior to recharge, also with no apparent variation with subsequent flow depth.
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