Mark Person

Putting aquifers into atmospheric simulation models: an example from the Mill Creek Watershed, northeastern Kansas

Aquifer–atmosphere interactions can be important in regions where the water table is shallow (<2 m). A shallow water table provides moisture for the soil and vegetation and thus acts as a source term for evapotranspiration to the atmosphere. A coupled aquifer–land surface–atmosphere model has been developed to study aquifer–atmosphere interactions in watersheds, on decadal timescales. A single column vertically discretized atmospheric model is linked to a distributed soil-vegetation–aquifer model. This physically based model was able to reproduce monthly and yearly trends in precipitation, stream discharge, and evapotranspiration, for a catchment in northeastern Kansas. However, the calculated soil moisture tended to drop to levels lower than were observed in drier years. The evapotranspiration varies spatially and seasonally and was highest in cells situated in topographic depressions where the water table is in the root zone. Annually, simulation results indicate that from 5–20% of groundwater supported evapotranspiration is drawn from the aquifer. The groundwater supported fraction of evapotranspiration is higher in drier years, when evapotranspiration exceeds precipitation. A long-term (40 year) simulation of extended drought conditions indicated that water table position is a function of groundwater hydrodynamics and cannot be predicted solely on the basis of topography. The response time of the aquifer to drought conditions was on the order of 200 years indicating that feedbacks between these two water reservoirs act on disparate time scales. With recent advances in the computational power of massively parallel supercomputers, it may soon become possible to incorporate physically based representations of aquifer hydrodynamics into general circulation models (GCM) land surface parameterization schemes. 2002 Elsevier Science Ltd. All rights reserved.

Basin‐scale hydrogeologic modeling

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration-driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, thermal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three-dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applications.

Hydrogeologic Controls on Induced Seismicity in Crystalline Basement Rocks Due to Fluid Injection into Basal Reservoirs

A series of Mb 3.8-5.5 induced seismic events in the midcontinent region, United States, resulted from injection of fluid either into a basal sedimentary reservoir with no underlying confining unit or directly into the underlying crystalline basement complex. The earthquakes probably occurred along faults that were likely critically stressed within the crystalline basement. These faults were located at a considerable distance (up to 10 km) from the injection wells and head increases at the hypocenters were likely relatively small (∼70-150 m). We present a suite of simulations that use a simple hydrogeologic-geomechanical model to assess what hydrogeologic conditions promote or deter induced seismic events within the crystalline basement across the midcontinent. The presence of a confining unit beneath the injection reservoir horizon had the single largest effect in preventing induced seismicity within the underlying crystalline basement. For a crystalline basement having a permeability of 2 × 10(-17)  m(2) and specific storage coefficient of 10(-7) /m, injection at a rate of 5455 m(3) /d into the basal aquifer with no underlying basal seal over 10 years resulted in probable brittle failure to depths of about 0.6 km below the injection reservoir. Including a permeable (kz  = 10(-13)  m(2) ) Precambrian normal fault, located 20 m from the injection well, increased the depth of the failure region below the reservoir to 3 km. For a large permeability contrast between a Precambrian thrust fault (10(-12)  m(2) ) and the surrounding crystalline basement (10(-18)  m(2) ), the failure region can extend laterally 10 km away from the injection well.

A sensitivity study of the driving forces on fluid flow during continental-rift basin evolution

Two-dimensional finite element models of coupled sediment compaction, variable-density ground-water flow, and conductive/convective heat transfer are used in this study to quantify basin hydrodynamics during the initial and flexural stages of continental rifting. The analysis also incorporates a two-stage stretching/cooling geodynamic model of the thermomechanical evolution of the lithosphere underlying the rift in order to specify geologically relevant boundary conditions for basin subsidence and basal heat flow. A sensitivity study is made using the model to explore the controls of both permeability and water table configuration in determining the dominant fluid flow drive (compaction, density, or topography) during basin evolution. The sensitivity analysis incorporates hydrologic conditions and rock properties representative of many extensional terrains . Assuming that rift basin subsidence and basal heat flow can be represented by the geodynamic model , two distinct ground-water flow systems evolve within continental rifts during basin evolution. During the initial (stretching) phase of rifting, subsidence is accommodated by fault block motion, and a topography-driven ground-water flow system develops within the permeable alluvial-fan deposits. Within the less permeable lacustrine facies located in the center of the basin, compaction-driven ground-water flow dominates. Here, the compacting lacustrine sediments focus pore fluids laterally from the basin center into the alluvial-fan deposits due to the relatively large permeability contrast between the two depositional environments. Thermal anomalies resulting from convective heat transfer are restricted to alluvial-fan facies near the basin-framing fault. During the thermal cooling (flexural) stage of basin development, laterally extensive onlap facies are deposited, and density-driven ground-water flow dominates in the permeable alluvial-fan deposits, while compaction-drivenflow continues within the lacustrine and onlap facies. The presence of a permeable aquifer within the onlap facies resulted in long-range fluid transport to the edge of the basin. During both stages of basin evolution, ground-water velocities varied from 10 -5 to 10 -1 m/yr between the lacustrine and alluvial-fan deposits, respectively. The observed presence of ore mineralization within alluvial-fan deposits of some continental-rift systems, such as the Cretaceous Rift Basin of Angola, supports the findings of this study.

Pleistocene hydrogeology of the Atlantic continental shelf, New England

Salinity data from the Atlantic continental shelf off New England indicate that the freshwater/saltwater interface is far out of equilibrium with modern sea-level conditions. More than 150 km offshore of Long Island, New York. aquifer salinity levels are less than 5 parts per thousand (5 ppt). Salinity levels within confining units beneath Nantucket Island, Massachusetts, are 30%. 70% of seawater levels and exhibit a par abolic profile consistent with ongoing vertical diffusion. Here, we evaluate two fluid-flow-inducing mechanisms that could explain the apparent flushing of these coastal-plain aquifers: (1) meteoric recharge during Pleistocene sea-level low stands, and (2) subglacial recharge from the Laurentide Ice Sheet. Analytical models of vertical solute diffusion for the Nantucket confining units suggest that flushing of aquifers beneath Nantucket began in the late Pleistocene between ca. 195 and 21 ka; the models assume a diffusion coefficient of 3.0 × 10 - 1 1 m 2 /s. Cross-sectional numerical models of variable-density groundwater flow, heat, and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island by applying boundary conditions consistent with Pleistocene seal-level fluctuations. Observed salinity conditions were most closely matched in the models by also including the effects of sub-glacial recharge from the Laurentide Ice Sheet and allowing groundwater to discharge from Miocene aquifers along submarine canyons near the continental slope. Simulated recharge induced by Laurentide Ice Sheet meltwater was probably short lived hut, on average, about two to ten times greater than modern subaerial levels. A sensitivity analysis performed using our cross-sectional model suggests that a narrow range of hydrologic conditions can drive fresh water long distances offshore across the continental shelf.

Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA

While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New Englands continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km(3) in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (10(4) km(3)) and passive margins globally (3 x 10(5) km(3)). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages.

Isotope transport and exchange within metamorphic core complexes

Field observations from the Shuswap metamorphic core complex in British Columbia indicate that meteoric fluids were focused along a sub-horizontal shear zone at a depth of at least 7 km. Fluid-rock interactions associated with this flow system resulted in oxygen isotope depletion of mylonitic rocks up to 4 permil in a region less than 900m wide. Dating of the recrystallized shear zone fabric and deformation-assisted fluid flow indicates that this paleo-fluid flow system was relatively short lived, (<1 Ma). Here we present idealized numerical representations of a metamorphic core complex system to assess the hydrologic and thermal controls on fluid-rock isotopic exchange. Our analysis focuses on understanding the relative importance of fault versus matrix controlled fluid flow, reactive-surface area, crustal permeability structure, and isotopic composition of the recharging fluids. The analysis permits us to bracket the possible permeability and surface area conditions that are consistent with field observations. We conclude that downward fluid flow along brittle fault systems and isotope exchange patterns could only be produced by a fracture flow dominated system. We found the fault permeability had to be greater than 10−16 m2 but less than or equal to 10−15 m2. Upper plate crystalline rocks adjacent to the fault zone had to have a permeability less than 10−17 m2. The above findings are valid assuming a lateral water table gradient of 5 percent, a shear zone surface area of 3.0×10−4 m2/mole, crustal rock surface area of 1.0×10−5 m2/mole, total duration of flow of 200,000 years, and a basal heat flux of 90 mW/m2. Fault zone surface areas are much too small to be consistent with pervasive grain boundary fluid-rock isotope interactions. Rather, the best fit surface areas were consistent with a fracture spacing of 0.25 m for the shear/fault zones and a 5 m spacing for surrounding upper and lower plate rocks. We found that fracture aperture widths of about 0.02 mm for the fault/shear zone units and 0.002 mm for the surrounding upper and lower plate rocks were consistent with the permeability values obtained from our generic modeling exercise. Imposing a more strongly 18O-depleted oxygen isotope composition for the meteoric recharge was directly reflected in lower computed δ18O rocks. However, the effects were non-unique and to some degree, masked by the large oxygen reservoir within the crustal rocks. Computed rock isotopic values consistent with field observations could have been produced with either heavier δ18O fluids in the recharge area over a longer period of infiltration or lighter δ18O fluid compositions in the recharge region over shorter periods of time.

Tectonic controls on the hydrogeology of the Rio Grande Rift, New Mexico

Mathematical modeling is used in this study to assess how tectonic movement of fault blocks and fault permeability influence the present-day and paleohydrogeology of the Rio Grande Rift near Socorro, New Mexico. Our analysis focuses on active and ancient groundwater flow patterns and hot spring development within the southern La Jencia and Socorro subbasins. The best agreement between model results and present-day and paleoheat flow data was achieved by representing faults as passive surfaces and incorporating 2 km of moderately permeable (10−14.0 m2) fractured crystalline rocks into the hydrogeologic model. Quantitative results indicate that changes in groundwater flow patterns across the basin are primarily generated by the truncation/reconnection of aquifers and confining units. Calculated flow patterns help to explain the annealing of apatite fission tracks within Eocene Baca Formation clasts to the east of Socorro, potassium metasomatism mass balance constraints within Oligocene volcanics and overlying Santa Fe Group deposits, and the timing of barite/fluorite ore mineralization within the Gonzales prospect on the eastern edge of the Rio Grande Rift. We estimate that about 5% of mountain front recharge penetrates to a depth of 2.8 km below the sedimentary pile. This may have implications for water resource planners who have historically focused on groundwater resource development within the shallow alluvial deposits along the Rio Grande Rift valley.

Modeling secondary oil migration with core-scale data: Viking Formation, Alberta basin

The Viking Formation in the Alberta basin contains approximately 88.7 x 106 m3 (5.579 x 108 bbl) of recoverable oil, which migrated more than 200 km, as indicated by oil-source rock correlation. Simulating the mechanisms controlling secondary oil migration (hydrodynamics, buoyancy, and permeability heterogeneity) is beneficial for exploration, but it remains extremely difficult to predict oil occurrences. Although core-scale petrophysical data for the Viking Formation are abundant (> 69,000 core plugs), the extent of fracture permeability and permeability alteration due to diagenesis are unknown. Moreover, sampling bias may affect the permeability distribution in unpredictable ways. Numerical simulations of oil migration were conducted using the highest core-plug measurement of permeability from each borehole to obtain an upper bound on oil migration velocities. This permeability model is not appropriate for simulating stratigraphic entrapment of oil, but it does reveal that core-scale data are in the appropriate range of magnitude to have allowed significant oil migration. Regional groundwater flow was essential for charging several of the largest and most distant oil fields in the Viking Formation. Maximum core-plug permeability data are useful for modeling the extent of secondary oil migration and may have applications to fluid flow and transport modeling in other foreland settings.

Stochastic permeability models of fluid flow during contact metamorphism

Stable isotope data from hydrothermally altered rocks often show significant scatter. Such scatter cannot be quantitatively interpreted by models in which each lithologic unit is assumed to have a uniform permeability. If a stochastic modeling approach is used instead, heterogeneous permeability maps can be constructed to approximate the statistical distributions of natural systems, and both overall isotopic trends and data scatter can be matched. Three models are presented for the Alta, Utah, contact aureole to show that the observed scatter in δ18O values is consistent with subhorizontal down-temperature fluid infiltration through carbonates with heterogeneous permeabilities. Infiltration through rocks with heterogeneous permeabilities creates irregular, lobate isotope patterns, so that the idealized isotope profiles predicted by one-dimensional homogeneous permeability models do not develop. Localized sampling is unlikely to yield an accurate estimate of the overall importance of fluid-rock interaction or of dominant flow directions. In heterogeneous systems, large-scale hydrothermal alteration and flow patterns can best be estimated from statistically unbiased and spatially distributed data sets.