Ward E. Sanford

Eolian transport, saline lake basins, and groundwater solutes

Eolian processes associated with saline lakes are shown to be important in determining solute concentration in groundwater in arid and semiarid areas. Steady state mass balance analyses of chloride in the groundwater at Double Lakes, a saline lake basin in the southern High Plains of Texas, United States, suggest that approximately 4.5 × 105 kg of chloride is removed from the relatively small (4.7 km2) basin floor each year by deflation. This mass enters the groundwater down the wind gradient from the lake, degrading the water quality. The estimates of mass transport were independently determined by evaluation of solutes in the unsaturated zone and by solute mass balance calculations of groundwater flux. Transport of salts from the lake was confirmed over a short term (2 years) by strategically placed dust collectors. Results consistent with those at Double Lake were obtained from dune surfaces collected upwind and downwind from a sabkha near the city of Abu Dhabi in the United Arab Emirates. The eolian transport process provides an explanation of the degraded groundwater quality associated with the 30–40 saline lake basins on the southern half of the southern High Plains of Texas and New Mexico and in many other arid and semiarid areas.

Quantifying Groundwater’s Role in Delaying Improvements to Chesapeake Bay Water Quality

A study has been undertaken to determine the time required for the effects of nitrogen-reducing best management practices (BMPs) implemented at the land surface to reach the Chesapeake Bay via groundwater transport to streams. To accomplish this, a nitrogen mass-balance regression (NMBR) model was developed and applied to seven watersheds on the Delmarva Peninsula. The model included the distribution of groundwater return times obtained from a regional groundwater-flow (GWF) model, the history of nitrogen application at the land surface over the last century, and parameters that account for denitrification. The model was (1) able to reproduce nitrate concentrations in streams and wells over time, including a recent decline in the rate at which concentrations have been increasing, and (2) used to forecast future nitrogen delivery from the Delmarva Peninsula to the Bay given different scenarios of nitrogen load reduction to the water table. The relatively deep porous aquifers of the Delmarva yield longer groundwater return times than those reported earlier for western parts of the Bay watershed. Accordingly, several decades will be required to see the full effects of current and future BMPs. The magnitude of this time lag is critical information for Chesapeake Bay watershed managers and stakeholders.

Constant-concentration boundary condition: Lessons from the HYDROCOIN variable-density groundwater benchmark problem

In a solute-transport model, if a constant-concentration boundary condition is applied at a node in an active flow field, a solute flux can occur by both advective and dispersive processes. The potential for advective release is demonstrated by reexamining the Hydrologic Code Intercomparison (HYDROCOIN) project case 5 problem, which represents a salt dome overlain by a shallow groundwater system. The resulting flow field includes significant salinity and fluid density variations. Several independent teams simulated this problem using finite difference or finite element numerical models. We applied a method-of-characteristics model (MOCDENSE). The previous numerical implementations by HYDROCOIN teams of a constant-concentration boundary to represent salt release by lateral dispersion only (as stipulated in the original problem definition) was flawed because this boundary condition allows the release of salt into the flow field by both dispersion and advection. When the constant-concentration boundary is modified to allow salt release by dispersion only, significantly less salt is released into the flow field. The calculated brine distribution for case 5 depends very little on which numerical model is used, as long as the selected model is solving the proper equations. Instead, the accuracy of the solution depends strongly on the proper conceptualization of the problem, including the detailed design of the constant-concentration boundary condition. The importance and sensitivity to the manner of specification of this boundary does not appear to have been recognized previously in the analysis of this problem.

Source of solutes to the coastal sabkha of Abu Dhabi

An ascending-brine model is proposed to address the observed isotope geochemistry, solute composition, and solute and water fluxes in the coastal sabkha of the Emirate of Abu Dhabi. Mass-balance measurements document that >95% of the solutes are derived from ascending continental brines; minor amounts are derived from rainfall and from groundwater entering from up- gradient areas. Nearly 100% of the annual water loss is from evaporation and not lateral discharge. Direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for ∼90% of the annual water input to the system; the remaining 10% comes from both lateral and ascending groundwater flow. Thus, the water and solutes in this system are from different sources. Solute concentrations of conservative (i.e., nonreactive) elements in the coastal, sabkha-covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties. Calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying Tertiary formations. Recharging rainwater dissolves halite and other soluble minerals on the surface, causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines. Solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes. Thus, the system becomes vertically mixed, consistent with the presence of tritium throughout the aquifer; but there is essentially no horizontal mixing of seawater with groundwater. The observed seawater solutes in the supratidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the Arabian Gulf and are not refluxed or laterally mixed. The ascending- brine model contrasts significantly with both the seawater-flooding and evaporative- pumping models previously proposed as a source of solutes to the coastal sabkha of the Emirate of Abu Dhabi. Use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record.

Porosity development in coastal carbonate aquifers

Geochemical mixing theory suggests that the mixing of seawater and calcite-saturated fresh ground water can result in a solution that is undersaturated with respect to calcite. Previous studies of the mixing of such waters in carbonate rocks along certain coastlines have indicated that this mixing effect may be responsible for significant amounts of calcite dissolution and porosity development. In this study, potential rates of porosity development by calcite dissolution are assessed by combining geochemical mixing theory with the hydrodynamics of fresh-water-salt-water mixing zones in a coupled reaction- transport model. Results from the reaction-path model PHREEQE are used with a variable-density ground-water flow and solute-transport model to simulate an idealized cross section of a coastal carbonate aquifer. Results of the simulations indicate that the dissolution process is sensitive to fresh-water chemistry, ground-water velocities, and sea-level movement. Dissolution potential was evaluated at three field sites, and evidence from those sites is in general agreement with the simulation results. Dissolution rates indicated by the model show that under the proper conditions this dissolution mechanism can produce significant increases in porosity over relatively short spans of geologic time (tens of thousands of years).

Deep Drilling into the Chesapeake Bay Impact Structure

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

Fate of reflux brines in carbonate platforms

Active reflux may occur during periods of platform-top brine generation, but the role and fate of these brines after reflux events are uncertain. We have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform-top salinity. Simulations suggest that reflux brines, originally concentrated to gypsum saturation (150‰), have a relatively long platform residence time, on the order of 100 times the duration of the reflux event. When brine-generating conditions cease, brines will continue to sink through the platform, entraining seawater, a variant of reflux circulation we term latent reflux. Mesosaline brines intercepted by drilling of carbonate margins by the Ocean Drilling Program may have originated from Pleistocene reflux event(s) on the adjacent platform top and be currently moving by latent reflux. Latent-reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates, including Mg for dolomitization.

Groundwater transport of crater-lake brine at Poa´s Volcano, Costa Rica

Abstract Poa´s Volcano is an active stratovolcano in Costa Rica that has a lake in its active crater. The crater lake has high temperatures (50–90 °C), high acidity (pH ≈ 0.0), and a high dissolved-solids content (100 g/kg). The volcano has numerous freshwater springs on its flanks, but a few on the northwestern flank are highly acidic (pH = 1.6–2.5) and have high dissolved-solids concentrations (2–22 g/kg). This study analyzes the regional groundwater system at Poa´s and demonstrates the likelihood that the water discharging from the acidic springs in the Rio Agrio watershed originates at the acidic crater lake. Both heat and solute transport are analyzed on a regional scale through numerical simulations using the HST3D finite-difference model, which solves the coupled equations for fluid flow, heat transport, and solute transport. The code allows fluid viscosity and density to be functions of both temperature and solute concentration. The simulations use estimates for recharge to the mountain and a range of values and various distributions of permeability and porosity. Several sensitivity analyses are performed to test how the uncertainty in many of the model parameters affects the simulation results. These uncertainties yield an estimated range of travel times from the crater lake to the Rio Agrio springs of 1–30 years, which is in close agreement with the results of tritium analyses of the springs. Calculated groundwater fluxes into and out of the crater lake are both about several hundred kg/s. These fluxes must be accounted for in water budgets of the crater lake.

Chesapeake Bay impact structure drilled

The Chesapeake Bay impact structure was formed by a meteorite crashing to Earth during the late Eocene, about 35.5 million years ago (Ma). In May 2006, a scientific drilling project, sponsored by the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS), completed a deep coring program into the impact structure. The deep drilling produced one of the most complete geologic sections ever obtained in an impact structure, and studies of the core samples will allow scientists to understand a shallow-marine impact event and its consequences at an unprecedented level. This buried structure is the seventh largest, and one of the best preserved, of the known impact structures on Earth [Poag et al., 2004]. It consists of a 38-kilometer-wide, highly deformed central zone, which approximates the dimensions and location of the transient impact crater, surrounded by a shallower outer zone of sediment collapse known as the annular trough [Horton et al., 2005]. Together, these zones have a diameter of about 85 kilometers and a distinctive shape similar to an ‘inverted sombrero.’

Numerical modelling of geothermal and reflux circulation in Enewetak Atoll: Implications for dolomitization

Abstract Two types of regional-scale seawater circulation have been proposed to explain the formation of Enewetak Atoll dolomites: geothermal and reflux circulation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulation mechanisms in Enewetak Atoll. Geothermal circulation is concentrated around the atoll-margin whereas refluxing mesosaline brines flow from the atoll interior towards the margin to restrict and eventually shut off geothermal circulation. Refluxing brines of 36–80‰ can account for the salinity signature recorded in dolomite fluid inclusions. Distributions of fluid flux and Mg mass-balance calculations suggest that both geothermal and reflux circulation mechanisms could account for the observed distribution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls.