Charles F. Harvey

Seasonal oscillations in water exchange between aquifers and the coastal ocean.

Ground water of both terrestrial and marine origin flows into coastal surface waters as submarine groundwater discharge, and constitutes an important source of nutrients, contaminants and trace elements to the coastal ocean. Large saline discharges have been observed by direct measurements and inferred from geochemical tracers, but sufficient seawater inflow has not been observed to balance this outflow. Geochemical tracers also suggest a time lag between changes in submarine groundwater discharge rates and the seasonal oscillations of inland recharge that drive groundwater flow towards the coast. Here we use measurements of hydraulic gradients and offshore fluxes taken at Waquoit Bay, Massachusetts, together with a modelling study of a generalized coastal groundwater system to show that a shift in the freshwater–saltwater interface—controlled by seasonal changes in water table elevation—can explain large saline discharges that lag inland recharge cycles. We find that sea water is drawn into aquifers as the freshwater–saltwater interface moves landward during winter, and discharges back into coastal waters as the interface moves seaward in summer. Our results demonstrate the connection between the seasonal hydrologic cycle inland and the saline groundwater system in coastal aquifers, and suggest a potentially important seasonality in the chemical loading of coastal waters.

Rate‐limited mass transfer or macrodispersion: Which dominates plume evolution at the macrodispersion experiment (MADE) site?

We present a model of solute transport that explains the large-scale behavior of the solute tracer-test plumes at the Macrodispersion Experiment (MADE) site as the result of advection and rate-limited mass transfer between mobile and small-scale immobile domains. This model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles. Compared to the macrodispersion model, the mass transfer model better represents the change in mobile dissolved mass with time, the peak of the concentration profile, and the profile asymmetry. Specifically, unlike the macrodispersion model, the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times. We suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain. The immobile domain consists of a combination of low-permeability zones on the scale of centimeters to decimeters (the Darcy-scale immobile domain), and intragranular porosity, dead-end pores, and surface sorption (the pore-scale immobile domain). We suggest that the mobile domain was sampled preferentially when water was extracted. Therefore, at early times, relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations. Similarly, the mass at late times was underestimated because solute trapped in the pore-scale immobile domain was not extracted during sampling and therefore ignored. The combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only ∼5 m by the termination of the experiment, as well as the different behavior of bromide and tritium tracers.

Permanent carbon dioxide storage in deep-sea sediments

Stabilizing the concentration of atmospheric CO2 may require storing enormous quantities of captured anthropogenic CO2 in near-permanent geologic reservoirs. Because of the subsurface temperature profile of terrestrial storage sites, CO2 stored in these reservoirs is buoyant. As a result, a portion of the injected CO2 can escape if the reservoir is not appropriately sealed. We show that injecting CO2 into deep-sea sediments <3,000-m water depth and a few hundred meters of sediment provides permanent geologic storage even with large geomechanical perturbations. At the high pressures and low temperatures common in deep-sea sediments, CO2 resides in its liquid phase and can be denser than the overlying pore fluid, causing the injected CO2 to be gravitationally stable. Additionally, CO2 hydrate formation will impede the flow of CO2(l) and serve as a second cap on the system. The evolution of the CO2 plume is described qualitatively from the injection to the formation of CO2 hydrates and finally to the dilution of the CO2(aq) solution by diffusion. If calcareous sediments are chosen, then the dissolution of carbonate host rock by the CO2(aq) solution will slightly increase porosity, which may cause large increases in permeability. Karst formation, however, is unlikely because total dissolution is limited to only a few percent of the rock volume. The total CO2 storage capacity within the 200-mile economic zone of the U.S. coastline is enormous, capable of storing thousands of years of current U.S. CO2 emissions.

Temporal Moment-Generating Equations: Modeling Transport and Mass Transfer in Heterogeneous Aquifers

We present an efficient method for determining temporal moments of concentration for a solute subject to first-order and diffusive mass transfer in steady velocity fields. The differential equations for the moments of all orders have the same form as the steady state nonreactive transport equation. Thus temporal moments can be calculated by a solute transport code that was written to simulate nonreactive steady state transport, even though the actual transport system is reactive and transient. Higher-order moments are found recursively from lower-order moments. For many cases a small number of moments sufficiently describe the movement of a solute plume. The first four moments describe the accumulated mass, mean, spread, and skewness of the concentration histories at all locations. Actual concentration histories at any location can be approximated from the moments by applying the principle of maximum entropy, a constraint consistent with the physical process of dispersion. The forms of the moment-generating equations for different mass transfer models provide insight into reactive transport through heterogeneous aquifers. For the mass transfer models we considered, the zeroth moment in a heterogeneous aquifer is independent of the mass transfer coefficients. Thus, if the velocity field is known, the mass transported past any point, or out any boundary, can be calculated without knowledge of the spatial pattern of mass transfer coefficients and, in fact, without knowledge of whether mass transfer is occurring. Also, for both first-order and diffusive mass transfer models, the mean arrival time depends on the distribution coefficient but is independent of the values of the rate coefficients, regardless of the spatial variability of groundwater velocity and mass transfer coefficients.

Arsenic in groundwater in Bangladesh: A geostatistical and epidemiological framework for evaluating health effects and potential remedies

[1] This paper examines the health crisis in Bangladesh due to dissolved arsenic in groundwater. First, we use geostatistical methods to construct a map of arsenic concentrations that divides Bangladesh into regions and estimate vertical concentration trends in these regions. Then, we use census data to estimate exposure distributions in the regions; we use epidemiological data from West Bengal and Taiwan to estimate dose response functions for arsenicosis and arsenic-induced cancers; and we combine the regional exposure distributions and the dose response models to estimate the health effects of groundwater arsenic in Bangladesh. We predict that long-term exposure to present arsenic concentrations will result in approximately 1,200,000 cases of hyperpigmentation, 600,000 cases of keratosis, 125,000 cases of skin cancer, and 3000 fatalities per year from internal cancers. Although these estimates are very uncertain, the method provides a framework for incorporating better data as it becomes available. Moreover, we examine the remedy of drilling deeper wells in selected regions of Bangladesh. By replacing 31% of the wells in the country with deeper wells the health effects of drinking groundwater arsenic could be reduced by approximately 70% provided that arsenic concentrations in deep wells remain relatively low. INDEX TERMS: 1831 Hydrology: Groundwater quality; 6309 Policy Sciences: Decision making under uncertainty; 6304 Policy Sciences: Benefit-cost analysis; 1829 Hydrology: Groundwater hydrology; KEYWORDS: arsenic, Bangladesh, geostatistics, health effects, risk assessment, mitigation

Mapping Hydraulic Conductivity: Sequential Conditioning with Measurements of Solute Arrival Time, Hydraulic Head, and Local Conductivity

We present a method for estimating the spatial pattern of aquifer hydraulic conductivity from three types of measurements: solute arrival time quantiles, hydraulic heads, and direct local measurements. Results indicate that arrival times and heads provide different, but complementary, information about the large features of the conductivity field which serve as flow paths and barriers. We compare the information provided by head and arrival time measurements by plotting the correlation between measurement and conductivity over the entire domain. We also compare the value of head and arrival time data by estimating the conductivity with only one type of data, and by incorporating the different data types into the estimation procedure in different sequences. Using quantiles of arrival time, rather than concentrations, has three advantages: (1) Arrival times are independent of the amount of dilute solute introduced into the aquifer. (2) Under some conditions, by measuring the median arrival time, the accuracy of the conductivity field estimate is not degraded by poor knowledge of the local dispersivity. (3) The estimation procedure is greatly simplified by relying on a single quantile to represent the critical information contained in a breakthrough curve that is constructed from many concentration measurements. For the case examined here, incorporating arrival time quantiles that describe the tails of the breakthrough curves did not significantly improve our estimate of the conductivity field. An accurate map of the hydraulic conductivity of an aquifer is vital for predicting groundwater flow and solute transport through the aquifer. Typically, the spatial pattern of conduc- tivity is qualitatively inferred from a few local values of con- ductivity, head, and concentration measured at wells which are sparsely distributed, leaving large volumes of the aquifer un- sampled. When monitoring wells fail to penetrate important flow channels or barriers, these features must be deduced from measurements of head and concentration at nearby wells. A number of studies (Kitanidis and Vomvoris, 1983; Hoek- sema and Kitanidis, 1984; Dagan, 1985; Sun and Yeh, 1992) have shown how measurements of hydraulic head can be used in conjunction with measurements of local transmissivity to estimate the spatial pattern of aquifer transmissivity. These studies solve the inverse problem with a procedure that com- bines solution of the flow equation with linear estimation. Some recent studies (Graham and McLaughlin, 1989a, b; Ru- bin, 1991a, b) describe how measurements of solute concen- trations can be used to predict or estimate concentrations at other times and locations. Although spatially variable conduc- tivity is the underlying cause of uncertainty in these studies, the procedures are not directed toward estimating the conductivity field. Wagner (1992) used both head and concentration mea- surements to estimate the value of conductivity within pre- scribed zones. Woodbury and Smith (1988) combined steady state temperature and head measurements to estimate spa-

The energy penalty of post-combustion CO2 capture & storage and its implications for retrofitting the U.S. installed base

A review of the literature has found a factor of 4 spread in the estimated values of the energy penalty for post-combustion capture and storage of CO2 from pulverized-coal (PC) fired power plants. We elucidate the cause of that spread by deriving an analytic relationship for the energy penalty from thermodynamic principles and by identifying which variables are most difficult to constrain. We define the energy penalty for CCS to be the fraction of fuel that must be dedicated to CCS for a fixed quantity of work output. That penalty can manifest itself as either the additional fuel required to maintain a power plants output or the loss of output for a constant fuel input. Of the 17 parameters that constitute the energy penalty, only the fraction of available waste heat that is recovered for use and the 2nd-law separation efficiency are poorly constrained. We provide an absolute lower bound for the energy penalty of ∼11%, and we demonstrate to what degree increasing the fraction of available-waste-heat recovery can reduce the energy penalty from the higher values reported. It is further argued that an energy penalty of ∼40% will be easily achieved while one of ∼29% represents a decent target value. Furthermore, we analyze the distribution of PC plants in the U.S. and calculate a distribution for the additional fuel required to operate all these plants with CO2 capture and storage (CCS).

Aquifer remediation: A method for estimating mass transfer rate coefficients and an evaluation of pulsed pumping

When pumping contaminated water from an aquifer, contaminant concentrations often decline rapidly, only to rebound when the pump is shut off. One reason for this behavior is that contaminants in the mobile phase are readily removed, but mass transfer from the immobile to the mobile phase is rate limited. Pulsed pumping, in which the pump is periodically turned off, has been suggested as a means to enhance remediation. We conducted a comprehensive comparison of pulsed pumping and continuous pumping for the case of a Gaussian plume, subject to first- order mass transfer during transport, in an aquifer with no regional gradient and constant mass transfer rate coefficients. We developed a Laplace domain Greens function solution for concentrations during pumping periods and coupled it with an analytic solution for concentrations during resting periods. First, we used this model to provide a simple type curve that can be used to estimate the mobile-immobile phase mass transfer coefficients from field data. Second, the mass and concentration removal histories were determined during pulsed pumping and during continuous pumping. The continuous pumping rate removed the same volume of water over the duration of remediation. We investigated the effects of physical parameters such as the mass transfer rate coefficients, and engineering design parameters such as the length of resting periods. Under the conditions considered, our evaluation shows that (1) for equal volumes of water removed, pulsed pumping does not remove more contaminant mass than pumping continuously at an average rate; (2) if the duration of the resting period is too large, then pulsed pumping removes much less mass than continuous pumping at the average rate, and (3) if the pulsed and continuous pumping rates are the same, pulsed pumping will take longer than continuous pumping to clean up the aquifer, but will require significantly less time during which the pump operates.

Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts

[1] A seepage meter study was performed in Waquoit Bay on Cape Cod, Massachusetts to characterize the amount, pattern, and origin of submarine groundwater discharge. Measurements from grids of 40 seepage meters provide a detailed representation of groundwater flux in both space and time. At the head of the bay, a distinct band of high, saline discharge was observed between 25 and 45 m from the shoreline. Slug tests indicated no pattern of permeability to explain the band of discharge, and the band was not observed offshore of an island where freshwater discharge is negligible. Experiments using clusters of seepage meters showed large variability in discharge at the meter scale and similar temporal variation throughout the domain, reflecting tidal influence primarily near shore. The small-scale variability challenges the assumption of locally homogeneous flow used in many models, and the band of discharge contradicts predictions that total outflow is largely fresh and decreases monotonically from shore. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 4235 Oceanography: General: Estuarine processes; 1894 Hydrology: Instruments and techniques; 4825 Oceanography: Biological and Chemical: Geochemistry. Citation: Michael, H. A., J. S. Lubetsky, and C. F. Harvey, Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts, Geophys. Res. Lett., 30(6), 1297, doi:10.1029/ 2002GL016000, 2003.

Using performance reference compounds in polyethylene passive samplers to deduce sediment porewater concentrations for numerous target chemicals.

Polymeric passive samplers are useful for assessing hydrophobic organic chemical contamination in sediment beds. Here, an improved method is described for measuring concentrations of contaminants in porewater by using performance reference compounds (deuterated phenanthrene, pyrene, and chrysene) to calibrate sampler/site-specific mass transfer behavior. The method employs a one-dimensional diffusion model of chemical exchange between a polymer sheet of finite thickness and an unmixed sediment bed. The model is parametrized by diffusivities and partition coefficients for both the sampler and sediment. This method was applied to estimate porewater concentrations for seventeen PAHs from polymeric samplers deployed for 3-10 days in homogenized sediment from a coal-tar contaminated site. The accuracy of the method was verified by comparing the passive sampler results to concentrations measured through liquid-liquid extraction of physically separated porewaters, with corrections for sorption to colloidal organic carbon. The measurements made using the two methods matched within about a factor of 2.0 (+/-0.9) for the 17 target PAHs.