Richelle M. Allen-King

Analytical method for the sorption of hydrophobic organic pollutants in clay-rich materials.

A method using dialysis tubing for phase separation was developed for measuring nonionic, hydrophobic organic compound sorption in clay-rich geologic media such as aquitards. The complications caused by nonsettling particles when centrifugation is used for phase separation are eliminated. Slurried sample was sealed in the tubing and placed into a sample bottle, which was filled with synthetic groundwater amended with the sorbate of interest. Particles are retained within the tubing during equilibration, whereas the sorbate may diffuse through the tubing yielding equal aqueous-phase concentrations inside and out. Following equilibration, the aqueous phase outside the tubing was analyzed, and sorbed mass was determined by difference. Equilibration occurred within 48-72 h

Nitrate in tile drainage of the semiarid Palouse Basin.

Topographically heterogeneous agricultural landscapes can complicate and accelerate agrochemical contamination of streams due to rapid transport of water and chemicals to poorly drained lower-landscape positions and shallow ground water. In the semiarid Palouse region, large parts of these landscapes have been tile drained to enhance crop yield. From 2000-2004 we monitored the discharge of a tile drain (TD) and a nearby profile of soil water for nitrate concentration ([NO(3)]), electrical conductivity level (EC), and water content to develop a conceptual framework describing soil nitrate occurrence and loss via subsurface pathways. Tile-drain baseflow [NO(3)] was consistently 4 mg N L(-1) and baseflow EC was 200 to 300 microS cm(-1). Each year sudden synoptic increases in TD discharge and [NO(3)] occurred in early winter following approximately 150 mm of fall precipitation, which saturated the soil and mobilized high-[NO(3)] soil water throughout the profile. The greatest TD [NO(3)] (20-30 mg N L(-1)) occurred approximately contemporaneous with greatest TD discharges. The EC decrease each year (to approximately 100 microS cm(-1)) during high discharge, a dilution effect, lagged approximately 1 mo behind the first appearance of high [NO(3)] and was consistent with advective transport of low-EC water from the shallow profile under saturated conditions. Water-budget considerations and temporal [NO(3)] patterns suggest that these processes deliver water to the TD from both lower- and upper-slope positions, the latter via lateral flow during the high-flow season. Management practices that reduce the fall reservoir of soil nitrate might be effective in reducing N loading to streams and shallow ground water in this region.

Hydrophobic organic contaminant transport property heterogeneity in the Borden Aquifer

We determined that the spatial heterogeneity in aquifer properties governing the reactive transport of volatile organic contaminants is defined by the arrangement of lithofacies. We measured permeability (k) and perchloroethene sorption distribution coefficient (Kd) for lithofacies that we delineated for samples from the Canadian Forces Base Borden Aquifer. We compiled existing data and collected 57 new cores to characterize a 30 m section of the aquifer near the test location of Mackay et al. (1986). The k and Kd were measured for samples taken at six elevations from all cores to create a data set consisting of nearly 400 colocated measurements. Through analysis of variance (corrected for multiple comparisons), we determined that the 12 originally mapped lithofacies could be grouped into five relatively distinct chemohydrofacies that capture the variability of both transport properties. The mean of ln k by lithofacies was related to the grain size and the variance was relatively consistent. In contrast, both the mean and variance of ln Kd were greater for more poorly sorted lithofacies, which were also typically more coarse-grained. Half of the aquifer sorption capacity occurred in the three highest-sorbing lithofacies but comprised only 20% of its volume. The model of the aquifer that emerged is that of discontinuous scour-fill deposits of medium sand, generally characterized by greater Kd and k, within laterally extensive fine-grained to very fine-grained sands of lower Kd and k. Our findings demonstrate the importance of considering source rock composition, transport, and deposition processes when constructing conceptual models of chemohydrofacies.

Distribution of Carbonaceous Matter in Lithofacies: Impacts on HOC Sorption Nonlinearity

Both the composition and distribution of the lithocomponents within an aquifer impact hydrophobic organic compound (HOC) transport. Using samples from the sandy, low fraction organic carbon content (f(oc)~0.02%) Borden aquifer, we demonstrate how HOC sorption is controlled by the carbonaceous matter (CM) associated with calcareous sedimentary lithocomponents. Two-point isotherms using perchloroethene (PCE) as a sorbate showed that medium-grained lithofacies have a broader range of K(f) (Freundlich coefficient), 1/n (Freundlich parameter) and f(oc) than fine-grained facies. Dual-mode (linear+Freundlich) sorption modeling, fraction inorganic carbon (f(ic)) and laboratory analyses confirm that both the magnitude and variability of PCE K(d) (sorption distribution coefficient) in the Borden aquifer are controlled by the presence of heterogeneous CM in dark and very dark carbonate lithocomponents. Laboratory analyses and model results confirmed that the CM type controlling PCE sorption behavior in the Borden aquifer is in a condensed form, likely kerogen, contained within the carbonate matrix of the grains. The dark carbonate grains comprise a small proportion of the aquifer sediment (≪1%) and are found predominantly in medium-grained lithofacies in the Borden aquifer. These results show that increased heterogeneity, HOC mass storage and sorption nonlinearity associated with medium-grained lithofacies impact HOC transport in historically contaminated sedimentary aquifers.

Temporal Hyporheic Zone Response to Water Table Fluctuations

Expansion and contraction of the hyporheic zone due to temporal hydrologic changes between stream and riparian aquifer influence the biogeochemical cycling capacity of streams. Theoretical studies have quantified the control of groundwater discharge on the depth of the hyporheic zone; however, observations of temporal groundwater controls are limited. In this study, we develop the concept of groundwater-dominated differential hyporheic zone expansion to explain the temporal control of groundwater discharge on the hyporheic zone in a third-order stream reach flowing through glacially derived terrain typical of the Great Lakes region. We define groundwater-dominated differential expansion of the hyporheic zone as: differing rates and magnitudes of hyporheic zone expansion in response to seasonal vs. storm-related water table fluctuation. Specific conductance and vertical hydraulic gradient measurements were used to map changes in the hyporheic zone during seasonal water table decline and storm events. Planar and riffle beds were monitored in order to distinguish the cause of increasing hyporheic zone depth. Planar bed seasonal expansion of the hyporheic zone was of a greater magnitude and longer in duration (weeks to months) than storm event expansion (hours to days). In contrast, the hyporheic zone beneath the riffle bed exhibited minimal expansion in response to seasonal groundwater decline compared to storm related expansion. Results indicated that fluctuation in the riparian water table controlled seasonal expansion of the hyporheic zone along the planar bed. This groundwater induced hyporheic zone expansion could increase the potential for biogeochemical cycling and natural attenuation.

Characterization and quantification of aquifer heterogeneity using outcrop analogs at the Canadian Forces Base Borden, Ontario, Canada

Accurate characterization of internal structures and geometries of aquifers is critical for evaluation of plume migration and dispersion of contaminants. For this reason, high-resolution transport study field sites, such as the Waterloo Groundwater Research Site at the Canadian Forces Base Borden, Ontario, Canada, have been established. However, geological characterization at this site is based primarily on cores and shallow geophysical data. Outcrop analog studies offer detailed horizontal data that can help to build quantified models of aquifer heterogeneity. We used a combination of ground-based light detection and ranging (LiDAR) and high-resolution photographs at a sand quarry cut into Borden aquifer sediment to evaluate and quantify the distribution of lithofacies and hydrofacies that control flow properties in the Borden aquifer. We exposed sixteen ∼20 m × 1.5 m outcrops, LiDAR surveyed, photographed, and field mapped lithofacies at each exposure, and segmented the lithofacies from photographs to produce maps of facies distributions. We grouped different lithofacies into hydrofacies based on overall facies geometry and estimated hydraulic properties. We used LiDAR scans and the segmented facies maps to produce a digital outcrop model (DOM) of the site. The resultant DOM combines lithofacies and hydrofacies derived from field observations with high-resolution (5 mm) LiDAR data to reconstruct hydrofacies distributions in a three-dimensional (3-D) geomodel. The DOM offers a relatively complete horizontal correlation structure that was used in transition probability geostatistical modeling to create realizations of hydrofacies distributions in the aquifer at the study site.

A Framework for Integrating Quantitative Geologic Problem Solving into Courses Across the Undergraduate Geology Curriculum

Forging a link between quantitative skills and geologic problem solving is a valuable instructional approach that can guide the development of quantitative course material. A conceptual framework is presented that shows how various research tasks (for example, data collection and hypothesis development) employ certain combinations of quantitative skills (for example, graphical presentation and algebra). The framework shows how this “repertoire” of skills can be explored and strengthened by posing course assignments as research problems. An example problem, ‘What is the major source of nitrogen to the South Fork of the Palouse River?,” illustrates implementation of all the tasks and skills in the framework via a four-week unit of coursework. Smaller units can focus on subsets of tasks and skills. Experience with units on structural geology, igneous petrology, hydrogeology, and isotope geochemistry suggests that the framework can be applied to virtually any geoscience topic at any level in the undergraduate curriculum.

Can chlorofluorocarbon sorption to black carbon (char) affect groundwater age determinations

Although adsorption is not generally considered important in low f(oc) (fraction organic carbon) aquifers, we show that chlorofluorocarbon (CFC) adsorption to black carbon (BC) is sufficiently large to retard transport and affect groundwater ages obtained with CFCs. Sorption isotherms of CFC-11, -12, and -113 to synthetic wood char were nonlinear (Freundlich n = 0.71-0.94) while humic acid isotherms were linear. Moreover, sorption to char was 10-1000 times greater than to humic acid for all three CFCs at the lowest observed concentrations, C(w)/S approximately 10(-8)-10(-7). We used the observed isotherms for char and humic acid to represent sorption to BC and amorphous organic matter, respectively, in a dual mode model to estimate retardation factors for a low f(oc) aquifer (= 0.06% gC g(-1)). The estimated retardation factors for the char-containing aquifer (presumed BC fraction = 9% of f(oc)) were approximately 6.8-10.6 at C(w)/S = 10(-8) and >5 times those estimated assuming amorphous organic matter partitioning only. The results indicate that unless CFC adsorption to BC is evaluated in transport, the groundwater age determined may be biased toward older than true ages. The CFC data archived in BC-containing aquifers may contain information about its adsorbent properties that could be useful to predict retardation of other chlorinated organic contaminants.

Heterogeneous carbonaceous matter in sedimentary rock lithocomponents causes significant trichloroethylene (TCE) sorption in a low organic carbon content aquifer/aquitard system.

This study evaluated the effects of heterogeneous thermally altered carbonaceous matter (CM) on trichloroethylene (TCE) sorption for a low fraction organic carbon content (foc) alluvial sedimentary aquifer and aquitard system (foc=0.046-0.105%). The equilibrium TCE sorption isotherms were highly nonlinear with Freundlich exponents of 0.46-0.58. Kerogen+black carbon was the dominant CM fraction extracted from the sediments and accounted for >60% and 99% of the total in the sands and silt, respectively. Organic petrological examination determined that the kerogen included abundant amorphous organic matter (bituminite), likely of marine origin. The dark calcareous siltstone exhibited the greatest TCE sorption among aquifer lithocomponents and accounted for most sorption in the aquifer. The results suggest that the source of the thermally altered CM, which causes nonlinear sorption, was derived from parent Paleozoic marine carbonate rocks that outcrop throughout much of New York State. A synthetic aquifer-aquitard unit system (10% aquitard) was used to illustrate the effect of the observed nonlinear sorption on mass storage potential at equilibrium. The calculation showed that >80% of TCE mass contained in the aquifer was sorbed on the aquifer sediment at aqueous concentration <1000 μgL(-1). These results show that sorption is likely a significant contributor to the persistence of a TCE groundwater plume in the aquifer studied. It is implied that sorption may similarly contribute to TCE persistence in other glacial alluvial aquifers with similar geologic characteristics, i.e., comprised of sedimentary rock lithocomponents that contain thermally altered CM.

Geochemical & Physical Aquifer Property Heterogeneity: A Multiscale Sedimentologic Approach to Reactive Solute Transport

This project is testing the hypothesis that sedimentary lithofacies determine the geochemical and physical hydrologic properties that control reactive solute transport (Figure 1). We are testing that hypothesis for one site, a portion of the saturated zone at the Hanford Site (Ringold Formation), and for a model solute, carbon tetrachloride (CT). The representative geochemical and physical aquifer properties selected for quantification in the proposed project are the properties that control CT transport: hydraulic conductivity (K) and reactivity (sorption distribution coefficient, Kd, and anaerobic transformation rate constant, kn). We are combining observations at outcrop analog sites (to measure lithofacies dimensions and statistical relations) with measurements from archived and fresh core samples (for geochemical experiments and to provide additional constraint to the stratigraphic model) from the Ringold Formation to place local-scale lithofacies successions, and their distinct hydrologic property distributions, into the basinal context, thus allowing us to estimate the spatial distributions of properties that control reactive solute transport in the subsurface.