Geology topics

K.M. Hess

Publications and source records attributed to K.M. Hess.

5 recordsLinked to original sources

USGS2010

Simulation of fluid distributions observed at a crude oil spill site incorporating hysteresis, oil entrapment, and spatial variability of hydraulic properties

Subsurface oil, water, and air saturation distributions were determined using 146 samples collected from seven boreholes along a 120-m transect at a crude oil spill site near Bemidji, Minnesota. The field data, collected 10 years after the spill, show a clearly defined oil body that has an oil saturation distribution that appears to be influenced by sediment heterogeneities and water table fluctuations. The center of the oil body has depressed the water-saturated zone boundary and the oil appears to have migrated laterally within the capillary fringe. A multiphase cross-sectional flow model was developed and used to simulate the movement of oil and water at the spill site. Comparisons between observed and simulated oil saturation distributions serve as an indicator of the appropriateness of using such models to predict the actual spread of organic immiscible liquids at spill sites. Sediment hydraulic properties used in the model were estimated from particle size data. The general large-scale features of the observed oil body were reproduced only when hysteresis with oil entrapment and representations of observed spatial variability of hydraulic properties were incorporated into the model. The small-scale details of the observed subsurface oil distribution were not reproduced in the simulations. The discrepancy between observed and simulated oil distributions reflects the considerable uncertainty in model parameter estimates and boundary conditions, three-phase capillary pressure-saturation-relative permeability functions, representations of spatial variability of hydraulic properties, and hydrodynamics of the groundwater flow system at the study site.

Water Resources Research↗

USGS2006

Long-term natural attenuation of carbon and nitrogen within a groundwater plume after removal of the treated wastewater source

Disposal of treated wastewater for more than 60 years onto infiltration beds on Cape Cod, Massachusetts produced a groundwater contaminant plume greater than 6 km long in a surficial sand and gravel aquifer. In December 1995 the wastewater disposal ceased. A long-term, continuous study was conducted to characterize the post-cessation attenuation of the plume from the source to 0.6 km downgradient. Concentrations and total pools of mobile constituents, such as boron and nitrate, steadily decreased within 1-4 years along the transect. Dissolved organic carbon loads also decreased, but to a lesser extent, particularly downgradient of the infiltration beds. After 4 years, concentrations and pools of carbon and nitrogen in groundwater were relatively constant with time and distance, but substantially elevated above background. The contaminant plume core remained anoxic for the entire 10-year study period; temporal patterns of integrated oxygen deficit decreased slowly at all sites. In 2004, substantial amounts of total dissolved carbon (7 mol C m-2) and fixed (dissolved plus sorbed) inorganic nitrogen (0.5 mol N m-2) were still present in a 28-m vertical interval at the disposal site. Sorbed constituents have contributed substantially to the dissolved carbon and nitrogen pools and are responsible for the long-term persistence of the contaminant plume. Natural aquifer restoration at the discharge location will take at least several decades, even though groundwater flow rates and the potential for contaminant flushing are relatively high.

Environmental Science & Technology↗

USGS2000

Multispecies reactive tracer test in an aquifer with spatially variable chemical conditions

A field investigation of multispecies reactive transport was conducted in a well‐characterized, sand and gravel aquifer on Cape Cod, Massachusetts. The aquifer is characterized by regions of differing chemical conditions caused by the disposal of secondary sewage effluent. Ten thousand liters of groundwater with added tracers (Br, Cr(VI), and EDTA complexed with Pb, Zn, Cu, and Ni) were injected into the aquifer and distributions of the tracers were monitored for 15 months. Most of the tracers were transported more than 200 m; transport was quantified using spatial moments computed from the results of a series of synoptic samplings. Cr(VI) transport was retarded relative to Br; the retardation factor varied from 1.1 to 2.4 and was dependent on chemical conditions. At 314 days after the injection, dissolved Cr(VI) mass in the tracer cloud had decreased 85%, with the likely cause being reduction to Cr(III) in a suboxic region of the aquifer. Transport of the metal‐EDTA complexes was affected by aqueous complexation, adsorption, and dissolution‐precipitation reactions of Fe oxyhydroxide minerals in the aquifer sediments. Dissolved Pb‐EDTA complexes disappeared from the tracer cloud within 85 days, probably due to metal exchange reactions with Fe and adsorbed Zn (present prior to the injection from contamination by the sewage effluent). About 30% of the Cu‐EDTA complexes remained within the tracer cloud 314 days after injection, even though the thermodynamic stability of the Pb‐EDTA complex is greater than Cu‐EDTA. It is hypothesized that stronger adsorption of Pb 2+ to the aquifer sediments causes the Pb‐EDTA complex to disassociate to a greater degree than the Cu‐EDTA complex. The mass of dissolved Zn‐EDTA increased during the first 175 days of the tracer test to 140% of the mass injected, with the increase due to desorption of sewage‐derived Zn. Dissolved Ni‐EDTA mass remained nearly constant throughout the tracer test, apparently only participating in reversible adsorption reactions. The results of the field experiment provide a chemically complex data set that can be used in the testing of reactive transport models of flow coupled with chemical reactions.

Water Resources Research↗

USGS1993

Simulated hydrologic responses of the Quashnet River stream-aquifer system to proposed ground-water withdrawals, Cape Cod, Massachusetts

An investigation of the Quashnet River stream- aquifer system on Cape Cod was initiated in response to concern over possible streamflow reduction and degradation of the sea-run brown trout habitat of the river resulting from proposed ground-water withdrawals. A two-layer finite-difference ground-water-flow model was developed to simulate the stream-aquifer system. Steady-state pumping rates of 0.5, 1.0, and 2.0 million gallons per day were simulated at three well sites 1,500 to 2,500 feet west of the river. No infiltration of water from the river to the aquifer was induced in any of the simulations. Maximum streamflow depletion along the river for the scenarios tested ranged from 3 to 15 percent of calculated steady-state prepumping streamflow. Mean monthly streamflow depletions determined by use of the transient model, for a constant withdrawal of 1.0 million gallons per day from a site 1,500 feet west of the river, range from 6 to 8 percent of the mean monthly streamflows measured at a gage located 0.3 miles from the mouth of the river. A particle-tracking postprocessor to the steady-state model was used to delineate contributing areas of the river and the proposed withdrawal sites. Although the simulated cone of depression produced by pumping extends beyond the river, the contributing area of the well does not include the river under any of the withdrawal schemes simulated.

Water-Resources Investigations Report↗

USGS1992

Determination of subsurface fluid contents at a crude-oil spill site

Measurement of the fluid-content distribution at sites contaminated by immiscible fluids, including crude oil, is needed to better understand the movement of these fluids in the subsurface and to provide data to calibrate and verify numerical models and geophysical methods. A laboratory method was used to quantify the fluid contents of 146 core sections retrieved from boreholes aligned along a 120-m longitudinal transect at a crude-oil spill site near Bemidji, Minnesota, U.S.A. The 47-mm-diameter, minimally disturbed cores spanned a 4-m vertical interval contaminated by oil. Cores were frozen on site in a dry ice-alcohol bath to prevent redistribution and loss of fluids while sectioning the cores. We gravimetrically determined oil and water contents using a two-step method: (1) samples were slurried and the oil was removed by absorption onto strips of hydrophobic porous polyethylene (PPE); and (2) the samples were oven-dried to remove the water. The resulting data show sharp vertical gradients in the water and oil contents and a clearly defined oil body. The subsurface distribution is complex and appears to be influenced by sediment heterogeneities and water-table fluctuations. The center of the oil body has depressed the water-saturated zone boundary, and the oil is migrating laterally within the capillary fringe. The oil contents are as high as 0.3cm 3 cm −3 , which indicates that oil is probably still mobile 10 years after the spill occurred. The thickness of oil measured in wells suggests that accumulated thickness in wells is a poor indicator of the actual distribution of oil in the subsurface. Several possible sources of error are identified with the field and laboratory methods. An error analysis indicates that adsorption of water and sediment into the PPE adds as much as 4% to the measured oil masses and that uncertainties in the calculated sample volume and the assumed oil density introduce an additional ±3% error when the masses are converted to fluid contents.

Journal of Contaminant Hydrology↗