Although numerous studies of hyporheic exchange and denitrification have been conducted in pristine, high‐gradient streams, few studies of this type have been conducted in nutrient‐rich, low‐gradient streams. This is a particularly important subject given the interest in nitrogen (N) inputs to the Gulf of Mexico and other eutrophic aquatic systems. A combination of hydrologic, mineralogical, chemical, dissolved gas, and isotopic data were used to determine the processes controlling transport and fate of NO3− in streambeds at five sites across the USA. Water samples were collected from streambeds at depths ranging from 0.3 to 3 m at three to five points across the stream and in two to five separate transects. Residence times of water ranging from 0.28 to 34.7 d m−1 in the streambeds of N‐rich watersheds played an important role in allowing denitrification to decrease NO3− concentrations. Where potential electron donors were limited and residence times were short, denitrification was limited. Consequently, in spite of reducing conditions at some sites, NO3− was transported into the stream. At two of the five study sites, NO3− in surface water infiltrated the streambeds and concentrations decreased, supporting current models that NO3− would be retained in N‐rich streams. At the other three study sites, hydrogeologic controls limited or prevented infiltration of surface water into the streambed, and ground‐water discharge contributed to NO3− loads. Our results also show that in these low hydrologic‐gradient systems, storm and other high‐flow events can be important factors for increasing surface‐water movement into streambeds.

A two-dimensional, multispecies reactive solute transport model with sequential aerobic and anaerobic degradation processes was developed and tested. The model was used to study the field-scale solute transport and degradation processes at the Bemidji, Minnesota, crude oil spill site. The simulations included the biodegradation of volatile and nonvolatile fractions of dissolved organic carbon by aerobic processes, manganese and iron reduction, and methanogenesis. Model parameter estimates were constrained by published Monod kinetic parameters, theoretical yield estimates, and field biomass measurements. Despite the considerable uncertainty in the model parameter estimates, results of simulations reproduced the general features of the observed groundwater plume and the measured bacterial concentrations. In the simulation, 46% of the total dissolved organic carbon (TDOC) introduced into the aquifer was degraded. Aerobic degradation accounted for 40% of the TDOC degraded. Anaerobic processes accounted for the remaining 60% of degradation of TDOC: 5% by Mn reduction, 19% by Fe reduction, and 36% by methanogenesis. Thus anaerobic processes account for more than half of the removal of DOC at this site. Introduction Experience obtained from remediation efforts at contaminated groundwater sites has demonstrated the limitations of pump-and-treat technology, especially at sites contaminated with nonaqueous phase liquids (NAPLs). Increasing effort isbeing devoted to the development and testing of alternative technologies [MacdonaM and Kavanaugh, 1994]. A promising alternative to traditional pump-and-treat methods is intrinsic bioremediation, a method that relies on the naturally occurring biodegradation processes at a site [Lee et al., 1988; Madsen, 1991; Bouwer and Zehnder, 1993; Salanitro, 1993]. Both aerobic and anaerobic biodegradation processes can be effective at removing hydrocarbons from the environment [Grbi&Gali•, 1991; Barker et al., 1987; Wilson et al., 1987; Chiang et al., 1989; Cozzarelli et al., 1990; Wilson et al., 1990; Acton and Barker, 1992; Lyngkilde and Christensen, 1992b; Thieftin et al., 1993; Baedecker et al., 1993; Eganhouse et al., 1993]. Long-term, detailed monitoring of the groundwater plume caused by a crude oil spill near Bemidji, Minnesota, has documented the importance of both aerobic and anaerobic biodegradation at this site [Baedecker et al., 1993]. The availability of electron acceptors determines the sequence of biodegradation processes. Based on the thermodynamics of reactions and redox potential, the theoretical sequence is aerobic degradation, followed by denitrification, manganese and iron reduction, sulfate reduction, and then methanogenesis. This sequence may cause zonation of a contaminant plume with different biodegradation processes dominating in each redox zone [Baedecker and Back, 1979; Chapelle This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union. Paper number 95WR02567. and Lovley, 1992; Lyngkilde and Christensen, 1992a; Vroblesky a...

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump-and-skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore-scale capillary pressure-saturation hysteresis and the presence of fine-grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound-specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field-scale reaction rates and hydrocarbon mass balance. Long-term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap-shot study of a hydrocarbon plume may not provide information that is of relevance to the long-term behavior of the plume during natural attenuation.

A quasi three-dimensional, finite difference model, that simulates freshwater and saltwater flow separated by a sharp interface, has been developed to study layered coastal aquifer systems. The model allows for regional simulation of coastal groundwater conditions, including the effects of saltwater dynamics on the freshwater system. Vertically integrated freshwater and saltwater flow equations incorporating the interface boundary condition are solved within each aquifer. Leakage through confining layers is calculated by Darcy's law, accounting for density differences across the layer. The locations of the interface tip and toe, within grid blocks, are tracked by linearly extrapolating the position of the interface. The model has been verified using available analytical solutions and experimental results. Application of the model to the Soquel-Aptos basin, Santa Cruz County, California, illustrates the use of the quasi three-dimensional, sharp interface approach for the examination of freshwater-saltwater dynamics in regional systems. Simulation suggests that the interface, today, is still responding to long-term Pleistocene sea level fluctuations and has not achieved equilibrium with present day sea level conditions. INTRODUCTION Coastal aquifers are an important water resource in areas bordering seas. In many coastal settings, aquifer systems are characterized by sequences of layers with varying hydraulic properties. Well-known examples of such systems are found in the north Ariantic and Israeli coastal plains and the Llobregat delta in Barcelona, Spain [Collins and Gelhar, 1971; Schmorak, 1967; Custodio, 1981]. A quasi threedimensional, numerical finite difference model to simulate freshwater and saltwater flow separated by a sharp interface in layered coastal aquifer systems is presented here. The modeling approach facilitates regional simulation of coastal groundwater conditions and includes the effects of saltwater dynamics on the freshwater system. Demonstration of the utility of this model is illustrated by an application to the multilayered aquifer system in the Soquel-Aptos basin, California. An idealized cross section through a layered coastal aquifer system extending offshore to a submarine canyon outcrop is shown in Figure 1. Under natural, undisturbed conditions an equilibrium seaward hydraulic gradient exists within each aquifer, with excess fresh water discharging to the sea. In the uppermost, unconfined aquifer the fresh water flows out to sea across the ocean floor. In the lower, confined aquifers the fresh water discharges to the sea by leaking upward through the overlying layers and/or by flowing out the canyon outcrop. Within each layer a wedgeshaped body of denser sea water will develop beneath the lighter fresh water. Any change in the flow regimen within the freshwater region, caused by changes in discharge or recharge inland, induces movement of the freshwater-saltwater interface. Reduction in freshwater flow toward the sea causes intrusion of salt water into the aquifers as the interface...

The U.S. Geological Survey (USGS) solute transport and biodegradation code BIOMOC was used in conjunction with the USGS universal inverse modeling code UCODE to quantify field-scale hydrocarbon dissolution and biodegradation at the USGS Toxic Substances Hydrology Program crude-oil spill research site located near Bemidji, MN. This inverse modeling effort used the extensive historical data compiled at the Bemidji site from 1986 to 1997 and incorporated a multicomponent transport and biodegradation model. Inverse modeling was successful when coupled transport and degradation processes were incorporated into the model and a single dissolution rate coefficient was used for all BTEX components. Assuming a stationary oil body, we simulated benzene, toluene, ethylbenzene, m,p-xylene, and o-xylene (BTEX) concentrations in the oil and ground water, respectively, as well as dissolved oxygen. Dissolution from the oil phase and aerobic and anaerobic degradation processes were represented. The parameters estimated were the recharge rate, hydraulic conductivity, dissolution rate coefficient, individual first-order BTEX anaerobic degradation rates, and transverse dispersivity. Results were similar for simulations obtained using several alternative conceptual models of the hydrologic system and biodegradation processes. The dissolved BTEX concentration data were not sufficient to discriminate between these conceptual models. The calibrated simulations reproduced the general large-scale evolution of the plume, but did not reproduce the observed small-scale spatial and temporal variability in concentrations. The estimated anaerobic biodegradation rates for toluene and o-xylene were greater than the dissolution rate coefficient. However, the estimated anaerobic biodegradation rates for benzene, ethylbenzene, and m,p-xylene were less than the dissolution rate coefficient. The calibrated model was used to determine the BTEX mass balance in the oil body and groundwater plume. This article is a U.S. government work, and is not subject to copyright in the United States.Dissolution from the oil body was greatest for compounds with large effective solubilities (benzene) and with large degradation rates (toluene and o-xylene). Anaerobic degradation removed 77% of the BTEX that dissolved into the water phase and aerobic degradation removed 17%. Although goodness-of-fit measures for the alternative conceptual models were not significantly different, predictions made with the models were quite variable. D