In order to gain regulatory approval for source zone natural attenuation (SZNA) at hydrocarbon-contaminated sites, knowledge regarding the extent of the contamination, its tendency to spread, and its longevity is required. However, reliable quantification of biodegradation rates, an important component of SZNA, remains a challenge. If the rate of CO(2) gas generation associated with contaminant degradation can be determined, it may be used as a proxy for the overall rate of subsurface biodegradation. Here, the CO(2)-efflux at the ground surface is measured using a dynamic closed chamber (DCC) method to evaluate whether this technique can be used to assess the areal extent of the contaminant source zone and the depth-integrated rate of contaminant mineralization. To this end, a field test was conducted at the Bemidji, MN, crude oil spill site. Results indicate that at the Bemidji site the CO(2)-efflux method is able to both delineate the source zone and distinguish between the rates of natural soil respiration and contaminant mineralization. The average CO(2)-efflux associated with contaminant degradation in the source zone is estimated at 2.6 μmol m(-2) s(-1), corresponding to a total petroleum hydrocarbon mineralization rate (expressed as C(10)H(22)) of 3.3 g m(-2) day(-1).

Nitrous oxide (N2O) emissions were monitored using the micrometeorological eddy covariance technique from manure-fertilized cropland on a large dairy farm in New York State in 2006 to 2009. Nitrous oxide emissions demonstrated episodic behavior with intermittent short-duration peak fluxes up to 39.7 mg N2O-N m~^ d'^ whereas most of background fluxes during the annual agricultural cycle were below 6.5 mg N2O-N m-^ d'^ This paper discusses temporal variability of measured N2O emissions using a "hot moment" approach. To identify and quantify peak events as potential hot moments and to determine whether or not they could be treated statistically as outliers, N2O daily fluxes were analyzed by the box plot method using multiple thresholds. Peak events exceeding outlier thresholds contributed up to 51% of cumulative annual N2O emissions, although they represented <7% of the total observation time. Individual N2O peaks were also categorized by their duration, as single day spikes and multiday events. The highest contributing instances were multiday N2O peaks during summer precipitation and early spring thaw, largely enhanced by manure fertilization. These high-intensity emission events demonstrated repetitive seasonal responses to a combination of environmental factors and were therefore identified as hot moments. Abrupt rises in both temperature and soil moisture appeared to trigger major hot moments, whereas the availability of manure N controlled their magnitude. In the absence of strong correlations between time-series of individual environmental factors and N2O emissions, the hot moment approach can be a promising tool for the integrated analysis of most significant N2O events in cultivated fields receiving manure applications.Abbreviations: EC, eddy covariance; WFPS, water-filled pore space.T he potential adverse effects of agriculturally-produced N2O as a greenhouse gas and stratospheric ozone destructor are a serious concern in environmental science and policy. Manure fertilization of agricultural lands considerably increases N2O emissions (Bouwman et al, 2002;Kroeze et al., 1999;Mosier et al., 1998). Nitrous oxide generated from manure-amended soils is formed through several microbiological and chemical processes, primarily via autotrophic nitrification of ammonium and hecerocrophic denitrification of soil nitrates and nicrices (Anderson et al., 1993; Barnard ec al., 2005;Bremner, 1997). Despite ongoing research into emission rates and underlying processes, there is still uncertainty in quantification and prediction of agricultural N2O emissions (Desjardins et al., 2010;Flechard et al., 2007), largely due to its high temporal variability.Measured N2O emissions from fertilized cropland fall within a wide range: 0.7 CO 51.8 mgN2O-N m"^ d'^ (Drury et al.

[1] We have conducted laboratory experiments to examine CaCO 3 dissolution and precipitation in saltwater-freshwater mixing zones, with a view to understanding and predicting porosity changes in coastal environments. Mixing of seawater or saline subsurface water with fresh water can be of major importance in the chemical diagenesis of carbonate rocks and sediments. We used artificial seawater and NaCl solutions of different concentrations under different CO 2 partial pressures and with different mixing ratios. Two-dimensional flow cells filled with glass beads and crushed calcium carbonate rock were used to measure calcium carbonate precipitation and dissolution, respectively. An important feature of these experiments is that the results are shown to agree well with a relatively simple transport theory describing mineral precipitation/dissolution that results from the nonlinear dependence of CaCO 3 saturation upon electrolyte concentration. The theory demonstrates that the rate of dissolution or precipitation depends on the curvature (and sign) of the solubility as a function of salinity, the square of the salinity gradient, and the macroscopic dispersion coefficient. The theory is largely scale independent and depends upon field parameters that can be determined. Analysis of data from three field sites (Yucatan peninsula, Bermuda, and Mallorca) demonstrates excellent agreement between field observations and theory.

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[1] The evolution of hydraulic conductivity and porosity during the process of dedolomitization was examined in a series of laboratory experiments by analyzing the effects of concurrent dolomite dissolution and calcium carbonate precipitation. Linear flow experiments were performed in columns of crushed sucrosic dolomite by injecting different concentrations of HCl at various flow rates. Temporal changes in head gradient were used to calculate overall hydraulic conductivities of each column, while chemical analyses of the effluent acid enabled estimation of porosity changes during the experiments. After each experiment, the rock samples were retrieved and sectioned in order to study the pore space geometry, micromorphology, and mineral concentrations. A range of injected HCl concentrations and flow rates was identified which leads to oscillations in the effective hydraulic conductivity and porosity of the evolving rock samples; in all cases, however, the porous medium ultimately clogged. Short-term experiments were also used to study the formation of dissolution and precipitation bands along the columns. Under the experimental conditions, dolomite dissolution is a reaction rate controlled process; experiments indicated that, as such, the flow rate and the pH of the injected fluid affect dissolution only during the initial stages, when calcium carbonate is dissolved. On the other hand, both the flow rate and the pH of the injected fluid strongly influence the precipitation process throughout the duration of the experiments because higher flow rates retard nucleation. These findings are in qualitative accordance with field observations of dolomite formations.