Critical loads and steady-state chemistry for streams in the state of Maryland

The critical loads to streams, steady-state stream chemistry and catchment chemical weathering rate in 73 catchments has been determined in the state of Maryland, USA. It was calculated with the PROFILE model from chemical limits for biological indicators, soil mineralogy, soil texture, annual average temperature, average soil moisture, net long-term uptake of base cations and nitrogen to the vegetation, annual precipitation and runoff and deposition of sulphur and nitrogen precursors of acid deposition. The results show a full range of critical loads from very low values in the sensitive catchments of western Maryland and the Coastal Plain on the Chesapeake Bay, to insensitive catchments in the Fredrick Valley and Ridge and the Piedmont plain. The critical loads will be used as an input to an integrated regional assessment of the quantitative sensitivity of streams to acid rain, and the assessment of regional stream alkalinity response to different abatement strategies. The mapping of steady-state stream chemistry indicates that streams in Maryland are still acidfying under the present deposition load. Land-use seems to play an important role in maintaining neutral pH in many of the streams of Maryland.     

Desorption kinetics studies on PAH-contaminated soil under varying temperatures

The purpose of this study was to investigate the effect of temperature on the release of polycyclic aromatic hydrocarbons (PAHs) from aged contaminated soil. The release of fluorene, phenanthrene, anthracene, fluoranthene and pyrene at 7, 15, 18 and 23 degrees C was studied using a column leaching method with a hydraulic retention time of 0.5 h. As the temperature declined from 23 to 7 degrees C the concentrations decreased by a factor of 11-12 for all the studied compounds except for anthracene, which only decreased by a factor 7. Rate constants at maximum release rate at the four studied temperatures were assessed. From temperature dependence studies, apparent activation energies of desorption, E*(des), were calculated. E*(des)-values appeared to be in the range of 105-137 kJ mol(-1) for the studied PAHs and increased with the LeBas molar volume of the compounds. The increase of E*(des) with increased molecular size indicates stronger sorption with increased hydrophobicity of the compounds.     

The Solling roof revisited--slow recovery from acidification observed and modeled despite a decade of "clean-rain" treatment

Soil chemistry under the Solling clean-rain roof was simulated using the dynamic multi-layer soil chemistry model SAFE, including sulfate adsorption. Soil was sampled in order to parameterize the pH and sulfate concentration dependent sulfate adsorption isotherm used in SAFE. Modeled soil solution chemistry was compared to the 14 year long time-series of monthly measurements of soil solution data at 10 and 100cm depth. The deposition of N and S under the roof has been reduced by 68% and 53%, respectively, compared to the surrounding area. Despite this the soil solution concentrations of sulfate are still high (a median of 420mumol(c)/L at 100cm depth between 2000 and 2002) and the soil base saturation low (approximately 3% in the whole profile in 1998). Sulfate adsorption is an important process in Solling. The soil capacity to adsorb sulfate is large, the modeled adsorbed pool in 2003 down to 100cm was 1030kg S/ha, and the measured sulfate concentration is high, due to release of adsorbed sulfate. The addition of sulfate adsorption improved the modeled sulfate dynamics although the model still slightly underestimated the sulfate concentration at 100cm. Model predictions show no recovery, based on the criteria of Bc/Al ratio above 1 in the rooting zone, before the year 2050, independent of future deposition cuts.     

A column method for determination of leaching of polycyclic aromatic hydrocarbons from aged contaminated soil

In this study a column leaching method for investigation of hydrophobic organic contaminants (HOCs) leaching from soil was developed. The method set-up is based on a recycled flow of sterile water through a soil column with a sedimentation chamber mounted on top of the column, in connection with on-line filtration. The combination of a sedimentation chamber and an on-line filtration enables the measurement of leaching concentrations from contaminated materials consisting of very fine particle fractions. In addition, by using on-line solid phase extraction, minute amounts of leaching HOCs may be captured and quantified with high accuracy and reproducibility. The method was applied successfully on a contaminated aged soil sample and the leaching behavior of seven PAHs, with three to six aromatic rings, was monitored for more than 1600 h under saturated conditions. The tested PAHs were fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)pyrene and benzo(ghi)perylene. The method proved to be reliable and capable of providing data on leachable amounts of the PAHs under field-like conditions and over a longer period of time. The results indicated low availability of the studied contaminants since only a minor fraction (0.3%) of the initial amount of PAHs in the soil was removed during the experiment (liquid/solid-ratio of 700 l/kg). Thus PAHs in aged contaminated soil are not to be expected to be released to any great extent only by leaching with water.     

Parameterization and evaluation of sulfate adsorption in a dynamic soil chemistry model

Sulfate adsorption was implemented in the dynamic, multi-layer soil chemistry model SAFE. The process is modeled by an isotherm in which sulfate adsorption is considered to be fully reversible and dependent on sulfate concentration as well as pH in soil solution. The isotherm was parameterized by a site-specific series of simple batch experiments at different pH (3.8-5.0) and sulfate concentration (10-260 micromol 1(-1)) levels. Application of the model to the Lake Gardsj6n roof covered site shows that including sulfate adsorption improves the dynamic behavior of the model and sulfate adsorption and desorption delay acidification and recovery of the soil. The modeled adsorbed pool of sulfate at the site reached a maximum level of 700 mmol/m(2) in the late 1980s, well in line with experimental data.     

Modelling long-term cation supply in acidified forest stands

A dynamic soil chemistry model was used to explain the observed decrease in soil base saturation between 1949 and 1984 at three stands in southern Sweden. The results show that acid deposition has caused soil acidification. The model, SAFE (Soil Acidification in Forest Ecosystems), includes the fundamental physical processes such as leaching and accumulation, and chemical processes such as cation exchange, mineral weathering, nutrient uptake and solute equilibrium reactions. The sources and sinks of base cations in the soil system were quantified, showing that weathering, deposition of base cations and depletion of exchangeable base cations supply cations to the soil solution in similar amounts in the upper 1 m during the acidification phase. This demonstrates that budget studies alone cannot be used to distinguish between long-term capacity to resist acidification, represented by weathering, from short-term buffering caused by cation exchange.     

Critical Loads – Is There A Need For A New Concept?

The concept of critical loads has been an important andsuccessful tool for the development of control strategiesfor transboundary air pollution in Europe. The use of theconcept has led us to a situation where very few areas inEurope will have an exceedance of critical loads foracidification in 2010, indicating that the benefits offurther control acidifying substances will be lessuseful. The critical loads concept does not, however,take into account the large benefits of further controlin damaged systems but where critical loads are nolonger exceeded. In this paper we discuss the importanceof widening the critical loads concept to include thesebenefits and we propose an additional effect-relatedmeasure, Dynamic Impact Analysis, to be included infurther control strategies and assessments. With such aconcept the actual situation and its further developmentwill be included in assessments and control strategies.     

Polycyclic aromatic hydrocarbons in ash: determination of total and leachable concentrations

Before wood ash can be used as a soil fertilizer, concentrations of environmentally hazardous compounds must be investigated. In this study, total and leachable concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) were determined in four ash samples and one green liquor sludge. The ash sample with the highest carbon content also contained high levels of PAHs; three of the ash samples had total concentrations exceeding the limit permitted by the Swedish Forest Agency for recycling to forest soils. The leachable concentrations were higher for the non-stabilized samples; this was probably due to colloid-facilitated transport of the contaminants in these samples. However, the leachable concentrations were overall relatively low in all the samples studied. The amounts of PAHs introduced to forest soils by additions of stabilized, recyclable ash products will be determined primarily by the rate of weathering of the ash particles and the total concentration of contaminants.