Peter Engelund Holm

Attenuation of landfill leachate pollutants in aquifers

Abstract Landfill leachate contains a variety of pollutants that may potentially contaminate the ground water and affect the quality of surface waters and well waters. The literature has been critically reviewed in order to assess the attenuation processes governing the contaminants in leachate‐affected aquifers. After an introductory section on leachate composition, the physical and chemical frameworks for the attenuation processes are discussed in terms of dilution/dispersion and redox zones in the plume, respectively. A separate section focuses on the microbiology in terms of the occurrence of bacteria in plumes, the fate of pathogens, and microbial mediation of redox processes. In individual sections, the attenuation of dissolved organic matter, anthropogenic‐specific organic compounds, inorganic macrocomponents as anions and cations, and heavy metals are discussed. The focus is on laboratory experiences and field investigations. The review shows that most leachate contamination plumes are relatively ...

Cadmium and zinc in plants and soil solutions from contaminated soils

In an experiment using ten heavy metal-contaminated soils from six European countries, soil solution was sampled by water displacement before and after the growth of radish. Concentrations of Cd, Zn and other elements in solution (K, Ca, Mg, Mn) generally decreased during plant growth, probably because of uptake by plants and the subsequent redistribution of ions onto soil exchange sites at lower ionic strength. Speciation analysis by a resin exchange method showed that most Cd and Zn in non-rhizosphere solutions was present as Cd2+ and Zn2+, respectively. The proportion of free ions was slightly lower in rhizosphere solutions, mainly due to an increase in dissolved organic carbon during plant growth. Solution pH increased during plant growth, although the bulk soil pH generally remained constant. Cd concentrations in leaves and tubers were more closely correlated with their total or free ionic concentrations in rhizosphere solutions (adjusted R2 ≥ 0.90) than with their concentrations in soils (adj. R2 ≥ 0.79). Cd concentrations in non-rhizosphere solutions were only poorly correlated with Cd concentrations in leaves and tubers. In contrast to Cd, there were no soil parameters that individually predicted Zn concentrations in leaves and tubers closely. However, multiple correlation analysis (including Zn concentrations in rhizosphere solutions and in bulk soils) closely predicted Zn concentrations in leaves and tubers (adj. R2 = 0.85 and 0.70, respectively). This suggests that the great variability among soils in the solubility of Zn affected the rate of release of Zn into solution, and thus Zn uptake. There was no such effect for Cd, for which solubility varied much less. Furthermore, the plants may have partly controlled Zn uptake, as they took up relatively less at high solution concentrations of Zn.Free ionic concentrations in soil solution did not predict concentrations of Cd or Zn in plants better than their total concentrations in solution. This suggests that with these soils, analysis of Cd and Zn speciation is of little practical importance when their bioavailability is assessed.

Metal uptake by plants from sludge-amended soils: caution is required in the plateau interpretation

Uptake of metals by plants growing in sewage sludge-amended soils frequently exhibits a plateau response at high sludge loading rates associated with high total concentrations of metals in the soil. This type of response has generally been attributed to attenuation of metal bioavailability by increased sorption sites provided by the sludge constituents at the high sludge loading rates. We grew Raphanus sativus L. in a soil historically amended with sewage sludge at different rates and examined concentrations of Cd and Zn in the plants and in corresponding rhizosphere soil solution. Metal concentrations in the plants displayed a plateau response. However, concentrations of total or free metals in the soil solution did not display a similar plateau response, therefore the pre-requisite for determining that metal uptake by plants was limited by sludge chemistry was not met. It was concluded that plant physiological factors were responsible for the plateau in plant metal concentrations observed in this study. Examination of data by other authors suggests that a plateau response due to plant physiological factors has routinely been misinterpreted as being the result of only attenuation by sludge chemistry. The serious implications of an incorrect interpretation of the factors underlying a plateau response are discussed.

Speciation of dissolved cadmium: Interpretation of dialysis, ion exchange and computer (GEOCHEM) methods

Equilibrium dialysis and ion exchange methods, as well as computer calculations (GEOCHEM), were applied for speciation of dissolved cadmium (Cd) in test solutions and leachate samples. The leachate samples originated from soil, compost, landfill waste and industrial waste. The ion exchange (IE) method separates dissolved Cd into free divalent Cd (Cd2+) and complexed Cd and furthermore separates the latter into the operationally defined forms: labile, slowly labile and stable complexes. The dialysis (ED) method determines high molecular weight Cd complexes (above 1000 mol. wt). For both method the reproducibility was good. By combining the results of the GEOCHEM calculations in terms of the inorganic complexes, and the IE results, the fractions of free and inorganically complexed Cd were estimated. The IE and ED results furthermore provided information about the organic complexes. Selected environmental leachates showed different Cd speciation patterns as expected. Some leachates were dominated by free divalent Cd (1–70%), some by inorganic complexes (1–87%), and some by organic complexes (7–98%).

Cadmium and nickel distribution coefficients for sandy aquifer materials

Distribution coefficients (Kd) were measured for cadmium (Cd) and nickel (Ni) in laboratory batch experiments for 18 sandy aquifer materials at environmentally relevant solute concentrations (Cd: 0.1–15 μg/1; Ni: 4–200 μg/1). The Kd-values ranged three orders of magnitude: Cd: 2–1770 1/kg and Ni: 3–7250 1/kg. Correlating observed Kd-values to characteristics of the aquifer materials (particle size fractions, organic-C content, surface area, pH) revealed very good correlation with pH in the pH range 4.9 to 8.9 (Cd: r2 = 0.942; Ni: r2 = 0.929). Including any other of the measured aquifer material characteristics did not improve the correlation significantly. The results indicate that at the same pH-values, the Kd-values for aquifer materials are 4–10 times lower than Kd-values for soils reported in the literature. However, the mobility of Cd and Ni, as reflected in the Kd-values, is still very limited in sandy aquifers, unless the pH is very low. The results indicate that at pH 6 the relative mobility of both Cd and Ni is of the order of 1%.

Measured Soil Water Concentrations of Cadmium and Zinc in Plant Pots and Estimated Leaching Outflows from Contaminated Soils

Soil water concentrations of cadmium and zinc were measured in plant pots with 15 contaminated soils which differed in origin, texture, pH (5.1 – 7.8) and concentrations of cadmium (0.2 – 17 mg Cd kg-1) and zinc (36 – 1300 mg Zn kg-1). The soil waters contained total concentrations of 0.5 to 17 µg Cd L-1 and 9 to 3600 µg Zn L-1, which were dominated by free metal ions as measured by an ion exchange-resin method. Annual leaching outflows were estimated from soil water concentrations to be 0.5 – 17 g Cd ha-1 y-1 and 9 – 3600 g Zn ha-1 y-1 per 100 mm of net percolation, corresponding to 0.1% per year of the total soil content of cadmium and zinc. The measured soil water concentrations of cadmium and zinc did not correlate linearly with the corresponding soil concentrations but correlated fairly well with concentrations measured in Ca(NO3)2 extracts of the soils and with soil water concentrations estimated from soil concentrations and pH. Such concentration estimates may be useful for estimating amounts of cadmium and zinc being leached from soils.

A field method for determination of groundwater and groundwater-sediment associated potentials for degradation of xenobiotic organic compounds

Abstract Determination of the degradation potentials for a mixture of eight organic trace contaminants (benzene, toluene, o -xylene, naphthalene, tetrachloromethane, 1,1,1-trichloroethane, trichloroethene, tetrachloroethene) has been made by specially developed in situ microcosms under aerobic and anaerobic aquifer conditions. The developed in situ microcosms allowed for determination of the degradation potentials in the aquifer as represented by the combined groundwater and sediment and by the groundwater only. Six out of eight microcosms functioned hydraulically well as determined by means of a hydraulic tracer. Control experiments showed that the in situ microcosms were not subject to unaccounted losses of the contaminants except from sorption to sediment in the beginning of the experiments. All compounds were studied at initial concentrations of approximately 120 μg/l for a 90 days period. In all experiments the groundwater alone exhibited a degradation potential for the same compounds as those in the case of groundwater plus sediment. These results may have implications for the use of groundwater test systems as means for an evaluation of biodegradation potentials in aquifers.

Solubility of Cadmium in Polluted Soil

Disposal of waste, industrial activities or sewage sludge application to land, may result in heavy metal polluted soils. Cadmium (Cd) is one of the heavy metals of most concern because of its relatively high mobility and its high toxicity.