Sibylle Grandel

The benthic silica cycle in the northeast Atlantic: annual mass balance, seasonality, and importance of non-steady-state processes for the early diagenesis of biogenic opal in deep-sea sediments

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment–water interface and the annual DSi efflux was close to 0.057 mol Si m−2 yr−1. Biogenic silica accumulation was close to 0.008 mol Si m−2 yr−1 and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si m−2 yr−1. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si m−2 yr−1). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.

Monitored natural attenuation of chlorinated solvents: assessment of potential and limitations.

Chlorinated solvent contaminations in groundwater and soil are a widespread problem. Their remediation either by active and/or passive measures is complicated though, due to their characteristic features of long plumes and DNAPL. In this study Monitored Natural Attenuation (MNA) is evaluated regarding its applicability to remediate chlorinated solvent sites. At first a short overview of legal requirements is given, by whose means the efficiency of MNA is supposed to be demonstrated and assessed. Thereby the focus is set on the circumstances in the United States and Germany, as in the first MNA is applied as remediation option, whereas in the latter implementation and definition of MNA is under discussion. Based on criteria given by several German guidelines investigation methods and suitable parameters are discussed with the objective to evaluate their potential and limitations to demonstrate MNA at a chlorinated solvent site. Regarding the description of source location/inventory and source emission the legal requirements cannot be accomplished mainly because of missing methods. With regard to the characterization of plume migration, and evaluation of the efficiency of NA processes the combination of hydrogeochemical data and model approaches can accomplish most of the legal requirements. Applicability of microbiological and isotope methods is limited which is also reflected by evaluation of field studies, where these methods have been used at less than 10% of the sites. With regard to chlorinated solvents current German guidelines should be modified to enable the feasible application of MNA as an alternative or additional remediation option.

Consequences of Different Kinetic Approaches for Simulation of Microbial Degradation on Contaminant Plume Development

This study investigates the consequences of assuming different kinetic approaches for calculation of microbial degradation on plume development in a simple case of natural attenuation on field scale. If the required parameters are properly chosen, all approaches can simulate similar plumes for a particular given time step. The differences of contaminant concentrations in the plumes are small and would not attract attention in a natural aquifer. On long term prognoses the kinetics result in very different plumes: A complex Monod approach considering microbial growth prognoses a further spreading of the plume, compared to a first order rate law, which results in a short and early stationary plume. Other approaches show plumes between these two extremes. On the other hand, the forecasts for plumes assuming Monod kinetics are similar, even if different values for parameterization are chosen. The reason for this insensibility is, that degradation is not limited by microbial kinetics in the simulation, but by dispersive mixing. Simplifying approaches may have few and well determinable parameters, but they are not suited for proper prognoses if they neglect the prerequisite, that contaminant and electron acceptor have to be present for a reaction.

Partial source treatment by in-situ technologies — a review of limits, advantages and challenges

Removal of contaminant sources or associated residual free phase pools often suffers from a combination of inefficiency, increased risk of contaminant spreading due to uncontrolled mobilization, and/or high treatment costs. The paper gives a brief overview of results from laboratory and field studies where chemical and biochemical in-situ source control actions are evaluated with respect to their efficiency on changes of source emission. The studies focused on the contaminant group of chlorinated ethenes. Chemical approaches aim to mobilize contaminant phases. By now these studies have been mainly executed in the laboratory and only few pilotscale field studies exist. The results indicate large emission rates at the beginning of the phase displacement but give less information on long-term emission rates. Biostimulation and bioaugmentation approaches revealed increased emission on short time scales and accumulation of cis- Dichloroethylene and Vinylchloride due to incomplete degradation of higher chlorinated solvents in the source zone. On long-term scales emission rates decreased and groundwater plumes were shrinking.