Can a pollution event be detected using a single biological effects monitoring method?

The trends of malformation prevalence in embryos of dab, Limanda limanda, in the southern North Sea after the year 1990 mirrored the drop in major pollutants in the rivers draining into the German Bight. Despite this general decline, we detected a pollution event in the southern North Sea in winter 1995/1996 employing the prevalence of malformations in pelagic dab embryos as an indicator. An abrupt rise in malformation prevalence in the embryos of dab, corresponded to a dramatic increase in DDT levels in parent fish from the same area, indicating a hitherto unnoticed introduction of considerable quantities of DDT into the system.     

Reactive modelling of CO2 intrusion into freshwater aquifers: current requirements, approaches and limitations to account for temperature and pressure effects

The modelling of CO₂ intrusion into virtual freshwater aquifers after a leakage from CO₂ storage formations is a well-established approach for the identification of monitoring parameters and for the risk assessment. At presence, standard or close-to-standard conditions in terms of temperature (T), i.e. 25?°C and pressure (P), i.e. 1?C5?bar, are assumed. This approach neglects the fact that temperature and pressure conditions change with the depth of the freshwater aquifer. This study tests the accuracy of T?CP corrections of the geochemical constants in the system gaseous CO₂?Cwater?Cmineral which are performed by the simulators PhreeqC (Parkhurst and Appelo in User??s guide to phreeqc (version 2)??a computer program for speciation, batch reaction, one-dimensional transport, and inverse geochemical calculations. Technical report, US Department of the Interior, 1999) and TOUGHREACT (Xu et?al. in Toughreact user??s guide: a simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media. Technical report, Lawrence Berkeley National Laboratory, 2004). It further identifies the impact of T and P variations on the predicted concentrations of the monitoring parameters pH and total inorganic carbon (TIC) and on the predicted concentration of the trace metal lead (Pb) in 3D multiphase-multicomponent simulations of virtual aquifers. The results reveal a strong imprecision in the correction of kinetic rates of mineral dissolution and a lack of corrections of sorption equilibrium states. The predicted pH and concentrations of TIC and lead depend strongly on the assumed T and P conditions. It is concluded that a neglect of T and P effects results in inaccurate predictions of groundwater chemistry. The impact assessment and monitoring strategies based on currently available modelling results consequently require strong improvements.     

Suitability of Existing Numerical Model Codes and Thermodynamic Databases for the Prognosis of Calcite Dissolution Processes in Near-Surface Sediments Due to a CO2 Leakage Investigated by Column Experiments

Among the risks of CO₂ storage is the potential of CO₂ leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO₂ can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO₂ storage site is essential for assessing the impact of CO₂ leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO₂ partial pressures are rare. Here, the first study for column experiments is presented applying CO₂ partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N₂-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi:10.2475/ajs.278.2.179, 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO₂ leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO₂ leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m.