Miljøprojekt, 883; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – GEOPROC - edb program til vurdering af geokemiske processer

GEOPROC - evaluation program for geochemical processes

Summary and conclusions

Soil and groundwater pollution often occurs in shallow aquifers with a complicated geology and poorly defined hydraulic conditions. A reliable delineation of the contaminant plume is a challenging task even when having optimal sampling locations. Groundwater samples may show apparent temporal variations of contaminant concentration and contaminants can bypass sampling locations due to the dynamic nature of groundwater systems. This may in turn lead to wrong conclusions regarding the rate of natural attenuation processes.

Usually, “the dilution problem” is addressed using non-reactive components as chloride and in some cases recalcitrant organic components as e.g. trimethylbenzenes. However, these key components may not be sufficiently available in the source zone or – in case of organic components – may themselves undergo transformation, adsorption etc. Biogeochemical transformation and mass transfer reactions with sediments and soil gases may lead to significant changes of the groundwater chemistry apart from changing the concentration of the anthropogenic components. In these cases detailed geochemical calculations using numerical solution (e.g. PHREEQC) might be an option to give an independent estimate of the dilution. Usually, only expert geochemists undertake this option as the calculations tends to be rather complicated.

To overcome this obstacle the Danish Environmental Protection Agency has developed an expert system (“GEOPROC”) for numerical geochemical calculations intended for free use by consultants, administrators etc. with experience but not expert skills.

GEOPROC has three major parts:

Input data and quality control. The user has to enter key data of the sampling points (distance, contaminant concentration) and as many as possible of the major components of the groundwater chemistry (Ca, Mg, Na, K, NH₄, Fe, Mn, HCO₃, Cl, SO₄, NO₃, P-PO₄, NVOC, O₂, CH₄, pH, temperature). GEOPROC will calculate the internal consistency of the input data (e.g. charge balance, total inorganic carbon, the CO₂-pressure, and the saturation indices of calcite and siderite).
Geochemical modelling. When entering two or more sample data GEOPROC determines the geochemistry upstream and downstream location comparing the available and already used Gibbs free energy of selected redox reactions (reduction of Fe₂O₃, SO₄, and NO₃, aerobic oxidation, and methanogenesis). The calculation is rather insensitive to dilution and is the sum of the Gibbs free energy release from potential and realized electron transfers (negative) using all of the available components (concentrations in mg/l) and assuming a reaction a standard organic (CH₂O), R-value:

R= (4/32*29.9)*[O₂]+(5/62*28.4)*[NO₃] +(7/96*6.1)*[SO₄]+
(-2/55*24.5)*[Mn²⁺] +(-1/56*12.5)*[Fe²⁺] +(-8/16*5.5)*[CH₄]
(units cal/liter)

The dilution factor is calculated using the generated alkalinity in the hot spot area as a conservative property equivalent to chloride. The alkalinity of the sample with the highest R-value is used as background value and the alkalinity corresponding to the lowest R-value as hot-spot value. The concept of using alkalinity as a conservative property was evaluated at two sites with good results.

The user can optionally change GEOPROCs default values and apply a detailed model to describe the geochemical changes between sampling points and internal data consistency.
1^st order degradation model. GEOPROC calculates apparent and intrinsic 1^st order degradation constants from the entered data. The intrinsic values are calculated using the dilution correction determined from the alkalinity calculations