Site description
The contaminated site (75% of the total area of 70,000 m2), located north-west of Berlin, Germany, was the site of a tank farm for the storage of petroleum products (especially fuel derivatives) from 1920 to 1976. Additionally, a tar distillation facility had been operated there for the first few decades.
Investigations at the site revealed that the main contamination was localized in the upper aquifer, which was separated from the deeper, second aquifer by glacial till. A contamination plume (250 m long and 80 m wide) currently exists in the upper aquifer, which is characterized by quaternary sediments mainly composed of fine-to-medium sand (average hydraulic conductivity K 2 × 10–4 m s−1). The depth to the water table is about 2–3 m and the thickness of the aquifer averages about 7–8 m. The groundwater has a temperature in the range of 10–12 °C, and flows with a low gradient (about 0.05%) and a velocity of 0.15 m d−1 in a south-westerly direction. Soil and water in the vicinity of the pollutant plume is contaminated with gasoline hydrocarbons originating from leaking underground storage tanks and surface spills. The main components are the BTEX compounds (benzene, toluene, ethyl benzene, and xylene isomers) and the three TMB isomers (1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene) (Table 1). Furthermore, polycyclic aromatic hydrocarbons (PAH) and alkyl phenols are detectable at significantly lower concentrations in the upper aquifer.
Table 1 Minimum and maximum pollutant concentrations in the vicinity of the pollutant plume in the groundwater in mg L−1 and in the soil in mg kg−1 Anoxic conditions predominate in the vicinity of the contaminant plume. The levels of potential electron acceptors are in the range of 0.5–10 mg L−1 for nitrate and 10–100 mg L−1 for sulfate. Due to the reducing conditions, iron is only present in bivalent form, with concentrations ranging from 13 to 17.5 mg L−1.
The electron acceptors nitrate and sulfate were infiltrated into the aquifer as part of an ENA project to boost the degradation of the pollutants. The successful anaerobic degradation of BTEX and TMB under nitrate and sulfate reducing conditions was confirmed in [31].
Column experiment—setup
Two stainless steel columns (one biologically active, the other abiotic as a blank) with a length of 47 cm and an internal diameter of 7 cm were used for the experiment. The inflow and outflow pipes were also made of stainless steel (Fig. 1).
The columns were filled with sandy soil material with low pollutant loading (Table 2) taken by tube core drilling at the edge of the contaminant plume from a depth of 5–6 m. The soil material was homogenized and packed into the columns by excluding oxygen. Before the beginning of the actual experiment, the hydraulic properties of the soil material were determined by means of sieving analysis [32] and tracer tests [33] (Table 2).
Table 2 Hydraulic properties and of the soil material used Anaerobic groundwater with low contamination, and distilled anaerobic water concentrated with nitrate were infiltrated into the test column by means of piston pumps in up flow mode to prevent channeling and differential gravity flow. The residence time of the infiltrated water in the column was 3 days, corresponding to the average velocity of the groundwater at the site of 15.6 cm d−1.
Pressure equalization in the flask containing the nitrate solution was carried out by means of a bag filled with nitrogen. To remove residual oxygen from the gas bag and the pipes, the nitrogen was passed through a solution of 10 mg L−1 sodium dithionite via a silicone tube. The test columns were operated at a temperature of 10–12 °C, simulating the conditions in the aquifer.
Sampling to analyze aromatic hydrocarbon as well as nitrate, nitrite, and sulfate took place at the inflow (P0) and outflow of the columns. In order to prevent the BTEX and TMB outgassing in the outflow, the samples were collected in gas-tight bags.
Column experiment—operating system
Conditioning phase (CP)
In the conditioning phase, anaerobic groundwater with low contamination taken from the edge of the contaminant plume was infiltrated without the addition of nitrate (Table 3). Specific stainless steel containers were used to transport large quantities of groundwater under strict anaerobic conditions from the site to the laboratory. The tanks were equipped with gas sampling bags to prevent the evaporation of volatile compounds. The tanks and gas bags were flushed with nitrogen to remove the oxygen before sampling. The water in the abiotic control column also contained sodium azide with a concentration of 1 g L−1, a common method to prevent biological activity [34].
Table 3 Concentrations of the compounds in the infiltrated groundwater Soil pore water associated with leaching BTEX and TMB from the soil material was exchanged until only low concentrations of these compounds were detectable in the outflow. This was expected to minimize the consumption of infiltrated nitrate by the biodegradation of BTEX and TMB in the following main experimental phase. Approximately 15 pore volumes were exchanged.
Main experimental phase (MEP)
Initially, oxidation of the bivalent iron contained in the anaerobic groundwater due to the addition of nitrate was observed. Therefore, distilled anaerobic water concentrated with nitrate (in the form of KNO3) in a concentration of 300 mg L−1 was used instead of anaerobic groundwater in the columns during the following main experimental phase to prevent blockage due to the precipitation of iron. In addition, the macro-nutrient phosphate (in the form of Na2HPO4) and a trace element solution was dosed into the column at a concentration of 1 mg L−1 (Table 4) to provide ideal conditions for the microorganisms [35].
Table 4 Composition of the trace element solution (TES) in mg L−1 The components of the trace element solution were extended to the elements magnesium, tungsten, selenium, and aluminum.
The consumption of nitrate for the biodegradation of BTEX was avoided by infiltrating pollutant-free water. Furthermore, the oxidation of the bivalent iron contained in the anaerobic groundwater and the resulting precipitation were prevented.
The soil pore water was exchanged until a concentration difference between the infiltrated sulfate and the sulfate contained in the pore water was no longer detectable. Approximately 45 pore volumes were exchanged.
Analysis
Aromatic hydrocarbon analysis was performed according to [36]. A Hewlett Packard 6890 gas chromatograph (GC) equipped with a splitless injection port, a 0.53 mm × 29.8 m DB624 capillary column with a film thickness of 3 μm, and a flame ionization detector was used. The chromatographic conditions were injection port temperature 250 °C, initial column temperature 90 °C, initial time 10.5 min, heating rate 5 °C min−1, final temperature 250 °C, final time 1.5 min, and column flow rate 4 mL min−1 helium. The detection limits for all the identified compounds varied from 0.005 to 1 mg L−1.
The ions nitrate, nitrite, and sulfate were analyzed according to [37]. A Metrohm 733 Separation Center equipped with a Metrosep separation column Chrompack 7414 (4.6 × 75 mm) and a conductivity detector were used for analysis. The detection limits were 0.5–25 mg L−1 for nitrate, sulfate, and phosphate, and 0.1–25 mg L−1 for nitrite.
Sulfate-S and sulfide-S concentrations in the soil were analyzed on an EA 2000 elemental analyzer multi (AnalytikJena GmbH, Jena, Germany) using the NDIR (non-dispersive infrared spectrometry) method. In the induction furnace, the sample was melted in a stream of pure oxygen at temperatures of 600 and 1400 °C, the sulfide-S and sulfate-S contained reacting to form sulfur dioxide (SO2). The sulfur dioxide was detected in infrared measuring cells, the detection limit being 2 mg kg−1.
Calculation of mass balance
The plausibility of nitrate consumption and sulfate formation for the oxidation of sulfides was analyzed by a mass balance between the inflow and outflow of the columns. This was done using Eqs. 1 and 2 based on the redox reaction of nitrate with iron disulfide (FeS2). It was assumed that nitrate was completely converted into nitrite (Eq. 1) or nitrogen (Eq. 2) by chemolithotrophic denitrification [10,11,12]:
$$7{\text{NO}}_{3}^{ - } + {\text{FeS}}_{2} + {\text{H}}_{2} {\text{O}} \to 7{\text{NO}}_{2}^{ - } + {\text{Fe}}^{2 + } + 2{\text{SO}}_{4}^{2 - } + 2{\text{H}}^{ + }$$
(1)
$$14{\text{NO}}_{3}^{ - } + 5{\text{FeS}}_{2} + 4{\text{H}}^{ + } \to 10{\text{SO}}_{4}^{2 - } + 5{\text{Fe}}^{2 + } + 7{\text{N}}_{2} + 2{\text{H}}_{2} {\text{O}}$$
(2)
The additional consumption of nitrate by the oxidation of the bivalent iron formed according to Eqs. 1 and 2 was calculated according to Eq. 3 [18, 23, 24]:
$$2{\text{NO}}_{3}^{ - } + 10{\text{Fe}}^{2 + } + 24{\text{H}}_{2} {\text{O}} \to {\text{N}}_{2} + 10{\text{Fe}}\left( {{\text{OH}}} \right)_{3} + 18{\text{H}}^{ + }$$
(3)
Furthermore, nitrate consumption by chemoorganotrophic denitrification must be taken into account when calculating mass balance. Nitrate consumption occurs through the oxidation of organic electron donors such as dissolved organic carbon (DOC) or bound organic material in the soil (BOM). Since the composition and structure of the carbon is often unknown, the stoichiometric calculation of nitrate consumption is done according to Eq. 4 [9, 26, 38]:
$$4{\text{NO}}_{3}^{ - } + 5{\text{CH}}_{2} {\text{O}} \to 2{\text{N}}_{2} + 4{\text{HCO}}_{3}^{ - } + {\text{CO}}_{2} + 3{\text{H}}_{2} {\text{O}}$$
(4)