Military and ammunition sites which are contaminated with explosives often cover great areas

1. Risk assessment and remediation of military and ammunition sites

Dr. André Gerth, Anja Hebner

Contamination of soil and water with explosives, especially 2,4,6-Trinitrotoluol (TNT), is a widespread problem on military sites, explosives producing plants and ammunition factories. Because the contaminated areas are often very large, off site soil treatment technologies are prohibitively costly.¹

Facilities for the treatment of explosive contaminated ground- and surface water have to be operated for decades. Since there is a need for cost-effective new technologies for cleaning up soil and water, biological technologies for the treatment of TNT in both media have been developed and applied. In the following chapters an overview about the in situ remediation of a former trickling pond by autochthonous microorganisms and the treatment of explosives polluted surface water from an ordinary detonation ground.

1.1 Remediation of a contaminated area on a ammunition plant

1.1.1 Introduction

Military and ammunition sites which are contaminated with explosives often cover great areas.² The soil contamination is often very inhomogeneous. This is on the fact of the physical qualities (relatively non-volatile, low aqueous solubility) of explosives.

Within view decimetre varieties on concentration up to hundredfold are reported.³ The variation coefficient of samples from 11 abandoned sites in the USA identified for TNT 248 % and for Hexogen 137 %.⁴ As a result of this, the sampling error is more important than the analytical error.

To obtain representative results and a well established risk assessment a great amount of soil samples, especially on extensive sites, are necessary.

Traditional sampling approach uses large sampling raster, a small number of discrete samples and off-site analysis (RP-HPLC with UV or DAD-detection, GC-ECD). These methods are very expensive. Because of the sample preparation the need a long period until results are available.

High analytical costs and expenditure of time can be reduced by using field analytical methods. The possibilities for TNT on site analytic are colorimetric methods, immunological methods, mobile GC´s and TNT sensors. An immunoassay for TNT (D-Tech®) from Strategic Diagnostics Inc., a Continuous Flow Immunosensor (FAST 2000/6000) from Research International1 and an Ion Mobility Spectrometers are also available.

The colorimetric methods based on the Janovsky-Reaction. Acetone which is added to the sample dissolves the NAC´s and forms red coloured adducts.⁵

These methods present a good quantitative agreement with lab methods, show the presence of also of other explosives (e.g. DNT´s) and need only few experiences.

By using on-site analytic:

• 
  lots of samples can be investigated in a short period of time
  
• 
  contaminated sites can be determined and outline very quick
  
• 
  provides information’s about the heterogeneity of explosives in the soil
  
• 
  selective variety for laboratory analyses
  
• 
  reduction of costs for laboratory analyses and risk assessment in whole.
  

The colorimetric tests have the disadvantages that they are not specific for single substances and yellow colour of humic substances can give interferences.

In the project colorimetric methods were used for on site analysis to identify highly contaminated hot spots and for the selection of samples for lab analysis.

The remediation of contaminated soil with traditional technologies is very expensive. A viable option for the remediation of these sites is biological treatment technique. TNT and other explosives can be biologically transformed, immobilised and degraded by indigenous microorganisms, if they are stimulated by the addition of nutrients. A very effective agent for bioremediation of explosives is molasses.⁶

A reasonable strategy for risk assessment should include a field analytical method. The processing of on site investigations can be done by extensive research (raster investigations) and individual objects.

1.1.2 Starting situation

On the site of an ammunition plant explosive contaminated waste water from the production of ammunition was seeped away in a trickling pond till 1982. The pond is about 300 m long and was overgrown by trees and shrubs. For the site a risk assessment was done. The soil in this area was highly contaminated with TNT and its transformation products.

On the site concentration of about 50,000 mg/kg TNT there measured. Concentration of around 1,000 mg/kg there found until 1 m under the top soil. Crystalline TNT (figure 1) partly can be found in the soil in the near of the inlet of the pond. Very high TNT concentration could be found in layers of clayey consistence in the sandy soil. The whole contaminated area was about 2,000 m².²

As a result of the risk assessment the remediation of the soil was necessary to protect the environment.

figure 1: crystalline TNT found on a contaminated site

1.1.3 Selection of remediation method

On the base on the risk assessment different possible remediation technologies were discussed. The excavation of the complete area and off site incineration would cause unacceptably high costs. For the remediation of the soil of the former trickling pond a site specific innovative strategy developed. This strategy includes 3 technological steps. At the beginning soil with a concentration until 1,000 mg/kg (hot spots) was excavated. After the excavation of the hot spots a biological in situ remediation step takes place. By establishing a vegetative cover (third step) on the area a long term protection of the groundwater was achieved.

In comparison to conventional treatment the in situ technologies are very favourable, environmentally and economically. The excavation and off site transport only of a small part of the contaminated soil are necessary.

1.1.4 Specification of the treatment steps
For the site of the trickling pond a site specific strategy for the remediation was established. Highly contaminated hot spots were excavated to protect the groundwater. The applied biological treatment technology was tested in pilot scale on site. The feasibility study was done for the estimation of remediation time and TNT reduction which can be achieved by the biological transformation.

Finally a vegetative cover was planned for the restoration and continuing groundwater protection.

1) Excavation of hot spots

The excavations of the hot spots were done in summer 2002. The residues which should be excavated were classified by on site analytical methods. The excavation residues were temporary stored on the site and afterwards disposed.

As a result of the site investigations by soil sampling the highest contaminations are supposed on the area of the former inlet of the pond. During the excavation a further hot spot was found with TNT concentrations of several thousand mg/kg TNT. Therefore on several places greater amounts of soil must be excavated than expected by the preliminary investigations. The TNT was found particularly in clayey soil.

figure 2: excavation of the hot spots

2) Treatment of soil by in situ bioremediation
On the base of feasibility studies an in situ technology was used for the biological remediation of TNT and his transformation products on the site. The microbiological transformation of TNT in the soil was stimulated by the addition of a carbon source and iron particles. The amount of additives is 5 litres of molasses and 5 kg iron particles per m². For the treatment of the site (2,000 m²) 10 m³ molasses and 10 t irons are needed. The addition of molasses and a following mechanical tilling (until 50 cm) was done quarterly and of iron ones a year.²

All in all the biological remediation takes two years and six treatment measures respectively. A treatment measure in winter time is not effective because of low microbiological activity in soil caused by low temperatures. The adjustment of additives carried out cost effective by using agricultural devices (figure 3 and 4).

Following on every soil measure grass and wild herbs were sowed on the treated area.

figure 3: addition of molasses

figure 4: mechanical tilling of the soil

During every treatment measure 50 soil samples were taken on the site. For the monitoring the following procedure applied:

1. 
   For gathering heterogeneity of TNT contamination in soil five samples per cluster were taken. The primary samples in the cluster were taken in a distance of 50 cm. The distance between two clusters was 10 metres.
   
2. 
   All primary samples were analysed on site by using photometric tests.
   
3. 
   The primary samples per cluster (five samples) were combined to one composite sample.
   
4. 
   The composite samples were analysed by GC/ECD.
   

In figure 5 the decrease of TNT concentration during the in situ treatment illustrated. TNT concentrations until 1,000 and 100 mg/kg TNT measured in at least one primary sample per cluster are characterized in this figure.

The TNT concentration was significantly reduced by in situ remediation. The remediation targets were obtained. The average concentration before realisation the first in situ treatment measures was 372 mg/kg TNT. After the sixth in situ treatment measure the average TNT concentration was about 23 – 24 mg/kg.
3) Vegetative covering
As final step the site of the former pond was capped by a layer of soil with a high water storage capacity (clay) and the layer of top soil. After a soil preparation grasses were sowed and trees (poplar, willow) are planted on the site. The established vegetation minimizes the formation of leachate. The plants effect an increase of evapotranspiration. The combination of grass and trees was chosen to develop a vegetative cover in short period of time.

1.1.5 Summary

A pond on an ammunition plant was used for the disposal of wastewater from ammunition production. The wastewater from the production buildings was discharged into the pond until 1982.

The soil remediation was necessary because groundwater continued to be polluted with leaching TNT. The highest concentration was found > 50,000 mg/kg TNT. TNT-concentrations above 1,000 mg/kg were only found in the first 100 cm of the soil profile.²

The excavation of the complete area and off site incineration would cause unacceptable high costs. A multi-stage treatment train was developed that incorporates an in situ bioremediation step. The in situ bioremediation as the only technology would have been risky, because it has never been tried with the high TNT concentration found on this site. The remediation time would be in order of decades. The in situ technology is very favourable, environmentally and economically.

The soil contaminated with < 1,000 mg/kg TNT (hot spots) was removed under filed analytical control and treated off site.

The biological treatment of TNT polluted soil on the described site was realised into two years. Altogether six treatment measures which include the combined addition of elemental iron and molasses and mechanical tilling were necessary.

The molasses was needed for the stimulation of the degradation processes and elemental iron to reach strongly reducing conditions. A repeated addition and mechanical tilling leads to an even distribution of the amendments, a homogenisation of the soil and the cycling of anoxic and oxic conditions. The process reaches the uppermost 50 cm oft the soil profile.

The TNT concentration was reduced by the biological transformation until 100 mg/kg.

The vegetation cap protects the groundwater from remaining contamination. The seepage water is reduced by a layer with a high water holding capacity and planting with trees. The long-term phytoremediation by the trees remaining contamination will be completely removed.

figure 5: TNT concentration in the soil

1.2 Biological treatment of explosives contaminated surface water in a wetland system

1.2.1 Starting situation

On a military training area in the north of Bavaria (Germany) 4 Million of soldiers were educated since 1936. A part of this training area is a still active used detonation ground. On this detonation ground the application of explosives were proved and tested. By every detonation about 1 to 2 % of the explosives are not burnt. The explosives were washed out by precipitation.

Through the continual detonation activities the underground (sand stone) was compacted and the sandstone becomes porous. The changes in the geological formation of the soil led to an increase of the discharge of explosives by leachate. Because of the loamy top soil rainwater was gathered on the ground. The availability of the detonation ground for the military activities was limited.

In a pilot project the active used detonation ground was reconstructed and a constructed wetland for the treatment of the collected drainage water was planned and built.

1.2.2 Reconstruction of the detonation ground

The top layer of the detonation ground was excavated until a depth of 0.5 metres. The excavated soil was intermediate stored on the site. A Bentonit sealing was installed on the ground of the area. A protective and a drainage layer were built up above the sealing. The excavated soil was built in the slopes of the reconstructed ground (figure 6).

The new top layer of the detonation ground was made from coarse-grained gravel. On the surface area of the street were compressed stone chips built in. The levelling of detonation crater on the reconstructed detonation site is necessary to protect the Bentonit sealing.

The drainage water flows trough several shafts and a grit chamber into the built treatment plant.

Bentonit

figure 6: scheme of the design from the reconstructed detonation ground

figure 7: laying of the Bentonit mats

1.2.3 Technical solution for the treatment of the explosives contaminated drainage water

A retention basin and a constructed wetland were planned and built for the treatment of the explosives (TNT, Hexogen, Octogen) polluted drainage water. The maximum concentrations of explosives measured in the surface water in 2000 were: 7,400 µg/l TNT, 370 µg/l Hexogen and 55 µg/l Octogen.⁷ The concentration of explosives depends on precipitation and explosives charge used for detonation on the ground.

The drainage water passes the retention basin and the wetland in line. Both basins are sealed with folia.

In the retention basin the drainage water was collected. The concentration of explosives in the drainage water was balanced. The retention basin contains reactive iron. The iron causes an oxygen removal. A wall inside of the basin made from gravel retains solid substances.

The pre-treated and oxygen reduced water flows into a shaft. The water flows with a defined flow rate from this shaft into the constructed wetland. For an effective microbiological transformation of the explosives molasses was added to the water.

After passing the constructed wetland the treated water was discharged into a nearby water trench. Sensors for the measurement of nitrate, pH and water temperature are installed in the outflow shaft of the wetland. The measurement of this parameter in the effluent is necessary for the regulation of the dosage of molasses. The data were registered and transferred via modem. The data registration is combined with a default announcement. Therefore critical values are defined. The default announcement enables fast reaction (decrease of the flow rate, increase of molasses dosage) by changing redox conditions in the wetland. The energy for these facilities was produced by a solar panel.

Since the beginning of operation of the treatment plant several technical problems take place. An intensive test phase was required for the optimization of effectivity of the treatment.

The reduction of the transformation products of the TNT, ADNT´s (2-ADNT, 4-ADNT), is critical by changing of air temperature in the autumn and in the spring. In the autumn and in the spring the detonation ground was much used and high precipitation occur on this location.

The limits for explosive compounds presently not exist in Germany. The responsible public authority had defined target values for the explosives. This target values reached in failure-free operation of the plant.

figure 8: after excavation of soil and make the profile placement of the folia

figure 9: placement of the inflow and outflow drainage, filling of the gravel

f
igure 10: treatment plant after plantation and start of operation

1.2.4 Summary

On an operating military detonation ground of the German army explosives were washed out by precipitation. The detonation ground was reconstructed including a drainage system. A treatment system was built for the explosives (TNT, Hexogen, Octogen) contaminated drainage water. The water was collected in drainages and fed into a retention basin and a constructed wetland. For the microbial transformation of explosives reducing conditions are needed. Because of this the retention basin contains reactive iron and molasses is added into the inlet of the wetland.

The water treatment system is very sensitive against modifications of the redox condition. The limits for the COD, BOD, total nitrogen and total phosphate can be unfailing reached. Through the site- specific optimisation of the operation parameters during the test phase the target values for the explosive compounds can also be met.

The reconstruction of the detonation ground and the construction of the treatment plant were done with Army personnel. The water treatment system needs only little maintenance and operating costs.

The surface water of another detonation ground on this military site is planned to add to the treatment plant.
Acknowledgement
This work was partially supported by Mr. Witt (Rheinmetall DeTec AG, Germany), Mr. Emmert (Standortverwaltung Hammelburg, Germany), and funded by BMBF under grant Nr. 02 WU 0101, 02 WU 0102).
References:
[1] Gerth, A., Hebner, A., Thomas, H.: Case Study Natural Remediation of TNT-Contaminated Water and Soil, Acta Biotechnol. 23 (2003) 2-3, 143-150

[2] Gerth, A., Thomas, H., Hebner, A.: Biologische In-situ-Sanierung von TNT kontaminiertem Boden, TerraTech 1-2 (2005), TT 21-23

[3] Jenkins, T.F. e.a.: Assessment of Sampling Error Associated with Collection and Analysis of Soil Samples at Explosives-Contaminated Sites. U.S. Army Corps of Engineers, Cold Regions Research & Engineering Laboratory, Special Report 96-15

[4] Crockett A.B., Jenkins T.F., Craig, H.D. Sisk, W.E.: Overview of On-Site Analytical Methods for Explosives in Soil (1998) U.S. Army Cold Regions & Engineering Laboratory, Special Report 98-4

[5] Jenkins, T.F.: Development of a Simplified Field Method for the Determination of TNT in Soil. U.S. Army Cold Regions Research & Engineering Laboratory, Special Report 90-38

[6] Thomas, H., Hebner, A., Gerth, A.: Nutzung des biologischen Abbaus von TNT zur Sanierung von Rüstungsaltlasten, GWF Wasser Abwasser, 144 (2003) Nr. 5, 355-358

[7] Emmert, H., Kuhne, A. Thomas, H.: Remediation and environmentally sustainable reconstruction of a Ordnance Detonation Site, Mutual Weapons Development Master Data Exchange Agreement between the Government of the united States and the Government of the Federal Republic of Germany, 14th General meeting 2004, Berlin, 16. – 20. February 2004