**5.1 Materials and methods**

With the progress of in situ instruments and sensors developed for spatial research and Mars exploration, a miniaturized Mössbauer spectrometer, Mimos II, was built up [13, 14]. The signal emitted by the source of the spectrometer is recorded using reflection-based geometry, which is not influenced by the thickness of the sample, unlike conventional transmission geometry [15]. Mimos II was placed in a tube, in the soil. Mössbauer spectra were recorded in a nondestructive way through windows at different depths till 1.20 m depth. Periodically the instrument is moved back to fixed positions that allow us to monitor the changes of iron mineralogy as a function of time [15].

The progress of in situ instrumentation also concerns the monitoring of water quality thanks to progress in the oceanographic research. Such a probe was used to monitor soil water in Brittany in the Fougères forest and in Camargue paddy field. The pH, redox potential, temperature, and electrical conductivity were hourly measured, and data were stored in the probe and collected every fortnight.

## **5.2 Results and discussion**

**Figure 7a** shows a typical Mössbauer spectrum obtained in the forested area, characterizing the fougerite mineral, which is a mixed hydroxide of Fe(II) and Fe(III) from green rust family [16, 17]. The points are the measures recorded by the instrument, and the lines are obtained by fitting Lorentzian functions to the signal. Thus the spectrum shows two doublets D1 and D2 characterizing the crystal environment of Fe(II) and one doublet D3 characterizing the crystal environment of Fe(III) in the mineral structure.

About 30 spectra were obtained during one hydrological year (i.e., between October 1998 and September 1999), and the variations of the ratio of Fe(III) to total iron are reported as a function of depth (**Figure 7b**). At a given depth, it changes in time, with fluctuations of the water table and anaerobiosis/aerobiosis conditions. When in reductive conditions, the signal of fougerite appears in less than 1 week [15].

Both at Fougères and in Camargue, plots of Eh-pH diagrams (**Figure 8**) show fast and large variations in short time in a quasi-identical range of variations, though soils are largely different, i.e., acidic in Brittany and carbonated in

**79**

S<sup>2</sup><sup>−</sup>/SO4

**Figure 8.**

**Figure 7.**

*Geochemical Methods to Assess Agriculture Sustainability*

Camargue. The observed measures both at Fougères and in Camargue show large variations covering the whole domain of existence of aqueous Fe(II) and Fe(III)

*Eh-pH variations in soil solution at Fougères and in Camargue recorded in situ with an oceanic probe. Each* 

*(a) Example of Mössbauer spectrum obtained in situ with Mimos II instrument; (b) variation of the* 

*FeIII/Fetotal ratio of iron minerals as function of depth and time (from [15]).*

rH = Eh/0.029 + 2 pH (1)

vary in between 4 and 14.4 at Fougères and in between 4 and 5.5 during the irrigation period in the Camargue. These values are much smaller than 20, which is the upper limit admitted for reducing media [18]. The "loops" of fast evolution, in a few hours, toward oxidizing conditions (i.e., Eh up to 100 mV) with small variations of pH are explained by the infiltration of oxygen-rich rainwater and then a return to

The representation of the thermodynamic equilibria of Fe2+/fougerite and of

shows that the principal controls of the water quality are exerted by Fe2+ versus

<sup>2</sup><sup>−</sup> is given by curve 1 and curve 2, respectively. The geochemical modeling

oxides. The rH values calculated according to the following equation:

*point is the average of sequence of 20 measurements recorded during 1 mn every hour.*

the redox state of the initial steady state.

*DOI: http://dx.doi.org/10.5772/intechopen.85336*

*Geochemical Methods to Assess Agriculture Sustainability DOI: http://dx.doi.org/10.5772/intechopen.85336*

**Figure 7.**

*Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…*

chemical element, the proposed model simulates very well the transformation of irrigation water into groundwater, describing for these 4 years a steady state. Thus, our findings suggest that irrigation, both with the water inputs and quality of water, have played a key role for the sustainability of hay production

The short time (1 hour intervals) of processes in agro- and ecosystems were recorded by in situ monitoring both of solid and water in a forested hydromorphic soil and in a paddy field. In both systems, the geochemistry of iron is marked by strong interactions between the solid minerals and the soil solution when oxidore-

With the progress of in situ instruments and sensors developed for spatial research and Mars exploration, a miniaturized Mössbauer spectrometer, Mimos II, was built up [13, 14]. The signal emitted by the source of the spectrometer is recorded using reflection-based geometry, which is not influenced by the thickness of the sample, unlike conventional transmission geometry [15]. Mimos II was placed in a tube, in the soil. Mössbauer spectra were recorded in a nondestructive way through windows at different depths till 1.20 m depth. Periodically the instrument is moved back to fixed positions that allow us to monitor the changes of iron

The progress of in situ instrumentation also concerns the monitoring of water quality thanks to progress in the oceanographic research. Such a probe was used to monitor soil water in Brittany in the Fougères forest and in Camargue paddy field. The pH, redox potential, temperature, and electrical conductivity were hourly measured, and data were stored in the probe and collected every

**Figure 7a** shows a typical Mössbauer spectrum obtained in the forested area, characterizing the fougerite mineral, which is a mixed hydroxide of Fe(II) and Fe(III) from green rust family [16, 17]. The points are the measures recorded by the instrument, and the lines are obtained by fitting Lorentzian functions to the signal. Thus the spectrum shows two doublets D1 and D2 characterizing the crystal environment of Fe(II) and one doublet D3 characterizing the crystal environment

About 30 spectra were obtained during one hydrological year (i.e., between October 1998 and September 1999), and the variations of the ratio of Fe(III) to total iron are reported as a function of depth (**Figure 7b**). At a given depth, it changes in time, with fluctuations of the water table and anaerobiosis/aerobiosis conditions. When in reductive conditions, the signal of fougerite appears in less

Both at Fougères and in Camargue, plots of Eh-pH diagrams (**Figure 8**) show fast and large variations in short time in a quasi-identical range of variations, though soils are largely different, i.e., acidic in Brittany and carbonated in

**5. Investigations of short time in agro- and ecosystems**

larger than 0.96, regardless of the year or the cut, or the

groundwater. With a R<sup>2</sup>

duction phenomena occurred [11, 12].

mineralogy as a function of time [15].

**5.1 Materials and methods**

till 60 years.

fortnight.

**5.2 Results and discussion**

of Fe(III) in the mineral structure.

**78**

than 1 week [15].

*(a) Example of Mössbauer spectrum obtained in situ with Mimos II instrument; (b) variation of the FeIII/Fetotal ratio of iron minerals as function of depth and time (from [15]).*

**Figure 8.**

*Eh-pH variations in soil solution at Fougères and in Camargue recorded in situ with an oceanic probe. Each point is the average of sequence of 20 measurements recorded during 1 mn every hour.*

Camargue. The observed measures both at Fougères and in Camargue show large variations covering the whole domain of existence of aqueous Fe(II) and Fe(III) oxides. The rH values calculated according to the following equation:

$$\text{rH} = \text{Eh/0.029} + \text{2 pH} \tag{1}$$

vary in between 4 and 14.4 at Fougères and in between 4 and 5.5 during the irrigation period in the Camargue. These values are much smaller than 20, which is the upper limit admitted for reducing media [18]. The "loops" of fast evolution, in a few hours, toward oxidizing conditions (i.e., Eh up to 100 mV) with small variations of pH are explained by the infiltration of oxygen-rich rainwater and then a return to the redox state of the initial steady state.

The representation of the thermodynamic equilibria of Fe2+/fougerite and of S<sup>2</sup><sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> is given by curve 1 and curve 2, respectively. The geochemical modeling shows that the principal controls of the water quality are exerted by Fe2+ versus

green rust fougerite, when reduction is moderate, and sulfide versus sulfate equilibria, when reduction is high.
